=item B<OSSL_CALLBACK>
-This is a function type for a generic callback function:
+This is a function type for a generic feedback callback function:
typedef int (OSSL_CALLBACK)(const OSSL_PARAM params[], void *arg);
expected to pass back, and pass that as I<params>, as well as
the caller data pointer it received, as I<arg>.
+=item B<OSSL_PASSPHRASE_CALLBACK>
+
+This is a function type for a generic pass phrase callback function:
+
+ typedef int (OSSL_PASSPHRASE_CALLBACK)(char *pass, size_t pass_size,
+ size_t *pass_len,
+ const OSSL_PARAM params[],
+ void *arg);
+
+This callback can be used to prompt the user for a passphrase. When
+calling it, a buffer to store the pass phrase needs to be given with
+I<pass>, and its size with I<pass_size>. The length of the prompted
+pass phrase will be given back in I<*pass_len>.
+
+Additional parameters can be passed with the B<OSSL_PARAM> array
+I<params>.
+
+A function that takes a pointer of this type should also take a
+pointer to caller data, which should be passed as I<arg> to this
+callback.
+
=back
=head1 SEE ALSO