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(gpgme.info)Passphrase Callback

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Passphrase Callback
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 - Data type: const char *(*GpgmePassphraseCb)(void *HOOK, const char
*DESC, void **R_HD)
     The `GpgmePassphraseCb' type is the type of functions usable as
     passphrase callback function.

     The string DESC contains a test usable to be displayed to the user
     of the application.  The function should return a passphrase for
     the context when invoked with DESC not being `NULL'.

     The user may store information about the resources associated with
     the returned passphrase in *R_HD.  When the passphrase is no longer
     needed by GPGME, the passphrase callback function will be called
     with DESC being NULL, and R_HD being the same as at the first
     invocation.

 - Function: void gpgme_set_passphrase_cb (GpgmeCtx CTX,
          GpgmePassphraseCb PASSFUNC, void *HOOK_VALUE)
     The function `gpgme_set_passphrase_cb' sets the function that is
     used when a passphrase needs to be provided by the user to
     PASSFUNC.  The function PASSFUNC needs to implemented by the user,
     and whenever it is called, it is called with its first argument
     being HOOK_VALUE.  By default, no passphrase callback function is
     set.

     Not all crypto engines require this callback to retrieve the
     passphrase.  It is better if the engine retrieves the passphrase
     from a trusted agent (a daemon process), rather than having each
     user to implement their own passphrase query.

     The user can disable the use of a passphrase callback function by
     calling `gpgme_set_passphrase_cb' with PASSFUNC being `NULL'.