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IETF RFC 4432

RSA Key Exchange for the Secure Shell (SSH) Transport Layer Protocol

Last modified on Monday, March 6th, 2006

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Network Working Group                                          B. Harris
Request for Comments: 4432                                    March 2006
Category: Standards Track 


              RSA Key Exchange for the Secure Shell (SSH)
                        Transport Layer Protocol

 Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

 Copyright Notice

   Copyright © The Internet Society (2006).

 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.

1.  Introduction

   Secure Shell (SSH) [RFC 4251] 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 [RFC 4251] that is similar to that used by the older
   version, and about as fast, while retaining the security advantages
   of the newer protocol.









Harris                      Standards Track                  PAGE 1 top


RFC 4432 SSH RSA Key Exchange March 2006 2. Conventions Used in This Document The key words "MUST" and "SHOULD" in this document are to be interpreted as described in [RFC 2119]. The data types "byte", "string", and "mpint" are defined in Section 5 of [RFC 4251]. Other terminology and symbols have the same meaning as in [RFC 4253]. 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 [RFC 4253]. This method provides explicit server identification as defined in Section 7 of [RFC 4253]. It requires a signature-capable host key. 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 Harris Standards Track PAGE 2 top

RFC 4432 SSH RSA Key Exchange March 2006 The key K_T is encoded according to the "ssh-rsa" scheme described in Section 6.6 of [RFC 4253]. Note that unlike an "ssh-rsa" host key, K_T is used only 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 [RFC 3447], 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 [RFC 3447]. This, combined with encoding the result as an SSH "string", gives a result that 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 [RFC 4253]. 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 Harris Standards Track PAGE 3 top

RFC 4432 SSH RSA Key Exchange March 2006 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 [RFC 3174] HLEN 160 MINKLEN 1024 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 [RFC 4251] 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 that used that key. As a result, servers SHOULD use each RSA key for as few key exchanges as possible. Harris Standards Track PAGE 4 top

RFC 4432 SSH RSA Key Exchange March 2006 [RFC 3447] 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 [RFC 4086] 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. For recommendations on this, see [RFC 3766]. The strength of RSAES-OAEP is in part dependent on the hash function it uses. [RFC 3447] 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 [RFC 4253], 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 has assigned the names "rsa1024-sha1" and "rsa2048-sha256" as Key Exchange Method Names in accordance with [RFC 4250]. Harris Standards Track PAGE 5 top

RFC 4432 SSH RSA Key Exchange March 2006 10. Acknowledgements 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 [RFC 4253]. 11. References 11.1. Normative References [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC 3174] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC 3174, September 2001. [RFC 3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003. [RFC 4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Protocol Architecture", RFC 4251, January 2006. [RFC 4253] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, January 2006. [RFC 4250] Lehtinen, S. and C. Lonvick, "The Secure Shell (SSH) Protocol Assigned Numbers", RFC 4250, January 2006. [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. [RFC 3766] Orman, H. and P. Hoffman, "Determining Strengths For Public Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766, April 2004. [RFC 4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. Harris Standards Track PAGE 6 top

RFC 4432 SSH RSA Key Exchange March 2006 Appendix A. On the Size of K The requirements on the size of K are intended to ensure that it is 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 "/" denotes integer division rounding down). The maximum length of message that can be encrypted using RSAEP-OAEP is defined by [RFC 3447] 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. Author's Address Ben Harris 2a Eachard Road CAMBRIDGE CB4 1XA UNITED KINGDOM EMail: bjh21@bjh21.me.uk Harris Standards Track PAGE 7 top

RFC 4432 SSH RSA Key Exchange March 2006 Full Copyright Statement Copyright © The Internet Society (2006). 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. 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. Intellectual Property 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. Acknowledgement Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Harris Standards Track PAGE 8 top

RSA Key Exchange for the Secure Shell (SSH) Transport Layer Protocol RFC TOTAL SIZE: 16077 bytes PUBLICATION DATE: Monday, March 6th, 2006 LEGAL RIGHTS: The IETF Trust (see BCP 78)


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