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Versions: 00 01 02 03 RFC 5647

Network Working Group                                          K.M. Igoe
Internet Draft                                  National Security Agency
Intended Status: Informational                              May 20, 2009
Expires: November 21, 2009                                  J.A. Solinas
                                                National Security Agency
                                                            May 20, 2009


  AES Galois Counter Mode for the Secure Shell Transport Layer Protocol
                      draft-igoe-secsh-aes-gcm-02


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   This Internet-Draft will expire on November 21, 2009.

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Abstract

   Secure Shell (SSH, RFC 4251) 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


Table of Contents

   1. Introduction.....................................................2
   2. Requirements Terminology.........................................2
   3. Applicability Statement..........................................2
   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
         5.2.1. Treatment of the Packet Length Field...................5
   6. Two New AEAD Algorithms..........................................6
      6.1. aead-aes-128-gcm-ssh........................................6
      6.2. aead-aes-256-gcm-ssh........................................6
   7. IV and Counter Management........................................7
   8. Size of the Message Authentication Code..........................7
   9. Security Considerations..........................................7
      9.1. Use of Packet Sequence Number in MAC........................8
      9.2. Non-encryption of Packet Length.............................8
   10. IANA Considerations.............................................9
   11. References......................................................9
      11.1. Normative References.......................................9


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 integrated
   into the Secure Shell Transport Layer Protocol, RFC 4253.


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



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   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
   which provides both confidentiality and data integrity.  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 6 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:


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            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 test, 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,
   as we shall see in section 5.2.1, this creates an incompatibility
   with the requirements of secure shell.


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 server-to-client communications.  Each
   tunnel is encrypted and a message authentications code is used to
   insure 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 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 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.



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


5.2.1. 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 can not be verified until we parse the binary packet
     2) The packet can not be parsed until the packet_length has been


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        decrypted.
     3) The packet_length can not 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 security considerations section of this
   document.


6. Two New AEAD Algorithms


6.1. 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 - 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 - 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 4.2 above.


6.2. 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 - 32 octets
      A_MAX       maximum additional               4 octets
                  authenticated data length


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      N_MIN       minimum nonce (IV) length        12 octets
      N_MAX       maximum nonce (IV) length        12 octets
      C_MAX       maximum cipher length            2^32 -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 4.2 above.


7. 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 which 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).


8. Size of the Message Authentication Code

   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".)


9. Security Considerations

   The security considerations in [RFC4251] apply.



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9.1. Use of Packet Sequence Number in MAC

   [RFC4253] requires that the formation of the mac 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 insures 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].


9.2. Non-encryption of Packet Length

   As discussed in section 5.2.1, there is an incompatability 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.



















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10. 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 5.1 |          5         |
          |                      |             |                    |
          | aead-aes-256-gcm-ssh | Section 5.2 |          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 5.1 |
                   |                      |             |
                   | aead-aes-256-gcm-ssh | Section 5.2 |
                   +----------------------+-------------+

   IANA will add the following two entries to the Secure Shell MAC
   Algorithm name Registry described in [RF4250]:

                   +----------------------+-------------+
                   |                      |             |
                   | Name                 |  Reference  |
                   +----------------------+-------------+
                   | aead-aes-128-gcm-ssh | Section 5.1 |
                   |                      |             |
                   | aead-aes-256-gcm-ssh | Section 5.2 |
                   +----------------------+-------------+


11. References


11.1. 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.


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   [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

   [RFC4344]  Bellare, M., Kohno, T., and C. Namprempre, "The Secure
              Shell (SSH) Transport Layer Encryption Modes", RFC 4344,
              January 2006.

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






































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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



Acknowledgement

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).




































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