Updated EIP712 filtering signature specs to start with a magic number

Making it impossible for a signature of one type to be valid as another

src_features/signMessageEIP712/filtering.c

@@ -65,6 +65,20 @@ static bool verify_filtering_signature(uint8_t dname_length,
 
     cx_sha256_init(&hash_ctx);
 
+    // Magic number, makes it so a signature of one type can't be used as another
+    switch (type) {
+        case FILTERING_STRUCT_FIELD:
+            hash_byte(FILTERING_MAGIC_STRUCT_FIELD, (cx_hash_t *) &hash_ctx);
+            break;
+        case FILTERING_CONTRACT_NAME:
+            hash_byte(FILTERING_MAGIC_CONTRACT_NAME, (cx_hash_t *) &hash_ctx);
+            break;
+        default:
+            apdu_response_code = APDU_RESPONSE_INVALID_DATA;
+            PRINTF("Invalid filtering type when verifying signature!\n");
+            return false;
+    }
+
     // Chain ID
     chain_id = __builtin_bswap64(eip712_context->chain_id);
     hash_nbytes((uint8_t *) &chain_id, sizeof(chain_id), (cx_hash_t *) &hash_ctx);

src_features/signMessageEIP712/filtering.h

@@ -6,6 +6,9 @@
 #include <stdbool.h>
 #include <stdint.h>
 
+#define FILTERING_MAGIC_CONTRACT_NAME 0b10110111  //  183
+#define FILTERING_MAGIC_STRUCT_FIELD  0b01001000  // ~183 = 72
+
 typedef enum { FILTERING_CONTRACT_NAME, FILTERING_STRUCT_FIELD } e_filtering_type;
 
 bool provide_filtering_info(const uint8_t *const payload, uint8_t length, e_filtering_type type);