IPSEC Working Group H. K. Orman
INTERNET-DRAFT Dept. of Computer Science, Univ. of Arizona
draft-ietf-ipsec-oakley-01.txt May 1996
The OAKLEY Key Determination Protocol
This document describes a protocol, named OAKLEY,
by which two authenticated parties can agree on secure and secret
keying material. The basic mechanism is the Diffie-Hellman key
exchange algorithm.
The OAKLEY protocol supports Perfect Forward Secrecy,
compatibility with the ISAKMP protocol for managing security
associations, user-defined abstract group structures for use with
the Diffie-Hellman algorithm, key updates, and incorporation of
keys distributed via out-of-band mechanisms.
Status of this Memo
This RFC is being distributed to members of the Internet community in
order to solicit their comments on the protocol described in it.
This draft expires six months from the day of issue. The expiration
date will be August 24, 1996.
Required text:
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1. INTRODUCTION
Key establishment is the heart of data protection that relies on
cryptography, and it is an essential component of the packet
protection mechanisms described in [RFC1825, RFC1826, RFC1827], for
example. A scalable and secure key distribution mechanism for the
Internet is a necessity. The goal of this protocol is to provide
that mechanism, coupled with a great deal of cryptographic strength.
The Diffie-Hellman key exchange algorithm provides such a mechanism.
It allows two parties to agree on a shared value without requiring
encryption. The shared value is immediately available for use in
encrypting subsequent conversation, e.g. data transmission and/or
authentication. The STS protocol [STS] provides a demonstration of
how to embed the algorithm in a secure protocol, one that ensures
that in addition to securely sharing a secret, the two parties can be
sure of each other's identities, even when an active attacker exists.
Because OAKLEY is a generic key exchange protocol, and because the
keys that it generates might be used for encrypting data with a long
privacy lifetime, 20 years or more, it is important that the
algorithms underlying the protocol be able to ensure the security of
the keys for that period of time, based on the best prediction
capabilities available for seeing into the mathematical future. The
protocol therefore has two options for adding to the difficulties
faced by an attacker who has a large amount of recorded key exchange
traffic at his disposal (a passive attacker). These options are
useful for deriving keys which will be used for encryption.
The OAKLEY protocol is related to STS, sharing the similarity of
authenticating the Diffie-Hellman exponentials and using them for
determining a shared key, and also of achieving Perfect Forward
Secrecy for the shared key, but it differs from the STS protocol in
several ways.
The first is the addition of a weak address identification
mechanism ("cookies", described by Phil Karn [Photuris]) to help
avoid denial of service attacks.
The second extension is to allow the two parties to select
mutually agreeable supporting algorithms for the protocol: the
encryption method, the key derivation method, and the
authentication method.
Thirdly, the authentication does not depend on encryption using
the Diffie-Hellman exponentials; instead, the authentication
validates the binding of the exponentials to the identities of the
parties.
The protocol does not require the two parties compute the shared
exponentials prior to authentication.
This protocol adds additional security to the derivation of keys
meant for use with encryption (as opposed to authentication) by
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including a dependence on an additional algorithm. The derivation
of keys for encryption is made to depend not only on the Diffie-
Hellman algorithm, but also on the cryptographic method used to
securely authenticate the communicating parties to each other.
Finally, this protocol explicitly defines how the two parties can
select the mathematical structures (group representation and
operation) for performing the Diffie-Hellman algorithm; they can
use standard groups or define their own. User-defined groups
provide an additional degree of long-term security.
OAKLEY has several options for distributing keys. In addition to the
classic Diffie-Hellman exchange, this protocol can be used to derive
a new key from an existing key and to distribute an externally
derived key by encrypting it.
The protocol allows two parties to use all or some of the anti-
clogging and perfect forward secrecy features. It also permits the
use of authentication based on symmetric encryption or non-encryption
algorithms. This flexibility is included in order to allow the
parties to use the features that are best suited to their security
and performance requirements.
This document draws extensively in spirit and approach from the
Photuris draft by Karn and Simpson [Photuris] (and from discussions
with the authors), specifics of the ISAKMP draft by Schertler et al.
[ISAKMP], and it was also influenced by papers by Paul van Oorschot
and Hugo Krawcyzk.
2. The Protocol Outline
2.1 General Remarks
The OAKLEY protocol is used to establish a shared key with an
assigned identifier and associated authenticated identities for the
two parties. The name of the key can be used later to derive
security associations for the RFC1826 and RFC1827 protocols (AH and
ESP) or to achieve other network security goals.
Each key is associated with algorithms that are used for
authentication, privacy, and one-way functions. These are ancillary
algorithms for OAKLEY; their appearance in subsequent security
association definitions derived with other protocols is neither
required nor prohibited.
The specification of the details of how to apply an algorithm to data
is called a transform. This document does not supply the transform
definitions; they will be in separate RFC's.
The anti-clogging tokens, or "cookies", provide a weak form of source
address identification for both parties; the cookie exchange can be
completed before they perform the computationally expensive part of
the protocol (large integer exponentiations).
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It is important to note that OAKLEY uses the cookies for two
purposes: anti-clogging and key naming. The two parties to the
protocol each contribute one cookie at the initiation of key
establishment; the pair of cookies becomes the key identifier
(KEYID), a reusable name for the keying material. Because of this
dual role, we will use the notation for the concatenation of the
cookies ("COOKIE-I, COOKIE-R") interchangeably with the symbol
"KEYID".
OAKLEY is designed to be a compatible component of the ISAKMP
protocol [ISAKMP], which runs over the UDP protocol using a well-
known port (see the RFC on port assignments, STD02-RFC-1700). The
only technical requirement for the protocol environment is that the
underlying protocol stack must be able to supply the Internet address
of the remote party for each message. Thus, OAKLEY could, in theory,
be used directly over the IP protocol or over UDP, if suitable
protocol or port number assignments were available.
The machine running OAKLEY must provide a good random number
generator, as described in [RFC1750], as the source of random numbers
required in this protocol description. Any mention of a "nonce"
implies that the nonce value is generated by such a generator. The
same is true for "pseudorandom" values.
2.2 Notation
The section describes the notation used in this document for message
sequences and content.
2.2.1 Message descriptions
The protocol exchanges below are written in an abbreviated notation
that is intended to convey the essential elements of the exchange in
a clear manner. A brief guide to the notation follows. The detailed
formats and assigned values are given in the appendices.
In order to represent message exchanges succinctly, this document
uses an abbreviated notation that describes each message in terms of
its source and destination and relevant fields.
Arrows ("->") indicate whether the message sent from the initiator to
the responder, or vice versa ("<-").
The fields in the message are named and comma separated. The
protocol uses the convention that the first several fields constitute
a fixed header format for all messages.
For example, consider a HYPOTHETICAL exchange of messages involving a
fixed format message, the four fixed fields being two "cookies", the
third field being a message type name, the fourth field being a
multi-precision integer representing a power of a number:
Initiator Responder
-> Cookie-I, 0, OK_KEYX, g^x ->
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<- Cookie-R, Cookie-I, OK_KEYX, g^y <-
The notation describes a two message sequence. The initiator begins
by sending a message with 4 fields to the responder; the first field
has the unspecified value "Cookie-I", second field has the numeric
value 0, the third field indicates the message type is OK_KEYX, the
fourth value is an abstract group element g to the x'th power.
The second line indicates that the responder replies with value
"Cookie-R" in the first field, a copy of the "Cookie-I" value in the
second field, message type OK_KEYX, and the number g raised to the
y'th power.
The value OK_KEYX is in capitals to indicate that it is a unique
constant (constants are defined the appendices).
2.2.2 Guide to symbols
Cookie-I and Cookie-R (or CKY-I and CKY-R) are 64-bit pseudo-random
numbers. The generation method must ensure with high probability
that the numbers are unique over some previous time period, such as
one hour.
KEYID is the concatenation of the initiator and responder cookies and
the domain of interpretation; it is the name of keying material.
sKEYID is used to denote the keying material named by the KEYID. It
is never transmitted, but it is used in various calculations
performed by the two parties.
OK_KEYX, OK_NEWGRP, and OK_SET_DEF are distinct message types.
IDP is a bit indicating whether or not material after the encryption
boundary (see appendix D), is encrypted.
g^x and g^y are encodings of group elements, where g is a special
group element indicated in the group description (see Appendix Group
Descriptors) and g^x indicates that element raised to the x'th power.
The type of the encoding is either a variable precision integer or a
pair of such integers, as indicated in the group operation in the
group description. Note that we will write g^xy as a short-hand for
g^(xy). See Appendix J for references that describe implementing
large integer computations and the relationship between various group
definitions and basic arithmetic operations.
EHAO is a list of encryption/hash/authentication choices. Each item
is a of pair values: a class name and an algorithm name.
EHAS is a set of three items selected from the EHAO list, one from
each of the classes for encryption, hash, authentication.
GRP is a name (32-bit value) for the group and its relevant
parameters: the size of the integers, the arithmetic operation, and
the generator element. There are a few pre-defined GRP's (for 768
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bit modular exponentiation groups, 1024 bit modexp, 2048 bit modexp,
155-bit elliptic curve, see Appendix H), but participants can share
other group descriptions in a later protocol stage (see the section
NEW GROUP).
The symbol vertical bar "|" is used to denote concatenation of bit
strings.
Ni and Nr are nonces selected by the initiator and responder,
respectively.
ID(I) and ID(R) are the identities to be used in authenticating the
initiator and responder respectively.
E{x}Ki indicates the encryption of x using the public key of the
initiator. Encryption is done using the algorithm associated with
the authentication method; usually this will be RSA.
S{x}Ki indicates the signature over x using the private key (signing
key) of the initiator. Usually this will be RSA or DSS.
prf(a, b) denotes the result of applying pseudo-random function "a"
to data "b".
prf(0, b) denotes the application of a one-way function to data "b".
The similarity with the previous notation is deliberate and indicates
that a single algorithm, e.g. MD5, might will used for both purposes.
In the first case a "keyed" MD5 transform would be used with key "a";
in the second case the transform would have the fixed key value zero,
resulting in a one-way function.
2.3 The Key Exchange Message Overview
The goal of key exchange processing of the secure establishment of
common keying information state in the two parties. This state
information is a key name, secret keying material, the identification
of the two parties, and three algorithms for use during
authentication: encryption (for privacy of the identities of the two
parties), hashing (a pseudorandom function for protecting the
integrity of the messages and for authentication), and authentication
(the algorithm on which the authentication is based). The encodings
and meanings for these choices are presented in Appendix B.
The main mode exchange has five optional features: stateless cookie
exchange, perfect forward secrecy for the keying material, secrecy
for the identities, perfect forward secrecy for identity secrecy, use
of signatures (for non-repudiation). The two parties can use all or
none of these features.
The general outline of processing is that the Initiator of the
exchange begins by specifying as much information as he wishes in his
first message. The Responder replies, supplying as much information
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as he wishes. The two sides exchange messages, supplying more
information each time, until their requirements are satisfied.
The choice of how much information to include in each message depends
on which options are desirable. For example, if stateless cookies
are not a requirement, and identity secrecy and perfect forward
secrecy for the keying material are not requirements, and if non-
repudiatable signatures are acceptable, then the exchange can be
completed in three messages.
Additional features may increase the number of roundtrips needed for
the keying material determination.
ISAKMP provides fields for specifying the security association
parameters for use with the AH and ESP protocols. These security
association payload types are specified in the ISAKMP draft; the
payload types can be protected with OAKLEY keying material and
algorithms, but this document does not discuss their use.
2.3.1 The Essential Key Exchange Message Fields
There are 12 fields in an OAKLEY key exchange message. Not all the
fields are relevant in every message; if a field is not relevant it
can have a null value or not be present (no payload).
CK-I originator cookie.
CK-R responder cookie.
MSGTYPE for key exchange, will be ISA_KE&AUTH_REQ or ISA_KE&AUTH_REP;
for new group definitions, will be ISA_NEW_GROUP_REQ
or ISA_NEW_GROUP_REP
GRP the name of the Diffie-Hellman group used for the exchange
g^x (or g^y) variable length integer representing a power of
group generator
EHAO or EHAS encryption, hash, authentication functions, offered
and selected
IDP an indicator as to whether or not encryption with
g^xy follows (perfect forward secrecy for ID's)
ID(I) the identity for the Initiator
ID(R) the identity for the Responder
Ni nonce supplied by the Initiator
Nr nonce supplied by the Responder
The construction of the cookies is implementation dependent. Phil
Karn has recommended making them the result of a one-way function
applied to a secret value (changed periodically), the local and
remote IP address, and the local and remote UDP port. In this way,
the cookies remain stateless and expire periodically. Note that with
OAKLEY, this would cause the KEYID's derived from the secret value to
also expire, necessitating the removal of any state information
associated with it.
The encryption functions used with OAKLEY must be cryptographic
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transforms which guarantee privacy and integrity for the message
data. Merely using DES in CBC mode is not permissible. The
MANDATORY and OPTIONAL transforms will include any that satisfy this
criteria and are defined for use with RFC1827 (ESP).
The one-way (hash) functions used with OAKLEY must be cryptographic
transforms which can be used as either keyed hash (pseudo-random) or
non-keyed transforms. The MANDATORY and OPTIONAL transforms will
include any that are defined for use with RFC1826 (AH).
Where nonces are indicated, they will be variable precision integers
with an entropy value that matches the "strength" attribute of the
GRP used with the exchange. If no GRP is indicated, the nonces must
be at least 90 bits long. The pseudo-random generator for the nonce
material should start with initial data that has at least 90 bits of
entropy; see RFC1750.
2.3.2 Mapping to ISAKMP Message Structures
All the OAKLEY message fields correspond to ISAKMP message payloads
or payload components. The relevant payload fields are the SA
payload, the AUTH payload, the Certificate Payload, the Key Exchange
Payload.
Some of the ISAKMP header and payload fields will have constant
values when used with OAKLEY:
DOI, the Domain of Interpretation, will have the value INTERNET. In this
document, the DOI will not be mentioned; it is assumed that the
software implementing OAKLEY will always be in the IPv4 or IPv6 DOI.
Unless otherwise noted, the Key Exchange Identifier is Oakley Main Mode.
In the SA Payload, the Situation is ISAKMPID
In the following we indicate where each OAKLEY field appears in the
ISAKMP message structure.
CK-I ISAKMP header
CK-R ISAKMP header
MSGTYPE Message Type in ISAKMP header
GRP In ISAKMP header (replaces SPI's)
g^x (or g^y) Key Exchange Payload, encoded as a variable precision integer
EHAO and EHAS SA payload, Proposal section
IDP Exchange field in the AUTH Payload header
ID(I) AUTH payload, Identity field
ID(R) AUTH payload, Identity field
Ni AUTH payload, Nonce Field
Nr AUTH payload, Nonce Field
S{...}Kx AUTH payload, Data Field
prf{K,...} AUTH payload, Data Field
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2.4 The Key Exchange Protocol
The exact number and content of messages exchanged during an OAKLEY
key exchange depends on which options the Initiator and Responder
want to use. A key exchange can be completed with three or more
messages, depending on those options.
The three components of the key determination protocol are the
1. cookie exchange (optionally stateless)
2. Diffie-Hellman half-key exchange (optional, but essential for
perfect forward secrecy)
3. authentication (options: privacy for ID's, privacy for ID's with PFS,
non-repudiatable)
The initiator can supply as little information as a bare exchange
exchange request, carrying no additional information. On the other
hand the initiator can begin by supplying all of the information
necessary for the responder to authenticate the request and complete
the key determination quickly, if the responder chooses to accept
this method. If not, the responder can reply with a minimal amount
of information (at the minimum, a cookie).
The Initiator is responsible for retransmitting messages if the
protocol does not terminate in a timely fashion. The Responder must
therefore avoid discarding reply information until it is acknowledged
by Initiator in the course of continuing the protocol.
The remainder of this section contains examples demonstrating how to
use OAKLEY options.
2.4.1 An Aggressive Example
The following example indicates how two parties can complete a key
exchange in two messages. The identities are not secret, the derived
keying material is protected by PFS.
By using digital signatures, the two parties will have a proof of
communication that can be recorded and presented later to a third
party.
The keying material implied by the group exponentials is not needed
for completing the exchange. If it is desirable to defer the
computation, the implementation can save the "x" and "g^y" values and
mark the keying material as "uncomputed". It can be computed from
this information later.
Initiator Responder
--------- ---------
-> CKY-I, 0, OK_KEYX, GRP, g^x, EHAO, NIDP, ->
ID(I), ID(R), Ni,
S{ID(I), ID(R), Ni, 0, GRP, g^x, EHAO}Ki
<- CKY-R, CKY-I, OK_KEYX, GRP, g^y, EHAS, NIDP,
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ID(R), ID(I), Nr, Ni
S{ID(R), ID(I), Nr, Ni, GRP, g^y, EHAS}Kr <-
-> CKY-I, CKY-R, OK_KEYX, 0, 0, 0, NIDP, ->
Ni, Nr, S{ID(I), ID(R), Ni, Nr}Ki
NB "NIDP" means that the PFS option for hiding identities is not used.
i.e., the identities are not encrypted using g^xy
The result of this exchange is a key with KEYID = CKY-I|CKY-R and
value
sKEYID = prf(Ni | Nr, g^xy | CKY-I | CKY-R).
The processing outline for this exchange is as follows:
Initiation
The initiator generates a unique cookie and associates it with the
expected IP address of the responder, and its chosen state
information: GRP, g^x, EHAO list. The first authentication choice
in the EHAO list is an algorithm that supports digital signatures,
and this is used to sign the ID's and the nonce and group id. The
initiator further
notes that the key is in the initial state of "unauthenticated",
and
sets a timer for possible retransmission and/or termination of the
request.
When responder receives the message, he may choose to ignore all the
information and treat it as merely a request for a cookie, creating
no state. If CKY-I is not already in use by the source address in
the IP header, the responder generates a unique cookie, CKY-R. The
next steps depend on the responders preferences. The minimal
required response is to reply with the first cookie field set to zero
and CKY-R in the second field. For this example we will assume that
responder is more aggressive and accepts the following:
group GRP,
first authentication choice (which must be a digital signature),
lack of perfect forward secrecy for protecting the identities,
identity ID(I),
identity ID(R)
The responder must validate the signature over the signed portion of
the message, and associate the pair (CKY-I, CKY-R) with the following
state information:
the network address of the message
key state of "unauthenticated"
the first algorithm from the authentication offer
group GRP and a g^y value in group GRP
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the nonce Ni and a pseudorandomly selected value Nr
a timer for possible destruction of the state.
The responder then signs the state information with the public key of
ID(R) and sends it to the initiator.
In this example, to expedite the protocol, the responder implicitly
accepts the first algorithm in the Authentication class of the EHAO
list. This because he cannot validate the initiator signature
without accepting the algorithm for doing the signature. The
responder's EHAS list will also reflect his acceptance.
The initiator receives the reply message and
validates that CKY-I is a valid association for the network
address of the incoming message,
adds the CKY-R value to the state for the pair (CKY-I, network
address), and associates all state information with the pair
(CKY-I, CKY-R),
adds g^y to its state information,
pseudorandomly chooses an exponent x and computes the
corresponding g^x value,
saves the EHA selections in the state,
optionally computes (g^x)^y (= g^xy) at this point, and
sends the reply message, signed with the public key of ID(I).
marks the KEYID (CKY-I|CKY-R) as authenticated.
When the responder receives this message, it marks the key as being
in the authenticated state. If it has not already done so, it should
compute g^xy and associate it with the KEYID.
Note that although PFS for identity protection is not used, PFS for
the derived keying material is still present because the Diffie-
Hellman half-keys g^x and g^y are exchanged.
2.4.1.1 Signature via Pseudo-Random Functions
The aggressive example is written to suggest that public key
technology is used for the signatures. However, a pseudorandom
function can be used, if the parties have previously agreed to such a
scheme and have a shared key.
If the first proposal in the EHAO list is an "existing key" method,
then the KEYID named in that proposal will supply the keying material
for the "signature" which is computed using the "H" algorithm
associated with the KEYID.
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Suppose the first proposal in EHAO is
EXISTING-KEY, 32
and the "H" algorithm for KEYID 32 is MD5-HMAC, by prior negotiation.
The keying material is some string of bits, call it sK32. Then in
the first message in the aggressive exchange, where the signature
S{ID(I), ID(R), Ni, 0, GRP, g^x, EHAO}Ki
is indicated, the signature computation would be performed by
MD5-HMAC_func(KEY=sK32, DATA = ID(I) | ID(R) | Ni | 0 | GRP | g^x
| EHAO)
(The exact definition of the algorithm corresponding to "MD5-HMAC-
func" will appear in the RFC defining that transform).
The result of this computation appears in the Authentication payload.
2.4.2 An Aggressive Example With Hidden Identities
The following example indicates how two parties can complete a key
exchange without using digital signatures. Public key cryptography
hides the identities during authentication. The group exponentials
are exchanged and authenticated, but the implied keying material
(g^xy) is not needed during the exchange.
This exchange has an important difference from the previous signature
scheme --- in the first message, an identity for the responder is
indicated as cleartext: ID(R'). However, the identity hidden with
the public key cryptography is different: ID(R). This happens
because the Initiator must somehow tell the Responder which
public/private key pair to use for the decryption, but at the same
time, the identity is hidden by encryption with that public key.
The Initiator might elect to forgo secrecy of the Responder identity,
but this is undesirable. Instead, if there is a well-known identity
for the Responder node, the public key for that identity can be used
to encrypt the actual Responder identity.
Initiator Responder
--------- ---------
-> CKY-I, 0, OK_KEYX, GRP, g^x, EHAO, NIDP, ->
ID(R'), E{ID(I), ID(R), E{Ni}Kr}Kr',
<- CKY-R, CKY-I, OK_KEYX, GRP, g^y, EHAS, NIDP,
E{ID(R), ID(I), Nr}Ki,
prf(Kir, ID(R) | ID(I) | Nr | Ni | GRP | g^y | g^x) <-
-> CKY-I, CKY-R, OK_KEYX, GRP, 0, 0, NIDP,
prf(Kir, ID(I)| ID(R) | Ni | Nr | GRP | g^x | g^y) ->
Kir = prf(0, Ni | Nr)
NB "NIDP" means that the PFS option for hiding identities is not used.
NB The ID(R') value is included in the Authentication payload as
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described in Appendix B.
The result of this exchange is a key with KEYID = CKY-I|CKY-R and
value sKEYID = prf(Ni | Nr, g^x | g^y | CKY-I | CKY-R).
The processing outline for this exchange is as follows:
Initiation
The initiator generates a unique cookie and associates it with the
expected IP address of the responder, and its chosen state
information: GRP, g^x, EHAO list. The first authentication choice
in the EHAO list is an algorithm that supports public key
encryption. The initiator also names the two identities to be
used for the connection and enters these into the state. A well-
known identity for the responder machine is also chosen, and the
public key for this identity is used to encrypt the nonce Ni and
the two connection identities. The initiator further
notes that the key is in the initial state of "unauthenticated",
and
sets a timer for possible retransmission and/or termination of the
request.
When responder receives the message, he may choose to ignore all the
information and treat it as merely a request for a cookie, creating
no state.
If CKY-I is not already in use by the source address in the IP
header, the responder generates a unique cookie, CKY-R. As before,
the next steps depend on the responders preferences. The minimal
required response is a message with the first cookie field set to
zero and CKY-R in the second field. For this example we will assume
that responder is more aggressive and accepts the following:
group GRP,
first authentication choice (which must be a public key encryption algorithm),
lack of perfect forward secrecy for protecting the identities,
identity ID(I),
identity ID(R)
The responder now associates the pair (CKY-I, CKY-R) with the
following state information:
the network address of the message
key state of "unauthenticated"
the first algorithm from each class in the EHAO (encryption-hash-
authentication algorithm offers) list
group GRP and a g^y value in group GRP
the nonce Ni and a pseudorandomly selected value Nr
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a timer for possible destruction of the state.
The responder then encrypts the state information with the public key
of ID(I) and sends it to the initiator.
The initiator receives the reply message and
validates that CKY-I is a valid association for the network
address of the incoming message,
adds the CKY-R value to the state for the pair (CKY-I, network
address), and associates all state information with the pair
(CKY-I, CKY-R),
adds g^y to its state information,
chooses an exponent x and computes the corresponding g^x value,
saves the EHA selections in the state,
optionally computes (g^x)^y (= g^xy) at this point, and
sends the reply message, encrypted with the public key of ID(R).
marks the KEYID (CKY-I|CKY-R) as authenticated.
When the responder receives this message, it marks the key as being
in the authenticated state. If it has not already done so, it should
compute g^xy and associate it with the KEYID.
The secret keying material sKEYID = prf(Ni | Nr, g^xy | CKY-I |
CKY-R)
Note that although PFS for identity protection is not used, PFS for
the derived keying material is still present because the Diffie-
Hellman half-keys g^x and g^y are exchanged.
2.4.3 An Aggressive Example With Private Identities and Without Diffie-
Hellman
Considerable computational expense can be avoided if perfect forward
secrecy is not a requirement for the session key derivation. The two
parties can exchange nonces and secret key parts to achieve the
authentication and derive keying material. The long-term privacy of
data protected with derived keying material is dependent on the
private keys of each of the parties.
In this exchange, the GRP has the value 0 and the field for the group
exponential is used to hold a nonce value instead.
As in the previous section, the first proposed algorithm must be a
public key encryption system; by responding with a cookie and a non-
zero exponential field, the Responder implicitly accepts the first
proposal and the lack of perfect forward secrecy for the identities
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and derived keying material.
Initiator Responder
--------- ---------
-> CKY-I, 0, OK_KEYX, 0, 0, EHAO, NIDP, ->
ID(R'), E{ID(I), ID(R), sKi}Kr',
<- CKY-R, CKY-I, OK_KEYX, 0, 0, EHAS, NIDP,
E{ID(R), ID(I), sKr}Ki,
prf(Kir, ID(R), ID(I), Nr, Ni) <-
-> CKY-I, CKY-R, OK_KEYX, EHAS, NIDP,
prf(Kir, ID(I), ID(R), Ni, Nr) ->
Kir = prf(0, sKi | sKr)
NB "NIDP" means that the PFS option for hiding identities is not used.
The result of this exchange is a key with KEYID = CKY-I|CKY-R and
value sKEYID = prf(Kir, CKY-I | CKY-R).
2.4.4 A Conservative Example
In this example the two parties are less aggressive; they use the
cookie exchange to delay creation of state, and they use perfect
forward secrecy to protect the identities.
The responder considers the ability of the initiator to repeat CKY-R
as weak evidence that the message originates from a "live"
correspondent on the network and the correspondent is associated with
the initiator's network address. The initiator makes similar
assumptions when CKY-I is repeated to the initiator.
All messages must have either have valid cookies or at least one zero
cookie. If both cookies are zero, this indicates a request for a
cookie; if only the initiator cookie is zero, it is a response to a
cookie request.
Information in messages violating the cookie rules cannot be used for
any OAKLEY operations.
Note that the Initiator must and Responder must agree on one set of
EHA algorithms; there is not one set for the Responder and one for
the Initiator. The Initiator must include at least MD5 and DES in
the initial offer.
Fields not indicated have null values.
Initiator Responder
--------- ---------
-> 0, 0, OK_KEYX ->
<- 0, CKY-R, OK_KEYX <-
-> CKY-I, CKY-R, OK_KEYX, GRP, g^x, EHAO ->
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<- CKY-R, CKY-I, OK_KEYX, GRP, g^y, EHAS <-
-> CKY-I, CKY-R, OK_KEYX, GRP, 0 , 0, IDP*,
ID(I), ID(R), E{Ni}Kr, ->
<- CKY-R, CKY-I, OK_KEYX, GRP, 0 , 0, IDP, <-
E{Nr, Ni}Ki, ID(R), ID(I),
prf(Nr | Ni, GRP, g^xy, ID(R), ID(I))
-> CKY-I, CKY-R, OK_KEYX, GRP, 0 , 0, IDP,
prf(Ni | Nr, GRP, g^xy, ID(I), ID(R)) ->
* when IDP is in effect, authentication payloads are encrypted with
the selected encryption algorithm using the key prf(0, g^xy.
This is in addition to and after any public key encryption.
See Appendix B.
The first exchange allows the Responder to use stateless cookies; if
the responder generates cookies in a manner that allows him to
validate them without saving them, as in Photuris, then this is
possible. Even if the Initiator includes a cookie in his initial
request, the responder can still use stateless cookies by merely
omitting the CKY-I from his reply and by declining to record the
Initiator cookie until it appears in a later message.
After the exchange is complete, both parties compute the shared key
material sKEYID as
prf(Ni | Nr, g^xy | CKY-I | CKY-R)
where "prf" is the pseudo-random function in class "hash" selected in
the EHA list.
As with the cookies, each party considers the ability of the remote
side to repeat the Ni or Nr value as a proof that Ka, the public key
of party a, speaks for the remote party and establishes its identity.
In analyzing this exchange, it is important to note that although the
IDP option ensures that the identities are protected with an
ephemeral key g^xy, the authentication itself does not depend on
g^xy. It is essential that the authentication steps validate the g^x
and g^y values, and it is thus imperative that the authentication not
involve a circular dependency on them. A third party could intervene
with a "man-in-middle" scheme to convince the initiator and responder
to use different g^xy values; although such an attack might result in
revealing the identities to the eavesdropper, the authentication
would fail.
2.4.5 Extra Strength for Protection of Encryption Keys
The nonces Ni and Nr are used to provide an extra dimension of
secrecy in deriving session keys. This makes the secrecy of the key
depend on two different problems: the discrete logarithm problem in
the group G, and the problem of breaking the nonce encryption scheme.
If RSA encryption is used, then this second problem is roughly
equivalent to factoring the RSA public keys of both the initiator and
responder.
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For authentication, the key type, the validation method, and the
certification requirement must be indicated.
2.5 Identity and Authentication
2.5.1 Identity
In OAKLEY exchanges the Initiator offers Initiator and Responder ID's
-- the former is the claimed identity for the Initiator, and the
latter is the requested ID for the Responder.
If neither ID is specified, the ID's are taken from the IP header
source and destination addresses.
If the Initiator doesn't supply a responder ID, the Responder can
reply by naming any identity that the local policy allows. The
Initiator can refuse acceptance by terminating the exchange.
The Responder can also reply with a different ID than the Initiator
suggested; the Initiator can accept this implicitly by continuing the
exchange or deny by terminating.
2.5.2 Authentication
The authentication of principals to one another is at the heart of
any key exchange scheme. The Internet community must decide on a
scalable standard for solving this problem, and OAKLEY must make use
of that standard. At the time of this writing, there is no such
standard, though several are emerging. This document attempts to
describe how a handful of standards could be incorporated into
OAKLEY, without attempting to pick and choose among them.
The following methods can appear in OAKLEY offers:
a. Pre-shared Keys
When two parties have arranged for a trusted method of
distributing secret keys for their mutual authentication, they can
be used for authentication. This has obvious scaling problems for
large systems, but it is an acceptable interim solution for some
situations. Support for pre-shared keys is REQUIRED.
The encryption, hash, and authentication algorithm for use with a
pre-shared key must be part of the state information distributed
with the key itself.
The pre-shared keys have a KEYID and keying material sKEYID; the
KEYID is used in a pre-shared key authentication option offer.
There can be more than one pre-shared key offer in a list.
Because the KEYID persists over different invocations of OAKLEY
(after a crash, etc.), it must occupy a reserved part of the KEYID
space for the two parties. A few bits can be set aside in each
party's "cookie space" to accommodate this.
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There is no certification authority for pre-shared keys. When a
pre-shared key is used to generate an authentication payload, the
certification authority is "None", the Authentication Type is
"Preshared", and the payload contains
the KEYID, encoded as two 64-bit quantities, and
the result of applying the pseudorandom hash function to the
message body with the sKEYID forming the key for the function
See Appendix B for details of formats for the Authentication
Payload.
b. DNS public keys
Security extensions to the DNS protocol [DNSSEC] provide a
convenient way to access public key information, especially for
public keys associated with hosts. RSA keys are a requirement for
secure DNS implementations; extensions to allow optional DSS keys
are a near-term possibility.
DNS KEY records have associated SIG records that are signed by a
zone authority, and a hierarchy of signatures back to the root
server establishes a foundation for trust. The SIG records
indicate the algorithm used for forming the signature.
OAKLEY implementations MUST support the use of DNS KEY and SIG
records for authenticating with respect to IPv4 and IPv6 addresses
and fully qualified domain names. However, implementations are
not required to support any particular algorithm (RSA, DSS, etc.).
c. RSA public keys w/o certification authority signature
PGP [Zimmerman] uses public keys with an informal method for
establishing trust. The format of PGP public keys and naming
methods will be described in a separate RFC. The RSA algorithm
can be used with PGP keys for either signing or encryption; the
authentication option should indicate either RSA-SIG or RSA-ENC,
respectively. Support for this is OPTIONAL.
d.1 RSA public keys w/ certificates
There are various formats and naming conventions for public keys
that are signed by one or more certification authorities. The
Public Key Interchange Protocol discusses X.509 encodings and
validation. Support for this is OPTIONAL.
d.2 DSS keys w/ certificates
Encoding for the Digital Signature Standard with X.509 is
described in in draft-ietf-ipsec-dss-cert-00.txt. Support for
this is OPTIONAL; an ISAKMP Authentication Type will be assigned.
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2.5.3 Validating Authentication Keys
The combination of the Authentication algorithm, the Authentication
Authority, the Authentication Type, and a key (usually public) define
how to validate the messages with respect to the claimed identity.
The key information will be available either from a pre-shared key,
or from some kind of certification authority.
Generally the certification authority produces a certificate binding
the entity name to a public key. OAKLEY implementations must be
prepared to fetch anad validate certificates before using the public
key for OAKLEY authentication purposes.
The ISAKMP Authentication Payload defines the Authentication
Authority field for specifying the authority that must be apparent in
the trust hierarchy for authentication.
Once an appropriate certificate is obtained (see 2.4.3), the
validation method will depend on the Authentication Type; if it is
PGP then the PGP signature validation routines can be called to
satisfy the local web-of-trust predicates; if it is RSA with X.509
certificates, the certificate must be examined to see if the
certification authority signature can be validated, and if the
hierarchy is recognized by the local policy.
2.5.4 Fetching Identity Objects
In addition to interpreting the certificate or other data structure
that contains an identity, users of OAKLEY must face the task of
retrieving certificates that bind a public key to an identifier and
also retrieving auxiliary certificates for certifying authorities or
co-signers (as in the PGP web of trust).
The ISAKMP Credentials Payload can be used to attach useful
certificates to OAKLEY messages. The Credentials Payload is defined
in Appendix B.
Support for accessing and revoking public key certificates via the
Secure DNS protocol [SECDNS] is MANDATORY for OAKLEY implementations.
Other retrieval methods can be used when the AUTH class indicates a
preference.
The Public Key Interchange Protocol discusses a full protocol that
might be used with X.509 encoded certificates.
2.6 Interface to Cryptographic Transforms
The keying material computed by the key exchange should have at least
90 bits of entropy, which means that it must be at least 90 bits in
length. This may be more or less than is required for keying the
encryption and/or pseudorandom function transforms.
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The transforms used with OAKLEY should have auxiliary algorithms
which take a variable precision integer and turn it into keying
material of the appropriate length. For example, a DES algorithm
could take the low order 56 bits, a triple DES algorithm might use
the following:
K1 = low 56 bits of md5(0|sKEYID)
K2 = low 56 bits of md5(1|sKEYID)
K3 = low 56 bits of md5(2|sKEYID)
The transforms will be called with the keying material encoded as a
variable precision integer, the length of the data, and the block of
memory with the data. Conversion of the keying material to a
transform key is the responsibility of the transform.
2.7 Retransmission, Timeouts, and Error Messages
If a response from the Responder is not elicited in an appropriate
amount of time, the message should be retransmitted by the Initiator.
These retransmissions must be handled gracefully by both parties; the
Responder must retain information for retransmitting until the
Initiator moves to the next message in the protocol or completes the
exchange.
Informational error messages present a problem because they cannot be
authenticated using only the information present in an incomplete
exchange; for this reason, the parties may wish to establish a
default key for OAKLEY error messages. A possible method for
establishing such a key is described in Appendix B, under the use of
ISA_INIT message types.
In the following the message type is OAKLEY Error, the KEYID supplies
the H algorithm and key for authenticating the message contents; this
value is carried in the Sig/Prf payload.
The Error payload contains the error code and the contents of the
rejected message.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Initiator-Cookie ~
/ ! !
KEYID +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ ! !
~ Responder-Cookie ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Domain of Interpretation !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Message Type ! Exch ! Vers ! Length !
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! SPI (unused) !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! SPI (unused) !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Error Payload !
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Sig/prf Payload
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The error message will contain the cookies as presented in the offending
message, the message type OAKLEY_ERROR, and the reason for the error,
followed by the rejected message.
Error messages are informational only, and the correctness of the
protocol does not depend on them.
Error reasons:
TIMEOUT exchange has taken too long, state destroyed
AEH_ERROR an unknown algorithm appears in an offer
GROUP_NOT_SUPPORTED GRP named is not supported
EXPONENTIAL_UNACCEPTABLE exponential too large/small
SELECTION_NOT_OFFERED selection does not occur in offer
NO_ACCEPTABLE_OFFERS no offer meets host requirements
AUTHENTICATION_FAILURE signature or hash function fails
RESOURCE_EXCEEDED too many exchanges or too much state info
NO_EXCHANGE_IN_PROGRESS a reply received with no request in progress
2.8 Additional Security for Privacy Keys: Private Groups
If the two parties have need to use a Diffie-Hellman key
determination scheme that does not depend on the standard group
definitions, they have the option of establishing a private group.
The authentication need not be repeated, because this stage of the
protocol will be protected by a pre-existing authentication key. As
an extra security measure, the two parties will establish a private
name for the shared keying material, so even if they use exactly the
same group to communicate with other parties, the re-use will not be
apparent to passive attackers.
Private groups have the advantage of making a widespread passive
attack much harder by increasing the number of groups that would have
to be exhaustively analyzed in order to recover a large number of
session keys. This contrasts with the case when only one or two
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groups are ever used; in that case, one would expect that years and
years of session keys would be compromised.
There are two technical challenges to face: how can a particular user
create a unique and appropriate group, and how can a second party
assure himself that the proposed group is reasonably secure?
The security of a modular exponentiation group depends on the largest
prime factor of the group size. In order to maximize this, one can
choose "strong" or Sophie-Germaine primes, P = 2Q + 1, where P and Q
are prime. However, if P = kQ + 1, where k is small, then the
strength of the group is still considerable. These groups are known
as Schnorr subgroups, and they can be found with much less
computational effort than Sophie-Germaine primes.
Schnorr subgroups can also be validated efficiently by using probable
prime tests.
It is also fairly easy to find P, k, and Q such that the largest
prime factor can be easily proven to be Q.
We estimate that it would take about 10 minutes to find a new group
of about 2^1024 elements, and this could be done once a day by a
scheduled process; validating a group proposed by a remote party
would take perhaps a minute on a 25 MHz RISC machine or a 66 MHz CISC
machine.
We note that validation is done only between previously mutually
authenticated parties, and that a new group definition always follows
and is protected by a key established using a well-known group.
There are four points to keep in mind:
a. The description and public identifier for the new group are
protected by the well-known group.
b. The responder can reject the attempt to establish the new
group, either because he is too busy or because he cannot validate
the largest prime factor as being sufficiently large.
c. The new modulus and generator can be cached for long periods of
time; they are not security critical and need not be associated
with ongoing activity.
d. Generating a new g^x value periodically will be more expensive
if there are many groups cached; however, the importance of
frequently generating new g^x values is reduced, so the time
period can be lengthened correspondingly.
2.8.1 Defining a New Group
This section describes how to define a new group. The description of
the group is hidden from eavesdroppers, and the identifier assigned
to the group is unique to the two parties. Use of the new group for
Diffie-Hellman key exchanges is described in the next section.
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The secrecy of the description and the identifier increases the
difficulty of a passive attack, because if the group descriptor is
not known to the attacker, there is no straightforward and efficient
way to gain information about keys calculated using the group.
Only the description of the new group need be encrypted in this
exchange. The hash algorithm is implied by the OAKLEY session named
by the group. The encryption is the authentication encryption
function of the OAKLEY session.
The descriptor of the new group is encoded in the new group payload.
The nonces are encoded in the Key Exchange Payload.
Data beyond the encryption boundary is encrypted using the transform
named by the KEYID.
The following message use the ISAKMP Key Exchange Identifier OAKLEY
New Group.
To define a new modular exponentiation group:
Initiator Responder
--------- ----------
-> KEYID, ->
INEWGRP,
Desc(New Group), Na
prf(sKEYID, Desc(New Group) | Na)
<- KEYID,
INEWGRPRS,
Na, Nb
prf(sKEYID, Na | Nb | Desc(New Group)) <-
-> KEYID,
INEWGRPACK
prf(sKEYID, Nb | Na | Desc(New Group)) ->
These messages are encrypted at the encryption boundary using the key
indicated. The hash value is placed in the "digital signature" field
(see Appendix C).
New GRP identifier = Na | Nb (the initiator and responder must use
nonces that are distinct from any cookies used for current
KEYID's; i.e., the initiator ensures that Na is distinct from any
CKY-I, the responder ensures that Nb is distinct from any CKY-R).
Desc(G) is the encoding of the descriptor for the group descriptor
(see Appendix A for the format of a group descriptor)
The two parties must store the mapping between the new group
identifier GRP and the group descriptor Desc(New Group). They must
also note the identities used for the KEYID and copy these to the
state for the new group.
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Note that one could have the same group descriptor associated with
several KEYID's. Pre-calculation of g^x values may be done based
only on the group descriptor, not the private group name.
2.8.2 Deriving a Key Using a Private Group
Once a private group has been established, its group id can be used
in the key exchange messages in the GRP position. No changes to the
protocol are required.
2.9 Quick Mode: New Keys From Old,
When an authenticated KEYID and associated keying material sKEYID
already exist, it is easy to derive additional KEYID's and keys,
using only hashing functions. The KEYID might be one that was
derived in Main Mode, for example.
On the other hand, the authenticated key may be a manually
distributed key, one that is shared by the initiator and responder
via some means external to OAKLEY. If the distribution method has
formed the KEYID using appropriately unique values for the two halves
(CKY-I and CKY-R), then this method is applicable.
In the following, the Key Exchange Identifier is OAKLEY Quick Mode.
The nonces are carried in the Key Exchange Payload, and the prf value
is carried in the Authentication Payload; the Authentication
Authority is "None" and the type is "Pre-Shared".
The protocol is:
Initiator Responder
--------- ---------
-> KEYID, INEWKRQ, Ni, prf(sKEYID, Ni) ->
<- KEYID, INEWKRS, Nr, prf(sKEYID, 1 | Nr | Ni) <-
-> KEYID, INEWKRP, 0, prf(sKEYID, 0 | Ni | Nr) ->
The New KEYID, NKEYID, is Ni | Nr
sNKEYID = prf(sKEYID, Ni | Nr )
The identities and EHA values associated with NKEYID are the same as
those associated with KEYID.
Each party must validate the hash values before using the new key for
any purpose.
2.10 Pre-Distributed Keys
If a key and an associated key identifier and state information have
been distributed manually, then the key can be used for any OAKLEY
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purpose. The key must be associated with the usual state
information: ID's and EHA algorithms.
Local policy dictates when a manual key can be included in the OAKLEY
database. For example, only privileged users would be permitted to
introduce keys associated with privileged ID's, an unprivileged user
could only introduce keys associated with her own ID.
2.11 Distribution of an External Key
Once an OAKLEY session key and ancillary algorithms are established,
the keying material and the "H" algorithm can be used to distribute
an externally generated key and to assign a KEYID to it.
In the following, KEYID represents an existing, authenticated OAKLEY
session key, and sNEWKEYID represents the externally generated keying
material.
In the following, the Key Exchange Identifier is OAKLEY External
Mode. The Key Exchange Payload contains the new key; the payload is
protected using the encryption transform associated with the KEYID
and associated key, sKEYID.
Initiator Responder
--------- ---------
-> KEYID, IEXTKEY, Ni, prf(sKEYID, Ni) ->
<- KEYID, IEXTKEY, Nr, prf(sKEYID, 1 | Nr | Ni) <-
-> KEYID, IEXTKEY, Kir xor sNEWKEYID*, prf(Kir, sNEWKEYID | Ni | Nr) ->
Kir = prf(sKEYID, Ni | Nr)
* this field is carried in the first section of the Authentication Payload,
encoded as a variable precision integer.
Each party must validate the hash values using the "H" function in
the KEYID state before changing any key state information.
The new key is recovered by the Responder by calculating the xor of
the field field in the Authentication Payload with the Kir value.
The new key identifier, naming the keying material sNEWKEYID, is
prf(sKEYID, 1 | Ni | Nr).
Note that this exchange does not require encryption. Hugo Krawcyzk
suggested the method and its advantage.
2.12. Cryptographic Strength Considerations
The strength of the key used to distribute the external key must be
at least equal to the strength of the external key. Generally, this
means that the length of the sKEYID material must be greater than or
equal to the length of the sNEWKEYID material.
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The derivation of the external key, its strength or intended use are
not addressed by this protocol; the parties using the key must have
some other method for determining these properties.
As of early 1996, it appears that for 90 bits of cryptographic
strength, one should use a modular exponentiation group modulus of
2000 bits. For 128 bits of strength, a 3000 bit modulus is required.
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3. Specifying and Deriving Security Associations
When a security association is defined, only the KEYID need be given.
The responder should be able to look up the state associated with the
KEYID value and find the appropriate keying material, sKEYID.
The OAKLEY protocol does not define security association encodings or
message formats. These can be defined through a protocol such as
ISAKMP. Compatibility with ISAKMP is a goal of the OAKLEY design,
and coordination of the message formats and use of identifiers is an
ongoing activity at this time.
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4. ISAKMP Compatibility
OAKLEY uses ISAKMP header and payload formats, as described in the
text and in Appendix B. There are particular noteworthy extensions
beyond the version 4 draft.
4.1 Authentication with Existing Keys
In the case that two parties do not have suitable public key
mechanisms in place for authenticating each other, they can use keys
that were distributed manually. After establishment of these keys
and their associated state in OAKLEY, they can be used for
authentication modes that depend on signatures, e.g. Aggressive Mode.
When an existing key is to appear in an offer list, it should be
indicated with an Authentication Authority of ISAKMP_EXISTING. The
value for this field must not conflict with any authentication
authority registered with IANA.
The authentication payload will have two parts:
the KEYID for the pre-existing key
the identifier for the party to be authenticated by the pre-
existing key.
The pseudo-random function "H" in the state information for that
KEYID will be the signature algorithm, and it will use the keying
material for that key (sKEYID) when generating or checking the
validity of message data.
E.g. if the existing key has an KEYID denoted by KID and 128 bits of
keying material denoted by sKID and "H" algorithm a transform named
HMAC, then to generate a "signature" for a data block, the output of
HMAC(sKID, data)
will be the corresponding signature payload.
The KEYID state will have the identities of the local and remote
parties for which the KEYID was assigned; it is up to the local
policy implementation to decide when it is appropriate to use such a
key for authenticating other parties. For example, a key distributed
for use between two Internet hosts A and B may be suitable for
authenticating all identities of the form "alice@A" and "bob@B".
4.2 Third Party Authentication
A local security policy might restrict key negotiation to trusted
parties. For example, two OAKLEY daemons running with equal
sensitivity labels on two machines might wish to be the sole arbiters
of key exchanges between users with that same sensitivity label. In
this case, some way of authenticating the provenance of key exchange
requests is needed. I.e., the identities of the two daemons should
be bound to a key, and that key will be used to form a "signature"
for the key exchange messages.
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The Signature Payload, in Appendix B, is for this purpose. This
payload names a KEYID that is in existence before the start of the
current exchange. The "H" transform for that KEYID is used to
calculate an integrity/authentication value for all payloads
preceding the signature.
Local policy can dictate which KEYID's are appropriate for signing
further exchanges.
4.3 New Group Mode
OAKLEY uses a new KEI for the exchange that defines a new group.
5. Security Implementation Notes
Timing attacks that are capable of recovering the exponent value used
in Diffie-Hellman calculations have been described by Paul Kocher
[Kocher]. In order to nullify the attack, implementors must take
pains to obscure the sequence of operations involved in carrying out
modular exponentiations.
A "blinding factor" can accomplish this goal. A group element, r, is
chosen at random. When an exponent x is chosen, the value r^(-x) is
also calculated. Then, when calculating (g^y)^x, the implementation
will calculate this sequence:
A = (rg^y)
B = A^x = (rg^y)^x = (r^x)(g^(xy))
C = B*r^(-x) = (r^x)(r^-(x))(g^(xy)) = g^(xy)
The blinding factor is only necessary if the exponent x is used more
than 100 times (estimate by Richard Schroeppel).
6. OAKLEY State Machine
There are many pathways through OAKLEY.
The initiator decides on an initial message in the following order:
1. Offer a cookie. This is not necessary but it helps with
aggressive exchanges.
2. Pick a group. The choices are the well-known groups or any
private groups that may have been negotiated. The very first
exchange between two Oakley daemons with no common state must
involve a well-known group.
3. Pick an exponent x and present g^x.
4. Offer Encryption, Hash, and Authentication lists.
5. Use PFS for hiding the identities
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If identity hiding is not used, then the initiator has this
option:
6. Name the identities and include authentication information
The information in the authentication section depends on the first
authentication offer. In this aggressive exchange, the Initiator
hopes that the Responder will accept all the offered information and
the first authentication method. The authentication method
determines the authentication payload as follows:
1. A signing method will be used to sign all the offered information.
2. A public key encryption method will be used to encrypt a nonce in
the public key of the requested reponder identity. There are two
cases possible, depending on whether or not identity hiding is used:
a. No identity hiding. The ID's will appear as plaintext.
b. Identity hiding. A well-known ID, call it R', will appear as
plaintext in the authentication payload. It will be followed
by two ID's and a nonce; these will be encrypted using the
public key for R'.
3. A pre-existing key method. The pre-existing key will be used to
encrypt a nonce. If identity hiding is used, the ID's will be
encrypted in place in the payload, using the "E" algorithm associated
with the pre-existing key.
The responder can accept all, part or none of the initial message.
The responder accepts as many of the fields as he wishes, using the
same decision order as the initiator. At any step he can stop,
implicitly rejecting further fields. The minimum response is a
cookie and the GRP.
1. Accept cookie. The responder may elect to record no state
information until the initiator successfully replies with a cookie
chosen by the responder. If so, the responder replies with a cookie,
the GRP, and no other information.
2. Accept GRP. If the group is not acceptable, the responder will
not reply. The responder may send an error message indicating the
the group is not acceptable (modulus too small, unknown identifier,
etc.)
3. Accept the g^x value. The responder indicates his acceptance of
the g^x value by including his own g^y value in his reply. He can
postpone this by ignoring g^x and putting a zero length g^y value in
his reply.
4. Accept one element from each of the EHA lists. The acceptance is
indicated by a non-zero proposal.
5. If PFS for identity hiding is requested, then no further data will
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follow.
6. If the authentication payload is present, and if the first item in
the offered authentication class is acceptable, then the responder
must should validate/decrypt the information in the authentication
payload and hash payload, if present. The responder should choose a
nonce and reply using the same authentication/hash algorithm as the
initiator used.
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APPENDIX A Group Descriptors
Three distinct group representations can be used with OAKLEY. Each
group is defined by its group operation and the kind of underlying
field used to represent group elements. The three types are modular
exponentiation groups (named MODP herein), elliptic curve groups
over the field GF[2^N] (named EC2N herein), and elliptic curve groups
over GF[P] (named ECP herein) For each representation, many distinct
realizations are possible, depending on parameter selection.
With a few exceptions, all the parameters are transmitted as if they
were non-negative multi-precision integers, using the format defined
in this appendix (note, this is distinct from the encoding in Appendix
D). Every multi-precision integer has a prefixed length field, even
where this information is redundant.
For the group type EC2N, the parameters are more properly thought of
as very long bit fields, but they are represented as multi-precision
integers, (with length fields, and right-justified). This is the
natural encoding.
MODP means the classical modular exponentiation group, where the
operation is to calculate G^X (mod P). The group is defined by
the numeric parameters P and G. P must be a prime. G is often 2,
but may be a larger number. 2 <= G <= P-2.
ECP is an elliptic curve group, modulo a prime number P.
The defining equation for this kind of group is
Y^2 = X^3 + AX + B
The group operation is taking a multiple of an elliptic-curve point.
The group is defined by 5 numeric parameters: The prime P, two curve
parameters A and B, and a generator (X,Y). A,B,X,Y are all
interpreted mod P, and must be (non-negative) integers less than P.
They must satisfy the defining equation, modulo P.
EC2N is an elliptic curve group, over the finite field F[2^N]. The
defining equation for this kind of group is
Y^2 + XY = X^3 + AX^2 + B
(This equation differs slightly from the mod P case: it has an XY
term, and an AX^2 term instead of an AX term.)
We must specify the field representation, and then the elliptic
curve. The field is specified by giving an irreducible polynomial
(mod 2) of degree N. This polynomial is represented as an integer of
size between 2^N and 2^(N+1), as if the defining polynomial were
evaluated at the value U=2.
For example, the field defined by the polynomial
U^155 + U^62 + 1
is represented by the integer 2^155 + 2^62 + 1. The group is defined
by 4 more parameters, A,B,X,Y. These parameters are elements of the
field F[2^N], and can be though of as polynomials of degree < N, with
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(mod 2) coefficients. They fit in N-bit fields, and are represented
as integers < 2^N, as if the polynomial were evaluated at U=2. For
example, the field element U^2 + 1 would be represented by the
integer 2^2+1, which is 5. The two parameters A and B define the
curve. A is frequently 0. B must not be 0. The parameters X and Y
select a point on the curve. The parameters A,B,X,Y must satisfy the
defining equation, modulo the defining polynomial, and mod 2.
Group descriptor formats:
Type of group: A two-byte field,
assigned values for the types "MODP", "ECP", "EC2N"
will be defined (see ISAKMP-04).
Size of a field element, in bits. This is either Ceiling(log2 P)
or the degree of the irreducible polynomial: a 32-bit integer.
The prime P or the irreducible field polynomial: a multi-precision integer.
The generator: 1 or 2 values, multi-precision integers.
EC only: The parameters of the curve: 2 values, multi-precision integers.
The following parameters are Optional (each of these may appear
independently):
a value of 0 may be used as a place-holder to represent an unspecified
parameter; any number of the parameters may be sent, from 0 to 3.
The largest prime factor: the encoded value that is the LPF of the group size,
a multi-precision integer.
EC only: The order of the group: multi-precision integer.
(The group size for MODP is always P-1.)
Strength of group: 32-bit integer.
The strength of the group is approximately the number of key-bits protected.
It is determined by the log2 of the effort to attack the group.
It may change as we learn more about cryptography.
This is a generic example for a "classic" modular exponentiation group:
Group type: "MODP"
Size of a field element in bits: Log2 (P) rounded *up*. A 32bit integer.
Defining prime P: a multi-precision integer.
Generator G: a multi-precision integer. 2 <= G <= P-2.
Largest prime factor of P-1: the multi-precision integer Q
Strength of group: a 32-bit integer. We will specify a formula
for calculating this number (TBD).
This is a generic example for elliptic curve group, mod P:
Group type: "ECP"
Size of a field element in bits: Log2 (P) rounded *up*,
a 32 bit integer.
Defining prime P: a multi-precision integer.
Generator (X,Y): 2 multi-precision integers, each < P.
Parameters of the curve A,B: 2 multi-precision integers, each < P.
Largest prime factor of the group order: a multi-precision integer.
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Order of the group: a multi-precision integer.
Strength of group: a 32-bit integer. Formula TBD.
This is a specific example for elliptic curve group:
Group type: "EC2N"
Degree of the irreducible polynomial: 155
Irreducible polynomial: U^155 + U^62 + 1, represented as the
multi-precision integer 2^155 + 2^62 + 1.
Generator (X,Y) : represented as 2 multi-precision integers, each < 2^155.
For our present curve, these are (decimal) 123 and 456. Each is represented
as a multi-precision integer.
Parameters of the curve A,B: represented as 2 multi-precision
integers, each < 2^155.
For our present curve these are 0 and (decimal) 471951, represented as two
multi-precision integers.
Largest prime factor of the group order:
3805993847215893016155463826195386266397436443,
represented as a multi-precision integer.
The order of the group:
45671926166590716193865565914344635196769237316
represented as a multi-precision integer.
Strength of group: 76, represented as a 32-bit integer.
The variable precision integer encoding for group descriptor fields
is the following. This is a slight variation on the format defined
in Appendix D in that a fixed 16-bit value is used first, and the
length is limited to 16 bits. However, the interpretation is
otherwise identical.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Fixed value (TBD) ! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. Integer .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The format of a group descriptor is:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!1!1! Group Description ! MODP !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!1!0! Field Size ! Length !
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! MPI !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!1!0! Prime ! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! MPI !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!1!0! Generator1 ! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! MPI !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!1!0! Generator2 ! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! MPI !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!1!0! Curve-p1 ! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! MPI !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!1!0! Curve-p2 ! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! MPI !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!1!0! Largest Prime Factor ! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! MPI !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!1!0! Order of Group ! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! MPI !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!0!0! Strength of Group ! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! MPI !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
APPENDIX B Message formats
1. The ISAKMP Message Types and Header
OAKLEY uses the ISAKMP Message Types ISA_KE&AUTH_REQ and
ISA_KE&AUTH_REP for all key exchanges.
OAKLEY uses the two SPI fields in an ISAKMP header for the name of
the abstract group (GRP) used in the negotiation. If no group is
used (forfeiting perfect forward secrecy), then the field will have
the value 0.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
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~ Initiator-Cookie ~
/ ! !
KEYID +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ ! !
~ Responder-Cookie ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Domain of Interpretation !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Message Type ! Exch ! Vers ! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Group ID !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Group ID !
eeee +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ eeee
! ... !
"eeee" represents the encryption boundary for messages requiring privacy.
The message after this point is subject to the encryption transform implied
by the KEYID.
The Group ID field is used for the group identifier for the key
exchange methods described in this document; in other ISAKMP messages
the field is used for a SPI.
The second SPI field is not used in OAKLEY. It must contain the
value zero.
The OAKLEY proposal format contains the SA attributes that are
exchanged in the ISA_INIT messages in order to establish the required
security at- tributes for the key and authentication exchange. See
[OAKLEY] for fur- ther details.
2. OAKLEY Use of ISA_AUTH&KE packets.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ISAKMP Header ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! Payload Len ! RESERVED !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
! Authentication Payload !
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! Payload Len ! RESERVED !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
! Key Exchange Payload !
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~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ISA_AUTH&KE_REQ and ISA_AUTH&KE_RESP Packet Format
The encodings of the OAKLEY parameters into these fields are
described in the next sections.
3. The Key Exchange Payload
The Key Exchange Payload carries values that are used to derive
secret keying material. Because OAKLEY uses both nonces and Diffie-
Hellman exponentials for deriving keys, its use of the Key Exchange
Payload is slightly different from the use described in ISAKMP; that
document expects only one Key Exchange Payload per packet, but OAKLEY
can have two, one for nonces, one for an exponential.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! Payload Len ! RESERVED !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! KEI ! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
! Key Exchange Data !
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Key Exchange Payload Format
o KEI (2 octets) - Key Exchange Identifier
o Length (2 octets) - Length of payload in octets
o Key Exchange Data (variable) - Data required to
create session key.
OAKLEY uses three KEI values: OAKLEY Main Mode, OAKLEY Quick Mode,
OAKLEY External Mode, OAKLEY New Group Mode.
The value encoded in the Key Exchange Data field will be the Diffie-
Hellman exponential (if it is used), encoded as variable precision
integers as shown in Appendix D.
3. The OAKLEY Authentication Payload
1 2 3
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! Payload Len ! RESERVED !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Authentication Authority ! Reserved !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Authentication Type ! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
! Authentication Data !
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Authentication Payload Format
The Authentication Payload will be used to carry three pieces of
essential information: the entity identifiers (ID's), the nonces, and
the output of a function proving proving knowledge of a secret.
The format of the ID's is described in the next section. A payload
will have two ID's, for the Initiator and Responder, in that order.
If the length of an ID is zero, the ID is unspecified.
If the low order bit of the Reserved field is set, the payload will
have three ID's; see section 2.4.2, An Aggressive Example With Hidden
Identities.
The nonce will follow the ID's; if a nonce is zero length, it is
considered to be not present.
The fourth part of the authentication payload will contain the result
of applying the pseudorandom function or signature algorithm to the
key exchange parameters, as described in the main text. For example,
the output might be the result of applying a keyed MD5 transform to
the ID's, the cookies, the nonces, and the exponentials.
The pseudorandom function output will encoded as a variable precision
integer as described in Appendix D.
4. The OAKLEY Proposal
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! OAKLEY ! Proposal # ! Proposal Len ! RESERVED !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! EHA Format !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Group Format !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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OAKLEY Proposal Format
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!0!1! Auth / Priv Flag ! PRIV !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!0!1! Encryption Algorithm ! DES !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!0!1! Hash Algorithm ! MD5 !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!1!1! Authentication Alg ! RSA !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!0!1! Authentication Mode ! KEYED !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
OAKLEY Proposal - EHA Format
5. Identity (ID) formats
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Identification Type ! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
! Identification Data !
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
There are three identification types: IP_ADDR (value 1), FQDN (value
2), USER_FQDN (value 3).
The length of the IP address will be 4 bytes for the IPv4 Domain of
Interpretation, 8 bytes for the IPv6 DOI.
FQDN is a fully qualified domain name, as used by the DNS protocol.
Its form is an ASCII character string. The domain components are
separated by "." characters, as in DNS.
USER_FQDN is a user id followed by a "." character, followed by a
fully qualified domain name, as used by the DNS protocol. Its form
is an ASCII character string.
6. OAKLEY's use ISA_INIT_REQ and ISA_INIT_RESP Packets
OAKLEY does not require the use the ISAKMP ISA_INIT_REQ and
ISA_INIT_RESP packets. Their optional use may include the
establishment of ISAKMP-to-ISAKMP daemon KEYID's for later use as
signatures over ISA_KE&AUTH packets, providing an extra level of
authenticity checking. In this case, the Situation field will have
the IP addresses of the two principals; the length of the IP address
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will depend on the Domain of Interpretation.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ISAKMP Header ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! Payload Len ! RESERVED !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Domain of Interpretation !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Situation ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ Proposal ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ISA_INIT_REQ and ISA_INIT_RESP Packet Format
7. Digital Signature/PRF Payload
The Digital Signature/PRF payload will carry a value for
authenticating the entire message. When it occurs, it will be the
last payload.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! KEYID !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
! Signature/hash data !
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The output of the signature or prf function will be encoded as a
variable precision integer as described in Appendix D. The KEYID
will indicate KEYID that names keying material and the Hash or
Signature function.
8. The Credential Payload
Useful certificates with public key information can be attached to
OAKLEY messages using Credential Payloads. The format of the payload
depends on the Authentication Type, and separate RFC's define the
formats. The encoding of the Authority and Type are the same as for
the Authentication Payload.
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1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Next Payload ! Payload Len ! RESERVED !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Authentication Authority ! Reserved !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Authentication Type ! Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
! Credential Data !
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Credential Payload Format
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APPENDIX D Encoding a variable precision integer.
Variable precision integers will be encoded as a 32-bit length field
followed by one or more 32-bit quantities containing the
representation of the integer, aligned with the most significant bit
in the first 32-bit item.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! first value word (most significant bits) !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! !
~ additional value words ~
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
An example of such an encoding is given below, for a number with 51
bits of significance. The length field indicates that 2 32-bit
quantities follow. The most significant non-zero bit of the number
is in bit 13 of the first 32-bit quantity, the low order bits are in
the second 32-bit quantity.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! 1 0!
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!0 0 0 0 0 0 0 0 0 0 0 0 0 1 x x x x x x x x x x x x x x x x x x!
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
!x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x!
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
APPENDIX E Cryptographic strengths
The Diffie-Hellman algorithm is used to compute keys that will be
used with symmetric algorithms. It should be no easier to break the
Diffie-Hellman computation than it is to do an exhaustive search over
the symmetric key space. A recent recommendation by an group of
cryptographers [Blaze-Diffie-et-al] has recommended a symmetric key
size of 75 bits for a practical level of security. For 20 year
security, they recommend 90 bits.
Based on that report, a conservative strategy for OAKLEY users would
be to ensure that their Diffie-Hellman computations were as secure as
at least a 90-bit key space. In order to accomplish this for modular
exponentiation groups, the size of the largest prime factor of the
modulus should be at least 180 bits, and the size of the modulus
should be at least 1400 bits. For elliptic curve groups, the LPF
should be at least 180 bits.
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If long-term secrecy of the encryption key is not an issue, then the
following parameters may be used for the modular exponentiation
group: 150 bits for the LPF, 980 bits for the modulus size.
The modulus size alone does not determine the strength of the
Diffie-Hellman calculation; the size of the exponent used in
computing powers within the group is also important. The size of the
exponent in bits should be at least twice the size of any symmetric
key that will be derived from it. We recommend that ISAKMP
implementors use at least 180 bits of exponent (twice the size of a
20-year symmetric key).
The mathematical justification for these estimates can be found in
texts that estimate the effort for solving the discrete log problem,
a task that is strongly related to the efficiency of using the Number
Field Sieve for factoring large integers. Readers are referred to
[Stinson] and [Schneier].
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APPENDIX F The Well-Known Groups
This section will have explicit descriptors for three modular
exponentiation groups and two elliptic curve over GF[2^n] groups.
The identifiers for the groups (the well-known GRP's) will also be
given here.
0 No group (used as a placeholder and for non-DH exchanges)
1 A modular exponentiation group with a 768 bit modulus (TBD)
2 A modular exponentiation group with a 1024 bit modulus (TBD)
3 A modular exponentiation group with a 2048 bit modulus (TBD)
4 An elliptic curve group over GF[2^155]
5 An elliptic curve group over GF[2^210]
values 2^32 and higher are used for private group identifiers
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Appendix K Implementing Group Operations
The group operation must be implemented as a sequence of arithmetic
operations; the exact operations depend on the type of group. For
modular exponentiation groups, the operation is multi-precision
integer multiplication and remainders by the group modulus. See
Knuth Vol. 2 [Knuth] for a discussion of how to implement these for
large integers. Implementation recommendations for elliptic curve
group operations over GF[2^N] are described in [Schroeppel].
H. K. Orman [Page 45]
INTERNET DRAFT May 1996
BIBLIOGRAPHY
[RFC1825] Atkinson, Randall, RFC's 1825-1827
[Blaze] Blaze, Matt et al., Recent symmetric key report
[STS] Diffie, van Oorschot, and Wiener, Authentication and
Authenticated Key Exchanges
[DSS] DSS draft-ietf-ipsec-dss-cert-00.txt
[SECDNS] DNS Signed Keys, Eastlake & Kaufman,
draft-ietf-dnssec-secext-09.txt
[Photuris] Karn, Phil and Simpson, William, Photuris, draft-ietf-
ipsec-photuris-09.txt
[Kocher] Kocher, Paul, Timing Attack
[Krawcyzk] Krawcyzk, Hugo, SKEME, ISOC, SNDS Symposium, San Diego,
1996
[PKIX] PKIX internet drafts, draft-ietf-pkix-ipki-00.txt
[Random] Random number RFC 1750
[ISAKMP] Schertler, Mark, ISAKMP, draft-ietf-ipsec-isakmp-03.txt and
draft-ietf-ipsec-isakmp-04.txt. The transition from version 3 to
version 4 was in progress at the time of this writing.
[Schneier] Schneier, Bruce, Applied cryptography: protocols,
algorithms, and source code in C, Second edition, John Wiley & Sons,
Inc. 1995, ISBN 0-471-12845-7, hardcover. ISBN 0-471-11709-9,
softcover.
[Schroeppel] Schroeppel, Richard, et al.; Fast Key Exchange with
Elliptic Curve Systems, Crypto '95, Santa Barbara, 1995. Available
on-line as ftp://ftp.cs.arizona.edu/reports/1995/TR95-03.ps (and .Z).
[Stinson] Stinson, Douglas, Cryptography Theory and Practice. CRC
Press, Inc., 2000, Corporate Blvd., Boca Raton, FL, 33431-9868, ISBN
0-8493-8521-0, 1995
[Zimmerman] Philip Zimmermann, The Official Pgp User's Guide,
Published by MIT Press Trade, Publication date: June 1995, ISBN:
0262740176
This draft expires six months from the day of issue. The
expiration date will be August 24, 1996.
H. K. Orman [Page 46]