generic array / stream encryptor type mismatch

[ousado]

The following code

use chacha20poly1305::{
    aead::{stream, OsRng},
    XChaCha20Poly1305, XNonce, Key, KeyInit, AeadCore
};

fn main() {
    let key   : Key               = XChaCha20Poly1305::generate_key(&mut OsRng);
    let nonce : XNonce            = XChaCha20Poly1305::generate_nonce(&mut OsRng);
    let aead  : XChaCha20Poly1305 = XChaCha20Poly1305::new(&key);
    let stream_encryptor          = stream::EncryptorBE32::from_aead(aead,&nonce);
}

produces the following error:

error[E0308]: mismatched types
   --> src/main.rs:11:78
    |
11  |     let stream_encryptor             = stream::EncryptorBE32::from_aead(aead,&nonce);
    |                                        --------------------------------      ^^^^^^ expected `B0`, found `B1`
    |                                        |
    |                                        arguments to this function are incorrect
    |
    = note: expected reference `&GenericArray<u8, UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B0>, B0>, B1>, B1>>`
               found reference `&GenericArray<u8, UInt<UInt<UInt<UInt<UInt<UTerm, B1>, B1>, B0>, B0>, B0>>`
note: associated function defined here
   --> / .. .. .. /cargo/registry/src/index.crates.io-6f17d22bba15001f/aead-0.5.2/src/stream.rs:337:1
    |
337 | / impl_stream_object!(
338 | |     Encryptor,
339 | |     encrypt_next,
340 | |     encrypt_next_in_place,
...   |
346 | |     "ℰ STREAM encryptor"
347 | | );
    | |_^
    = note: this error originates in the macro `impl_stream_object` (in Nightly builds, run with -Z macro-backtrace for more info)

Here's a repository with this example:
https://github.com/ousado/xcc20poly1305issue/blob/main/src/main.rs

[tarcieri]

The nonce size for *BE32 is 5-bytes smaller (32-bit counter + last block flag byte) than the 24-byte XChaCha20Poly1305 nonce so 24 - 5 = 19 bytes.

See also RustCrypto/traits#1306

[ousado]

I assumed it's somehow generic over the cipher's NonceSize due to these definitions:

pub type Nonce<A, S> = GenericArray<u8, NonceSize<A, S>>;

pub type NonceSize<A, S> = <<A as AeadCore>::NonceSize as Sub<<S as StreamPrimitive>::NonceOverhead>>::Output;

Does that mean there is currently no stream::Encryptor that can be used with XChaCha20Poly1305?

[tarcieri]

No, it just means you're passing the wrong nonce size.

You need to use GenericArray<u8, U19>.

You can use those type aliases if you'd like, but right now you're using XNonce which is 24-byte.

[ousado]

Ah, I see, thanks for the quick clarification!