Network Working Group B. Harris Internet-Draft August 25, 2005 Expires: February 26, 2006 Rivest-Shamir-Adleman (RSA) key exchange for the Secure Shell (SSH) Transport Layer Protocol draft-harris-ssh-rsa-kex-05 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 February 26, 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 February 26, 2006 [Page 1] Internet-Draft SSH RSA key exchange August 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]. 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 [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. 4. Details Harris Expires February 26, 2006 [Page 2] Internet-Draft SSH RSA key exchange August 2005 The RSA key exchange method has the following parameters, which are defined by the method name: HASH hash algorithm for calculating exchange hash etc. HLEN output length of HASH in bits MINKLEN minimum transient RSA modulus length in bits 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-2HLEN-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]. 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 Harris Expires February 26, 2006 [Page 3] Internet-Draft SSH RSA key exchange August 2005 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 version string (CR and NL excluded) string V_S, the server's version string (CR and NL 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 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 Harris Expires February 26, 2006 [Page 4] Internet-Draft SSH RSA key exchange August 2005 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. K, the random integer generated by the client, is the only secret shared by client and server. Thus its entropy directly determines the security of the session against eavesdropping. 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 [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-512 for those requiring up to 256 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 Harris Expires February 26, 2006 [Page 5] Internet-Draft SSH RSA key exchange August 2005 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 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] Harris Expires February 26, 2006 [Page 6] Internet-Draft SSH RSA key exchange August 2005 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. 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-2HLEN-49+1+7)/8 + 4 = (KLEN-2HLEN-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+7-2HLEN-16)/8 = (KLEN-2HLEN-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 February 26, 2006 [Page 7] Internet-Draft SSH RSA key exchange August 2005 Author's Address Ben Harris 37 Milton Road CAMBRIDGE CB4 1XA GB Email: bjh21@bjh21.me.uk Harris Expires February 26, 2006 [Page 8] Internet-Draft SSH RSA key exchange August 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. 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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 February 26, 2006 [Page 9]