gpg: Print info about the used AEAD algorithm in the compliance msg.

* g10/misc.c (openpgp_cipher_algo_mode_name): New.
* g10/decrypt-data.c (decrypt_data): Use function here.
--

Note that openpgp_cipher_algo_mode_name is different from the version
2.2 becuase we append ".CFB" here.

Without this change we would see

  gpg: cipher algorithm 'AES256' may not be used in
  --compliance=de-vs mode

This is confusing because AES256 is compliant.  Now we see

  gpg: cipher algorithm 'AES256.OCB' may not be used in
  --compliance=de-vs mode

which gives a hint on the problem.

1 files changed, 13 insertions, 0 deletions

diff --git a/g10/misc.c b/g10/misc.c
index 2a431b137..581601209 100644
--- a/g10/misc.c
+++ b/g10/misc.c
@@ -614,6 +614,19 @@ openpgp_cipher_algo_name (cipher_algo_t algo)
 }
 
 
+/* Same as openpgp_cipher_algo_name but returns a string in the form
+ * "ALGO.MODE".  If AEAD is 0 "CFB" is used for the mode.  */
+const char *
+openpgp_cipher_algo_mode_name (cipher_algo_t algo, aead_algo_t aead)
+{
+  return map_static_strings ("openpgp_cipher_algo_mode_name", algo, aead,
+                             openpgp_cipher_algo_name (algo),
+                             ".",
+                             aead? openpgp_aead_algo_name (aead) : "CFB",
+                             NULL);
+}
+
+
 /* Return 0 if ALGO is supported.  Return an error if not. */
 gpg_error_t
 openpgp_aead_test_algo (aead_algo_t algo)