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RFC 6239

Suite B Cryptographic Suites for Secure Shell (SSH)

Pages: 14
Historic

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Internet Engineering Task Force (IETF)                           K. Igoe
Request for Comments: 6239                      National Security Agency
Category: Informational                                         May 2011
ISSN: 2070-1721


          Suite B Cryptographic Suites for Secure Shell (SSH)

Abstract

This document describes the architecture of a Suite B compliant implementation of the Secure Shell Transport Layer Protocol and the Secure Shell Authentication Protocol. Suite B Secure Shell makes use of the elliptic curve Diffie-Hellman (ECDH) key agreement, the elliptic curve digital signature algorithm (ECDSA), the Advanced Encryption Standard running in Galois/Counter Mode (AES-GCM), two members of the SHA-2 family of hashes (SHA-256 and SHA-384), and X.509 certificates. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6239.
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   Copyright (c) 2011 IETF Trust and the persons identified as the
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

1. Introduction ....................................................3 2. Suite B and Secure Shell ........................................3 2.1. Minimum Levels of Security (minLOS) ........................4 2.2. Digital Signatures and Certificates ........................4 2.3. Non-Signature Primitives ...................................5 3. Security Mechanism Negotiation and Initialization ...............6 3.1. Algorithm Negotiation: SSH_MSG_KEXINIT .....................7 4. Key Exchange and Server Authentication ..........................8 4.1. SSH_MSG_KEXECDH_INIT .......................................9 4.2. SSH_MSG_KEXECDH_REPLY ......................................9 4.3. Key and Initialization Vector Derivation ..................10 5. User Authentication ............................................10 5.1. First SSH_MSG_USERAUTH_REQUEST Message ....................10 5.2. Second SSH_MSG_USERAUTH_REQUEST Message ...................11 6. Confidentiality and Data Integrity of SSH Binary Packets .......12 6.1. Galois/Counter Mode .......................................12 6.2. Data Integrity ............................................12 7. Rekeying .......................................................12 8. Security Considerations ........................................13 9. References .....................................................13 9.1. Normative References ......................................13 9.2. Informative References ....................................13
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1. Introduction

This document describes the architecture of a Suite B compliant implementation of the Secure Shell Transport Layer Protocol and the Secure Shell Authentication Protocol. Suite B Secure Shell makes use of the elliptic curve Diffie-Hellman (ECDH) key agreement, the elliptic curve digital signature algorithm (ECDSA), the Advanced Encryption Standard running in Galois/Counter Mode (AES-GCM), two members of the SHA-2 family of hashes (SHA-256 and SHA-384), and X.509 certificates. 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 RFC 2119 [RFC2119].

2. Suite B and Secure Shell

Several RFCs have documented how each of the Suite B components are to be integrated into Secure Shell (SSH): kex algorithms ecdh-sha2-nistp256 [SSH-ECC] ecdh-sha2-nistp384 [SSH-ECC] server host key algorithms x509v3-ecdsa-sha2-nistp256 [SSH-X509] x509v3-ecdsa-sha2-nistp384 [SSH-X509] encryption algorithms (both client_to_server and server_to_client) AEAD_AES_128_GCM [SSH-GCM] AEAD_AES_256_GCM [SSH-GCM] MAC algorithms (both client_to_server and server_to_client) AEAD_AES_128_GCM [SSH-GCM] AEAD_AES_256_GCM [SSH-GCM] In Suite B, public key certificates used to verify signatures MUST be compliant with the Suite B Certificate Profile specified in RFC 5759 [SUITEBCERT]. The purpose of this document is to draw upon all of these documents to provide guidance for Suite B compliant implementations of Secure Shell (hereafter referred to as "SecSh-B"). Note that while SecSh-B MUST follow the guidance in this document, that requirement does not in and of itself imply that a given implementation of Secure Shell is suitable for use in protecting classified data. An implementation of SecSh-B must be validated by the appropriate authority before such usage is permitted.
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   The two elliptic curves used in Suite B appear in the literature
   under two different names.  For the sake of clarity, we list both
   names below.

      Curve        NIST name        SECG name     OID [SEC2]
      ---------------------------------------------------------------
      P-256        nistp256         secp256r1     1.2.840.10045.3.1.7
      P-384        nistp384         secp384r1     1.3.132.0.34

   A description of these curves can be found in [NIST] or [SEC2].

   For the sake of brevity, ECDSA-256 will be used to denote ECDSA on
   P-256 using SHA-256, and ECDSA-384 will be used to denote ECDSA on
   P-384 using SHA-384.

2.1. Minimum Levels of Security (minLOS)

Suite B provides for two levels of cryptographic security, namely a 128-bit minimum level of security (minLOS_128) and a 192-bit minimum level of security (minLOS_192). As we shall see below, the ECDSA-256/384 signature algorithms and corresponding X.509v3 certificates are treated somewhat differently than the non-signature primitives (kex algorithms, encryption algorithms, and Message Authentication Code (MAC) algorithms in Secure Shell parlance).

2.2. Digital Signatures and Certificates

SecSh-B uses ECDSA-256/384 for server authentication, user authentication, and in X.509 certificates. [SSH-X509] defines two methods, x509v3-ecdsa-sha2-nistp256 and x509v3-ecdsa-sha2-nistp384, that are to be used for server and user authentication. The following conditions must be met: 1) The server MUST share its public key with the host using an X.509v3 certificate as described in [SSH-X509]. This public key MUST be used to authenticate the server to the host using ECDSA-256 or ECDSA-384 as appropriate (see Section 3). 2) User authentication MUST begin with public key authentication using ECDSA-256/384 with X.509v3 certificates (see Section 4). Additional user authentication methods MAY be used, but only after the certificate-based ECDSA authentication has been successfully completed. 3) The X.509v3 certificates MUST use only the two Suite B digital signatures, ECDSA-256 and ECDSA-384. 4) ECDSA-256 MUST NOT be used to sign an ECDSA-384 public key.
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   5) ECDSA-384 MAY be used to sign an ECDSA-256 public key.

   6) At minLOS_192, all SecSh-B implementations MUST be able to verify
      ECDSA-384 signatures.

   7) At minLOS_128, all SecSh-B implementations MUST be able to verify
      ECDSA-256 signatures and SHOULD be able to verify ECDSA-384
      signatures, unless it is absolutely certain that the
      implementation will never need to verify certificates originating
      from an authority that uses an ECDSA-384 signing key.

   8) At minLOS_128, each SecSh-B server and each SecSh-B user MUST have
      either an ECDSA-256 signing key with a corresponding X.509v3
      certificate, an ECDSA-384 signing key with a corresponding X.509v3
      certificate, or both.

   9) At minLOS_192, each SecSh-B server and each SecSh-B user MUST have
      an ECDSA-384 signing key with a corresponding X.509v3 certificate.

   The selection of the signature algorithm to be used for server
   authentication is governed by the server_host_key_algorithms name-
   list in the SSH_MSG_KEXINIT packet (see Section 3.1).  The key
   exchange and server authentication are performed by the
   SSH_MSG_KEXECDH_REPLY packets (see Section 4).  User authentication
   is done via the SSH_MSG_USERAUTH_REQUEST messages (see Section 5).

2.3. Non-Signature Primitives

This section covers the constraints that the choice of minimum security level imposes upon the selection of a key agreement protocol (kex algorithm), encryption algorithm, and data integrity algorithm (MAC algorithm). We divide the non-signature algorithms into two families, as shown in Table 1. +--------------+----------------------+----------------------+ | Algorithm | Family 1 | Family 2 | +==============+======================+======================+ | kex | ecdh-sha2-nistp256 | ecdh-sha2-nistp384 | +--------------+----------------------+----------------------+ | encryption | AEAD_AES_128_GCM | AEAD_AES_256_GCM | +--------------+----------------------+----------------------+ | MAC | AEAD_AES_128_GCM | AEAD_AES_256_GCM | +--------------+-----------------------+---------------------+ Table 1. Families of Non-Signature Algorithms in SecSh-B
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   At the 128-bit minimum level of security:

   o  The non-signature algorithms MUST either come exclusively from
      Family 1 or exclusively from Family 2.

   o  The selection of Family 1 versus Family 2 is independent of the
      choice of server host key algorithm.

   At the 192-bit minimum level of security:

   o  The non-signature algorithms MUST all come from Family 2.

   Most of the constraints described in this section can be achieved by
   severely restricting the kex_algorithm, encryption_algorithm, and
   mac_algorithm name lists offered in the SSH_MSG_KEXINIT packet.  See
   Section 3.1 for details.

3. Security Mechanism Negotiation and Initialization

As described in [SSH-Tran], the exchange of SSH_MSG_KEXINIT between the server and the client establishes which key agreement algorithm, MAC algorithm, host key algorithm (server authentication algorithm), and encryption algorithm are to be used. This section describes how the Suite B components are to be used in the Secure Shell algorithm negotiation, key agreement, server authentication, and user authentication. Negotiation and initialization of a Suite B Secure Shell connection involves the following Secure Shell messages (where C->S denotes a message from the client to the server, and S->C denotes a server-to- client message): SSH_MSG_KEXINIT C->S Contains lists of algorithms acceptable to the client. SSH_MSG_KEXINIT S->C Contains lists of algorithms acceptable to the server. SSH_MSG_KEXECDH_INIT C->S Contains the client's ephemeral elliptic curve Diffie-Hellman key. SSH_MSG_KEXECDH_REPLY S->C Contains a certificate with the server's ECDSA public signature key, the server's ephemeral ECDH contribution, and an ECDSA digital signature of the newly formed exchange hash value.
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      SSH_MSG_USERAUTH_REQUEST  C->S  Contains the user's name, the
                                      name of the service the user is
                                      requesting, the name of the
                                      authentication method the client
                                      wishes to use, and method-specific
                                      fields.

   When not in the midst of processing a key exchange, either party may
   initiate a key re-exchange by sending an SSH_MSG_KEXINIT packet.  All
   packets exchanged during the re-exchange are encrypted and
   authenticated using the current keys until the conclusion of the
   re-exchange, at which point an SSH_MSG_NEWKEYS initiates a change to
   the newly established keys.  Otherwise, the re-exchange protocol is
   identical to the initial key exchange protocol.  See Section 9 of
   [SSH-Tran].

3.1. Algorithm Negotiation: SSH_MSG_KEXINIT

The choice of all but the user authentication methods are determined by the exchange of SSH_MSG_KEXINIT between the client and the server. As described in [SSH-Tran], the SSH_MSG_KEXINIT packet has the following structure: byte SSH_MSG_KEXINIT byte[16] cookie (random bytes) name-list kex_algorithms name-list server_host_key_algorithms name-list encryption_algorithms_client_to_server name-list encryption_algorithms_server_to_client name-list mac_algorithms_client_to_server name-list mac_algorithms_server_to_client name-list compression_algorithms_client_to_server name-list compression_algorithms_server_to_client name-list languages_client_to_server name-list languages_server_to_client boolean first_kex_packet_follows uint32 0 (reserved for future extension) The SSH_MSG_KEXINIT name lists can be used to constrain the choice of non-signature and host key algorithms in accordance with the guidance given in Section 2. Table 2 lists three allowable name lists for the non-signature algorithms. One of these options MUST be used.
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       Family 1 only (min_LOS 128):
          kex_algorithm name_list         := { ecdh_sha2_nistp256 }
          encryption_algorithm name_list  := { AEAD_AES_128_GCM   }
          mac_algorithm name_list         := { AEAD_AES_128_GCM   }

       Family 2 only (min_LOS 128 or 192):
          kex_algorithm name_list         := { ecdh_sha2_nistp384 }
          encryption_algorithm name_list  := { AEAD_AES_256_GCM   }
          mac_algorithm name_list         := { AEAD_AES_256_GCM   }

       Family 1 or Family 2 (min_LOS 128):
          kex_algorithm name_list         := { ecdh_sha2_nistp256,
                                               ecdh_sha2_nistp384 }
          encryption_algorithm name_list  := { AEAD_AES_128_GCM,
                                               AEAD_AES_256_GCM   }
          mac_algorithm name_list         := { AEAD_AES_128_GCM,
                                               AEAD_AES_256_GCM   }

           Table 2.  Allowed Non-Signature Algorithm Name Lists

   Table 3 lists three allowable name lists for the server host key
   algorithms.  One of these options MUST be used.

            ECDSA-256 only (min_LOS 128):
               server_host_key_algorithms name_list :=
                                { x509v3-ecdsa-sha2-nistp256 }

            ECDSA-384 only (min_LOS 128 or 192):
               server_host_key_algorithms name_list :=
                                { x509v3-ecdsa-sha2-nistp384 }

            ECDSA-256 or ECDSA-384 (min_LOS 128):
               server_host_key_algorithms name_list :=
                                { x509v3-ecdsa-sha2-nistp256,
                                  x509v3-ecdsa-sha2-nistp384 }

          Table 3.  Allowed Server Host Key Algorithm Name Lists

4. Key Exchange and Server Authentication

SecSh-B uses ECDH to establish a shared secret value between the client and the server. An X.509v3 certificate containing the server's public signing ECDSA key and an ECDSA signature on the exchange hash value derived from the newly established shared secret value are used to authenticate the server to the client.
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4.1. SSH_MSG_KEXECDH_INIT

The key exchange to be used in Secure Shell is determined by the name lists exchanged in the SSH_MSG_KEXINIT packets. In Suite B, one of the following key agreement methods MUST be used to generate a shared secret value (SSV): ecdh-sha2-nistp256 ephemeral-ephemeral elliptic curve Diffie-Hellman on nistp256 with SHA-256 ecdh-sha2-nistp384 ephemeral-ephemeral elliptic curve Diffie-Hellman on nistp384 with SHA-384 and the format of the SSH_MSG_KEXECDH_INIT message is: byte SSH_MSG_KEXDH_INIT string Q_C // the client's ephemeral contribution to the // ECDH exchange, encoded as an octet string where the encoding of the elliptic curve point Q_C as an octet string is as specified in Section 2.3.3 of [SEC1].

4.2. SSH_MSG_KEXECDH_REPLY

The SSH_MSG_KEXECDH_REPLY contains the server's contribution to the ECDH exchange, the server's public signature key, and a signature of the exchange hash value formed from the newly established shared secret value. As stated in Section 3.1, in SecSh-B, the server host key algorithm MUST be either x509v3-ecdsa-sha2-nistp256 or x509v3-ecdsa-sha2-nistp384. The format of the SSH_MSG_KEXECDH_REPLY is: byte SSH_MSG_KEXECDH_REPLY string K_S // a string encoding an X.509v3 certificate // containing the server's ECDSA public host key string Q_S // the server's ephemeral contribution to the // ECDH exchange, encoded as an octet string string Sig_S // an octet string containing the server's // signature of the newly established exchange // hash value
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   Details on the structure and encoding of the X.509v3 certificate can
   be found in Section 2 of [SSH-X509].  The encoding of the elliptic
   curve point Q_C as an octet string is as specified in Section 2.3.3
   of [SEC1], and the encoding of the ECDSA signature Sig_S as an octet
   string is as described in Section 3.1.2 of [SSH-ECC].

4.3. Key and Initialization Vector Derivation

As specified in [SSH-Tran], the encryption keys and initialization vectors needed by Secure Shell are derived directly from the SSV using the hash function specified by the key agreement algorithm (SHA-256 for ecdh-sha2-nistp256 and SHA-384 for ecdh-sha2-nistp384). The client-to-server channel and the server-to-client channel will have independent keys and initialization vectors. These keys will remain constant until a re-exchange results in the formation of a new SSV.

5. User Authentication

The Secure Shell Transport Layer Protocol authenticates the server to the host but does not authenticate the user (or the user's host) to the server. For this reason, condition (2) of Section 2.2 requires that all users of SecSh-B MUST be authenticated using ECDSA-256/384 signatures and X.509v3 certificates. [SSH-X509] provides two methods, x509v3-ecdsa-sha2-nistp256 and x509v3-ecdsa-sha2-nistp384, that MUST be used to achieve this goal. At minLOS 128, either one of these methods may be used, but at minLOS 192, x509v3-ecdsa-sha2-nistp384 MUST be used.

5.1. First SSH_MSG_USERAUTH_REQUEST Message

The user's public key is sent to the server using an SSH_MSG_USERAUTH_REQUEST message. When an x509v3-ecdsa-sha2-* user authentication method is being used, the structure of the SSH_MSG_USERAUTH_REQUEST message should be: byte SSH_MSG_USERAUTH_REQUEST string user_name // in ISO-10646 UTF-8 encoding string service_name // service name in US-ASCII string "publickey" boolean FALSE
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      string    public_key_algorithm_name  // x509v3-ecdsa-sha2-nistp256
                                        // or x509v3-ecdsa-sha2-nistp384

      string    public_key_blob // X.509v3 certificate

   Details on the structure and encoding of the X.509v3 certificate can
   be found in Section 2 of [SSH-X509].

5.2. Second SSH_MSG_USERAUTH_REQUEST Message

Once the server has responded to the request message with an SSH_MSG_USERAUTH_PK_OK message, the client uses a second SSH_MSG_USERAUTH_REQUEST message to perform the actual authentication: byte SSH_MSG_USERAUTH_REQUEST string user_name // in ISO-10646 UTF-8 encoding string service_name // service name in US-ASCII string "publickey" boolean TRUE string public_key_algorithm_name // x509v3-ecdsa-sha2-nistp256 // or x509v3-ecdsa-sha2-nistp384 string Sig_U The signature field Sig_U is an ECDSA signature of the concatenation of several values, including the session identifier, user name, service name, public key algorithm name, and the user's public signing key. The user's public signing key MUST be the signing key conveyed in the X.509v3 certificate sent in the first SSH_MSG_USERAUTH_REQUEST message. The encoding of the ECDSA signature Sig_U as an octet string is as described in Section 3.1.2 of [SSH-ECC]. The server MUST respond with either SSH_MSG_USERAUTH_SUCCESS (if no more authentications are needed) or SSH_MSG_USERAUTH_FAILURE (if the request failed, or more authentications are needed).
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6. Confidentiality and Data Integrity of SSH Binary Packets

Secure Shell transfers data between the client and the server using its own binary packet structure. The SSH binary packet structure is independent of any packet structure on the underlying data channel. The contents of each binary packet and portions of the header are encrypted, and each packet is authenticated with its own message authentication code. AES GCM will both encrypt the packet and form a 16-octet authentication tag to ensure data integrity.

6.1. Galois/Counter Mode

[SSH-GCM] describes how AES Galois/Counter Mode is to be used in Secure Shell. Suite B SSH implementations MUST support AEAD_AES_GCM_128 and SHOULD support AEAD_AES_GCM_256 to both provide confidentiality and ensure data integrity. No other confidentiality or data integrity algorithms are permitted. These algorithms rely on two counters: Invocation Counter: A 64-bit integer, incremented after each call to AES-GCM to process an SSH binary packet. The initial value of the invocation counter is determined by the SSH initialization vector. Block Counter: A 32-bit integer, set to one at the start of each new SSH binary packet and incremented as each 16-octet block of data is processed. Ensuring that these counters are properly implemented is crucial to the security of the system. The reader is referred to [SSH-GCM] for details on the format, initialization, and usage of these counters and their relationship to the initialization vector and the SSV.

6.2. Data Integrity

The reader is reminded that, as specified in [SSH-GCM], Suite B requires that all 16 octets of the authentication tag MUST be used as the SSH data integrity value of the SSH binary packet.

7. Rekeying

Secure Shell allows either the server or client to request that the Secure Shell connection be rekeyed. Suite B places no constraints on how frequently this is to be done, but it does require that the cipher suite being employed MUST NOT be changed when a rekey occurs.
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8. Security Considerations

When using ecdh_sha2_nistp256, each exponent used in the key exchange must have 256 bits of entropy. Similarly, when using ecdh_sha2_nistp384, each exponent used in the key exchange must have 384 bits of entropy. The security considerations of [SSH-Arch] apply.

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [SUITEBCERT] Solinas, J. and L. Zieglar, "Suite B Certificate and Certificate Revocation List (CRL) Profile", RFC 5759, January 2010. [SSH-Arch] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Protocol Architecture", RFC 4251, January 2006. [SSH-Tran] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, January 2006. [SSH-ECC] Stebila, D. and J. Green, "Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer", RFC 5656, December 2009. [SSH-GCM] Igoe, K. and J. Solinas, "AES Galois Counter Mode for the Secure Shell Transport Layer Protocol", RFC 5647, August 2009. [SSH-X509] Igoe, K. and D. Stebila, "X.509v3 Certificates for Secure Shell Authentication", RFC 6187, March 2011.

9.2. Informative References

[NIST] National Institute of Standards and Technology, "Digital Signature Standard (DSS)", Federal Information Processing Standards Publication 186-3. [SEC1] Standards for Efficient Cryptography Group, "Elliptic Curve Cryptography", SEC 1 v2.0, May 2009, <http://www.secg.org/download/aid-780/sec1-v2.pdf>.
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   [SEC2]       Standards for Efficient Cryptography Group, "Recommended
                Elliptic Curve Domain Parameters", SEC 2 v1.0, September
                2000.  <http://www.secg.org/download/aid-386/
                sec2_final.pdf>.

Author's Address

Kevin M. Igoe NSA/CSS Commercial Solutions Center National Security Agency EMail: kmigoe@nsa.gov