encrypted-dns-server/src/dnscrypt.rs

use std::sync::Arc;

use libsodium_sys::*;
use rand::prelude::*;

use crate::crypto::*;
use crate::dns::*;
use crate::dnscrypt_certs::*;
use crate::errors::*;

pub const DNSCRYPT_FULL_NONCE_SIZE: usize =
    crypto_box_curve25519xchacha20poly1305_NONCEBYTES as usize;
pub const DNSCRYPT_MAC_SIZE: usize = crypto_box_curve25519xchacha20poly1305_MACBYTES as usize;

pub const DNSCRYPT_QUERY_MAGIC_SIZE: usize = 8;
pub const DNSCRYPT_QUERY_PK_SIZE: usize = 32;
pub const DNSCRYPT_QUERY_NONCE_SIZE: usize = DNSCRYPT_FULL_NONCE_SIZE / 2;
pub const DNSCRYPT_QUERY_HEADER_SIZE: usize =
    DNSCRYPT_QUERY_MAGIC_SIZE + DNSCRYPT_QUERY_PK_SIZE + DNSCRYPT_QUERY_NONCE_SIZE;
pub const DNSCRYPT_QUERY_MIN_PADDING_SIZE: usize = 1;
pub const DNSCRYPT_QUERY_MIN_OVERHEAD: usize =
    DNSCRYPT_QUERY_HEADER_SIZE + DNSCRYPT_MAC_SIZE + DNSCRYPT_QUERY_MIN_PADDING_SIZE;

pub const DNSCRYPT_RESPONSE_MAGIC_SIZE: usize = 8;
pub const DNSCRYPT_RESPONSE_MAGIC: [u8; DNSCRYPT_RESPONSE_MAGIC_SIZE] =
    [0x72, 0x36, 0x66, 0x6e, 0x76, 0x57, 0x6a, 0x38];
pub const DNSCRYPT_RESPONSE_CERT_PREFIX_OFFSET: usize = 4;
pub const DNSCRYPT_RESPONSE_NONCE_SIZE: usize = DNSCRYPT_FULL_NONCE_SIZE;
pub const DNSCRYPT_RESPONSE_HEADER_SIZE: usize =
    DNSCRYPT_RESPONSE_MAGIC_SIZE + DNSCRYPT_RESPONSE_NONCE_SIZE;
pub const DNSCRYPT_RESPONSE_MIN_PADDING_SIZE: usize = 1;
pub const DNSCRYPT_RESPONSE_MIN_OVERHEAD: usize =
    DNSCRYPT_RESPONSE_HEADER_SIZE + DNSCRYPT_MAC_SIZE + DNSCRYPT_RESPONSE_MIN_PADDING_SIZE;

pub const DNSCRYPT_UDP_QUERY_MIN_SIZE: usize = DNSCRYPT_QUERY_MIN_OVERHEAD + DNS_HEADER_SIZE;
pub const DNSCRYPT_UDP_QUERY_MAX_SIZE: usize = DNS_MAX_PACKET_SIZE;
pub const DNSCRYPT_TCP_QUERY_MIN_SIZE: usize = DNSCRYPT_QUERY_MIN_OVERHEAD + DNS_HEADER_SIZE;
pub const DNSCRYPT_TCP_QUERY_MAX_SIZE: usize = DNSCRYPT_QUERY_MIN_OVERHEAD + DNS_MAX_PACKET_SIZE;

pub const DNSCRYPT_UDP_RESPONSE_MIN_SIZE: usize = DNSCRYPT_RESPONSE_MIN_OVERHEAD + DNS_HEADER_SIZE;
pub const DNSCRYPT_UDP_RESPONSE_MAX_SIZE: usize = DNS_MAX_PACKET_SIZE;
pub const DNSCRYPT_TCP_RESPONSE_MIN_SIZE: usize = DNSCRYPT_RESPONSE_MIN_OVERHEAD + DNS_HEADER_SIZE;
pub const DNSCRYPT_TCP_RESPONSE_MAX_SIZE: usize =
    DNSCRYPT_RESPONSE_MIN_OVERHEAD + DNS_MAX_PACKET_SIZE;

pub fn decrypt(
    wrapped_packet: &[u8],
    dnscrypt_encryption_params_set: &[Arc<DNSCryptEncryptionParams>],
) -> Result<(SharedKey, [u8; DNSCRYPT_FULL_NONCE_SIZE], Vec<u8>), Error> {
    ensure!(
        wrapped_packet.len()
            >= DNSCRYPT_QUERY_MAGIC_SIZE
                + DNSCRYPT_QUERY_PK_SIZE
                + DNSCRYPT_QUERY_NONCE_SIZE
                + DNS_HEADER_SIZE,
        "Short packet"
    );
    let client_magic = &wrapped_packet[..DNSCRYPT_QUERY_MAGIC_SIZE];
    let client_pk = &wrapped_packet
        [DNSCRYPT_QUERY_MAGIC_SIZE..DNSCRYPT_QUERY_MAGIC_SIZE + DNSCRYPT_QUERY_PK_SIZE];
    let client_nonce = &wrapped_packet[DNSCRYPT_QUERY_MAGIC_SIZE + DNSCRYPT_QUERY_PK_SIZE
        ..DNSCRYPT_QUERY_MAGIC_SIZE + DNSCRYPT_QUERY_PK_SIZE + DNSCRYPT_QUERY_NONCE_SIZE];
    let encrypted_packet = &wrapped_packet[DNSCRYPT_QUERY_HEADER_SIZE..];

    let dnscrypt_encryption_params = dnscrypt_encryption_params_set
        .iter()
        .find(|p| p.client_magic() == client_magic)
        .ok_or_else(|| anyhow!("Client magic not found"))?;

    let mut nonce = [0u8; DNSCRYPT_FULL_NONCE_SIZE];
    nonce[..DNSCRYPT_QUERY_NONCE_SIZE].copy_from_slice(client_nonce);

    let cached_shared_key = {
        let mut key_cache = dnscrypt_encryption_params
            .key_cache
            .as_ref()
            .unwrap()
            .lock();
        key_cache
            .get(client_pk)
            .map(|cached_shared_key| (*cached_shared_key).clone())
    };
    let shared_key = match cached_shared_key {
        Some(cached_shared_key) => cached_shared_key,
        None => {
            let shared_key = dnscrypt_encryption_params
                .resolver_kp()
                .compute_shared_key(client_pk)?;
            let mut client_pk_ = [0u8; DNSCRYPT_QUERY_PK_SIZE];
            client_pk_.copy_from_slice(client_pk);
            dnscrypt_encryption_params
                .key_cache
                .as_ref()
                .unwrap()
                .lock()
                .insert(client_pk_, shared_key.clone());
            shared_key
        }
    };
    let packet = shared_key.decrypt(&nonce, encrypted_packet)?;
    rand::thread_rng().fill_bytes(&mut nonce[DNSCRYPT_QUERY_NONCE_SIZE..]);

    Ok((shared_key, nonce, packet))
}

pub fn encrypt(
    packet: Vec<u8>,
    shared_key: &SharedKey,
    nonce: &[u8; DNSCRYPT_FULL_NONCE_SIZE],
    max_packet_size: usize,
) -> Result<Vec<u8>, Error> {
    let mut wrapped_packet = Vec::with_capacity(DNS_MAX_PACKET_SIZE);
    wrapped_packet.extend_from_slice(&DNSCRYPT_RESPONSE_MAGIC);
    wrapped_packet.extend_from_slice(nonce);
    ensure!(
        max_packet_size >= wrapped_packet.len(),
        "Max packet size too short"
    );
    let max_encrypted_size = max_packet_size - wrapped_packet.len();
    shared_key.encrypt_into(
        &mut wrapped_packet,
        nonce,
        &nonce[..DNSCRYPT_QUERY_NONCE_SIZE],
        packet,
        max_encrypted_size,
    )?;
    Ok(wrapped_packet)
}

pub fn may_be_quic(packet: &[u8]) -> bool {
    !packet.is_empty() && ((80..=127).contains(&packet[0]) || (192..=255).contains(&packet[0]))
}