AES GCM implementation with authentication Tag in Java

Question

I\'m using AES GCM authentication in my android project and it works fine. But getting some issues with authentication tag when it compare with openssl API generate tag. Ple

Answer 1

In Java the tag is unfortunately added at the end of the ciphertext. You can configure the size (in bits, using a multiple of 8) using GCMParameterSpec. You can therefore grab it using Arrays.copyOfRange(ciphertext, ciphertext.length - (tagSize / Byte.SIZE), ciphertext.length) if you really want to.

It is unfortunate since the tag does not have to be put at the end, and it makes messes up the online nature of GCM decryption - requiring internal buffering instead of being able to directly return the plaintext. On the other hand, the tag is automatically verified during decryption.