Network Working Group                                            K. Igoe
Request for Comments: 5647                                    J. Solinas
Category: Informational                         National Security Agency
                                                             August 2009

AES Galois Counter Mode for
the Secure Shell Transport Layer Protocol

Abstract

   Secure shell (SSH) is a secure remote-login protocol.  SSH provides
   for algorithms that provide authentication, key agreement,
   confidentiality, and data-integrity services.  The purpose of this
   document is to show how the AES Galois Counter Mode can be used to
   provide both confidentiality and data integrity to the SSH Transport
   Layer Protocol.

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

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   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   1. Introduction ....................................................2
   2. Requirements Terminology ........................................2
   3. Applicability Statement .........................................3
   4. Properties of Galois Counter Mode ...............................3
      4.1. AES GCM Authenticated Encryption ...........................3
      4.2. AES GCM Authenticated Decryption ...........................3
   5. Review of Secure Shell ..........................................4
      5.1. Key Exchange ...............................................4
      5.2. Secure Shell Binary Packets ................................5
   6. AES GCM Algorithms for Secure Shell .............................6
      6.1. AEAD_AES_128_GCM ...........................................6
      6.2. AEAD_AES_256_GCM ...........................................6
      6.3. Size of the Authentication Tag .............................6
   7. Processing Binary Packets in AES-GCM Secure Shell ...............7
      7.1. IV and Counter Management ..................................7
      7.2. Formation of the Binary Packet .............................7
      7.3. Treatment of the Packet Length Field .......................8
   8. Security Considerations .........................................8
      8.1. Use of the Packet Sequence Number in the AT ................8
      8.2. Non-Encryption of Packet Length ............................8
   9. IANA Considerations .............................................9
   10. References ....................................................10
      10.1. Normative References .....................................10

1. Introduction

   Galois Counter Mode (GCM) is a block-cipher mode of operation that
   provides both confidentiality and data-integrity services.  GCM uses
   counter mode to encrypt the data, an operation that can be
   efficiently pipelined.  Further, GCM authentication uses operations
   that are particularly well suited to efficient implementation in
   hardware, making it especially appealing for high-speed
   implementations or for implementations in an efficient and compact
   circuit.  The purpose of this document is to show how GCM with either
   AES-128 or AES-256 can be integrated into the Secure Shell Transport
   Layer Protocol [RFC4253].

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. Properties of Galois Counter Mode

   Galois Counter Mode (GCM) is a mode of operation for block ciphers
   that provides both confidentiality and data integrity.  National
   Institute of Standards and Technology (NIST) Special Publication SP
   800 38D [GCM] gives an excellent explanation of Galois Counter Mode.
   In this document, we shall focus on AES GCM, the use of the Advanced
   Encryption Algorithm (AES) in Galois Counter Mode.  AES-GCM is an
   example of an "algorithm for authenticated encryption with associated
   data" (AEAD algorithm) as described in [RFC5116].

4.1. AES GCM Authenticated Encryption

   An invocation of AES GCM to perform an authenticated encryption has
   the following inputs and outputs:

     GCM Authenticated Encryption

         Inputs:
            octet_string PT ;   // Plain Text, to be both
                                //    authenticated and encrypted
            octet_string AAD;   // Additional Authenticated Data,
                                //    authenticated but not encrypted
            octet_string IV;    // Initialization Vector
            octet_string BK;    // Block Cipher Key

         Outputs:
            octet_string  CT;   // Cipher Text
            octet_string  AT;   // Authentication Tag

   Note: in [RFC5116], the IV is called the nonce.

   For a given block-cipher key BK, it is critical that no IV be used
   more than once.  Section 7.1 addresses how this goal is to be
   achieved in secure shell.

4.2. AES GCM Authenticated Decryption

An invocation of AES GCM to perform an authenticated decryption has
the following inputs and outputs:

     GCM Authenticated Decryption

         Inputs:
            octet_string CT ;   // Cipher text, to be both
                                //    authenticated and decrypted
            octet_string AAD;   // Additional Authenticated Data,
                                //    authenticated only
            octet_string AT;    // Authentication Tag
            octet_string IV;    // Initialization Vector
            octet_string BK;    // Block Cipher Key

         Output:
            Failure_Indicator;  // Returned if the authentication tag
                                //   is invalid
            octet_string  PT;   // Plain Text, returned if and only if
                                //    the authentication tag is valid

   AES-GCM is prohibited from returning any portion of the plaintext
   until the authentication tag has been validated.  Though this feature
   greatly simplifies the security analysis of any system using AES-GCM,
   this creates an incompatibility with the requirements of secure
   shell, as we shall see in Section 7.3.

5. Review of Secure Shell

   The goal of secure shell is to establish two secure tunnels between a
   client and a server, one tunnel carrying client-to-server
   communications and the other carrying server-to-client
   communications.  Each tunnel is encrypted, and a message
   authentication code is used to ensure data integrity.

5.1. Key Exchange

   These tunnels are initialized using the secure shell key exchange
   protocol as described in Section 7 of [RFC4253].  This protocol
   negotiates a mutually acceptable set of cryptographic algorithms and
   produces a secret value K and an exchange hash H that are shared by
   the client and server.  The initial value of H is saved for use as
   the session_id.

   If AES-GCM is selected as the encryption algorithm for a given
   tunnel, AES-GCM MUST also be selected as the Message Authentication
   Code (MAC) algorithm.  Conversely, if AES-GCM is selected as the MAC
   algorithm, it MUST also be selected as the encryption algorithm.

   As described in Section 7.2 of [RFC4253], a hash-based key derivation
   function (KDF) is applied to the shared secret value K to generate
   the required symmetric keys.  Each tunnel gets a distinct set of

   symmetric keys.  The keys are generated as shown in Figure 1.  The
   sizes of these keys varies depending upon which cryptographic
   algorithms are being used.

      Initial IV
         Client-to-Server     HASH( K || H ||"A"|| session_id)
         Server-to-Client     HASH( K || H ||"B"|| session_id)
      Encryption Key
         Client-to-Server     HASH( K || H ||"C"|| session_id)
         Server-to-Client     HASH( K || H ||"D"|| session_id)
      Integrity Key
         Client-to-Server     HASH( K || H ||"E"|| session_id)
         Server-to-Client     HASH( K || H ||"F"|| session_id)

             Figure 1: Key Derivation in Secure Shell

   As we shall see below, SSH AES-GCM requires a 12-octet Initial IV and
   an encryption key of either 16 or 32 octets.  Because an AEAD
   algorithm such as AES-GCM uses the encryption key to provide both
   confidentiality and data integrity, the integrity key is not used
   with AES-GCM.

   Either the server or client may at any time request that the secure
   shell session be rekeyed.  The shared secret value K, the exchange
   hash H, and all the above symmetric keys will be updated.  Only the
   session_id will remain unchanged.

5.2. Secure Shell Binary Packets

   Upon completion of the key exchange protocol, all further secure
   shell traffic is parsed into a data structure known as a secure shell
   binary packet as shown below in Figure 2 (see also Section 6 of
   [RFC4253]).

     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

         Figure 2: Structure of a Secure Shell Binary Packet

   The authentication tag produced by AES-GCM authenticated encryption
   will be placed in the MAC field at the end of the secure shell binary
   packet.

6. AES GCM Algorithms for Secure Shell

6.1. AEAD_AES_128_GCM

   AEAD_AES_128_GCM is specified in Section 5.1 of [RFC5116].  Due to
   the format of secure shell binary packets, the buffer sizes needed to
   implement AEAD_AES_128_GCM are smaller than those required in
   [RFC5116].  Using the notation defined in [RFC5116], the input and
   output lengths for AEAD_AES_128_GCM in secure shell are as follows:

      PARAMETER   Meaning                          Value

      K_LEN       AES key length                   16 octets
      P_MAX       maximum plaintext length         2^32 - 32 octets
      A_MAX       maximum additional               4 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

6.2. AEAD_AES_256_GCM

   AEAD_AES_256_GCM is specified in Section 5.2 of [RFC5116].  Due to
   the format of secure shell binary packets, the buffer sizes needed
   to implement AEAD_AES_256_GCM are smaller than those required in
   [RFC5116].  Using the notation defined in [RFC5116], the input and
   output lengths for AEAD_AES_256_GCM in secure shell are as follows:

      PARAMETER   Meaning                          Value

      K_LEN       AES key length                   32 octets
      P_MAX       maximum plaintext length         2^32 - 32 octets
      A_MAX       maximum additional               4 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

6.3. Size of the Authentication Tag

   Both AEAD_AES_128_GCM and AEAD_AES_256_GCM produce a 16-octet
   Authentication Tag ([RFC5116] calls this a "Message Authentication
   Code").  Some applications allow use of a truncated version of this
   tag.  This is not allowed in AES-GCM secure shell.  All
   implementations of AES-GCM secure shell MUST use the full 16-octet
   Authentication Tag.

7. Processing Binary Packets in AES-GCM Secure Shell

7.1. IV and Counter Management

   With AES-GCM, the 12-octet IV is broken into two fields: a 4-octet
   fixed field and an 8-octet invocation counter field.  The invocation
   field is treated as a 64-bit integer and is incremented after each
   invocation of AES-GCM to process a binary packet.

         uint32  fixed;                  // 4 octets
         uint64  invocation_counter;     // 8 octets

           Figure 3: Structure of an SSH AES-GCM Nonce

   AES-GCM produces a keystream in blocks of 16-octets that is used to
   encrypt the plaintext.  This keystream is produced by encrypting the
   following 16-octet data structure:

         uint32  fixed;                  // 4 octets
         uint64  invocation_counter;     // 8 octets
         uint32  block_counter;          // 4 octets

           Figure 4: Structure of an AES Input for SSH AES-GCM

   The block_counter is initially set to one (1) and incremented as each
   block of key is produced.

   The reader is reminded that SSH requires that the data to be
   encrypted MUST be padded out to a multiple of the block size
   (16-octets for AES-GCM).

7.2. Formation of the Binary Packet

   In AES-GCM secure shell, the inputs to the authenticated encryption
   are:

     PT (Plain Text)
        byte      padding_length; // 4 <= padding_length < 256
        byte[n1]  payload;        // n1 = packet_length-padding_length-1
        byte[n2]  random_padding; // n2 = padding_length
     AAD (Additional Authenticated Data)
        uint32    packet_length;  // 0 <= packet_length < 2^32
     IV (Initialization Vector)
        As described in section 7.1.
     BK (Block Cipher Key)
        The appropriate Encryption Key formed during the Key Exchange.

   As required in [RFC4253], the random_padding MUST be at least 4
   octets in length but no more than 255 octets.  The total length of
   the PT MUST be a multiple of 16 octets (the block size of AES).  The
   binary packet is the concatenation of the 4-octet packet_length, the
   cipher text (CT), and the 16-octet authentication tag (AT).

7.3. Treatment of the Packet Length Field

   Section 6.3 of [RFC4253] requires that the packet length, padding
   length, payload, and padding fields of each binary packet be
   encrypted.  This presents a problem for SSH AES-GCM because:

   1) The tag cannot be verified until we parse the binary packet.

   2) The packet cannot be parsed until the packet_length has been
      decrypted.

   3) The packet_length cannot be decrypted until the tag has been
      verified.

   When using AES-GCM with secure shell, the packet_length field is to
   be treated as additional authenticated data, not as plaintext.  This
   violates the requirements of [RFC4253].  The repercussions of this
   decision are discussed in the following Security Considerations
   section.

8. Security Considerations

The security considerations in [RFC4251] apply.

8.1. Use of the Packet Sequence Number in the AT

   [RFC4253] requires that the formation of the AT involve the packet
   sequence_number, a 32-bit value that counts the number of binary
   packets that have been sent on a given SSH tunnel.  Since the
   sequence_number is, up to an additive constant, just the low 32 bits
   of the invocation_counter, the presence of the invocation_counter
   field in the IV ensures that the sequence_number is indeed involved
   in the formation of the integrity tag, though this involvement
   differs slightly from the requirements in Section 6.4 of [RFC4253].

8.2. Non-Encryption of Packet Length

   As discussed in Section 7.3, there is an incompatibility between
   GCM's requirement that no plaintext be returned until the
   authentication tag has been verified, secure shell's requirement that
   the packet length be encrypted, and the necessity of decrypting the
   packet length field to locate the authentication tag.  This document

   addresses this dilemma by requiring that, in AES-GCM, the packet
   length field will not be encrypted but will instead be processed as
   additional authenticated data.

   In theory, one could argue that encryption of the entire binary
   packet means that the secure shell dataflow becomes a featureless
   octet stream.  But in practice, the secure shell dataflow will come
   in bursts, with the length of each burst strongly correlated to the
   length of the underlying binary packets.  Encryption of the packet
   length does little in and of itself to disguise the length of the
   underlying binary packets.  Secure shell provides two other
   mechanisms, random padding and SSH_MSG_IGNORE messages, that are far
   more effective than encrypting the packet length in masking any
   structure in the underlying plaintext stream that might be revealed
   by the length of the binary packets.

9. IANA Considerations

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
              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.

   [RFC4253]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, January 2006.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, January 2008.

Authors' Addresses

   Kevin M. Igoe
   NSA/CSS Commercial Solutions Center
   National Security Agency
   USA

   EMail: kmigoe@nsa.gov

   Jerome A. Solinas
   National Information Assurance Research Laboratory
   National Security Agency
   USA

   EMail: jasolin@orion.ncsc.mil