There are different ways to specify a user ID to GnuPG.  Some of them are only valid for gpg others are only good for gpgsm.  Here is the entire list of ways to specify a key:

  * By key Id. This format is deduced from the length of the string and its content or 0x prefix. The key Id of an X.509 certificate are the low 64 bits of its SHA-1 fingerprint.  The use of key Ids is just a shortcut, for all automated processing the fingerprint should be used.

     When using gpg an exclamation mark (!) may be appended to force using the specified primary or secondary key and not to try and calculate which primary or secondary key to use.

     The last four lines of the example give the key ID in their long form as internally used by the OpenPGP protocol. You can see the long key ID using the option --with-colons.

          234567C4
          0F34E556E
          01347A56A
          0xAB123456

          234AABBCC34567C4
          0F323456784E56EAB
          01AB3FED1347A5612
          0x234AABBCC34567C4

  * By fingerprint. This format is deduced from the length of the string and its content or the 0x prefix.  Note, that only the 20 byte version fingerprint is available with gpgsm (i.e. the SHA-1 hash of the certificate).

     When using gpg an exclamation mark (!) may be appended to force using the specified primary or secondary key and not to try and calculate which primary or secondary key to use.

     The best way to specify a key Id is by using the fingerprint.  This avoids any ambiguities in case that there are duplicated key IDs.

          1234343434343434C434343434343434
          123434343434343C3434343434343734349A3434
          0E12343434343434343434EAB3484343434343434
          0xE12343434343434343434EAB3484343434343434

     gpgsm also accepts colons between each pair of hexadecimal digits because this is the de-facto standard on how to present X.509 fingerprints.  gpg also allows the use of the space separated SHA-1 fingerprint as printed by the key listing commands.

  * By exact match on OpenPGP user ID. This is denoted by a leading equal sign. It does not make sense for X.509 certificates.

          =Heinrich Heine <heinrichh@uni-duesseldorf.de>

  * By exact match on an email address. This is indicated by enclosing the email address in the usual way with left and right angles.

          <heinrichh@uni-duesseldorf.de>

  * By partial match on an email address. This is indicated by prefixing the search string with an @. This uses a substring search but considers only the mail address (i.e. inside the angle brackets).

          @heinrichh

  * By exact match on the subject's DN. This is indicated by a leading slash, directly followed by the RFC-2253 encoded DN of the subject.  Note that you can't use the string printed by gpgsm --list-keys because that one has been reordered and modified for better readability; use --with-colons to print the raw (but standard escaped) RFC-2253 string.

          /CN=Heinrich Heine,O=Poets,L=Paris,C=FR

  * By exact match on the issuer's DN. This is indicated by a leading hash mark, directly followed by a slash and then directly followed by the RFC-2253 encoded DN of the issuer. This should return the Root cert of the issuer.  See note above.

          #/CN=Root Cert,O=Poets,L=Paris,C=FR

  * By exact match on serial number and issuer's DN. This is indicated by a hash mark, followed by the hexadecimal representation of the serial number, then followed by a slash and the RFC-2253 encoded DN of the issuer. See note above.

          #4F03/CN=Root Cert,O=Poets,L=Paris,C=FR

  * By keygrip. This is indicated by an ampersand followed by the 40 hex digits of a keygrip.  gpgsm prints the keygrip when using the command --dump-cert.  It does not yet work for OpenPGP keys.

          &D75F22C3F86E355877348498CDC92BD21010A480

  * By substring match. This is the default mode but applications may want to explicitly indicate this by putting the asterisk in front.  Match is not case sensitive.

          Heine
          *Heine

  * . and + prefixes These prefixes are reserved for looking up mails anchored at the end and for a word search mode.  They are not yet implemented and using them is undefined.

Please note that we have reused the hash mark identifier which was used in old GnuPG versions to indicate the so called local-id.  It is not anymore used and there should be no conflict when used with X.509 stuff.

Using the RFC-2253 format of DNs has the drawback that it is not possible to map them back to the original encoding, however we don't have to do this because our key database stores this encoding as meta data.