0

I'm looking to Implement Galois Counter Mode (not, use, implement) for a proprietary TLS implementation. The problem I'm having is that I can't figure out if the 128-bit sizes for the standard NIST design are coupled to the 128-bit sizes for the underlying ciphers prescribed. I.e. GCM was designed to pair with AES-128.

When I run GCM with AES-256, do I still use 128 bit blocks for the GCM operations, or do I up them to 256 bit blocks?

dciliske
  • 157
  • 2
  • 9
  • 2
    Key size and block size are decoupled in most (all?) block ciphers. Changing one doesn't mean that you need to change the other. – Artjom B. Jun 13 '16 at 20:58
  • Well, I missed that one. Yea, that is the key size. I always assumed it was the block size. – dciliske Jun 13 '16 at 21:25

1 Answers1

0

Due to the size of the Galois field, you should use GCM with a 128 bit block cipher. Fortunately AES-128, -192 and -256 all have a 128 bit block size; obviously the key sizes differ.

Maarten Bodewes
  • 90,524
  • 13
  • 150
  • 263