Restrict the meaning of digests_t to sha1+sha256.

This saves CPU and RAM when handling consensuses and x509 certs.

Closes ticket 17795; bug not in any released tor.

src/common/crypto.c

@@ -1655,7 +1655,7 @@ crypto_digest_all(digests_t *ds_out, const char *m, size_t len)
   memset(ds_out, 0, sizeof(*ds_out));
   if (crypto_digest(ds_out->d[DIGEST_SHA1], m, len) < 0)
     return -1;
-  for (i = DIGEST_SHA256; i < N_DIGEST_ALGORITHMS; ++i) {
+  for (i = DIGEST_SHA256; i < N_COMMON_DIGEST_ALGORITHMS; ++i) {
       switch (i) {
         case DIGEST_SHA256: /* FALLSTHROUGH */
         case DIGEST_SHA3_256:
@@ -1664,6 +1664,7 @@ crypto_digest_all(digests_t *ds_out, const char *m, size_t len)
           break;
         case DIGEST_SHA512:
         case DIGEST_SHA3_512: /* FALLSTHROUGH */
+          tor_assert(0); /* These won't fit. */
           if (crypto_digest512(ds_out->d[i], m, len, i) < 0)
             return -1;
           break;

src/common/crypto.h

@@ -100,8 +100,10 @@ typedef enum {
   DIGEST_SHA3_512 = 4,
 } digest_algorithm_t;
 #define  N_DIGEST_ALGORITHMS (DIGEST_SHA3_512+1)
+#define  N_COMMON_DIGEST_ALGORITHMS (DIGEST_SHA256+1)
 
-/** A set of all the digests we know how to compute, taken on a single
+
+/** A set of all the digests we commonly compute, taken on a single
  * string.  Any digests that are shorter than 512 bits are right-padded
  * with 0 bits.
  *
@@ -110,7 +112,7 @@ typedef enum {
  * once.
  **/
 typedef struct {
-  char d[N_DIGEST_ALGORITHMS][DIGEST512_LEN];
+  char d[N_COMMON_DIGEST_ALGORITHMS][DIGEST256_LEN];
 } digests_t;
 
 typedef struct crypto_pk_t crypto_pk_t;