From 76afaed65e3b0ddfa4923cb577ada43217dd4b18 Mon Sep 17 00:00:00 2001 From: Daniel Kahn Gillmor Date: Sun, 18 Oct 2015 17:35:32 -0400 Subject: [PATCH] gpg: Add option --weak-digest to gpg and gpgv. * g10/options.h: Add additional_weak_digests linked list to opts. * g10/main.h: Declare weakhash linked list struct and additional_weak_digest() function to insert newly-declared weak digests into opts. * g10/misc.c: (additional_weak_digest): New function. (print_digest_algo_note): Check for deprecated digests; use proper gcry_md_algos type. * g10/sig-check.c: (do_check): Reject weak digests in addition to MD5. * g10/gpg.c: Add --weak-digest option to gpg. * doc/gpg.texi: Document gpg --weak-digest option. * g10/gpgv.c: Add --weak-digest option to gpgv. * doc/gpgv.texi: Document gpgv --weak-digest option. -- gpg and gpgv treat signatures made over MD5 as unreliable, unless the user supplies --allow-weak-digests to gpg. Signatures over any other digest are considered acceptable. Despite SHA-1 being a mandatory-to-implement digest algorithm in RFC 4880, the collision-resistance of SHA-1 is weaker than anyone would like it to be. Some operators of high-value targets that depend on OpenPGP signatures may wish to require their signers to use a stronger digest algorithm than SHA1, even if the OpenPGP ecosystem at large cannot deprecate SHA1 entirely today. This changeset adds a new "--weak-digest DIGEST" option for both gpg and gpgv, which makes it straightforward for anyone to treat any signature or certification made over the specified digest as unreliable. This option can be supplied multiple times if the operator wishes to deprecate multiple digest algorithms, and will be ignored completely if the operator supplies --allow-weak-digests (as before). MD5 is still always considered weak, regardless of any further --weak-digest options supplied. Signed-off-by: Daniel Kahn Gillmor Capitialized some comments, shorted a line in do_check, and changed subject to name the option. -wk --- doc/gpg.texi | 17 ++++++++++++++--- doc/gpgv.texi | 8 ++++++++ g10/gpg.c | 6 ++++++ g10/gpgv.c | 5 +++++ g10/main.h | 7 +++++++ g10/misc.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++--- g10/options.h | 1 + g10/sig-check.c | 11 +++++++---- 8 files changed, 96 insertions(+), 10 deletions(-) diff --git a/doc/gpg.texi b/doc/gpg.texi index a70204043..ffd7a976e 100644 --- a/doc/gpg.texi +++ b/doc/gpg.texi @@ -2789,9 +2789,20 @@ message was tampered with intentionally by an attacker. @item --allow-weak-digest-algos @opindex allow-weak-digest-algos -Signatures made with the broken MD5 algorithm are normally rejected -with an ``invalid digest algorithm'' message. This option allows the -verification of signatures made with such weak algorithms. +Signatures made with known-weak digest algorithms are normally +rejected with an ``invalid digest algorithm'' message. This option +allows the verification of signatures made with such weak algorithms. +MD5 is the only digest algorithm considered weak by default. See also +@option{--weak-digest} to reject other digest algorithms. + +@item --weak-digest @code{name} +@opindex weak-digest +Treat the specified digest algorithm as weak. Signatures made over +weak digests algorithms are normally rejected. This option can be +supplied multiple times if multiple algorithms should be considered +weak. See also @option{--allow-weak-digest-algos} to disable +rejection of weak digests. MD5 is always considered weak, and does +not need to be listed explicitly. @item --no-default-keyring @opindex no-default-keyring diff --git a/doc/gpgv.texi b/doc/gpgv.texi index 6bcbc0add..280966bf8 100644 --- a/doc/gpgv.texi +++ b/doc/gpgv.texi @@ -118,6 +118,14 @@ checks into warnings. @include opt-homedir.texi +@item --weak-digest @code{name} +@opindex weak-digest +Treat the specified digest algorithm as weak. Signatures made over +weak digests algorithms are normally rejected. This option can be +supplied multiple times if multiple algorithms should be considered +weak. MD5 is always considered weak, and does not need to be listed +explicitly. + @end table @mansect return value diff --git a/g10/gpg.c b/g10/gpg.c index ada913c0a..cb610a11b 100644 --- a/g10/gpg.c +++ b/g10/gpg.c @@ -389,6 +389,7 @@ enum cmd_and_opt_values oPrintDANERecords, oTOFUDefaultPolicy, oTOFUDBFormat, + oWeakDigest, oNoop }; @@ -749,6 +750,7 @@ static ARGPARSE_OPTS opts[] = { ARGPARSE_s_s (oPersonalCompressPreferences, "personal-compress-preferences", "@"), ARGPARSE_s_s (oFakedSystemTime, "faked-system-time", "@"), + ARGPARSE_s_s (oWeakDigest, "weak-digest","@"), /* Aliases. I constantly mistype these, and assume other people do as well. */ @@ -2208,6 +2210,7 @@ main (int argc, char **argv) set_homedir (default_homedir ()); opt.passphrase_repeat = 1; opt.emit_version = 1; /* Limit to the major number. */ + opt.additional_weak_digests = NULL; /* Check whether we have a config file on the command line. */ orig_argc = argc; @@ -3124,6 +3127,9 @@ main (int argc, char **argv) break; case oAgentProgram: opt.agent_program = pargs.r.ret_str; break; case oDirmngrProgram: opt.dirmngr_program = pargs.r.ret_str; break; + case oWeakDigest: + additional_weak_digest(pargs.r.ret_str); + break; case oDisplay: set_opt_session_env ("DISPLAY", pargs.r.ret_str); diff --git a/g10/gpgv.c b/g10/gpgv.c index 0807622a6..23e761047 100644 --- a/g10/gpgv.c +++ b/g10/gpgv.c @@ -61,6 +61,7 @@ enum cmd_and_opt_values { oStatusFD, oLoggerFD, oHomedir, + oWeakDigest, aTest }; @@ -78,6 +79,7 @@ static ARGPARSE_OPTS opts[] = { N_("|FD|write status info to this FD")), ARGPARSE_s_i (oLoggerFD, "logger-fd", "@"), ARGPARSE_s_s (oHomedir, "homedir", "@"), + ARGPARSE_s_s (oWeakDigest, "weak-digest", "@"), ARGPARSE_end () }; @@ -192,6 +194,9 @@ main( int argc, char **argv ) log_set_fd (translate_sys2libc_fd_int (pargs.r.ret_int, 1)); break; case oHomedir: opt.homedir = pargs.r.ret_str; break; + case oWeakDigest: + additional_weak_digest(pargs.r.ret_str); + break; case oIgnoreTimeConflict: opt.ignore_time_conflict = 1; break; default : pargs.err = ARGPARSE_PRINT_ERROR; break; } diff --git a/g10/main.h b/g10/main.h index c9521ad1d..0226c6418 100644 --- a/g10/main.h +++ b/g10/main.h @@ -69,6 +69,12 @@ struct groupitem struct groupitem *next; }; +struct weakhash +{ + enum gcry_md_algos algo; + struct weakhash *next; +}; + /*-- gpg.c --*/ extern int g10_errors_seen; @@ -82,6 +88,7 @@ void print_pubkey_algo_note (pubkey_algo_t algo); void print_cipher_algo_note (cipher_algo_t algo); void print_digest_algo_note (digest_algo_t algo); void print_md5_rejected_note (void); +void additional_weak_digest (const char* digestname); /*-- armor.c --*/ char *make_radix64_string( const byte *data, size_t len ); diff --git a/g10/misc.c b/g10/misc.c index 9134b2823..c135059d8 100644 --- a/g10/misc.c +++ b/g10/misc.c @@ -307,6 +307,10 @@ print_cipher_algo_note (cipher_algo_t algo) void print_digest_algo_note (digest_algo_t algo) { + int deprecated = 0; + const enum gcry_md_algos galgo = map_md_openpgp_to_gcry (algo); + const struct weakhash *weak; + if(algo >= 100 && algo <= 110) { static int warn=0; @@ -315,14 +319,21 @@ print_digest_algo_note (digest_algo_t algo) warn=1; es_fflush (es_stdout); log_info (_("WARNING: using experimental digest algorithm %s\n"), - gcry_md_algo_name (algo)); + gcry_md_algo_name (galgo)); } } - else if(algo==DIGEST_ALGO_MD5) + else if(algo == DIGEST_ALGO_MD5) + deprecated = 1; + else + for (weak = opt.additional_weak_digests; weak != NULL; weak = weak->next) + if (weak->algo == galgo) + deprecated = 1; + + if (deprecated) { es_fflush (es_stdout); log_info (_("WARNING: digest algorithm %s is deprecated\n"), - gcry_md_algo_name (algo)); + gcry_md_algo_name (galgo)); } } @@ -1676,3 +1687,37 @@ ecdsa_qbits_from_Q (unsigned int qbits) qbits /= 2; return qbits; } + + +/* Ignore signatures and certifications made over certain digest + * algorithms by default, MD5 is considered weak. This allows users + * to deprecate support for other algorithms as well. + */ +void +additional_weak_digest (const char* digestname) +{ + struct weakhash *weak = NULL; + const enum gcry_md_algos algo = string_to_digest_algo(digestname); + + if (algo == GCRY_MD_MD5) + return; /* MD5 is always considered weak, no need to add it. */ + + if (algo == GCRY_MD_NONE) + { + log_error(_("Unknown weak digest '%s'\n"), digestname); + return; + } + + /* Check to ensure it's not already present. */ + for (weak = opt.additional_weak_digests; weak != NULL; weak = weak->next) + { + if (algo == weak->algo) + return; + } + + /* Add it to the head of the list. */ + weak = xmalloc(sizeof(*weak)); + weak->algo = algo; + weak->next = opt.additional_weak_digests; + opt.additional_weak_digests = weak; +} diff --git a/g10/options.h b/g10/options.h index 2135aa0b9..c1ea9ddd8 100644 --- a/g10/options.h +++ b/g10/options.h @@ -170,6 +170,7 @@ struct prefitem_t *personal_cipher_prefs; prefitem_t *personal_digest_prefs; prefitem_t *personal_compress_prefs; + struct weakhash *additional_weak_digests; int no_perm_warn; int no_mdc_warn; char *temp_dir; diff --git a/g10/sig-check.c b/g10/sig-check.c index d45a9f378..84930d615 100644 --- a/g10/sig-check.c +++ b/g10/sig-check.c @@ -274,15 +274,18 @@ do_check( PKT_public_key *pk, PKT_signature *sig, gcry_md_hd_t digest, { gcry_mpi_t result = NULL; int rc = 0; + const struct weakhash *weak; if( (rc=do_check_messages(pk,sig,r_expired,r_revoked)) ) return rc; - if (sig->digest_algo == GCRY_MD_MD5 - && !opt.flags.allow_weak_digest_algos) + if (!opt.flags.allow_weak_digest_algos) { - print_md5_rejected_note (); - return GPG_ERR_DIGEST_ALGO; + if (sig->digest_algo == GCRY_MD_MD5) + return GPG_ERR_DIGEST_ALGO; + for (weak = opt.additional_weak_digests; weak; weak = weak->next) + if (sig->digest_algo == weak->algo) + return GPG_ERR_DIGEST_ALGO; } /* Make sure the digest algo is enabled (in case of a detached