gnupg-pkcs11-scd - Man Page

GnuPG-compatible smart-card daemon with PKCS#11 support


gnupg-pkcs11-scd[--server] [--multi-server] [--daemon] [--homedir dir] [--uid-acl uid] [--verbose] [--quiet] [--sh] [--csh] [--options file] [--no-detach] [--log-file file] [--help]


gnupg-pkcs11-scd is a drop-in replacement for the smart-card daemon (scd) shipped with the next-generation GnuPG (gnupg-2). The daemon interfaces to smart-cards by using RSA Security Inc. PKCS#11 Cryptographic Token Interface (Cryptoki).

The interface with GnuPG is restricted to feching existing keys from the card. Neither new key generation nor key transfer is possible through this interface. Instead, when the smart-card is asked to generate a key in a particular slot, the existing public key in that slot is returned. This facilitates the transfer of keys on the smart-card to usage as a subkey on an existing GnuPG master key. See the Gnupg Integration section for example usage.

The following options are available:


Run in server mode (foreground). If not redirected, input and output are over stdin/stdout.


Run in multi-server mode (foreground). In addition to communicating over stdin/stdout, the server also opens an additional listening UNIX socket.


Detach and run in background.

--homedir dir

Use this home directory instead of guessing.

--uid-acl uid

Create unix socket as world read/write and apply access control that accepts only remote processes of this uid. Usable for proxy scenario.


Be verbose while running.


Be as quiet as possible.


Output sh-style environment variable definition.


Output csh-style environment variable definition.

--options file

Read options from file. Some of the configuration options can only be set in the configuration file (see the Configuration section).


Do not detach from console (useful for debugging purposes).

--log-file file

Output log to file.


Print help information.

When the daemon receives any of the SIGHUP, SIGTERM and SIGINT signals, it cleans up and exits.

gnupg-pkcs11-scd works only with already personalized cards, and supports (for the time being) only RSA keypairs. The following constraints must be satisfied:

  1. For each private key object, a certificate object must exist on the card. The existence of the corresponding public key object is not important (since the certificate includes public key).

  2. The certificate and the corresponding private key must have identical CKA_ID attribute.

The PKCS#11 implementation is not obliged to enforce any of the above rules. However, practice has shown that popular PKCS#11 implementations found "in the wild" seem to respect them.


Unlike gpg-agent, gnupg-pkcs11-scd supports more than one token available at the same time. In order to make gpg-agent happy, gnupg-pkcs11-scd always returns the same card serial number to gpg-agent. When unavailable token is requested, gnupg-pkcs11-scd will use NEEDPIN callback in order to ask for the requested token. When and if gpg-agent will support more than one serial number or NEEDTOKEN callback, this behavior will be modified.



Used to locate the home directory.


Used instead of ~/.gnupg.


Used only on Win32 to locate the home directory.


Create sockets in this directory, default to TMPDIR.

Additionally, the \\Software\\GNU\\GnuPG\\HomeDir registry key is used on Win32 to locate the default GNUPGHOME.


Files affecting the operation of gnupg-pkcs11-scd:


gnupg-pkcs11-scd uses this as a default configuration file.


gnupg-pkcs11-scd uses this as a default system wide configuration file.


Default configuration file for gpg-agent.


To tell gpg-agent to use another smart-card daemon, the following needs to be put in ~/.gnupg/gpg-agent.conf:

scdaemon-program /usr/bin/gnupg-pkcs11-scd
pinentry-program /usr/bin/pinentry-qt

The first line is mandatory in order to use gnupg-pkcs11-scd. With the second line you can set your preferred pinentry program (it has to be one compatible with GnuPG). Of course, you need to adjust the paths according to your system setup.

An example ~/.gnupg/gnupg-pkcs11-scd.conf file (lines beginning with # are comments):

# Log file.
#log-file log1

# Default is not verbose.

# Default is no debugging.

# Pin cache period in seconds; default is infinite.
#pin-cache 20

# Comma-separated list of available provider names. Then set
# attributes for each provider using the provider-[name]-attribute
# syntax.
providers p1

# Provider attributes (see below for detailed description)
provider-p1-library /usr/lib/pkcs11/
#provider-p1-private-mask 0

# The following are for gnupg-2.0 mode
#openpgp-sign 5C661B8C07CFD957F7D98D5B9A0F31D236BFAC2A
#openpgp-encr D2DC0BD1EDD185969748B6025B452816F97CBA57
#openpgp-auth A7B8C1A3A8F71FCEC018886F8767927B9C8D871F

The following attributes can be set for each provider:


Full path to the PKCS#11 shared library (= provider).


Allow protected authentication for provider. This needs to be supported by the provider and you should have appropriate reader hardware.


Authentication is required before certificates can be accessed. Most configurations store certificates as public, so there is no need to use this option.


Private key mask mode. Use this only when you have problem using private key operations. The value is hex encoded mask number.


Determine automatically.


Force sign.


Force sign with recovery.


Force decrypt.


Force decrypt with unwrap.


[gnupg-2.0] Hex string (Upper letter, no space) SHA1 of signing public key see Gnupg Integration how to obtain.


[gnupg-2.0] Hex string (Upper letter, no space) SHA1 of encryption public key see Gnupg Integration how to obtain.


[gnupg-2.0] Hex string (Upper letter, no space) SHA1 of authentication public key see Gnupg Integration how to obtain.

Gnupg Integration

Typical steps to set up a card for gpgsm usage:

  1. Import the CA certificate of your issuer:

    gpgsm --import < ca-certificate

    You should also manually import all self-signed certificates.

  2. Instruct GnuPG to discover all useful certificates on the card:

    gpgsm --learn-card

Signing, verification, etc. work as usual with gpgsm.

Typical steps to set up a card for gpg-2.0 usage:

  1. Acquire key ids:

    gpg-agent --server gpg-connect-agent

    Enter "SCD LEARN" and look for "KEY-FRIEDNLY" responses, the first field is the hash, the second is the subject name.

  2. Instruct GnuPG to discover all useful information of card:

    gpg --card-status

    You should see valid card status.

  3. Now, you should virtual generate keys, the keys are not actually generated, but returned to gpg to be registered.

    gpg --card-edit


    generate (DO NOT BACKUP KEYS)

    Kill gpg-agent and modify configuration to have sign, encrypt, authenticate key hex ids.

  4. Alternatively, you can add the existing keys as subkeys on an existing GPG master key:

    gpg --edit-key MASTER_KEY_ID


  5. In order to reattach a key to smartcard, remove secret key using:

    gpg --delete-secret-keys KEY_ID

    Then regenerate but without replace keys using:

    gpg --card-edit


    generate (DO NOT GENERATE KEYS)

Signing, verification, etc. work as usual with gpg.

Typical steps to set up a card for >=gpg-2.1.19 usage:

  1. Refresh local key store:

    gpg --card-status

  2. Acquire key ids:

    gpg-agent --server gpg-connect-agent

    Enter "SCD LEARN" and look for "KEY-FRIEDNLY" responses, the first field is the keygrip, the second is the subject name.

  3. Create master key based on existing key using:

    gpg --expert --full-generate-key


    (13) Existing key

    Enter keygrip to be used as primary key.

  4. Continue as usual to setup your primary key, you should probably use signature for master key.

  5. Add subkey using:

    gpg --expert --edit-key ${MASTER_KEY_ID}

    gpg> addkey

    (13) Existing key

    Enter keygrip to be used as subkey.

  6. Continue as usual to setup your subkey.

Signing, verification, etc. work as usual with gpg.

Security Considerations

All communication between components is currently unprotected and in plain text (that's how the Assuan protocol operates). It is trivial to trace (using e.g. the strace(1) program) individual components (e.g. pinentry) and steal sensitive data (such as the smart-card PIN) or even change it (e.g. the hash to be signed).

When using the software in production scenario, be sure to turn off debugging/verbose options in configuration of all components. Otherwise, some sensitive data might be displayed on the screen (most notably, the PIN).

See Also

strace(1) truss(1) gnupg(7)

GnuPG Home Page,

gnupg-pkcs11 Home Page,


October 15, 2017