auth_tag

        # OpenSSL::Cipher.auth_tag

(from ruby core)
### Implementation from Cipher
---
    cipher.auth_tag(tag_len = 16) -> String

---

Gets the authentication tag generated by Authenticated Encryption Cipher
modes (GCM for example). This tag may be stored along with the
ciphertext, then set on the decryption cipher to authenticate the
contents of the ciphertext against changes. If the optional integer
parameter *tag_len* is given, the returned tag will be *tag_len* bytes
long. If the parameter is omitted, the default length of 16 bytes or the
length previously set by #auth_tag_len= will be used. For maximum
security, the longest possible should be chosen.

The tag may only be retrieved after calling Cipher#final.