This describes the protocol used by OpenSSH's ssh-agent.
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OpenSSH's agent supports managing keys for the standard SSH protocol
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2 as well as the legacy SSH protocol 1. Support for these key types
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is almost completely disjoint - in all but a few cases, operations on
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protocol 2 keys cannot see or affect protocol 1 keys and vice-versa.
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Protocol 1 and protocol 2 keys are separated because of the differing
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cryptographic usage: protocol 1 private RSA keys are used to decrypt
|
challenges that were encrypted with the corresponding public key,
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whereas protocol 2 RSA private keys are used to sign challenges with
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a private key for verification with the corresponding public key. It
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is considered unsound practice to use the same key for signing and
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encryption.
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With a couple of exceptions, the protocol message names used in this
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document indicate which type of key the message relates to. SSH_*
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messages refer to protocol 1 keys only. SSH2_* messages refer to
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protocol 2 keys. Furthermore, the names also indicate whether the
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message is a request to the agent (*_AGENTC_*) or a reply from the
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agent (*_AGENT_*). Section 3 below contains the mapping of the
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protocol message names to their integer values.
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1. Data types
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Because of support for legacy SSH protocol 1 keys, OpenSSH's agent
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protocol makes use of some data types not defined in RFC 4251.
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1.1 uint16
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The "uint16" data type is a simple MSB-first 16 bit unsigned integer
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encoded in two bytes.
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1.2 mpint1
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The "mpint1" type represents an arbitrary precision integer (bignum).
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Its format is as follows:
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uint16 bits
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byte[(bits + 7) / 8] bignum
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"bignum" contains an unsigned arbitrary precision integer encoded as
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eight bits per byte in big-endian (MSB first) format.
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Note the difference between the "mpint1" encoding and the "mpint"
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encoding defined in RFC 4251. Also note that the length of the encoded
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integer is specified in bits, not bytes and that the byte length of
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the integer must be calculated by rounding up the number of bits to the
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nearest eight.
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2. Protocol Messages
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All protocol messages are prefixed with their length in bytes, encoded
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as a 32 bit unsigned integer. Specifically:
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uint32 message_length
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byte[message_length] message
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The following message descriptions refer only to the content the
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"message" field.
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2.1 Generic server responses
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The following generic messages may be sent by the server in response to
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requests from the client. On success the agent may reply either with:
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byte SSH_AGENT_SUCCESS
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or a request-specific success message.
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On failure, the agent may reply with:
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byte SSH_AGENT_FAILURE
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SSH_AGENT_FAILURE messages are also sent in reply to unknown request
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types.
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2.2 Adding keys to the agent
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Keys are added to the agent using the SSH_AGENTC_ADD_RSA_IDENTITY and
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SSH2_AGENTC_ADD_IDENTITY requests for protocol 1 and protocol 2 keys
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respectively.
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Two variants of these requests are SSH_AGENTC_ADD_RSA_ID_CONSTRAINED
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and SSH2_AGENTC_ADD_ID_CONSTRAINED - these add keys with optional
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"constraints" on their usage.
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OpenSSH may be built with support for keys hosted on a smartcard
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or other hardware security module. These keys may be added
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to the agent using the SSH_AGENTC_ADD_SMARTCARD_KEY and
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SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED requests.
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2.2.1 Key constraints
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The OpenSSH agent supports some basic optional constraints on key usage.
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At present there are two constraints defined.
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The first constraint limits the validity duration of a key. It is
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encoded as:
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byte SSH_AGENT_CONSTRAIN_LIFETIME
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uint32 seconds
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Where "seconds" contains the number of seconds that the key shall remain
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valid measured from the moment that the agent receives it. After the
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validity period has expired, OpenSSH's agent will erase these keys from
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memory.
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The second constraint requires the agent to seek explicit user
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confirmation before performing private key operations with the loaded
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key. This constraint is encoded as:
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byte SSH_AGENT_CONSTRAIN_CONFIRM
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Zero or more constraints may be specified when adding a key with one
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of the *_CONSTRAINED requests. Multiple constraints are appended
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consecutively to the end of the request:
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byte constraint1_type
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.... constraint1_data
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byte constraint2_type
|
.... constraint2_data
|
....
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byte constraintN_type
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.... constraintN_data
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Such a sequence of zero or more constraints will be referred to below
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as "constraint[]". Agents may determine whether there are constraints
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by checking whether additional data exists in the "add key" request
|
after the key data itself. OpenSSH will refuse to add a key if it
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contains unknown constraints.
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2.2.2 Add protocol 1 key
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A client may add a protocol 1 key to an agent with the following
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request:
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byte SSH_AGENTC_ADD_RSA_IDENTITY or
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SSH_AGENTC_ADD_RSA_ID_CONSTRAINED
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uint32 ignored
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mpint1 rsa_n
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mpint1 rsa_e
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mpint1 rsa_d
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mpint1 rsa_iqmp
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mpint1 rsa_q
|
mpint1 rsa_p
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string key_comment
|
constraint[] key_constraints
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Note that there is some redundancy in the key parameters; a key could be
|
fully specified using just rsa_q, rsa_p and rsa_e at the cost of extra
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computation.
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"key_constraints" may only be present if the request type is
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SSH_AGENTC_ADD_RSA_ID_CONSTRAINED.
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The agent will reply with a SSH_AGENT_SUCCESS if the key has been
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successfully added or a SSH_AGENT_FAILURE if an error occurred.
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2.2.3 Add protocol 2 key
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The OpenSSH agent supports DSA, ECDSA and RSA keys for protocol 2. DSA
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keys may be added using the following request
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byte SSH2_AGENTC_ADD_IDENTITY or
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SSH2_AGENTC_ADD_ID_CONSTRAINED
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string "ssh-dss"
|
mpint dsa_p
|
mpint dsa_q
|
mpint dsa_g
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mpint dsa_public_key
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mpint dsa_private_key
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string key_comment
|
constraint[] key_constraints
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DSA certificates may be added with:
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byte SSH2_AGENTC_ADD_IDENTITY or
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SSH2_AGENTC_ADD_ID_CONSTRAINED
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string "ssh-dss-cert-v00@openssh.com"
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string certificate
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mpint dsa_private_key
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string key_comment
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constraint[] key_constraints
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ECDSA keys may be added using the following request
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byte SSH2_AGENTC_ADD_IDENTITY or
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SSH2_AGENTC_ADD_ID_CONSTRAINED
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string "ecdsa-sha2-nistp256" |
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"ecdsa-sha2-nistp384" |
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"ecdsa-sha2-nistp521"
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string ecdsa_curve_name
|
string ecdsa_public_key
|
mpint ecdsa_private
|
string key_comment
|
constraint[] key_constraints
|
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ECDSA certificates may be added with:
|
byte SSH2_AGENTC_ADD_IDENTITY or
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SSH2_AGENTC_ADD_ID_CONSTRAINED
|
string "ecdsa-sha2-nistp256-cert-v01@openssh.com" |
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"ecdsa-sha2-nistp384-cert-v01@openssh.com" |
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"ecdsa-sha2-nistp521-cert-v01@openssh.com"
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string certificate
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mpint ecdsa_private_key
|
string key_comment
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constraint[] key_constraints
|
|
ED25519 keys may be added using the following request
|
byte SSH2_AGENTC_ADD_IDENTITY or
|
SSH2_AGENTC_ADD_ID_CONSTRAINED
|
string "ssh-ed25519"
|
string ed25519_public_key
|
string ed25519_private_key || ed25519_public_key
|
string key_comment
|
constraint[] key_constraints
|
|
ED25519 certificates may be added with:
|
byte SSH2_AGENTC_ADD_IDENTITY or
|
SSH2_AGENTC_ADD_ID_CONSTRAINED
|
string "ssh-ed25519-cert-v01@openssh.com"
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string certificate
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string ed25519_public_key
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string ed25519_private_key || ed25519_public_key
|
string key_comment
|
constraint[] key_constraints
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|
For both ssh-ed25519 and ssh-ed25519-cert-v01@openssh.com keys, the private
|
key has the public key appended (for historical reasons).
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RSA keys may be added with this request:
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byte SSH2_AGENTC_ADD_IDENTITY or
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SSH2_AGENTC_ADD_ID_CONSTRAINED
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string "ssh-rsa"
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mpint rsa_n
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mpint rsa_e
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mpint rsa_d
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mpint rsa_iqmp
|
mpint rsa_p
|
mpint rsa_q
|
string key_comment
|
constraint[] key_constraints
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RSA certificates may be added with this request:
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|
byte SSH2_AGENTC_ADD_IDENTITY or
|
SSH2_AGENTC_ADD_ID_CONSTRAINED
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string "ssh-rsa-cert-v00@openssh.com"
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string certificate
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mpint rsa_d
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mpint rsa_iqmp
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mpint rsa_p
|
mpint rsa_q
|
string key_comment
|
constraint[] key_constraints
|
|
Note that the 'rsa_p' and 'rsa_q' parameters are sent in the reverse
|
order to the protocol 1 add keys message. As with the corresponding
|
protocol 1 "add key" request, the private key is overspecified to avoid
|
redundant processing.
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For DSA, ECDSA and RSA key add requests, "key_constraints" may only be
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present if the request type is SSH2_AGENTC_ADD_ID_CONSTRAINED.
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The agent will reply with a SSH_AGENT_SUCCESS if the key has been
|
successfully added or a SSH_AGENT_FAILURE if an error occurred.
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2.2.4 Loading keys from a smartcard
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The OpenSSH agent may have optional smartcard support built in to it. If
|
so, it supports an operation to load keys from a smartcard. Technically,
|
only the public components of the keys are loaded into the agent so
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this operation really arranges for future private key operations to be
|
delegated to the smartcard.
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byte SSH_AGENTC_ADD_SMARTCARD_KEY or
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SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED
|
string reader_id
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string pin
|
constraint[] key_constraints
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|
"reader_id" is an identifier to a smartcard reader and "pin"
|
is a PIN or passphrase used to unlock the private key(s) on the
|
device. "key_constraints" may only be present if the request type is
|
SSH_AGENTC_ADD_SMARTCARD_KEY_CONSTRAINED.
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This operation may load all SSH keys that are unlocked using the
|
"pin" on the specified reader. The type of key loaded (protocol 1
|
or protocol 2) will be specified by the smartcard itself, it is not
|
client-specified.
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The agent will reply with a SSH_AGENT_SUCCESS if one or more keys have
|
been successfully loaded or a SSH_AGENT_FAILURE if an error occurred.
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The agent will also return SSH_AGENT_FAILURE if it does not support
|
smartcards.
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2.3 Removing multiple keys
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A client may request that an agent delete all protocol 1 keys using the
|
following request:
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byte SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES
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This message requests the deletion of all protocol 2 keys:
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byte SSH2_AGENTC_REMOVE_ALL_IDENTITIES
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|
On success, the agent will delete all keys of the requested type and
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reply with a SSH_AGENT_SUCCESS message. If an error occurred, the agent
|
will reply with SSH_AGENT_FAILURE.
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Note that, to delete all keys (both protocol 1 and 2), a client
|
must send both a SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES and a
|
SSH2_AGENTC_REMOVE_ALL_IDENTITIES request.
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2.4 Removing specific keys
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2.4.1 Removing a protocol 1 key
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|
Removal of a protocol 1 key may be requested with the following message:
|
|
byte SSH_AGENTC_REMOVE_RSA_IDENTITY
|
uint32 key_bits
|
mpint1 rsa_e
|
mpint1 rsa_n
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|
Note that key_bits is strictly redundant, as it may be inferred by the
|
length of rsa_n.
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|
The agent will delete any private key matching the specified public key
|
and return SSH_AGENT_SUCCESS. If no such key was found, the agent will
|
return SSH_AGENT_FAILURE.
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|
2.4.2 Removing a protocol 2 key
|
|
Protocol 2 keys may be removed with the following request:
|
|
byte SSH2_AGENTC_REMOVE_IDENTITY
|
string key_blob
|
|
Where "key_blob" is encoded as per RFC 4253 section 6.6 "Public Key
|
Algorithms" for any of the supported protocol 2 key types.
|
|
The agent will delete any private key matching the specified public key
|
and return SSH_AGENT_SUCCESS. If no such key was found, the agent will
|
return SSH_AGENT_FAILURE.
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|
2.4.3 Removing keys loaded from a smartcard
|
|
A client may request that a server remove one or more smartcard-hosted
|
keys using this message:
|
|
byte SSH_AGENTC_REMOVE_SMARTCARD_KEY
|
string reader_id
|
string pin
|
|
"reader_id" the an identifier to a smartcard reader and "pin" is a PIN
|
or passphrase used to unlock the private key(s) on the device.
|
|
When this message is received, and if the agent supports
|
smartcard-hosted keys, it will delete all keys that are hosted on the
|
specified smartcard that may be accessed with the given "pin".
|
|
The agent will reply with a SSH_AGENT_SUCCESS if one or more keys have
|
been successfully removed or a SSH_AGENT_FAILURE if an error occurred.
|
The agent will also return SSH_AGENT_FAILURE if it does not support
|
smartcards.
|
|
2.5 Requesting a list of known keys
|
|
An agent may be requested to list which keys it holds. Different
|
requests exist for protocol 1 and protocol 2 keys.
|
|
2.5.1 Requesting a list of protocol 1 keys
|
|
To request a list of protocol 1 keys that are held in the agent, a
|
client may send the following message:
|
|
byte SSH_AGENTC_REQUEST_RSA_IDENTITIES
|
|
The agent will reply with the following message:
|
|
byte SSH_AGENT_RSA_IDENTITIES_ANSWER
|
uint32 num_keys
|
|
Followed by zero or more consecutive keys, encoded as:
|
|
uint32 bits
|
mpint1 rsa_e
|
mpint1 rsa_n
|
string key_comment
|
|
2.5.2 Requesting a list of protocol 2 keys
|
|
A client may send the following message to request a list of
|
protocol 2 keys that are stored in the agent:
|
|
byte SSH2_AGENTC_REQUEST_IDENTITIES
|
|
The agent will reply with the following message header:
|
|
byte SSH2_AGENT_IDENTITIES_ANSWER
|
uint32 num_keys
|
|
Followed by zero or more consecutive keys, encoded as:
|
|
string key_blob
|
string key_comment
|
|
Where "key_blob" is encoded as per RFC 4253 section 6.6 "Public Key
|
Algorithms" for any of the supported protocol 2 key types.
|
|
2.6 Private key operations
|
|
The purpose of the agent is to perform private key operations, such as
|
signing and encryption without requiring a passphrase to unlock the
|
key and without allowing the private key itself to be exposed. There
|
are separate requests for the protocol 1 and protocol 2 private key
|
operations.
|
|
2.6.1 Protocol 1 private key challenge
|
|
The private key operation used in version 1 of the SSH protocol is
|
decrypting a challenge that has been encrypted with a public key.
|
It may be requested using this message:
|
|
byte SSH_AGENTC_RSA_CHALLENGE
|
uint32 ignored
|
mpint1 rsa_e
|
mpint1 rsa_n
|
mpint1 encrypted_challenge
|
byte[16] session_id
|
uint32 response_type /* must be 1 */
|
|
"rsa_e" and "rsa_n" are used to identify which private key to use.
|
"encrypted_challenge" is a challenge blob that has (presumably)
|
been encrypted with the public key and must be in the range
|
1 <= encrypted_challenge < 2^256. "session_id" is the SSH protocol 1
|
session ID (computed from the server host key, the server semi-ephemeral
|
key and the session cookie).
|
|
"ignored" and "response_type" exist for compatibility with legacy
|
implementations. "response_type" must be equal to 1; other response
|
types are not supported.
|
|
On receiving this request, the server decrypts the "encrypted_challenge"
|
using the private key matching the supplied (rsa_e, rsa_n) values. For
|
the response derivation, the decrypted challenge is represented as an
|
unsigned, big-endian integer encoded in a 32 byte buffer (i.e. values
|
smaller than 2^248 will have leading 0 bytes).
|
|
The response value is then calculated as:
|
|
response = MD5(decrypted_challenge || session_id)
|
|
and returned in the following message
|
|
byte SSH_AGENT_RSA_RESPONSE
|
byte[16] response
|
|
If the agent cannot find the key specified by the supplied (rsa_e,
|
rsa_n) then it will return SSH_AGENT_FAILURE.
|
|
2.6.2 Protocol 2 private key signature request
|
|
A client may use the following message to request signing of data using
|
a protocol 2 key:
|
|
byte SSH2_AGENTC_SIGN_REQUEST
|
string key_blob
|
string data
|
uint32 flags
|
|
Where "key_blob" is encoded as per RFC 4253 section 6.6 "Public Key
|
Algorithms" for any of the supported protocol 2 key types. "flags" is
|
a bit-mask, but at present only one possible value is defined (see below
|
for its meaning):
|
|
SSH_AGENT_OLD_SIGNATURE 1
|
|
Upon receiving this request, the agent will look up the private key that
|
corresponds to the public key contained in key_blob. It will use this
|
private key to sign the "data" and produce a signature blob using the
|
key type-specific method described in RFC 4253 section 6.6 "Public Key
|
Algorithms".
|
|
An exception to this is for "ssh-dss" keys where the "flags" word
|
contains the value SSH_AGENT_OLD_SIGNATURE. In this case, a legacy
|
signature encoding is used in lieu of the standard one. In this case,
|
the DSA signature blob is encoded as:
|
|
byte[40] signature
|
|
The signature will be returned in the response message:
|
|
byte SSH2_AGENT_SIGN_RESPONSE
|
string signature_blob
|
|
If the agent cannot find the key specified by the supplied key_blob then
|
it will return SSH_AGENT_FAILURE.
|
|
2.7 Locking or unlocking an agent
|
|
The agent supports temporary locking with a passphrase to suspend
|
processing of sensitive operations until it has been unlocked with the
|
same passphrase. To lock an agent, a client send the following request:
|
|
byte SSH_AGENTC_LOCK
|
string passphrase
|
|
Upon receipt of this message and if the agent is not already locked,
|
it will suspend processing requests and return a SSH_AGENT_SUCCESS
|
reply. If the agent is already locked, it will return SSH_AGENT_FAILURE.
|
|
While locked, the agent will refuse all requests except
|
SSH_AGENTC_UNLOCK, SSH_AGENTC_REQUEST_RSA_IDENTITIES and
|
SSH2_AGENTC_REQUEST_IDENTITIES. The "request identities" requests are
|
treated specially by a locked agent: it will always return an empty list
|
of keys.
|
|
To unlock an agent, a client may request:
|
|
byte SSH_AGENTC_UNLOCK
|
string passphrase
|
|
If the passphrase matches and the agent is locked, then it will resume
|
processing all requests and return SSH_AGENT_SUCCESS. If the agent
|
is not locked or the passphrase does not match then it will return
|
SSH_AGENT_FAILURE.
|
|
Locking and unlocking affects both protocol 1 and protocol 2 keys.
|
|
3. Protocol message numbers
|
|
3.1 Requests from client to agent for protocol 1 key operations
|
|
SSH_AGENTC_REQUEST_RSA_IDENTITIES 1
|
SSH_AGENTC_RSA_CHALLENGE 3
|
SSH_AGENTC_ADD_RSA_IDENTITY 7
|
SSH_AGENTC_REMOVE_RSA_IDENTITY 8
|
SSH_AGENTC_REMOVE_ALL_RSA_IDENTITIES 9
|
SSH_AGENTC_ADD_RSA_ID_CONSTRAINED 24
|
|
3.2 Requests from client to agent for protocol 2 key operations
|
|
SSH2_AGENTC_REQUEST_IDENTITIES 11
|
SSH2_AGENTC_SIGN_REQUEST 13
|
SSH2_AGENTC_ADD_IDENTITY 17
|
SSH2_AGENTC_REMOVE_IDENTITY 18
|
SSH2_AGENTC_REMOVE_ALL_IDENTITIES 19
|
SSH2_AGENTC_ADD_ID_CONSTRAINED 25
|
|
3.3 Key-type independent requests from client to agent
|
|
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
|
|
3.4 Generic replies from agent to client
|
|
SSH_AGENT_FAILURE 5
|
SSH_AGENT_SUCCESS 6
|
|
3.5 Replies from agent to client for protocol 1 key operations
|
|
SSH_AGENT_RSA_IDENTITIES_ANSWER 2
|
SSH_AGENT_RSA_RESPONSE 4
|
|
3.6 Replies from agent to client for protocol 2 key operations
|
|
SSH2_AGENT_IDENTITIES_ANSWER 12
|
SSH2_AGENT_SIGN_RESPONSE 14
|
|
3.7 Key constraint identifiers
|
|
SSH_AGENT_CONSTRAIN_LIFETIME 1
|
SSH_AGENT_CONSTRAIN_CONFIRM 2
|
|
$OpenBSD: PROTOCOL.agent,v 1.11 2016/05/19 07:45:32 djm Exp $
|
