Internet DRAFT - draft-ssorce-gss-keyex-sha2
draft-ssorce-gss-keyex-sha2
Internet Engineering Task Force S. Sorce
Internet-Draft H. Kario
Updates: 4462 (if approved) Red Hat, Inc.
Intended status: Standards Track December 13, 2016
Expires: June 16, 2017
GSS-API Key Exchange with SHA2
draft-ssorce-gss-keyex-sha2-00
Abstract
This document specifies additions and amendments to SSH GSS-API
Methods [RFC4462]. It defines a new key exchange method that uses
SHA-2 for integrity and deprecates weak DH groups. The purpose of
this specification is to modernize the cryptographic primitives used
by GSS Key Exchanges.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 16, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Document Conventions . . . . . . . . . . . . . . . . . . . . 2
4. New Diffie-Hellman Key Exchange methods . . . . . . . . . . . 3
4.1. gss-group14-sha256-* . . . . . . . . . . . . . . . . . . 3
4.2. gss-group15-sha512-* . . . . . . . . . . . . . . . . . . 3
4.3. gss-group16-sha512-* . . . . . . . . . . . . . . . . . . 3
4.4. gss-group17-sha512-* . . . . . . . . . . . . . . . . . . 4
4.5. gss-group18-sha512-* . . . . . . . . . . . . . . . . . . 4
5. New Elliptic Curve Diffie-Hellman Key Exchange methods . . . 4
5.1. Generic GSS-API Key Exchange with ECDH . . . . . . . . . 4
5.2. ECDH Key Exchange Methods . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7.1. New Finite Field DH mechanisms . . . . . . . . . . . . . 10
7.2. New Elliptic Curve DH mechanisms . . . . . . . . . . . . 10
8. Normative References . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
SSH GSS-API Methods [RFC4462] allows the use of GSSAPI for
authentication and key exchange in SSH. It defines three exchange
methods all based on DH groups and SHA-1. The new methods described
in this document are intended to support environments that desired to
use the SHA-2 cryptographic hash functions.
2. Rationale
Due to security concerns with SHA-1 [RFC6194] and with MODP groups
with less than 2048 bits [NIST-SP-800-131Ar1] we propose the use of
the SHA-2 based hashes with DH group14, group15, group16, group17 and
group18 [RFC3526]. Additionally we add support for key exchange
based on Elliptic Curve Diffie Hellman with NIST P-256, P-384 and
P-521 as well as X25519 and X448 curves. Following the rationale of
[I-D.ietf-curdle-ssh-kex-sha2] only SHA-256 and SHA-512 hashes are
used.
3. Document Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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4. New Diffie-Hellman Key Exchange methods
This document adopts the same naming convention defined in [RFC4462]
to define families of methods that cover any GSS-API mechanism used
with a specific Diffie-Hellman group and SHA-2 Hash combination.
The following new key exchange algorithms are defined:
+--------------------------+--------------------------------+
| Key Exchange Method Name | Implementation Recommendations |
+--------------------------+--------------------------------+
| gss-group14-sha256-* | SHOULD/RECOMMENDED |
| gss-group15-sha512-* | MAY/OPTIONAL |
| gss-group16-sha512-* | SHOULD/RECOMMENDED |
| gss-group17-sha512-* | MAY/OPTIONAL |
| gss-group18-sha512-* | MAY/OPTIONAL |
+--------------------------+--------------------------------+
Each key exchange method is implicitly registered by this document.
The IESG is considered to be the owner of all these key exchange
methods; this does NOT imply that the IESG is considered to be the
owner of the underlying GSS-API mechanism.
4.1. gss-group14-sha256-*
Each of these methods specifies GSS-API-authenticated Diffie-Hellman
key exchange as described in Section 2.1 of [RFC4462] with SHA-256 as
HASH, and the group defined in Section 8.2 of [RFC4253] The method
name for each method is the concatenation of the string "gss-
group14-sha256-" with the Base64 encoding of the MD5 hash [RFC1321]
of the ASN.1 DER encoding [ISO-IEC-8825-1] of the underlying GSS-API
mechanism's OID. Base64 encoding is described in Section 6.8 of
[RFC2045].
4.2. gss-group15-sha512-*
Each of these methods specifies GSS-API-authenticated Diffie-Hellman
key exchange as described in Section 2.1 of [RFC4462] with SHA-512 as
HASH, and the group defined in Section 5 of [RFC3526] The method name
for each method is the concatenation of the string "gss-
group15-sha512-" with the Base64 encoding of the MD5 hash of the
ASN.1 DER encoding of the underlying GSS-API mechanism's OID.
4.3. gss-group16-sha512-*
Each of these methods specifies GSS-API-authenticated Diffie-Hellman
key exchange as described in Section 2.1 of [RFC4462] with SHA-512 as
HASH, and the group defined in Section 5 of [RFC3526] The method name
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for each method is the concatenation of the string "gss-
group16-sha512-" with the Base64 encoding of the MD5 hash of the
ASN.1 DER encoding of the underlying GSS-API mechanism's OID.
4.4. gss-group17-sha512-*
Each of these methods specifies GSS-API-authenticated Diffie-Hellman
key exchange as described in Section 2.1 of [RFC4462] with SHA-512 as
HASH, and the group defined in Section 5 of [RFC3526] The method name
for each method is the concatenation of the string "gss-
group17-sha512-" with the Base64 encoding of the MD5 hash of the
ASN.1 DER encoding of the underlying GSS-API mechanism's OID.
4.5. gss-group18-sha512-*
Each of these methods specifies GSS-API-authenticated Diffie-Hellman
key exchange as described in Section 2.1 of [RFC4462] with SHA-512 as
HASH, and the group defined in Section 7 of [RFC3526] The method name
for each method is the concatenation of the string "gss-
group18-sha512-" with the Base64 encoding of the MD5 hash of the
ASN.1 DER encoding of the underlying GSS-API mechanism's OID.
5. New Elliptic Curve Diffie-Hellman Key Exchange methods
In [RFC5656] new SSH key exchange algorithms based on Elliptic Curve
Cryptography are introduced. We reuse much of section 4 to implement
GSS-API-authenticated ECDH Key Exchanges.
Additionally we utilize also the curves defined in
[I-D.ietf-curdle-ssh-curves] to complement the 3 classic NIST defined
curves required by [RFC5656].
5.1. Generic GSS-API Key Exchange with ECDH
This section reuses much of the scheme defined in Section 2.1 of
[RFC4462] and combines it with the scheme defined in Section 4 of
[RFC5656]; in particular, all checks and verification steps
prescribed in Section 4 of [RFC5656] apply here as well.
The symbols used in this description conform to the symbols used in
Section 2.1 of [RFC4462]. Additionally, the following symbols are
defined:
Q_C is the client ephemeral public key octet string
Q_S is the server ephemeral public key octet string
The Client:
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1. C generates an ephemeral key pair with public key Q_C
2. C calls GSS_Init_sec_context(), using the most recent reply token
received from S during this exchange, if any. For this call, the
client MUST set mutual_req_flag to "true" to request that mutual
authentication be performed. It also MUST set integ_req_flag to
"true" to request that per-message integrity protection be supported
for this context. In addition, deleg_req_flag MAY be set to "true"
to request access delegation, if requested by the user. Since the
key exchange process authenticates only the host, the setting of
anon_req_flag is immaterial to this process. If the client does not
support the "gssapi-keyex" user authentication method described in
Section 4 of [RFC4462], or does not intend to use that method in
conjunction with the GSS-API context established during key exchange,
then anon_req_flag SHOULD be set to "true". Otherwise, this flag MAY
be set to true if the client wishes to hide its identity. Since the
key exchange process will involve the exchange of only a single token
once the context has been established, it is not necessary that the
GSS-API context support detection of replayed or out-of-sequence
tokens. Thus, replay_det_req_flag and sequence_req_flag need not be
set for this process. These flags SHOULD be set to "false".
If the resulting major_status code is GSS_S_COMPLETE and the
mutual_state flag is not true, then mutual authentication has not
been established, and the key exchange MUST fail.
If the resulting major_status code is GSS_S_COMPLETE and the
integ_avail flag is not true, then per-message integrity
protection is not available, and the key exchange MUST fail.
If the resulting major_status code is GSS_S_COMPLETE and both the
mutual_state and integ_avail flags are true, the resulting output
token is sent to S.
If the resulting major_status code is GSS_S_CONTINUE_NEEDED, the
output_token is sent to S, which will reply with a new token to be
provided to GSS_Init_sec_context().
The client MUST also include Q_C with the first message it sends
to the server during this process; if the server receives more
than one Q_C or none at all, the key exchange MUST fail.
It is an error if the call does not produce a token of non- zero
length to be sent to the server. In this case, the key exchange
MUST fail.
3. When a Q_C key is received, S verifies that the key is valid. If
the key is not valid the key exchange MUST fail.
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4. S calls GSS_Accept_sec_context(), using the token received from
C.
If the resulting major_status code is GSS_S_COMPLETE and the
mutual_state flag is not true, then mutual authentication has not
been established, and the key exchange MUST fail.
If the resulting major_status code is GSS_S_COMPLETE and the
integ_avail flag is not true, then per-message integrity
protection is not available, and the key exchange MUST fail.
If the resulting major_status code is GSS_S_COMPLETE and both the
mutual_state and integ_avail flags are true, then the security
context has been established, and processing continues with step
5.
If the resulting major_status code is GSS_S_CONTINUE_NEEDED, then
the output token is sent to C, and processing continues with step
2.
If the resulting major_status code is GSS_S_COMPLETE, but a non-
zero-length reply token is returned, then that token is sent to
the client.
5. S generates an ephemeral key pair with public key Q_S and
performs the following computations:
K a shared secret obtained using Q_C and Q_S.
H = hash(V_C || V_S || I_C || I_S || K_S || Q_C || Q_S || K).
MIC is the GSS-API message integrity code for H computed by
calling GSS_GetMIC().
6. This step is performed only if the server's final call to
GSS_Accept_sec_context() produced a non-zero-length final reply token
to be sent to the client and if no previous call by the client to
GSS_Init_sec_context() has resulted in a major_status of
GSS_S_COMPLETE. Under these conditions, the client makes an
additional call to GSS_Init_sec_context() to process the final reply
token. This call is made exactly as described above. However, if
the resulting major_status is anything other than GSS_S_COMPLETE, or
a non-zero-length token is returned, it is an error and the key
exchange MUST fail.
7. C verifies that the key Q_S is valid. If the key is not valid
the key exchange MUST fail.
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8. C computes the shared secret K and H the same way it is done in
step 5. It then calls GSS_VerifyMIC() to check that the MIC sent by
S verifies H's integrity. If the MIC is not successfully verified,
the key exchange MUST fail.
If any GSS_Init_sec_context() or GSS_Accept_sec_context() returns a
major_status other than GSS_S_COMPLETE or GSS_S_CONTINUE_NEEDED, or
any other GSS-API call returns a major_status other than
GSS_S_COMPLETE, the key exchange MUST fail. The same recommendations
expressed in Section 2.1 of [RFC4462] are followed with regards to
error reporting.
This excahnge is implemented with the following messages:
The client sends:
byte SSH_MSG_KEXGSS_INIT
string output_token (from GSS_Init_sec_context())
string Q_C, client's ephemeral public key octet string
The server may responds with:
byte SSH_MSG_KEXGSS_HOSTKEY
string server public host key and certificates (K_S)
Since this key exchange method does not require the host key to be
used for any encryption operations, this message is OPTIONAL. If the
"null" host key algorithm described in Section 5 of [RFC4462]is used,
this message MUST NOT be sent.
Each time the server's call to GSS_Accept_sec_context() returns a
major_status code of GSS_S_CONTINUE_NEEDED
The server replies:
byte SSH_MSG_KEXGSS_CONTINUE
string output_token (from GSS_Accept_sec_context())
If the client receives this message after a call to
GSS_Init_sec_context() has returned a major_status code of
GSS_S_COMPLETE, a protocol error has occurred and the key exchange
MUST fail.
Each time the client receives the message described above, it makes
another call to GSS_Init_sec_context().
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The client sends:
byte SSH_MSG_KEXGSS_CONTINUE
string output_token (from GSS_Init_sec_context())
The server and client continue to trade these two messages as long as
the server's calls to GSS_Accept_sec_context() result in major_status
codes of GSS_S_CONTINUE_NEEDED. When a call results in a
major_status code of GSS_S_COMPLETE, it sends one of two final
messages.
If the server's final call to GSS_Accept_sec_context() (resulting in
a major_status code of GSS_S_COMPLETE) returns a non-zero-length
token to be sent to the client, it sends the following:
byte SSH_MSG_KEXGSS_COMPLETE
string Q_S, server's ephemeral public key octet string
string mic_token (MIC of H)
boolean TRUE
string output_token (from GSS_Accept_sec_context())
If the client receives this message after a call to
GSS_Init_sec_context() has returned a major_status code of
GSS_S_COMPLETE, a protocol error has occurred and the key exchange
MUST fail.
If the server's final call to GSS_Accept_sec_context() (resulting in
a major_status code of GSS_S_COMPLETE) returns a zero-length token or
no token at all, it sends the following:
byte SSH_MSG_KEXGSS_COMPLETE
string Q_S, server's ephemeral public key octet string
string mic_token (MIC of H)
boolean FALSE
If the client receives this message when no call to
GSS_Init_sec_context() has yet resulted in a major_status code of
GSS_S_COMPLETE, a protocol error has occurred and the key exchange
MUST fail.
In case of errors the messages described in Section 2.1 of [RFC4462]
are used as well as the recommendation about the messages' order.
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The hash H is computed as the HASH hash of the concatenation of the
following:
string V_C, the client's version string (CR, NL excluded)
string V_S, server's identification string (CR and LF excluded)
string I_C, payload of the client's SSH_MSG_KEXINIT
string I_S, payload of the server's SSH_MSG_KEXINIT
string K_S, server's public host key
string Q_C, client's ephemeral public key octet string
string Q_S, server's ephemeral public key octet string
mpint K, shared secret
This value is called the exchange hash, and it is used to
authenticate the key exchange. The exchange hash SHOULD be kept
secret. If no SSH_MSG_KEXGSS_HOSTKEY message has been sent by the
server or received by the client, then the empty string is used in
place of K_S when computing the exchange hash.
The GSS_GetMIC call MUST be applied over H, not the original data.
5.2. ECDH Key Exchange Methods
The following new key exchange methods are defined:
+--------------------------+--------------------------------+
| Key Exchange Method Name | Implementation Recommendations |
+--------------------------+--------------------------------+
| gss-secp256r1-sha256-* | SHOULD/RECOMMENDED |
| gss-secp384r1-sha512-* | MAY/OPTIONAL |
| gss-secp521r1-sha512-* | MAY/OPTIONAL |
| gss-curve25519-sha256-* | SHOULD/RECOMMENDED |
| gss-curve448-sha512-* | MAY/OPTIONAL |
+--------------------------+--------------------------------+
Each key exchange method is implicitly registered by this document.
The IESG is considered to be the owner of all these key exchange
methods; this does NOT imply that the IESG is considered to be the
owner of the underlying GSS-API mechanism.
6. IANA Considerations
This document augments the SSH Key Exchange Method Names in
[RFC4462].
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IANA is requested to update the SSH algorithm registry with the
following entries:
+--------------------------+------------+------------------------+
| Key Exchange Method Name | Reference | Implementation Support |
+--------------------------+------------+------------------------+
| gss-group14-sha256-* | This draft | SHOULD |
| gss-group15-sha512-* | This draft | MAY |
| gss-group16-sha512-* | This draft | SHOULD |
| gss-group17-sha512-* | This draft | MAY |
| gss-group18-sha512-* | This draft | MAY |
| gss-secp256r1-sha256-* | This draft | SHOULD |
| gss-secp384r1-sha512-* | This draft | MAY |
| gss-secp521r1-sha512-* | This draft | MAY |
| gss-curve25519-sha256-* | This draft | SHOULD |
| gss-curve448-sha512-* | This draft | MAY |
+--------------------------+------------+------------------------+
7. Security Considerations
7.1. New Finite Field DH mechanisms
Except for the use of a different secure hash function and larger DH
groups, no significant changes has been made to the protocol
described by [RFC4462]; therefore all the original Security
Considerations apply.
7.2. New Elliptic Curve DH mechanisms
Although a new cryptographic primitive is used with these methods the
actual key exchange closely follows the key exchange defined in
[RFC5656]; therefore all the original Security Considerations as well
as those expressed in [RFC5656] apply.
8. Normative References
[FIPS-180-4]
National Institute of Standards and Technology, "FIPS PUB
180-4: Secure Hash Standard (SHS)", FIPS PUB 180-4, August
2015, <http://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.180-4.pdf>.
[I-D.ietf-curdle-ssh-curves]
Adamantiadis, A. and S. Josefsson, "Secure Shell (SSH) Key
Exchange Method using Curve25519 and Curve448", draft-
ietf-curdle-ssh-curves-00 (work in progress), March 2016.
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[I-D.ietf-curdle-ssh-kex-sha2]
Baushke, M., "Key Exchange (KEX) Method Updates and
Recommendations for Secure Shell (SSH)", draft-ietf-
curdle-ssh-kex-sha2-05 (work in progress), September 2016.
[ISO-IEC-8825-1]
International Organization for Standardization /
International Electrotechnical Commission, "ASN.1 encoding
rules: Specification of Basic Encoding Rules (BER),
Canonical Encoding Rules (CER) and Distinguished Encoding
Rules (DER)", ISO/IEC 8825-1, November 2015,
<http://standards.iso.org/ittf/PubliclyAvailableStandards/
c068345_ISO_IEC_8825-1_2015.zip>.
[NIST-SP-800-131Ar1]
National Institute of Standards and Technology,
"Transitions: Recommendation for Transitioning of the Use
of Cryptographic Algorithms and Key Lengths", NIST Special
Publication 800-131A Revision 1, November 2015,
<http://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.800-131Ar1.pdf>.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<http://www.rfc-editor.org/info/rfc1321>.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<http://www.rfc-editor.org/info/rfc2045>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, DOI 10.17487/RFC3526, May 2003,
<http://www.rfc-editor.org/info/rfc3526>.
[RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
January 2006, <http://www.rfc-editor.org/info/rfc4253>.
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[RFC4462] Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch,
"Generic Security Service Application Program Interface
(GSS-API) Authentication and Key Exchange for the Secure
Shell (SSH) Protocol", RFC 4462, DOI 10.17487/RFC4462, May
2006, <http://www.rfc-editor.org/info/rfc4462>.
[RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm
Integration in the Secure Shell Transport Layer",
RFC 5656, DOI 10.17487/RFC5656, December 2009,
<http://www.rfc-editor.org/info/rfc5656>.
[RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
Considerations for the SHA-0 and SHA-1 Message-Digest
Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
<http://www.rfc-editor.org/info/rfc6194>.
Authors' Addresses
Simo Sorce
Red Hat, Inc.
140 Broadway
24th Floor
New York, NY 10025
USA
Email: simo@redhat.com
Hubert Kario
Red Hat, Inc.
Purkynova 99/71
Brno 612 45
Czech Republic
Email: hkario@redhat.com
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