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core: Always fail if an OpenPG message is not integrity protected.

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* src/decrypt.c (struct op_data_t): Add field not_integrity_protected.
(parse_decryption_info): Set this.  Also rename mode to aead_algo for
clarity.
(_gpgme_decrypt_status_handler): Force failure in case of a missing
MDC.
--

This extra check makes sure that a missing or stripped MDC in
 - gpg < 2.1
 - or gpg 2.2 with an old cipher algorithm
will lead to a decryption failure.  gpg 2.3 will always fail in this
case.  Implementing this check here and not backporting the 2.3 change
to 2.2 has the benefit that all GPGME using applications are protected
but scripts relying on rfc2440 (i.e. without MDC) will only break when
migrating to 2.3.

Note that S/MIME has no integrity protection mechanism but gpgsm
neither emits a DECRYPTION_INFO status line, so an error will not be
triggered.  If in the future gpgsm supports authenticated encryption
it may issue a DECRYPTION_INFO line to force a failure here but it
will in that case also emit a DECRYPTION_FAILED anyway.

GnuPG-bug-id: 3981
Signed-off-by: Werner Koch <wk@gnupg.org>
This commit is contained in:
Werner Koch 2018-05-17 09:14:40 +02:00
parent e54b110aec
commit 8a0c8c5251
No known key found for this signature in database
GPG Key ID: E3FDFF218E45B72B
3 changed files with 24 additions and 8 deletions
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3
NEWS
View File

@ -1,6 +1,9 @@
Noteworthy changes in version 1.11.2 (unreleased)
-------------------------------------------------
* Even for old versions of gpg a missing MDC will now lead to a
decryption failure.
Noteworthy changes in version 1.11.1 (2018-04-20)
-------------------------------------------------

2
doc/gpgme.texi
View File

@ -5408,7 +5408,7 @@ or @code{gpgme_get_ctx_flag (ctx, "export-session-key")} returns true
@since{1.11.0}
A string with the symmetric encryption algorithm and mode using the
format "<algo>.<mode>". Note that old non-MDC encryption mode of
format "<algo>.<mode>". Note that the deprecated non-MDC encryption mode of
OpenPGP is given as "PGPCFB".
@end table

27
src/decrypt.c
View File

@ -56,6 +56,11 @@ typedef struct
* message that the general DECRYPTION_FAILED. */
int any_no_seckey;
/* If the engine emits a DECRYPTION_INFO status and that does not
* indicate that an integrity proetction mode is active, this flag
* is set. */
int not_integrity_protected;
/* A pointer to the next pointer of the last recipient in the list.
This makes appending new invalid signers painless while
preserving the order. */
@ -280,7 +285,7 @@ parse_decryption_info (char *args, op_data_t opd, gpgme_protocol_t protocol)
char *field[3];
int nfields;
char *args2;
int mdc, mode;
int mdc, aead_algo;
const char *algostr, *modestr;
if (!args)
@ -296,19 +301,22 @@ parse_decryption_info (char *args, op_data_t opd, gpgme_protocol_t protocol)
mdc = atoi (field[0]);
algostr = _gpgme_cipher_algo_name (atoi (field[1]), protocol);
mode = nfields < 3? 0 : atoi (field[2]);
modestr = _gpgme_cipher_mode_name (mode, protocol);
aead_algo = nfields < 3? 0 : atoi (field[2]);
modestr = _gpgme_cipher_mode_name (aead_algo, protocol);
free (args2);
free (opd->result.symkey_algo);
if (!mode && mdc != 2)
if (!aead_algo && mdc != 2)
opd->result.symkey_algo = _gpgme_strconcat (algostr, ".PGPCFB", NULL);
else
opd->result.symkey_algo = _gpgme_strconcat (algostr, ".", modestr, NULL);
if (!opd->result.symkey_algo)
return gpg_error_from_syserror ();
if (!mdc && !aead_algo)
opd->not_integrity_protected = 1;
return 0;
}
@ -338,13 +346,18 @@ _gpgme_decrypt_status_handler (void *priv, gpgme_status_code_t code,
break;
case GPGME_STATUS_EOF:
/* FIXME: These error values should probably be attributed to
the underlying crypto engine (as error source). */
/* We force an encryption failure if we know that integrity
* protection is missing. For modern version of gpg using
* modern cipher algorithms this is not required because gpg
* will issue a failure anyway. However older gpg versions emit
* only a warning.
* Fixme: These error values should probably be attributed to
* the underlying crypto engine (as error source). */
if (opd->failed && opd->pkdecrypt_failed)
return opd->pkdecrypt_failed;
else if (opd->failed && opd->any_no_seckey)
return gpg_error (GPG_ERR_NO_SECKEY);
else if (opd->failed)
else if (opd->failed || opd->not_integrity_protected)
return gpg_error (GPG_ERR_DECRYPT_FAILED);
else if (!opd->okay)
return gpg_error (GPG_ERR_NO_DATA);