Internet Engineering Task Force (IETF) K. Igoe

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 Notice

Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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: [email protected]

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