| Internet-Draft | SSH Agent | August 2023 |
| Miller | Expires 15 February 2024 | [Page] |
This document describes a key agent protocol for use in the Secure Shell (SSH) protocol.¶
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 15 February 2024.¶
Copyright (c) 2023 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 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.¶
Secure Shell (SSH) is a protocol for secure remote connections and login over untrusted networks. It supports multiple authentication mechanisms, including public key authentication. This document describes the protocol for interacting with an agent that holds private keys. Clients (and possibly servers) can use 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. Moreover, the agent implements a simple protocol and presents a smaller attack surface than a key loaded into a full SSH server or client.¶
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.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].¶
The agent protocol is a packetised request-response protocol, solely driven by the client. It consists of a number of requests sent from the client to the server and a set of reply messages that are sent in response. At no time does the server send messages except in response to a client request. Replies are sent in order.¶
All values in the agent protocol are encoded using the SSH wire representations specified by [RFC4251]. Messages consist of a length, type and contents.¶
uint32 message length byte message type byte[message length - 1] message contents¶
The following generic messages may be sent by the server in response to requests from the client. On success the agent may reply either with:¶
byte SSH_AGENT_SUCCESS¶
or a request-specific success message. On failure, the agent may reply with:¶
byte SSH_AGENT_FAILURE¶
SSH_AGENT_FAILURE messages are also sent in reply to requests with unknown types.¶
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 contents
string key comment
¶
Here "type" is the specified key type name, for example "ssh-rsa" for a RSA key as defined by [RFC4253]. "contents" consists of the public and private components of the key and vary by key type, they are listed below for standard and commonly used key types. "comment" is an optional human-readable key name or comment as a UTF-8 string that may serve to identify the key in user-visible messages.¶
The SSH_AGENTC_ADD_ID_CONSTRAINED is similar, but adds a extra field:¶
byte SSH_AGENTC_ADD_ID_CONSTRAINED
string type
byte[] contents
string comment
constraint[] constraints
¶
Constraints are used to place limits on the validity or use of keys. Section 4.2.6 details constraint types and their format.¶
An agent should reply with SSH_AGENT_SUCCESS if the key was successfully loaded as a result of one of these messages, or SSH_AGENT_FAILURE otherwise.¶
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.¶
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 [FIPS.186-4].¶
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 [FIPS.186-4].¶
Ed25519 keys have key type "ssh-ed25519" and are defined in [I-D.ietf-curdle-ssh-ed25519]. They may be added to the agent using the following message. The "key 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"
string ENC(A)
string k || ENC(A)
string comment
constraint[] constraints
¶
The first value is the 32 byte Ed25519
public key
ENC(A).
The second value is a concatenation of
the 32 byte private key
k and
32 byte public
ENC(A) key.
The contents and interpretation of the
ENC(A)
and k values are
defined by
[I-D.irtf-cfrg-eddsa].¶
RSA keys have key type "ssh-rsa" and are defined in [RFC4253]. They may be added to the agent using the following message. The "key 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. "p" and "q" are its constituent private prime factors. "e" is the public exponent. "iqmp" is the inverse of "q" modulo "p". All these values except "iqmp" (which can be calculated from the others) are defined by [FIPS.186-4].¶
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.¶
byte SSH_AGENTC_ADD_SMARTCARD_KEY or
SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED
string id
string PIN
constraint[] constraints
¶
Here "id" is an opaque identifier for the hardware token and "PIN" is an optional password on PIN to unlock the key. The interpretation of "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 should 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 should also return SSH_AGENT_FAILURE if the token "id" was not recognised or if the agent doesn't support token-hosted keys at all.¶
A number of constraints and 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
¶
If an agent does not recognise or support a requested constraint it MUST refuse the request and return a SSH_AGENT_FAILURE message to the client.¶
The following constraints are defined.¶
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
¶
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
¶
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 4.2 of [RFC4251], e.g. "foo@example.com".¶
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 shall delete all keys that it is holding and 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. SSH protocol key encodings are defined in [RFC4253] for "ssh-rsa" and "ssh-dss" keys, in [RFC5656] for "ecdsa-sha2-*" keys and in [I-D.ietf-curdle-ssh-ed25519] for "ssh-ed25519" keys.¶
An agent shall reply with SSH_AGENT_SUCCESS if the key was deleted or SSH_AGENT_FAILURE if it was not found.¶
Smartcard keys may be removed using:¶
byte SSH_AGENTC_REMOVE_SMARTCARD_KEY
string reader id
string PIN
¶
Where "reader id" is an opaque identifier for the smartcard reader and "PIN" is an optional password or PIN (not typically used). Requesting deletion of smartcard-hosted keys will cause the agent to remove all keys loaded from that smartcard.¶
An agent shall reply with SSH_AGENT_SUCCESS if the key was deleted or SSH_AGENT_FAILURE if it was not found.¶
A client may request a list of keys from an agent using the following message:¶
byte SSH_AGENTC_REQUEST_IDENTITIES
¶
The agent shall 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, each encoded as:¶
string key blob
string comment
¶
Where "key blob" is the wire encoding of the public key and "comment" is a human-readable comment encoded as a UTF-8 string.¶
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, "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 a SSH_AGENT_FAILURE message.¶
On success, the agent shall 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 [I-D.ietf-curdle-ssh-ed25519] for "ssh-ed25519" keys.¶
Two flags are currently defined for signature request messages: SSH_AGENT_RSA_SHA2_256 and SSH_AGENT_RSA_SHA2_512. 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 [I-D.ietf-curdle-rsa-sha2].¶
The agent protocol supports requesting that an agent temporarily lock itself with a pass-phrase. When locked an agent should suspend processing of sensitive operations (private key 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 shall 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 should unlock and reply with SSH_AGENT_SUCCESS. If the agent is unlocked or if the the pass-phrase does not match it should reply with SSH_AGENT_FAILURE. An agent SHOULD take countermeasures against brute-force guessing attacks against the pass-phrase.¶
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 contents
¶
The extension type indicates the type of the extension message as a UTF-8 string. Implementation-specific extensions should be suffixed by the implementation domain following the extension naming scheme defined in Section 4.2 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. Extension requests may return SSH_AGENT_SUCCESS on success or some other extension-specific message.¶
Extension failure should be signaled using the SSH_AGENT_EXTENSION_FAILURE code - extensions should not use the standard SSH_AGENT_FAILURE message. This allows failed requests to be distinguished from the extension not being supported.¶
A single, optional extension request
query
is defined to allow a client to query
which, if any, extensions are supported
by an agent.¶
If an agent supports the
query
extension is should reply with
a list of supported extension names.¶
byte SSH_AGENT_SUCCESS
string[] extension type
¶
The following numbers are used 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_ID_CONSTRAINED 25
SSH_AGENTC_ADD_SMARTCARD_KEY 20
SSH_AGENTC_REMOVE_SMARTCARD_KEY 21
SSH_AGENTC_LOCK 22
SSH_AGENTC_UNLOCK 23
SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED 26
SSH_AGENTC_EXTENSION 27
¶
The following numbers are used for replies from the agent to the client.¶
SSH_AGENT_FAILURE 5
SSH_AGENT_SUCCESS 6
SSH_AGENT_EXTENSION_FAILURE 28
SSH_AGENT_IDENTITIES_ANSWER 12
SSH_AGENT_SIGN_RESPONSE 14
¶
The following message numbers are reserved for implementations that implement support for the legacy SSH protocol version 1: 1-4, 7-9 and 24 (inclusive). These message numbers MAY be used by an implementation supporting the legacy protocol but MUST NOT be reused otherwise.¶
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 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 2
SSH_AGENT_RSA_SHA2_512 4
¶
The flag value 1 is reserved for historical implementations.¶
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 who reviewed and helped improve this document.¶
This protocol requires three registries be established, one for message numbers, one for constraints and one for signature request flags.¶
This registry, titled "SSH agent protocol numbers" records the message numbers for client requests and agent responses. Its initial state should consist of the following numbers and reservations. Future message number allocations shall require specification in the form of an RFC (RFC REQUIRED as per [RFC5226]).¶
| Number | Identifier | Reference |
|---|---|---|
| 1 | reserved | Section 5.1 |
| 2 | reserved | Section 5.1 |
| 3 | reserved | Section 5.1 |
| 4 | reserved | Section 5.1 |
| 5 | SSH_AGENT_FAILURE | Section 5.1 |
| 6 | SSH_AGENT_SUCCESS | Section 5.1 |
| 7 | reserved | Section 5.1 |
| 8 | reserved | Section 5.1 |
| 9 | reserved | Section 5.1 |
| 10 | reserved | Section 5.1 |
| 11 | SSH_AGENTC_REQUEST_IDENTITIES | Section 5.1 |
| 12 | SSH_AGENT_IDENTITIES_ANSWER | Section 5.1 |
| 13 | SSH_AGENTC_SIGN_REQUEST | Section 5.1 |
| 14 | SSH_AGENT_SIGN_RESPONSE | Section 5.1 |
| 15 | reserved | Section 5.1 |
| 16 | reserved | Section 5.1 |
| 17 | SSH_AGENTC_ADD_IDENTITY | Section 5.1 |
| 18 | SSH_AGENTC_REMOVE_IDENTITY | Section 5.1 |
| 19 | SSH_AGENTC_REMOVE_ALL_IDENTITIES | Section 5.1 |
| 20 | SSH_AGENTC_ADD_SMARTCARD_KEY | Section 5.1 |
| 21 | SSH_AGENTC_REMOVE_SMARTCARD_KEY | Section 5.1 |
| 22 | SSH_AGENTC_LOCK | Section 5.1 |
| 23 | SSH_AGENTC_UNLOCK | Section 5.1 |
| 24 | reserved | Section 5.1 |
| 25 | SSH_AGENTC_ADD_ID_CONSTRAINED | Section 5.1 |
| 26 | SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED | Section 5.1 |
| 27 | SSH_AGENTC_EXTENSION | Section 5.1 |
| 28 | SSH_AGENT_EXTENSION_FAILURE | Section 5.1 |
This registry, titled "SSH agent key constraint numbers" records the message numbers for key use constraints. Its initial state should consist of the following numbers. Future constraint number allocations shall require specification in the form of an RFC (RFC REQUIRED as per [RFC5226]).¶
| Number | Identifier | Reference |
|---|---|---|
| 1 | SSH_AGENT_CONSTRAIN_LIFETIME | Section 5.2 |
| 2 | SSH_AGENT_CONSTRAIN_CONFIRM | Section 5.2 |
| 255 | SSH_AGENT_CONSTRAIN_EXTENSION | Section 5.2 |
This registry, titled "SSH agent signature flags records the values for signature request (SSH_AGENTC_SIGN_REQUEST) flag values. 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.¶
Future constraint number allocations shall require specification in the form of an RFC (RFC REQUIRED as per [RFC5226]).¶
| Number | Identifier | Reference |
|---|---|---|
| 0x01 | reserved | Section 5.3 |
| 0x02 | SSH_AGENT_RSA_SHA2_256 | Section 5.3 |
| 0x04 | SSH_AGENT_RSA_SHA2_512 | Section 5.3 |
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.¶
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 in to 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 direct theft of loaded keys. This typically include 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.¶