How to serialize a Nonce in aes_gcm

[Antosser]

When creating a random nonce, Rust automagically gives it the following type:

use aes_gcm::AeadCore;

fn main() {
    let mut rng = rand::thread_rng();

    let _nonce = aes_gcm::Aes256Gcm::generate_nonce(&mut rng);
}

// size = 12 (0xC), align = 0x1
let _nonce: GenericArray<u8, UInt<UInt<UInt<UInt<UTerm, B1>, B1>, B0>, B0>>

My goal is to put the Nonce into a struct that implements both serde::Serialize and serde::Deserialize. And actually writing that whole type doesn't seem intended.

The library provides two Nonce types: aes_gcm::Nonce and aes_gcm::aead::Nonce with the latter one being returned from the generate_nonce function.

Signature of generate_nonce:

fn generate_nonce(mut rng: impl CryptoRng + RngCore) -> Nonce<Self> where Nonce<Self>: Default

Type Nonce:

pub type Nonce<A> = GenericArray<u8, <A as AeadCore>::NonceSize>;

However, when I try to give the nonce the type myself, like in the following, I get the following error:

use aes_gcm::{aead::Nonce, AeadCore};

fn main() {
    let mut rng = rand::thread_rng();

    let _nonce: Nonce<AeadCore> = aes_gcm::Aes256Gcm::generate_nonce(&mut rng);
}

error[E0191]: the value of the associated types `TagSize`, `NonceSize` and `CiphertextOverhead` in `AeadCore` must be specified
 --> src/main.rs:6:23
  |
6 |     let _nonce: Nonce<AeadCore> = aes_gcm::Aes256Gcm::generate_nonce(&mut rng);
  |                       ^^^^^^^^ help: specify the associated types: `AeadCore<NonceSize = Type, TagSize = Type, CiphertextOverhead = Type>`

All this is getting way too complicated, which I hope is not intended. What would be the intended solution to (de)?serialize a Nonce?

[tarcieri]

Nonce is just a type alias for GenericArray<u8, AeadCore::NonceSize>: https://docs.rs/generic-array/0.14.7/generic_array/struct.GenericArray.html

There are several ways to convert normal core arrays like [0u8; 12] to/from GenericArray, such as the From impls:

https://docs.rs/generic-array/0.14.7/generic_array/struct.GenericArray.html#impl-From%3C%26'a+%5BT;+12%5D%3E-for-%26'a+GenericArray%3CT,+U12%3E

https://docs.rs/generic-array/0.14.7/generic_array/struct.GenericArray.html#impl-From%3CGenericArray%3CT,+UInt%3CUInt%3CUInt%3CUInt%3CUTerm,+B1%3E,+B1%3E,+B0%3E,+B0%3E%3E%3E-for-%5BT;+12%5D

You can also use the AsRef/AsMut impls to obtain &[u8; 12]: https://docs.rs/generic-array/0.14.7/generic_array/struct.GenericArray.html#impl-AsRef%3C%5BT;+12%5D%3E-for-GenericArray%3CT,+U12%3E

aes_gcm::aead is a re-export of the aead crate. aead::Nonce is generic around an AeadCore algorithm such as AesGcm.

To avoid you having to specify the algorithm, aes_gcm::Nonce is a type alias for AesGcm: https://docs.rs/aes-gcm/latest/aes_gcm/type.Nonce.html

So you should really work with that, not aes_gcm::aead::Nonce, and you can use any of the above converts to go to/from [u8; 12].

[Antosser]

This crate uses a very old version of generic_array (0.14.7), and not the latest stable version (1.1.0). That means, you have to add that specific version to your project because the different versions are incompatible. Would re-exporting the generic_array crate make sense? Or even upgrading?

And thanks for helping!

[tarcieri]

We re-export generic_array from the aead crate: https://docs.rs/aead/latest/aead/

It's available as aes_gcm::aead::generic_array

In our next breaking releases, we're migrating to hybrid-array, which should have better ergonomics: https://github.com/RustCrypto/hybrid-array