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gpg: Hard fail on a missing MDC even for legacy algorithms.

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* g10/mainproc.c (proc_encrypted): Require an MDC or AEAD
* tests/openpgp/defs.scm (create-gpghome): Use --ignore-mdc-error to
allow testing with the current files.

Signed-off-by: Werner Koch <wk@gnupg.org>
This commit is contained in:
Werner Koch 2018-05-15 12:33:03 +02:00
parent 96350c5d5a
commit d1431901f0
No known key found for this signature in database
GPG Key ID: E3FDFF218E45B72B
3 changed files with 11 additions and 12 deletions
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9
doc/gpg.texi
View File

@ -3230,10 +3230,11 @@ to ignore CRC errors.
@item --ignore-mdc-error
@opindex ignore-mdc-error
This option changes a MDC integrity protection failure into a warning.
This can be useful if a message is partially corrupt, but it is
necessary to get as much data as possible out of the corrupt message.
However, be aware that a MDC protection failure may also mean that the
message was tampered with intentionally by an attacker.
It is required to decrypt old messages which did not use an MDC. It
may also be useful if a message is partially garbled, but it is
necessary to get as much data as possible out of that garbled message.
Be aware that a missing or failed MDC can be an indication of an
attack. Use with caution.
@item --allow-weak-digest-algos
@opindex allow-weak-digest-algos

13
g10/mainproc.c
View File

@ -738,15 +738,12 @@ proc_encrypted (CTX c, PACKET *pkt)
else if (!result
&& !opt.ignore_mdc_error
&& !pkt->pkt.encrypted->mdc_method
&& !pkt->pkt.encrypted->aead_algo
&& openpgp_cipher_get_algo_blklen (c->dek->algo) != 8
&& c->dek->algo != CIPHER_ALGO_TWOFISH)
&& !pkt->pkt.encrypted->aead_algo)
{
/* The message has been decrypted but has no MDC despite that a
modern cipher (blocklength != 64 bit, except for Twofish) is
used and the option to ignore MDC errors is not used: To
avoid attacks changing an MDC message to a non-MDC message,
we fail here. */
/* The message has been decrypted but does not carry an MDC or
* uses AEAD encryption. --ignore-mdc-error has also not been
* used. To avoid attacks changing an MDC message to a non-MDC
* message, we fail here. */
log_error (_("WARNING: message was not integrity protected\n"));
if (opt.verbose > 1)
log_info ("decryption forced to fail\n");

1
tests/openpgp/defs.scm
View File

@ -341,6 +341,7 @@
"no-auto-key-retrieve"
"no-auto-key-locate"
"allow-weak-digest-algos"
"ignore-mdc-error"
(if have-opt-always-trust
"no-auto-check-trustdb" "#no-auto-check-trustdb")
(string-append "agent-program "