challenge_response 0.5.46

use crate::error::ChallengeResponseError;
use crate::sec::{crc16, CRC_RESIDUAL_OK};
use aes::cipher::{Block, BlockCipherDecrypt, KeyInit};
use aes::Aes128;
use rand::{Rng, RngExt};
use std;

#[repr(C)]
#[repr(packed)]
#[derive(Default)]
pub struct Otp {
    /// The private ID, XORed with the challenge.
    pub uid: [u8; 6],
    /// A counter incremented each time the YubiKey is powered up.
    pub use_counter: u16,
    /// A timestamp, encoded little-endian, starting from the time the YubiKey is powered up.
    pub timestamp: [u8; 3],
    /// A counter incremented with each response.
    pub session_counter: u8,
    /// A random number (not cryptographically strong).
    pub random_number: u16,
    /// CRC of the other fields.
    pub crc: u16,
}

/// A secret key for AES128 / OTP challenge-response.
#[derive(Debug)]
pub struct Aes128Key(pub [u8; 16]);
impl Drop for Aes128Key {
    fn drop(&mut self) {
        for i in self.0.iter_mut() {
            *i = 0;
        }
    }
}

impl Aes128Key {
    pub fn from_slice(s: &[u8]) -> Self {
        let mut key = Aes128Key([0; 16]);
        (&mut key.0).clone_from_slice(s);
        key
    }

    pub fn generate<R: Rng>(mut rng: R) -> Self {
        let mut key = Aes128Key([0; 16]);
        for i in key.0.iter_mut() {
            *i = rng.random()
        }
        key
    }
}

#[derive(Debug)]
pub struct Aes128Block {
    pub block: Block<Aes128>,
}

impl Drop for Aes128Block {
    fn drop(&mut self) {
        for i in self.block.iter_mut() {
            *i = 0;
        }
    }
}

impl std::ops::Deref for Aes128Block {
    type Target = [u8];
    fn deref(&self) -> &Self::Target {
        &self.block
    }
}

impl Aes128Block {
    /// Decrypts an AES block as returned by the YubiKey. The caller
    /// must check that the `uid` field is equal to the known private
    /// id, and that the `(use_counter, session_counter)` is strictly
    /// larger than the last value seen.
    pub fn check(&self, key: &Aes128Key, challenge: &[u8]) -> Result<Otp, ChallengeResponseError> {
        let aes_dec = Aes128::new(&key.0.into());
        let mut tmp = Otp::default();
        {
            let tmp_slice = unsafe { std::slice::from_raw_parts_mut(&mut tmp as *mut Otp as *mut u8, 16) };
            let mut block_copy = self.block.clone();
            aes_dec.decrypt_block(&mut block_copy);
            tmp_slice.copy_from_slice(&block_copy);

            if crc16(tmp_slice) != CRC_RESIDUAL_OK {
                return Err(ChallengeResponseError::WrongCRC);
            }
        }

        for i in 0..6 {
            tmp.uid[i] ^= challenge[i]
        }

        Ok(tmp)
    }
}