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// Copyright 2023 The Matrix.org Foundation C.I.C.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//! ☣️ Compat support for libolm's PkEncryption and PkDecryption
//!
//! This implements the `m.megolm_backup.v1.curve25519-aes-sha2` described in
//! the Matrix [spec]. This is a hybrid encryption scheme utilizing Curve25519
//! and AES-CBC. X25519 ECDH is performed between an ephemeral key pair and a
//! long-lived backup key pair to establish a shared secret, from which
//! symmetric encryption and message authentication (MAC) keys are derived.
//!
//! **WARNING**: Please note the algorithm contains a critical flaw and does not
//! provide authentication of the ciphertext.
//!
//! # Examples
//!
//! ```
//! use anyhow::Result;
//! use vodozemac::pk_encryption::{PkDecryption, PkEncryption};
//!
//! fn main() -> Result<()> {
//! let plaintext = b"It's a secret to everybody";
//!
//! let decryption = PkDecryption::new();
//! let encryption = PkEncryption::from_key(decryption.public_key());
//!
//! let message = encryption.encrypt(plaintext);
//! let decrypted = decryption.decrypt(&message)?;
//!
//! assert_eq!(decrypted.as_slice(), plaintext);
//!
//! Ok(())
//! }
//! ```
//!
//! [spec]: https://spec.matrix.org/v1.11/client-server-api/#backup-algorithm-mmegolm_backupv1curve25519-aes-sha2
use aes::cipher::{
block_padding::{Pkcs7, UnpadError},
BlockDecryptMut as _, BlockEncryptMut as _, KeyIvInit as _,
};
use hmac::{digest::MacError, Mac as _};
use matrix_pickle::{Decode, Encode};
use thiserror::Error;
use zeroize::{Zeroize, ZeroizeOnDrop};
use crate::{
base64_decode,
cipher::{
key::{CipherKeys, ExpandedKeys},
Aes256CbcDec, Aes256CbcEnc, HmacSha256, Mac,
},
Curve25519PublicKey, Curve25519SecretKey, KeyError,
};
const PICKLE_VERSION: u32 = 1;
/// An error type describing failures which can happen during the decryption
/// step.
#[derive(Debug, Error)]
pub enum Error {
/// The message has invalid [Pkcs7] padding.
#[error("failed to decrypt, invalid padding: {0}")]
InvalidPadding(#[from] UnpadError),
/// The message failed to be authenticated.
#[error("the MAC of the ciphertext didn't pass validation: {0}")]
Mac(#[from] MacError),
}
/// An error type describing failures which can happen during the decoding of an
/// encrypted [`Message`].
#[derive(Debug, Error)]
pub enum MessageDecodeError {
/// One of the message parts wasn't valid Base64.
#[error(transparent)]
Base64(#[from] crate::Base64DecodeError),
/// The ephemeral Curve25519 key isn't valid.
#[error(transparent)]
Key(#[from] KeyError),
}
/// A message that was encrypted using a [`PkEncryption`] object.
#[derive(Debug)]
pub struct Message {
/// The ciphertext of the message.
pub ciphertext: Vec<u8>,
/// The message authentication code of the message.
///
/// **WARNING**: As stated in the module description, this does not
/// authenticate the message.
pub mac: Vec<u8>,
/// The ephemeral [`Curve25519PublicKey`] used to derive the individual
/// message key.
pub ephemeral_key: Curve25519PublicKey,
}
impl Message {
/// Attempt to decode a PkEncryption [`Message`] from a Base64-encoded
/// triplet of ciphertext, MAC, and ephemeral key.
pub fn from_base64(
ciphertext: &str,
mac: &str,
ephemeral_key: &str,
) -> Result<Self, MessageDecodeError> {
Ok(Self {
ciphertext: base64_decode(ciphertext)?,
mac: base64_decode(mac)?,
ephemeral_key: Curve25519PublicKey::from_base64(ephemeral_key)?,
})
}
}
/// The decryption component of the PkEncryption support.
///
/// The public key can be shared with others, allowing them to encrypt messages
/// which can be decrypted using the corresponding private key.
pub struct PkDecryption {
secret_key: Curve25519SecretKey,
public_key: Curve25519PublicKey,
}
impl PkDecryption {
/// Create a new random [`PkDecryption`] object.
///
/// This contains a fresh [`Curve25519SecretKey`] which is used as a
/// long-term key to derive individual message keys and effectively serves
/// as the decryption secret.
pub fn new() -> Self {
let secret_key = Curve25519SecretKey::new();
let public_key = Curve25519PublicKey::from(&secret_key);
Self { secret_key, public_key }
}
/// Create a [`PkDecryption`] object from a [`Curve25519SecretKey`] key.
///
/// The [`Curve25519SecretKey`] will be used as the long-term key to derive
/// individual message keys.
pub fn from_key(secret_key: Curve25519SecretKey) -> Self {
let public_key = Curve25519PublicKey::from(&secret_key);
Self { secret_key, public_key }
}
/// Get the [`Curve25519SecretKey`] of this [`PkDecryption`] object.
///
/// If persistence is required, securely serialize and store this key. It
/// can be used to reconstruct the [`PkDecryption`] object for decrypting
/// associated messages.
pub const fn secret_key(&self) -> &Curve25519SecretKey {
&self.secret_key
}
/// Get the associated ephemeral [`Curve25519PublicKey`]. This key can be
/// used to reconstruct the [`PkEncryption`] object to encrypt messages.
pub const fn public_key(&self) -> Curve25519PublicKey {
self.public_key
}
/// Create a [`PkDecryption`] object by unpickling a PkDecryption pickle in
/// libolm legacy pickle format.
///
/// Such pickles are encrypted and need to first be decrypted using a
/// `pickle_key`.
pub fn from_libolm_pickle(
pickle: &str,
pickle_key: &[u8],
) -> Result<Self, crate::LibolmPickleError> {
use crate::utilities::unpickle_libolm;
unpickle_libolm::<PkDecryptionPickle, _>(pickle, pickle_key, PICKLE_VERSION)
}
/// Pickle a [`PkDecryption`] into a libolm pickle format.
///
/// This pickle can be restored using the
/// `[PkDecryption::from_libolm_pickle]` method, or can be used in the
/// [`libolm`] C library.
///
/// The pickle will be encrypted using the pickle key.
///
/// ⚠️ ***Security Warning***: The pickle key will get expanded into both
/// an AES key and an IV in a deterministic manner. If the same pickle
/// key is reused, this will lead to IV reuse. To prevent this, users
/// have to ensure that they always use a globally (probabilistically)
/// unique pickle key.
///
/// [`libolm`]: https://gitlab.matrix.org/matrix-org/olm/
///
/// # Examples
/// ```
/// use vodozemac::pk_encryption::PkDecryption;
/// use olm_rs::{pk::OlmPkDecryption, PicklingMode};
///
/// let decrypt = PkDecryption::new();
///
/// let pickle = decrypt
/// .to_libolm_pickle(&[0u8; 32])
/// .expect("We should be able to pickle a freshly created PkDecryption");
///
/// let unpickled = OlmPkDecryption::unpickle(
/// pickle,
/// PicklingMode::Encrypted { key: [0u8; 32].to_vec() },
/// ).expect("We should be able to unpickle our exported PkDecryption");
/// ```
pub fn to_libolm_pickle(&self, pickle_key: &[u8]) -> Result<String, crate::LibolmPickleError> {
use crate::utilities::pickle_libolm;
pickle_libolm::<PkDecryptionPickle>(self.into(), pickle_key)
}
/// Decrypt a [`Message`] which was encrypted for this [`PkDecryption`]
/// object.
pub fn decrypt(&self, message: &Message) -> Result<Vec<u8>, Error> {
let shared_secret = self.secret_key.diffie_hellman(&message.ephemeral_key);
let expanded_keys = ExpandedKeys::new_helper(shared_secret.as_bytes(), b"");
let cipher_keys = CipherKeys::from_expanded_keys(expanded_keys);
let hmac = HmacSha256::new_from_slice(cipher_keys.mac_key())
.expect("We should be able to create a Hmac object from a 32 byte key");
// BUG: This is a know issue, we check the MAC of an empty message instead of
// updating the `hmac` object with the ciphertext bytes.
hmac.verify_truncated_left(&message.mac)?;
let cipher = Aes256CbcDec::new(cipher_keys.aes_key(), cipher_keys.iv());
let decrypted = cipher.decrypt_padded_vec_mut::<Pkcs7>(&message.ciphertext)?;
Ok(decrypted)
}
}
impl Default for PkDecryption {
fn default() -> Self {
Self::new()
}
}
impl TryFrom<PkDecryptionPickle> for PkDecryption {
type Error = crate::LibolmPickleError;
fn try_from(pickle: PkDecryptionPickle) -> Result<Self, Self::Error> {
let secret_key = Curve25519SecretKey::from_slice(&pickle.private_curve25519_key);
let public_key = Curve25519PublicKey::from(&secret_key);
Ok(Self { secret_key, public_key })
}
}
/// A libolm compatible and picklable form of [`PkDecryption`].
#[derive(Encode, Decode, Zeroize, ZeroizeOnDrop)]
struct PkDecryptionPickle {
version: u32,
public_curve25519_key: [u8; 32],
#[secret]
private_curve25519_key: Box<[u8; 32]>,
}
impl From<&PkDecryption> for PkDecryptionPickle {
fn from(decrypt: &PkDecryption) -> Self {
Self {
version: PICKLE_VERSION,
public_curve25519_key: decrypt.public_key.to_bytes(),
private_curve25519_key: decrypt.secret_key.to_bytes(),
}
}
}
/// The encryption component of PkEncryption support.
///
/// This struct can be created from a [`Curve25519PublicKey`] corresponding to
/// a [`PkDecryption`] object, allowing encryption of messages for that object.
pub struct PkEncryption {
public_key: Curve25519PublicKey,
}
impl PkEncryption {
/// Create a new [`PkEncryption`] object from a [`Curve25519PublicKey`].
///
/// The public key should be obtained from an existing [`PkDecryption`]
/// object.
pub const fn from_key(public_key: Curve25519PublicKey) -> Self {
Self { public_key }
}
/// Encrypt a message using this [`PkEncryption`] object.
pub fn encrypt(&self, message: &[u8]) -> Message {
let ephemeral_key = Curve25519SecretKey::new();
let shared_secret = ephemeral_key.diffie_hellman(&self.public_key);
let expanded_keys = ExpandedKeys::new_helper(shared_secret.as_bytes(), b"");
let cipher_keys = CipherKeys::from_expanded_keys(expanded_keys);
let cipher = Aes256CbcEnc::new(cipher_keys.aes_key(), cipher_keys.iv());
let ciphertext = cipher.encrypt_padded_vec_mut::<Pkcs7>(message);
let hmac = HmacSha256::new_from_slice(cipher_keys.mac_key())
.expect("We should be able to create a Hmac object from a 32 byte key");
// BUG: This is a know issue, we create a MAC of an empty message instead of
// updating the `hmac` object with the ciphertext bytes.
let mut mac = hmac.finalize().into_bytes().to_vec();
mac.truncate(Mac::TRUNCATED_LEN);
Message { ciphertext, mac, ephemeral_key: Curve25519PublicKey::from(&ephemeral_key) }
}
}
impl From<&PkDecryption> for PkEncryption {
fn from(value: &PkDecryption) -> Self {
Self::from_key(value.public_key())
}
}
impl From<Curve25519PublicKey> for PkEncryption {
fn from(public_key: Curve25519PublicKey) -> Self {
Self { public_key }
}
}
#[cfg(test)]
mod tests {
use olm_rs::pk::{OlmPkDecryption, OlmPkEncryption, PkMessage};
use super::{Message, MessageDecodeError, PkDecryption, PkEncryption};
use crate::{base64_encode, Curve25519PublicKey, Curve25519SecretKey};
/// Conversion from the libolm type to the vodozemac type. To make some
/// tests easier on the eyes.
impl TryFrom<PkMessage> for Message {
type Error = MessageDecodeError;
fn try_from(value: PkMessage) -> Result<Self, Self::Error> {
Self::from_base64(&value.ciphertext, &value.mac, &value.ephemeral_key)
}
}
/// Conversion from the vodozemac type to the libolm type, in a similar
/// manner to the above [TryFrom] implementation.
impl From<Message> for PkMessage {
fn from(val: Message) -> Self {
PkMessage {
ciphertext: base64_encode(val.ciphertext),
mac: base64_encode(val.mac),
ephemeral_key: val.ephemeral_key.to_base64(),
}
}
}
#[test]
fn decrypt_libolm_encrypted_message() {
let decryptor = PkDecryption::new();
let public_key = decryptor.public_key();
let encryptor = OlmPkEncryption::new(&public_key.to_base64());
let message = "It's a secret to everybody";
let encrypted = encryptor.encrypt(message);
let encrypted =
encrypted.try_into().expect("We should be able to decode a message libolm created");
let decrypted = decryptor
.decrypt(&encrypted)
.expect("We should be able to decrypt a message libolm encrypted");
assert_eq!(
message.as_bytes(),
decrypted,
"The plaintext should match the decrypted message"
);
}
#[test]
fn encrypt_for_libolm_pk_decryption() {
let decryptor = OlmPkDecryption::new();
let public_key = Curve25519PublicKey::from_base64(decryptor.public_key())
.expect("libolm should provide us with a valid Curve25519 public key");
let encryptor = PkEncryption::from_key(public_key);
let message = "It's a secret to everybody";
let encrypted = encryptor.encrypt(message.as_ref());
let encrypted = encrypted.into();
let decrypted = decryptor
.decrypt(encrypted)
.expect("We should be able to decrypt a message vodozemac encrypted using libolm");
assert_eq!(message, decrypted, "The plaintext should match the decrypted message");
}
#[test]
fn encryption_roundtrip() {
let decryptor = PkDecryption::new();
let public_key = decryptor.public_key();
let encryptor = PkEncryption::from_key(public_key);
let message = "It's a secret to everybody";
let encrypted = encryptor.encrypt(message.as_ref());
let decrypted = decryptor
.decrypt(&encrypted)
.expect("We should be able to decrypt a message we encrypted");
assert_eq!(message.as_ref(), decrypted, "The plaintext should match the decrypted message");
}
#[test]
fn from_bytes() {
let decryption = PkDecryption::default();
let bytes = decryption.secret_key().to_bytes();
let secret_key = Curve25519SecretKey::from_slice(&bytes);
let restored = PkDecryption::from_key(secret_key);
assert_eq!(
decryption.public_key(),
restored.public_key(),
"The public keys of the restored and original PK decryption should match"
);
}
#[test]
fn libolm_unpickling() {
let olm = OlmPkDecryption::new();
let key = b"DEFAULT_PICKLE_KEY";
let pickle = olm.pickle(olm_rs::PicklingMode::Encrypted { key: key.to_vec() });
let unpickled = PkDecryption::from_libolm_pickle(&pickle, key)
.expect("We should be able to unpickle a key pickled by libolm");
assert_eq!(
olm.public_key(),
unpickled.public_key().to_base64(),
"The public keys of libolm and vodozemac should match"
);
}
#[test]
fn libolm_pickle_cycle() {
let olm = OlmPkDecryption::new();
let key = b"DEFAULT_PICKLE_KEY";
let pickle = olm.pickle(olm_rs::PicklingMode::Encrypted { key: key.to_vec() });
let decrypt = PkDecryption::from_libolm_pickle(&pickle, key)
.expect("We should be able to unpickle a key pickled by libolm");
let vodozemac_pickle =
decrypt.to_libolm_pickle(key).expect("We should be able to pickle a key");
let _ = PkDecryption::from_libolm_pickle(&vodozemac_pickle, key)
.expect("We should be able to unpickle a key pickled by vodozemac");
let unpickled = OlmPkDecryption::unpickle(
vodozemac_pickle,
olm_rs::PicklingMode::Encrypted { key: key.to_vec() },
)
.expect("Libolm should be able to unpickle a key pickled by vodozemac");
assert_eq!(
olm.public_key(),
unpickled.public_key(),
"The public keys of the restored and original libolm PK decryption should match"
);
}
}