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finished implementing ratchet count salting

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mamoniot 2022-12-28 10:39:44 -05:00
parent c90faab4c0
commit 53fe95c923
1 changed files with 62 additions and 42 deletions
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104
zssp/src/zssp.rs
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@ -3,6 +3,7 @@
// ZSSP: ZeroTier Secure Session Protocol
// FIPS compliant Noise_IK with Jedi powers and built-in attack-resistant large payload (fragmentation) support.
use std::sync::atomic::{AtomicU64, Ordering};
use std::sync::{Mutex, RwLock};
use zerotier_crypto::aes::{Aes, AesGcm};
@ -113,6 +114,7 @@ pub struct Session<Application: ApplicationLayer> {
/// An arbitrary application defined object associated with each session
pub application_data: Application::Data,
ratchet_counts: [AtomicU64; 2], // Number of preceding session keys in ratchet
header_check_cipher: Aes, // Cipher used for header check codes (not Noise related)
receive_windows: [CounterWindow; 2], // Receive window for anti-replay and deduplication
state: RwLock<SessionMutableState>, // Mutable parts of state (other than defrag buffers)
@ -144,7 +146,6 @@ struct SessionKey {
send_key: Secret<AES_KEY_SIZE>, // Send side AES-GCM key
receive_cipher_pool: Mutex<Vec<Box<AesGcm>>>, // Pool of reusable sending ciphers
send_cipher_pool: Mutex<Vec<Box<AesGcm>>>, // Pool of reusable receiving ciphers
ratchet_count: u64, // Number of preceding session keys in ratchet
jedi: bool, // True if Kyber1024 was used (both sides enabled)
}
@ -246,6 +247,7 @@ impl<Application: ApplicationLayer> Session<Application> {
&bob_s_public_blob_hash,
&noise_ss,
None,
1,
None,
mtu,
current_time,
@ -256,6 +258,7 @@ impl<Application: ApplicationLayer> Session<Application> {
return Ok(Self {
id: local_session_id,
application_data,
ratchet_counts: [AtomicU64::new(1), AtomicU64::new(0)],
header_check_cipher,
receive_windows: [CounterWindow::new(), CounterWindow::new_invalid()],
state: RwLock::new(SessionMutableState {
@ -293,12 +296,14 @@ impl<Application: ApplicationLayer> Session<Application> {
debug_assert!(mtu_sized_buffer.len() >= MIN_TRANSPORT_MTU);
let state = self.state.read().unwrap();
if let Some(remote_session_id) = state.remote_session_id {
if let Some(session_key) = state.session_keys[state.cur_session_key_id as usize].as_ref() {
let key_id = state.cur_session_key_id;
if let Some(session_key) = state.session_keys[key_id as usize].as_ref() {
// Total size of the armored packet we are going to send (may end up being fragmented)
let packet_len = data.len() + HEADER_SIZE + AES_GCM_TAG_SIZE;
//key ratchet count to be used for salting
let ratchet_count = self.ratchet_counts[key_id as usize].load(Ordering::Relaxed);
// This outgoing packet's nonce counter value.
let counter = state.send_counters[state.cur_session_key_id as usize].next();
let counter = state.send_counters[key_id as usize].next();
////////////////////////////////////////////////////////////////
// packet encoding for post-noise transport
@ -312,7 +317,7 @@ impl<Application: ApplicationLayer> Session<Application> {
PACKET_TYPE_DATA,
remote_session_id.into(),
counter,
state.cur_session_key_id
key_id
)?;
// Get an initialized AES-GCM cipher and re-initialize with a 96-bit IV built from remote session ID,
@ -331,7 +336,7 @@ impl<Application: ApplicationLayer> Session<Application> {
let fragment_size = fragment_data_size + HEADER_SIZE;
c.crypt(&data[..fragment_data_size], &mut mtu_sized_buffer[HEADER_SIZE..fragment_size]);
data = &data[fragment_data_size..];
set_header_check_code(mtu_sized_buffer, &self.header_check_cipher);
set_header_check_code(mtu_sized_buffer, ratchet_count, &self.header_check_cipher);
send(&mut mtu_sized_buffer[..fragment_size]);
debug_assert!(header[15].wrapping_shr(2) < 63);
@ -352,7 +357,7 @@ impl<Application: ApplicationLayer> Session<Application> {
c.crypt(data, &mut mtu_sized_buffer[HEADER_SIZE..payload_end]);
let gcm_tag = c.finish_encrypt();
mtu_sized_buffer[payload_end..last_fragment_size].copy_from_slice(&gcm_tag);
set_header_check_code(mtu_sized_buffer, &self.header_check_cipher);
set_header_check_code(mtu_sized_buffer, ratchet_count, &self.header_check_cipher);
send(&mut mtu_sized_buffer[..last_fragment_size]);
// Check reusable AES-GCM instance back into pool.
@ -380,8 +385,9 @@ impl<Application: ApplicationLayer> Session<Application> {
/// and whether Kyber1024 was used. None is returned if the session isn't established.
pub fn status(&self) -> Option<([u8; 16], i64, u64, bool)> {
let state = self.state.read().unwrap();
if let Some(key) = state.session_keys[state.cur_session_key_id as usize].as_ref() {
Some((key.secret_fingerprint, key.creation_time, key.ratchet_count, key.jedi))
let key_id = state.cur_session_key_id;
if let Some(key) = state.session_keys[key_id as usize].as_ref() {
Some((key.secret_fingerprint, key.creation_time, self.ratchet_counts[key_id as usize].load(Ordering::Relaxed), key.jedi))
} else {
None
}
@ -419,6 +425,7 @@ impl<Application: ApplicationLayer> Session<Application> {
//mark the previous key as no longer being supported because it is about to be overwritten
self.receive_windows[(!current_key_id) as usize].invalidate();
let mut offer = None;
//TODO: what happens here if the session is in a limbo state due to dropped packets?
if send_ephemeral_offer(
&mut send,
state.send_counters[current_key_id as usize].next(),
@ -432,6 +439,7 @@ impl<Application: ApplicationLayer> Session<Application> {
&self.remote_s_public_blob_hash,
&self.noise_ss,
state.session_keys[current_key_id as usize].as_ref(),
self.ratchet_counts[current_key_id as usize].load(Ordering::Relaxed),
if state.remote_session_id.is_some() {
Some(&self.header_check_cipher)
} else {
@ -497,7 +505,10 @@ impl<Application: ApplicationLayer> ReceiveContext<Application> {
if let Some(local_session_id) = SessionId::new_from_u64(u64::from_le(memory::load_raw(&incoming_packet[8..16])) & 0xffffffffffffu64)
{
if let Some(session) = app.lookup_session(local_session_id) {
if verify_header_check_code(incoming_packet, &session.header_check_cipher) {
//this is the only time ratchet_counts is ever accessed outside of a lock
//as such this read can be wrong, but that is incredibly unlikely since we are tracking the last two ratchet counts, and if it's wrong it just means we drop a packet that would have been dropped anyways for being too old or too new
let ratchet_count = session.ratchet_counts[key_id as usize].load(Ordering::SeqCst);
if verify_header_check_code(incoming_packet, ratchet_count, &session.header_check_cipher) {
if session.receive_windows[key_id as usize].message_received(counter) {
let canonical_header = CanonicalHeader::make(local_session_id, packet_type, counter);
if fragment_count > 1 {
@ -556,8 +567,8 @@ impl<Application: ApplicationLayer> ReceiveContext<Application> {
}
} else {
unlikely_branch(); // we want data receive to be the priority branch, this is only occasionally used
if verify_header_check_code(incoming_packet, &self.incoming_init_header_check_cipher) {
//salt with a known value so new sessions can be established
if verify_header_check_code(incoming_packet, 1u64, &self.incoming_init_header_check_cipher) {
let canonical_header = CanonicalHeader::make(SessionId::NIL, packet_type, counter);
if fragment_count > 1 {
let mut defrag = self.initial_offer_defrag.lock().unwrap();
@ -839,7 +850,7 @@ impl<Application: ApplicationLayer> ReceiveContext<Application> {
// Perform checks and match ratchet key if there's an existing session, or gate (via host) and
// then create new sessions.
let (new_session, reply_counter, new_key_id, ratchet_key, last_ratchet_count) = if let Some(session) = session.as_ref() {
let (new_session, reply_counter, new_key_id, ratchet_key) = if let Some(session) = session.as_ref() {
// Existing session identity must match the one in this offer.
if !secure_eq(&session.remote_s_public_blob_hash, &SHA384::hash(&alice_s_public_blob)) {
return Err(Error::FailedAuthentication);
@ -848,7 +859,6 @@ impl<Application: ApplicationLayer> ReceiveContext<Application> {
// Match ratchet key fingerprint and fail if no match, which likely indicates an old offer packet.
let alice_ratchet_key_fingerprint = alice_ratchet_key_fingerprint.unwrap();
let mut ratchet_key = None;
let mut last_ratchet_count = 0;
let state = session.state.read().unwrap();
//key_id here is the key id of the key being rekeyed and replaced
//it must be equal to the current session key, and not the previous session key
@ -858,14 +868,13 @@ impl<Application: ApplicationLayer> ReceiveContext<Application> {
if let Some(k) = state.session_keys[key_id as usize].as_ref() {
if public_fingerprint_of_secret(k.ratchet_key.as_bytes())[..16].eq(alice_ratchet_key_fingerprint) {
ratchet_key = Some(k.ratchet_key.clone());
last_ratchet_count = k.ratchet_count;
}
}
if ratchet_key.is_none() {
return Ok(ReceiveResult::Ignored); // old packet?
}
(None, state.send_counters[state.cur_session_key_id as usize].next(), !key_id, ratchet_key, last_ratchet_count)
(None, state.send_counters[state.cur_session_key_id as usize].next(), !key_id, ratchet_key)
} else {
if key_id != false {
return Ok(ReceiveResult::Ignored);
@ -882,6 +891,7 @@ impl<Application: ApplicationLayer> ReceiveContext<Application> {
Some(Session::<Application> {
id: new_session_id,
application_data: associated_object,
ratchet_counts: [AtomicU64::new(1), AtomicU64::new(0)],
header_check_cipher,
receive_windows: [CounterWindow::new(), CounterWindow::new_invalid()],
state: RwLock::new(SessionMutableState {
@ -900,8 +910,7 @@ impl<Application: ApplicationLayer> ReceiveContext<Application> {
}),
reply_counter,
false,
None,
0,
None
)
} else {
return Err(Error::NewSessionRejected);
@ -1037,32 +1046,34 @@ impl<Application: ApplicationLayer> ReceiveContext<Application> {
session_key,
Role::Bob,
current_time,
last_ratchet_count + 1,
hybrid_kk.is_some(),
);
//TODO: check for correct orderings
let ratchet_count;
let mut state = session.state.write().unwrap();
let _ = state.session_keys[new_key_id as usize].replace(session_key);
if existing_session.is_some() {
debug_assert!(new_key_id != key_id);
// receive_windows only has race conditions with the counter of the remote party. It is theoretically possible that the local host receives counters under new_key_id while the receive_window is still in the process of resetting, but this is very unlikely. If it does happen, two things could happen:
// 1) The received counter is less than what is currently stored in the window, so a valid packet is rejected
// 2) The received counter is greater than what is currently stored in the window, so a valid packet is accepted *but* its counter is deleted from the window so it can be replayed
// 1 is completely acceptable behavior; 2 is unacceptable, but extremely extremely unlikely. Since it is utterly impractical for an adversary to trigger 2 intentionally, and preventing 2 is expensive, we do not currently plan to prevent it.
// if receive_window is ever reimplemented, double check it maintains the above properties.
// To prevent these race conditions, we only update the ratchet_count for salting the check code after the window has reset. So if a counter passes the initial check code: it either means the thread sees ratchet count has been update, therefore it either sees receive_window has been reset (due to memory orderings), or it means a rare accidental check forge has occurred.
session.receive_windows[new_key_id as usize].reset_for_initial_offer();
ratchet_count = session.ratchet_counts[new_key_id as usize].fetch_add(2, Ordering::SeqCst) + 1;
let _ = state.remote_session_id.replace(alice_session_id);
// if this wasn't done inside a lock, a theoretical race condition exists where a thread uses the new key id before the counter is reset, or worse: a thread has held onto the previous key_id == new_key_id, and attempts to use the reset counter
// if the following wasn't done inside a lock, a theoretical race condition exists where a thread uses the new key id before the counter is reset, or worse: a thread has held onto the previous key_id == new_key_id, and attempts to use the reset counter
// for this reason do not access send_counters without holding the read lock
state.cur_session_key_id = new_key_id;
state.send_counters[new_key_id as usize].reset_for_initial_offer();
} else {
ratchet_count = 1;
}
drop(state);
// Bob now has final key state for this exchange. Yay! Now reply to Alice so she can construct it.
send_with_fragmentation(send, &mut reply_buf[..packet_end], mtu, &session.header_check_cipher);
send_with_fragmentation(send, &mut reply_buf[..packet_end], mtu, ratchet_count, &session.header_check_cipher);
if let Some(new_session) = new_session {
return Ok(ReceiveResult::OkNewSession(new_session));
@ -1176,21 +1187,20 @@ impl<Application: ApplicationLayer> ReceiveContext<Application> {
session_key,
Role::Alice,
current_time,
last_ratchet_count + 1,
hybrid_kk.is_some(),
);
let new_key_id = offer.key_id;
let is_new_session = offer.ratchet_count == 0;
drop(state);
//TODO: check for correct orderings
let mut state = session.state.write().unwrap();
let _ = state.remote_session_id.replace(bob_session_id);
let _ = state.session_keys[new_key_id as usize].replace(session_key);
if !is_new_session {
if last_ratchet_count > 0 {
//when an brand new key offer is sent, it is sent using the new_key_id==false counter, we cannot reset it in that case.
//NOTE: the following code should be properly threadsafe, see the large comment above at the end of KEY_OFFER decoding for more info
session.receive_windows[new_key_id as usize].reset_for_initial_offer();
let _ = session.ratchet_counts[new_key_id as usize].fetch_add(2, Ordering::SeqCst).wrapping_add(2);
state.cur_session_key_id = new_key_id;
state.send_counters[new_key_id as usize].reset_for_initial_offer();
}
@ -1229,6 +1239,7 @@ fn send_ephemeral_offer<SendFunction: FnMut(&mut [u8])>(
bob_s_public_blob_hash: &[u8],
noise_ss: &Secret<48>,
current_key: Option<&SessionKey>,
ratchet_count: u64,
header_check_cipher: Option<&Aes>, // None to use one based on the recipient's public key for initial contact
mtu: usize,
current_time: i64,
@ -1248,10 +1259,10 @@ fn send_ephemeral_offer<SendFunction: FnMut(&mut [u8])>(
};
// Get ratchet key for current key if one exists.
let (ratchet_key, ratchet_count) = if let Some(current_key) = current_key {
(Some(current_key.ratchet_key.clone()), current_key.ratchet_count)
let ratchet_key = if let Some(current_key) = current_key {
Some(current_key.ratchet_key.clone())
} else {
(None, 0)
None
};
// Random ephemeral offer ID
@ -1348,12 +1359,13 @@ fn send_ephemeral_offer<SendFunction: FnMut(&mut [u8])>(
let packet_end = idx;
if let Some(header_check_cipher) = header_check_cipher {
send_with_fragmentation(send, &mut packet_buf[..packet_end], mtu, header_check_cipher);
send_with_fragmentation(send, &mut packet_buf[..packet_end], mtu, ratchet_count, header_check_cipher);
} else {
send_with_fragmentation(
send,
&mut packet_buf[..packet_end],
mtu,
ratchet_count,
&Aes::new(kbkdf512(&bob_s_public_blob_hash, KBKDF_KEY_USAGE_LABEL_HEADER_CHECK).first_n::<HEADER_CHECK_AES_KEY_SIZE>()),
);
}
@ -1418,6 +1430,7 @@ fn send_with_fragmentation<SendFunction: FnMut(&mut [u8])>(
send: &mut SendFunction,
packet: &mut [u8],
mtu: usize,
ratchet_count: u64,
header_check_cipher: &Aes,
) {
let packet_len = packet.len();
@ -1426,7 +1439,7 @@ fn send_with_fragmentation<SendFunction: FnMut(&mut [u8])>(
let mut header: [u8; 16] = packet[..HEADER_SIZE].try_into().unwrap();
loop {
let fragment = &mut packet[fragment_start..fragment_end];
set_header_check_code(fragment, header_check_cipher);
set_header_check_code(fragment, ratchet_count, header_check_cipher);
send(fragment);
if fragment_end < packet_len {
debug_assert!(header[15].wrapping_shr(2) < 63);
@ -1442,19 +1455,28 @@ fn send_with_fragmentation<SendFunction: FnMut(&mut [u8])>(
}
/// Set 32-bit header check code, used to make fragmentation mechanism robust.
fn set_header_check_code(packet: &mut [u8], header_check_cipher: &Aes) {
fn set_header_check_code(packet: &mut [u8], ratchet_count: u64, header_check_cipher: &Aes) {
debug_assert!(packet.len() >= MIN_PACKET_SIZE);
let mut check_code = 0u128.to_ne_bytes();
header_check_cipher.encrypt_block(&packet[4..20], &mut check_code);
packet[..4].copy_from_slice(&check_code[..4]);
let mut header_mac = 0u128.to_le_bytes();
memory::store_raw((ratchet_count as u16).to_le_bytes(), &mut header_mac[0..2]);
header_mac[2..16].copy_from_slice(&packet[4..18]);
header_check_cipher.encrypt_block_in_place(&mut header_mac);
packet[..4].copy_from_slice(&header_mac[..4]);
}
/// Verify 32-bit header check code.
fn verify_header_check_code(packet: &[u8], header_check_cipher: &Aes) -> bool {
/// This is not nearly enough entropy to be cryptographically secure, it only is meant for making DOS attacks very hard
fn verify_header_check_code(packet: &[u8], ratchet_count: u64, header_check_cipher: &Aes) -> bool {
debug_assert!(packet.len() >= MIN_PACKET_SIZE);
let mut header_mac = 0u128.to_ne_bytes();
header_check_cipher.encrypt_block(&packet[4..20], &mut header_mac);
memory::load_raw::<u32>(&packet[..4]) == memory::load_raw::<u32>(&header_mac)
//2 bytes is the ratchet key
//12 bytes is the header we want to verify
//2 bytes is random salt from the encrypted message
let mut header_mac = 0u128.to_le_bytes();
memory::store_raw((ratchet_count as u16).to_le_bytes(), &mut header_mac[0..2]);
header_mac[2..16].copy_from_slice(&packet[4..18]);
header_check_cipher.encrypt_block_in_place(&mut header_mac);
memory::load_raw::<u32>(&packet[..4]) == memory::load_raw::<u32>(&header_mac[..4])
}
/// Parse KEY_OFFER and KEY_COUNTER_OFFER starting after the unencrypted public key part.
@ -1511,7 +1533,6 @@ impl SessionKey {
key: Secret<64>,
role: Role,
current_time: i64,
ratchet_count: u64,
jedi: bool,
) -> Self {
let a2b: Secret<AES_KEY_SIZE> = kbkdf512(key.as_bytes(), KBKDF_KEY_USAGE_LABEL_AES_GCM_ALICE_TO_BOB).first_n_clone();
@ -1529,7 +1550,6 @@ impl SessionKey {
send_key,
receive_cipher_pool: Mutex::new(Vec::with_capacity(2)),
send_cipher_pool: Mutex::new(Vec::with_capacity(2)),
ratchet_count,
jedi,
}
}