diff --git a/include/hydrolink/protocol.h b/include/hydrolink/protocol.h
index e506bac..4ae4311 100644
--- a/include/hydrolink/protocol.h
+++ b/include/hydrolink/protocol.h
@@ -8,7 +8,42 @@
 #define HYDROLINK_MAX_PAYLOAD_LEN 6
 #define HYDROLINK_MAX_MSG_LEN (HYDROLINK_MAX_PAYLOAD_LEN + HYDROLINK_NON_PAYLOAD_LEN)
 
-#define HYDROLINK_MSG_CRCS {73, 58, 250, 38, 28}
+typedef struct {
+  uint8_t msg_id;
+  uint8_t crc_extra;
+} hydrolink_crc_extra_t;
+
+#define HYDROLINK_MSG_CRCS {{1, 73}, {2, 58}, {3, 250}, {4, 38}, {5, 28}}
+
+/**
+* @brief Get the crc_extra byte corresponding to msg_id
+*
+* @param msg_id the message's id
+* @param crc[out] the crc_extra byte as output of this function
+* @return Return 1 if successfull, 0 otherwise.
+static inline uint8_t hydrolink_get_crc_extra(uint8_t msg_id, uint8_t *crc) {
+  static const hydrolink_crc_extra_t crcs[] = HYDROLINK_MSG_CRCS;
+  uint8_t left = 0;
+  uint8_t right = sizeof(crcs) / sizeof(crcs[0]) - 1;
+  while (left < right) {
+    uint8_t center = (left+right+1) / 2;
+    if (msg_id < crcs[center].msg_id) {
+      right = center - 1;
+      continue;
+    }
+    if (msg_id > crcs[center].msg_id) {
+      left = center;
+      continue;
+    }
+    left = mid;
+    break;
+  }
+  if (crcs[left].msg_id != msg_id) {
+    return 0;
+  }
+  *crc = crcs[left].crc_extra;
+  return 1;
+}
 
 struct hydrolink_msg_s;