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gpg: Do not allow old cipher algorithms for encryption.

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* g10/gpg.c: New option --allow-old-cipher-algos.
(set_compliance_option): Set --rfc4880bis explictly to SHA256 and
AES256.  Allow old cipher algos for OpenPGP, rfc4880, and rfc2440.
* g10/options.h (opt): Add flags.allow_old_cipher_algos.
* g10/misc.c (print_sha1_keysig_rejected_note): Always print the note
unless in --quiet mode.
* g10/encrypt.c (setup_symkey): Disallow by default algos with a
blocklengt < 128.
(encrypt_crypt): Ditto.  Fallback by default to AES instead of 3DES.
* g10/pkclist.c (algo_available): Take care of old cipher also.
(select_algo_from_prefs): Use AES as implicit algorithm by default.

* tests/openpgp/defs.scm (create-gpghome): Set allow-old-cipher-algos.
--

GnuPG-bug-id: 3415
This commit is contained in:
Werner Koch 2021-02-10 14:31:34 +01:00
parent 6e730c1881
commit 825dd7220f
No known key found for this signature in database
GPG key ID: E3FDFF218E45B72B
7 changed files with 92 additions and 16 deletions
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21
doc/gpg.texi
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@ -2848,16 +2848,17 @@ different compliance option in the gpg.conf file.
@item --openpgp
@opindex openpgp
Reset all packet, cipher and digest options to strict OpenPGP
behavior. Use this option to reset all previous options like
@option{--s2k-*}, @option{--cipher-algo}, @option{--digest-algo} and
behavior. This option implies @option{--allow-old-cipher-algos}. Use
this option to reset all previous options like @option{--s2k-*},
@option{--cipher-algo}, @option{--digest-algo} and
@option{--compress-algo} to OpenPGP compliant values. All PGP
workarounds are disabled.
@item --rfc4880
@opindex rfc4880
Reset all packet, cipher and digest options to strict RFC-4880
behavior. Note that this is currently the same thing as
@option{--openpgp}.
behavior. This option implies @option{--allow-old-cipher-algos}.
Note that this is currently the same thing as @option{--openpgp}.
@item --rfc4880bis
@opindex rfc4880bis
@ -2869,7 +2870,8 @@ proposed updates of RFC-4880.
Reset all packet, cipher and digest options to strict RFC-2440
behavior. Note that by using this option encryption packets are
created in a legacy mode without MDC protection. This is dangerous
and should thus only be used for experiments. See also option
and should thus only be used for experiments. This option implies
@option{--allow-old-cipher-algos}. See also option
@option{--ignore-mdc-error}.
@item --pgp6
@ -3391,6 +3393,15 @@ 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 great caution; see also option @option{--rfc2440}.
@item --allow-old-cipher-algos
@opindex allow-old-cipher-algos
Old cipher algorithms like 3DES, IDEA, or CAST5 encrypt data using
blocks of 64 bits; modern algorithms use blocks of 128 bit instead.
To avoid certain attack on these old algorithms it is suggested not to
encrypt more than 150 MiByte using the same key. For this reason gpg
does not allow the use of 64 bit block size algorithms for encryption
unless this option is specified.
@item --allow-weak-digest-algos
@opindex allow-weak-digest-algos
Signatures made with known-weak digest algorithms are normally