chore: document why SHA1 is the only supported hash algorithm for cert_id generation in OCSP response

api/s2n.h

@@ -999,6 +999,12 @@ S2N_API extern int s2n_config_set_verify_host_callback(struct s2n_config *config
  *
  * The default value is 1 if the underlying libCrypto implementation supports OCSP. 
  *
+ * @note SHA-1 is the only supported hash algorithm for the `certID` field. If a different hash 
+ * algorithm is used, validation will fail and manual verification will be needed. This is 
+ * different from the hash algorithm used for the signature algorithm. See 
+ * [RFC6960](https://datatracker.ietf.org/doc/html/rfc6960#section-4.1.1) for details about 
+ * `certID` field.
+ * 
  * @param config The configuration object being updated
  * @param check_ocsp The desired OCSP response check configuration
  * @returns S2N_SUCCESS on success. S2N_FAILURE on failure

tls/s2n_x509_validator.c

@@ -874,7 +874,13 @@ S2N_RESULT s2n_x509_validator_validate_cert_stapled_ocsp_response(struct s2n_x50
     int status = 0;
     int reason = 0;
 
-    /* sha1 is the only supported OCSP digest */
+    /* SHA-1 is the only supported OCSP digest due to its wide compatibility and established use
+    * in existing systems. Supporting additional hash algorithms would require changes to error 
+    * handling and compatibility checks, which are not currently justified by user demand. For 
+    * verifying OCSP response with non-SHA-1 hash algorithm, users can call 
+    * s2n_connection_get_ocsp_response() to retrieve the received OCSP stapling information for 
+    * manual verification. 
+    */
     OCSP_CERTID *cert_id = OCSP_cert_to_id(EVP_sha1(), subject, issuer);
     RESULT_ENSURE_REF(cert_id);