Expose client/server-side ECH (#48)

* RTG-3333 Support X25519MLKEM768 by default, but don't sent it as client

X25519MLKEM768 is the standardised successor of the preliminary
X25519Kyber768Draft00. Latest browsers have switched to X25519MLKEM768.
Cloudflare supports both on the edge.

We've had support for X25519MLKEM768 in this crate for a while, but
didn't enable by default. We're now enabling serverside support by
default. We also let clients advertise support when set
to kx-client-pq-supported.

We don't enable support by default yet for clients set to
kx-client-pq-preferred, as that would cause an extra round-trip due to
HelloRetryRequest if the server doesn't support X25519MLKEM768 yet.

BoringSSL against which we build must support X25519MLKEM768, otherwise
this will fail.

* replace once_cell with LazyLock

We can drop the once_cell dependency since the same functionality is
implemented in std now.

Requires bumping MSRV to 1.80.

* fix manual_c_str_literals clippy warning

* chore: Fix docs on SslRef::replace_ex_data

* Detailed error codes

* Clean up boring_sys::init()

We don't need the workaround that was initially introduced for a bug in
openssl, and OPENSSL_init_ssl always calls into CRYPTO_library_init on
boringssl, so just call it explicitly.

* Expose EVP_HPKE_KEY

* Expose client/server-side ECH

Resolves cloudflare/boring#282

---------

Co-authored-by: Bas Westerbaan <[email protected]>
Co-authored-by: Alessandro Ghedini <[email protected]>
Co-authored-by: Evan Rittenhouse <[email protected]>
Co-authored-by: Kornel <[email protected]>
Co-authored-by: Rushil Mehra <[email protected]>

Author: 6 people

boring/src/hpke.rs

@@ -0,0 +1,31 @@
+use crate::error::ErrorStack;
+use crate::{cvt_0i, cvt_p, ffi};
+
+use foreign_types::ForeignType;
+
+foreign_type_and_impl_send_sync! {
+    type CType = ffi::EVP_HPKE_KEY;
+    fn drop = ffi::EVP_HPKE_KEY_free;
+
+    pub struct HpkeKey;
+}
+
+impl HpkeKey {
+    /// Allocates and initializes a key with the `EVP_HPKE_KEY` type using the
+    /// `EVP_hpke_x25519_hkdf_sha256` KEM algorithm.
+    pub fn dhkem_p256_sha256(pkey: &[u8]) -> Result<HpkeKey, ErrorStack> {
+        unsafe {
+            ffi::init();
+            let hpke = cvt_p(ffi::EVP_HPKE_KEY_new()).map(|p| HpkeKey::from_ptr(p))?;
+
+            cvt_0i(ffi::EVP_HPKE_KEY_init(
+                hpke.as_ptr(),
+                ffi::EVP_hpke_x25519_hkdf_sha256(),
+                pkey.as_ptr(),
+                pkey.len(),
+            ))?;
+
+            Ok(hpke)
+        }
+    }
+}

boring/src/lib.rs

@@ -123,6 +123,8 @@ pub mod error;
 pub mod ex_data;
 pub mod fips;
 pub mod hash;
+#[cfg(not(feature = "fips"))]
+pub mod hpke;
 pub mod memcmp;
 pub mod nid;
 pub mod pkcs12;

boring/src/ssl/ech.rs

@@ -0,0 +1,43 @@
+use crate::ffi;
+use foreign_types::{ForeignType, ForeignTypeRef};
+use libc::c_int;
+
+use crate::error::ErrorStack;
+use crate::hpke::HpkeKey;
+use crate::{cvt_0i, cvt_p};
+
+foreign_type_and_impl_send_sync! {
+    type CType = ffi::SSL_ECH_KEYS;
+    fn drop = ffi::SSL_ECH_KEYS_free;
+
+    pub struct SslEchKeys;
+}
+
+impl SslEchKeys {
+    pub fn new() -> Result<SslEchKeys, ErrorStack> {
+        unsafe {
+            ffi::init();
+            cvt_p(ffi::SSL_ECH_KEYS_new()).map(|p| SslEchKeys::from_ptr(p))
+        }
+    }
+}
+
+impl SslEchKeysRef {
+    pub fn add_key(
+        &mut self,
+        is_retry_config: bool,
+        ech_config: &[u8],
+        key: HpkeKey,
+    ) -> Result<(), ErrorStack> {
+        unsafe {
+            cvt_0i(ffi::SSL_ECH_KEYS_add(
+                self.as_ptr(),
+                is_retry_config as c_int,
+                ech_config.as_ptr(),
+                ech_config.len(),
+                key.as_ptr(),
+            ))
+            .map(|_| ())
+        }
+    }
+}

boring/src/ssl/mod.rs

@@ -87,6 +87,8 @@ use crate::pkey::{HasPrivate, PKeyRef, Params, Private};
 use crate::srtp::{SrtpProtectionProfile, SrtpProtectionProfileRef};
 use crate::ssl::bio::BioMethod;
 use crate::ssl::callbacks::*;
+#[cfg(not(feature = "fips"))]
+use crate::ssl::ech::SslEchKeys;
 use crate::ssl::error::InnerError;
 use crate::stack::{Stack, StackRef, Stackable};
 use crate::x509::store::{X509Store, X509StoreBuilderRef, X509StoreRef};
@@ -114,6 +116,8 @@ mod callbacks;
 #[cfg(feature = "cert-compression")]
 mod cert_compression;
 mod connector;
+#[cfg(not(feature = "fips"))]
+mod ech;
 mod error;
 mod mut_only;
 #[cfg(test)]
@@ -2014,6 +2018,16 @@ impl SslContextBuilder {
         }
     }
 
+    /// Registers a list of ECH keys on the context. This list should contain new and old
+    /// ECHConfigs to allow stale DNS caches to update. Unlike most `SSL_CTX` APIs, this function
+    /// is safe to call even after the `SSL_CTX` has been associated with connections on various
+    /// threads.
+    #[cfg(not(feature = "fips"))]
+    #[corresponds(SSL_CTX_set1_ech_keys)]
+    pub fn set_ech_keys(&mut self, keys: SslEchKeys) -> Result<(), ErrorStack> {
+        unsafe { cvt(ffi::SSL_CTX_set1_ech_keys(self.as_ptr(), keys.as_ptr())).map(|_| ()) }
+    }
+
     /// Consumes the builder, returning a new `SslContext`.
     pub fn build(self) -> SslContext {
         self.ctx
@@ -3573,6 +3587,77 @@ impl SslRef {
     pub fn add_chain_cert(&mut self, cert: &X509Ref) -> Result<(), ErrorStack> {
         unsafe { cvt(ffi::SSL_add1_chain_cert(self.as_ptr(), cert.as_ptr())).map(|_| ()) }
     }
+
+    /// Configures `ech_config_list` on `SSL` for offering ECH during handshakes. If the server
+    /// cannot decrypt the encrypted ClientHello, `SSL` will instead handshake using
+    /// the cleartext parameters of the ClientHelloOuter.
+    ///
+    /// Clients should use `get_ech_name_override` to verify the server certificate in case of ECH
+    /// rejection, and follow up with `get_ech_retry_configs` to retry the connection with a fresh
+    /// set of ECHConfigs. If the retry also fails, clients should report a connection failure.
+    #[cfg(not(feature = "fips"))]
+    #[corresponds(SSL_set1_ech_config_list)]
+    pub fn set_ech_config_list(&mut self, ech_config_list: &[u8]) -> Result<(), ErrorStack> {
+        unsafe {
+            cvt_0i(ffi::SSL_set1_ech_config_list(
+                self.as_ptr(),
+                ech_config_list.as_ptr(),
+                ech_config_list.len(),
+            ))
+            .map(|_| ())
+        }
+    }
+
+    /// This function returns a serialized `ECHConfigList` as provided by the
+    /// server, if one exists.
+    ///
+    /// Clients should call this function when handling an `SSL_R_ECH_REJECTED` error code to
+    /// recover from potential key mismatches. If the result is `Some`, the client should retry the
+    /// connection using the returned `ECHConfigList`.
+    #[cfg(not(feature = "fips"))]
+    #[corresponds(SSL_get0_ech_retry_configs)]
+    pub fn get_ech_retry_configs(&self) -> Option<&[u8]> {
+        unsafe {
+            let mut data = ptr::null();
+            let mut len: usize = 0;
+            ffi::SSL_get0_ech_retry_configs(self.as_ptr(), &mut data, &mut len);
+
+            if data.is_null() {
+                None
+            } else {
+                Some(slice::from_raw_parts(data, len))
+            }
+        }
+    }
+
+    /// If `SSL` is a client and the server rejects ECH, this function returns the public name
+    /// associated with the ECHConfig that was used to attempt ECH.
+    ///
+    /// Clients should call this function during the certificate verification callback to
+    /// ensure the server's certificate is valid for the public name, which is required to
+    /// authenticate retry configs.
+    #[cfg(not(feature = "fips"))]
+    #[corresponds(SSL_get0_ech_name_override)]
+    pub fn get_ech_name_override(&self) -> Option<&[u8]> {
+        unsafe {
+            let mut data: *const c_char = ptr::null();
+            let mut len: usize = 0;
+            ffi::SSL_get0_ech_name_override(self.as_ptr(), &mut data, &mut len);
+
+            if data.is_null() {
+                None
+            } else {
+                Some(slice::from_raw_parts(data as *const u8, len))
+            }
+        }
+    }
+
+    // Whether or not `SSL` negotiated ECH.
+    #[cfg(not(feature = "fips"))]
+    #[corresponds(SSL_ech_accepted)]
+    pub fn ech_accepted(&self) -> bool {
+        unsafe { ffi::SSL_ech_accepted(self.as_ptr()) != 0 }
+    }
 }
 
 /// An SSL stream midway through the handshake process.

boring/src/ssl/test/ech.rs

@@ -0,0 +1,67 @@
+use crate::hpke::HpkeKey;
+use crate::ssl::ech::SslEchKeys;
+use crate::ssl::test::Server;
+use crate::ssl::HandshakeError;
+
+// For future reference, these configs are generated by building the bssl tool (the binary is built
+// alongside boringssl) and running the following command:
+//
+// ./bssl generate-ech -out-ech-config-list ./list -out-ech-config ./config -out-private-key ./key
+// -public-name ech.com -config-id 1
+static ECH_CONFIG_LIST: &[u8] = include_bytes!("../../../test/echconfiglist");
+static ECH_CONFIG: &[u8] = include_bytes!("../../../test/echconfig");
+static ECH_KEY: &[u8] = include_bytes!("../../../test/echkey");
+
+static ECH_CONFIG_2: &[u8] = include_bytes!("../../../test/echconfig-2");
+static ECH_KEY_2: &[u8] = include_bytes!("../../../test/echkey-2");
+
+#[test]
+fn ech() {
+    let server = {
+        let key = HpkeKey::dhkem_p256_sha256(ECH_KEY).unwrap();
+        let mut ech_keys = SslEchKeys::new().unwrap();
+        ech_keys.add_key(true, ECH_CONFIG, key).unwrap();
+
+        let mut builder = Server::builder();
+        builder.ctx().set_ech_keys(ech_keys).unwrap();
+
+        builder.build()
+    };
+
+    let mut client = server.client_with_root_ca().build().builder();
+    client.ssl().set_ech_config_list(ECH_CONFIG_LIST).unwrap();
+    client.ssl().set_hostname("foobar.com").unwrap();
+
+    let ssl_stream = client.connect();
+    assert!(ssl_stream.ssl().ech_accepted())
+}
+
+#[test]
+fn ech_rejection() {
+    let server = {
+        let key = HpkeKey::dhkem_p256_sha256(ECH_KEY_2).unwrap();
+        let mut ech_keys = SslEchKeys::new().unwrap();
+        ech_keys.add_key(true, ECH_CONFIG_2, key).unwrap();
+
+        let mut builder = Server::builder();
+        builder.ctx().set_ech_keys(ech_keys).unwrap();
+
+        builder.build()
+    };
+
+    let mut client = server.client_with_root_ca().build().builder();
+    // Server is initialized using `ECH_CONFIG_2`, so using `ECH_CONFIG_LIST` instead of
+    // `ECH_CONFIG_LIST_2` should trigger rejection.
+    client.ssl().set_ech_config_list(ECH_CONFIG_LIST).unwrap();
+    client.ssl().set_hostname("foobar.com").unwrap();
+    let HandshakeError::Failure(failed_ssl_stream) = client.connect_err() else {
+        panic!("wrong HandshakeError failure variant!");
+    };
+
+    assert_eq!(
+        failed_ssl_stream.ssl().get_ech_name_override(),
+        Some(b"ech.com".to_vec().as_ref())
+    );
+    assert!(failed_ssl_stream.ssl().get_ech_retry_configs().is_some());
+    assert!(!failed_ssl_stream.ssl().ech_accepted())
+}

boring/src/ssl/test/mod.rs

@@ -26,6 +26,8 @@ use super::CompliancePolicy;
 
 mod cert_verify;
 mod custom_verify;
+#[cfg(not(feature = "fips"))]
+mod ech;
 mod private_key_method;
 mod server;
 mod session;