gpg: Be prepared for chosen-prefix SHA-1 collisions in key signatures.

* g10/sig-check.c (check_signature_over_key_or_uid): Reject cewrtain SHA-1 based signatures. -- Signed-off-by: Werner Koch <wk@gnupg.org> (cherry picked from c4f2d9e) Adjusted for changed added arguments in a function.

Author: dd9jn

g10/sig-check.c

@@ -824,6 +824,10 @@ check_signature_over_key_or_uid (ctrl_t ctrl, PKT_public_key *signer,
 PKT_public_key*pripk=kb->pkt->pkt.public_key;
 gcry_md_hd_tmd;
 intsigner_alloced=0;
+intstub_is_selfsig;
+
+if (!is_selfsig)
+is_selfsig=&stub_is_selfsig;
 
 rc=openpgp_pk_test_algo (sig->pubkey_algo);
 if (rc)
@@ -857,14 +861,11 @@ check_signature_over_key_or_uid (ctrl_t ctrl, PKT_public_key *signer,
 
 if (signer)
  {
-if (is_selfsig)
- {
-if (signer->keyid[0] ==pripk->keyid[0]
-&&signer->keyid[1] ==pripk->keyid[1])
-*is_selfsig=1;
-else
-*is_selfsig=0;
- }
+if (signer->keyid[0] ==pripk->keyid[0]
+&&signer->keyid[1] ==pripk->keyid[1])
+*is_selfsig=1;
+else
+*is_selfsig=0;
  }
 else
  {
@@ -874,8 +875,7 @@ check_signature_over_key_or_uid (ctrl_t ctrl, PKT_public_key *signer,
  {
 /* Issued by the primary key. */
 signer=pripk;
-if (is_selfsig)
-*is_selfsig=1;
+*is_selfsig=1;
  }
 else
  {
@@ -904,8 +904,7 @@ check_signature_over_key_or_uid (ctrl_t ctrl, PKT_public_key *signer,
 if (! signer)
  {
 /* Signer by some other key. */
-if (is_selfsig)
-*is_selfsig=0;
+*is_selfsig=0;
 if (ret_pk)
  {
 signer=ret_pk;
@@ -966,9 +965,22 @@ check_signature_over_key_or_uid (ctrl_t ctrl, PKT_public_key *signer,
 elseif (IS_UID_SIG (sig) ||IS_UID_REV (sig))
  {
 log_assert (packet->pkttype==PKT_USER_ID);
-hash_public_key (md, pripk);
-hash_uid_packet (packet->pkt.user_id, md, sig);
-rc=check_signature_end_simple (signer, sig, md);
+if (sig->digest_algo==DIGEST_ALGO_SHA1&& !*is_selfsig
+&&sig->timestamp>1547856000)
+ {
+/* If the signature was created using SHA-1 we consider this
+ * signature invalid because it makes it possible to mount a
+ * chosen-prefix collision. We don't do this for
+ * self-signatures or for signatures created before the
+ * somewhat arbitrary cut-off date 2019-01-19. */
+rc=gpg_error (GPG_ERR_DIGEST_ALGO);
+ }
+else
+ {
+hash_public_key (md, pripk);
+hash_uid_packet (packet->pkt.user_id, md, sig);
+rc=check_signature_end_simple (signer, sig, md);
+ }
  }
 else
  {