Get NonceSize from the algorithm

[tisonkun]

Is it reasonable to add a method nonce_size() to algorithms like Aes256Gcm?

Usage:

let nonce_size = Aes256Gcm::nonce_size();

To avoid inline magic numbers (12 in this case).

[tarcieri]

Nonce size is available as AeadCore::NonceSize.

The typenum::Unsigned::USIZE associated constant in particular can be used to query it as a usize.

[tisonkun]

@tarcieri Yeah. I can see the NonceSize associated type. In what syntax I can get the size from the Aes256Gcm type or an instant of it?

[tarcieri]

<Aes256Gcm as AeadCore>::NonceSize::USIZE

[tisonkun]

@tarcieri Thank you! Works perfectly:

use aes_gcm::aead::Aead;
use aes_gcm::aes::cipher::Unsigned;
use aes_gcm::AeadCore;

let nonce_size = <aes_gcm::Aes256Gcm as AeadCore>::NonceSize::USIZE;