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Hardened TLS-F ServerHello selection

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Co-Authored-By: brekotis <93345790+brekotis@users.noreply.github.com>
This commit is contained in:
Alexey
2026-06-11 13:07:40 +03:00
parent db7ff8737c
commit c4b58ad374
6 changed files with 126 additions and 128 deletions
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52
src/protocol/tls.rs
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@@ -561,7 +561,6 @@ pub fn build_server_hello(
fake_cert_len,
rng,
cipher_suite::TLS_AES_128_GCM_SHA256,
TLS_NAMED_GROUP_X25519MLKEM768,
alpn,
new_session_tickets,
)
@@ -579,7 +578,6 @@ pub(crate) fn build_server_hello_with_cipher(
fake_cert_len: usize,
rng: &SecureRandom,
selected_cipher_suite: [u8; 2],
selected_key_share_group: u16,
alpn: Option<Vec<u8>>,
new_session_tickets: u8,
) -> Vec<u8> {
@@ -588,21 +586,12 @@ 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 server_hello = if selected_key_share_group == TLS_NAMED_GROUP_X25519MLKEM768 {
let key_share = gen_fake_x25519mlkem768_server_key_share(rng);
ServerHelloBuilder::new(session_id.to_vec())
.with_cipher_suite(selected_cipher_suite)
.with_key_share(TLS_NAMED_GROUP_X25519MLKEM768, &key_share)
.with_tls13_version()
.build_record()
} else {
let key_share = gen_fake_x25519_key(rng);
ServerHelloBuilder::new(session_id.to_vec())
.with_cipher_suite(selected_cipher_suite)
.with_key_share(TLS_NAMED_GROUP_X25519, &key_share)
.with_tls13_version()
.build_record()
};
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_tls13_version()
.build_record();
// Build Change Cipher Spec record
let change_cipher_spec = [
@@ -1201,20 +1190,23 @@ fn is_tls13_cipher_suite(suite: [u8; 2]) -> bool {
/// Select the ServerHello cipher suite from the already-received ClientHello.
///
/// This is intentionally a borrowed, zero-allocation scan. It runs only for an
/// authenticated success response and keeps malformed or unexpected ClientHello
/// shapes on the previous fallback behavior.
pub(crate) fn select_server_hello_cipher_suite(handshake: &[u8], preferred: [u8; 2]) -> [u8; 2] {
/// authenticated success response and fails closed for malformed or unsupported
/// ClientHello shapes that cannot produce a DPI-consistent ServerHello.
pub(crate) fn select_server_hello_cipher_suite(
handshake: &[u8],
preferred: [u8; 2],
) -> Option<[u8; 2]> {
let preferred = if is_tls13_cipher_suite(preferred) {
preferred
} else {
cipher_suite::TLS_AES_128_GCM_SHA256
};
let Some(range) = client_hello_cipher_suites_range(handshake) else {
return preferred;
return None;
};
if client_hello_offers_cipher_suite(handshake, range, preferred) {
return preferred;
return Some(preferred);
}
for fallback in [
@@ -1223,26 +1215,26 @@ pub(crate) fn select_server_hello_cipher_suite(handshake: &[u8], preferred: [u8;
cipher_suite::TLS_AES_256_GCM_SHA384,
] {
if client_hello_offers_cipher_suite(handshake, range, fallback) {
return fallback;
return Some(fallback);
}
}
preferred
None
}
/// Select the ServerHello key_share named group from the authenticated ClientHello.
/// Select the hybrid ServerHello key_share named group from the authenticated ClientHello.
///
/// Malformed key_share structures intentionally keep the legacy X25519 response
/// to avoid breaking older clients that do not advertise the hybrid group.
pub(crate) fn select_server_hello_key_share_group(handshake: &[u8]) -> u16 {
/// 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(
handshake,
TLS_NAMED_GROUP_X25519MLKEM768,
X25519MLKEM768_CLIENT_KEY_SHARE_LEN,
) {
TLS_NAMED_GROUP_X25519MLKEM768
Some(TLS_NAMED_GROUP_X25519MLKEM768)
} else {
TLS_NAMED_GROUP_X25519
None
}
}