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sm,dirmngr: Restrict allowed parameters used with rsaPSS.

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* sm/certcheck.c (extract_pss_params): Check the used PSS params.
* dirmngr/crlcache.c (finish_sig_check): Ditto.
* dirmngr/validate.c (check_cert_sig): Ditto.
--

GnuPG-bug-id: 4538

# ------------------------ >8 ------------------------
See
https://www.metzdowd.com/pipermail/cryptography/2019-November/035449.html

Signed-off-by: Werner Koch <wk@gnupg.org>
This commit is contained in:
Werner Koch 2020-04-15 11:05:41 +02:00
parent 50b320952e
commit c0d5c67354
No known key found for this signature in database
GPG Key ID: E3FDFF218E45B72B
3 changed files with 87 additions and 2 deletions
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23
dirmngr/crlcache.c
View File

@ -1731,6 +1731,29 @@ finish_sig_check (ksba_crl_t crl, gcry_md_hd_t md, int algo,
algo, hashalgo);
return gpg_error (GPG_ERR_INV_CRL);
}
/* Add some restrictions; see ../sm/certcheck.c for details. */
switch (algo)
{
case GCRY_MD_SHA1:
case GCRY_MD_SHA256:
case GCRY_MD_SHA384:
case GCRY_MD_SHA512:
case GCRY_MD_SHA3_256:
case GCRY_MD_SHA3_384:
case GCRY_MD_SHA3_512:
break;
default:
log_error ("PSS hash algorithm '%s' rejected\n",
gcry_md_algo_name (algo));
return gpg_error (GPG_ERR_DIGEST_ALGO);
}
if (gcry_md_get_algo_dlen (algo) != saltlen)
{
log_error ("PSS hash algorithm '%s' rejected due to salt length %u\n",
gcry_md_algo_name (algo), saltlen);
return gpg_error (GPG_ERR_DIGEST_ALGO);
}
}

26
dirmngr/validate.c
View File

@ -1014,7 +1014,31 @@ check_cert_sig (ksba_cert_t issuer_cert, ksba_cert_t cert)
gcry_sexp_release (s_sig);
return gpg_error (GPG_ERR_DIGEST_ALGO);
}
/* log_debug ("PSS hash=%d saltlen=%u\n", algo, saltlen); */
/* Add some restrictions; see ../sm/certcheck.c for details. */
switch (algo)
{
case GCRY_MD_SHA1:
case GCRY_MD_SHA256:
case GCRY_MD_SHA384:
case GCRY_MD_SHA512:
case GCRY_MD_SHA3_256:
case GCRY_MD_SHA3_384:
case GCRY_MD_SHA3_512:
break;
default:
log_error ("PSS hash algorithm '%s' rejected\n",
gcry_md_algo_name (algo));
gcry_sexp_release (s_sig);
return gpg_error (GPG_ERR_DIGEST_ALGO);
}
if (gcry_md_get_algo_dlen (algo) != saltlen)
{
log_error ("PSS hash algorithm '%s' rejected due to salt length %u\n",
gcry_md_algo_name (algo), saltlen);
gcry_sexp_release (s_sig);
return gpg_error (GPG_ERR_DIGEST_ALGO);
}
}
algo_name = hash_algo_to_string (algo);

40
sm/certcheck.c
View File

@ -300,7 +300,45 @@ extract_pss_params (gcry_sexp_t s_sig, int *r_algo, unsigned int *r_saltlen)
{
return gpg_error (GPG_ERR_DIGEST_ALGO);
}
/* log_debug ("PSS hash=%d saltlen=%u\n", *r_algo, *r_saltlen); */
/* PSS has no hash function firewall like PKCS#1 and thus offers
* a path for hash algorithm replacement. To avoid this it makes
* sense to restrict the allowed hash algorithms and also allow only
* matching salt lengths. According to Peter Gutmann:
* "Beware of bugs in the above signature scheme;
* I have only proved it secure, not implemented it"
* - Apologies to Donald Knuth.
* https://www.metzdowd.com/pipermail/cryptography/2019-November/035449.html
*
* Given the set of supported algorithms currently available in
* Libgcrypt and the extra hash checks we have in some compliance
* modes, it would be hard to trick gpgsm to verify a forged
* signature. However, if eventually someone adds the xor256 hash
* algorithm (1.3.6.1.4.1.3029.3.2) to Libgcrypt we would be doomed.
*/
switch (*r_algo)
{
case GCRY_MD_SHA1:
case GCRY_MD_SHA256:
case GCRY_MD_SHA384:
case GCRY_MD_SHA512:
case GCRY_MD_SHA3_256:
case GCRY_MD_SHA3_384:
case GCRY_MD_SHA3_512:
break;
default:
log_error ("PSS hash algorithm '%s' rejected\n",
gcry_md_algo_name (*r_algo));
return gpg_error (GPG_ERR_DIGEST_ALGO);
}
if (gcry_md_get_algo_dlen (*r_algo) != *r_saltlen)
{
log_error ("PSS hash algorithm '%s' rejected due to salt length %u\n",
gcry_md_algo_name (*r_algo), *r_saltlen);
return gpg_error (GPG_ERR_DIGEST_ALGO);
}
return 0;
}