sendme: Do not poke at crypto.sendme_digest directly

As per review from nickm, keep as much as we can the relay_crypto_t object
opaque.

Part of #26288

Signed-off-by: David Goulet <dgoulet@torproject.org>

src/core/crypto/relay_crypto.c

@@ -91,6 +91,14 @@ relay_crypt_one_payload(crypto_cipher_t *cipher, uint8_t *in)
   crypto_cipher_crypt_inplace(cipher, (char*) in, CELL_PAYLOAD_SIZE);
 }
 
+/** Return the sendme_digest within the <b>crypto</b> object. */
+uint8_t *
+relay_crypto_get_sendme_digest(relay_crypto_t *crypto)
+{
+  tor_assert(crypto);
+  return crypto->sendme_digest;
+}
+
 /** Record the b_digest from <b>crypto</b> and put it in the sendme_digest. */
 void
 relay_crypto_record_sendme_digest(relay_crypto_t *crypto)

src/core/crypto/relay_crypto.h

@@ -27,6 +27,7 @@ void relay_crypto_clear(relay_crypto_t *crypto);
 
 void relay_crypto_assert_ok(const relay_crypto_t *crypto);
 
+uint8_t *relay_crypto_get_sendme_digest(relay_crypto_t *crypto);
 void relay_crypto_record_sendme_digest(relay_crypto_t *crypto);
 
 #endif /* !defined(TOR_RELAY_CRYPTO_H) */

src/core/or/sendme.c

@@ -388,10 +388,10 @@ sendme_circuit_consider_sending(circuit_t *circ, crypt_path_t *layer_hint)
     log_debug(LD_CIRC,"Queuing circuit sendme.");
     if (layer_hint) {
       layer_hint->deliver_window += CIRCWINDOW_INCREMENT;
-      digest = layer_hint->crypto.sendme_digest;
+      digest = relay_crypto_get_sendme_digest(&layer_hint->crypto);
     } else {
       circ->deliver_window += CIRCWINDOW_INCREMENT;
-      digest = TO_OR_CIRCUIT(circ)->crypto.sendme_digest;
+      digest = relay_crypto_get_sendme_digest(&TO_OR_CIRCUIT(circ)->crypto);
     }
     if (send_circuit_level_sendme(circ, layer_hint, digest) < 0) {
       return; /* The circuit's closed, don't continue */
@@ -597,7 +597,7 @@ sendme_record_cell_digest(circuit_t *circ)
   }
 
   /* Add the digest to the last seen list in the circuit. */
-  digest = TO_OR_CIRCUIT(circ)->crypto.sendme_digest;
+  digest = relay_crypto_get_sendme_digest(&TO_OR_CIRCUIT(circ)->crypto);
   if (circ->sendme_last_digests == NULL) {
     circ->sendme_last_digests = smartlist_new();
   }