Utility functions to get the algorithm from a public key.

Every time I've had to do this before, I've always done the three-line
dance of initialising a BinarySource and calling get_string on it.
It's long past time I wrapped that up into a convenient subroutine.

cmdgen.c

@@ -1280,9 +1280,8 @@ int main(int argc, char **argv)
             } else {
                 assert(ssh2blob);
 
-                BinarySource src[1];
+                ptrlen algname = pubkey_blob_to_alg_name(
-                BinarySource_BARE_INIT_PL(src, ptrlen_from_strbuf(ssh2blob));
+                    ptrlen_from_strbuf(ssh2blob));
-                ptrlen algname = get_string(src);
                 const ssh_keyalg *alg = find_pubkey_alg_len(algname);
                 if (!alg) {
                     fprintf(stderr, "puttygen: cannot extract key components "

ssh.h

@@ -1331,6 +1331,9 @@ extern const size_t n_keyalgs;
 const ssh_keyalg *find_pubkey_alg(const char *name);
 const ssh_keyalg *find_pubkey_alg_len(ptrlen name);
 
+ptrlen pubkey_blob_to_alg_name(ptrlen blob);
+const ssh_keyalg *pubkey_blob_to_alg(ptrlen blob);
+
 /* Convenient wrappers on the LoadedFile mechanism suitable for key files */
 LoadedFile *lf_load_keyfile(const Filename *filename, const char **errptr);
 LoadedFile *lf_load_keyfile_fp(FILE *fp, const char **errptr);

ssh/userauth2-client.c

@@ -353,10 +353,8 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
 
                 /* Also, extract the algorithm string from the start
                  * of the public-key blob. */
-                BinarySource src[1];
+                s->agent_keys[i].algorithm = pubkey_blob_to_alg_name(
-                BinarySource_BARE_INIT_PL(src, ptrlen_from_strbuf(
+                    ptrlen_from_strbuf(s->agent_keys[i].blob));
-                    s->agent_keys[i].blob));
-                s->agent_keys[i].algorithm = get_string(src);
             }
 
             ppl_logevent("Pageant has %"SIZEu" SSH-2 keys", nkeys);

sshpubk.c

@@ -585,6 +585,18 @@ const ssh_keyalg *find_pubkey_alg(const char *name)
     return find_pubkey_alg_len(ptrlen_from_asciz(name));
 }
 
+ptrlen pubkey_blob_to_alg_name(ptrlen blob)
+{
+    BinarySource src[1];
+    BinarySource_BARE_INIT_PL(src, blob);
+    return get_string(src);
+}
+
+const ssh_keyalg *pubkey_blob_to_alg(ptrlen blob)
+{
+    return find_pubkey_alg_len(pubkey_blob_to_alg_name(blob));
+}
+
 struct ppk_cipher {
     const char *name;
     size_t blocklen, keylen, ivlen;

windows/pageant.c

@@ -368,10 +368,8 @@ static void keylist_update_callback(
          * overflow past the bit-count tab stop and leave out a tab
          * character. Urgh.
          */
-        BinarySource src[1];
+        const ssh_keyalg *alg = pubkey_blob_to_alg(
-        BinarySource_BARE_INIT_PL(src, ptrlen_from_strbuf(key->blob));
+            ptrlen_from_strbuf(key->blob));
-        ptrlen algname = get_string(src);
-        const ssh_keyalg *alg = find_pubkey_alg_len(algname);
 
         bool include_bit_count = (alg == &ssh_dsa || alg == &ssh_rsa);