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Generate Valid X25519MLKEM768 ServerHello key shares

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Co-Authored-By: brekotis <93345790+brekotis@users.noreply.github.com>
This commit is contained in:
Alexey
2026-06-11 14:14:09 +03:00
parent eba55e755d
commit 62af515504
9 changed files with 477 additions and 70 deletions
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src/protocol/tls.rs
+86 -25
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@@ -63,6 +63,7 @@
use super::constants::*;
use crate::crypto::{SecureRandom, sha256_hmac};
use ml_kem::{B32, EncapsulationKey as MlKemEncapsulationKey, Key as MlKemKey, MlKem768};
#[cfg(test)]
use crate::error::ProxyError;
use std::time::{SystemTime, UNIX_EPOCH};
@@ -123,6 +124,7 @@ pub(crate) const TLS_NAMED_GROUP_X25519MLKEM768: u16 = named_curve::X25519MLKEM7
const X25519_KEY_SHARE_LEN: usize = 32;
const X25519MLKEM768_CLIENT_KEY_SHARE_LEN: usize = 1216;
const X25519MLKEM768_SERVER_KEY_SHARE_LEN: usize = 1120;
const MLKEM768_CLIENT_ENCAPSULATION_KEY_LEN: usize = 1184;
const MLKEM768_SERVER_CIPHERTEXT_LEN: usize = 1088;
// ============= TLS Validation Result =============
@@ -521,9 +523,15 @@ fn validate_tls_handshake_at_time_with_boot_cap(
/// Uses RFC 7748 X25519 scalar multiplication over the canonical basepoint,
/// yielding distribution-consistent public keys for anti-fingerprinting.
pub fn gen_fake_x25519_key(rng: &SecureRandom) -> [u8; 32] {
let (_scalar, public_key) = gen_x25519_key_pair(rng);
public_key
}
fn gen_x25519_key_pair(rng: &SecureRandom) -> ([u8; 32], [u8; 32]) {
let mut scalar = [0u8; X25519_KEY_SHARE_LEN];
scalar.copy_from_slice(&rng.bytes(X25519_KEY_SHARE_LEN));
x25519(scalar, X25519_BASEPOINT_BYTES)
rng.fill(&mut scalar);
let public_key = x25519(scalar, X25519_BASEPOINT_BYTES);
(scalar, public_key)
}
/// Generate a fake X25519MLKEM768 ServerHello key_share payload.
@@ -537,6 +545,49 @@ pub(crate) fn gen_fake_x25519mlkem768_server_key_share(rng: &SecureRandom) -> Ve
key_share
}
fn mlkem768_encapsulate_to_client(client_key: &[u8], rng: &SecureRandom) -> Option<Vec<u8>> {
let key_bytes = MlKemKey::<MlKemEncapsulationKey<MlKem768>>::try_from(client_key).ok()?;
let encapsulation_key = MlKemEncapsulationKey::<MlKem768>::new(&key_bytes).ok()?;
let mut randomness = [0u8; 32];
rng.fill(&mut randomness);
let randomness = B32::try_from(randomness.as_slice()).ok()?;
let (ciphertext, _shared_key) = encapsulation_key.encapsulate_deterministic(&randomness);
let ciphertext = ciphertext.as_slice().to_vec();
if ciphertext.len() == MLKEM768_SERVER_CIPHERTEXT_LEN {
Some(ciphertext)
} else {
None
}
}
/// Build a valid X25519MLKEM768 ServerHello key_share for the authenticated ClientHello.
pub(crate) fn build_x25519mlkem768_server_key_share(
handshake: &[u8],
rng: &SecureRandom,
) -> Option<Vec<u8>> {
let client_key_exchange = client_hello_key_share_group_entry(
handshake,
TLS_NAMED_GROUP_X25519MLKEM768,
X25519MLKEM768_CLIENT_KEY_SHARE_LEN,
)?;
let client_mlkem_key = client_key_exchange.get(..MLKEM768_CLIENT_ENCAPSULATION_KEY_LEN)?;
let client_x25519_key = client_key_exchange.get(MLKEM768_CLIENT_ENCAPSULATION_KEY_LEN..)?;
let mlkem_ciphertext = mlkem768_encapsulate_to_client(client_mlkem_key, rng)?;
let mut client_x25519 = [0u8; X25519_KEY_SHARE_LEN];
client_x25519.copy_from_slice(client_x25519_key);
let (server_x25519_scalar, server_x25519_key) = gen_x25519_key_pair(rng);
let x25519_shared = x25519(server_x25519_scalar, client_x25519);
if bool::from(x25519_shared.ct_eq(&[0u8; X25519_KEY_SHARE_LEN])) {
return None;
}
let mut key_share = Vec::with_capacity(X25519MLKEM768_SERVER_KEY_SHARE_LEN);
key_share.extend_from_slice(&mlkem_ciphertext);
key_share.extend_from_slice(&server_x25519_key);
Some(key_share)
}
/// Build TLS ServerHello response
///
/// This builds a complete TLS 1.3-like response including:
@@ -561,6 +612,7 @@ pub fn build_server_hello(
fake_cert_len,
rng,
cipher_suite::TLS_AES_128_GCM_SHA256,
&gen_fake_x25519mlkem768_server_key_share(rng),
alpn,
new_session_tickets,
)
@@ -578,6 +630,7 @@ pub(crate) fn build_server_hello_with_cipher(
fake_cert_len: usize,
rng: &SecureRandom,
selected_cipher_suite: [u8; 2],
server_key_share: &[u8],
alpn: Option<Vec<u8>>,
new_session_tickets: u8,
) -> Vec<u8> {
@@ -586,10 +639,9 @@ pub(crate) fn build_server_hello_with_cipher(
let fake_cert_len = fake_cert_len.clamp(MIN_APP_DATA, MAX_APP_DATA);
// Build ServerHello
let key_share = gen_fake_x25519mlkem768_server_key_share(rng);
let server_hello = ServerHelloBuilder::new(session_id.to_vec())
.with_cipher_suite(selected_cipher_suite)
.with_key_share(TLS_NAMED_GROUP_X25519MLKEM768, &key_share)
.with_key_share(TLS_NAMED_GROUP_X25519MLKEM768, server_key_share)
.with_tls13_version()
.build_record();
@@ -1096,49 +1148,56 @@ fn client_hello_extensions_range(handshake: &[u8]) -> Option<(usize, usize)> {
Some((pos, extensions_end))
}
fn key_share_extension_has_group(
data: &[u8],
fn key_share_extension_group_entry<'a>(
data: &'a [u8],
group: u16,
expected_key_exchange_len: usize,
) -> bool {
) -> Option<&'a [u8]> {
if data.len() < 2 {
return false;
return None;
}
let shares_len = u16::from_be_bytes([data[0], data[1]]) as usize;
if shares_len != data.len().saturating_sub(2) {
return false;
return None;
}
let mut pos = 2usize;
let shares_end = 2 + shares_len;
let mut found_group = false;
let mut found_group = None;
while pos + 4 <= shares_end {
let entry_group = u16::from_be_bytes([data[pos], data[pos + 1]]);
let key_exchange_len = u16::from_be_bytes([data[pos + 2], data[pos + 3]]) as usize;
pos += 4;
let Some(key_exchange_end) = pos.checked_add(key_exchange_len) else {
return false;
return None;
};
if key_exchange_end > shares_end {
return false;
return None;
}
if entry_group == group && key_exchange_len == expected_key_exchange_len {
found_group = true;
if entry_group == group {
if key_exchange_len != expected_key_exchange_len || found_group.is_some() {
return None;
}
found_group = Some(&data[pos..key_exchange_end]);
}
pos = key_exchange_end;
}
found_group && pos == shares_end
if pos == shares_end {
found_group
} else {
None
}
}
fn client_hello_offers_key_share_group(
handshake: &[u8],
fn client_hello_key_share_group_entry<'a>(
handshake: &'a [u8],
group: u16,
expected_key_exchange_len: usize,
) -> bool {
) -> Option<&'a [u8]> {
let Some((mut pos, extensions_end)) = client_hello_extensions_range(handshake) else {
return false;
return None;
};
while pos + 4 <= extensions_end {
@@ -1146,14 +1205,14 @@ fn client_hello_offers_key_share_group(
let ext_len = u16::from_be_bytes([handshake[pos + 2], handshake[pos + 3]]) as usize;
pos += 4;
let Some(ext_end) = pos.checked_add(ext_len) else {
return false;
return None;
};
if ext_end > extensions_end {
return false;
return None;
}
if ext_type == extension_type::KEY_SHARE {
return key_share_extension_has_group(
return key_share_extension_group_entry(
&handshake[pos..ext_end],
group,
expected_key_exchange_len,
@@ -1163,7 +1222,7 @@ fn client_hello_offers_key_share_group(
pos = ext_end;
}
false
None
}
fn client_hello_offers_cipher_suite(
@@ -1227,11 +1286,13 @@ pub(crate) fn select_server_hello_cipher_suite(
/// Malformed or non-hybrid key_share structures fail closed so authenticated
/// but DPI-inconsistent ClientHellos take the ordinary masking fallback path.
pub(crate) fn select_server_hello_key_share_group(handshake: &[u8]) -> Option<u16> {
if client_hello_offers_key_share_group(
if client_hello_key_share_group_entry(
handshake,
TLS_NAMED_GROUP_X25519MLKEM768,
X25519MLKEM768_CLIENT_KEY_SHARE_LEN,
) {
)
.is_some()
{
Some(TLS_NAMED_GROUP_X25519MLKEM768)
} else {
None