metalssh 0.0.1

//! `chacha20-poly1305@openssh.com` cipher.

use aws_lc_rs::aead::chacha20_poly1305_openssh::OpeningKey;
use aws_lc_rs::aead::chacha20_poly1305_openssh::PACKET_LENGTH_LEN;
use aws_lc_rs::aead::chacha20_poly1305_openssh::SealingKey;
use aws_lc_rs::aead::chacha20_poly1305_openssh::TAG_LEN;

use crate::crypto::cipher::Cipher;
use crate::types::Error;
use crate::types::Result;
use crate::wire::Packet;

/// See module level docs.
pub struct ChaCha20Poly1305 {
    key_material: [u8; 64],
}

impl ChaCha20Poly1305 {
    #[must_use]
    pub const fn new(key_material: [u8; 64]) -> Self {
        Self { key_material }
    }
}

impl Cipher for ChaCha20Poly1305 {
    const AEAD_LENGTH: Option<usize> = Some(TAG_LEN);

    fn encrypt_packet<'buf, B>(
        &self,
        packet: &'buf mut Packet<&'buf mut B>,
        sequence_number: u32,
    ) -> Result<()>
    where
        B: AsRef<[u8]> + AsMut<[u8]> + ?Sized,
    {
        let key = SealingKey::new(&self.key_material);

        let packet_len = packet.packet_length()?;
        let (data, mac) = packet.packet_mut()?.split_at_mut(4 + packet_len as usize);

        let mac = mac.as_mut_array().ok_or(Error::Crypto)?;

        key.seal_in_place(sequence_number, data, mac);

        Ok(())
    }

    // Packet length can't be decrypted in place by itself, because then the
    // subsequent full packet authenticated decrypt would fail.
    fn decrypt_packet_length<B>(&self, packet: &Packet<B>, sequence_number: u32) -> Result<u32>
    where
        B: AsRef<[u8]>,
    {
        let key = OpeningKey::new(&self.key_material);
        let packet_len_encrypted = packet.packet_length_bytes()?;
        let packet_len_decrypted = key.decrypt_packet_length(sequence_number, packet_len_encrypted);
        Ok(u32::from_be_bytes(packet_len_decrypted))
    }

    fn decrypt_packet<'buf, B>(
        &self,
        packet: &'buf mut Packet<&'buf mut B>,
        sequence_number: u32,
    ) -> Result<()>
    where
        B: AsRef<[u8]> + AsMut<[u8]> + ?Sized,
    {
        let packet_len = self.decrypt_packet_length(packet, sequence_number)?;
        let (data, mac) = packet.packet_mut()?.split_at_mut(4 + packet_len as usize);
        let packet_len_bytes = packet_len.to_be_bytes();

        let mut tag = [0u8; TAG_LEN];
        tag.copy_from_slice(mac);

        // Does not update the packet length in place, so we have to manually
        let key = OpeningKey::new(&self.key_material);
        key.open_in_place(sequence_number, data, &tag)
            .map_err(|_unspecified| Error::Crypto)?;
        data[..PACKET_LENGTH_LEN].copy_from_slice(&packet_len_bytes);

        Ok(())
    }
}

#[cfg(test)]
mod tests {
    // Test vectors:
    // https://github.com/rus-cert/ssh-chacha20-poly1305-drafts/blob/3931a96c7b2212907386e4c75104aa8aae9fb441/ssh-chacha20-poly1305%40openssh.md

    use aws_lc_rs::aead::chacha20_poly1305_openssh::KEY_LEN;
    use rstest::rstest;

    use super::*;

    // From test vectors, see above
    const KEY_MATERIAL: [u8; KEY_LEN] = {
        let mut bytes = [0u8; KEY_LEN];
        bytes[KEY_LEN - 1] = 1;
        bytes
    };

    #[rstest]
    #[case(
        0,
        "00000008061500010203040500000000000000000000000000000000",
        "4540f0529912e7bf57523c7f66022017cfefd3278ac13f40f8523faf"
    )]
    fn encrypt_works(
        #[case] sequence_number: u32,
        #[case] packet_clear: &str,
        #[case] packet_cipher_should: &str,
    ) {
        // Start with unencrypted binary packet data
        let mut data = hex::decode(packet_clear).unwrap();
        let data_len_should = data.len();

        // Take a mutable view over the packet and encrypt it in place
        {
            let mut packet = Packet::new(&mut data, TAG_LEN as u8);
            let cipher = ChaCha20Poly1305::new(KEY_MATERIAL);
            cipher.encrypt_packet(&mut packet, sequence_number).unwrap();
        }

        // Ensure that the buffer was not enlarged
        assert_eq!(data.len(), data_len_should);

        // Ensure that the packet bytes were updated in place
        let data = hex::encode(&data);
        assert_eq!(data, packet_cipher_should);
    }

    #[rstest]
    #[case(
        "4540f0529912e7bf57523c7f66022017cfefd3278ac13f40f8523faf",
        0,
        "00000008061500010203040566022017cfefd3278ac13f40f8523faf",
        8,
        6,
        &[0x15],
        &[0x00, 0x01, 0x02 , 0x03 , 0x04 , 0x05]
    )]
    fn decrypt_works(
        #[case] input_data: &str,
        #[case] input_sequence_number: u32,
        #[case] should_data: &str,
        #[case] should_packet_length: u32,
        #[case] should_padding_length: u8,
        #[case] should_payload: &[u8],
        #[case] should_padding: &[u8],
    ) {
        let mut data = hex::decode(input_data).unwrap();

        // Contain the mutable borrow of `data` to a limited lifetime. If we didn't, we
        // couldn't access any of the methods after decryption
        {
            let mut packet = Packet::new(&mut data, TAG_LEN as u8);

            let mut key_material = [0u8; KEY_LEN];
            key_material[63] = 1;
            let cipher = ChaCha20Poly1305::new(key_material);

            cipher
                .decrypt_packet(&mut packet, input_sequence_number)
                .unwrap();
        }

        // Ensure that the packet bytes were updated in place
        let got_dec_data = hex::encode(&data);
        assert_eq!(got_dec_data, should_data);

        // This time take an immutable borrow of the packet
        let packet = Packet::new(&data, TAG_LEN as u8);
        assert_eq!(packet.packet_length().unwrap(), should_packet_length);
        assert_eq!(packet.padding_length().unwrap(), should_padding_length);
        assert_eq!(packet.payload().unwrap(), should_payload);
        assert_eq!(packet.padding().unwrap(), should_padding);
    }
}