Key Signatures - The `GnuPG Made Easy' Reference Manual

The following interfaces are deprecated and only provided for backward compatibility. Don't use them. They will be removed in a future version of GPGME.

The signatures on a key are only available if the key was retrieved via a listing operation with the GPGME_KEYLIST_MODE_SIGS mode enabled, because it is expensive to retrieve all signatures of a key.

So, before using the below interfaces to retrieve the signatures on a key, you have to make sure that the key was listed with signatures enabled. One convenient, but blocking, way to do this is to use the function gpgme_get_key.

— Data type: gpgme_attr_t

The gpgme_attr_t type is used to specify a key signature attribute. The following attributes are defined:

GPGME_ATTR_KEYID
This is the key ID of the key which was used for the signature. It is representable as a string.
GPGME_ATTR_ALGO
This is the crypto algorithm used to create the signature. It is representable as a string and as a number. The numbers correspond to the enum gcry_pk_algos values in the gcrypt library.
GPGME_ATTR_CREATED
This is the timestamp at creation time of the signature. It is representable as a number.
GPGME_ATTR_EXPIRE
This is the expiration time of the signature. It is representable as a number.
GPGME_ATTR_USERID
This is the user ID associated with the signing key. The user ID is representable as a number.
GPGME_ATTR_NAME
This is the name belonging to a user ID. It is representable as a string.
GPGME_ATTR_EMAIL
This is the email address belonging to a user ID. It is representable as a string.
GPGME_ATTR_COMMENT
This is the comment belonging to a user ID. It is representable as a string.
GPGME_ATTR_KEY_REVOKED
This specifies if a key signature is a revocation signature. It is representable as a number, and is 1 if the key is revoked, and 0 otherwise.
GPGME_ATTR_SIG_CLASS
This specifies the signature class of a key signature. It is representable as a number. The meaning is specific to the crypto engine.
GPGME_ATTR_SIG_CLASS
This specifies the signature class of a key signature. It is representable as a number. The meaning is specific to the crypto engine.
GPGME_ATTR_SIG_STATUS
This is the same value as returned by gpgme_get_sig_status.

— Function: const char * gpgme_key_sig_get_string_attr (gpgme_key_t key, int uid_idx, gpgme_attr_t what, const void *reserved, int idx)

The function gpgme_key_sig_get_string_attr returns the value of the string-representable attribute what of the signature idx on the user ID uid_idx in the key key. The argument reserved is reserved for later use and should be NULL.
The string returned is only valid as long as the key is valid.
The function returns 0 if an attribute can't be returned as a string, key is not a valid pointer, uid_idx or idx out of range, or reserved not NULL.

— Function: unsigned long gpgme_key_sig_get_ulong_attr (gpgme_key_t key, int uid_idx, gpgme_attr_t what, const void *reserved, int idx)

The function gpgme_key_sig_get_ulong_attr returns the value of the number-representable attribute what of the signature idx on the user ID uid_idx in the key key. The argument reserved is reserved for later use and should be NULL.
The function returns 0 if an attribute can't be returned as a string, key is not a valid pointer, uid_idx or idx out of range, or reserved not NULL.

7.4.3 Key Signatures