When downloading certificates, distinguish requesting by identity digest from requesting by ID digest, signing key pair; fixes bug 5595

5 files changed, 428 insertions, 91 deletions

diff --git a/src/or/directory.c b/src/or/directory.c
index f235bf3b4..8dd5a7cdc 100644
--- a/src/or/directory.c
+++ b/src/or/directory.c
@@ -809,13 +809,34 @@ connection_dir_download_cert_failed(dir_connection_t *conn, int status)
   if (!conn->requested_resource)
     return;
   failed = smartlist_new();
-  dir_split_resource_into_fingerprints(conn->requested_resource+3,
-                                       failed, NULL, DSR_HEX);
-  SMARTLIST_FOREACH(failed, char *, cp,
-  {
-    authority_cert_dl_failed(cp, status);
-    tor_free(cp);
-  });
+  /*
+   * We have two cases download by fingerprint (resource starts
+   * with "fp/") or download by fingerprint/signing key pair
+   * (resource starts with "fp-sk/").
+   */
+  if (!strcmpstart(conn->requested_resource, "fp/")) {
+    /* Download by fingerprint case */
+    dir_split_resource_into_fingerprints(conn->requested_resource + 3,
+                                         failed, NULL, DSR_HEX);
+    SMARTLIST_FOREACH_BEGIN(failed, char *, cp) {
+      /* Null signing key digest indicates download by fp only */
+      authority_cert_dl_failed(cp, NULL, status);
+      tor_free(cp);
+    } SMARTLIST_FOREACH_END(cp);
+  } else if (!strcmpstart(conn->requested_resource, "fp-sk/")) {
+    /* Download by (fp,sk) pairs */
+    dir_split_resource_into_fingerprint_pairs(conn->requested_resource + 5,
+                                              failed);
+    SMARTLIST_FOREACH_BEGIN(failed, fp_pair_t *, cp) {
+      authority_cert_dl_failed(cp->first, cp->second, status);
+      tor_free(cp);
+    } SMARTLIST_FOREACH_END(cp);
+  } else {
+    log_warn(LD_DIR,
+             "Don't know what to do with failure for cert fetch %s",
+             conn->requested_resource);
+  }
+
   smartlist_free(failed);
 
   update_certificate_downloads(time(NULL));
@@ -1589,6 +1610,7 @@ connection_dir_client_reached_eof(dir_connection_t *conn)
                        conn->_base.purpose == DIR_PURPOSE_FETCH_MICRODESC);
   int was_compressed=0;
   time_t now = time(NULL);
+  int src_code;
 
   switch (connection_fetch_from_buf_http(TO_CONN(conn),
                               &headers, MAX_HEADERS_SIZE,
@@ -1857,14 +1879,33 @@ connection_dir_client_reached_eof(dir_connection_t *conn)
     }
     log_info(LD_DIR,"Received authority certificates (size %d) from server "
              "'%s:%d'", (int)body_len, conn->_base.address, conn->_base.port);
-    if (trusted_dirs_load_certs_from_string(body, 0, 1)<0) {
-      log_warn(LD_DIR, "Unable to parse fetched certificates");
-      /* if we fetched more than one and only some failed, the successful
-       * ones got flushed to disk so it's safe to call this on them */
-      connection_dir_download_cert_failed(conn, status_code);
+    /*
+     * Tell trusted_dirs_load_certs_from_string() whether it was by fp
+     * or fp-sk pair.
+     */
+    src_code = -1;
+    if (!strcmpstart(conn->requested_resource, "fp/")) {
+      src_code = TRUSTED_DIRS_CERTS_SRC_DL_BY_ID_DIGEST;
+    } else if (!strcmpstart(conn->requested_resource, "fp-sk/")) {
+      src_code = TRUSTED_DIRS_CERTS_SRC_DL_BY_ID_SK_DIGEST;
+    }
+
+    if (src_code != -1) {
+      if (trusted_dirs_load_certs_from_string(body, src_code, 1)<0) {
+        log_warn(LD_DIR, "Unable to parse fetched certificates");
+        /* if we fetched more than one and only some failed, the successful
+         * ones got flushed to disk so it's safe to call this on them */
+        connection_dir_download_cert_failed(conn, status_code);
+      } else {
+        directory_info_has_arrived(now, 0);
+        log_info(LD_DIR, "Successfully loaded certificates from fetch.");
+      }
     } else {
-      directory_info_has_arrived(now, 0);
-      log_info(LD_DIR, "Successfully loaded certificates from fetch.");
+      log_warn(LD_DIR,
+               "Couldn't figure out what to do with fetched certificates for "
+               "unknown resource %s",
+               conn->requested_resource);
+      connection_dir_download_cert_failed(conn, status_code);
     }
   }
   if (conn->_base.purpose == DIR_PURPOSE_FETCH_STATUS_VOTE) {
diff --git a/src/or/dirvote.c b/src/or/dirvote.c
index 144859ae0..959e05a8a 100644
--- a/src/or/dirvote.c
+++ b/src/or/dirvote.c
@@ -2947,7 +2947,7 @@ dirvote_add_vote(const char *vote_body, const char **msg_out, int *status_out)
     /* Hey, it's a new cert! */
     trusted_dirs_load_certs_from_string(
                                vote->cert->cache_info.signed_descriptor_body,
-                               0 /* from_store */, 1 /*flush*/);
+                               TRUSTED_DIRS_CERTS_SRC_FROM_VOTE, 1 /*flush*/);
     if (!authority_cert_get_by_digests(vote->cert->cache_info.identity_digest,
                                        vote->cert->signing_key_digest)) {
       log_warn(LD_BUG, "We added a cert, but still couldn't find it.");
diff --git a/src/or/router.c b/src/or/router.c
index 1ace8e249..c68a30923 100644
--- a/src/or/router.c
+++ b/src/or/router.c
@@ -758,7 +758,8 @@ init_keys(void)
   if (cert) { /* add my own cert to the list of known certs */
     log_info(LD_DIR, "adding my own v3 cert");
     if (trusted_dirs_load_certs_from_string(
-                      cert->cache_info.signed_descriptor_body, 0, 0)<0) {
+                      cert->cache_info.signed_descriptor_body,
+                      TRUSTED_DIRS_CERTS_SRC_SELF, 0)<0) {
       log_warn(LD_DIR, "Unable to parse my own v3 cert! Failing.");
       return -1;
     }
diff --git a/src/or/routerlist.c b/src/or/routerlist.c
index 3c39e362d..20d511025 100644
--- a/src/or/routerlist.c
+++ b/src/or/routerlist.c
@@ -19,6 +19,7 @@
 #include "directory.h"
 #include "dirserv.h"
 #include "dirvote.h"
+#include "fp_pair.h"
 #include "geoip.h"
 #include "hibernate.h"
 #include "main.h"
@@ -38,6 +39,24 @@
 
 /****************************************************************************/
 
+DECLARE_TYPED_DIGESTMAP_FNS(sdmap_, digest_sd_map_t, signed_descriptor_t)
+DECLARE_TYPED_DIGESTMAP_FNS(rimap_, digest_ri_map_t, routerinfo_t)
+DECLARE_TYPED_DIGESTMAP_FNS(eimap_, digest_ei_map_t, extrainfo_t)
+DECLARE_TYPED_DIGESTMAP_FNS(dsmap_, digest_ds_map_t, download_status_t)
+#define SDMAP_FOREACH(map, keyvar, valvar)                              \
+  DIGESTMAP_FOREACH(sdmap_to_digestmap(map), keyvar, signed_descriptor_t *, \
+                    valvar)
+#define RIMAP_FOREACH(map, keyvar, valvar) \
+  DIGESTMAP_FOREACH(rimap_to_digestmap(map), keyvar, routerinfo_t *, valvar)
+#define EIMAP_FOREACH(map, keyvar, valvar) \
+  DIGESTMAP_FOREACH(eimap_to_digestmap(map), keyvar, extrainfo_t *, valvar)
+#define DSMAP_FOREACH(map, keyvar, valvar) \
+  DIGESTMAP_FOREACH(dsmap_to_digestmap(map), keyvar, download_status_t *, \
+                    valvar)
+
+/* Forward declaration for cert_list_t */
+typedef struct cert_list_t cert_list_t;
+
 /* static function prototypes */
 static const routerstatus_t *router_pick_directory_server_impl(
                                            dirinfo_type_t auth, int flags);
@@ -56,19 +75,14 @@ static const char *signed_descriptor_get_body_impl(
                                               int with_annotations);
 static void list_pending_downloads(digestmap_t *result,
                                    int purpose, const char *prefix);
+static void list_pending_fpsk_downloads(fp_pair_map_t *result);
 static void launch_dummy_descriptor_download_as_needed(time_t now,
                                    const or_options_t *options);
-
-DECLARE_TYPED_DIGESTMAP_FNS(sdmap_, digest_sd_map_t, signed_descriptor_t)
-DECLARE_TYPED_DIGESTMAP_FNS(rimap_, digest_ri_map_t, routerinfo_t)
-DECLARE_TYPED_DIGESTMAP_FNS(eimap_, digest_ei_map_t, extrainfo_t)
-#define SDMAP_FOREACH(map, keyvar, valvar)                              \
-  DIGESTMAP_FOREACH(sdmap_to_digestmap(map), keyvar, signed_descriptor_t *, \
-                    valvar)
-#define RIMAP_FOREACH(map, keyvar, valvar) \
-  DIGESTMAP_FOREACH(rimap_to_digestmap(map), keyvar, routerinfo_t *, valvar)
-#define EIMAP_FOREACH(map, keyvar, valvar) \
-  DIGESTMAP_FOREACH(eimap_to_digestmap(map), keyvar, extrainfo_t *, valvar)
+static void download_status_reset_by_sk_in_cl(cert_list_t *cl,
+                                              const char *digest);
+static int download_status_is_ready_by_sk_in_cl(cert_list_t *cl,
+                                                const char *digest,
+                                                time_t now, int max_failures);
 
 /****************************************************************************/
 
@@ -78,10 +92,17 @@ static smartlist_t *trusted_dir_servers = NULL;
 
 /** List of for a given authority, and download status for latest certificate.
  */
-typedef struct cert_list_t {
-  download_status_t dl_status;
+struct cert_list_t {
+  /*
+   * The keys of download status map are cert->signing_key_digest for pending
+   * downloads by (identity digest/signing key digest) pair; functions such
+   * as authority_cert_get_by_digest() already assume these are unique.
+   */
+  struct digest_ds_map_t *dl_status_map;
+  /* There is also a dlstatus for the download by identity key only */
+  download_status_t dl_status_by_id;
   smartlist_t *certs;
-} cert_list_t;
+};
 /** Map from v3 identity key digest to cert_list_t. */
 static digestmap_t *trusted_dir_certs = NULL;
 /** True iff any key certificate in at least one member of
@@ -125,6 +146,72 @@ get_n_authorities(dirinfo_type_t type)
   return n;
 }
 
+/** Reset the download status of a specified element in a dsmap */
+static void
+download_status_reset_by_sk_in_cl(cert_list_t *cl, const char *digest)
+{
+  download_status_t *dlstatus = NULL;
+
+  tor_assert(cl);
+  tor_assert(digest);
+
+  /* Make sure we have a dsmap */
+  if (!(cl->dl_status_map)) {
+    cl->dl_status_map = dsmap_new();
+  }
+  /* Look for a download_status_t in the map with this digest */
+  dlstatus = dsmap_get(cl->dl_status_map, digest);
+  /* Got one? */
+  if (!dlstatus) {
+    /* Insert before we reset */
+    dlstatus = tor_malloc_zero(sizeof(*dlstatus));
+    dsmap_set(cl->dl_status_map, digest, dlstatus);
+  }
+  tor_assert(dlstatus);
+  /* Go ahead and reset it */
+  download_status_reset(dlstatus);
+}
+
+/**
+ * Return true if the download for this signing key digest in cl is ready
+ * to be re-attempted.
+ */
+static int
+download_status_is_ready_by_sk_in_cl(cert_list_t *cl,
+                                     const char *digest,
+                                     time_t now, int max_failures)
+{
+  int rv = 0;
+  download_status_t *dlstatus = NULL;
+
+  tor_assert(cl);
+  tor_assert(digest);
+
+  /* Make sure we have a dsmap */
+  if (!(cl->dl_status_map)) {
+    cl->dl_status_map = dsmap_new();
+  }
+  /* Look for a download_status_t in the map with this digest */
+  dlstatus = dsmap_get(cl->dl_status_map, digest);
+  /* Got one? */
+  if (dlstatus) {
+    /* Use download_status_is_ready() */
+    rv = download_status_is_ready(dlstatus, now, max_failures);
+  } else {
+    /*
+     * If we don't know anything about it, return 1, since we haven't
+     * tried this one before.  We need to create a new entry here,
+     * too.
+     */
+    dlstatus = tor_malloc_zero(sizeof(*dlstatus));
+    download_status_reset(dlstatus);
+    dsmap_set(cl->dl_status_map, digest, dlstatus);
+    rv = 1;
+  }
+
+  return rv;
+}
+
 #define get_n_v2_authorities() get_n_authorities(V2_DIRINFO)
 
 /** Helper: Return the cert_list_t for an authority whose authority ID is
@@ -138,8 +225,9 @@ get_cert_list(const char *id_digest)
   cl = digestmap_get(trusted_dir_certs, id_digest);
   if (!cl) {
     cl = tor_malloc_zero(sizeof(cert_list_t));
-    cl->dl_status.schedule = DL_SCHED_CONSENSUS;
+    cl->dl_status_by_id.schedule = DL_SCHED_CONSENSUS;
     cl->certs = smartlist_new();
+    cl->dl_status_map = dsmap_new();
     digestmap_set(trusted_dir_certs, id_digest, cl);
   }
   return cl;
@@ -159,7 +247,9 @@ trusted_dirs_reload_certs(void)
   tor_free(filename);
   if (!contents)
     return 0;
-  r = trusted_dirs_load_certs_from_string(contents, 1, 1);
+  r = trusted_dirs_load_certs_from_string(
+        contents,
+        TRUSTED_DIRS_CERTS_SRC_FROM_STORE, 1);
   tor_free(contents);
   return r;
 }
@@ -182,17 +272,23 @@ already_have_cert(authority_cert_t *cert)
 }
 
 /** Load a bunch of new key certificates from the string <b>contents</b>.  If
- * <b>from_store</b> is true, the certificates are from the cache, and we
- * don't need to flush them to disk. If <b>flush</b> is true, we need
- * to flush any changed certificates to disk now.  Return 0 on success, -1
- * if any certs fail to parse. */
+ * <b>source</b> is TRUSTED_DIRS_CERTS_SRC_FROM_STORE, the certificates are
+ * from the cache, and we don't need to flush them to disk.  If we are a
+ * dirauth loading our own cert, source is TRUSTED_DIRS_CERTS_SRC_SELF.
+ * Otherwise, source is download type: TRUSTED_DIRS_CERTS_SRC_DL_BY_ID_DIGEST
+ * or TRUSTED_DIRS_CERTS_SRC_DL_BY_ID_SK_DIGEST.  If <b>flush</b> is true, we
+ * need to flush any changed certificates to disk now.  Return 0 on success,
+ * -1 if any certs fail to parse.
+ */
+
 int
-trusted_dirs_load_certs_from_string(const char *contents, int from_store,
+trusted_dirs_load_certs_from_string(const char *contents, int source,
                                     int flush)
 {
   trusted_dir_server_t *ds;
   const char *s, *eos;
   int failure_code = 0;
+  int from_store = (source == TRUSTED_DIRS_CERTS_SRC_FROM_STORE);
 
   for (s = contents; *s; s = eos) {
     authority_cert_t *cert = authority_cert_parse_from_string(s, &eos);
@@ -229,7 +325,18 @@ trusted_dirs_load_certs_from_string(const char *contents, int from_store,
                    ds ? ds->nickname : "an old or new authority");
         }
 
-        authority_cert_dl_failed(cert->cache_info.identity_digest, 404);
+        /*
+         * This is where we care about the source; authority_cert_dl_failed()
+         * needs to know whether the download was by fp or (fp,sk) pair to
+         * twiddle the right bit in the download map.
+         */
+        if (source == TRUSTED_DIRS_CERTS_SRC_DL_BY_ID_DIGEST) {
+          authority_cert_dl_failed(cert->cache_info.identity_digest,
+                                   NULL, 404);
+        } else if (source == TRUSTED_DIRS_CERTS_SRC_DL_BY_ID_SK_DIGEST) {
+          authority_cert_dl_failed(cert->cache_info.identity_digest,
+                                   cert->signing_key_digest, 404);
+        }
       }
 
       authority_cert_free(cert);
@@ -445,17 +552,53 @@ authority_cert_get_all(smartlist_t *certs_out)
 }
 
 /** Called when an attempt to download a certificate with the authority with
- * ID <b>id_digest</b> fails with HTTP response code <b>status</b>: remember
- * the failure, so we don't try again immediately. */
+ * ID <b>id_digest</b> and, if not NULL, signed with key signing_key_digest
+ * fails with HTTP response code <b>status</b>: remember the failure, so we
+ * don't try again immediately. */
 void
-authority_cert_dl_failed(const char *id_digest, int status)
+authority_cert_dl_failed(const char *id_digest,
+                         const char *signing_key_digest, int status)
 {
   cert_list_t *cl;
+  download_status_t *dlstatus = NULL;
+  char id_digest_str[2*DIGEST_LEN+1];
+  char sk_digest_str[2*DIGEST_LEN+1];
+
   if (!trusted_dir_certs ||
       !(cl = digestmap_get(trusted_dir_certs, id_digest)))
     return;
 
-  download_status_failed(&cl->dl_status, status);
+  /*
+   * Are we noting a failed download of the latest cert for the id digest,
+   * or of a download by (id, signing key) digest pair?
+   */
+  if (!signing_key_digest) {
+    /* Just by id digest */
+    download_status_failed(&cl->dl_status_by_id, status);
+  } else {
+    /* Reset by (id, signing key) digest pair
+     *
+     * Look for a download_status_t in the map with this digest
+     */
+    dlstatus = dsmap_get(cl->dl_status_map, signing_key_digest);
+    /* Got one? */
+    if (dlstatus) {
+      download_status_failed(dlstatus, status);
+    } else {
+      /*
+       * Do this rather than hex_str(), since hex_str clobbers
+       * old results and we call twice in the param list.
+       */
+      base16_encode(id_digest_str, sizeof(id_digest_str),
+                    id_digest, DIGEST_LEN);
+      base16_encode(sk_digest_str, sizeof(sk_digest_str),
+                    signing_key_digest, DIGEST_LEN);
+      log_warn(LD_DIR,
+               "Got failure for cert fetch with (fp,sk) = (%s,%s), with "
+               "status %d, but knew nothing about the download.",
+               id_digest_str, sk_digest_str, status);
+    }
+  }
 }
 
 /** Return true iff when we've been getting enough failures when trying to
@@ -471,7 +614,7 @@ authority_cert_dl_looks_uncertain(const char *id_digest)
       !(cl = digestmap_get(trusted_dir_certs, id_digest)))
     return 0;
 
-  n_failures = download_status_get_n_failures(&cl->dl_status);
+  n_failures = download_status_get_n_failures(&cl->dl_status_by_id);
   return n_failures >= N_AUTH_CERT_DL_FAILURES_TO_BUG_USER;
 }
 
@@ -487,20 +630,38 @@ authority_cert_dl_looks_uncertain(const char *id_digest)
 void
 authority_certs_fetch_missing(networkstatus_t *status, time_t now)
 {
-  digestmap_t *pending;
+  /*
+   * The pending_id digestmap tracks pending certificate downloads by
+   * identity digest; the pending_cert digestmap tracks pending downloads
+   * by (identity digest, signing key digest) pairs.
+   */
+  digestmap_t *pending_id;
+  fp_pair_map_t *pending_cert;
   authority_cert_t *cert;
-  smartlist_t *missing_digests;
+  /*
+   * The missing_id_digests smartlist will hold a list of id digests
+   * we want to fetch the newest cert for; the missing_cert_digests
+   * smartlist will hold a list of fp_pair_t with an identity and
+   * signing key digest.
+   */
+  smartlist_t *missing_cert_digests, *missing_id_digests;
   char *resource = NULL;
   cert_list_t *cl;
   const int cache = directory_caches_unknown_auth_certs(get_options());
+  fp_pair_t *fp_tmp = NULL;
+  char id_digest_str[2*DIGEST_LEN+1];
+  char sk_digest_str[2*DIGEST_LEN+1];
 
   if (should_delay_dir_fetches(get_options()))
     return;
 
-  pending = digestmap_new();
-  missing_digests = smartlist_new();
+  pending_cert = fp_pair_map_new();
+  pending_id = digestmap_new();
+  missing_cert_digests = smartlist_new();
+  missing_id_digests = smartlist_new();
 
-  list_pending_downloads(pending, DIR_PURPOSE_FETCH_CERTIFICATE, "fp/");
+  list_pending_downloads(pending_id, DIR_PURPOSE_FETCH_CERTIFICATE, "fp/");
+  list_pending_fpsk_downloads(pending_cert);
   if (status) {
     SMARTLIST_FOREACH_BEGIN(status->voters, networkstatus_voter_info_t *,
                             voter) {
@@ -516,16 +677,43 @@ authority_certs_fetch_missing(networkstatus_t *status, time_t now)
                                              sig->signing_key_digest);
         if (cert) {
           if (now < cert->expires)
-            download_status_reset(&cl->dl_status);
+            download_status_reset_by_sk_in_cl(cl, sig->signing_key_digest);
           continue;
         }
-        if (download_status_is_ready(&cl->dl_status, now,
-                                     MAX_CERT_DL_FAILURES) &&
-            !digestmap_get(pending, voter->identity_digest)) {
-          log_info(LD_DIR, "We're missing a certificate from authority "
-                   "with signing key %s: launching request.",
-                   hex_str(sig->signing_key_digest, DIGEST_LEN));
-          smartlist_add(missing_digests, sig->identity_digest);
+        if (download_status_is_ready_by_sk_in_cl(
+              cl, sig->signing_key_digest,
+              now, MAX_CERT_DL_FAILURES) &&
+            !fp_pair_map_get_by_digests(pending_cert,
+                                        voter->identity_digest,
+                                        sig->signing_key_digest)) {
+          /*
+           * Do this rather than hex_str(), since hex_str clobbers
+           * old results and we call twice in the param list.
+           */
+          base16_encode(id_digest_str, sizeof(id_digest_str),
+                        voter->identity_digest, DIGEST_LEN);
+          base16_encode(sk_digest_str, sizeof(sk_digest_str),
+                        sig->signing_key_digest, DIGEST_LEN);
+
+          if (voter->nickname) {
+            log_info(LD_DIR,
+                     "We're missing a certificate from authority %s "
+                     "(ID digest %s) with signing key %s: "
+                     "launching request.",
+                     voter->nickname, id_digest_str, sk_digest_str);
+          } else {
+            log_info(LD_DIR,
+                     "We're missing a certificate from authority ID digest "
+                     "%s with signing key %s: launching request.",
+                     id_digest_str, sk_digest_str);
+          }
+
+          /* Allocate a new fp_pair_t to append */
+          fp_tmp = tor_malloc(sizeof(*fp_tmp));
+          memcpy(fp_tmp->first, voter->identity_digest, sizeof(fp_tmp->first));
+          memcpy(fp_tmp->second, sig->signing_key_digest,
+                 sizeof(fp_tmp->second));
+          smartlist_add(missing_cert_digests, fp_tmp);
         }
       } SMARTLIST_FOREACH_END(sig);
     } SMARTLIST_FOREACH_END(voter);
@@ -534,60 +722,118 @@ authority_certs_fetch_missing(networkstatus_t *status, time_t now)
     int found = 0;
     if (!(ds->type & V3_DIRINFO))
       continue;
-    if (smartlist_digest_isin(missing_digests, ds->v3_identity_digest))
+    if (smartlist_digest_isin(missing_id_digests, ds->v3_identity_digest))
       continue;
     cl = get_cert_list(ds->v3_identity_digest);
     SMARTLIST_FOREACH(cl->certs, authority_cert_t *, cert, {
       if (now < cert->expires) {
         /* It's not expired, and we weren't looking for something to
          * verify a consensus with.  Call it done. */
-        download_status_reset(&cl->dl_status);
+        download_status_reset(&(cl->dl_status_by_id));
+        /* No sense trying to download it specifically by signing key hash */
+        download_status_reset_by_sk_in_cl(cl, cert->signing_key_digest);
         found = 1;
         break;
       }
     });
     if (!found &&
-        download_status_is_ready(&cl->dl_status, now,MAX_CERT_DL_FAILURES) &&
-        !digestmap_get(pending, ds->v3_identity_digest)) {
-      log_info(LD_DIR, "No current certificate known for authority %s; "
-               "launching request.", ds->nickname);
-        smartlist_add(missing_digests, ds->v3_identity_digest);
+        download_status_is_ready(&(cl->dl_status_by_id), now,
+                                 MAX_CERT_DL_FAILURES) &&
+        !digestmap_get(pending_id, ds->v3_identity_digest)) {
+      log_info(LD_DIR,
+               "No current certificate known for authority %s "
+               "(ID digest %s); launching request.",
+               ds->nickname, hex_str(ds->v3_identity_digest, DIGEST_LEN));
+      smartlist_add(missing_id_digests, ds->v3_identity_digest);
     }
   } SMARTLIST_FOREACH_END(ds);
 
-  if (!smartlist_len(missing_digests)) {
-    goto done;
-  } else {
+  /* Do downloads by identity digest */
+  if (smartlist_len(missing_id_digests) > 0) {
     smartlist_t *fps = smartlist_new();
     smartlist_add(fps, tor_strdup("fp/"));
-    SMARTLIST_FOREACH(missing_digests, const char *, d, {
-        char *fp;
-        if (digestmap_get(pending, d))
-          continue;
-        fp = tor_malloc(HEX_DIGEST_LEN+2);
-        base16_encode(fp, HEX_DIGEST_LEN+1, d, DIGEST_LEN);
-        fp[HEX_DIGEST_LEN] = '+';
-        fp[HEX_DIGEST_LEN+1] = '\0';
-        smartlist_add(fps, fp);
-      });
-    if (smartlist_len(fps) == 1) {
-      /* we didn't add any: they were all pending */
-      SMARTLIST_FOREACH(fps, char *, cp, tor_free(cp));
-      smartlist_free(fps);
-      goto done;
+
+    SMARTLIST_FOREACH(missing_id_digests, const char *, d, {
+      char *fp;
+      if (digestmap_get(pending_id, d))
+        continue;
+      fp = tor_malloc(HEX_DIGEST_LEN+2);
+      base16_encode(fp, HEX_DIGEST_LEN+1, d, DIGEST_LEN);
+      fp[HEX_DIGEST_LEN] = '+';
+      fp[HEX_DIGEST_LEN+1] = '\0';
+      smartlist_add(fps, fp);
+    });
+
+    if (smartlist_len(fps) > 1) {
+      resource = smartlist_join_strings(fps, "", 0, NULL);
+      resource[strlen(resource)-1] = '\0';
+      directory_get_from_dirserver(DIR_PURPOSE_FETCH_CERTIFICATE, 0,
+                                   resource, PDS_RETRY_IF_NO_SERVERS);
+      tor_free(resource);
     }
-    resource = smartlist_join_strings(fps, "", 0, NULL);
-    resource[strlen(resource)-1] = '\0';
+    /* else we didn't add any: they were all pending */
+
     SMARTLIST_FOREACH(fps, char *, cp, tor_free(cp));
     smartlist_free(fps);
   }
-  directory_get_from_dirserver(DIR_PURPOSE_FETCH_CERTIFICATE, 0,
-                               resource, PDS_RETRY_IF_NO_SERVERS);
 
- done:
-  tor_free(resource);
-  smartlist_free(missing_digests);
-  digestmap_free(pending, NULL);
+  /* Do downloads by identity digest/signing key pair */
+  if (smartlist_len(missing_cert_digests) > 0) {
+    smartlist_t *fp_pairs = smartlist_new();
+    int need_plus = 0, offset = 0;
+
+    smartlist_add(fp_pairs, tor_strdup("fp-sk/"));
+
+    SMARTLIST_FOREACH_BEGIN(missing_cert_digests, const fp_pair_t *, d) {
+      char *fp_pair;
+
+      if (fp_pair_map_get(pending_cert, d))
+        continue;
+
+      fp_pair = tor_malloc(2*HEX_DIGEST_LEN+3);
+      offset = 0;
+      if (need_plus) {
+        fp_pair[offset++] = '+';
+      } else {
+        /* Prepend a '+' to all but the first in the list */
+        need_plus = 1;
+      }
+
+      /* Encode the first fingerprint */
+      base16_encode(fp_pair + offset, HEX_DIGEST_LEN+1,
+                    d->first, DIGEST_LEN);
+      offset += HEX_DIGEST_LEN;
+      /* Add a '-' to separate them */
+      fp_pair[offset++] = '-';
+      /* Encode the second fingerprint */
+      base16_encode(fp_pair + offset, HEX_DIGEST_LEN+1,
+                    d->second, DIGEST_LEN);
+      offset += HEX_DIGEST_LEN;
+      /* Add a NUL */
+      fp_pair[offset++] = '\0';
+
+      /* Add it to the list of pairs to request */
+      smartlist_add(fp_pairs, fp_pair);
+    } SMARTLIST_FOREACH_END(d);
+
+    if (smartlist_len(fp_pairs) > 1) {
+      resource = smartlist_join_strings(fp_pairs, "", 0, NULL);
+      resource[strlen(resource)-1] = '\0';
+      directory_get_from_dirserver(DIR_PURPOSE_FETCH_CERTIFICATE, 0,
+                                   resource, PDS_RETRY_IF_NO_SERVERS);
+      tor_free(resource);
+    }
+    /* else they were all pending */
+
+    SMARTLIST_FOREACH(fp_pairs, char *, p, tor_free(p));
+    smartlist_free(fp_pairs);
+  }
+
+  smartlist_free(missing_id_digests);
+  SMARTLIST_FOREACH(missing_cert_digests, fp_pair_t *, p, tor_free(p));
+  smartlist_free(missing_cert_digests);
+  digestmap_free(pending_id, NULL);
+  fp_pair_map_free(pending_cert, NULL);
 }
 
 /* Router descriptor storage.
@@ -4283,6 +4529,41 @@ list_pending_microdesc_downloads(digestmap_t *result)
   list_pending_downloads(result, DIR_PURPOSE_FETCH_MICRODESC, "d/");
 }
 
+/** For every certificate we are currently downloading by (identity digest,
+ * signing key digest) pair, set result[fp_pair] to (void *1).
+ */
+static void
+list_pending_fpsk_downloads(fp_pair_map_t *result)
+{
+  const char *pfx = "fp-sk/";
+  smartlist_t *tmp;
+  smartlist_t *conns;
+  const char *resource;
+
+  tor_assert(result);
+
+  tmp = smartlist_new();
+  conns = get_connection_array();
+
+  SMARTLIST_FOREACH_BEGIN(conns, connection_t *, conn) {
+    if (conn->type == CONN_TYPE_DIR &&
+        conn->purpose == DIR_PURPOSE_FETCH_CERTIFICATE &&
+        !conn->marked_for_close) {
+      resource = TO_DIR_CONN(conn)->requested_resource;
+      if (!strcmpstart(resource, pfx))
+        dir_split_resource_into_fingerprint_pairs(resource + strlen(pfx),
+                                                  tmp);
+    }
+  } SMARTLIST_FOREACH_END(conn);
+
+  SMARTLIST_FOREACH_BEGIN(tmp, fp_pair_t *, fp) {
+    fp_pair_map_set(result, fp, (void*)1);
+    tor_free(fp);
+  } SMARTLIST_FOREACH_END(fp);
+
+  smartlist_free(tmp);
+}
+
 /** Launch downloads for all the descriptors whose digests or digests256
  * are listed as digests[i] for lo <= i < hi.  (Lo and hi may be out of
  * range.)  If <b>source</b> is given, download from <b>source</b>;
diff --git a/src/or/routerlist.h b/src/or/routerlist.h
index 8dcc6eb02..ba6f930a5 100644
--- a/src/or/routerlist.h
+++ b/src/or/routerlist.h
@@ -13,7 +13,20 @@
 
 int get_n_authorities(dirinfo_type_t type);
 int trusted_dirs_reload_certs(void);
-int trusted_dirs_load_certs_from_string(const char *contents, int from_store,
+
+/*
+ * Pass one of these as source to trusted_dirs_load_certs_from_string()
+ * to indicate whence string originates; this controls error handling
+ * behavior such as marking downloads as failed.
+ */
+
+#define TRUSTED_DIRS_CERTS_SRC_SELF 0
+#define TRUSTED_DIRS_CERTS_SRC_FROM_STORE 1
+#define TRUSTED_DIRS_CERTS_SRC_DL_BY_ID_DIGEST 2
+#define TRUSTED_DIRS_CERTS_SRC_DL_BY_ID_SK_DIGEST 3
+#define TRUSTED_DIRS_CERTS_SRC_FROM_VOTE 4
+
+int trusted_dirs_load_certs_from_string(const char *contents, int source,
                                         int flush);
 void trusted_dirs_flush_certs_to_disk(void);
 authority_cert_t *authority_cert_get_newest_by_id(const char *id_digest);
@@ -21,7 +34,8 @@ authority_cert_t *authority_cert_get_by_sk_digest(const char *sk_digest);
 authority_cert_t *authority_cert_get_by_digests(const char *id_digest,
                                                 const char *sk_digest);
 void authority_cert_get_all(smartlist_t *certs_out);
-void authority_cert_dl_failed(const char *id_digest, int status);
+void authority_cert_dl_failed(const char *id_digest,
+                              const char *signing_key_digest, int status);
 void authority_certs_fetch_missing(networkstatus_t *status, time_t now);
 int router_reload_router_list(void);
 int authority_cert_dl_looks_uncertain(const char *id_digest);