Internet Engineering Task Force (IETF) D. Miller
Request for Comments: 9987 OpenSSH
Category: Standards Track May 2026
ISSN: 2070-1721
Secure Shell (SSH) Agent Protocol
Abstract
This document specifies a key agent protocol for use in the Secure
Shell (SSH) protocol.
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9987.
Copyright Notice
Copyright (c) 2026 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
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Table of Contents
1. Introduction
2. Requirements Language
3. Protocol Overview
4. Terminology and Units
5. Protocol Messages
5.1. Generic Agent Responses
5.2. Adding Keys to the Agent
5.2.1. DSA Keys
5.2.2. ECDSA Keys
5.2.3. EdDSA Keys
5.2.4. RSA Keys
5.2.5. Other Keys
5.2.6. Adding Keys from a Token
5.2.7. Key Constraints
5.3. Public Key Encoding
5.4. Removing Keys from the Agent
5.5. Requesting a List of Keys
5.6. Private Key Operations
5.6.1. Signature Flags
5.7. Locking and Unlocking an Agent
5.8. Extension Mechanism
5.8.1. Query Extension
6. Connecting to an Agent
7. Forwarding Access to an Agent
7.1. Advertising Agent Forwarding Support
7.2. Requesting Agent Forwarding
7.3. Agent Connection Requests
8. Protocol Numbers
8.1. Message Type Numbers
8.1.1. Reserved Message Type Numbers
8.2. Constraint Identifiers
8.3. Signature Flags
9. IANA Considerations
9.1. Guidance for Designated Experts
9.2. "SSH Agent Protocol Message Type Numbers" Registry
9.3. "SSH Agent Key Constraint Numbers" Registry
9.4. "SSH Agent Key Constraint Extension Names" Registry
9.5. "SSH Agent Signature Flags" Registry
9.6. "SSH Agent Extension Request Names" Registry
9.7. Additions to the "Extension Names" Registry
9.8. Additions to the "Connection Protocol Channel Request
Names" Registry
9.9. Additions to the "Connection Protocol Channel Types"
Registry
10. Security Considerations
11. References
11.1. Normative References
11.2. Informative References
Acknowledgments
Author's Address
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 passphrase. 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 that may be subject to special
protection from the wider system.
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.
3. Protocol Overview
The agent protocol is a packetised request-response protocol that is
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 perform signature operations
using previously loaded keys.
Agents MAY implement support for only a subset of available key types
and MAY additionally refuse some operations in particular contexts.
For example, an agent may allow only clients local to itself to add
keys or may 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.
4. 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 Secure Shell client as
described in [RFC4251], then the client 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.
5. 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 8.1.
5.1. 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 with 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 with 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.
5.2. 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 SSH_AGENTC_ADD_IDENTITY message is:
byte SSH_AGENTC_ADD_IDENTITY
string key type
byte[] key data
string comment
Here "key type" is the specified key type name, for example, "ssh-
rsa" for an RSA key as defined by [RFC4253]. The "key data" consists
of the public and private components of the key and varies by key
type, as specified in Sections 5.2.1 through 5.2.4 for commonly used
key types. A "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 message 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 5.2.7 details constraint types and their formats. Clients
SHOULD prefer the SSH_AGENTC_ADD_IDENTITY message over sending an
SSH_AGENTC_ADD_ID_CONSTRAINED message 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.
Adding a key that is already present in an agent SHOULD replace any
constraints it was previously loaded with those (if any) that are
present in the subsequent add request, as this ensures that security-
relevant constraints on a loaded key best match user expectations.
Otherwise, an agent MAY refuse to load a key that has already been
loaded.
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 an SSH_AGENT_FAILURE reply.
5.2.1. DSA Keys
Digital Signature Algorithm (DSA) keys have key type name "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.
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", and "g" values are the DSA domain parameters. The "y"
and "x" values are the public and private keys, respectively. These
values are as defined by Section 4.1 of [FIPS.186-4].
5.2.2. ECDSA Keys
Elliptic Curve Digital Signature Algorithm (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].
5.2.3. EdDSA Keys
[RFC8709] defines Edwards-curve Digital Signature Algorithm (EdDSA)
keys (see [RFC8032]) 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
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].
5.2.4. RSA Keys
RSA keys have key type name "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 of
these values, except "iqmp" (which can be calculated from the
others), are defined by Section 5.1 of [FIPS.186-5].
5.2.5. 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
codepoints are allocated by IANA [IANA-PUBKEYS].
5.2.6. 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 the
"constraints" field is only included for the
SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED variant of this message.
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: it 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; thus, strictly speaking,
this message really arranges for 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 with
SSH_AGENT_FAILURE if no keys were found. The agent MUST also return
SSH_AGENT_FAILURE if the "token id" was not recognised, if the
request was against agent policy, or if the agent doesn't support
token-hosted keys at all.
5.2.7. 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; 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 subsections describe the constraints that have been
defined.
5.2.7.1. 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
5.2.7.2. 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
5.2.7.3. Constraint Extensions
Agents may implement experimental or private-use constraints through
an 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. Vendor
extensions MUST be 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 the
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 safely).
5.3. 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.
5.4. 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 SHOULD delete all keys that it
is holding and reply with SSH_AGENT_SUCCESS; otherwise, it MUST reply
with SSH_AGENT_FAILURE if the request was refused.
This request SHOULD be honoured regardless of any agent policy that
limits actions that a given client may take; otherwise, a user would
be unable to quickly and completely remove their keys in an urgent
situation.
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 5.3).
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 SHOULD
cause the agent to remove all keys it loaded from the device matching
"token id". Similarly to SSH_AGENTC_REMOVE_ALL_IDENTITIES, agents
SHOULD honour this request regardless of local policy to allow fast
and complete removal of keys. Note: this operation Removing a token-hosted key affects
the agent only; it SHOULD NOT cause the keys be deleted from the
token itself.
An agent MUST reply with SSH_AGENT_SUCCESS if the keys were deleted
or SSH_AGENT_FAILURE if none were found.
5.5. 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
Where "nkeys" indicates the number of keys to follow. Following the
preamble are zero or more keys, representing the keys the agent makes
available to the client with each encoded as:
string key blob
string comment
Where "key blob" is the standard public key encoding of the key
(Section 5.3) and "comment" is a human-readable comment encoded as a
UTF-8 string.
5.6. Private Key Operations
A client may request that 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 5.3), "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 (for example, 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.
5.6.1. 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 8.3). 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].
5.7. Locking and Unlocking an Agent
The agent protocol supports instructing an agent to temporarily lock
itself with a passphrase. 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
passphrase.
The following message instructs an agent to lock itself:
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 passphrase 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
passphrase does not match, it MUST reply with SSH_AGENT_FAILURE.
5.8. 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.
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 an
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.
5.8.1. 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
6. 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 10.
Multiple clients may access a single agent by making connections to
these sockets.
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. Alternatively, agents MAY use an implicit
mechanism for clients to locate their endpoint, such as a default
per-user location.
7. Forwarding Access to an Agent
The
Using the connection protocol described in [RFC4254], the agent
protocol may be forwarded over an SSH connection, using the
[RFC4254] connection protocol, allowing connection. This allows agent
forwarding to be requested for any session channel, 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 the SSH
protocol and agent implementations.
7.1. Advertising Agent Forwarding Support
Support for agent forwarding may be advertised by an SSH server using
the extension mechanism described in [RFC8308] 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
extension mechanism described in [RFC8308]. Further note that
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 advertisement (see [RFC8308]) using the vendor-specific names
mentioned below. Likewise, SSH servers MAY implement the vendor-
specific names in addition to the one described here.
7.2. Requesting Agent Forwarding
An SSH client may request agent forwarding for a previously opened
session (see 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
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 an 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 7.1.
7.3. Agent Connection Requests
After an SSH client has requested that a session have agent
forwarding enabled, the SSH server 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 7.1.
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 transport (see [RFC4253]), which may each request use of the
agentcw
agent 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
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).
8. Protocol Numbers
8.1. 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
8.1.1. 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 is reserved for Private Use
extensions to the agent protocol and MUST NOT be used by generic
implementations (see [RFC8126] for more information on Private Use).
8.2. 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.
8.3. 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.
9. IANA Considerations
This protocol describes the establishment of five registries: one for
message type numbers, one for constraint numbers, one for signature
request flags, one for constraint extension names, and one for
extension request names. Additionally, new codepoints are requested
in three existing registries.
9.1. Guidance for Designated Experts
When a Designated Expert (DE) is asked to review additions to the new
registries described in this document (Section 9.2, Section 9.3,
Section 9.5, and Section 9.6), they are requested to verify that
suitable documentation as described in [RFC8126] exists and is
permanently and publicly available. The DE is also requested to
check the clarity of purpose and use of the requested codepoints.
The DE should also verify that specifications produced in the IETF
that request codepoints in these registries have been made available
to the SSHM Working Group and the ssh@ietf.org mailing list for
review. Requests for codepoints made for specifications produced
outside the IETF should not conflict with active IETF work or prior
IETF specifications.
The available number of codepoints in the SSH agent protocol numbers
(Section 9.2), constraint numbers (Section 9.3), and SSH agent
signature flags (Section 9.5) registries are limited, so the DE is
requested to ensure the use of codepoints is very well justified.
For the SSH agent protocol message numbers, named extension requests
(Section 9.6) provide an alternative for most uses with no practical
limitation on the number of available codepoints. For key constraint
numbers, the constraint extension mechanism (Section 5.2.7.3)
provides a similar alternative that is not limited by available
codepoints.
9.2. "SSH Agent Protocol Message Type Numbers" Registry
The "SSH Agent Protocol Message Type Numbers" registry records the
message type numbers for client requests and agent responses. It is
located in the "Secure Shell (SSH) Protocol Parameters" registry
group [IANA-SSH]. Its initial state consists of the following
numbers and reservations. Future message number allocations shall
occur via Expert Review as per [RFC8126].
+=========+==========================================+=============+
| Number | Identifier | Reference |
+=========+==========================================+=============+
| 0 | Reserved | RFC 9987, |
| | | Section |
| | | 8.1.1 |
+---------+------------------------------------------+-------------+
| 1 | Reserved | RFC 9987, |
| | | Section |
| | | 8.1.1 |
+---------+------------------------------------------+-------------+
| 2 | Reserved | RFC 9987, |
| | | Section |
| | | 8.1.1 |
+---------+------------------------------------------+-------------+
| 3 | Reserved | RFC 9987, |
| | | Section |
| | | 8.1.1 |
+---------+------------------------------------------+-------------+
| 4 | Reserved | RFC 9987, |
| | | Section |
| | | 8.1.1 |
+---------+------------------------------------------+-------------+
| 5 | SSH_AGENT_FAILURE | RFC 9987, |
| | | Sections |
| | | 5.1 and 8.1 |
+---------+------------------------------------------+-------------+
| 6 | SSH_AGENT_SUCCESS | RFC 9987, |
| | | Sections |
| | | 5.1 and 8.1 |
+---------+------------------------------------------+-------------+
| 7 | Reserved | RFC 9987, |
| | | Section |
| | | 8.1.1 |
+---------+------------------------------------------+-------------+
| 8 | Reserved | RFC 9987, |
| | | Section |
| | | 8.1.1 |
+---------+------------------------------------------+-------------+
| 9 | Reserved | RFC 9987, |
| | | Section |
| | | 8.1.1 |
+---------+------------------------------------------+-------------+
| 10 | Reserved | RFC 9987, |
| | | Section |
| | | 8.1.1 |
+---------+------------------------------------------+-------------+
| 11 | SSH_AGENTC_REQUEST_IDENTITIES | RFC 9987, |
| | | Sections |
| | | 5.5 and 8.1 |
+---------+------------------------------------------+-------------+
| 12 | SSH_AGENT_IDENTITIES_ANSWER | RFC 9987, |
| | | Sections |
| | | 5.5 and 8.1 |
+---------+------------------------------------------+-------------+
| 13 | SSH_AGENTC_SIGN_REQUEST | RFC 9987, |
| | | Sections |
| | | 5.6 and 8.1 |
+---------+------------------------------------------+-------------+
| 14 | SSH_AGENT_SIGN_RESPONSE | RFC 9987, |
| | | Sections |
| | | 5.6 and 8.1 |
+---------+------------------------------------------+-------------+
| 15 | Reserved | RFC 9987, |
| | | Section |
| | | 8.1.1 |
+---------+------------------------------------------+-------------+
| 16 | Reserved | RFC 9987, |
| | | Section |
| | | 8.1.1 |
+---------+------------------------------------------+-------------+
| 17 | SSH_AGENTC_ADD_IDENTITY | RFC 9987, |
| | | Sections |
| | | 5.2 and 8.1 |
+---------+------------------------------------------+-------------+
| 18 | SSH_AGENTC_REMOVE_IDENTITY | RFC 9987, |
| | | Sections |
| | | 5.4 and 8.1 |
+---------+------------------------------------------+-------------+
| 19 | SSH_AGENTC_REMOVE_ALL_IDENTITIES | RFC 9987, |
| | | Sections |
| | | 5.4 and 8.1 |
+---------+------------------------------------------+-------------+
| 20 | SSH_AGENTC_ADD_SMARTCARD_KEY | RFC 9987, |
| | | Sections |
| | | 5.2.6 and |
| | | 8.1 |
+---------+------------------------------------------+-------------+
| 21 | SSH_AGENTC_REMOVE_SMARTCARD_KEY | RFC 9987, |
| | | Sections |
| | | 5.4 and 8.1 |
+---------+------------------------------------------+-------------+
| 22 | SSH_AGENTC_LOCK | RFC 9987, |
| | | Sections |
| | | 5.7 and 8.1 |
+---------+------------------------------------------+-------------+
| 23 | SSH_AGENTC_UNLOCK | RFC 9987, |
| | | Sections |
| | | 5.7 and 8.1 |
+---------+------------------------------------------+-------------+
| 24 | Reserved | RFC 9987, |
| | | Section |
| | | 8.1.1 |
+---------+------------------------------------------+-------------+
| 25 | SSH_AGENTC_ADD_ID_CONSTRAINED | RFC 9987, |
| | | Sections |
| | | 5.2 and 8.1 |
+---------+------------------------------------------+-------------+
| 26 | SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED | RFC 9987, |
| | | Sections |
| | | 5.2.6 and |
| | | 8.1 |
+---------+------------------------------------------+-------------+
| 27 | SSH_AGENTC_EXTENSION | RFC 9987, |
| | | Sections |
| | | 5.8 and 8.1 |
+---------+------------------------------------------+-------------+
| 28 | SSH_AGENT_EXTENSION_FAILURE | RFC 9987, |
| | | Sections |
| | | 5.8 and 8.1 |
+---------+------------------------------------------+-------------+
| 29 | SSH_AGENT_EXTENSION_RESPONSE | RFC 9987, |
| | | Sections |
| | | 5.8 and 8.1 |
+---------+------------------------------------------+-------------+
| 240-255 | Private Use | RFC 9987, |
| | | Section 8.1 |
+---------+------------------------------------------+-------------+
Table 1
9.3. "SSH Agent Key Constraint Numbers" Registry
The "SSH Agent Key Constraint Numbers" registry records the message
numbers for key use constraints. It is located in the "Secure Shell
(SSH) Protocol Parameters" registry group [IANA-SSH]. Its initial
state is as follows. Future key constraint number allocations shall
occur via Expert Review as per [RFC8126].
+========+===============================+=======================+
| Number | Identifier | Reference |
+========+===============================+=======================+
| 0 | Reserved | RFC 9987, Section 8.2 |
+--------+-------------------------------+-----------------------+
| 1 | SSH_AGENT_CONSTRAIN_LIFETIME | RFC 9987, Section 8.2 |
+--------+-------------------------------+-----------------------+
| 2 | SSH_AGENT_CONSTRAIN_CONFIRM | RFC 9987, Section 8.2 |
+--------+-------------------------------+-----------------------+
| 255 | SSH_AGENT_CONSTRAIN_EXTENSION | RFC 9987, Section 8.2 |
+--------+-------------------------------+-----------------------+
Table 2
9.4. "SSH Agent Key Constraint Extension Names" Registry
The "SSH Agent Key Constraint Extension Names" registry records the
names used in the SSH_AGENT_CONSTRAIN_EXTENSION constraint extension
type (Section 5.2.7.3). It is located in the "Secure Shell (SSH)
Protocol Parameters" registry group [IANA-SSH]. Its initial state is
empty. Future key constraint extension name allocations shall occur
via Expert Review as per [RFC8126].
9.5. "SSH Agent Signature Flags" Registry
The "SSH Agent Signature Flags" registry records the values for
signature request (SSH_AGENTC_SIGN_REQUEST) flag values. It is
located in the "Secure Shell (SSH) Protocol Parameters" registry
group [IANA-SSH]. Its initial state consists 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.
Future signature flag allocations shall occur via Expert Review as
per [RFC8126].
+========+========================+=======================+
| Number | Identifier | Reference |
+========+========================+=======================+
| 0x01 | Reserved | RFC 9987, Section 8.3 |
+--------+------------------------+-----------------------+
| 0x02 | SSH_AGENT_RSA_SHA2_256 | RFC 9987, Section 8.3 |
+--------+------------------------+-----------------------+
| 0x04 | SSH_AGENT_RSA_SHA2_512 | RFC 9987, Section 8.3 |
+--------+------------------------+-----------------------+
Table 3
9.6. "SSH Agent Extension Request Names" Registry
The "SSH Agent Extension Request Names" registry records the names
used in the generic extension request message (SSH_AGENTC_EXTENSION).
It is located in the "Secure Shell (SSH) Protocol Parameters"
registry group [IANA-SSH]. Its initial state consists of the
following names.
Future name allocations shall occur via Expert Review as per
[RFC8126].
+================+=========================+
| Extension Name | Reference |
+================+=========================+
| query | RFC 9987, Section 5.8.1 |
+----------------+-------------------------+
Table 4
9.7. Additions to the "Extension Names" Registry
IANA has added the following entries to the "Extension Names"
registry [IANA-SSH-EXT] in the "Secure Shell (SSH) Protocol
Parameters" registry group [IANA-SSH].
+================+=======================+
| Extension Name | Reference |
+================+=======================+
| agent-forward | RFC 9987, Section 7.1 |
+----------------+-----------------------+
Table 5
9.8. Additions to the "Connection Protocol Channel Request Names"
Registry
IANA has added the following entries to the "Connection Protocol
Channel Request Names" registry [IANA-SSH-CHANREQ] in the "Secure
Shell (SSH) Protocol Parameters" registry group [IANA-SSH].
+==============+=======================+
| Request Type | Reference |
+==============+=======================+
| agent-req | RFC 9987, Section 7.2 |
+--------------+-----------------------+
Table 6
9.9. Additions to the "Connection Protocol Channel Types" Registry
IANA has added the following entries to the "Connection Protocol
Channel Types" registry [IANA-SSH-CHANTYPE] under the "Secure Shell
(SSH) Protocol Parameters" registry group [IANA-SSH].
+===============+=======================+
| Channel Type | Reference |
+===============+=======================+
| agent-connect | RFC 9987, Section 7.3 |
+---------------+-----------------------+
Table 7
10. 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 be controlled by attaching a security
descriptor at the time of its creation.
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
from being read by other processes to prevent theft of loaded keys.
Typically, this includes disabling debugging interfaces and
preventing process memory dumps on abnormal termination.
Another, more subtle, means by which keys may be stolen is 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 7)
inherently creates a transitive trust relationship. SSH
implementations SHOULD NOT forward use of an agent by default, and
users SHOULD NOT forward use of an agent to hosts that are not fully
trusted, as doing so could expose access to the user's keys to
attackers on remote hosts. Agents SHOULD implement additional
controls over key visibility and use for forwarded agent connections;
otherwise, the user has only an all-or-nothing choice about whether
to forward an agent.
Implementation of token/smartcard-hosted keys hosted by a token or smartcard requires some
care, too. On some systems, tokens may be invoked by providing a
path to a 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 the loading of
potentially hostile code, agents that support loading token-hosted
keys via a library path SHOULD ensure that only trusted token
provider libraries are loadable. Additionally, 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 keys from a token
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 5.7, an agent SHOULD take countermeasures against brute-force
guessing attacks on the passphrase. 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.
11. References
11.1. Normative References
[FIPS.186-4]
NIST, "Digital Signature Standard (DSS)", NIST FIPS 186-4,
DOI 10.6028/NIST.FIPS.186-4, June 2013,
<https://doi.org/10.6028/NIST.FIPS.186-4>.
[FIPS.186-5]
NIST, "Digital Signature Standard (DSS)", NIST FIPS 186-5,
DOI 10.6028/NIST.FIPS.186-5, February 2023,
<https://doi.org/10.6028/NIST.FIPS.186-5>.
[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>.
[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]
NIST, "Digital Signature Standard (DSS)", NIST FIPS 186-4,
DOI 10.6028/NIST.FIPS.186-4, June 2013,
<https://doi.org/10.6028/NIST.FIPS.186-4>.
[FIPS.186-5]
NIST, "Digital Signature Standard (DSS)", NIST FIPS 186-5,
DOI 10.6028/NIST.FIPS.186-5, February 2023,
<https://doi.org/10.6028/NIST.FIPS.186-5>.
11.2. Informative References
[IANA-PUBKEYS]
IANA, "Public Key Algorithm Names",
<https://www.iana.org/assignments/ssh-parameters/>.
[IANA-SSH] IANA, "Secure Shell (SSH) Protocol Parameters",
<https://www.iana.org/assignments/ssh-parameters/>.
[IANA-SSH-CHANREQ]
IANA, "Connection Protocol Channel Types",
<https://www.iana.org/assignments/ssh-parameters/>.
[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/>.
[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>.
[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/>.
[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/>.
[IANA-PUBKEYS]
IANA, "Public Key Algorithm Names",
<https://www.iana.org/assignments/ssh-parameters/>.
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.
Author's Address
Damien Miller
OpenSSH
Email: djm@openssh.com
URI: https://www.openssh.com/