serai/crypto/dkg/src/encryption.rs

use core::{ops::Deref, fmt};
use std::{io, collections::HashMap};

use thiserror::Error;

use zeroize::{Zeroize, Zeroizing};
use rand_core::{RngCore, CryptoRng};

use chacha20::{
  cipher::{crypto_common::KeyIvInit, StreamCipher},
  Key as Cc20Key, Nonce as Cc20Iv, ChaCha20,
};

use transcript::{Transcript, RecommendedTranscript};

#[cfg(test)]
use ciphersuite::group::ff::Field;
use ciphersuite::{group::GroupEncoding, Ciphersuite};
use multiexp::BatchVerifier;

use schnorr::SchnorrSignature;
use dleq::DLEqProof;

use crate::{Participant, ThresholdParams};

mod sealed {
  use super::*;

  pub trait ReadWrite: Sized {
    fn read<R: io::Read>(reader: &mut R, params: ThresholdParams) -> io::Result<Self>;
    fn write<W: io::Write>(&self, writer: &mut W) -> io::Result<()>;

    fn serialize(&self) -> Vec<u8> {
      let mut buf = vec![];
      self.write(&mut buf).unwrap();
      buf
    }
  }

  pub trait Message: Clone + PartialEq + Eq + fmt::Debug + Zeroize + ReadWrite {}
  impl<M: Clone + PartialEq + Eq + fmt::Debug + Zeroize + ReadWrite> Message for M {}

  pub trait Encryptable: Clone + AsRef<[u8]> + AsMut<[u8]> + Zeroize + ReadWrite {}
  impl<E: Clone + AsRef<[u8]> + AsMut<[u8]> + Zeroize + ReadWrite> Encryptable for E {}
}
pub(crate) use sealed::*;

/// Wraps a message with a key to use for encryption in the future.
#[derive(Clone, PartialEq, Eq, Debug, Zeroize)]
pub struct EncryptionKeyMessage<C: Ciphersuite, M: Message> {
  msg: M,
  enc_key: C::G,
}

// Doesn't impl ReadWrite so that doesn't need to be imported
impl<C: Ciphersuite, M: Message> EncryptionKeyMessage<C, M> {
  pub fn read<R: io::Read>(reader: &mut R, params: ThresholdParams) -> io::Result<Self> {
    Ok(Self { msg: M::read(reader, params)?, enc_key: C::read_G(reader)? })
  }

  pub fn write<W: io::Write>(&self, writer: &mut W) -> io::Result<()> {
    self.msg.write(writer)?;
    writer.write_all(self.enc_key.to_bytes().as_ref())
  }

  pub fn serialize(&self) -> Vec<u8> {
    let mut buf = vec![];
    self.write(&mut buf).unwrap();
    buf
  }

  #[cfg(any(test, feature = "tests"))]
  pub(crate) fn enc_key(&self) -> C::G {
    self.enc_key
  }
}

/// An encrypted message, with a per-message encryption key enabling revealing specific messages
/// without side effects.
#[derive(Clone, Zeroize)]
pub struct EncryptedMessage<C: Ciphersuite, E: Encryptable> {
  key: C::G,
  // Also include a proof-of-possession for the key.
  // If this proof-of-possession wasn't here, Eve could observe Alice encrypt to Bob with key X,
  // then send Bob a message also claiming to use X.
  // While Eve's message would fail to meaningfully decrypt, Bob would then use this to create a
  // blame argument against Eve. When they do, they'd reveal bX, revealing Alice's message to Bob.
  // This is a massive side effect which could break some protocols, in the worst case.
  // While Eve can still reuse their own keys, causing Bob to leak all messages by revealing for
  // any single one, that's effectively Eve revealing themselves, and not considered relevant.
  pop: SchnorrSignature<C>,
  msg: Zeroizing<E>,
}

fn ecdh<C: Ciphersuite>(private: &Zeroizing<C::F>, public: C::G) -> Zeroizing<C::G> {
  Zeroizing::new(public * private.deref())
}

// Each ecdh must be distinct. Reuse of an ecdh for multiple ciphers will cause the messages to be
// leaked.
fn cipher<C: Ciphersuite>(context: [u8; 32], ecdh: &Zeroizing<C::G>) -> ChaCha20 {
  // Ideally, we'd box this transcript with ZAlloc, yet that's only possible on nightly
  // TODO: https://github.com/serai-dex/serai/issues/151
  let mut transcript = RecommendedTranscript::new(b"DKG Encryption v0.2");
  transcript.append_message(b"context", context);

  transcript.domain_separate(b"encryption_key");

  let mut ecdh = ecdh.to_bytes();
  transcript.append_message(b"shared_key", ecdh.as_ref());
  ecdh.as_mut().zeroize();

  let zeroize = |buf: &mut [u8]| buf.zeroize();

  let mut key = Cc20Key::default();
  let mut challenge = transcript.challenge(b"key");
  key.copy_from_slice(&challenge[.. 32]);
  zeroize(challenge.as_mut());

  // Since the key is single-use, it doesn't matter what we use for the IV
  // The issue is key + IV reuse. If we never reuse the key, we can't have the opportunity to
  // reuse a nonce
  // Use a static IV in acknowledgement of this
  let mut iv = Cc20Iv::default();
  // The \0 is to satisfy the length requirement (12), not to be null terminated
  iv.copy_from_slice(b"DKG IV v0.2\0");

  // ChaCha20 has the same commentary as the transcript regarding ZAlloc
  // TODO: https://github.com/serai-dex/serai/issues/151
  let res = ChaCha20::new(&key, &iv);
  zeroize(key.as_mut());
  res
}

fn encrypt<R: RngCore + CryptoRng, C: Ciphersuite, E: Encryptable>(
  rng: &mut R,
  context: [u8; 32],
  from: Participant,
  to: C::G,
  mut msg: Zeroizing<E>,
) -> EncryptedMessage<C, E> {
  /*
  The following code could be used to replace the requirement on an RNG here.
  It's just currently not an issue to require taking in an RNG here.
  let last = self.last_enc_key.to_bytes();
  self.last_enc_key = C::hash_to_F(b"encryption_base", last.as_ref());
  let key = C::hash_to_F(b"encryption_key", last.as_ref());
  last.as_mut().zeroize();
  */

  // Generate a new key for this message, satisfying cipher's requirement of distinct keys per
  // message, and enabling revealing this message without revealing any others
  let key = Zeroizing::new(C::random_nonzero_F(rng));
  cipher::<C>(context, &ecdh::<C>(&key, to)).apply_keystream(msg.as_mut().as_mut());

  let pub_key = C::generator() * key.deref();
  let nonce = Zeroizing::new(C::random_nonzero_F(rng));
  let pub_nonce = C::generator() * nonce.deref();
  EncryptedMessage {
    key: pub_key,
    pop: SchnorrSignature::sign(
      &key,
      nonce,
      pop_challenge::<C>(context, pub_nonce, pub_key, from, msg.deref().as_ref()),
    ),
    msg,
  }
}

impl<C: Ciphersuite, E: Encryptable> EncryptedMessage<C, E> {
  pub fn read<R: io::Read>(reader: &mut R, params: ThresholdParams) -> io::Result<Self> {
    Ok(Self {
      key: C::read_G(reader)?,
      pop: SchnorrSignature::<C>::read(reader)?,
      msg: Zeroizing::new(E::read(reader, params)?),
    })
  }

  pub fn write<W: io::Write>(&self, writer: &mut W) -> io::Result<()> {
    writer.write_all(self.key.to_bytes().as_ref())?;
    self.pop.write(writer)?;
    self.msg.write(writer)
  }

  pub fn serialize(&self) -> Vec<u8> {
    let mut buf = vec![];
    self.write(&mut buf).unwrap();
    buf
  }

  #[cfg(test)]
  pub(crate) fn invalidate_pop(&mut self) {
    self.pop.s += C::F::ONE;
  }

  #[cfg(test)]
  pub(crate) fn invalidate_msg<R: RngCore + CryptoRng>(
    &mut self,
    rng: &mut R,
    context: [u8; 32],
    from: Participant,
  ) {
    // Invalidate the message by specifying a new key/Schnorr PoP
    // This will cause all initial checks to pass, yet a decrypt to gibberish
    let key = Zeroizing::new(C::random_nonzero_F(rng));
    let pub_key = C::generator() * key.deref();
    let nonce = Zeroizing::new(C::random_nonzero_F(rng));
    let pub_nonce = C::generator() * nonce.deref();
    self.key = pub_key;
    self.pop = SchnorrSignature::sign(
      &key,
      nonce,
      pop_challenge::<C>(context, pub_nonce, pub_key, from, self.msg.deref().as_ref()),
    );
  }

  // Assumes the encrypted message is a secret share.
  #[cfg(test)]
  pub(crate) fn invalidate_share_serialization<R: RngCore + CryptoRng>(
    &mut self,
    rng: &mut R,
    context: [u8; 32],
    from: Participant,
    to: C::G,
  ) {
    use ciphersuite::group::ff::PrimeField;

    let mut repr = <C::F as PrimeField>::Repr::default();
    for b in repr.as_mut() {
      *b = 255;
    }
    // Tries to guarantee the above assumption.
    assert_eq!(repr.as_ref().len(), self.msg.as_ref().len());
    // Checks that this isn't over a field where this is somehow valid
    assert!(!bool::from(C::F::from_repr(repr).is_some()));

    self.msg.as_mut().as_mut().copy_from_slice(repr.as_ref());
    *self = encrypt(rng, context, from, to, self.msg.clone());
  }

  // Assumes the encrypted message is a secret share.
  #[cfg(test)]
  pub(crate) fn invalidate_share_value<R: RngCore + CryptoRng>(
    &mut self,
    rng: &mut R,
    context: [u8; 32],
    from: Participant,
    to: C::G,
  ) {
    use ciphersuite::group::ff::PrimeField;

    // Assumes the share isn't randomly 1
    let repr = C::F::ONE.to_repr();
    self.msg.as_mut().as_mut().copy_from_slice(repr.as_ref());
    *self = encrypt(rng, context, from, to, self.msg.clone());
  }
}

/// A proof that the provided encryption key is a legitimately derived shared key for some message.
#[derive(Clone, PartialEq, Eq, Debug, Zeroize)]
pub struct EncryptionKeyProof<C: Ciphersuite> {
  key: Zeroizing<C::G>,
  dleq: DLEqProof<C::G>,
}

impl<C: Ciphersuite> EncryptionKeyProof<C> {
  pub fn read<R: io::Read>(reader: &mut R) -> io::Result<Self> {
    Ok(Self { key: Zeroizing::new(C::read_G(reader)?), dleq: DLEqProof::read(reader)? })
  }

  pub fn write<W: io::Write>(&self, writer: &mut W) -> io::Result<()> {
    writer.write_all(self.key.to_bytes().as_ref())?;
    self.dleq.write(writer)
  }

  pub fn serialize(&self) -> Vec<u8> {
    let mut buf = vec![];
    self.write(&mut buf).unwrap();
    buf
  }

  #[cfg(test)]
  pub(crate) fn invalidate_key(&mut self) {
    *self.key += C::generator();
  }

  #[cfg(test)]
  pub(crate) fn invalidate_dleq(&mut self) {
    let mut buf = vec![];
    self.dleq.write(&mut buf).unwrap();
    // Adds one to c since this is serialized c, s
    // Adding one to c will leave a validly serialized c
    // Adding one to s may leave an invalidly serialized s
    buf[0] = buf[0].wrapping_add(1);
    self.dleq = DLEqProof::read::<&[u8]>(&mut buf.as_ref()).unwrap();
  }
}

// This doesn't need to take the msg. It just doesn't hurt as an extra layer.
// This still doesn't mean the DKG offers an authenticated channel. The per-message keys have no
// root of trust other than their existence in the assumed-to-exist external authenticated channel.
fn pop_challenge<C: Ciphersuite>(
  context: [u8; 32],
  nonce: C::G,
  key: C::G,
  sender: Participant,
  msg: &[u8],
) -> C::F {
  let mut transcript = RecommendedTranscript::new(b"DKG Encryption Key Proof of Possession v0.2");
  transcript.append_message(b"context", context);

  transcript.domain_separate(b"proof_of_possession");

  transcript.append_message(b"nonce", nonce.to_bytes());
  transcript.append_message(b"key", key.to_bytes());
  // This is sufficient to prevent the attack this is meant to stop
  transcript.append_message(b"sender", sender.to_bytes());
  // This, as written above, doesn't hurt
  transcript.append_message(b"message", msg);
  // While this is a PoK and a PoP, it's called a PoP here since the important part is its owner
  // Elsewhere, where we use the term PoK, the important part is that it isn't some inverse, with
  // an unknown to anyone discrete log, breaking the system
  C::hash_to_F(b"DKG-encryption-proof_of_possession", &transcript.challenge(b"schnorr"))
}

fn encryption_key_transcript(context: [u8; 32]) -> RecommendedTranscript {
  let mut transcript = RecommendedTranscript::new(b"DKG Encryption Key Correctness Proof v0.2");
  transcript.append_message(b"context", context);
  transcript
}

#[derive(Clone, Copy, PartialEq, Eq, Debug, Error)]
pub(crate) enum DecryptionError {
  #[error("accused provided an invalid signature")]
  InvalidSignature,
  #[error("accuser provided an invalid decryption key")]
  InvalidProof,
}

// A simple box for managing decryption.
#[derive(Clone, Debug)]
pub(crate) struct Decryption<C: Ciphersuite> {
  context: [u8; 32],
  enc_keys: HashMap<Participant, C::G>,
}

impl<C: Ciphersuite> Decryption<C> {
  pub(crate) fn new(context: [u8; 32]) -> Self {
    Self { context, enc_keys: HashMap::new() }
  }
  pub(crate) fn register<M: Message>(
    &mut self,
    participant: Participant,
    msg: EncryptionKeyMessage<C, M>,
  ) -> M {
    assert!(
      !self.enc_keys.contains_key(&participant),
      "Re-registering encryption key for a participant"
    );
    self.enc_keys.insert(participant, msg.enc_key);
    msg.msg
  }

  // Given a message, and the intended decryptor, and a proof for its key, decrypt the message.
  // Returns None if the key was wrong.
  pub(crate) fn decrypt_with_proof<E: Encryptable>(
    &self,
    from: Participant,
    decryptor: Participant,
    mut msg: EncryptedMessage<C, E>,
    // There's no encryption key proof if the accusation is of an invalid signature
    proof: Option<EncryptionKeyProof<C>>,
  ) -> Result<Zeroizing<E>, DecryptionError> {
    if !msg.pop.verify(
      msg.key,
      pop_challenge::<C>(self.context, msg.pop.R, msg.key, from, msg.msg.deref().as_ref()),
    ) {
      Err(DecryptionError::InvalidSignature)?;
    }

    if let Some(proof) = proof {
      // Verify this is the decryption key for this message
      proof
        .dleq
        .verify(
          &mut encryption_key_transcript(self.context),
          &[C::generator(), msg.key],
          &[self.enc_keys[&decryptor], *proof.key],
        )
        .map_err(|_| DecryptionError::InvalidProof)?;

      cipher::<C>(self.context, &proof.key).apply_keystream(msg.msg.as_mut().as_mut());
      Ok(msg.msg)
    } else {
      Err(DecryptionError::InvalidProof)
    }
  }
}

// A simple box for managing encryption.
#[derive(Clone)]
pub(crate) struct Encryption<C: Ciphersuite> {
  context: [u8; 32],
  i: Participant,
  enc_key: Zeroizing<C::F>,
  enc_pub_key: C::G,
  decryption: Decryption<C>,
}

impl<C: Ciphersuite> fmt::Debug for Encryption<C> {
  fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
    fmt
      .debug_struct("Encryption")
      .field("context", &self.context)
      .field("i", &self.i)
      .field("enc_pub_key", &self.enc_pub_key)
      .field("decryption", &self.decryption)
      .finish_non_exhaustive()
  }
}

impl<C: Ciphersuite> Zeroize for Encryption<C> {
  fn zeroize(&mut self) {
    self.enc_key.zeroize();
    self.enc_pub_key.zeroize();
    for (_, mut value) in self.decryption.enc_keys.drain() {
      value.zeroize();
    }
  }
}

impl<C: Ciphersuite> Encryption<C> {
  pub(crate) fn new<R: RngCore + CryptoRng>(
    context: [u8; 32],
    i: Participant,
    rng: &mut R,
  ) -> Self {
    let enc_key = Zeroizing::new(C::random_nonzero_F(rng));
    Self {
      context,
      i,
      enc_pub_key: C::generator() * enc_key.deref(),
      enc_key,
      decryption: Decryption::new(context),
    }
  }

  pub(crate) fn registration<M: Message>(&self, msg: M) -> EncryptionKeyMessage<C, M> {
    EncryptionKeyMessage { msg, enc_key: self.enc_pub_key }
  }

  pub(crate) fn register<M: Message>(
    &mut self,
    participant: Participant,
    msg: EncryptionKeyMessage<C, M>,
  ) -> M {
    self.decryption.register(participant, msg)
  }

  pub(crate) fn encrypt<R: RngCore + CryptoRng, E: Encryptable>(
    &self,
    rng: &mut R,
    participant: Participant,
    msg: Zeroizing<E>,
  ) -> EncryptedMessage<C, E> {
    encrypt(rng, self.context, self.i, self.decryption.enc_keys[&participant], msg)
  }

  pub(crate) fn decrypt<R: RngCore + CryptoRng, I: Copy + Zeroize, E: Encryptable>(
    &self,
    rng: &mut R,
    batch: &mut BatchVerifier<I, C::G>,
    // Uses a distinct batch ID so if this batch verifier is reused, we know its the PoP aspect
    // which failed, and therefore to use None for the blame
    batch_id: I,
    from: Participant,
    mut msg: EncryptedMessage<C, E>,
  ) -> (Zeroizing<E>, EncryptionKeyProof<C>) {
    msg.pop.batch_verify(
      rng,
      batch,
      batch_id,
      msg.key,
      pop_challenge::<C>(self.context, msg.pop.R, msg.key, from, msg.msg.deref().as_ref()),
    );

    let key = ecdh::<C>(&self.enc_key, msg.key);
    cipher::<C>(self.context, &key).apply_keystream(msg.msg.as_mut().as_mut());
    (
      msg.msg,
      EncryptionKeyProof {
        key,
        dleq: DLEqProof::prove(
          rng,
          &mut encryption_key_transcript(self.context),
          &[C::generator(), msg.key],
          &self.enc_key,
        ),
      },
    )
  }

  pub(crate) fn into_decryption(self) -> Decryption<C> {
    self.decryption
  }
}