



Internet Engineering Task Force                                D. Miller
Internet-Draft                                                   OpenSSH
Intended status: Standards Track                        27 November 2025
Expires: 31 May 2026


                           SSH Agent Protocol
                      draft-ietf-sshm-ssh-agent-13

Abstract

   This document specifies a key agent protocol for use in the Secure
   Shell (SSH) protocol.

Note

   This note is to be removed before publishing as an RFC.

   In the IANA considerations section, please replace "thisRFC" with
   "RFC" and the assigned number, when known.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on 31 May 2026.

Copyright Notice

   Copyright (c) 2025 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 (https://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



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   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Terminology and units . . . . . . . . . . . . . . . . . .   4
   3.  Protocol Messages . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Generic agent responses . . . . . . . . . . . . . . . . .   5
     3.2.  Adding keys to the agent  . . . . . . . . . . . . . . . .   5
       3.2.1.  DSA keys  . . . . . . . . . . . . . . . . . . . . . .   6
       3.2.2.  ECDSA keys  . . . . . . . . . . . . . . . . . . . . .   7
       3.2.3.  EDDSA keys  . . . . . . . . . . . . . . . . . . . . .   7
       3.2.4.  RSA keys  . . . . . . . . . . . . . . . . . . . . . .   8
       3.2.5.  Other keys  . . . . . . . . . . . . . . . . . . . . .   8
       3.2.6.  Adding keys from a token  . . . . . . . . . . . . . .   8
       3.2.7.  Key Constraints . . . . . . . . . . . . . . . . . . .   9
     3.3.  Public key encoding . . . . . . . . . . . . . . . . . . .  10
     3.4.  Removing keys from the agent  . . . . . . . . . . . . . .  11
     3.5.  Requesting a list of keys . . . . . . . . . . . . . . . .  11
     3.6.  Private key operations  . . . . . . . . . . . . . . . . .  12
       3.6.1.  Signature flags . . . . . . . . . . . . . . . . . . .  12
     3.7.  Locking and unlocking an agent  . . . . . . . . . . . . .  13
     3.8.  Extension mechanism . . . . . . . . . . . . . . . . . . .  13
       3.8.1.  Query extension . . . . . . . . . . . . . . . . . . .  14
   4.  Connecting to an agent  . . . . . . . . . . . . . . . . . . .  14
   5.  Forwarding access to an agent . . . . . . . . . . . . . . . .  15
     5.1.  Advertising agent forwarding support  . . . . . . . . . .  15
     5.2.  Requesting agent forwarding . . . . . . . . . . . . . . .  15
     5.3.  Agent connection requests . . . . . . . . . . . . . . . .  16
   6.  Protocol numbers  . . . . . . . . . . . . . . . . . . . . . .  17
     6.1.  Message type numbers  . . . . . . . . . . . . . . . . . .  17
       6.1.1.  Reserved message type numbers . . . . . . . . . . . .  18
     6.2.  Constraint identifiers  . . . . . . . . . . . . . . . . .  18
     6.3.  Signature flags . . . . . . . . . . . . . . . . . . . . .  18
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     7.1.  Guidance for Designated Experts . . . . . . . . . . . . .  19
     7.2.  New registry: SSH agent protocol message type numbers . .  19
     7.3.  New registry: SSH agent key constraint numbers  . . . . .  22
     7.4.  New registry: SSH agent signature flags . . . . . . . . .  22
     7.5.  New registry: SSH agent extension request names . . . . .  23
     7.6.  Additions to SSH Extension Names  . . . . . . . . . . . .  23




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     7.7.  Additions to SSH Connection Protocol Channel Request
           Names . . . . . . . . . . . . . . . . . . . . . . . . . .  24
     7.8.  Additions to SSH Connection Protocol Channel Types  . . .  24
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  24
   9.  Implementation Status . . . . . . . . . . . . . . . . . . . .  26
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  27
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  27
     10.2.  Informative References . . . . . . . . . . . . . . . . .  28
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  29
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Introduction

   Secure Shell (SSH) [RFC4251] is a protocol for secure remote
   connections [RFC4253] and login [RFC4254] over untrusted networks.
   It supports multiple authentication mechanisms [RFC4252], including
   public key authentication.  This document specifies the protocol for
   interacting with a key management component, usually referred to as
   "an agent", that holds private keys.  SSH clients (and possibly SSH
   servers) can invoke the agent via this protocol to perform operations
   using public and private keys held in the agent.

   Holding keys in an agent offers usability and security advantages to
   loading and unwrapping them at each use, as each key unwrapping may
   require entry of a pass-phrase.  Access to an agent may optionally be
   forwarded across an SSH connection, thereby allowing remote systems
   to use stored keys without directly exposing the key material to the
   remote system.  Finally, the agent may be implemented as a dedicated
   component that presents a smaller attack surface than a key loaded
   into a full SSH server or client, and which may be subject to special
   protection from the wider system.

1.1.  Background

   This section is to be removed before publishing as an RFC.

   This agent protocol is already widely used and a de-facto standard,
   having been implemented by a number of popular SSH clients and
   servers for many years.  The purpose of this document is to describe
   the protocol as it has been implemented.

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.



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2.  Protocol Overview

   The agent protocol is a packetised request-response protocol, solely
   driven by the client.  It consists of a number of requests sent from
   a client to an agent and a set of reply messages that are sent in
   response.  At no time does the agent send messages except in response
   to a client request.  Replies are sent in order.

   These requests include the ability to load keys into an agent, remove
   some or all keys from an agent and to perform signature operations
   using previously-loaded keys.

   Agents MAY implement support for only a subset of operations or
   available key types, and MAY additionally refuse arbitrary operations
   in particular contexts.  For example, an agent may allow only clients
   local to itself to add or remove keys, or make particular subsets of
   keys available to a given client.  For this reason, clients of the
   agent SHOULD be prepared to fail gracefully if any operation is
   refused.

2.1.  Background

   This section is to be removed before publishing as an RFC.

   Note that this protocol is separate to and incompatible with the one
   described in the similarly-named [draft-ietf-secsh-agent-02] which
   expired in 2004.

2.2.  Terminology and units

   Henceforth in this document, "agent" will be used to refer to a key
   management component that implements the responder side of this
   protocol.  "Client" will refer to a tool that implements the
   requester side of the protocol to communicate with an agent.  If it
   is pertinent that the client in question is a [RFC4251] Secure Shell
   client, then it will be explicitly referred to as an "SSH client".
   Similarly "SSH server" will be used to refer to Secure Shell servers.

   All encoding data types ("byte", "uint32", "string", etc.) are as
   specified in Section 5 of [RFC4251].  Additionally, the type "byte[]"
   without a specified length within the square brackets indicates an
   unadorned sequence of zero or more bytes where the length is
   determined by context.

   All length units are given in bytes unless otherwise specified.






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3.  Protocol Messages

   Messages consist of a length, type and contents.

       uint32            length
       byte              type
       byte[length - 1]  contents

   In the sections below, the "length" field is omitted.  For clarity,
   the symbolic names of the message types are shown; their numeric
   values are listed in Section 6.1 below.

3.1.  Generic agent responses

   The following generic messages may be sent by the agent in response
   to requests from the client.  On success the agent MUST reply either
   with the single-byte response:

       byte              SSH_AGENT_SUCCESS

   or a request-specific success message that may contain additional
   fields.  On failure, the agent MUST reply with the single-byte
   response:

       byte              SSH_AGENT_FAILURE

   or a request-specific failure message that may contain additional
   fields.  SSH_AGENT_FAILURE messages MUST also be sent in reply to
   requests with unknown or unsupported types.

3.2.  Adding keys to the agent

   Keys may be added to the agent using the SSH_AGENTC_ADD_IDENTITY or
   SSH_AGENTC_ADD_ID_CONSTRAINED messages.  The latter variant allows
   adding keys with optional constraints on their usage.

   The generic format for the key SSH_AGENTC_ADD_IDENTITY message is:

       byte             SSH_AGENTC_ADD_IDENTITY
       string           key type
       byte[]           key data
       string           comment









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   Here "key type" is the specified key type name, for example "ssh-rsa"
   for a RSA key as defined by [RFC4253].  "key data" consists of the
   public and private components of the key and vary by key type, as
   specified in sub-sections 3.2.1 through 3.2.4 for commonly used key
   types.  "comment" is a human-readable key name or comment as a UTF-8
   string that may serve to identify the key in user-visible messages.
   This string may be of zero length.

   The SSH_AGENTC_ADD_ID_CONSTRAINED is similar, but adds an extra
   field:

       byte             SSH_AGENTC_ADD_ID_CONSTRAINED
       string           key type
       byte[]           key data
       string           comment
       constraint[]     constraints

   Constraints are used to place limits on the validity or use of keys.
   Section 3.2.7 details constraint types and their format.  Clients
   SHOULD prefer the SSH_AGENTC_ADD_IDENTITY message over sending a
   SSH_AGENTC_ADD_ID_CONSTRAINED with an empty constraints field, though
   both are valid and equivalent.

   An agent MUST reply with SSH_AGENT_SUCCESS if the key was
   successfully loaded as a result of one of these messages, or
   SSH_AGENT_FAILURE otherwise.

   An agent MAY support only a subset of the key types defined here and
   MAY support additional key types as described below.  If an agent
   does not recognise the type name in a request to add a key, then it
   MUST respond with a SSH_AGENT_FAILURE reply.

3.2.1.  DSA keys

   DSA keys have key type "ssh-dss" and are defined in [RFC4253].  They
   may be added to the agent using the following message.  The
   "constraints" field is only present for the
   SSH_AGENTC_ADD_ID_CONSTRAINED message.













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       byte             SSH_AGENTC_ADD_IDENTITY or
                        SSH_AGENTC_ADD_ID_CONSTRAINED
       string           "ssh-dss"
       mpint            p
       mpint            q
       mpint            g
       mpint            y
       mpint            x
       string           comment
       constraint[]     constraints

   The "p", "q", "g" values are the DSA domain parameters. "y" and "x"
   are the public and private keys respectively.  These values are as
   defined by Section 4.1 of [FIPS.186-4].

3.2.2.  ECDSA keys

   ECDSA keys have key types starting with "ecdsa-sha2-" and are defined
   in [RFC5656].  They may be added to the agent using the following
   message.  The "constraints" field is only present for the
   SSH_AGENTC_ADD_ID_CONSTRAINED message.

       byte             SSH_AGENTC_ADD_IDENTITY or
                        SSH_AGENTC_ADD_ID_CONSTRAINED
       string           key type
       string           ecdsa_curve_name
       string           Q
       mpint            d
       string           comment
       constraint[]     constraints

   The values "Q" and "d" are the ECDSA public and private values
   respectively.  Both are defined by Section 6.2 of [FIPS.186-5].

3.2.3.  EDDSA keys

   [RFC8709] defines Ed25519 and Ed448 with key type names "ssh-ed25519"
   and "ssh-ed448" respectively.  These may be added to the agent using
   the following message.  The "constraints" field is only present for
   the SSH_AGENTC_ADD_ID_CONSTRAINED message.

       byte             SSH_AGENTC_ADD_IDENTITY or
                        SSH_AGENTC_ADD_ID_CONSTRAINED
       string           "ssh-ed25519" or "ssh-ed448"
       string           ENC(A)
       string           k || ENC(A)
       string           comment
       constraint[]     constraints



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   The first value is the EDDSA public key ENC(A).  The second value is
   a concatenation of the private key k and the public ENC(A) key (this
   redundant repetition of the public key is to maintain compatibility
   with widely deployed implementations).  The contents and
   interpretation of the ENC(A) and k values are defined by Section 3.2
   of [RFC8032].

3.2.4.  RSA keys

   RSA keys have key type "ssh-rsa" and are defined in [RFC4253].  They
   may be added to the agent using the following message.  The
   "constraints" field is only present for the
   SSH_AGENTC_ADD_ID_CONSTRAINED message.

       byte             SSH_AGENTC_ADD_IDENTITY or
                        SSH_AGENTC_ADD_ID_CONSTRAINED
       string           "ssh-rsa"
       mpint            n
       mpint            e
       mpint            d
       mpint            iqmp
       mpint            p
       mpint            q
       string           comment
       constraint[]     constraints

   "n" is the public composite modulus.  "e" is the public exponent.
   "d" is the private exponent.  "p" and "q" are its constituent private
   prime factors.  "iqmp" is the inverse of "q" modulo "p".  All these
   values except "iqmp" (which can be calculated from the others) are
   defined by Section 5.1 of [FIPS.186-5].

3.2.5.  Other keys

   Agents and their clients MAY support additional key types not
   documented here.  Vendor-specific key types MUST use the domain-
   qualified naming convention defined in Section 6 of [RFC4251] until
   code-points are allocated by IANA.

3.2.6.  Adding keys from a token

   Keys hosted on smart-cards or other hardware tokens may be added
   using the SSH_AGENTC_ADD_SMARTCARD_KEY and
   SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED requests.  Note that
   "constraints" field is only included for the
   SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED variant of this message.





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       byte             SSH_AGENTC_ADD_SMARTCARD_KEY or
                        SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED
       string           token id
       string           PIN
       constraint[]     constraints

   Here "token id" is an opaque identifier for the hardware token and
   "PIN" is an optional password or PIN to unlock the key.  The
   interpretation of "token id" is not defined by the protocol but is
   left solely up to the agent.

   Typically only the public components of any keys supported on a
   hardware token will be loaded into an agent so, strictly speaking,
   this message really arranges future private key operations to be
   delegated to the hardware token in question.

   An agent MUST reply with SSH_AGENT_SUCCESS if one or more keys were
   successfully loaded as a result of one of these messages, or
   SSH_AGENT_FAILURE if no keys were found.  The agent MUST also return
   SSH_AGENT_FAILURE if the "token id" was not recognised or if the
   agent doesn't support token-hosted keys at all.

3.2.7.  Key Constraints

   A number of constraints may be used in the constrained variants of
   the key add messages.  Each constraint is represented by a type byte
   followed by zero or more value bytes.

   Zero or more constraints may be specified when adding a key with one
   of the *_CONSTRAINED requests.  Multiple constraints are appended
   consecutively to the end of the request:

       byte             constraint1_type
       byte[]           constraint1_data
       byte             constraint2_type
       byte[]           constraint2_data
       ....
       byte             constraintN_type
       byte[]           constraintN_data

   To fully parse a constraint, it is necessary to know its structure
   beforehand and it is not possible to safely recover when an
   unrecognised constraint is encountered.  Given this, if an agent does
   not recognise or support a requested constraint it MUST abort
   parsing, refuse the request and return an SSH_AGENT_FAILURE message
   to the client.

   The following constraints are defined.



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3.2.7.1.  Key lifetime constraint

   This constraint requests that the agent limit the key's lifetime by
   deleting it after the specified duration (in seconds) has elapsed
   from the time the key was added to the agent.

       byte             SSH_AGENT_CONSTRAIN_LIFETIME
       uint32           seconds

3.2.7.2.  Key confirmation constraint

   This constraint requests that the agent require explicit user
   confirmation for each private key operation using the key.  For
   example, the agent could present a confirmation dialog before
   completing a signature operation.

       byte             SSH_AGENT_CONSTRAIN_CONFIRM

3.2.7.3.  Constraint extensions

   Agents may implement experimental or private-use constraints through
   a extension constraint that supports named constraints.

       byte             SSH_AGENT_CONSTRAIN_EXTENSION
       string           extension name
       byte[]           extension-specific details

   The extension name MUST consist of a UTF-8 string suffixed by the
   implementation domain following the naming scheme defined in
   Section 6 of [RFC4251], e.g.  "foo@example.com".

   Note, given the above requirement to reject keys with unsupported
   constraints, a constraint extension is only usable when both client
   and agent support it.  Otherwise, the agent will be required to
   reject the key.  This is desirable, as the constraint extension may
   specify limits on the key that, if ignored, may result in the key
   being available in situations the user did not intend (i.e. the agent
   will fail in a safe way).

3.3.  Public key encoding

   Keys previously loaded into an agent are referred to by their public
   key blob, which is the standard SSH wire encoding for public keys.
   SSH protocol key encodings are defined in [RFC4253] for "ssh-rsa" and
   "ssh-dss" keys, in [RFC5656] for "ecdsa-sha2-*" keys and in [RFC8709]
   for "ssh-ed25519" and "ssh-ed448" keys.





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3.4.  Removing keys from the agent

   A client may request that an agent remove all keys that it stores:

       byte             SSH_AGENTC_REMOVE_ALL_IDENTITIES

   On receipt of such a message, an agent MUST delete all keys that it
   is holding and MUST reply with SSH_AGENT_SUCCESS.

   Specific keys may also be removed:

       byte             SSH_AGENTC_REMOVE_IDENTITY
       string           key blob

   Where "key blob" is the standard public key encoding of the key to be
   removed (Section 3.3).

   An agent MUST reply with SSH_AGENT_SUCCESS if the key was deleted or
   SSH_AGENT_FAILURE if it was not found.

   Token-hosted keys may be removed from an agent using:

       byte             SSH_AGENTC_REMOVE_SMARTCARD_KEY
       string           token id
       string           PIN

   Where "token id" is an opaque identifier for the hardware token and
   "PIN" is an optional password or PIN (not typically used), both
   encoded using UTF-8.  Requesting deletion of token-hosted keys MUST
   cause the agent to remove all keys it loaded from the device matching
   "token id".  Note: this operation affects the agent only, it SHOULD
   NOT caese the keys be deleted from the token itself.

   An agent MUST reply with SSH_AGENT_SUCCESS if the key was deleted or
   SSH_AGENT_FAILURE if it was not found.

3.5.  Requesting a list of keys

   A client may request a list of keys from an agent using the following
   message:

       byte             SSH_AGENTC_REQUEST_IDENTITIES

   The agent MUST reply with a message with the following preamble.

       byte             SSH_AGENT_IDENTITIES_ANSWER
       uint32           nkeys




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   Where "nkeys" indicates the number of keys to follow.  Following the
   preamble are zero or more keys, each encoded as:

       string           key blob
       string           comment

   Where "key blob" is the standard public key encoding of the key
   (Section 3.3) and "comment" is a human-readable comment encoded as a
   UTF-8 string.

3.6.  Private key operations

   A client may request the agent perform a private key signature
   operation using the following message:

       byte             SSH_AGENTC_SIGN_REQUEST
       string           key blob
       string           data
       uint32           flags

   Where "key blob" is the key requested to perform the signature
   (encoded as per Section 3.3), "data" is the data to be signed and
   "flags" is a bitfield containing the bitwise OR of zero or more
   signature flags (see below).

   If the agent does not support the requested flags, or is otherwise
   unable or unwilling to generate the signature (e.g.  because it
   doesn't have the specified key, or the user refused confirmation of a
   constrained key), it MUST reply with an SSH_AGENT_FAILURE message.

   On success, the agent MUST reply with:

       byte             SSH_AGENT_SIGN_RESPONSE
       string           signature

   The signature format is specific to the algorithm of the key type in
   use.  SSH protocol signature formats are defined in [RFC4253] for
   "ssh-rsa" and "ssh-dss" keys, in [RFC5656] for "ecdsa-sha2-*" keys
   and in [RFC8709] for "ssh-ed25519" and "ssh-ed448" keys.

3.6.1.  Signature flags

   Two flags are currently defined for signature request messages:
   SSH_AGENT_RSA_SHA2_256 and SSH_AGENT_RSA_SHA2_512 (defined in
   Section 6.3).  These two flags are only valid for "ssh-rsa" keys and
   request that the agent return a signature using the "rsa-sha2-256" or
   "rsa-sha2-512" signature methods respectively.  These signature
   schemes are defined in [RFC8332].



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3.7.  Locking and unlocking an agent

   The agent protocol supports requesting that an agent temporarily lock
   itself with a pass-phrase.  When locked, an agent MUST suspend
   processing of sensitive operations (private key signature operations
   at the very least) until it has been unlocked with the same pass-
   phrase.

   The following message requests agent locking

       byte             SSH_AGENTC_LOCK
       string           passphrase

   The agent MUST reply with SSH_AGENT_SUCCESS if locked successfully or
   SSH_AGENT_FAILURE otherwise (e.g. if the agent was already locked).

   The following message requests unlocking an agent:

       byte             SSH_AGENTC_UNLOCK
       string           passphrase

   If the agent is already locked and the pass-phrase matches the one
   used to lock it then it MUST unlock and reply with SSH_AGENT_SUCCESS.
   If the agent is already unlocked or if the pass-phrase does not match
   it MUST reply with SSH_AGENT_FAILURE.

3.8.  Extension mechanism

   The agent protocol includes an optional extension mechanism that
   allows vendor-specific and experimental messages to be sent via the
   agent protocol.  Extension requests from the client consist of:

       byte             SSH_AGENTC_EXTENSION
       string           extension type
       byte[]           extension request-specific contents

   The extension type indicates the type of the extension message as a
   UTF-8 string.  Implementation-specific extensions MUST be suffixed by
   the implementation domain following the extension naming scheme
   defined in Section 6 of [RFC4251], e.g. "foo@example.com".

   An agent that does not support extensions of the supplied type MUST
   reply with an empty SSH_AGENT_FAILURE message.  This reply is also
   sent by agents that do not support the extension mechanism at all.







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   The contents of successful extension reply messages are specific to
   the extension type.  Successful extension requests MUST return either
   SSH_AGENT_SUCCESS on success or an extension-specific response
   message:

       byte             SSH_AGENT_EXTENSION_RESPONSE
       string           extension type
       byte[]           extension response-specific contents

   Where the extension type is the same as that in the request.

   Extension failure SHOULD be signaled using a
   SSH_AGENT_EXTENSION_FAILURE message:

       byte             SSH_AGENT_EXTENSION_FAILURE

   Extensions SHOULD NOT use the standard SSH_AGENT_FAILURE message.
   This allows failed requests to be distinguished from the extension
   not being supported.

3.8.1.  Query extension

   A single, optional extension request "query" is defined to allow a
   client to query which, if any, extensions are supported by an agent.

       byte             SSH_AGENTC_EXTENSION
       string           "query"

   If an agent supports the query extension it SHOULD reply with a list
   of supported extension names.

       byte             SSH_AGENT_EXTENSION_RESPONSE
       string           "query"
       string[]         supported extension types

4.  Connecting to an agent

   Agents are exposed to the local system using a connection-oriented
   endpoint.  On Unix-like systems, it is typical to arrange for the
   agent to listen on a filesystem-based Unix domain socket.  On
   Microsoft Windows, it is usual to use a Windows Named Pipe.  Access
   to these endpoints should be controlled as discussed in Section 8.









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   In both cases, it is common to expose the name or address of the
   listening endpoint via an environment variable named "SSH_AUTH_SOCK".
   Clients of an agent will use this variable to locate and connect to
   the listening agent.  Agents alternately MAY use an implicit
   mechanism for clients to locate their endpoint, such as a default
   per-user location.

5.  Forwarding access to an agent

   The agent protocol may be forwarded over an SSH connection, using the
   [RFC4254] connection protocol, allowing agent forwarding to be
   requested for any session channel, using a model that is similar to
   the connection protocol's support for X11 Forwarding (Section 6.3 of
   [RFC4254]).  This feature is OPTIONAL for SSH protocol and agent
   implementations.

5.1.  Advertising agent forwarding support

   Support for agent forwarding may be advertised by an SSH server using
   the [RFC8308] extension mechanism using the name "agent-forward" in
   the SSH_MSG_EXT_INFO message.

       string           "agent-forward"
       string           "0" (version)

   Note that this protocol substantially predates the existence of the
   [RFC8308] extension mechanism and several widely-deployed SSH
   implementations that support agent forwarding do not advertise their
   ability to do so.  SSH Clients MAY opportunistically attempt to
   request agent forwarding in the absence of an [RFC8308] advertisement
   using the vendor-specific names mentioned below.  Likewise, SSH
   servers MAY implement the vendor-specific names in addition to the
   one described here.

5.2.  Requesting agent forwarding

   An SSH client may request agent forwarding for a previously-opened
   session (Section 6.1 of [RFC4254]) using the following channel
   request.  This request is sent after the channel has been opened, but
   before a shell, command or subsystem has been executed.

       byte             SSH_MSG_CHANNEL_REQUEST
       uint32           channel_id
       string           "agent-req" or "auth-agent-req@openssh.com"
       boolean          want_reply






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   Where channel_id is the identifier for an established session channel
   (as returned from a previous SSH_MSG_CHANNEL_OPEN request, and the
   want_reply flag indicates whether the SSH server should respond with
   a confirmation of whether the request was successful (as specified in
   Section 5.4 of [RFC4254])

   If an SSH server accepts this request, typically it will arrange to
   make a endpoint (e.g. a listening socket) available and advertise
   this fact to the subordinate session.  Most implementations on Unix-
   like systems do this by providing a user-private listening Unix
   domain socket and recording its location in an environment variable
   SSH_AUTH_SOCK.

   As mentioned previously, many deployed implementations only support
   the pre-standardisation "auth-agent-req@openssh.com" request name.
   The "agent-req" name SHOULD only be used if support was explicitly
   advertised as per Section 5.1.

5.3.  Agent connection requests

   After an SSH client has requested that a session have agent
   forwarding enabled, the SSH server later may request a connection to
   the forwarded agent.  The SSH server does this by requesting a
   dedicated channel to communicate with the SSH client's agent.

       byte             SSH_MSG_CHANNEL_OPEN
       string           "agent-connect" or "auth-agent@openssh.com"
       uint32           channel_id
       uint32           local_window
       uint32           local_maxpacket

   The channel_id, local_window and local_maxpacket fields should be
   interpreted as specified by Section 5.1 of [RFC4254].

   As above, the "agent-connect" open type name SHOULD only be used if
   support was explicitly advertised as per Section 5.1.

   An SSH client SHOULD be prepared to handle multiple concurrent
   forwarded connections to a client-side agent, otherwise requests to
   access the agent from the remote side that happen to overlap prior
   requests may fail.  Overlapping requests may occur because the SSH
   connection protocol [RFC4254] allows multiple user sessions over a
   single [RFC4253] transport, which may each request use of the agentcw
   independently and potentially concurrently.

   An SSH client MAY accept agent connection requests (subject to
   authorisation) without a prior agent forwarding request having been
   made to support the situation where agent forwarding without opening



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   a session is desired.  Similarly, an SSH client MAY continue to
   accept agent connection requests after the session for which agent
   forwarding was requested has closed.

   An SSH client MUST refuse unauthorised agent connection requests,
   when agent forwarding is neither requested nor desired by the SSH
   client but an SSH server sends an agent connection request anyway.

   Because the "agent-connect" request contains no identifier to
   distinguish which session channel originated the connection request,
   an SSH connection can effectively forward access to only a single SSH
   client-side agent using this protocol (although there may be multiple
   concurrent connections to that single agent).

6.  Protocol numbers

6.1.  Message type numbers

   The following numbers are used as message types for requests from the
   client to the agent.

       SSH_AGENTC_REQUEST_IDENTITIES                  11
       SSH_AGENTC_SIGN_REQUEST                        13
       SSH_AGENTC_ADD_IDENTITY                        17
       SSH_AGENTC_REMOVE_IDENTITY                     18
       SSH_AGENTC_REMOVE_ALL_IDENTITIES               19
       SSH_AGENTC_ADD_SMARTCARD_KEY                   20
       SSH_AGENTC_REMOVE_SMARTCARD_KEY                21
       SSH_AGENTC_LOCK                                22
       SSH_AGENTC_UNLOCK                              23
       SSH_AGENTC_ADD_ID_CONSTRAINED                  25
       SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED       26
       SSH_AGENTC_EXTENSION                           27

   The following numbers are used as message types for replies from the
   agent to the client.

       SSH_AGENT_FAILURE                               5
       SSH_AGENT_SUCCESS                               6
       SSH_AGENT_IDENTITIES_ANSWER                     12
       SSH_AGENT_SIGN_RESPONSE                         14
       SSH_AGENT_EXTENSION_FAILURE                     28
       SSH_AGENT_EXTENSION_RESPONSE                    29








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6.1.1.  Reserved message type numbers

   The following message type numbers are reserved for implementations
   that implement support for the legacy SSH protocol version 1: 1-4,
   7-10, 15-16 and 24 (inclusive).  These message numbers MAY be used by
   an implementation supporting the legacy protocol but MUST NOT be
   reused otherwise.

   Message number 0 is also reserved and MUST NOT be used.

   The range of message numbers 240-255 are reserved for Private Use
   extensions to the agent protocol and MUST NOT be used by generic
   implementations.

6.2.  Constraint identifiers

   The following numbers are used to identify key constraints.  These
   are only used in key constraints and are not sent as message numbers.

       SSH_AGENT_CONSTRAIN_LIFETIME                    1
       SSH_AGENT_CONSTRAIN_CONFIRM                     2
       SSH_AGENT_CONSTRAIN_EXTENSION                   255

   The constraint identifier 0 is reserved.

6.3.  Signature flags

   The following numbers may be present in signature request
   (SSH_AGENTC_SIGN_REQUEST) messages.  These flags form a bit field by
   taking the logical OR of zero or more flags.

       SSH_AGENT_RSA_SHA2_256                          0x00000002
       SSH_AGENT_RSA_SHA2_512                          0x00000004

   The flag value 1 is reserved for historical implementations.

7.  IANA Considerations

   This protocol requires four registries be established, one for
   message type numbers, one for constraints, one for signature request
   flags and one for extension request names.  Additionally, new
   codepoints are requested in three existing registries.









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7.1.  Guidance for Designated Experts

   When a Designated Expert (DE) is asked to review additions to the new
   registries described above (Section 7.2, Section 7.3, Section 7.4 and
   Section 7.5), they are requested to verify that suitable
   documentation as described in [RFC5226] exists and is permanently and
   publicly available.  The DE is also requested to check the clarity of
   purpose and use of the requested code points.  The DE should also
   verify that specifications produced in the IETF that request code
   points in these registries have been made available to the SSHM
   working group and the ssh@ietf.org mailing list for review.  Requests
   for code points made for specifications produced outside the IETF
   should not conflict with active IETF work or prior IETF
   specifications.

   The available number of code points in the SSH agent protocol numbers
   (Section 7.2) and SSH agent signature flags (Section 7.4) registries
   are limited, so the DE is requested to ensure the use of code points
   is very well justified.  For the SSH agent protocol numbers, named
   extension requests (Section 7.5) provide an alternative for most uses
   with no practical limitation on the number of available code points.

7.2.  New registry: SSH agent protocol message type numbers

   This registry, titled "SSH agent protocol message type numbers"
   records the message type numbers for client requests and agent
   responses.  It should be created in the Secure Shell (SSH) Protocol
   Parameters registry group [IANA-SSH].  Its initial state should
   consist of the following numbers and reservations.  Future message
   number allocations shall occur via EXPERT REVIEW as per [RFC8126].

   +===========+==========================================+===========+
   | Number(s) | Identifier                               | Reference |
   +===========+==========================================+===========+
   | 0         | reserved                                 | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1.1     |
   +-----------+------------------------------------------+-----------+
   | 1         | reserved                                 | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1.1     |
   +-----------+------------------------------------------+-----------+
   | 2         | reserved                                 | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1.1     |
   +-----------+------------------------------------------+-----------+
   | 3         | reserved                                 | thisrfc,  |
   |           |                                          | Section   |



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   |           |                                          | 6.1.1     |
   +-----------+------------------------------------------+-----------+
   | 4         | reserved                                 | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1.1     |
   +-----------+------------------------------------------+-----------+
   | 5         | SSH_AGENT_FAILURE                        | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1       |
   +-----------+------------------------------------------+-----------+
   | 6         | SSH_AGENT_SUCCESS                        | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1       |
   +-----------+------------------------------------------+-----------+
   | 7         | reserved                                 | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1.1     |
   +-----------+------------------------------------------+-----------+
   | 8         | reserved                                 | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1.1     |
   +-----------+------------------------------------------+-----------+
   | 9         | reserved                                 | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1.1     |
   +-----------+------------------------------------------+-----------+
   | 10        | reserved                                 | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1.1     |
   +-----------+------------------------------------------+-----------+
   | 11        | SSH_AGENTC_REQUEST_IDENTITIES            | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1       |
   +-----------+------------------------------------------+-----------+
   | 12        | SSH_AGENT_IDENTITIES_ANSWER              | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1       |
   +-----------+------------------------------------------+-----------+
   | 13        | SSH_AGENTC_SIGN_REQUEST                  | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1       |
   +-----------+------------------------------------------+-----------+
   | 14        | SSH_AGENT_SIGN_RESPONSE                  | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1       |
   +-----------+------------------------------------------+-----------+
   | 15        | reserved                                 | thisrfc,  |
   |           |                                          | Section   |



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   |           |                                          | 6.1.1     |
   +-----------+------------------------------------------+-----------+
   | 16        | reserved                                 | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1.1     |
   +-----------+------------------------------------------+-----------+
   | 17        | SSH_AGENTC_ADD_IDENTITY                  | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1       |
   +-----------+------------------------------------------+-----------+
   | 18        | SSH_AGENTC_REMOVE_IDENTITY               | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1       |
   +-----------+------------------------------------------+-----------+
   | 19        | SSH_AGENTC_REMOVE_ALL_IDENTITIES         | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1       |
   +-----------+------------------------------------------+-----------+
   | 20        | SSH_AGENTC_ADD_SMARTCARD_KEY             | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1       |
   +-----------+------------------------------------------+-----------+
   | 21        | SSH_AGENTC_REMOVE_SMARTCARD_KEY          | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1       |
   +-----------+------------------------------------------+-----------+
   | 22        | SSH_AGENTC_LOCK                          | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1       |
   +-----------+------------------------------------------+-----------+
   | 23        | SSH_AGENTC_UNLOCK                        | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1       |
   +-----------+------------------------------------------+-----------+
   | 24        | reserved                                 | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1.1     |
   +-----------+------------------------------------------+-----------+
   | 25        | SSH_AGENTC_ADD_ID_CONSTRAINED            | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1       |
   +-----------+------------------------------------------+-----------+
   | 26        | SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1       |
   +-----------+------------------------------------------+-----------+
   | 27        | SSH_AGENTC_EXTENSION                     | thisrfc,  |
   |           |                                          | Section   |



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   |           |                                          | 6.1       |
   +-----------+------------------------------------------+-----------+
   | 28        | SSH_AGENT_EXTENSION_FAILURE              | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1       |
   +-----------+------------------------------------------+-----------+
   | 29        | SSH_AGENT_EXTENSION_RESPONSE             | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1       |
   +-----------+------------------------------------------+-----------+
   | 240-255   | Private Use                              | thisrfc,  |
   |           |                                          | Section   |
   |           |                                          | 6.1       |
   +-----------+------------------------------------------+-----------+

                                 Table 1

7.3.  New registry: SSH agent key constraint numbers

   This registry, titled "SSH agent key constraint numbers" records the
   message numbers for key use constraints.  It should be created in the
   Secure Shell (SSH) Protocol Parameters registry group [IANA-SSH].
   Its initial state should consist of the following numbers.  Future
   key constraint number allocations shall occur via EXPERT REVIEW as
   per [RFC8126].

     +========+===============================+======================+
     | Number | Identifier                    | Reference            |
     +========+===============================+======================+
     | 1      | SSH_AGENT_CONSTRAIN_LIFETIME  | thisrfc, Section 6.2 |
     +--------+-------------------------------+----------------------+
     | 2      | SSH_AGENT_CONSTRAIN_CONFIRM   | thisrfc, Section 6.2 |
     +--------+-------------------------------+----------------------+
     | 255    | SSH_AGENT_CONSTRAIN_EXTENSION | thisrfc, Section 6.2 |
     +--------+-------------------------------+----------------------+

                                  Table 2

7.4.  New registry: SSH agent signature flags

   This registry, titled "SSH agent signature flags" records the values
   for signature request (SSH_AGENTC_SIGN_REQUEST) flag values.  It
   should be created in the Secure Shell (SSH) Protocol Parameters
   registry group [IANA-SSH].  Its initial state should consist of the
   following numbers.  Note that as the flags are combined by bitwise
   OR, all flag values must be powers of two and the maximum available
   flag value is 0x80000000.




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   Future signature flag allocations shall occur via EXPERT REVIEW as
   per [RFC8126].

        +========+========================+======================+
        | Number | Identifier             | Reference            |
        +========+========================+======================+
        | 0x01   | reserved               | thisrfc, Section 6.3 |
        +--------+------------------------+----------------------+
        | 0x02   | SSH_AGENT_RSA_SHA2_256 | thisrfc, Section 6.3 |
        +--------+------------------------+----------------------+
        | 0x04   | SSH_AGENT_RSA_SHA2_512 | thisrfc, Section 6.3 |
        +--------+------------------------+----------------------+

                                 Table 3

7.5.  New registry: SSH agent extension request names

   This registry, titled "SSH agent extension request names" records the
   names used in the generic extension request message
   (SSH_AGENTC_EXTENSION).  It should be created in the Secure Shell
   (SSH) Protocol Parameters registry group [IANA-SSH].  Its initial
   state should consist of the following names.

   Future name allocations shall occur via EXPERT REVIEW as per
   [RFC8126].

                +================+========================+
                | Extension Name | Reference              |
                +================+========================+
                | query          | thisrfc, Section 3.8.1 |
                +----------------+------------------------+

                                  Table 4

7.6.  Additions to SSH Extension Names

   IANA is requested to insert the following entries into the table
   Extension Names [IANA-SSH-EXT] in the Secure Shell (SSH) Protocol
   Parameters registry group [IANA-SSH].

                 +================+======================+
                 | Extension Name | Reference            |
                 +================+======================+
                 | agent-forward  | thisrfc, Section 5.1 |
                 +----------------+----------------------+

                                  Table 5




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7.7.  Additions to SSH Connection Protocol Channel Request Names

   IANA is requested to insert the following entries into the table
   Connection Protocol Channel Request Names [IANA-SSH-CHANREQ] in the
   Secure Shell (SSH) Protocol Parameters registry group [IANA-SSH].

                  +==============+======================+
                  | Request Type | Reference            |
                  +==============+======================+
                  | agent-req    | thisrfc, Section 5.2 |
                  +--------------+----------------------+

                                  Table 6

7.8.  Additions to SSH Connection Protocol Channel Types

   IANA is requested to insert the following entries into the table
   Connection Protocol Channel Types [IANA-SSH-CHANTYPE] under Secure
   Shell (SSH) Protocol Parameters [IANA-SSH].

                 +===============+======================+
                 | Request Type  | Reference            |
                 +===============+======================+
                 | agent-connect | thisrfc, Section 5.3 |
                 +---------------+----------------------+

                                 Table 7

8.  Security Considerations

   The agent is a service that is tasked with retaining and providing
   controlled access to what are typically long-lived login
   authentication credentials.  It is by nature a sensitive and trusted
   software component.  Moreover, the agent protocol itself does not
   include any authentication or transport security; ability to
   communicate with an agent is usually sufficient to invoke it to
   perform private key operations.

   Since being able to access an agent is usually sufficient to perform
   private key operations, it is critically important that the agent
   only be exposed to its owner and their authorised delegates.  On
   Unix-like systems this may be achieved via filesystem permissions on
   the agent socket and/or identity checks on the client connected to a
   socket (e.g. SO_PEERCRED on some Unix-like systems).  On Windows,
   access to a named pipe may controlled by attaching a security
   descriptor at the time of its creation.





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   The primary design intention of an agent is that an attacker with
   unprivileged access to their victim's agent should be prevented from
   gaining a copy of any keys that have been loaded into it.  This may
   not preclude the attacker from stealing use of those keys (e.g.  if
   they have been loaded without a confirmation constraint).

   Given this, the agent should, as far as possible, prevent its memory
   being read by other processes to prevent theft of loaded keys.  This
   typically includes disabling debugging interfaces and preventing
   process memory dumps on abnormal termination.

   Another, more subtle, means by which keys may be stolen are via
   cryptographic side-channels.  Private key operations may leak
   information about the contents of keys via differences in timing,
   power use or by side-effects in the memory subsystems (e.g. CPU
   caches) of the host running the agent.  For the case of a local
   attacker and an agent holding unconstrained keys, the only limit on
   the number of private key operations the attacker may be able to
   observe is the rate at which the CPU can perform signatures.  This
   grants the attacker an almost ideal oracle for side-channel attacks.
   While a full treatment of side-channel attacks is beyond the scope of
   this specification, agents SHOULD use cryptographic implementations
   that are resistant to side-channel attacks and MAY take additional
   measures to hide the actual time spent processing private key
   operations.  Failure to do so may expose keys to recovery through
   these side-channels.

   Forwarding access to a local agent over an SSH connection (Section 5)
   inherently creates a transitive trust relationship.  SSH
   implementations SHOULD NOT forward use of an agent by default, as
   doing so could expose access to the user's keys to untrusted hosts
   they connect to.  Agents SHOULD implement additional controls over
   key visibility and use for forwarded agent connections, otherwise the
   user has only an all-or-nothing choice over whether to forward an
   agent.

   Implementation of token/smartcard-hosted keys requires some care too.
   On some systems, tokens may invoked by providing a path to shared
   library that must be loaded to make use of keys hosted on the device
   (a path to a library for a particular PKCS#11 module, for example).
   Loading a shared library on most platforms implies automatic
   execution of code in that library in the address space of the process
   that loads it.  To avoid loading of potentially-hostile code, agents
   that support loading token-hosted keys via library path SHOULD ensure
   that only trusted token provider libraries are loadable.  Additionaly
   agents SHOULD ensure that loaded token library code cannot gain
   access to other keys loaded in the agent and MAY disallow remote
   clients from loading token keys entirely.  Protection for existing



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   keys from tokey library code may be achieved by loading the token
   library into a separate process to the agent and arranging for the
   agent to invoke token operations to this process via IPC.

   Finally, with respect to the agent locking functionality in
   Section 3.7, an agent SHOULD take countermeasures against brute-force
   guessing attacks against the pass-phrase.  This may take the form of
   enforced delays when an unlock attempt is made with an incorrect
   password (potentially increasing for subsequent failures), a lockout
   period where the agent refuses to accept further requests after some
   threshold of failed unlock attempts has been made and/or deletion of
   all keys held by the agent after a threshold of failed unlock
   attempts.

9.  Implementation Status

   This section is to be removed before publishing as an RFC.

   Note to editor: please also remove the orphaned reference to RFC7942.

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   According to [RFC7942], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

   The following projects maintain an implementation of this protocol:

   OpenSSH  OpenSSH is the originating implementation of this protocol
      and has supported it since 2000.

      Website: https://www.openssh.com/

   PuTTY  PuTTY is a popular SSH client implementation for multiple



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      platforms that has included a compatible agent client since 2001.

      Website: https://www.chiark.greenend.org.uk/~sgtatham/putty/

   Dropbear  Dropbear is an SSH client and server implementation for
      Unix- like systems.  It has supported the agent protocol since
      2005.

      Website: https://matt.ucc.asn.au/dropbear/dropbear.html

   Paramiko  Paramiko is an SSH client and server implementation in the
      Python programming language.  It has supported an agent protocol
      implementation since 2005.

      Website: https://www.paramiko.org/

   Golang x/crypto/ssh/agent  The Go programming language project has
      supported an implementation of this protocol in its external "x"
      repository since 2015.

      Website: https://pkg.go.dev/golang.org/x/crypto/ssh/agent

   This list is not exhaustive.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4251]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251,
              January 2006, <https://www.rfc-editor.org/info/rfc4251>.

   [RFC4253]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
              January 2006, <https://www.rfc-editor.org/info/rfc4253>.

   [RFC4254]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Connection Protocol", RFC 4254, DOI 10.17487/RFC4254,
              January 2006, <https://www.rfc-editor.org/info/rfc4254>.







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   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <https://www.rfc-editor.org/info/rfc5226>.

   [RFC5656]  Stebila, D. and J. Green, "Elliptic Curve Algorithm
              Integration in the Secure Shell Transport Layer",
              RFC 5656, DOI 10.17487/RFC5656, December 2009,
              <https://www.rfc-editor.org/info/rfc5656>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8308]  Bider, D., "Extension Negotiation in the Secure Shell
              (SSH) Protocol", RFC 8308, DOI 10.17487/RFC8308, March
              2018, <https://www.rfc-editor.org/info/rfc8308>.

   [RFC8332]  Bider, D., "Use of RSA Keys with SHA-256 and SHA-512 in
              the Secure Shell (SSH) Protocol", RFC 8332,
              DOI 10.17487/RFC8332, March 2018,
              <https://www.rfc-editor.org/info/rfc8332>.

   [RFC8709]  Harris, B. and L. Velvindron, "Ed25519 and Ed448 Public
              Key Algorithms for the Secure Shell (SSH) Protocol",
              RFC 8709, DOI 10.17487/RFC8709, February 2020,
              <https://www.rfc-editor.org/info/rfc8709>.

   [FIPS.186-4]
              National Institute of Standards and Technology, "Digital
              Signature Standard (DSS)", FIPS PUB 186-4,
              DOI 10.6028/NIST.FIPS.186-4, July 2013,
              <https://doi.org/10.6028/NIST.FIPS.186-4>.

   [FIPS.186-5]
              National Institute of Standards and Technology, "Digital
              Signature Standard (DSS)", FIPS PUB 186-5,
              DOI 10.6028/NIST.FIPS.186-5, February 2023,
              <https://doi.org/10.6028/NIST.FIPS.186-5>.

10.2.  Informative References





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   [RFC4252]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252,
              January 2006, <https://www.rfc-editor.org/info/rfc4252>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [IANA-SSH-CHANREQ]
              IANA, "Connection Protocol Channel Types",
              <https://www.iana.org/assignments/ssh-parameters/>.

   [IANA-SSH] IANA, "Secure Shell (SSH) Protocol Parameters",
              <https://www.iana.org/assignments/ssh-parameters/ssh-
              parameters.xhtml>.

   [IANA-SSH-CHANTYPE]
              IANA, "Extension Names",
              <https://www.iana.org/assignments/ssh-parameters/>.

   [IANA-SSH-EXT]
              IANA, "Connection Protocol Channel Request Names",
              <https://www.iana.org/assignments/ssh-parameters/>.

   [draft-ietf-secsh-agent-02]
              Ylonen, T., Rinne, T. J., and S. Lehtinen, "Secure Shell
              Authentication Agent Protocol", January 2004,
              <https://datatracker.ietf.org/doc/html/draft-ietf-secsh-
              agent-02>.

Acknowledgments

   This protocol was designed and first implemented by Markus Friedl,
   based on a similar protocol for an agent to support the legacy SSH
   version 1 by Tatu Ylonen.

   Thanks to Simon Tatham, Niels Möller, James Spencer, Simon Josefsson,
   Matt Johnston, Jakub Jelen, Rich Salz, Caspar Schutijser, Florian
   Obser, Martin Thomson, Deb Cooley and Tero Kivinen who reviewed and
   helped improve this document.





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Author's Address

   Damien Miller
   OpenSSH
   Email: djm@openssh.com
   URI:   https://www.openssh.com/













































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