Adding upstream version 2.2.40.upstream/2.2.40upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1229 insertions, 0 deletions

diff --git a/g10/sig-check.c b/g10/sig-check.c
new file mode 100644
index 0000000..eeaf6f0
--- /dev/null
+++ b/g10/sig-check.c
@@ -0,0 +1,1229 @@
+/* sig-check.c -  Check a signature
+ * Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003,
+ *               2004, 2006 Free Software Foundation, Inc.
+ * Copyright (C) 2015, 2016 g10 Code GmbH
+ *
+ * This file is part of GnuPG.
+ *
+ * GnuPG is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * GnuPG is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <https://www.gnu.org/licenses/>.
+ */
+
+#include <config.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "gpg.h"
+#include "../common/util.h"
+#include "packet.h"
+#include "keydb.h"
+#include "main.h"
+#include "../common/status.h"
+#include "../common/i18n.h"
+#include "options.h"
+#include "pkglue.h"
+#include "../common/compliance.h"
+
+static int check_signature_end (PKT_public_key *pk, PKT_signature *sig,
+				gcry_md_hd_t digest,
+				int *r_expired, int *r_revoked,
+				PKT_public_key *ret_pk);
+
+static int check_signature_end_simple (PKT_public_key *pk, PKT_signature *sig,
+                                       gcry_md_hd_t digest);
+
+
+/* Statistics for signature verification.  */
+struct
+{
+  unsigned int total;  /* Total number of verifications.  */
+  unsigned int cached; /* Number of seen cache entries.  */
+  unsigned int goodsig;/* Number of good verifications from the cache.  */
+  unsigned int badsig; /* Number of bad verifications from the cache.  */
+} cache_stats;
+
+
+/* Dump verification stats.  */
+void
+sig_check_dump_stats (void)
+{
+  log_info ("sig_cache: total=%u cached=%u good=%u bad=%u\n",
+            cache_stats.total, cache_stats.cached,
+            cache_stats.goodsig, cache_stats.badsig);
+}
+
+
+static gpg_error_t
+check_key_verify_compliance (PKT_public_key *pk)
+{
+  gpg_error_t err = 0;
+
+  if (!gnupg_pk_is_allowed (opt.compliance, PK_USE_VERIFICATION,
+                            pk->pubkey_algo, 0, pk->pkey,
+                            nbits_from_pk (pk),
+                            NULL))
+    {
+      /* Compliance failure.  */
+      log_info (_("key %s may not be used for signing in %s mode\n"),
+                 keystr_from_pk (pk),
+                 gnupg_compliance_option_string (opt.compliance));
+      if (opt.flags.override_compliance_check)
+        log_info (_("continuing verification anyway due to option %s\n"),
+                  "--override-compliance-failure");
+      else
+        {
+          log_inc_errorcount (); /* We used log info above.  */
+          err = gpg_error (GPG_ERR_PUBKEY_ALGO);
+        }
+    }
+
+  return err;
+}
+
+
+
+/* Check a signature.  This is shorthand for check_signature2 with
+   the unnamed arguments passed as NULL.  */
+int
+check_signature (ctrl_t ctrl, PKT_signature *sig, gcry_md_hd_t digest)
+{
+  return check_signature2 (ctrl, sig, digest, NULL, NULL, NULL, NULL, NULL);
+}
+
+
+/* Check a signature.
+ *
+ * Looks up the public key that created the signature (SIG->KEYID)
+ * from the key db.  Makes sure that the signature is valid (it was
+ * not created prior to the key, the public key was created in the
+ * past, and the signature does not include any unsupported critical
+ * features), finishes computing the hash of the signature data, and
+ * checks that the signature verifies the digest.  If the key that
+ * generated the signature is a subkey, this function also verifies
+ * that there is a valid backsig from the subkey to the primary key.
+ * Finally, if status fd is enabled and the signature class is 0x00 or
+ * 0x01, then a STATUS_SIG_ID is emitted on the status fd.
+ *
+ * SIG is the signature to check.
+ *
+ * DIGEST contains a valid hash context that already includes the
+ * signed data.  This function adds the relevant meta-data from the
+ * signature packet to compute the final hash.  (See Section 5.2 of
+ * RFC 4880: "The concatenation of the data being signed and the
+ * signature data from the version number through the hashed subpacket
+ * data (inclusive) is hashed.")
+ *
+ * If FORCED_PK is not NULL this public key is used to verify the
+ * signature and no other public key is looked up.
+ *
+ * If R_EXPIREDATE is not NULL, R_EXPIREDATE is set to the key's
+ * expiry.
+ *
+ * If R_EXPIRED is not NULL, *R_EXPIRED is set to 1 if PK has expired
+ * (0 otherwise).  Note: PK being expired does not cause this function
+ * to fail.
+ *
+ * If R_REVOKED is not NULL, *R_REVOKED is set to 1 if PK has been
+ * revoked (0 otherwise).  Note: PK being revoked does not cause this
+ * function to fail.
+ *
+ * If R_PK is not NULL, the public key is stored at that address if it
+ * was found; other wise NULL is stored.
+ *
+ * Returns 0 on success.  An error code otherwise.  */
+gpg_error_t
+check_signature2 (ctrl_t ctrl,
+                  PKT_signature *sig, gcry_md_hd_t digest,
+                  PKT_public_key *forced_pk,
+                  u32 *r_expiredate,
+		  int *r_expired, int *r_revoked, PKT_public_key **r_pk)
+{
+  int rc=0;
+  PKT_public_key *pk;
+
+  if (r_expiredate)
+    *r_expiredate = 0;
+  if (r_expired)
+    *r_expired = 0;
+  if (r_revoked)
+    *r_revoked = 0;
+  if (r_pk)
+    *r_pk = NULL;
+
+  pk = xtrycalloc (1, sizeof *pk);
+  if (!pk)
+    return gpg_error_from_syserror ();
+
+  if  ((rc=openpgp_md_test_algo(sig->digest_algo)))
+    {
+      /* We don't have this digest. */
+    }
+  else if (!gnupg_digest_is_allowed (opt.compliance, 0, sig->digest_algo))
+    {
+      /* Compliance failure.  */
+      log_info (_("digest algorithm '%s' may not be used in %s mode\n"),
+                gcry_md_algo_name (sig->digest_algo),
+                gnupg_compliance_option_string (opt.compliance));
+      rc = gpg_error (GPG_ERR_DIGEST_ALGO);
+    }
+  else if ((rc=openpgp_pk_test_algo(sig->pubkey_algo)))
+    {
+      /* We don't have this pubkey algo. */
+    }
+  else if (!gcry_md_is_enabled (digest,sig->digest_algo))
+    {
+      /* Sanity check that the md has a context for the hash that the
+       * sig is expecting.  This can happen if a onepass sig header
+       * does not match the actual sig, and also if the clearsign
+       * "Hash:" header is missing or does not match the actual sig. */
+      log_info(_("WARNING: signature digest conflict in message\n"));
+      rc = gpg_error (GPG_ERR_GENERAL);
+    }
+  else if (get_pubkey_for_sig (ctrl, pk, sig, forced_pk))
+    rc = gpg_error (GPG_ERR_NO_PUBKEY);
+  else if ((rc = check_key_verify_compliance (pk)))
+    ;/* Compliance failure.  */
+  else if (!pk->flags.valid)
+    {
+      /* You cannot have a good sig from an invalid key.  */
+      rc = gpg_error (GPG_ERR_BAD_PUBKEY);
+    }
+  else
+    {
+      if (r_expiredate)
+        *r_expiredate = pk->expiredate;
+
+      rc = check_signature_end (pk, sig, digest, r_expired, r_revoked, NULL);
+
+      /* Check the backsig.  This is a back signature (0x19) from
+       * the subkey on the primary key.  The idea here is that it
+       * should not be possible for someone to "steal" subkeys and
+       * claim them as their own.  The attacker couldn't actually
+       * use the subkey, but they could try and claim ownership of
+       * any signatures issued by it.  */
+      if (!rc && !pk->flags.primary && pk->flags.backsig < 2)
+        {
+          if (!pk->flags.backsig)
+            {
+              log_info (_("WARNING: signing subkey %s is not"
+                          " cross-certified\n"),keystr_from_pk(pk));
+              log_info (_("please see %s for more information\n"),
+                        "https://gnupg.org/faq/subkey-cross-certify.html");
+              /* The default option --require-cross-certification
+               * makes this warning an error.  */
+              if (opt.flags.require_cross_cert)
+                rc = gpg_error (GPG_ERR_GENERAL);
+            }
+          else if(pk->flags.backsig == 1)
+            {
+              log_info (_("WARNING: signing subkey %s has an invalid"
+                          " cross-certification\n"), keystr_from_pk(pk));
+              rc = gpg_error (GPG_ERR_GENERAL);
+            }
+        }
+
+    }
+
+  if (!rc && sig->sig_class < 2 && is_status_enabled ())
+    {
+      /* This signature id works best with DLP algorithms because
+       * they use a random parameter for every signature.  Instead of
+       * this sig-id we could have also used the hash of the document
+       * and the timestamp, but the drawback of this is, that it is
+       * not possible to sign more than one identical document within
+       * one second.	Some remote batch processing applications might
+       * like this feature here.
+       *
+       * Note that before 2.0.10, we used RIPE-MD160 for the hash
+       * and accidentally didn't include the timestamp and algorithm
+       * information in the hash.  Given that this feature is not
+       * commonly used and that a replay attacks detection should
+       * not solely be based on this feature (because it does not
+       * work with RSA), we take the freedom and switch to SHA-1
+       * with 2.0.10 to take advantage of hardware supported SHA-1
+       * implementations.  We also include the missing information
+       * in the hash.  Note also the SIG_ID as computed by gpg 1.x
+       * and gpg 2.x didn't matched either because 2.x used to print
+       * MPIs not in PGP format.  */
+      u32 a = sig->timestamp;
+      int nsig = pubkey_get_nsig (sig->pubkey_algo);
+      unsigned char *p, *buffer;
+      size_t n, nbytes;
+      int i;
+      char hashbuf[20];
+
+      nbytes = 6;
+      for (i=0; i < nsig; i++ )
+        {
+          if (gcry_mpi_print (GCRYMPI_FMT_USG, NULL, 0, &n, sig->data[i]))
+            BUG();
+          nbytes += n;
+        }
+
+      /* Make buffer large enough to be later used as output buffer.  */
+      if (nbytes < 100)
+        nbytes = 100;
+      nbytes += 10;  /* Safety margin.  */
+
+      /* Fill and hash buffer.  */
+      buffer = p = xmalloc (nbytes);
+      *p++ = sig->pubkey_algo;
+      *p++ = sig->digest_algo;
+      *p++ = (a >> 24) & 0xff;
+      *p++ = (a >> 16) & 0xff;
+      *p++ = (a >>  8) & 0xff;
+      *p++ =  a & 0xff;
+      nbytes -= 6;
+      for (i=0; i < nsig; i++ )
+        {
+          if (gcry_mpi_print (GCRYMPI_FMT_PGP, p, nbytes, &n, sig->data[i]))
+            BUG();
+          p += n;
+          nbytes -= n;
+        }
+      gcry_md_hash_buffer (GCRY_MD_SHA1, hashbuf, buffer, p-buffer);
+
+      p = make_radix64_string (hashbuf, 20);
+      sprintf (buffer, "%s %s %lu",
+               p, strtimestamp (sig->timestamp), (ulong)sig->timestamp);
+      xfree (p);
+      write_status_text (STATUS_SIG_ID, buffer);
+      xfree (buffer);
+    }
+
+  if (r_pk)
+    *r_pk = pk;
+  else
+    {
+      release_public_key_parts (pk);
+      xfree (pk);
+    }
+
+  return rc;
+}
+
+
+/* The signature SIG was generated with the public key PK.  Check
+ * whether the signature is valid in the following sense:
+ *
+ *   - Make sure the public key was created before the signature was
+ *     generated.
+ *
+ *   - Make sure the public key was created in the past
+ *
+ *   - Check whether PK has expired (set *R_EXPIRED to 1 if so and 0
+ *     otherwise)
+ *
+ *   - Check whether PK has been revoked (set *R_REVOKED to 1 if so
+ *     and 0 otherwise).
+ *
+ * If either of the first two tests fail, returns an error code.
+ * Otherwise returns 0.  (Thus, this function doesn't fail if the
+ * public key is expired or revoked.)  */
+static int
+check_signature_metadata_validity (PKT_public_key *pk, PKT_signature *sig,
+				   int *r_expired, int *r_revoked)
+{
+  u32 cur_time;
+
+  if (r_expired)
+    *r_expired = 0;
+  if (r_revoked)
+    *r_revoked = 0;
+
+  if (pk->timestamp > sig->timestamp )
+    {
+      ulong d = pk->timestamp - sig->timestamp;
+      if ( d < 86400 )
+        {
+          log_info (ngettext
+                    ("public key %s is %lu second newer than the signature\n",
+                     "public key %s is %lu seconds newer than the signature\n",
+                     d), keystr_from_pk (pk), d);
+        }
+      else
+        {
+          d /= 86400;
+          log_info (ngettext
+                    ("public key %s is %lu day newer than the signature\n",
+                     "public key %s is %lu days newer than the signature\n",
+                     d), keystr_from_pk (pk), d);
+        }
+      if (!opt.ignore_time_conflict)
+        return GPG_ERR_TIME_CONFLICT; /* pubkey newer than signature.  */
+    }
+
+  cur_time = make_timestamp ();
+  if (pk->timestamp > cur_time)
+    {
+      ulong d = pk->timestamp - cur_time;
+      if (d < 86400)
+        {
+          log_info (ngettext("key %s was created %lu second"
+                             " in the future (time warp or clock problem)\n",
+                             "key %s was created %lu seconds"
+                             " in the future (time warp or clock problem)\n",
+                             d), keystr_from_pk (pk), d);
+        }
+      else
+        {
+          d /= 86400;
+          log_info (ngettext("key %s was created %lu day"
+                             " in the future (time warp or clock problem)\n",
+                             "key %s was created %lu days"
+                             " in the future (time warp or clock problem)\n",
+                             d), keystr_from_pk (pk), d);
+        }
+      if (!opt.ignore_time_conflict)
+        return GPG_ERR_TIME_CONFLICT;
+    }
+
+  /* Check whether the key has expired.  We check the has_expired
+   * flag which is set after a full evaluation of the key (getkey.c)
+   * as well as a simple compare to the current time in case the
+   * merge has for whatever reasons not been done.  */
+  if (pk->has_expired || (pk->expiredate && pk->expiredate < cur_time))
+    {
+      char buf[11];
+      if (opt.verbose)
+        log_info (_("Note: signature key %s expired %s\n"),
+                  keystr_from_pk(pk), asctimestamp( pk->expiredate ) );
+      snprintf (buf, sizeof buf, "%lu",(ulong)pk->expiredate);
+      write_status_text (STATUS_KEYEXPIRED, buf);
+      if (r_expired)
+        *r_expired = 1;
+    }
+
+  if (pk->flags.revoked)
+    {
+      if (opt.verbose)
+        log_info (_("Note: signature key %s has been revoked\n"),
+                  keystr_from_pk(pk));
+      if (r_revoked)
+        *r_revoked=1;
+    }
+
+  return 0;
+}
+
+
+/* Finish generating a signature and check it.  Concretely: make sure
+ * that the signature is valid (it was not created prior to the key,
+ * the public key was created in the past, and the signature does not
+ * include any unsupported critical features), finish computing the
+ * digest by adding the relevant data from the signature packet, and
+ * check that the signature verifies the digest.
+ *
+ * DIGEST contains a hash context, which has already hashed the signed
+ * data.  This function adds the relevant meta-data from the signature
+ * packet to compute the final hash.  (See Section 5.2 of RFC 4880:
+ * "The concatenation of the data being signed and the signature data
+ * from the version number through the hashed subpacket data
+ * (inclusive) is hashed.")
+ *
+ * SIG is the signature to check.
+ *
+ * PK is the public key used to generate the signature.
+ *
+ * If R_EXPIRED is not NULL, *R_EXPIRED is set to 1 if PK has expired
+ * (0 otherwise).  Note: PK being expired does not cause this function
+ * to fail.
+ *
+ * If R_REVOKED is not NULL, *R_REVOKED is set to 1 if PK has been
+ * revoked (0 otherwise).  Note: PK being revoked does not cause this
+ * function to fail.
+ *
+ * If RET_PK is not NULL, PK is copied into RET_PK on success.
+ *
+ * Returns 0 on success.  An error code other.  */
+static int
+check_signature_end (PKT_public_key *pk, PKT_signature *sig,
+		     gcry_md_hd_t digest,
+		     int *r_expired, int *r_revoked, PKT_public_key *ret_pk)
+{
+  int rc = 0;
+
+  if ((rc = check_signature_metadata_validity (pk, sig,
+                                               r_expired, r_revoked)))
+    return rc;
+
+  if ((rc = check_signature_end_simple (pk, sig, digest)))
+    return rc;
+
+  if (!rc && ret_pk)
+    copy_public_key(ret_pk,pk);
+
+  return rc;
+}
+
+
+/* This function is similar to check_signature_end, but it only checks
+ * whether the signature was generated by PK.  It does not check
+ * expiration, revocation, etc.  */
+static int
+check_signature_end_simple (PKT_public_key *pk, PKT_signature *sig,
+                            gcry_md_hd_t digest)
+{
+  gcry_mpi_t result = NULL;
+  int rc = 0;
+
+  if (!opt.flags.allow_weak_digest_algos)
+    {
+      if (is_weak_digest (sig->digest_algo))
+        {
+          print_digest_rejected_note (sig->digest_algo);
+          return GPG_ERR_DIGEST_ALGO;
+        }
+    }
+
+  /* For key signatures check that the key has a cert usage.  We may
+   * do this only for subkeys because the primary may always issue key
+   * signature.  The latter may not be reflected in the pubkey_usage
+   * field because we need to check the key signatures to extract the
+   * key usage.  */
+  if (!pk->flags.primary
+      && IS_CERT (sig) && !(pk->pubkey_usage & PUBKEY_USAGE_CERT))
+    {
+      rc = gpg_error (GPG_ERR_WRONG_KEY_USAGE);
+      if (!opt.quiet)
+        log_info (_("bad key signature from key %s: %s (0x%02x, 0x%x)\n"),
+                  keystr_from_pk (pk), gpg_strerror (rc),
+                  sig->sig_class, pk->pubkey_usage);
+      return rc;
+    }
+
+  /* For data signatures check that the key has sign usage.  */
+  if (!IS_BACK_SIG (sig) && IS_SIG (sig)
+      && !(pk->pubkey_usage & PUBKEY_USAGE_SIG))
+    {
+      rc = gpg_error (GPG_ERR_WRONG_KEY_USAGE);
+      if (!opt.quiet)
+        log_info (_("bad data signature from key %s: %s (0x%02x, 0x%x)\n"),
+                  keystr_from_pk (pk), gpg_strerror (rc),
+                  sig->sig_class, pk->pubkey_usage);
+      return rc;
+    }
+
+  /* Make sure the digest algo is enabled (in case of a detached
+   * signature).  */
+  gcry_md_enable (digest, sig->digest_algo);
+
+  /* Complete the digest. */
+  if (sig->version >= 4)
+    gcry_md_putc (digest, sig->version);
+
+  gcry_md_putc( digest, sig->sig_class );
+  if (sig->version < 4)
+    {
+      u32 a = sig->timestamp;
+      gcry_md_putc (digest, ((a >> 24) & 0xff));
+      gcry_md_putc (digest, ((a >> 16) & 0xff));
+      gcry_md_putc (digest, ((a >>  8) & 0xff));
+      gcry_md_putc (digest, ( a        & 0xff));
+    }
+  else
+    {
+      byte buf[6];
+      size_t n;
+      gcry_md_putc (digest, sig->pubkey_algo);
+      gcry_md_putc (digest, sig->digest_algo);
+      if (sig->hashed)
+        {
+          n = sig->hashed->len;
+          gcry_md_putc (digest, (n >> 8) );
+          gcry_md_putc (digest,  n       );
+          gcry_md_write (digest, sig->hashed->data, n);
+          n += 6;
+	}
+      else
+        {
+	  /* Two octets for the (empty) length of the hashed
+           * section. */
+          gcry_md_putc (digest, 0);
+	  gcry_md_putc (digest, 0);
+	  n = 6;
+	}
+      /* Add some magic per Section 5.2.4 of RFC 4880.  */
+      buf[0] = sig->version;
+      buf[1] = 0xff;
+      buf[2] = n >> 24;
+      buf[3] = n >> 16;
+      buf[4] = n >>  8;
+      buf[5] = n;
+      gcry_md_write( digest, buf, 6 );
+    }
+  gcry_md_final( digest );
+
+  /* Convert the digest to an MPI.  */
+  result = encode_md_value (pk, digest, sig->digest_algo );
+  if (!result)
+    return GPG_ERR_GENERAL;
+
+  /* Verify the signature.  */
+  rc = pk_verify (pk->pubkey_algo, result, sig->data, pk->pkey);
+  gcry_mpi_release (result);
+
+  if (!rc && sig->flags.unknown_critical)
+    {
+      log_info(_("assuming bad signature from key %s"
+                 " due to an unknown critical bit\n"),keystr_from_pk(pk));
+      rc = GPG_ERR_BAD_SIGNATURE;
+    }
+
+  return rc;
+}
+
+
+/* Add a uid node to a hash context.  See section 5.2.4, paragraph 4
+ * of RFC 4880.  */
+static void
+hash_uid_packet (PKT_user_id *uid, gcry_md_hd_t md, PKT_signature *sig )
+{
+  if (uid->attrib_data)
+    {
+      if (sig->version >=4)
+        {
+          byte buf[5];
+          buf[0] = 0xd1;		   /* packet of type 17 */
+          buf[1] = uid->attrib_len >> 24;  /* always use 4 length bytes */
+          buf[2] = uid->attrib_len >> 16;
+          buf[3] = uid->attrib_len >>  8;
+          buf[4] = uid->attrib_len;
+          gcry_md_write( md, buf, 5 );
+	}
+      gcry_md_write( md, uid->attrib_data, uid->attrib_len );
+    }
+  else
+    {
+      if (sig->version >=4)
+        {
+          byte buf[5];
+          buf[0] = 0xb4;	      /* indicates a userid packet */
+          buf[1] = uid->len >> 24;    /* always use 4 length bytes */
+          buf[2] = uid->len >> 16;
+          buf[3] = uid->len >>  8;
+          buf[4] = uid->len;
+          gcry_md_write( md, buf, 5 );
+	}
+      gcry_md_write( md, uid->name, uid->len );
+    }
+}
+
+static void
+cache_sig_result ( PKT_signature *sig, int result )
+{
+  if (!result)
+    {
+      sig->flags.checked = 1;
+      sig->flags.valid = 1;
+    }
+  else if  (gpg_err_code (result) == GPG_ERR_BAD_SIGNATURE)
+    {
+      sig->flags.checked = 1;
+      sig->flags.valid = 0;
+    }
+  else
+    {
+      sig->flags.checked = 0;
+      sig->flags.valid = 0;
+    }
+}
+
+
+/* SIG is a key revocation signature.  Check if this signature was
+ * generated by any of the public key PK's designated revokers.
+ *
+ *   PK is the public key that SIG allegedly revokes.
+ *
+ *   SIG is the revocation signature to check.
+ *
+ * This function avoids infinite recursion, which can happen if two
+ * keys are designed revokers for each other and they revoke each
+ * other.  This is done by observing that if a key A is revoked by key
+ * B we still consider the revocation to be valid even if B is
+ * revoked.  Thus, we don't need to determine whether B is revoked to
+ * determine whether A has been revoked by B, we just need to check
+ * the signature.
+ *
+ * Returns 0 if sig is valid (i.e. pk is revoked), non-0 if not
+ * revoked.  We are careful to make sure that GPG_ERR_NO_PUBKEY is
+ * only returned when a revocation signature is from a valid
+ * revocation key designated in a revkey subpacket, but the revocation
+ * key itself isn't present.
+ *
+ * XXX: This code will need to be modified if gpg ever becomes
+ * multi-threaded.  Note that this guarantees that a designated
+ * revocation sig will never be considered valid unless it is actually
+ * valid, as well as being issued by a revocation key in a valid
+ * direct signature.  Note also that this is written so that a revoked
+ * revoker can still issue revocations: i.e. If A revokes B, but A is
+ * revoked, B is still revoked.  I'm not completely convinced this is
+ * the proper behavior, but it matches how PGP does it. -dms */
+int
+check_revocation_keys (ctrl_t ctrl, PKT_public_key *pk, PKT_signature *sig)
+{
+  static int busy=0;
+  int i;
+  int rc = GPG_ERR_GENERAL;
+
+  log_assert (IS_KEY_REV(sig));
+  log_assert ((sig->keyid[0]!=pk->keyid[0]) || (sig->keyid[0]!=pk->keyid[1]));
+
+  /* Avoid infinite recursion.  Consider the following:
+   *
+   *   - We want to check if A is revoked.
+   *
+   *   - C is a designated revoker for B and has revoked B.
+   *
+   *   - B is a designated revoker for A and has revoked A.
+   *
+   * When checking if A is revoked (in merge_selfsigs_main), we
+   * observe that A has a designed revoker.  As such, we call this
+   * function.  This function sees that there is a valid revocation
+   * signature, which is signed by B.  It then calls check_signature()
+   * to verify that the signature is good.  To check the sig, we need
+   * to lookup B.  Looking up B means calling merge_selfsigs_main,
+   * which checks whether B is revoked, which calls this function to
+   * see if B was revoked by some key.
+   *
+   * In this case, the added level of indirection doesn't hurt.  It
+   * just means a bit more work.  However, if C == A, then we'd end up
+   * in a loop.  But, it doesn't make sense to look up C anyways: even
+   * if B is revoked, we conservatively consider a valid revocation
+   * signed by B to revoke A.  Since this is the only place where this
+   * type of recursion can occur, we simply cause this function to
+   * fail if it is entered recursively.  */
+  if (busy)
+    {
+      /* Return an error (i.e. not revoked), but mark the pk as
+         uncacheable as we don't really know its revocation status
+         until it is checked directly.  */
+      pk->flags.dont_cache = 1;
+      return rc;
+    }
+
+  busy=1;
+
+  /*  es_printf("looking at %08lX with a sig from %08lX\n",(ulong)pk->keyid[1],
+      (ulong)sig->keyid[1]); */
+
+  /* is the issuer of the sig one of our revokers? */
+  if( !pk->revkey && pk->numrevkeys )
+     BUG();
+  else
+      for(i=0;i<pk->numrevkeys;i++)
+	{
+	  /* The revoker's keyid.  */
+          u32 keyid[2];
+
+          keyid_from_fingerprint (ctrl, pk->revkey[i].fpr,
+                                  MAX_FINGERPRINT_LEN, keyid);
+
+          if(keyid[0]==sig->keyid[0] && keyid[1]==sig->keyid[1])
+	    /* The signature was generated by a designated revoker.
+	       Verify the signature.  */
+	    {
+              gcry_md_hd_t md;
+
+              if (gcry_md_open (&md, sig->digest_algo, 0))
+                BUG ();
+              hash_public_key(md,pk);
+	      /* Note: check_signature only checks that the signature
+		 is good.  It does not fail if the key is revoked.  */
+              rc = check_signature (ctrl, sig, md);
+	      cache_sig_result(sig,rc);
+              gcry_md_close (md);
+	      break;
+	    }
+	}
+
+  busy=0;
+
+  return rc;
+}
+
+/* Check that the backsig BACKSIG from the subkey SUB_PK to its
+ * primary key MAIN_PK is valid.
+ *
+ * Backsigs (0x19) have the same format as binding sigs (0x18), but
+ * this function is simpler than check_key_signature in a few ways.
+ * For example, there is no support for expiring backsigs since it is
+ * questionable what such a thing actually means.  Note also that the
+ * sig cache check here, unlike other sig caches in GnuPG, is not
+ * persistent.  */
+int
+check_backsig (PKT_public_key *main_pk,PKT_public_key *sub_pk,
+	       PKT_signature *backsig)
+{
+  gcry_md_hd_t md;
+  int rc;
+
+  /* Always check whether the algorithm is available.  Although
+     gcry_md_open would throw an error, some libgcrypt versions will
+     print a debug message in that case too. */
+  if ((rc=openpgp_md_test_algo (backsig->digest_algo)))
+    return rc;
+
+  if(!opt.no_sig_cache && backsig->flags.checked)
+    return backsig->flags.valid? 0 : gpg_error (GPG_ERR_BAD_SIGNATURE);
+
+  rc = gcry_md_open (&md, backsig->digest_algo,0);
+  if (!rc)
+    {
+      hash_public_key(md,main_pk);
+      hash_public_key(md,sub_pk);
+      rc = check_signature_end (sub_pk, backsig, md, NULL, NULL, NULL);
+      cache_sig_result(backsig,rc);
+      gcry_md_close(md);
+    }
+
+  return rc;
+}
+
+
+/* Check that a signature over a key is valid.  This is a
+ * specialization of check_key_signature2 with the unnamed parameters
+ * passed as NULL.  See the documentation for that function for more
+ * details.  */
+int
+check_key_signature (ctrl_t ctrl, kbnode_t root, kbnode_t node,
+                     int *is_selfsig)
+{
+  return check_key_signature2 (ctrl, root, node, NULL, NULL,
+                               is_selfsig, NULL, NULL);
+}
+
+
+/* Returns whether SIGNER generated the signature SIG over the packet
+ * PACKET, which is a key, subkey or uid, and comes from the key block
+ * KB.  (KB is PACKET's corresponding keyblock; we don't assume that
+ * SIG has been added to the keyblock.)
+ *
+ * If SIGNER is set, then checks whether SIGNER generated the
+ * signature.  Otherwise, uses SIG->KEYID to find the alleged signer.
+ * This parameter can be used to effectively override the alleged
+ * signer that is stored in SIG.
+ *
+ * KB may be NULL if SIGNER is set.
+ *
+ * Unlike check_key_signature, this function ignores any cached
+ * results!  That is, it does not consider SIG->FLAGS.CHECKED and
+ * SIG->FLAGS.VALID nor does it set them.
+ *
+ * This doesn't check the signature's semantic mean.  Concretely, it
+ * doesn't check whether a non-self signed revocation signature was
+ * created by a designated revoker.  In fact, it doesn't return an
+ * error for a binding generated by a completely different key!
+ *
+ * Returns 0 if the signature is valid.  Returns GPG_ERR_SIG_CLASS if
+ * this signature can't be over PACKET.  Returns GPG_ERR_NOT_FOUND if
+ * the key that generated the signature (according to SIG) could not
+ * be found.  Returns GPG_ERR_BAD_SIGNATURE if the signature is bad.
+ * Other errors codes may be returned if something else goes wrong.
+ *
+ * IF IS_SELFSIG is not NULL, sets *IS_SELFSIG to 1 if this is a
+ * self-signature (by the key's primary key) or 0 if not.
+ *
+ * If RET_PK is not NULL, returns a copy of the public key that
+ * generated the signature (i.e., the signer) on success.  This must
+ * be released by the caller using release_public_key_parts ().  */
+gpg_error_t
+check_signature_over_key_or_uid (ctrl_t ctrl, PKT_public_key *signer,
+                                 PKT_signature *sig, KBNODE kb, PACKET *packet,
+                                 int *is_selfsig, PKT_public_key *ret_pk)
+{
+  int rc;
+  PKT_public_key *pripk = kb->pkt->pkt.public_key;
+  gcry_md_hd_t md;
+  int signer_alloced = 0;
+  int stub_is_selfsig;
+
+  if (!is_selfsig)
+    is_selfsig = &stub_is_selfsig;
+
+  rc = openpgp_pk_test_algo (sig->pubkey_algo);
+  if (rc)
+    return rc;
+  rc = openpgp_md_test_algo (sig->digest_algo);
+  if (rc)
+    return rc;
+
+  /* A signature's class indicates the type of packet that it
+     signs.  */
+  if (IS_BACK_SIG (sig) || IS_KEY_SIG (sig) || IS_KEY_REV (sig))
+    {
+      /* Key revocations can only be over primary keys.  */
+      if (packet->pkttype != PKT_PUBLIC_KEY)
+        return gpg_error (GPG_ERR_SIG_CLASS);
+    }
+  else if (IS_SUBKEY_SIG (sig) || IS_SUBKEY_REV (sig))
+    {
+      if (packet->pkttype != PKT_PUBLIC_SUBKEY)
+        return gpg_error (GPG_ERR_SIG_CLASS);
+    }
+  else if (IS_UID_SIG (sig) || IS_UID_REV (sig))
+    {
+      if (packet->pkttype != PKT_USER_ID)
+        return gpg_error (GPG_ERR_SIG_CLASS);
+    }
+  else
+    return gpg_error (GPG_ERR_SIG_CLASS);
+
+  /* PACKET is the right type for SIG.  */
+
+  if (signer)
+    {
+      if (signer->keyid[0] == pripk->keyid[0]
+          && signer->keyid[1] == pripk->keyid[1])
+        *is_selfsig = 1;
+      else
+        *is_selfsig = 0;
+    }
+  else
+    {
+      /* Get the signer.  If possible, avoid a look up.  */
+      if (sig->keyid[0] == pripk->keyid[0]
+          && sig->keyid[1] == pripk->keyid[1])
+        {
+          /* Issued by the primary key.  */
+          signer = pripk;
+          *is_selfsig = 1;
+        }
+      else
+        {
+          /* See if one of the subkeys was the signer (although this
+           * is extremely unlikely).  */
+          kbnode_t ctx = NULL;
+          kbnode_t n;
+
+          while ((n = walk_kbnode (kb, &ctx, 0)))
+            {
+              PKT_public_key *subk;
+
+              if (n->pkt->pkttype != PKT_PUBLIC_SUBKEY)
+                continue;
+
+              subk = n->pkt->pkt.public_key;
+              if (sig->keyid[0] == subk->keyid[0]
+                  && sig->keyid[1] == subk->keyid[1])
+                {
+                  /* Issued by a subkey.  */
+                  signer = subk;
+                  break;
+                }
+            }
+
+          if (! signer)
+            {
+              /* Signer by some other key.  */
+              *is_selfsig = 0;
+              if (ret_pk)
+                {
+                  signer = ret_pk;
+                  /* FIXME: Using memset here is probematic because it
+                   * assumes that there are no allocated fields in
+                   * SIGNER.  */
+                  memset (signer, 0, sizeof (*signer));
+                  signer_alloced = 1;
+                }
+              else
+                {
+                  signer = xmalloc_clear (sizeof (*signer));
+                  signer_alloced = 2;
+                }
+
+              if (IS_CERT (sig))
+                signer->req_usage = PUBKEY_USAGE_CERT;
+
+              rc = get_pubkey_for_sig (ctrl, signer, sig, NULL);
+              if (rc)
+                {
+                  xfree (signer);
+                  signer = NULL;
+                  signer_alloced = 0;
+                  goto leave;
+                }
+            }
+        }
+    }
+
+  /* We checked above that we supported this algo, so an error here is
+   * a bug.  */
+  if (gcry_md_open (&md, sig->digest_algo, 0))
+    BUG ();
+
+  /* Hash the relevant data.  */
+
+  if (IS_KEY_SIG (sig) || IS_KEY_REV (sig))
+    {
+      log_assert (packet->pkttype == PKT_PUBLIC_KEY);
+      hash_public_key (md, packet->pkt.public_key);
+      rc = check_signature_end_simple (signer, sig, md);
+    }
+  else if (IS_BACK_SIG (sig))
+    {
+      log_assert (packet->pkttype == PKT_PUBLIC_KEY);
+      hash_public_key (md, packet->pkt.public_key);
+      hash_public_key (md, signer);
+      rc = check_signature_end_simple (signer, sig, md);
+    }
+  else if (IS_SUBKEY_SIG (sig) || IS_SUBKEY_REV (sig))
+    {
+      log_assert (packet->pkttype == PKT_PUBLIC_SUBKEY);
+      hash_public_key (md, pripk);
+      hash_public_key (md, packet->pkt.public_key);
+      rc = check_signature_end_simple (signer, sig, md);
+    }
+  else if (IS_UID_SIG (sig) || IS_UID_REV (sig))
+    {
+      log_assert (packet->pkttype == PKT_USER_ID);
+      if (sig->digest_algo == DIGEST_ALGO_SHA1 && !*is_selfsig
+          && sig->timestamp > 1547856000
+          && !opt.flags.allow_weak_key_signatures)
+        {
+          /* 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.  */
+          print_sha1_keysig_rejected_note ();
+          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
+    {
+      /* We should never get here.  (The first if above should have
+       * already caught this error.)  */
+      BUG ();
+    }
+
+  gcry_md_close (md);
+
+ leave:
+  if (! rc && ret_pk && ret_pk != signer)
+    copy_public_key (ret_pk, signer);
+
+  if (signer_alloced)
+    {
+      /* We looked up SIGNER; it is not a pointer into KB.  */
+      release_public_key_parts (signer);
+      /* Free if we also allocated the memory.  */
+      if (signer_alloced == 2)
+        xfree (signer);
+    }
+
+  return rc;
+}
+
+
+/* Check that a signature over a key (e.g., a key revocation, key
+ * binding, user id certification, etc.) is valid.  If the function
+ * detects a self-signature, it uses the public key from the specified
+ * key block and does not bother looking up the key specified in the
+ * signature packet.
+ *
+ * ROOT is a keyblock.
+ *
+ * NODE references a signature packet that appears in the keyblock
+ * that should be verified.
+ *
+ * If CHECK_PK is set, the specified key is sometimes preferred for
+ * verifying signatures.  See the implementation for details.
+ *
+ * If RET_PK is not NULL, the public key that successfully verified
+ * the signature is copied into *RET_PK.
+ *
+ * If IS_SELFSIG is not NULL, *IS_SELFSIG is set to 1 if NODE is a
+ * self-signature.
+ *
+ * If R_EXPIREDATE is not NULL, *R_EXPIREDATE is set to the expiry
+ * date.
+ *
+ * If R_EXPIRED is not NULL, *R_EXPIRED is set to 1 if PK has been
+ * expired (0 otherwise).  Note: PK being revoked does not cause this
+ * function to fail.
+ *
+ *
+ * If OPT.NO_SIG_CACHE is not set, this function will first check if
+ * the result of a previous verification is already cached in the
+ * signature packet's data structure.
+ *
+ * TODO: add r_revoked here as well.  It has the same problems as
+ * r_expiredate and r_expired and the cache.  */
+int
+check_key_signature2 (ctrl_t ctrl,
+                      kbnode_t root, kbnode_t node, PKT_public_key *check_pk,
+                      PKT_public_key *ret_pk, int *is_selfsig,
+                      u32 *r_expiredate, int *r_expired )
+{
+  PKT_public_key *pk;
+  PKT_signature *sig;
+  int algo;
+  int rc;
+
+  if (is_selfsig)
+    *is_selfsig = 0;
+  if (r_expiredate)
+    *r_expiredate = 0;
+  if (r_expired)
+    *r_expired = 0;
+  log_assert (node->pkt->pkttype == PKT_SIGNATURE);
+  log_assert (root->pkt->pkttype == PKT_PUBLIC_KEY);
+
+  pk = root->pkt->pkt.public_key;
+  sig = node->pkt->pkt.signature;
+  algo = sig->digest_algo;
+
+  /* Check whether we have cached the result of a previous signature
+   * check.  Note that we may no longer have the pubkey or hash
+   * needed to verify a sig, but can still use the cached value.  A
+   * cache refresh detects and clears these cases. */
+  if ( !opt.no_sig_cache )
+    {
+      cache_stats.total++;
+      if (sig->flags.checked) /* Cached status available.  */
+        {
+          cache_stats.cached++;
+          if (is_selfsig)
+            {
+              u32 keyid[2];
+
+              keyid_from_pk (pk, keyid);
+              if (keyid[0] == sig->keyid[0] && keyid[1] == sig->keyid[1])
+                *is_selfsig = 1;
+	    }
+          /* BUG: This is wrong for non-self-sigs... needs to be the
+           * actual pk.  */
+          rc = check_signature_metadata_validity (pk, sig, r_expired, NULL);
+          if (rc)
+            return rc;
+          if (sig->flags.valid)
+            {
+              cache_stats.goodsig++;
+              return 0;
+            }
+          cache_stats.badsig++;
+          return gpg_error (GPG_ERR_BAD_SIGNATURE);
+        }
+    }
+
+  rc = openpgp_pk_test_algo(sig->pubkey_algo);
+  if (rc)
+    return rc;
+  rc = openpgp_md_test_algo(algo);
+  if (rc)
+    return rc;
+
+  if (IS_KEY_REV (sig))
+    {
+      u32 keyid[2];
+      keyid_from_pk( pk, keyid );
+
+      /* Is it a designated revoker? */
+      if (keyid[0] != sig->keyid[0] || keyid[1] != sig->keyid[1])
+        rc = check_revocation_keys (ctrl, pk, sig);
+      else
+        {
+          rc = check_signature_metadata_validity (pk, sig,
+                                                  r_expired, NULL);
+          if (! rc)
+            rc = check_signature_over_key_or_uid (ctrl, pk, sig,
+                                                  root, root->pkt,
+                                                  is_selfsig, ret_pk);
+        }
+    }
+  else if (IS_SUBKEY_REV (sig) || IS_SUBKEY_SIG (sig))
+    {
+      kbnode_t snode = find_prev_kbnode (root, node, PKT_PUBLIC_SUBKEY);
+
+      if (snode)
+        {
+          rc = check_signature_metadata_validity (pk, sig,
+                                                  r_expired, NULL);
+          if (! rc)
+            {
+              /* A subkey revocation (0x28) must be a self-sig, but a
+               * subkey signature (0x18) needn't be.  */
+              rc = check_signature_over_key_or_uid (ctrl,
+                                                    IS_SUBKEY_SIG (sig)
+                                                    ? NULL : pk,
+                                                    sig, root, snode->pkt,
+                                                    is_selfsig, ret_pk);
+            }
+        }
+      else
+        {
+          if (opt.verbose)
+            {
+              if (IS_SUBKEY_REV (sig))
+                log_info (_("key %s: no subkey for subkey"
+                            " revocation signature\n"), keystr_from_pk(pk));
+              else if (sig->sig_class == 0x18)
+                log_info(_("key %s: no subkey for subkey"
+                           " binding signature\n"), keystr_from_pk(pk));
+            }
+          rc = GPG_ERR_SIG_CLASS;
+        }
+    }
+  else if (IS_KEY_SIG (sig)) /* direct key signature */
+      {
+        rc = check_signature_metadata_validity (pk, sig,
+                                                r_expired, NULL);
+        if (! rc)
+          rc = check_signature_over_key_or_uid (ctrl, pk, sig, root, root->pkt,
+                                                is_selfsig, ret_pk);
+      }
+    else if (IS_UID_SIG (sig) || IS_UID_REV (sig))
+      {
+	kbnode_t unode = find_prev_kbnode (root, node, PKT_USER_ID);
+
+	if (unode)
+          {
+            rc = check_signature_metadata_validity (pk, sig, r_expired, NULL);
+            if (! rc)
+              {
+                /* If this is a self-sig, ignore check_pk.  */
+                rc = check_signature_over_key_or_uid
+                  (ctrl,
+                   keyid_cmp (pk_keyid (pk), sig->keyid) == 0 ? pk : check_pk,
+                   sig, root, unode->pkt, NULL, ret_pk);
+              }
+          }
+	else
+	  {
+            if (!opt.quiet)
+	      log_info ("key %s: no user ID for key signature packet"
+			" of class %02x\n",keystr_from_pk(pk),sig->sig_class);
+	    rc = GPG_ERR_SIG_CLASS;
+	  }
+      }
+  else
+    {
+      log_info ("sig issued by %s with class %d (digest: %02x %02x)"
+                " is not valid over a user id or a key id, ignoring.\n",
+                keystr (sig->keyid), sig->sig_class,
+                sig->digest_start[0], sig->digest_start[1]);
+      rc = gpg_error (GPG_ERR_BAD_SIGNATURE);
+    }
+
+  cache_sig_result  (sig, rc);
+
+  return rc;
+}