Network Working Group K.M. Igoe Internet Draft National Security Agency Intended Status: Informational J.A. Solinas Expires: December 27, 2008 National Security Agency June 27, 2008 AES Galois Counter Mode for the Secure Shell Transport Layer Protocol draft-igoe-secsh-aes-gcm-00 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 December 06, 2008 Copyright Notice Copyright (C) The IETF Trust (2008). Igoe and Solinas Informational [Page 1] Internet Draft draft-igoe-secsh-aes-gcm-00 June 27, 2008 Abstract Secure Shell (SSH) [RFC4251] is a secure remote-login protocol. SSH provides for algorithms that provide authentication , key agreement. confidentiality and data integrity services. This purpose of this document is to show how the AES Galois/Counter Mode can be used to provide both confidentiality and data integrity. Table of Contents 1. Introduction.....................................................1 2. Requirements Terminology.........................................1 3. Applicability Statement..........................................1 4. Two New AEAD Algorithms..........................................1 4.1. aead-aes-128-gcm-ssh........................................2 4.2. aead-aes-256-gcm-ssh........................................2 5. Size of the Message Authentication Code..........................2 6. Maximum Payload Size.............................................3 7. Linkage of Confidentiality and Data Integrity....................3 8. Security Considerations..........................................3 9. IANA Considerations..............................................4 10. References......................................................4 10.1. Normative References.......................................4 1. Introduction Galois/Counter Mode (GCM) is a block cipher mode of operation that provides both confidentiality and data integrity services. The purpose of this document is to show how AES-GCM can be intergrated into the Secure Shell Transport Layer Protocol. 2. Requirements Terminology 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 [RFC2119]. 3. Applicability Statement Using AES-GCM to provide both confidentiality and data integrity is generally more efficient than using two separate algorithms to provide these security services. 4. Two New AEAD Algorithms Igoe and Solinas Informational [Page 2] Internet Draft draft-igoe-secsh-aes-gcm-00 June 27, 2008 4.1. aead-aes-128-gcm-ssh aead-aes-128-gcm-ssh is a variant of the algorithm AEAD_AES_128_GCM specified in section 5.1 of [RFC5116]. The only differences between the two algorithms are in the input and output lengths. Using the notation defined in [RFC5116], the input and output lengths for aead-aes-128-gcm-ssh are as follows: PARAMETER Meaning Value K_LEN AES key length 16 octets P_MAX maximum plaintext length 2^32 octets A_MAX maximum additional 0 octets authenticated data length N_MIN minimum nonce (IV) length 12 octets N_MAX maximum nonce (IV) length 12 octets C_MAX maximum cipher length 2^32 octets Test cases are provided in the appendix of [GCM]. The reader is reminded that due to the presence of length fields and padding in SSH packets, the plaintext length is not the same as the payload length. See section 6 below. 4.2. aead-aes-256-gcm-ssh aead-aes-256-gcm-ssh is a variant of the algorithm AEAD_AES_256_GCM specified in section 5.2 of [RFC5116]. The only differences between the two algorithms are in the input and output lengths. Using the notation defined in [RFC5116], the input and output lengths for aead-aes-256-gcm-ssh are as follows: PARAMETER Meaning Value K_LEN AES key length 32 octets P_MAX maximum plaintext length 2^32 octets A_MAX maximum additional 0 octets authenticated data length N_MIN minimum nonce (IV) length 12 octets N_MAX maximum nonce (IV) length 12 octets C_MAX maximum cipher length 2^32 octets Test cases are provided in the appendix of [GCM]. The reader is reminded that due to the presence of length fields and padding in SSH packets, the plaintext length is not the same as the payload length. See section 6 below. 5. Size of the Message Authentication Code Igoe and Solinas Informational [Page 3] Internet Draft draft-igoe-secsh-aes-gcm-00 June 27, 2008 Both aead-aes-128-gcm-ssh and aead-aes-256-gcm-ssh produce a 16-octet message authentication code. ([RFC5116] calls this an "authentication tag" rather than a "message authentication code".) 6. Maximum Payload Size The value of P_MAX and C_MAX listed above are determined by constraints on the structure of an SSH packet. Referring to [RFC 4253], one finds that an SSH packet consists of five fields: uint32 packet_length; // 0 <= packet_length < 2^32 byte padding_length; // 4 <= padding_length < 256 byte[n1] payload; // n1 = packet_length - padding_length - 1 byte[n2] random_padding; // n2 = padding_length byte[m] mac; // m = mac_length (= 16 for aes128/256_gcm) All save the mac field are encrypted, and the total length of the data to be encrypted (plaintext length) must be a multiple of the block length. When using either aead-aes-128-gcm-ssh or aead-aes-256-gcm-ssh (or any other algorithm with either a 16 octet or 8 octet block size) the largest possible payload is achieved when payload length = 2^32-9 octets padding length = 4 octets packet length = 1 + (2^32-9) + 4 = 2^32 - 4 octets plaintext length = 4 + 1 + (2^32 - 9) + 4 = 2^32 octets. 7. Linkage of Confidentiality and Data Integrity When either aead-aes-128-gcm-ssh or aead-aes-256-gcm-ssh is being employed it SHOULD be used both as the confidentiality mechanism and as the data integrity mechanism. 8. Security Considerations The security considerations in [RFC4251] apply. Igoe and Solinas Informational [Page 4] Internet Draft draft-igoe-secsh-aes-gcm-00 June 27, 2008 9. IANA Considerations IANA will add the following two entries to the AEAD Registry described in [RFC5116]: +----------------------+-------------+--------------------+ | | | Proposed | | Name | Reference | Numeric Identifier | +----------------------+-------------+--------------------+ | aead-aes-128-gcm-ssh | Section 4.1 | 5 | | | | | | aead-aes-256-gcm-ssh | Section 4.2 | 6 | +----------------------+-------------+--------------------+ IANA will add the following two entries to the Secure Shell Encryption Algorithm name Registry described in [RFC4250]: +----------------------+-------------+ | | | | Name | Reference | +----------------------+-------------+ | aead-aes-128-gcm-ssh | Section 4.1 | | | | | aead-aes-256-gcm-ssh | Section 4.2 | +----------------------+-------------+ IANA will add the following two entries to the Secure Shell MAC Algorithm name Registry described in [RFC4250]: +----------------------+-------------+ | | | | Name | Reference | +----------------------+-------------+ | aead-aes-128-gcm-ssh | Section 4.1 | | | | | aead-aes-256-gcm-ssh | Section 4.2 | +----------------------+-------------+ 10. References 10.1. Normative References [GCM] Dworkin, M, "Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC", NIST Special Publication 800-30D, November 2007. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Igoe and Solinas Informational [Page 5] Internet Draft draft-igoe-secsh-aes-gcm-00 June 27, 2008 Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4250] Lehtinen, S. and C. Lonvick, Ed., "The Secure Shell (SSH) Protocol Assigned Numbers", RFC 4250, January 2006. [RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Protocol Architecture", RFC 4251, January 2006. [RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated Encryptions", RFC 5116, January 2008. Igoe and Solinas Informational [Page 6] Internet Draft draft-igoe-secsh-aes-gcm-00 June 27, 2008 Author's Addresses Kevin M. Igoe NSA/CSS Commercial Solutions Center National Security Agency EMail: kmigoe@nsa.gov Jerome A. Solinas National Information Assurance Research Laboratory National Security Agency EMail: jasolin@orion.ncsc.mil Trademark Notice "SSH" is a registered trademark in the United States. Full Copyright Statement Copyright (C) The IETF Trust (2008). 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, THE IETF TRUST 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. 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