IPSEC Working Group D. Harkins, D. Carrel
INTERNET-DRAFT cisco Systems
draft-ietf-ipsec-isakmp-oakley-03.txt February 1997
Expire in six months
The resolution of ISAKMP with Oakley
<draft-ietf-ipsec-isakmp-oakley-03.txt>
Status of this Memo
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1. Abstract
[MSST96] (ISAKMP) provides a framework for authentication and key
exchange but does not define them. ISAKMP is designed to be key
exchange independant; that is, it is designed to support many
different key exchanges.
[Orm96] (Oakley) describes a series of key exchanges-- called
"modes"-- and details the services provided by each (e.g. perfect
forward secrecy for keys, identity protection, and authentication).
This document describes a proposal for using the Oakley Key Exchange
Protocol in conjunction with ISAKMP to obtain authenticated keying
material for use with ISAKMP, and for other security associations
such as AH and ESP for the IETF IPsec DOI.
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2. Discussion
This draft combines ISAKMP and Oakley. The purpose is to negotiate,
and provide authenticated keying material for, security associations
in a protected manner.
Processes which implement this draft can be used for negotiating
virtual private networks (VPNs) and also for providing a remote user
from a remote site (whose IP address need not be known beforehand)
access to a secure host or network.
Proxy negotiation is supported. Proxy mode is where the negotiating
parties are not the endpoints for which security association
negotiation is taking place. When used in proxy mode, the identities
of the end parties remain hidden.
This proposal does not implement the entire Oakley protocol, but only
a subset necessary to satisfy its goals. It does not claim
conformance or compliance with the entire Oakley protocol. For
greater understanding of the Oakley protocol and the mathematics
behind it, please refer to [Orm96].
3. Terms and Definitions
3.1 Requirements Terminology
In this document, the words that are used to define the significance
of each particular requirement are capitalised. These words are:
- MUST
This word or the adjective "REQUIRED" means that the item is an
absolute requirement of the specification.
- SHOULD
This word or the adjective "RECOMMENDED" means that there might
exist valid reasons in particular circumstances to ignore this
item, but the full implications should be understood and the case
carefully weighed before taking a different course.
- MAY
This word or the adjective "OPTIONAL" means that this item is
truly optional. One vendor might choose to include the item
because a particular marketplace requires it or because it
enhances the product, for example; another vendor may omit the
same item.
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3.2 Notation
The following notation is used throughout this draft.
HDR is an ISAKMP header whose exchange type is the mode. When
writen as HDR* it indicates payload encryption.
SA is an SA negotiation payload with one or more proposals. An
initiator MAY provide multiple proposals for negotiation; a
responder MUST reply with only one.
SAp is the entire body of the SA payload (minus the SA header)--
i.e. all proposals and transforms offered by the Initiator.
g^xi and g^xr are the Diffie-Hellman public values of the
initiator and responder respectively.
KE is the key exchange payload.
Nx is the nonce payload; x can be: i or r for the ISAKMP initiator
and responder respectively.
IDx is the identity payload for "x". x can be: "ii" or "ir" for
the ISAKMP initiator and responder respectively during phase one
negotiation; or "ui" or "ur" for the user initiator and responder
respectively during phase two. The ID payload format for the
Internet DOI is defined in [Pip96].
SIG is the signature payload. The data to sign is exchange-
specific.
CERT is the certificate payload.
HASH (and any derivitive such as HASH(2) or HASH_I) is the hash
payload. The contents of the hash are specific to the
authentication method.
prf(key, msg) is the keyed pseudo-random function-- often a keyed
hash function-- used to generate a deterministic output that
appears pseudo-random. prf's are used both for key derivations
and for authentication (i.e. as a keyed MAC). (See [KBC96]).
SKEYID is a string derived from secret material known only to the
active players in the exchange.
SKEYID_e is the keying material used by the ISAKMP SA to protect
it's messages.
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SKEYID_a is the keying material used by the ISAKMP SA to
authenticate it's messages.
SKEYID_d is the keying material used to derive keys for non-ISAKMP
security associations.
<x>y indicates that "x" is encrypted with the key "y".
--> signifies "initiator to responder" communication (requests).
<-- signifies "responder to initiator" communication (replies).
| signifies concatenation of information-- e.g. X | Y is the
concatentation of X with Y.
[x] indicates that x is optional.
Payload encryption (when noted by a '*' after the ISAKMP header) MUST
begin immediately after the ISAKMP header. When communication is
protected, all payloads following the ISAKMP header MUST be
encrypted. Encryption keys are generated from SKEYID_e in a manner
that is defined for each algorithm.
When used to describe the payloads contained in complete message
exchanges, the ISAKMP generic header is implicitly included. When
used as part of a prf computation, the ISAKMP generic header is not
included. For example, the initiator may send the responder the
following message:
HDR, KE, Ni
The ISAKMP header is included in the KE and Ni payloads. But if the
initiator generates the following pseudo-random output:
HASH = prf(key, Ni | Nr)
the ISAKMP headers of the two nonce payloads are not included-- only
the body of the payload-- the nonce itself-- is used.
3.3 Perfect Forward Secrecy
When used in the draft Perfect Forward Secrecy (PFS) refers to the
notion that compromise of a single key will permit access to only
data protected by a single key. For PFS to exist the key used to
protect transmission of data MUST NOT be used to derive any
additional keys, and if the key used to protect transmission of data
was derived from some other keying material, that material MUST NOT
be used to derive any more keys.
Perfect Forward Secrecy for both keys and identities is provided in
this protocol. (Sections 5.8 and 7).
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3.4 Security Association
A security association (SA) is a set of policy and key used to
protect information. The ISAKMP SA is the shared policy and key used
by the negotiating peers in this protocol to protect their
communication.
4. Introduction
Oakley defines a method to establish an authenticated key exchange.
This includes how payloads are constructed, the information they
carry, the order in which they are processed and how they are used.
While Oakley defines "modes", ISAKMP defines "phases". The
relationship between the two is very straightforward. ISAKMP's phase
1 is where the two ISAKMP peers establish a secure, authenticated
channel with which to communicate. This is called the ISAKMP
Security Association (SA). Oakley's "Main Mode" and "Aggressive Mode"
each accomplish a phase 1 exchange. "Main Mode" and "Aggressive
Mode" MUST ONLY be used in phase 1.
Phase 2 is where Security Associations are negotiated on behalf of
services such as IPsec or any other service which needs key material
and/or parameter negotiation. Oakley's "Quick Mode" accomplishes a
phase 2 exchange. "Quick Mode" MUST ONLY be used in phase 2.
Oakley's "New Group Mode" is not really a phase 1 or phase 2. It
follows phase 1, but serves to establish a new group which can be
used in future negotiations. "New Group Mode" MUST ONLY be used in
phase 2.
With the use of ISAKMP phases, an implementation can accomplish very
fast keying when necessary. A single phase 1 negotiation may be used
for more than one phase 2 negotiation. Additionally a single phase 2
negotiation can request multiple Security Associations. With these
optimizations, an implementation can see less than one round trip per
SA as well as less than one DH exponentiation per SA. "Main Mode"
for phase 1 provides identity protection. When identity protection
is not needed, "Aggressive Mode" can be used to reduce round trips
even further. Developer hints for doing these optimizations are
included below. It should also be noted that using public key
encryption to authenticate an Aggressive Mode exchange will still
provide identity protection.
The following attributes are used by Oakley and are negotiated as
part of the ISAKMP Security Association. (These attributes pertain
only to the ISAKMP Security Association and not to any Security
Associations that ISAKMP may be negotiating on behalf of other
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services.)
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- encryption algorithm (e.g. DES, IDEA, Blowfish).
- hash algorithm (e.g. MD5, SHA)
- authentication method (e.g. DSS sig, RSA sig, RSA encryption,
pre-shared key)
- information about a group over which to do Diffie-Hellman.
- prf (e.g. 3DES-CBC-MAC)
All of these attributes are mandatory and MUST be negotiated except
for the "prf". The "prf" MAY be negotiated, but if it is not, the
HMAC (see [KBC96]) version of the negotiated hash algorithm is used
as a pseudo-random function. Other non-mandatory attributes are
described in Appendix A. The selected hash algorithm MUST support
both native and HMAC modes.
Oakley implementations MUST support the following attribute values:
- DES-CBC with a weak, and semi-weak, key check (weak and semi-
weak keys are referenced in [Sch94] and listed in Appendix A). The
key is derived according to Appendix B.
- MD5 and SHA.
- Authentication via pre-shared keys. The Digital Signature
Standard SHOULD be supported; RSA SHOULD also be supported.
- MODP over the default Oakley group (see below). ECP groups MAY
be supported.
The Oakley modes described here MUST be implemented whenever the IETF
IPsec DOI [Pip96] is implemented. Other DOIs MAY use the Oakley modes
described here.
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5. Exchanges
There are two basic methods used to establish an authenticated key
exchange: Oakley Main Mode and Oakley Aggressive Mode. Each generates
authenticated keying material from an ephemeral Diffie-Hellman
exchange. Oakley in Main Mode MUST be implemented; Oakley Aggressive
Mode SHOULD be implemented. In addition, Oakley Quick Mode MUST be
implemented as a mechanism to generate fresh keying material and
negotiate non-ISAKMP security services. In additon, Oakley New Group
Mode SHOULD be implemented as a mechanism to define private groups
for Diffie-Hellman exchanges. Implementations MUST NOT switch
exchange types in the middle of an exchange.
Exchanges conform to standard ISAKMP [MSST96] payload syntax.
attribute encoding, timeouts and retransmits of messages, and
informational messages-- e.g a notify response is sent when, for
example, a proposal is unacceptable, or a signature verification or
decryption was unsuccessful, etc.
Oakley Main Mode is an instantiation of the ISAKMP Identity Protect
Exchange: The first two messages negotiate policy; the next two
exchange Diffie-Hellman public values and ancillary data (e.g.
nonces) necessary for the exchange; and the last two messages
authenticate the Diffie-Hellman Exchange. The authentication method
negotiated as part of the initial ISAKMP exchange influences the
composition of the payloads but not their purpose. The XCHG for
Oakley Main Mode is ISAKMP Identity Protect.
Similarly, Oakley Aggressive Mode is an instantiation of the ISAKMP
Aggressive Exchange. The first two messages negotiate policy,
exchange Diffie-Hellman public values and ancillary data necessary
for the exchange, and identities. In addition the second message
authenticates the responder. The third message authenticates the
initiator and provides a proof of participation in the exchange. The
XCHG for Oakley Aggressive Mode is ISAKMP Aggressive. The final
message is not sent under protection of the ISAKMP SA allowing each
party to postpone exponentiation, if desired, until negotiation of
this exchange is complete.
Quick Mode and New Group Mode have no analog in ISAKMP. The XCHG
values for Quick Mode and New Group Mode are defined in Appendix A.
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Three different authentication methods are allowed with either Main
Mode or Aggressive Mode-- digital signature, public key encryption,
or pre-shared key. The value SKEYID is computed seperately for each
authentication method.
For signatures: SKEYID = prf(Ni | Nr, g^xy)
For public key encryption: SKEYID = hash(Ni | Nr)
For pre-shared keys: SKEYID = prf(pre-shared-key, Ni | Nr)
The result of either Main Mode or Aggressive Mode is three groups of
authenticated keying material:
SKEYID_d = prf(SKEYID, g^xy | CKY-I | CKY-R, 0)
SKEYID_a = prf(SKEYID, SKEYID_d | g^xy | CKY-I | CKY-R, 1)
SKEYID_e = prf(SKEYID, SKEYID_a | g^xy | CKY-I | CKY-R, 2)
and agreed upon policy to protect further communications. The values
of 0, 1, and 2 above are represented by a single octet. The key used
for encryption is derived from SKEYID_e in an algorithm-specific
manner (see appendix B).
To authenticate either exchange the initiator of the protocol
generates HASH_I and the responder generates HASH_R where:
HASH_I = prf(SKEYID, g^xi | g^xr | CKY-I | CKY-R | SAp | IDii)
HASH_R = prf(SKEYID, g^xr | g^xi | CKY-R | CKY-I | SAp | IDir)
For authentication with digital signatures, HASH_I and HASH_R are
signed and verified; for authentication with either public key
encryption or pre-shared keys, HASH_I and HASH_R directly
authenticate the exchange.
As mentioned above, the negotiated authentication method influences
the content and use of messages for Phase 1 Oakley Modes, but not
their intent. When using public keys for authentication, the Phase 1
Oakley can be accomplished either by using signatures or by using
public key encryption (if the algorithm supports it). Following are
Main Mode Exchanges with different authentication options.
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5.1 ISAKMP/Oakley Phase 1 Authenticated With Signatures
Using signatures, the ancillary information exchanged during the
second roundtrip are nonces; the exchange is authenticated by signing
a mutually obtainable hash. Oakley Main Mode with signature
authentication is described as follows:
Initiator Responder
---------- -----------
HDR, SA -->
<-- HDR, SA
HDR, KE, Ni -->
<-- HDR, KE, Nr
HDR*, IDii, [ CERT, ] SIG_I -->
<-- HDR*, IDir, [ CERT, ] SIG_R
Oakley Aggressive mode with signatures in conjunction with ISAKMP is
described as follows:
Initiator Responder
----------- -----------
HDR, SA, KE, Ni, IDii -->
<-- HDR, SA, KE, Nr, IDir,
[ CERT, ] SIG_R
HDR, [ CERT, ] SIG_I -->
In both modes, the signed data, SIG_I or SIG_R, is the result of the
negotiated digital signature algorithm applied to HASH_I or HASH_R
respectively.
In general the signature will be over HASH_I and HASH_R as above
using the negotiated prf, or the HMAC version of the negotiated hash
function (if no prf is negotiated). However, this can be overridden
for construction of the signature if the signature algorithm is tied
to a particular hash algorithm (e.g. DSS is only defined with SHA's
160 bit output). In this case, the signature will be over HASH_I and
HASH_R as above, except using the HMAC version of the hash algorithm
associated with the signature method. The negotiated prf and hash
function would continue to be used for all other proscribed pseudo-
random functions.
RSA signatures MUST be encoded in PKCS #1 format. DSS signatures
MUST be encoded as r followed by s.
One or more certificate payloads MAY be optionally passed.
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5.2 Oakley Phase 1 Authenticated With Public Key Encryption
Using public key encryption to authenticate the exchange, the
ancillary information exchanged is encrypted nonces. Each party's
ability to reconstruct a hash (proving that the other party decrypted
the nonce) authenticates the exchange.
In order to perform the public key encryption, the initiator must
already have the responder's public key. In the case where a party
has multiple public keys, a hash of the certificate the initiator is
using to encrypt the ancillary information is passed as part of the
third message. In this way the responder can determine which
corresponding private key to use to decrypt the encrypted payloads
and identity protection is retained.
In addition to the nonce, the identities of the parties (IDii and
IDir) are also encrypted with the other parties public key. If the
authentication method is public key encryption, the nonce and
identity payloads MUST be encrypted with the public key of the other
party. Only the body of the payloads are encrypted, the payload
headers are left in the clear.
When using encrytion for authentication with Oakley, Main Mode is
defined as follows.
Initiator Responder
----------- -----------
HDR, SA -->
<-- HDR, SA
HDR, KE, [ HASH(1), ]
<IDii>PubKey_r,
<Ni>PubKey_r -->
HDR, KE, <IDir>PubKey_i,
<-- <Nr>PubKey_i
HDR*, HASH_I -->
<-- HDR*, HASH_R
Oakley Aggressive Mode authenticated with encryption is described as
follows:
Initiator Responder
----------- -----------
HDR, SA, [ HASH(1),] KE,
<IDii>Pubkey_r,
<Ni>Pubkey_r -->
HDR, SA, KE, <IDir>PubKey_i,
<-- <Nr>PubKey_r, HASH_R
HDR, HASH_I -->
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Where HASH(1) is a hash (using the negotiated hash function) of the
certificate which the initiator is using to encrypt the nonce and
identity.
RSA encryption MUST be encoded in PKCS #1 format. The payload length
is the length of the entire encrypted payload plus header. The PKCS
#1 encoding allows for determination of the actual length of the
cleartext payload upon decryption.
Using encryption for authentication provides for a plausably deniable
exchange. There is no proof (as with a digital signature) that the
conversation ever took place since each party can completely
reconstruct both sides of the exchange. In addition, security is
added to secret generation since an attacker would have to
successfully break not only the Diffie-Hellman exchange but also both
RSA encryptions. This exchange was motivated by [Kra96].
Note that, unlike other authentication methods, authentication with
public key encryption allows for identity protection with Aggressive
Mode.
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5.3 Oakley Phase 1 Authenticated With a Pre-Shared Key
A key derived by some out-of-band mechanism may also be used to
authenticate the exchange. The actual establishment of this key is
out of the scope of this document.
When doing a pre-shared key authentication with Oakley, Main Mode is
defined as follows
Initiator Responder
---------- -----------
HDR, SA -->
<-- HDR, SA
HDR, KE, Ni -->
<-- HDR, KE, Nr
HDR*, IDii, HASH_I -->
<-- HDR*, IDir, HASH_R
Oakley Aggressive mode with a pre-shared key is described as follows:
Initiator Responder
----------- -----------
HDR, SA, KE, Ni, IDii -->
<-- HDR, SA, KE, Nr, IDir, HASH_R
HDR, HASH_I -->
When using pre-shared key authentication with Main Mode the key can
only be identified by the IP address of the peers since HASH_I must
be computed before the initiator has processed IDir. Aggressive Mode
allows for a wider range of identifiers of the pre-shared secret to
be used. In addition, Aggressive Mode allows two parties to maintain
multiple, different pre-shared keys and identify the correct one for
a particular exchange.
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5.4 Oakley Phase 2 - Quick Mode
Oakley Quick Mode is not a complete exchange itself, but is used as
part of the ISAKMP SA negotiation process (phase 2) to derive keying
material and negotiate shared policy for non-ISAKMP SAs. The
information exchanged along with Oakley Quick Mode MUST be protected
by the ISAKMP SA-- i.e. all payloads except the ISAKMP header are
encrypted.
Quick Mode is essentially an exchange of nonces that provides replay
protection. The nonces are used to generate fresh key material and
prevent replay attacks from generating bogus security associations.
An optional Key Exchange payload can be exchanged to allow for an
additional Diffie-Hellman exchange and exponentiation per Quick Mode.
While use of the key exchange payload with Quick Mode is optional it
MUST be supported.
Base Quick Mode (without the KE payload) refreshens the keying
material derived from the exponentiation in phase 1. This does not
provide PFS. Using the optional KE payload, an additional
exponentiation is performed and PFS is provided for the keying
material. If a KE payload is sent, a Diffie-Hellman group (see
section 5.6.1 and [Pip96]) MUST be sent as attributes of the SA being
negotiated.
Quick Mode is defined as follows:
Initiator Responder
----------- -----------
HDR*, HASH(1), SA, Ni
[, KE ] [, IDui, IDur ] -->
<-- HDR*, HASH(2), SA, Nr
[, KE ] [, IDui, IDur ]
HDR*, HASH(3) -->
Where:
HASH(1) and HASH(2) are the prf over the message id (M-ID) from
the ISAKMP header concatenated with the entire message that follows
the hash including payload headers, but excluding any padding added
for encryption. HASH(3)-- for liveliness-- is the prf over the value
zero represented as a single octet, followed by a concatenation of
the message id and the two nonces-- the initiator's followed by the
responder's. In other words, the hashes for the above exchange are:
HASH(1) = prf(SKEYID_a, M-ID | SA | Ni [ | KE ] [ | IDui | IDur ])
HASH(2) = prf(SKEYID_a, M-ID | SA | Nr [ | KE ] [ | IDui | IDur ])
HASH(3) = prf(SKEYID_a, 0 | M-ID | Ni | Nr)
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If PFS is not needed, and KE payloads are not exchanged, the new
keying material is defined as KEYMAT = prf(SKEYID_d, SPI | Ni | Nr).
If PFS is desired and KE payloads were exchanged, the new keying
material is defined as KEYMAT = prf(SKEYID_d, g(qm)^xy | SPI | Ni |
Nr), where g(qm)^xy is the shared secret from the ephemeral Diffie-
Hellman exchange of this Quick Mode.
The SPI is the value from the SPI field in the SA payload.
A single SA negotiation results in two security assocations-- one
inbound and one outbound. Different SPIs for each SA (one chosen by
the initiator, the other by the responder) guarantee a different key
for each direction. The SPI chosen by the destination of the SA is
used to derive KEYMAT for that SA.
For situations where the amount of keying material desired is greater
than that supplied by the prf, KEYMAT is expanded by feeding the
results of the prf back into itself and concatenating results until
the required keying material has been reached. In other words,
KEYMAT = K1 | K2 | K3 | ...
where
K1 = prf(SKEYID_d, [ g(qm)^xy | ] SPI | Ni | Nr)
K2 = prf(SKEYID_d, K1 | [ g(qm)^xy | ] SPI | Ni | Nr)
K3 = prf(SKEYID_d, K2 | [ g(qm)^xy | ] SPI | Ni | Nr)
etc.
This keying material (whether with PFS or without, and whether
derived directly or through concatenation) MUST be used with the
negotiated SA. It is up to the service to define how keys are derived
from the keying material (see Appendix B).
In the case of an ephemeral Diffie-Hellman exchange in Quick Mode,
the exponential (g(qm)^xy) is irretreivably removed from the current
state and SKEYID_e and SKEYID_a (derived from phase 1 negotiation)
continue to protect and authenticate the ISAKMP SA and SKEYID_d
continues to be used to derive keys.
If ISAKMP is acting as a proxy negotiator on behalf of another party
the identities of the parties MUST be passed as IDui and IDur. Local
policy will dictate whether the proposals are acceptible for the
identities specified. If IDs are not exchanged, the negotiation is
assumed to be done on behalf of each ISAKMP peer. If an ID range
(see Appendix A of [Pip96]) is not acceptable (for example, the
specified subnet is too large) a BAD_ID_RANGE notify message followed
by an acceptible ID range, in an ID payload, MUST be sent.
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Using Quick Mode, multiple SA's and keys can be negotiated with one
exchange as follows:
Initiator Responder
----------- -----------
HDR*, HASH(1), SA0, SA1, Ni,
[, KE ] [, IDui, IDur ] -->
<-- HDR*, HASH(2), SA0, SA1, Nr,
[, KE ] [, IDui, IDur ]
HDR*, HASH(3) -->
The keying material is derived identically as in the case of a single
SA. In this case (negotiation of two SA payloads) the result would be
four security associations-- two each way for both SAs.
5.5 Oakley New Group Mode
Oakley New Group Mode MUST NOT be used prior to establishment of an
ISAKMP SA. The description of a new group MUST only follow phase 1
negotiation. (It is not a phase 2 exchange, though).
Initiator Responder
----------- -----------
HDR*, HASH(1), SA -->
<-- HDR*, HASH(2), SA
where HASH(1) is the prf output, using SKEYID_a as the key, and the
entire SA proposal, body and header, as the data; HASH(2) is the prf
output, using SKEYID_a as the key, and the reply as the data.
The proposal will be an Oakley proposal which specifies the
characteristics of the group (see appendix A, "Oakley Attribute
Assigned Numbers"). Group descriptions for private Oakley Groups MUST
be greater than or equal to 2^15. If the group is not acceptable,
the responder MUST reply with a Notify payload with the message type
set to GROUP_NOT_ACCEPTABLE (13).
ISAKMP implementations MAY require private groups to expire with the
SA under which they were established.
Groups may be directly negotiated in the SA proposal with Oakley Main
Mode. To do this the Prime, Generator (using the Generator One
attribute class from Appendix A), and Group Type are passed as SA
attributes (see Appendix A in [MSST96]). Alternately, the nature of
the group can be hidden using Oakley New Group Mode and only the
group identifier is passed in the clear during phase 1 negotiation.
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5.6 Oakley Groups
[Orm96] defines several groups. The value 0 indicates no group. The
value 1 indicates the default group described below. The attribute
class for "Group" is defined in Appendix A. Other values are also
defined in [Orm96]. All values 2^15 and higher are used for private
group identifiers.
5.6.1 Oakley Default Group
Oakley implementations MUST support a MODP group with the following
prime and generator. This group is assigned id 1 (one).
The prime is: 2^768 - 2 ^704 - 1 + 2^64 * { [2^638 pi] + 149686 }
Its hexadecimal value is
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
E485B576 625E7EC6 F44C42E9 A63A3620 FFFFFFFF FFFFFFFF
The generator is: 2.
other groups can be defined using Oakley New Group Mode. This default
group was generated by Richard Schroeppel at the University of
Arizona. Properties of this prime are described by the following
exerpt from [Orm96]:
The prime for this group was selected to have certain
properties. The high order 64 bits are forced to 1. This
helps the classical remainder algorithm, because the trial
quotient digit can always be taken as the high order word of
the dividend, possibly +1. The low order 64 bits are forced to
1. This helps the Montgomery-style remainder algorithms,
because the multiplier digit can always be taken to be the low
order word of the dividend. The middle bits are taken from the
binary expansion of pi. This guarantees that they are
effectively random, while avoiding any suspicion that the
primes have secretly been selected to be weak.
The prime is chosen to be a Sophie-Germain prime (i.e., (P-1)/2
is also prime), to have the maximum strength against the
square-root attack. The starting trial numbers were repeatedly
incremented by 2^64 until suitable primes were located.
Because this prime is congruent to 7 (mod 8), 2 is a quadratic
residue. All powers of 2 will also be quadratic residues.
This prevents an opponent from learning the low order bit of
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the Diffie-Hellman exponent. Using 2 as a generator is
efficient for some modular exponentiation algorithms. [Note
that 2 is technically not a generator in the number theory
sense, because it omits half of the possible residues mod P.
From a cryptographic viewpoint, this is a virtue.]
A further discussion of the properties of this group, the motivation
behind its creation, as well as the definition of several more groups
can be found in [Orm96].
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5.7 Payload Explosion for Complete ISAKMP-Oakley Exchange
This section illustrates how ISAKMP payloads are used with Oakley to:
- establish a secure and authenticated channel between ISAKMP
processes (phase 1); and
- generate key material for, and negotiate, an IPsec SA (phase 2).
5.7.1 Phase 1 using Oakley Main Mode
The following diagram illustrates the payloads exchanged between the
two parties in the first round trip exchange. The initiator MAY
propose several proposals; the responder MUST reply with one.
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 with XCHG of Oakley Main Mode, ~
~ and Next Payload of ISA_SA ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! 0 ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Domain of Interpretation (IPsec DOI) !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Situation !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! 0 ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Proposal #1 ! Proto=ISAKMP ! # of Transforms !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ SPI (8 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ISA_TRANS ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Transform #1 ! RESERVED | OAKLEY !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ preffered SA attributes ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! 0 ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Transform #2 ! RESERVED | OAKLEY !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ alternate SA attributes ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The responder replies in kind but selects, and returns, one transform
proposal (the ISAKMP SA attributes).
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The second exchange consists of the following payloads:
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 with XCHG of Oakley Main Mode, ~
~ and Next Payload of ISA_KE ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ISA_NONCE ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ D-H Public Value (g^xi from initiator g^xr from responder) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! 0 ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Ni (from initiator) or Nr (from responder) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The shared keys, SKEYID_e and SKEYID_a, are now used to protect and
authenticate all further communication. Note that both SKEYID_e and
SKEYID_a are unauthenticated.
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 with XCHG of Oakley Main Mode, ~
~ and Next Payload of ISA_ID and the encryption bit set ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ISA_SIG ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Identification Data of the ISAKMP negotiator ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! 0 ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ signature verified by the public key of the ID above ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The key exchange is authenticated over a signed hash as described in
section 5.1. Once the signature has been verified using the
authentication algorithm negotiated as part of the ISAKMP SA, the
shared keys, SKEYID_e and SKEYID_a can be marked as authenticated.
(For brevity, certificate payloads were not exchanged).
5.7.2 Phase 2 using Oakley Quick Mode
The following payloads are exchanged in the first round of Oakley
Quick Mode with ISAKMP SA negotiation. In this hypothetical exchange,
the ISAKMP negotiators are proxies for other parties which have
requested authentication.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ISAKMP Header with XCHG of Oakley Quick Mode, ~
~ Next Payload of ISA_HASH and the encryption bit set ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ISA_SA ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ keyed hash of message ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ISA_NONCE ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Domain Of Interpretation (DOI) !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Situation !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! 0 ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Proposal #1 ! Protocol=AH ! # of Transforms !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ SPI (8 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ISA_TRANS ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Transform #1 ! RESERVED | SHA !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! other SA attributes !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! 0 ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Transform #1 ! RESERVED | MD5 !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! other SA attributes !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ISA_ID ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ nonce ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ISA_ID ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ID of source for which ISAKMP is a proxy ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! 0 ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ID of destination for which ISAKMP is a proxy ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where the contents of the hash are described in 5.4 above. The
responder replies with a similar message which only contains one
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transform-- the selected AH transform. Upon receipt, the initiator
can provide the key engine with the negotiated security association
and the keying material. As a check against replay attacks, the
responder waits until receipt of the next message.
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 with XCHG of Oakley Quick Mode, ~
~ Next Payload of ISA_HASH and the encryption bit set ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! 0 ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ hash data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where the contents of the hash are described in 5.4 above.
5.8 Perfect Forward Secrecy Example
This protocol can provide PFS of both keys and identities. The
identies of both the ISAKMP negotiating peer and, if applicable, the
proxy for whom the peers are negotiating can be protected with PFS.
To provide Perfect Forward Secrecy of both keys and all identities,
two parties would perform the following:
o A Main Mode Exchange to protect the identities of the ISAKMP
peers.
This establishes an ISAKMP SA. o A Quick Mode Exchange to
negotiate other security protocol protection.
This establishes a SA on each end for this protocol. o Delete
the ISAKMP SA and its associated state.
Since the key for use in the non-ISAKMP SA was derived from the
single ephemeral Diffie-Hellman exchange PFS is preserved.
To provide Perfect Forward Secrecy of merely the keys of a non-ISAKMP
security association, it in not necessary to do a phase 1 exchange if
an ISAKMP SA exists between the two peers. A single Quick Mode in
which the optional KE payload is passed, and an additional Diffie-
Hellman exchange is performed, is all that is required. At this point
the state derived from this Quick Mode must be deleted from the
ISAKMP SA as described in section 5.4.
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6. Implementation Hints
Using a single ISAKMP Phase 1 negotiation makes subsequent Phase 2
negotiations extremely quick. As long as the Phase 1 state remains
cached, and PFS is not needed, Phase 2 can proceed without any
exponentiation. How many Phase 2 negotiations can be performed for a
single Phase 1 is a local policy issue. The decision will depend on
the strength of the algorithms being used and level of trust in the
peer system.
An implementation may wish to negotiate a range of SAs when
performing Quick Mode. By doing this they can speed up the "re-
keying". Quick Mode defines how KEYMAT is defined for a range of SAs.
When one peer feels it is time to change SAs they simple use the next
one within the stated range. A range of SAs can be established by
negotiating multiple SAs (identical attributes, different SPIs) with
one Quick Mode.
An optimization that is often useful is to establish Security
Associations with peers before they are needed so that when they
become needed they are already in place. This ensures there would be
no delays due to key management before initial data transmission.
This optimization is easily implemented by setting up more than one
Security Association with a peer for each requested Security
Association and caching those not immediately used.
Also, if an ISAKMP implementation is alerted that a SA will soon be
needed (e.g. to replace an existing SA that will expire in the near
future), then it can establish the new SA before that new SA is
needed.
7. Security Considerations
This entire draft discusses a hybrid protocol, combining Oakley with
ISAKMP, to negotiate, and derive keying material for, security
associations in a secure and authenticated manner.
Confidentiality is assured by the use of a negotiated encryption
algorithm. Authentication is assured by the use of a negotiated
method: a digital signature algorithm; a public key algorithm which
supports encryption; or, a pre-shared key. The confidentiality and
authentication of this exchange is only as good as the attributes
negotiated as part of the ISAKMP security association.
Repeated re-keying using Quick Mode can consume the entropy of the
Diffie- Hellman shared secret. Implementors should take note of this
fact and set a limit on Quick Mode Exchanges between exponentiations.
This draft does not proscribe such a limit.
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Perfect Forward Secrecy (PFS) of both keying material and identities
is possible with this protocol. By specifying a Diffie-Hellman group,
and passing public values in KE payloads, ISAKMP peers can establish
PFS of keys-- the identities would be protected by SKEYID_e from the
ISAKMP SA and would therefore not be protected by PFS. If PFS of both
keying material and identities is desired, an ISAKMP peer MUST
establish only one non-ISAKMP security association (e.g. IPsec
Security Association) per ISAKMP SA. PFS for keys and identities is
accomplished by deleting the ISAKMP SA (and optionally issuing a
DELETE message) upon establishment of the single non-ISAKMP SA. In
this way a phase one negotiation is uniquely tied to a single phase
two negotiation, and the ISAKMP SA established during phase one
negotiation is never used again.
Implementations SHOULD not use Diffie-Hellman exponents of less than
200 bits to avoid adversely effecting the security of both parties.
It is assumed that the Diffie-Hellman exponents in this exchange are
erased from memory after use. In particular, these exponents must not
be derived from long-lived secrets like the seed to a pseudo-random
generator.
8. Acknowledgements
This document is the result of close consultation with Hilarie Orman,
Douglas Maughan, Mark Schertler, Mark Schneider, and Jeff Turner. It
relies completely on protocols which were written by them. Without
their interest and dedication, this would not have been written.
We would also like to thank Cheryl Madson, Harry Varnis, Elfed
Weaver, and Hugo Krawcyzk for technical input.
9. References
[ACM97] Adams, C.M., "Constructing Symmetric Ciphers Using the CAST
Design Procedure", Designs, Codes and Cryptorgraphy (to appear).
[KBC96] Krawczyk, H., Bellare, M., Canetti, R., "HMAC: Keyed-Hashing
for Message Authentication", draft-ietf-ipsec-hmac-md5-01.txt
[Kra96] Krawczyk, H., "SKEME: A Versatile Secure Key Exchange
Mechanism for Internet", from IEEE Proceedings of the 1996 Symposium
on Network and Distributed Systems Security.
[MSST96] Maughhan, D., Schertler, M., Schneider, M., and Turner, J.,
"Internet Security Association and Key Management Protocol (ISAKMP)",
version 7, draft-ietf-ipsec-isakmp-07.{ps,txt}.
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[Orm96] Orman, H., "The Oakley Key Determination Protocol", version
1, draft-ietf-ipsec-oakley-01.txt.
[Pip96] Piper, D., "The Internet IP Security Domain Of Interpretation
for ISAKMP", version 2, draft-ietf-ipsec-ipsec-doi-02.txt.
[Sch94] Schneier, B., "Applied Cryptography, Protocols, Algorithms,
and Source Code in C", 2nd edition.
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Appendix A
This is a list of DES Weak and Semi-Weak keys. The keys come from
[Sch94]. All keys are listed in hexidecimal.
DES Weak Keys
0101 0101 0101 0101
1F1F 1F1F E0E0 E0E0
E0E0 E0E0 1F1F 1F1F
FEFE FEFE FEFE FEFE
DES Semi-Weak Keys
01FE 01FE 01FE 01FE
1FE0 1FE0 0EF1 0EF1
01E0 01E0 01F1 01F1
1FFE 1FFE 0EFE 0EFE
011F 011F 010E 010E
E0FE E0FE F1FE F1FE
FE01 FE01 FE01 FE01
E01F E01F F10E F10E
E001 E001 F101 F101
FE1F FE1F FE0E FE0E
1F01 1F01 0E01 0E01
FEE0 FEE0 FEF1 FEF1
Attribute Assigned Numbers
Attributes negotiated during phase one use the following definitions.
Phase two attributes are defined in the applicable DOI specification
(for example, IPsec attributes are defined in the IPsec DOI), with
the exception of a group description when Quick Mode includes an
ephemeral Diffie-Hellman exchange. Attribute types can be either
Basic (B) or Variable-length (V). Encoding of these attributes is
defined in the base ISAKMP specification.
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Attribute Classes
class value type
-------------------------------------------------------------------
Encryption Algorithm 1 B
Hash Algorithm 2 B
Authentication Method 3 B
Group Description 4 B
Group Type 5 B
Group Prime 6 V
Group Generator One 7 V
Group Generator Two 8 V
Group Curve A 9 V
Group Curve B 10 V
Life Type 11 B
Life Duration 12 B/V
PRF 13 B
Key Length 14 B
Class Values
- Encryption Algorithm
DEC-CBC 1
IDEA-CBC 2
Blowfish-CBC 3
RC5-R12-B64-CBC 4
3DES-CBC 5
CAST-CBC 6
values 7-65000 are reserved. Values 65001-65535 are for private use
among mutually consenting parties.
- Hash Algorithm
MD5 1
SHA 2
Tiger 3
values 4-65000 are reserved. Values 65001-65535 are for private use
among mutually consenting parties.
- Authentication Method
pre-shared key 1
DSS signatures 2
RSA signatures 3
RSA encryption 4
values 5-65000 are reserved. Values 65001-65535 are for private use
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among mutually consenting parties.
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- Group Description
default group (section 5.6.1) 1
values 2-32767 are reserved. Values 32768-65535 are for private use
among mutually consenting parties.
- Group Type
MODP (modular exponentiation group) 1
ECP (elliptic curve group) 2
values 3-65000 are reserved. Values 65001-65535 are for private use
among mutually consenting parties.
- Life Type
seconds 1
kilobytes 2
values 3-65000 are reserved. Values 65001-65535 are for private use
among mutually consenting parties. For a given "Life Type" the
value of the "Life Duration" attribute defines the actual length of
the SA life-- either a number of seconds, or a number of kbytes
protected.
- PRF
3DES-CBC-MAC 1
values 2-65000 are reserved. Values 65001-65535 are for private use
among mutually consenting parties
- Key Length
When using an Encryption Algorithm that has a variable length key,
this attribute specifies the key length in bits. (MUST use network
byte order).
Additional Exchanges Defined-- XCHG values
Quick Mode 32
New Group Mode 33
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Appendix B
This appendix describes encryption details to be used ONLY when
encrypting ISAKMP messages. When a service (such as an IPSEC
transform) utilizes ISAKMP to generate keying material, all
encryption algorithm specific details (such as key and IV generation,
padding, etc...) MUST be defined by that service. ISAKMP does not
purport to ever produce keys that are suitable for any encryption
algorithm. ISAKMP produces the requested amount of keying material
from which the service MUST generate a suitable key. Details, such
as weak key checks, are the responsibility of the service.
Encryption keys used to protect the ISAKMP SA are derived from
SKEYID_e in an algorithm-specific manner. When SKEYID_e is not long
enough to supply all the necessary keying material an algorithm
requires, the key is derived from feeding the results of a pseudo-
random function into itself, concatenating the results, and taking
the highest necessary bits.
For example, if (ficticious) algorithm AKULA requires 320-bits of key
(and has no weak key check) and the prf used to generate SKEYID_e
only generates 120 bits of material, the key for AKULA, would be the
first 320-bits of Ka, where:
Ka = K1 | K2 | K3
and
K1 = prf(SKEYID_e, 0)
K2 = prf(SKEYID_e, K1)
K3 = prf(SKEYID_e, K2)
where prf is the HMAC version of the negotiated hash function or the
negotiated prf. Each result of the prf provides 120 bits of material
for a total of 360 bits. AKULA would use the first 320 bits of that
360 bit string.
In phase 1, material for the initialization vector (IV material) for
CBC mode encryption algorithms is derived from a hash of a
concatenation of the initiator's public Diffie-Hellman value and the
responder's public Diffie-Hellman value using the negotiated hash
algorithm. This is used for the first message only. Each message
should be padded up to the nearest block size using bytes containing
0x00. The message length in the header MUST include the length of the
pad since this reflects the size of the cyphertext. Subsequent
messages MUST use the last CBC encryption block from the previous
message as their initialization vector.
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In phase 2, material for the initialization vector for CBC mode
encryption of the first message of a Quick Mode exchange is derived
from a hash of a concatenation of the last phase 1 CBC output block
and the phase 2 message id using the negotiated hash algorithm. The
IV for subsequent messages within a Quick Mode exchange is the CBC
output block from the previous message. Padding and IVs for
subsequent messages are done as in phase 1.
Note that the final phase 1 CBC output block, the result of
encryption/decryption of the last phase 1 message, must be retained
in the ISAKMP SA state to allow for generation of unique IVs for each
Quick Mode. Each phase 2 exchange generates IVs independantly to
prevent IVs from getting out of sync when two different Quick Modes
are started simultaneously.
The key for DES-CBC is derived from the first eight (8) non-weak and
semi-weak (see Appendix A) bytes of SKEYID_e. The IV is the first 8
bytes of the IV material derived above.
The key for IDEA-CBC is derived from the first sixteen (16) bytes of
SKEYID_e. The IV is the first eight (8) bytes of the IV material
derived above.
The key for Blowfish-CBC is either the negotiated key size, or the
first fifty-six (56) bytes of a key (if no key size is negotiated)
derived in the aforementioned pseudo-random function feedback method.
The IV is the first eight (8) bytes of the IV material derived above.
The key for RC5-R12-B64-CBC is the negotiated key length, or the
first sixteen (16) bytes of a key (if no key size is negotiated)
derived from the aforementioned pseudo-random function feedback
method if necessary. The IV is the first eight (8) bytes of the IV
material derived above. The number of rounds MUST be 12 and the block
size MUST be 64.
The key for 3DES-CBC is the first twenty-four (24) bytes of a key
derived in the aforementioned pseudo-random function feedback method.
3DES-CBC is an encrypt-decrypt-encrypt operation using the first,
middle, and last eight (8) bytes of the entier 3DES-CBC key. The IV
is the first eight (8) bytes of the IV material derived above.
The key for CAST-CBC is either the negotiated key size, or the first
sixteen (16) bytes of a key derived in the aforementioned pseudo-
random function feedback method. The IV is the first eight (8) bytes
of the IV material derived above.
Support for algorithms other than DES-CBC is purely optional. Some
optional algorithms may be subject to intellectual property claims.
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Authors' Addresses:
Dan Harkins <dharkins@cisco.com>
cisco Systems
170 W. Tasman Dr.
San Jose, California, 95134-1706
United States of America
+1 408 526 4000
Dave Carrel <carrel@ipsec.org>
76 Lippard Ave.
San Francisco, CA 94131-2947
United States of America
+1 415 337 8469
Authors' Note:
The authors encourage independent implementation, and
interoperability testing, of this hybrid exchange.
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