gpg: Allow decryption of symencr even for non-compliant cipher.

* g10/decrypt-data.c (decrypt_data): Add arg compliance_error.  Adjust
all callers.  Fail on compliance error only in --require-compliance
mode.  Make sure to return an error if the buffer is missing; actually
that should be an assert.
* g10/mainproc.c (proc_encrypted): Delay printing of the compliance
mode status.  Consult the compliance error now returned by
decrypt_data.
--

The actual case here is that we fail hard if a message has been AEAD
encrypted with one AEAD capable key and also with one passphrase.  In
general the preference system takes care of not using AEAD if one
recipient's key does not support it.  However, if the sender uses her
own AEAD-capable key _and_ a passphrase the message will be AEAD
encrypted.  This change allows to decrypt that anyway along with a
warning message.

Note that this does currently not work in 2.3 due to a non-compliant
libgcrypt.  We will however, backport this to 2.2.

1 files changed, 17 insertions, 4 deletions

diff --git a/g10/decrypt-data.c b/g10/decrypt-data.c
index 8b48d18bf..6c1d6ebcd 100644
--- a/g10/decrypt-data.c
+++ b/g10/decrypt-data.c
@@ -214,10 +214,14 @@ aead_checktag (decode_filter_ctx_t dfx, int final, const void *tagbuf)
 
 
 /****************
- * Decrypt the data, specified by ED with the key DEK.
+ * Decrypt the data, specified by ED with the key DEK.  On return
+ * COMPLIANCE_ERROR is set to true iff the decryption can claim that
+ * it was compliant in the current mode; otherwise this flag is set to
+ * false.
  */
 int
-decrypt_data (ctrl_t ctrl, void *procctx, PKT_encrypted *ed, DEK *dek)
+decrypt_data (ctrl_t ctrl, void *procctx, PKT_encrypted *ed, DEK *dek,
+              int *compliance_error)
 {
   decode_filter_ctx_t dfx;
   enum gcry_cipher_modes ciphermode;
@@ -228,6 +232,8 @@ decrypt_data (ctrl_t ctrl, void *procctx, PKT_encrypted *ed, DEK *dek)
   unsigned int blocksize;
   unsigned int nprefix;
 
+  *compliance_error = 0;
+
   dfx = xtrycalloc (1, sizeof *dfx);
   if (!dfx)
     return gpg_error_from_syserror ();
@@ -261,8 +267,14 @@ decrypt_data (ctrl_t ctrl, void *procctx, PKT_encrypted *ed, DEK *dek)
       log_error (_("cipher algorithm '%s' may not be used in %s mode\n"),
 		 openpgp_cipher_algo_name (dek->algo),
 		 gnupg_compliance_option_string (opt.compliance));
-      rc = gpg_error (GPG_ERR_CIPHER_ALGO);
-      goto leave;
+      *compliance_error = 1;
+      if (opt.flags.require_compliance)
+        {
+          /* We fail early in this case because it does not make sense
+           * to first decrypt everything.  */
+          rc = gpg_error (GPG_ERR_CIPHER_ALGO);
+          goto leave;
+        }
     }
 
   write_status_printf (STATUS_DECRYPTION_INFO, "%d %d %d",
@@ -424,6 +436,7 @@ decrypt_data (ctrl_t ctrl, void *procctx, PKT_encrypted *ed, DEK *dek)
       if (!ed->buf)
         {
           log_error (_("problem handling encrypted packet\n"));
+          rc = gpg_error (GPG_ERR_INV_PACKET);
           goto leave;
         }