IP Security Protocol Working Group (IPSEC) T. Kivinen
INTERNET-DRAFT SSH Communications Security
draft-ietf-ipsec-ike-hash-revised-00.txt 23 November 1999
Expires: 23 May 2000
Fixing IKE Phase 1 Authentication HASH
Status of This memo
This document is a submission to the IETF IP Security Protocol
(IPSEC) Working Group. Comments are solicited and should be
addressed to the working group mailing list (ipsec@lists.tislabs.com)
or to the editor.
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
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Abstract
This document defines new method of calculating the authentication HASH
of the IKE [RFC-2409] protocol. It fixes known problems with the IKE.
The way the HASH is currently defined in the [RFC-2409] does not authen-
ticate the generic ISAKMP [RFC-2408] header, nor does it authenticate
any extra payloads inside phase 1 packets. This causes a security prob-
lem when using extra payloads as already defined in the IKE and DOI
[RFC-2407] (vendor ID payload, INITIAL-CONTACT notification etc). This
document defines three methods how to negotiate the new HASH method. All
methods tries to be backward compatible as much as possible. Only one of
those methods must be selected by the working group and all the other
should be removed from this document.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Specification of Requirements . . . . . . . . . . . . . . . . . 3
3. Revised HASH Calculation . . . . . . . . . . . . . . . . . . . . 3
4. Negotiating Revised HASH . . . . . . . . . . . . . . . . . . . . 4
5. Selecting Revised HASH Using New Authentication Methods . . . . 4
6. Selecting Revised HASH Using New HASH Algorithms . . . . . . . . 5
7. Selecting Revised HASH Using New PRF Algorithm . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
In the IKE [RFC-2409] protocol there is a clear security problem,
because of the way the authentication HASH is calculated.
The HASH is defined in the [RFC-2409] like this:
HASH_I = prf(SKEYID, g^xi | g^xr | CKY-I | CKY-R | SAi_b | IDii_b )
HASH_R = prf(SKEYID, g^xr | g^xi | CKY-R | CKY-I | SAi_b | IDir_b )
The HASH does not include all payloads, nor it does not include generic
ISAKMP [RFC-2408] header, which contains version numbers, exchange type
etc.
This problem is quite easy to fix, but the bigger problem is how to fix
it in the backward compatible way, so we do not break all existing
implementations out there.
Most of the implementations out there will break immediately if we
update the ISAKMP major/minor number or the phase 1 transform
identifier. Most of the implementators say that their implementation
should be able to ignore unknown SA data attributes inside the SA
payload. They will not select that transform, so there should also be
transform that is supported by them to really be backward compatible.
This means that the most backword compatible way to negotiate the new
HASH method is to negotiate it inside the SA payload.
One advantage of negotiating new HASH inside the SA payload is that it
is authenticated, so if attacker removes new revised HASH proposals from
the SA payload the initiator will detect this when checking the HASH.
There are three different alternatives that can be used. One is to
define new, revised, authentication method for each existing
authentication method. I.e define "Pre Shared Keys Revised HASH
Authentication Method", "RSA Signatures Revised HASH Authentication
Method" etc. Another alternative is to duplicate all HASH types ("MD5
Revised Authentication HASH"), and the third way is to define PRF method
to describe new authentication HASH usage ("Revised Authentication
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PRF").
All of the cases are identical in sense that the reserved number range
is large enough (16 bits), so we can "consume" some extra numbers. The
PRF method is might have more problems, because currently it is not used
at all (there is no defined PRFs), so some implementations might break
when they see such attribute class.
2. Specification of Requirements
This document shall use the keywords "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED, "MAY", and
"OPTIONAL" to describe requirements. They are to be interpreted as
described in [RFC-2119] document.
3. Revised HASH Calculation
The new HASH is defined so that all the packet received and sent before
are included in the HASH calculation. This also includes the packet
currently being generated.
The HASH includes the ISAKMP generic headers, ISAKMP payload headers
etc, i.e the exact bits sent to the wire from the beginning of the
ISAKMP generic header to the end of the packet. The HASH includes the
padding added because of the encryption. When the length of the packet
(inside the ISAKMP header) is calculated in to the HASH, it MUST be set
to the real length of packet including the padding. Packet is added to
the HASH as plaintext. Retransmission packets are not added to the HASH.
The authentication payloads (HASH or SIG) MUST be the last payload in
the packet, and when it is calculated to the authentication HASH its
contents MUST be all zeros, with proper length (either determined by the
HASH algorithm or the public key used in the authentication).
So in the main mode the initiator HASH is calculated as follows:
HASH_I = prf(SKEYID, packet_1 | packet_2 | packet_3 | packet_4 |
packet_5)
Where the packet_1 is the first packet initiator sends to the network
(starting from the beginning of the generic header and continuing to the
length specified in the ISAKMP header). Same goes for packets 2 to 4.
The packet 5 is special, because it is this packet we are currently
sending out. It is calculated to the HASH before encryption, but
including the padding. The HASH/SIG payload MUST be in its place and
must contain all zeros. After the HASH has been calculated, the value is
filled in to the place holder inside the packet and then the packet is
encrypted before sent out.
When the responder is checking the HASH it first decrypts the packet_5
and then it copies the HASH/SIG away and clears it from the packet.
Then it calculates the exactly same HASH_I the initiator did, which can
then be used authenticate the exchange (either direct comparison of the
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HASH, or signature verification).
In the main mode the responder HASH is calculated as follows:
HASH_R = prf(SKEYID, packet_1 | packet_2 | packet_3 | packet_4 |
packet_5 | packet_6)
The packets 1 to 5 are identical to initiator case, i.e the SIG/HASH
payload inside the payload 5 contains zeros. The packet 6 is similar
than packet 5 in the initiator case, i.e the HASH/SIG payload is in its
place and must contain all zeros.
In the aggressive mode the HASH is calculated as follows:
HASH_I = prf(SKEYID, packet_1 | packet_2)
HASH_R = prf(SKEYID, packet_1 | packet_2 | packet_3)
With same kind of processing of packet 2 and 3 than was for packets 5
and 6 in the main mode. Note, that the encryption of the final packet in
the aggressive mode does affect the HASH, because there might be padding
added to the packet 3 which must be then be included to the HASH.
4. Negotiating Revised HASH
The revised HASH is negotiated using defined attribute values inside the
SA payload. This means that current implementations are able to ignore
those proposal and select the old HASH method. New implementations
SHOULD add revised HASH method before normal method, so that the new
revised HASH method is preferred.
XXX Next paragraph will disappear from the final document:
Next three sections describe three different ways to negotiate the new
revised HASH method. The working group must select one of those and
after that two of the other methods are removed from this document.
Each of sections are currently written as they would be there alone,
thus there is some duplication of text inside of them.
5. Selecting Revised HASH Using New Authentication Methods
The revised HASH method is negotiated by adding 5 new authentication
methods, i.e:
XXX+1
Pre-Shared Keys with Revised Authentication HASH
XXX+2
DSS Signatures with Revised Authentication HASH
XXX+3
RSA Signatures with Revised Authentication HASH
XXX+4
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Encryption with RSA with Revised Authentication HASH
XXX+5
Revised Encryption with RSA with Revised Authentication HASH
Note, The XXX should be replaced as the last defined authentication
method number from the RFC2409.
Each authentication method is exactly identical to the old ones, except
the HASH_I and HASH_R are calculated as described in the section
``Revised HASH Calculation''.
In the signature modes the final SIG_I or SIG_R is the result of the
negotiated digital signature algorithm applied to HASH_I or HASH_R
respectively.
In the RSA Encryption mode the authentication of the other party takes
place in the generation of the SKEYID, because to generate it correctly
the other end must be able to decrypt the encrypted NONCE payload. Note
that the ID and NONCE payloads are already encrypted using public key
when they are calculated to the authentication HASH.
6. Selecting Revised HASH Using New HASH Algorithms
The revised HASH method is negotiated by adding 3 new HASH algorithms,
i.e:
XXX+1
MD5 with Revised Authentication HASH
XXX+2
SHA with Revised Authentication HASH
XXX+3
Tiger with Revised Authentication HASH
Note, The XXX should be replaced as the last defined hash algorithm
number from the RFC2409.
Each HASH is defined so that it doesn't affect the HASH algorithm
itself, but the new revised HASH methods changes the way how the HASH_I
and HASH_R are calculated. This is described in the section ``Revised
HASH Calculation''.
In the signature modes the final SIG_I or SIG_R, is the result of the
negotiated digital signature algorithm applied to HASH_I or HASH_R
respectively.
In the RSA Encryption mode the authentication of the other party happens
in the generation of the SKEYID, because to generate it correctly the
other end must be able to decrypt the encrypted NONCE payload. Note that
the ID and NONCE payloads are already encrypted, when they are
calculated to the authentication HASH.
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7. Selecting Revised HASH Using New PRF Algorithm
The revised HASH method is negotiated by adding a PRF algorithm, i.e:
XXX+1
HMAC-HASH PRF with Revised Authentication HASH
Note, The XXX should be replaced as the last defined PRF algorithm
number from the RFC2409.
Each HASH is defined so that it doesn't affect the prf algorithm itself,
but the new revised HASH method change the way how the HASH_I and HASH_R
are calculated. This is described in the section ``Revised HASH
Calculation''.
In the signature modes the final SIG_I or SIG_R, is the result of the
negotiated digital signature algorithm applied to HASH_I or HASH_R
respectively.
In the RSA Encryption mode the authentication of the other party happens
in the generation of the SKEYID, because to generate it correctly the
other end must be able to decrypt the encrypted NONCE payload. Note that
the ID and NONCE payloads are already encrypted, when they are
calculated to the authentication HASH.
8. Security Considerations
This document describes a way to fix the security problem inside the
IKE. In the IKE defined in RFC2409 only some payloads are authenticated.
This means that generic ISAKMP header (version numbers, exchange type,
flags etc) and extra payloads (Notifications, Vendor ID, CERT, and CR
payloads) are not authenticated. This document fixes that security
problem.
This document does NOT fix the other exchanges methods, like the quick
mode or the new group mode exchanges. In those exchanges the ISAKMP
generic header is still unauthenticated, all other payloads inside the
those exchanges are already authenticated.
9. References
[RFC-2408] Maughan D., Schertler M., Schneider M., Turner J., "Internet
Security Association and Key Management Protocol (ISAKMP)", November
1998.
[RFC-2409] Harkins D., Carrel D., "The Internet Key Exchange (IKE)",
November 1998
[RFC-2119] Bradner, S., "Key words for use in RFCs to indicate
Requirement Levels", March 1997
[RFC-2407] Piper D., "The Internet IP Security Domain of Interpretation
for ISAKMP", November 1998
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10. Authors' Addresses
Tero Kivinen
SSH Communications Security Ltd.
Tekniikantie 12
FIN-02150 ESPOO
Finland
E-mail: kivinen@ssh.fi
T. Kivinen [page 7]
