[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 RFC 4432

Network Working Group                                          B. Harris
Internet-Draft                                         December 19, 2005
Expires: June 22, 2006


  Rivest-Shamir-Adleman (RSA) key exchange for the Secure Shell (SSH)
                        Transport Layer Protocol
                      draft-harris-ssh-rsa-kex-06

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on June 22, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This memo describes a key-exchange method for the Secure Shell (SSH)
   protocol based on Rivest-Shamir-Adleman (RSA) public-key encryption.
   It uses much less client CPU time than the Diffie-Hellman algorithm
   specified as part of the core protocol, and hence is particularly
   suitable for slow client systems.






Harris                    Expires June 22, 2006                 [Page 1]

Internet-Draft            SSH RSA key exchange             December 2005


1.  Introduction

   Secure Shell (SSH) [I-D.ietf-secsh-architecture] is a secure remote-
   login protocol.  The core protocol uses Diffie-Hellman key exchange.
   On slow CPUs, this key exchange can take tens of seconds to complete,
   which can be irritating for the user.  A previous version of the SSH
   protocol, described in [SSH1] uses a key-exchange method based on
   Rivest-Shamir-Adleman (RSA) public-key encryption, which consumes an
   order of magnitude less CPU time on the client, and hence is
   particularly suitable for slow client systems such as mobile devices.
   This memo describes a key-exchange mechanism for the version of SSH
   described in [I-D.ietf-secsh-architecture] which is similar to that
   used by the older version, and about as fast, while retaining the
   security advantages of the newer protocol.


2.  Conventions Used in this Document

   The key words "MUST" and "SHOULD" in this document are to be
   interpreted as described in [RFC2119].

   The data types "byte", "string", and "mpint" are defined in section 5
   of [I-D.ietf-secsh-architecture].

   Other terminology and symbols have the same meaning as in [I-D.ietf-
   secsh-transport].


3.  Overview

   The RSA key-exchange method consists of three messages.  The server
   sends to the client an RSA public key, K_T, to which the server holds
   the private key.  This may be a transient key generated solely for
   this SSH connection, or it may be re-used for several connections.
   The client generates a string of random bytes, K, encrypts it using
   K_T, and sends the result back to the server, which decrypts it.  The
   client and server each hash K, K_T, and the various key-exchange
   parameters to generate the exchange hash, H, which is used to
   generate the encryption keys for the session, and the server signs H
   with its host key and sends the signature to the client.  The client
   then verifies the host key as described in section 8 of [I-D.ietf-
   secsh-transport].

   This method provides explicit server identification as defined in
   section 7 of [I-D.ietf-secsh-transport].  It requires a signature-
   capable host key.





Harris                    Expires June 22, 2006                 [Page 2]

Internet-Draft            SSH RSA key exchange             December 2005


4.  Details

   The RSA key exchange method has the following parameters:

       HASH     hash algorithm for calculating exchange hash etc.
       HLEN     output length of HASH in bits
       MINKLEN  minimum transient RSA modulus length in bits

   Their values are defined in Section 5 and Section 6 for the two
   methods defined by this document.

   The method uses the following messages.

   First, the server sends:

       byte      SSH_MSG_KEXRSA_PUBKEY
       string    server public host key and certificates (K_S)
       string    K_T, transient RSA public key

   The key K_T is encoded according to the "ssh-rsa" scheme described in
   section 6.6 of [I-D.ietf-secsh-transport].  Note that unlike an "ssh-
   rsa" host key, K_T is only used for encryption, and not for
   signature.  The modulus of K_T MUST be at least MINKLEN bits long.

   The client generates a random integer, K, in the range
   0 <= K < 2^(KLEN-2*HLEN-49), where KLEN is the length of the modulus
   of K_T, in bits.  The client then uses K_T to encrypt:

       mpint     K, the shared secret

   The encryption is performed according to the RSAES-OAEP scheme of
   [RFC3447], with a mask generation function of MGF1-with-HASH, a hash
   of HASH, and an empty label.  See Appendix A for a proof that the
   encoding of K is always short enough to be thus encrypted.  Having
   performed the encryption, the client sends:

       byte      SSH_MSG_KEXRSA_SECRET
       string    RSAES-OAEP-ENCRYPT(K_T, K)

   Note that the last stage of RSAES-OAEP-ENCRYPT is to encode an
   integer as an octet-string using the I2OSP primitive of [RFC3447].
   This, combined with encoding the result as an SSH "string", gives a
   result which is similar, but not identical, to the SSH "mpint"
   encoding applied to that integer.  This is the same encoding as is
   used by "ssh-rsa" signatures in [I-D.ietf-secsh-transport].






Harris                    Expires June 22, 2006                 [Page 3]

Internet-Draft            SSH RSA key exchange             December 2005


   The server decrypts K. If a decryption error occurs, the server
   SHOULD send SSH_MESSAGE_DISCONNECT with a reason code of
   SSH_DISCONNECT_KEY_EXCHANGE_FAILED and MUST disconnect.  Otherwise,
   the server responds with:

       byte      SSH_MSG_KEXRSA_DONE
       string    signature of H with host key

   The hash H is computed as the HASH hash of the concatenation of the
   following:

       string    V_C, the client's identification string
                 (CR and LF excluded)
       string    V_S, the server's identification string
                 (CR and LF excluded)
       string    I_C, the payload of the client's SSH_MSG_KEXINIT
       string    I_S, the payload of the server's SSH_MSG_KEXINIT
       string    K_S, the host key
       string    K_T, the transient RSA key
       string    RSAES_OAEP_ENCRYPT(K_T, K), the encrypted secret
       mpint     K, the 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.

   The signature algorithm MUST be applied over H, not the original
   data.  Most signature algorithms include hashing and additional
   padding.  For example, "ssh-dss" specifies SHA-1 hashing.  In such
   cases, the data is first hashed with HASH to compute H, and H is then
   hashed again as part of the signing operation.


5.  rsa1024-sha1

   The "rsa1024-sha1" method specifies RSA key exchange as described
   above with the following parameters:

       HASH     SHA-1, as defined in [RFC3174]
       HLEN     160
       MINKLEN  1024










Harris                    Expires June 22, 2006                 [Page 4]

Internet-Draft            SSH RSA key exchange             December 2005


6.  rsa2048-sha256

   The "rsa2048-sha256" method specifies RSA key exchange as described
   above with the following parameters:

       HASH     SHA-256, as defined in [FIPS-180-2]
       HLEN     256
       MINKLEN  2048


7.  Message numbers

   The following message numbers are defined:

       SSH_MSG_KEXRSA_PUBKEY  30
       SSH_MSG_KEXRSA_SECRET  31
       SSH_MSG_KEXRSA_DONE    32


8.  Security Considerations

   The security considerations in [I-D.ietf-secsh-architecture] apply.

   If the RSA private key generated by the server is revealed then the
   session key is revealed.  The server should thus arrange to erase
   this from memory as soon as it is no longer required.  If the same
   RSA key is used for multiple SSH connections, an attacker who can
   find the private key (either by factorising the public key or by
   other means) will gain access to all of the sessions which used that
   key.  As a result, servers SHOULD use each RSA key for as few key
   exchanges as possible.

   [RFC3447] recommends that RSA keys used with RSAES-OAEP not be used
   with other schemes, or with RSAES-OAEP using a different hash
   function.  In particular, this means that K_T should not be used as a
   host key, or as a server key in earlier versions of the SSH protocol.

   Like all key-exchange mechanisms, this one depends for its security
   on the randomness of the secrets generated by the client (the random
   number K) and the server (the transient RSA private key).  In
   particular, it is essential that the client use a high-quality
   cryptographic pseudo-random number generator to generate K. Using a
   bad random number generator will allow an attacker to break all the
   encryption and integrity protection of the Secure Shell transport
   layer.  See [RFC4086] for recommendations on random-number generation

   The size of transient key used should be sufficient to protect the
   encryption and integrity keys generated by the key exchange method.



Harris                    Expires June 22, 2006                 [Page 5]

Internet-Draft            SSH RSA key exchange             December 2005


   For recommendations on this, see [RFC3766].  The strength of RSAES-
   OAEP is in part dependent on the hash function it uses.  [RFC3447]
   suggests using a hash with an output length of twice the security
   level required, so SHA-1 is appropriate for applications requiring up
   to 80 bits of security, and SHA-256 for those requiring up to 128
   bits.

   Unlike the Diffie-Hellman key exchange method defined by [I-D.ietf-
   secsh-transport], this method allows the client to fully determine
   the shared secret, K. This is believed not to be significant, since K
   is only ever used when hashed with data provided in part by the
   server (usually in the form of the exchange hash, H).  If an
   extension to SSH were to use K directly and to assume that it had
   been generated by Diffie-Hellman key exchange, this could produce a
   security weakness.  Protocol extensions using K directly should be
   viewed with extreme suspicion.

   This key-exchange method is designed to be resistant to collision
   attacks on the exchange hash, by ensuring that neither side is able
   to freely choose its input to the hash after seeing all of the other
   side's input.  The server's last input is in SSH_MSG_KEXRSA_PUBKEY,
   before it has seen the client's choice of K. The client's last input
   is K and its RSA encryption, and the one-way nature of RSA encryption
   should ensure that the client cannot choose K so as to cause a
   collision.


9.  IANA Considerations

   IANA should assign the names "rsa1024-sha1" and "rsa2048-sha256" as
   Key Exchange Method Names in accordance with [I-D.ietf-secsh-
   assignednumbers].


10.  Acknowledgments

   The author acknowledges the assistance of Simon Tatham with the
   design of this key exchange method.

   The text of this document is derived in part from [I-D.ietf-secsh-
   transport].


11.  References







Harris                    Expires June 22, 2006                 [Page 6]

Internet-Draft            SSH RSA key exchange             December 2005


11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3174]  Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1
              (SHA1)", RFC 3174, September 2001.

   [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, February 2003.

   [I-D.ietf-secsh-architecture]
              Ylonen, T. and C. Lonvick, "SSH Protocol Architecture",
              draft-ietf-secsh-architecture-22 (work in progress),
              March 2005.

   [I-D.ietf-secsh-transport]
              Lonvick, C., "SSH Transport Layer Protocol",
              draft-ietf-secsh-transport-24 (work in progress),
              March 2005.

   [I-D.ietf-secsh-assignednumbers]
              Lehtinen, S. and C. Lonvick, "SSH Protocol Assigned
              Numbers", draft-ietf-secsh-assignednumbers-12 (work in
              progress), March 2005.

   [FIPS-180-2]
              National Institute of Standards and Technology (NIST),
              "Secure Hash Standard (SHS)", FIPS PUB 180-2, August 2002.

11.2.  Informative References

   [SSH1]     Ylonen, T., "SSH -- Secure Login Connections over the
              Internet", 6th USENIX Security Symposium, pp. 37-42,
              July 1996.

   [RFC3766]  Orman, H. and P. Hoffman, "Determining Strengths For
              Public Keys Used For Exchanging Symmetric Keys", BCP 86,
              RFC 3766, April 2004.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.


Appendix A.  On the size of K

   The requirements on the size of K are intended to ensure that it is



Harris                    Expires June 22, 2006                 [Page 7]

Internet-Draft            SSH RSA key exchange             December 2005


   always possible to encrypt it under K_T. The mpint encoding of K
   requires a leading zero bit, padding to a whole number of bytes, and
   a four-byte length field, giving a maximum length in bytes,
   B = (KLEN-2*HLEN-49+1+7)/8 + 4 = (KLEN-2*HLEN-9)/8 (where "/"
   represents integer division rounding down).

   The maximum length of message that can be encrypted using RSAEP-OAEP
   is defined by [RFC3447] in terms of the key length in bytes, which is
   (KLEN+7)/8.  The maximum length is thus L = (KLEN+7-2*HLEN-16)/8 =
   (KLEN-2*HLEN-9)/8.  Thus, the encoded version of K is always small
   enough to be encrypted under K_T.

Trademark Notice

   "SSH" is a registered trademark in the United States.




































Harris                    Expires June 22, 2006                 [Page 8]

Internet-Draft            SSH RSA key exchange             December 2005


Author's Address

   Ben Harris
   2a Eachard Road
   CAMBRIDGE  CB4 1XA
   GB

   Email: a7250@bjh21.me.uk











































Harris                    Expires June 22, 2006                 [Page 9]

Internet-Draft            SSH RSA key exchange             December 2005


Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.


Disclaimer of Validity

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


Copyright Statement

   Copyright (C) The Internet Society (2005).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.


Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.




Harris                    Expires June 22, 2006                [Page 10]


Html markup produced by rfcmarkup 1.109, available from https://tools.ietf.org/tools/rfcmarkup/