Add mlkem768x25519 hybrid PQ quantum key exchange

Tested to interoperate with OpenSSH, PuTTY, and Dropbear.
It adds at least ~3kB to session state, and probably a few kB of stack
size. It's enabled by default for std feature only at the moment.

Author: mkj

Cargo.lock

@@ -55,6 +55,7 @@ subtle = { version = "2.4", default-features = false }
 ed25519-dalek = { version = "2.1", default-features = false, features = ["zeroize", "rand_core"] }
 x25519-dalek = { version = "2.0", default-features = false, features = ["zeroize"] }
 curve25519-dalek = { version = "4.1", default-features = false, features = ["zeroize"] }
+ml-kem = { version = "0.2.1", default-features = false, features = ["zeroize"], optional = true }
 # p521 = { version = "0.13.2", default-features = false, features = ["ecdh", "ecdsa"] }
 rsa = { version = "0.9", default-features = false, optional = true, features = ["sha2"] }
 # TODO: getrandom feature is a workaround for missing ssh-key dependency with rsa. fixed in pending 0.6
@@ -66,9 +67,11 @@ embedded-io = { version = "0.6", optional = true }
 pretty-hex = { version = "0.4", default-features = false }
 
 [features]
-std = ["snafu/std", "ssh-key/alloc", "larger"]
+default = []
+std = ["snafu/std", "ssh-key/alloc", "larger", "mlkem"]
 backtrace = ["snafu/backtrace"]
 rsa = ["dep:rsa", "ssh-key/rsa"]
+mlkem = ["dep:ml-kem"]
 # allows conversion to/from OpenSSH key formats
 openssh-key = ["ssh-key"]
 # implements embedded_io::Error for sunset::Error

Cargo.toml

@@ -790,8 +790,8 @@ mod tests {
                     SessId::from_slice(&Sha256::digest("some sessid".as_bytes()))
                         .unwrap();
                 let sharedkey = b"hello";
-                let ko = KexOutput::new_test(sharedkey, &algos, &h);
-                let ko_b = KexOutput::new_test(sharedkey, &algos, &h);
+                let ko = KexOutput::new_test(sharedkey, &h);
+                let ko_b = KexOutput::new_test(sharedkey, &h);
 
                 trace!("algos enc {algos:?}");
                 let newkeys = Keys::derive(ko, &sess_id, &algos).unwrap();
@@ -829,7 +829,7 @@ mod tests {
                 let sess_id =
                     SessId::from_slice(&Sha256::digest(b"some sessid")).unwrap();
                 let sharedkey = b"hello";
-                let ko = KexOutput::new_test(sharedkey, &algos, &h);
+                let ko = KexOutput::new_test(sharedkey, &h);
                 let newkeys = Keys::derive(ko, &sess_id, &algos).unwrap();
 
                 keys.rekey(newkeys);