Use features to set default algorithm preferences

Overwrite boringSSL's default key exchange preferences with safe
defaults using feature flags:

* "pq-kx-supported" enables support for PQ key exchange algorithms by
  default. Classical key exchange is still preferred, but will be
  upgraded to PQ if requested.

* "pq-kx-preferred"  enables preference for PQ key exchange by default,
  with fallback to classical key exchange if requested.

" "fips-required" disables non-FIPS-compliant algorithms by default.

Cargo.toml

@@ -18,6 +18,7 @@ boring = { version = "3", path = "./boring" }
 tokio-boring = { version = "3", path = "./tokio-boring" }
 
 bindgen = { version = "0.66.1", default-features = false, features = ["runtime"] }
+cfg-if = "1.0.0"
 cmake = "0.1"
 fs_extra = "1.3.0"
 fslock = "0.2"

boring/Cargo.toml

@@ -30,8 +30,26 @@ rpk = ["boring-sys/rpk"]
 # Enables experimental post-quantum crypto (https://blog.cloudflare.com/post-quantum-for-all/)
 pq-experimental = ["boring-sys/pq-experimental"]
 
+# Support PQ key exchange by default. The client will prefer classical key
+# exchange, but will upgrade to PQ key exchange if requested by the server.
+# This is the safest option if you don't know if the peer supports PQ key
+# exchange.
+pq-kx-supported = ["pq-experimental"]
+
+# Prefer PQ key exchange by default. The client will prefer PQ exchange, but
+# fallback to classical key exchange if requested by the server. This is the
+# best option if you know the peer supports PQ key exchange.
+pq-kx-preferred = ["pq-kx-supported"]
+
+# Disable support for non-FIPS-compliant algorithm by default.
+#
+# TODO(cjpatton) Disable non-compliant ciphersuites. Currently this feature
+# only disables non-compliant key exchange algorithms.
+fips-required = []
+
 [dependencies]
 bitflags = { workspace = true }
+cfg-if = { workspace = true }
 foreign-types = { workspace = true }
 once_cell = { workspace = true }
 libc = { workspace = true }

boring/src/ssl/mod.rs

@@ -696,16 +696,20 @@ pub struct SslContextBuilder {
 impl SslContextBuilder {
     /// Creates a new `SslContextBuilder` to be used with Raw Public Key.
     ///
-    /// This corresponds to [`SSL_CTX_new`].
+    /// This corresponds to calling [`SSL_CTX_new`] then overwriting the default key exchange and
+    /// cipher preferences. The defaults are only overwritten if certain feature flags are set.
     ///
     /// [`SSL_CTX_new`]: https://www.openssl.org/docs/manmaster/man3/SSL_CTX_new.html
     pub fn new_rpk() -> Result<SslContextBuilder, ErrorStack> {
-        unsafe {
+        let mut ctx = unsafe {
             init();
             let ctx = cvt_p(ffi::SSL_CTX_new(SslMethod::tls_with_buffer().as_ptr()))?;
 
-            Ok(SslContextBuilder::from_ptr(ctx, true))
-        }
+            SslContextBuilder::from_ptr(ctx, true)
+        };
+
+        ctx.set_safe_defaults()?;
+        Ok(ctx)
     }
 
     /// Sets raw public key certificate in DER format.
@@ -739,24 +743,28 @@ impl SslContextBuilder {
 impl SslContextBuilder {
     /// Creates a new `SslContextBuilder`.
     ///
-    /// This corresponds to [`SSL_CTX_new`].
+    /// This corresponds to calling [`SSL_CTX_new`] then overwriting the default key exchange and
+    /// cipher preferences. The defaults are only overwritten if certain feature flags are set.
     ///
     /// [`SSL_CTX_new`]: https://www.openssl.org/docs/manmaster/man3/SSL_CTX_new.html
     pub fn new(method: SslMethod) -> Result<SslContextBuilder, ErrorStack> {
-        unsafe {
+        let mut ctx = unsafe {
             init();
             let ctx = cvt_p(ffi::SSL_CTX_new(method.as_ptr()))?;
 
             #[cfg(feature = "rpk")]
             {
-                Ok(SslContextBuilder::from_ptr(ctx, false))
+                SslContextBuilder::from_ptr(ctx, false)
             }
 
             #[cfg(not(feature = "rpk"))]
             {
-                Ok(SslContextBuilder::from_ptr(ctx))
+                SslContextBuilder::from_ptr(ctx)
             }
-        }
+        };
+
+        ctx.set_safe_defaults()?;
+        Ok(ctx)
     }
 
     /// Creates an `SslContextBuilder` from a pointer to a raw OpenSSL value.
@@ -1707,6 +1715,58 @@ impl SslContextBuilder {
         }
     }
 
+    fn set_safe_defaults(&mut self) -> Result<(), ErrorStack> {
+        // Override boringSSL's default key exchange and cipher preferences based on the set of
+        // features we're compiling this crate with.
+        //
+        // For "fips-required" we could have considered the following:
+        //
+        // unsafe {
+        //     let _ = cvt_0i(ffi::SSL_CTX_set_compliance_policy(self.as_ptr(), ffi::ssl_compliance_policy_t::ssl_compliance_policy_fips_202205))?;
+        // }
+        //
+        // However this is stricter than what our unit tests expect because it disables older
+        // version of TLS:
+        // ssl::test::test_connect_with_srtp_ctx
+        // ssl::test::test_connect_with_srtp_ssl
+        //
+        // It would also imply disabling PQ key exchange even though P256_KYBER768_DRAFT00 is
+        // FIPS-compliant.
+        //
+        // TODO(cjpatton) For "fips-required" we also need to disable ciphersuites (TLS 1.3, called
+        // the "cipher list" in 1.2 and below) with ChaCha20Poly1305.
+        cfg_if::cfg_if! {
+            if #[cfg(feature = "pq-kx-preferred")] {
+                self.set_curves(&[
+                    #[cfg(not(feature = "fips-required"))]
+                    SslCurve::X25519_KYBER512_DRAFT00,
+                    SslCurve::P256_KYBER768_DRAFT00,
+                    #[cfg(not(feature = "fips-required"))]
+                    SslCurve::X25519,
+                    SslCurve::SECP256R1,
+                    SslCurve::SECP384R1,
+                ])?;
+            } else if #[cfg(feature = "pq-kx-supported")] {
+                self.set_curves(&[
+                    #[cfg(not(feature = "fips-required"))]
+                    SslCurve::X25519,
+                    SslCurve::SECP256R1,
+                    SslCurve::SECP384R1,
+                    #[cfg(not(feature = "fips-required"))]
+                    SslCurve::X25519_KYBER512_DRAFT00,
+                    SslCurve::P256_KYBER768_DRAFT00,
+                ])?;
+            } else if #[cfg(feature = "fips-required")] {
+               self.set_curves(&[
+                    SslCurve::SECP256R1,
+                    SslCurve::SECP384R1,
+                ])?;
+            }
+        }
+
+        Ok(())
+    }
+
     /// Consumes the builder, returning a new `SslContext`.
     pub fn build(self) -> SslContext {
         self.ctx
@@ -1741,7 +1801,7 @@ impl ToOwned for SslContextRef {
     }
 }
 
-// TODO: add useful info here
+// TODOu: add useful info here
 impl fmt::Debug for SslContext {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         write!(fmt, "SslContext")