feature request: add block_size method to BlockCipher

[hxzhao527]

describe & question

BlockCipher.encrypt work in-place, and no allocate. So I need to calculate the buffer size manually.
my question :

• Is there any way to get block-size easily? not the wordy one.
• can we add block_size method to BlockCipher?

my code

use std::error;

use aes::block_cipher::{generic_array::typenum::Unsigned, BlockCipher};
use aes::Aes128;
use block_modes::{block_padding::Pkcs7, BlockMode, Cbc};

type Aes128Cbc = Cbc<Aes128, Pkcs7>;
type Byte = u8;

fn aes128_cbc_encrypt(
    plaintext: &[Byte],
    key: &[Byte],
    iv: &[Byte],
) -> Result<Vec<Byte>, Box<dyn error::Error>> {
    // pading只是做对齐, 但是这个对齐是in-place, 这个buf还是需要自己申请大小以满足条件
    // pading just fill, no allocate
    // =======================================
    let block_size = <Aes128 as BlockCipher>::BlockSize::to_usize();
    let padding_length = block_size - plaintext.len() % block_size;
    // =======================================
    let cipher = Aes128Cbc::new_var(&key, &iv)?;

    let mut buffer = Vec::with_capacity(padding_length + plaintext.len());
    buffer.resize(padding_length + plaintext.len(), 0u8);
    // unsafe { buffer.set_len(padding_length+plaintext.len()); }
    buffer[..plaintext.len()].copy_from_slice(plaintext);

    //let ciphertext = cipher.encrypt(&mut buffer, plaintext.len())?;
    cipher.encrypt(&mut buffer, plaintext.len())?;
    Ok(buffer)
}

Cargo.toml

[dependencies]
block-modes="0.4.0"
aes="0.4.0

[tarcieri]

I think adding a block_size() method makes sense.

Kind of curious what you're trying to do there specifically and if we could add some additional higher level methods to help. Do you specifically need (unauthenticated) AES-CBC encryption?

[hxzhao527]

Just transform golang to rust. 😂

In go, https://golang.org/pkg/crypto/cipher/#Block, provide a method BlockSize(), it is
convenient.

[tarcieri]

Eventually they'll just be associated consts (after const generics land), but I agree for now having a method is easier than <Aes128 as BlockCipher>::BlockSize::to_usize()

[hxzhao527]

you mean something like this?

pub trait BlockCipher {
    const Size: usize;
    fn block_size() -> usize {
        Self::Size
    }

    fn new() -> Self;
}

pub struct Aes {}

impl BlockCipher for Aes {
    const Size: usize = 1;
    fn new() -> Self {
        Self {}
    }
}

pub struct Cbc<C: BlockCipher> {
    cipher: C,
}
impl<C> Cbc<C>
where
    C: BlockCipher,
{
    fn encrypt(self, data: &str) {
        for _ in 0..C::block_size() {
            println!("data: {}", data);
        }
    }

    fn new() -> Self {
        Self { cipher: C::new() }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    type AesCbc = Cbc<Aes>;
    #[test]
    pub fn test_size() {
        println!("block_size: {}", Aes::block_size());
        let encryptor = AesCbc::new();
        encryptor.encrypt("hello");
    }
}

[tarcieri]

What I'm talking about involves changes to the Rust language. Here's a blog post I wrote on the topic:

https://tonyarcieri.com/rust-in-2019-security-maturity-stability#const-generics-and-cryptography_2

[hxzhao527]

Got it.

======
For others:
The Rust RFC Book : 2000-const_generics
Tracking issue for const generics (RFC 2000)