In relay_digest_matches(), use stack instead of heap.

We'd been using crypto_digest_dup() and crypto_digest_assign() here,
but they aren't necessary.  Instead we can just use the stack to
store the previous state of the SHA_CTX and avoid a malloc/free pair.

Closes ticket 24914.

src/common/crypto.c

@@ -1822,6 +1822,7 @@ crypto_digest_alloc_bytes(digest_algorithm_t alg)
   /* Gives the length of crypto_digest_t through the end of the field 'd' */
 #define END_OF_FIELD(f) (offsetof(crypto_digest_t, f) + \
                          STRUCT_FIELD_SIZE(crypto_digest_t, f))
+
   switch (alg) {
     case DIGEST_SHA1:
       return END_OF_FIELD(d.sha1);
@@ -2007,6 +2008,33 @@ crypto_digest_dup(const crypto_digest_t *digest)
   return tor_memdup(digest, alloc_bytes);
 }
 
+/** Temporarily save the state of <b>digest</b> in <b>checkpoint</b>.
+ * Asserts that <b>digest</b> is a SHA1 digest object.
+ */
+void
+crypto_digest_checkpoint(crypto_digest_checkpoint_t *checkpoint,
+                         const crypto_digest_t *digest)
+{
+  tor_assert(digest->algorithm == DIGEST_SHA1);
+  /* The optimizer should turn this into a constant... */
+  const size_t bytes = crypto_digest_alloc_bytes(DIGEST_SHA1);
+  /* ... and remove this assertion entirely. */
+  tor_assert(bytes <= sizeof(checkpoint->mem));
+  memcpy(checkpoint->mem, digest, bytes);
+}
+
+/** Restore the state of  <b>digest</b> from <b>checkpoint</b>.
+ * Asserts that <b>digest</b> is a SHA1 digest object. Requires that the
+ * state was previously stored with crypto_digest_checkpoint() */
+void
+crypto_digest_restore(crypto_digest_t *digest,
+                      const crypto_digest_checkpoint_t *checkpoint)
+{
+  tor_assert(digest->algorithm == DIGEST_SHA1);
+  const size_t bytes = crypto_digest_alloc_bytes(DIGEST_SHA1);
+  memcpy(digest, checkpoint->mem, bytes);
+}
+
 /** Replace the state of the digest object <b>into</b> with the state
  * of the digest object <b>from</b>.  Requires that 'into' and 'from'
  * have the same digest type.

src/common/crypto.h

@@ -98,6 +98,13 @@ typedef struct crypto_digest_t crypto_digest_t;
 typedef struct crypto_xof_t crypto_xof_t;
 typedef struct crypto_dh_t crypto_dh_t;
 
+#define DIGEST_CHECKPOINT_BYTES (SIZEOF_VOID_P + SIZEOF_SHA_CTX)
+/** Structure used to temporarily save the a digest object. Only implemented
+ * for SHA1 digest for now. */
+typedef struct crypto_digest_checkpoint_t {
+  uint8_t mem[DIGEST_CHECKPOINT_BYTES];
+} crypto_digest_checkpoint_t;
+
 /* global state */
 int crypto_early_init(void) ATTR_WUR;
 int crypto_global_init(int hardwareAccel,
@@ -235,6 +242,10 @@ void crypto_digest_add_bytes(crypto_digest_t *digest, const char *data,
 void crypto_digest_get_digest(crypto_digest_t *digest,
                               char *out, size_t out_len);
 crypto_digest_t *crypto_digest_dup(const crypto_digest_t *digest);
+void crypto_digest_checkpoint(crypto_digest_checkpoint_t *checkpoint,
+                              const crypto_digest_t *digest);
+void crypto_digest_restore(crypto_digest_t *digest,
+                           const crypto_digest_checkpoint_t *checkpoint);
 void crypto_digest_assign(crypto_digest_t *into,
                           const crypto_digest_t *from);
 void crypto_hmac_sha256(char *hmac_out,