fastly.error

declare local var.key STRING = ...;
declare local var.iv STRING = ...;
declare local var.ciphertext STRING = ...;
declare local var.plaintext STRING;

# ... unrelated calls that may produce errors ...

# Clear fastly.error before an important operation
unset fastly.error;
set var.plaintext = crypto.decrypt_hex(aes256, cbc, nopad, var.key, var.iv, var.ciphertext);

# If fastly.error is set here, we're certain it's due to the call we're concerned with
if (fastly.error == "EBADDECRYPT") {
  error 403 "Wrong key";
} else if (fastly.error) {
  error 503;
}