docs/reference/prerequisites-ssh-key.md (33a5295) - derivepassphrase.git

prerequisites-ssh-key.md

# Prerequisites for using `derivepassphrase vault` with an SSH key

Using `derivepassphrase vault` with an SSH key requires:

1. [a running SSH agent](#ssh-agent),
 2. [a Python installation that can talk to the SSH
    agent](#python-support), and
 3. [a supported SSH key.](#ssh-key)

### A running SSH agent { #ssh-agent }

SSH agents are usually packaged as part of SSH client distributions.
Pageant from [PuTTY][] is known to work, on both UNIX and Windows.
`ssh-agent` from [OpenSSH][] is known to work well on UNIX, and
somewhat poorly on Windows; see notes below.
`gpg-agent` (v2) from [GnuPG][] is also known to work, but comes with
caveats; again, see notes below.

If in doubt, we recommend OpenSSH on UNIX and Pageant on Windows,
because they are the <i>de-facto</i> canonical SSH agent implementation
on their respective operating system.

!!! note "Agent-specific features"

* OpenSSH's `ssh-agent` supports limiting the time the agent holds
      the key in memory ("key lifetime"), on UNIX only.
      We recommend its usage.
    * `ssh-agent` and GnuPG's `gpg-agent` support requiring confirmation
      upon each use for a specific key.
      (Again, for `ssh-agent` only on UNIX.)
      We recommend its usage as well.

!!! note "Other agent-specific notes"

=== "OpenSSH on Windows"

Using OpenSSH on Windows with `derivepassphrase vault` is
        possible, but currently *not recommended*.
        [The Windows port of OpenSSH lacks support for key constraints
        [1]][OPENSSH_ON_WINDOWS_NO_KEY_CONSTRAINTS_1]
        [[2]][OPENSSH_ON_WINDOWS_NO_KEY_CONSTRAINTS_2], which are the
        main advantage OpenSSH had over Pageant.
        Without this functionality, Pageant is the better choice: more
        supported key formats, manually lockable and unlockable key
        material in the agent, and no Administrator credentials needed
        to configure the system.

Additionally, the ported SSH agent implements the agent
        protocol incorrectly because it terminates the connection when
        encountering an unsupported request, instead of returning an
        error code.
        (The original agent, on UNIX, implements the protocol
        correctly.)
        This makes it difficult for `derivepassphrase vault` to
        correctly *detect* or *verify* that it is talking to the
        OpenSSH agent on Windows, instead of merely *assuming* it is
        doing so.

[OPENSSH_ON_WINDOWS_NO_KEY_CONSTRAINTS_1]: https://github.com/PowerShell/Win32-OpenSSH/issues/1056#issuecomment-362494167 "Issue #1056: ssh-agent should support '-c' and '-t' options of ssh-add"
        [OPENSSH_ON_WINDOWS_NO_KEY_CONSTRAINTS_2]: https://github.com/PowerShell/Win32-OpenSSH/issues/2314#issuecomment-2529160589 'Issue #2314: Adding key with confirmation option to ssh-agent gives "agent refused operation"'

=== "GnuPG/`gpg-agent`"

* `gpg-agent` v2.0 and later uses a *persistent* database of
          known keys, SSH or otherwise.
          "Adding" a key to the agent actually means *importing* it, and
          requires choosing an "import passphrase" to protect the key on
          disk, in the persistent database.
          `gpg-agent` will cache the import passphrase in memory, and if
          that cache entry expires, then the *import passphrase* must be
          provided to unlock the key.

* The GnuPG distribution does not contain tools to generate
          native SSH keys or interactively add keys to a running
          `gpg-agent`, because its purpose is to expose keys in
          a different format (OpenPGP) to other (agent-compatible) SSH
          clients.
          A third-party tool (such as a full SSH client distribution) is
          necessary to load/import native SSH keys into `gpg-agent`.

* As a design consequence of the persistent database,
          `gpg-agent` always lists all known SSH keys as available in
          the agent.
          It is impossible to remove an SSH key from `gpg-agent` using
          standard SSH agent operations.

* `gpg-agent` does not advertise its communication socket by
          default, contrary to other SSH agents, so it must be manually
          advertised:

=== "UNIX"

~~~~ console
                $ SSH_AUTH_SOCK="$(gpgconf --list-dirs agent-ssh-socket)"
                $ export SSH_AUTH_SOCK
                ~~~~

=== "Windows (`libassuan` socket)"

`gpg-agent` on Windows contains a native emulation of
                UNIX domain sockets by the `assuan` library, which GnuPG
                internally uses for network connectivity and
                inter-process communication.
                No specific configuration is necessary, and the agent
                address can be directly obtained with `gpgconf`:

~~~~ console
                $ gpgconf --list-dirs agent-ssh-socket
                ~~~~

This mode, sadly, is not currently supported.
                The system is specific to GnuPG/`libassuan`, and
                unlikely to be deployed widely enough to make
                implementing this a priority for us… at least, relative
                to the other, more common interprocess communication
                channels for SSH agents on Windows.

=== "Windows (OpenSSH emulation)"

From v2.4 onwards, `gpg-agent` supports masquerading as
                OpenSSH's `ssh-agent` on Windows, by starting the agent
                with the `--enable-win32-openssh-support` command-line
                argument.
                (Usually, this would be added to the `gpg-agent`
                configuration file instead of being manually supplied on
                the command-line.)

This mode is untested.
                We, the `derivepassphrase` authors, do not know how to
                run `gpg-agent` on Windows with masquerading as OpenSSH
                enabled: the steps outlined in the official
                documentation for `gpg-agent` do not work as advertised.
                [We have asked for clarification on GnuPG's developer
                mailing list.][GPG_AGENT_ON_WINDOWS_WTF]

[GPG_AGENT_ON_WINDOWS_WTF]: https://lists.gnupg.org/pipermail/gnupg-devel/2025-December/036116.html

</section>

### A Python installation that can talk to the SSH agent { #python-support }

As of version 0.6, Windows is supported.
[Your Python installation must support interfacing with the Windows
system DLLs.][ctypes.WinDLL]

On non-Windows operating systems, the SSH agent is expected to advertise
its communication socket via the `SSH_AUTH_SOCK` environment variable,
which is common procedure.
Therefore, [your Python installation must support UNIX domain
sockets][socket.AF_UNIX].

`derivepassphrase vault --version` will report on supported and on
unavailable features, including "master SSH key":[^1]

[^1]: This indicates support in principle, on this
    hardware/software/system combination, for interfacing with an SSH
    agent.
    At runtime, this could still fail, e.g. because the SSH agent isn't
    actually running, because `derivepassphrase` is trying to connect to
    the wrong address, etc.

=== "supported"

~~~~ console
    $ derivepassphrase vault --version
    derivepassphrase 0.5
    Using cryptography 44.0.0
    Using click 8.1.8

Supported features: master SSH key.
    ~~~~

=== "unavailable"

~~~~ console
    $ derivepassphrase vault --version
    derivepassphrase 0.5
    Using cryptography 44.0.0
    Using click 8.1.8

Unavailable features: master SSH key.
    ~~~~

### A supported SSH key { #ssh-key }

For an SSH key to be usable by `derivepassphrase`, the SSH agent must
always generate the same signature for the same input, i.e. the
signature must be deterministic for this key type.
Commonly used SSH key types (as of August 2025) include [ECDSA][],
[Ed25519][], [RSA][], and, somewhat less commonly, [Ed448][] and
[DSA][].

[RSA]: https://en.wikipedia.org/wiki/RSA_(cryptosystem)
  [DSA]: https://en.wikipedia.org/wiki/Digital_Signature_Algorithm
  [ECDSA]: https://en.wikipedia.org/wiki/Elliptic_Curve_Digital_Signature_Algorithm
  [Ed25519]: https://en.wikipedia.org/wiki/EdDSA#Ed25519
  [Ed448]: https://en.wikipedia.org/wiki/EdDSA#Ed448

* RSA, Ed25519 and Ed448 signatures are deterministic by definition.
  Thus RSA, Ed25519 and Ed448 keys are supported under any SSH agent
  that implements them.

* DSA and ECDSA signatures require choosing a value specific to each
  signature (a "cryptographic nonce"), which must be unpredictable.
  Typical DSA/ECDSA implementations therefore generate a suitably large
  random number as the nonce.
  This makes signatures non-deterministic, and thus unsuitable for
  `derivepassphrase`.

??? info "Exception: PuTTY/Pageant and RFC 6979"

[RFC 6979][] specifies a method to *calculate* the nonce from
        the DSA/ECDSA key and the message to be signed.
        DSA/ECDSA signatures from SSH agents implementing RFC 6979 are
        therefore deterministic, and thus *also* suitable for
        `derivepassphrase`.
        Pageant 0.81 implements RFC 6979.

!!! warning "Warning: Pageant < 0.81"

Pageant 0.80 and earlier uses a different, homegrown method
            to calculate the nonce deterministically.
            Those versions are *also* prinicipally suitable for use with
            `derivepassphrase`, but **they generate different signatures
            -- and different derived passphrases -- than Pageant 0.81
            and later**.

??? info "What SSH key type do I have?"

If, according to your SSH agent, your key's type…

* …ends with `-cert-v01@openssh.com`, then, for the purposes of
        this list, ignore the `-cert-v01@openssh.com` suffix.
      * …is `dsa` or `ssh-dss`, or is `dsa` followed by a number, then
        your key type is **DSA**.
      * …is `rsa` or `ssh-rsa`, or is `rsa` followed by a number, then
        your key type is **RSA**.
      * …is `ecdsa` followed by a number, or is `ecdsa-sha2-nistp`
        followed by a number, then your key type is **ECDSA**.
      * …is `ssh-ed25519`, then your key type is **Ed25519**.
      * …is `ssh-ed448`, then your key type is **Ed448**.

If you do not yet have a (supported) SSH key, we recommend Ed25519 for
maximum speed and reasonable availability, otherwise RSA for maximum
availability.
We do not in general recommend Ed448 because it is not widely
implemented (as of August 2025).

??? example "Generating new SSH keys for `derivepassphrase`"

=== "OpenSSH"

The resulting key will be stored in
        `~/.ssh/my-vault-ed25519-key`, using "vault key" as a comment.
        Replace `-t ed25519` with `-t rsa` if generating an RSA key, and
        adapt the filename accordingly.

~~~~ console
        $ ssh-keygen -t ed25519 -f ~/.ssh/my-vault-ed25519-key -C "vault key"
        Generating public/private ed25519 key pair.
        Enter passphrase for ".../.ssh/my-vault-ed25519-key" (empty for no passphrase): 
        Enter same passphrase again:
        Your identification has been saved in .../.ssh/my-vault-ed25519-key
        Your public key has been saved in .../.ssh/my-vault-ed25519-key.pub
        The key fingerprint is:
        SHA256:0h+WAokssfhzfzVyuMLJlIcWyCtk5WiXI8BHyhXYxC0 vault key
        The key's randomart image is:
        +--[ED25519 256]--+
        |o B=+            |
        |.=oE = .         |
        |.oX @ +          |
        | = + o * . .     |
        |  + o * S B      |
        |   + * + O o     |
        |      * o .      |
        |       o         |
        |                 |
        +----[SHA256]-----+
        ~~~~

(The key fingerprint and the randomart image will naturally
        differ, as they are key-specific.)

=== "PuTTY"

The resulting key will be stored in
        `~/.ssh/my-vault-ed25519-key.ppk`, using "vault key" as a comment.
        Replace `-t ed25519` with `-t rsa` if generating an RSA key, and
        adapt the filename accordingly.

~~~~ console
        $ puttygen -t ed25519 -o ~/.ssh/my-vault-ed25519-key.ppk -C "vault key"
        Enter passphrase to save key: 
        Re-enter passphrase to verify: 
        ~~~~

=== "GnuPG"

Not supported natively.
        A different SSH client distribution such as OpenSSH or PuTTY is
        necessary to create SSH keys specifically.

Alternatively, GnuPG supports reusing keys in its native OpenPGP
        format for SSH as long as the underlying key type is compatible.
        First, obtain GnuPG's internal identifier (the "keygrip") for
        the correct key you may want to use.
        (Warning: OpenPGP subkeys have a different keygrip, so be sure
        to use the correct one.)

~~~~ console
        $ gpg --list-keys --with-keygrip sample-key@example.com
        pub   rsa4096 2025-07-27 [SC] [expires: 2025-08-01]
              675F056879A81925E3E0DE60370C2A7D2E40FF4C
              Keygrip = C71CB33DC50C9972EF9C135B0FB70D87B1491923
        uid           [ultimate] Sample Key <sample-key@example.com>
        sub   rsa4096 2025-07-27 [E] [expires: 2025-08-01]
              Keygrip = 84129D49C9A0654BDFAE2DACBC7A9D8C563FF884
        ~~~~

=== "before v2.3.7"

Add the keygrip (on a line of its own) to the `sshcontrol`
            file in the GnuPG configuration directory.

~~~~ console
            $ echo C71CB33DC50C9972EF9C135B0FB70D87B1491923 >> ~/.gnupg/sshcontrol
            ~~~~

=== "v2.3.7 and later"

Set a key attribute to permit this key's use in SSH:

~~~~ console
            $ gpg-connect-agent 'keyattr C71CB33DC50C9972EF9C135B0FB70D87B1491923 Use-for-ssh: true' /bye
            ~~~~

---

!!! abstract "Further reading"

→ [How to set up `derivepassphrase vault` with an SSH key][HOWTO]

[HOWTO]: ../how-tos/ssh-key.md
[GnuPG]: https://gnupg.org/
[BUG_WINDOWS_SSH_AGENT_SUPPORT]: ../wishlist/windows-ssh-agent-support.md
[OpenSSH]: https://www.openssh.com/
[PuTTY]: https://www.chiark.greenend.org.uk/~sgtatham/putty/
[PYTHON_AF_UNIX]: https://docs.python.org/3/library/socket.html#socket.AF_UNIX
[RFC 6979]: https://www.rfc-editor.org/rfc/rfc6979

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