IPSEC Working Group D. Harkins, D. Carrel, Editors
INTERNET-DRAFT cisco Systems
draft-ietf-ipsec-isakmp-oakley-01.txt November 1996
Expire in six months
The resolution of ISAKMP with Oakley
<draft-ietf-ipsec-isakmp-oakley-01.txt>
Status of this Memo
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documents of the Internet Engineering Task Force (IETF), its areas,
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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.
2. Discussion
This draft combines ISAKMP and Oakley. The purpose is to negotiate,
and provide authenticated keying material for, security associations
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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 usually 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.
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 is the hash payload. The contents of the hash are exchange
specific.
prf() is the keyed hash function negotiated as part of the ISAKMP
SA.
SKEYID_e is the keying material used by the ISAKMP SA to protect
it's messages.
SKEYID_a is the keying material used by the ISAKMP SA to
authenticate it's messages.
<x>y indicates that "x" is encrypted with the key "y".
--> signifies "initiator to responder" communication (requests).
<-- signifies "responder to initiator" communication (replies).
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| 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.
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 5.7 and 7).
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.
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4. Introducttion
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 AH, ESP 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.
The following attributes are required by Oakley and MUST be
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 services.)
- 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)
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- information about a group over which to do Diffie-Hellman.
The selected hash algorithm MUST support both keyed and unkeyed
modes.
Oakley implementations MUST support the following default attributes:
- 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 in native and HMAC mode [KBC96].
- 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 and EC2N
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.
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
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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
Base 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 Base Exchange. 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.
The result of either exchange is two groups of authenticated keying
material:
SKEYID_e = prf(g^xy, Ni | Nr | CKY-I | CKY-R | IDii | IDir | 0)
SKEYID_a = prf(g^xy, Ni | Nr | CKY-I | CKY-R | IDii | IDir | 1)
and agreed upon policy to protect further communications. The values
of 0 and 1 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).
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.
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 -->
<-- HDR*, IDir, [ CERT, ] SIG
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
HDR, [ CERT, ] SIG -->
In both modes, the signed data in SIG is a signature of a keyed hash
of the concatenation of the nonces, cookies, the entire SA offer--
everything following the SA header-- that was sent from Initiator to
Responder, and the sender's ID, with g^xy as the key to the hash
function. The order of the nonces, and cookies are specific to the
direction. In other words, the sender signs, HASH_I, and the
responder signs HASH_R where:
HASH_I = prf(g^xy, Ni | Nr | CKY-I | CKY-R | SAp | IDii))
HASH_R = prf(g^xy, Nr | Ni | CKY-R | CKY-I | SAp | IDir))
In general the keyed hash will be the HMAC version of the negotiated
hash function. This can be overridden for construction of the
signature if the signature algorithm is tied to a particular hash
algorithm. In this case, the negotiated hash function would continue
to be used for all other proscribed hashing functions.
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 nonce and identity
protection is retained.
In addition, 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.
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_r,
<-- <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, <Nr>PubKey_i,
<-- IDir, HASH_R
HDR, HASH_I -->
Where HASH(1) is a hash (using the negotiated hash function) of the
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certificate which the initiator is using to encrypt the nonce and
identity. The contents of the other hashes (HASH_I and HASH_R) are
the results of the HMAC version of the hash algorithm negotiated in
the first roundtrip, with a concatenation of the nonces as the key
and a concatenation of the shared secret, the cookies, the entire SA
offer-- everything following the SA header-- that was sent from
Initiator to Responder, and the sender's ID as the hashed data. The
order of the nonces and cookies are specific to the direction. In
other words:
HASH_I = prf(Ni | Nr, g^xy | CKY-I | CKY-R | SAp | IDii))
HASH_R = prf(Nr | Ni, g^xy | CKY-R | CKY-I | SAp | IDir))
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.
Note that, unlike other authentication methods, authentication with
public key encryption allows for identity protection with Aggressive
Mode.
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 -->
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The hash is the result of the HMAC version of the hash algorithm
negotiated in the first roundtrip with the pre-shared key as the key
to the HMAC, and a concatenation of the shared secret, the nonces,
the cookies, the complete SA offer-- everything following the SA
header-- sent from Initiator to Responder, and the sender's ID. The
order of the cookies and nonces are specific to the direction. In
other words,
HASH_I = prf(pre-shared-key, g^xy | Ni | Nr | CKY-I | CKY-R | SAp
| IDii)
HASH_R = prf(pre-shared-key, g^xy | Nr | Ni | CKY-R | CKY-I | SAp
| IDir)
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 5.5.1 and Appendix A) 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) -->
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Where:
HASH(1) and HASH(2) are keyed hashes of the entire message that
follows the hash including payload headers, and HASH(3)-- for
liveliness-- is a keyed hash of the value zero represented as a
single octet, followed by a concatenation of the two nonces. For
example, the hashes for the above exchange would be:
HASH(1) = prf( SKEYID_a, SA | Ni [ | KE ] [ | IDui | IDur ] )
HASH(2) = prf( SKEYID_a, SA | Nr [ | KE ] [ | IDui | IDur ] )
HASH(3) = prf( SKEYID_a, 0 | Ni | Nr )
If PFS is not needed, and KE payloads are not exchanged, the new
keying material is defined as KEYMAT = prf(SKEYID_e, Ni | Nr | 0).
If PFS is desired and KE payloads were exchanged, the new keying
material is defined as KEYMAT = prf(g(qm)^xy, Ni | Nr | 0), where
g(qm)^xy is the shared secret from the ephemeral Diffie-Hellman
exchange of this Quick Mode.
In either case, 0 is represented by a single octet.
For situations where the amount of keying material desired is greater
than that supplied by the prf, KEYMAT is expanded by concatenation
and rehashing with a monotonically increasing number represented by a
single octet, i.e.
KEYMAT = prf(SKEYID_e, Ni | Nr | 0) | prf(SKEYID_e, Ni | Nr | 1)
...
repeated until the required keying material has been reached.
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.
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
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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) -->
and the keying material for SAx is prf(SKEYID_e, Ni | Nr | x) where x
is the number of the SA negotiated (starting with zero). (In the case
where one, or all, of the SAs required keys longer than that supplied
by the prf, the number merely monotonically increases for this entire
exchange-- e.g. SA0 uses 0 and 1; SA1 uses 2 and 3; etc). For ease of
processing the HASH payloads MUST immediately follow the ISAKMP
header and precede all other payloads.
5.4 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 a keyed hash, using SKEYID_a as the key, and the
entire SA proposal, body and header, as the data; HASH(2) is a keyed
hash, 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, 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
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group identifier is passed in the clear during Main Mode.
5.5 Oakley Groups
[Orm96] defines several groups. The value 0 indicates no group. The
value 1 indicates the default group described below. The attriute
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.
5.5.1 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
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residue. All powers of 2 will also be quadratic residues.
This prevents an opponent from learning the low order bit of
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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5.6 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).
5.6.1 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 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ISA_PROP ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Domain of Interpretation (IPsec DOI) !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Situation !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ISA_TRANS ! 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^x from initiator g^y 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).
5.6.2 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_PROP ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Domain Of Interpretation (DOI) !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Situation !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ISA_TRANS ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Proposal #1 ! Protocol=AH ! # of Transforms !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ SPI (8 octets) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ISA_TRANS ! RESERVED ! Payload Length !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! Transform #1 ! RESERVED | SHA !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! other SA attributes !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
! ISA_NONCE ! 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.
5.7 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.
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
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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 ISAKMP is 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.
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
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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.
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
[KBC96] Krawczyk, H., Bellare, M., Canetti, R., "HMAC: Keyed-Hashing
for Message Authentication", draft-ietf-ipsec-hmac-md5-01.txt
[Kra96] Krawcyzk, 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 6, draft-ietf-ipsec-isakmp-06.{ps,txt}.
[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 1, draft-ietf-ipsec-ipsec-doi-01.txt.
[Sch94] Schneier, B., "Applied Cryptography, Protocols, Algorithms,
and Source Code in C", 1st 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.
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
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Group Curve A 9 V
Group Curve B 10 V
Life Type 11 B
Life Duration 12 B/V
Class Values
Encryption Algorithm
DEC-CBC 1
IDEA-CBC 2
Blowfish-CBC 3
values 4-65000 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 among mutually consenting parties.
Group Description
default group (section 5.5.1) 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
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"
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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.
Additional Exchanges Defined-- XCHG values
Quick Mode 32
New Group Mode 33
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Appendix B
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
a concatenation of SKEYID_e and successive keyed hashes of a single
character which contains a monotonically increasing counter beginning at
one (1), and SKEYID_e as the key, using the negotiated hash function.
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 = SKEYID_e | prf(SKEYID_e | 1) | prf(SKEYID_e | 2)
where prf is the HMAC version of the negotiated hash function. SKEYID_e
provides 120-bits, and each of the two additional hashes provide 120-bits,
for a total of 360 bits.
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.
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 8 bytes of the IV material derived above.
The key for Blowfish-CBC is derived from the first fifty-six (56) bytes of
a key derived in the method described above, by concatenating successive
hashes onto SKEYID_e until the requesite number of bytes has been achieved.
The IV is the first 8 bytes of the IV material derived above.
Editors' Addresses:
Dan Harkins <dharkins@cisco.com>
Dave Carrel <carrel@cisco.com>
cisco Systems
170 W. Tasman Dr.
San Jose, California, 95134-1706
United States of America
+1 408 526 4000
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Editors' Note:
The editors encourage independent implementation, and
interoperability testing, of this hybrid exchange.
Harkins, Carrel [Page 27]
