diff --git a/Cargo.lock b/Cargo.lock
index 97c4d00..ea1b659 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -453,6 +453,7 @@ dependencies = [
  "p256",
  "poly1305",
  "rand_core",
+ "secrecy",
  "sha2",
  "subtle",
  "tracing",
@@ -474,6 +475,8 @@ dependencies = [
  "libc",
  "postcard",
  "rustix",
+ "seccompiler",
+ "secrecy",
  "serde",
  "thiserror",
  "tokio",
@@ -481,6 +484,7 @@ dependencies = [
  "tracing",
  "tracing-subscriber",
  "users",
+ "zeroize",
 ]
 
 [[package]]
@@ -1392,6 +1396,25 @@ dependencies = [
  "zeroize",
 ]
 
+[[package]]
+name = "seccompiler"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "345a3e4dddf721a478089d4697b83c6c0a8f5bf16086f6c13397e4534eb6e2e5"
+dependencies = [
+ "libc",
+]
+
+[[package]]
+name = "secrecy"
+version = "0.8.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9bd1c54ea06cfd2f6b63219704de0b9b4f72dcc2b8fdef820be6cd799780e91e"
+dependencies = [
+ "serde",
+ "zeroize",
+]
+
 [[package]]
 name = "semver"
 version = "1.0.23"
diff --git a/Cargo.toml b/Cargo.toml
index 8b74aa3..5e8b572 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -14,8 +14,3 @@ useless_format = "allow"
 [workspace.dependencies]
 tracing = "0.1.40"
 eyre = "0.6.12"
-
-# Blowfish (bcrypt) is the critical path for private key encryption (KDF),
-# and not optimizing it makes the test suite a lot slower.
-[profile.dev.package.blowfish]
-opt-level = 3
diff --git a/bin/cluelessh-faked/src/main.rs b/bin/cluelessh-faked/src/main.rs
index 5dfec82..6c6520e 100644
--- a/bin/cluelessh-faked/src/main.rs
+++ b/bin/cluelessh-faked/src/main.rs
@@ -4,7 +4,7 @@ use std::{net::SocketAddr, sync::Arc};
 
 use cluelessh_keys::private::EncryptedPrivateKeys;
 use cluelessh_tokio::{server::ServerAuth, Channel};
-use eyre::{Context, OptionExt, Result};
+use eyre::{eyre, Context, OptionExt, Result};
 use tokio::{
     net::{TcpListener, TcpStream},
     sync::Mutex,
@@ -77,15 +77,25 @@ async fn main() -> eyre::Result<()> {
             !! DO NOT ENTER PASSWORDS YOU DON'T WANT STOLEN !!\r\n"
                 .to_owned(),
         ),
-        sign_with_hostkey: Arc::new(move |msg| {
+        do_key_exchange: Arc::new(move |msg| {
             let host_keys = host_keys.clone();
             Box::pin(async move {
                 let private = host_keys
                     .iter()
-                    .find(|privkey| privkey.private_key.public_key() == msg.public_key)
+                    .find(|privkey| {
+                        privkey.private_key.public_key()
+                            == msg.server_host_key_algorithm.public_key()
+                    })
                     .ok_or_eyre("missing private key")?;
 
-                Ok(private.private_key.sign(&msg.hash))
+                // TODO: non-shitty error handling here
+
+                cluelessh_protocol::transport::server::do_key_exchange(
+                    msg,
+                    private,
+                    &mut cluelessh_protocol::OsRng,
+                )
+                .map_err(|_| eyre!("error during key exchange"))
             })
         }),
     };
diff --git a/bin/cluelesshd/Cargo.toml b/bin/cluelesshd/Cargo.toml
index 84eede1..588ec54 100644
--- a/bin/cluelesshd/Cargo.toml
+++ b/bin/cluelesshd/Cargo.toml
@@ -22,6 +22,9 @@ toml = "0.8.19"
 clap = { version = "4.5.16", features = ["derive"] }
 postcard = { version = "1.0.10", features = ["alloc"] }
 libc = "0.2.158"
+seccompiler = "0.4.0"
+secrecy = { version = "0.8.0", features = ["serde"] }
+zeroize = "1.8.1"
 
 [lints]
 workspace = true
diff --git a/bin/cluelesshd/cluelesshd.toml b/bin/cluelesshd/cluelesshd.toml
index 4961e0c..3ce6d0a 100644
--- a/bin/cluelesshd/cluelesshd.toml
+++ b/bin/cluelesshd/cluelesshd.toml
@@ -16,3 +16,4 @@ banner = "welcome to my server!!!\r\ni hope you enjoy your stay.\r\n"
 unprivileged_uid = 355353
 unprivileged_gid = 355353
 #unprivileged_user = "sshd"
+experimental_seccomp = true
diff --git a/bin/cluelesshd/src/config.rs b/bin/cluelesshd/src/config.rs
index 3c7dcb1..109d842 100644
--- a/bin/cluelesshd/src/config.rs
+++ b/bin/cluelesshd/src/config.rs
@@ -46,6 +46,10 @@ pub struct SecurityConfig {
     pub unprivileged_gid: Option<u32>,
     /// The username of an unprivileged user.
     pub unprivileged_user: Option<String>,
+
+    /// Apply experimental seccomp filters.
+    #[serde(default = "default_false")]
+    pub experimental_seccomp: bool,
 }
 
 impl Config {
@@ -72,6 +76,11 @@ fn default_true() -> bool {
     true
 }
 
+fn default_false() -> bool {
+    false
+}
+
+
 fn addr_default() -> IpAddr {
     IpAddr::V4(Ipv4Addr::UNSPECIFIED)
 }
diff --git a/bin/cluelesshd/src/connection.rs b/bin/cluelesshd/src/connection.rs
index 9f27478..6ff8719 100644
--- a/bin/cluelesshd/src/connection.rs
+++ b/bin/cluelesshd/src/connection.rs
@@ -1,7 +1,9 @@
 use std::{
+    io,
     os::fd::{FromRawFd, OwnedFd},
     pin::Pin,
     sync::Arc,
+    task::{ready, Poll},
 };
 
 use crate::{
@@ -18,8 +20,7 @@ use cluelessh_tokio::{
 };
 use eyre::{bail, ensure, Result, WrapErr};
 use tokio::{
-    fs::File,
-    io::{AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt},
+    io::{unix::AsyncFd, AsyncRead, AsyncReadExt, AsyncWrite, AsyncWriteExt},
     net::TcpStream,
     sync::mpsc,
 };
@@ -89,9 +90,9 @@ async fn connection_inner(state: SerializedConnectionState) -> Result<()> {
             })
         })),
         auth_banner: config.auth.banner,
-        sign_with_hostkey: Arc::new(move |msg| {
+        do_key_exchange: Arc::new(move |msg| {
             let rpc_client = rpc_client3.clone();
-            Box::pin(async move { rpc_client.sign(msg.hash, msg.public_key).await })
+            Box::pin(async move { rpc_client.kex_exchange(msg).await })
         }),
     };
 
@@ -400,10 +401,8 @@ impl SessionState {
 
         self.pty_term = Some(term);
 
-        self.writer = Some(Box::pin(File::from_std(std::fs::File::from(
-            controller.try_clone()?,
-        ))));
-        self.reader = Some(Box::pin(File::from_std(std::fs::File::from(controller))));
+        self.writer = Some(Box::pin(AsyncFdWrapper::from_fd(controller.try_clone()?)?));
+        self.reader = Some(Box::pin(AsyncFdWrapper::from_fd(controller)?));
         Ok(())
     }
 
@@ -429,9 +428,9 @@ impl SessionState {
                 fds.len()
             );
 
-            let stdin = File::from_std(std::fs::File::from(fds.remove(0)));
-            let stdout = File::from_std(std::fs::File::from(fds.remove(0)));
-            let stderr = File::from_std(std::fs::File::from(fds.remove(0)));
+            let stdin = AsyncFdWrapper::from_fd(fds.remove(0))?;
+            let stdout = AsyncFdWrapper::from_fd(fds.remove(0))?;
+            let stderr = AsyncFdWrapper::from_fd(fds.remove(0))?;
 
             self.writer = Some(Box::pin(stdin));
             self.reader = Some(Box::pin(stdout));
@@ -448,3 +447,73 @@ impl SessionState {
         Ok(())
     }
 }
+
+struct AsyncFdWrapper {
+    fd: AsyncFd<OwnedFd>,
+}
+
+impl AsyncFdWrapper {
+    fn from_fd(fd: OwnedFd) -> Result<Self> {
+        rustix::io::ioctl_fionbio(&fd, true).wrap_err("putting fd into nonblocking mode")?;
+        Ok(Self {
+            fd: AsyncFd::new(fd).wrap_err("failed to register async event")?,
+        })
+    }
+}
+
+impl AsyncRead for AsyncFdWrapper {
+    fn poll_read(
+        self: Pin<&mut Self>,
+        cx: &mut std::task::Context<'_>,
+        buf: &mut tokio::io::ReadBuf<'_>,
+    ) -> Poll<Result<(), io::Error>> {
+        loop {
+            let mut guard = ready!(self.fd.poll_read_ready(cx))?;
+
+            let unfilled = buf.initialize_unfilled();
+            match guard.try_io(|inner| {
+                rustix::io::read(inner.get_ref(), unfilled).map_err(io::Error::from)
+            }) {
+                Ok(Ok(len)) => {
+                    buf.advance(len);
+                    return Poll::Ready(Ok(()));
+                }
+                Ok(Err(err)) => return Poll::Ready(Err(err)),
+                Err(_would_block) => continue,
+            }
+        }
+    }
+}
+
+impl AsyncWrite for AsyncFdWrapper {
+    fn poll_write(
+        self: Pin<&mut Self>,
+        cx: &mut std::task::Context<'_>,
+        buf: &[u8],
+    ) -> Poll<Result<usize, io::Error>> {
+        loop {
+            let mut guard = ready!(self.fd.poll_write_ready(cx))?;
+
+            match guard
+                .try_io(|inner| rustix::io::write(inner.get_ref(), buf).map_err(io::Error::from))
+            {
+                Ok(result) => return Poll::Ready(result),
+                Err(_would_block) => continue,
+            }
+        }
+    }
+
+    fn poll_flush(
+        self: Pin<&mut Self>,
+        _: &mut std::task::Context<'_>,
+    ) -> Poll<Result<(), io::Error>> {
+        Poll::Ready(Ok(()))
+    }
+
+    fn poll_shutdown(
+        self: Pin<&mut Self>,
+        _: &mut std::task::Context<'_>,
+    ) -> Poll<Result<(), io::Error>> {
+        Poll::Ready(Ok(()))
+    }
+}
diff --git a/bin/cluelesshd/src/rpc.rs b/bin/cluelesshd/src/rpc.rs
index 8044906..6a98249 100644
--- a/bin/cluelesshd/src/rpc.rs
+++ b/bin/cluelesshd/src/rpc.rs
@@ -14,6 +14,7 @@ use cluelessh_keys::public::PublicKey;
 use cluelessh_keys::signature::Signature;
 use cluelessh_protocol::auth::CheckPubkey;
 use cluelessh_protocol::auth::VerifySignature;
+use cluelessh_transport::crypto::AlgorithmName;
 use eyre::bail;
 use eyre::ensure;
 use eyre::eyre;
@@ -26,6 +27,8 @@ use rustix::net::SendAncillaryBuffer;
 use rustix::net::SendAncillaryMessage;
 use rustix::net::SendFlags;
 use rustix::termios::Winsize;
+use secrecy::ExposeSecret;
+use secrecy::Secret;
 use serde::de::DeserializeOwned;
 use serde::{Deserialize, Serialize};
 use tokio::io::Interest;
@@ -36,20 +39,16 @@ use tracing::debug;
 use tracing::trace;
 use users::os::unix::UserExt;
 use users::User;
+use zeroize::Zeroizing;
 
 #[derive(Debug, Serialize, Deserialize)]
 enum Request {
-    // TODO: This is a bit... not good, it's not good.
-    // It can be used to sign any arbitrary message, or any arbitary exchange!
-    // I think we need to let the monitor do the DH Key Exchange.
-    // Basically, it should generate the private key for the exchange (and give that to the client)
-    // and then when signing, we compute the shared secret ourselves for the hash.
-    // This should ensure that the connection process cannot sign anything except an SSH kex has
-    // but only with our specific chosen shared secret, which should make it entirely useless for anything else.
-    Sign {
-        hash: [u8; 32],
-        public_key: PublicKey,
-    },
+    /// Performs the key exchange by generating a private key, deriving the shared secret,
+    /// computing the hash and signing it.
+    /// This is combined into one operation to ensure that no signature forgery can happen,
+    /// as the only thing we sign here is a hash, and this hash is guaranteed to contain
+    /// some random bytes from us, making it entirely unpredictable and useless to forge anything.
+    KeyExchange(KeyExchangeRequest),
     CheckPublicKey {
         user: String,
         session_identifier: [u8; 32],
@@ -76,6 +75,56 @@ enum Request {
     Wait,
 }
 
+#[derive(Serialize, Deserialize)]
+pub struct KeyExchangeRequest {
+    pub client_ident: Vec<u8>,
+    pub server_ident: Vec<u8>,
+    pub client_kexinit: Vec<u8>,
+    pub server_kexinit: Vec<u8>,
+    pub eph_client_public_key: Vec<u8>,
+    pub server_host_key: PublicKey,
+    pub kex_algorithm: String,
+}
+
+impl Debug for KeyExchangeRequest {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("KeyExchangeRequest")
+            .field("client_ident", &"[...]")
+            .field("server_ident", &"[...]")
+            .field("client_kexinit", &"[...]")
+            .field("server_kexinit", &"[...]")
+            .field("eph_client_public_key", &self.eph_client_public_key)
+            .field("server_host_key", &self.server_host_key)
+            .field("kex_algorithm", &self.kex_algorithm)
+            .finish()
+    }
+}
+
+#[derive(Clone, Serialize, Deserialize)]
+pub struct SerializableSharedSecret(Vec<u8>);
+impl zeroize::Zeroize for SerializableSharedSecret {
+    fn zeroize(&mut self) {
+        self.0.zeroize()
+    }
+}
+impl Debug for SerializableSharedSecret {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("SerializableSharedSecret")
+            .finish_non_exhaustive()
+    }
+}
+impl secrecy::CloneableSecret for SerializableSharedSecret {}
+impl secrecy::SerializableSecret for SerializableSharedSecret {}
+impl secrecy::DebugSecret for SerializableSharedSecret {}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct KeyExchangeResponse {
+    pub hash: [u8; 32],
+    pub server_ephemeral_public_key: Vec<u8>,
+    pub shared_secret: secrecy::Secret<SerializableSharedSecret>,
+    pub signature: Signature,
+}
+
 #[derive(Debug, Serialize, Deserialize)]
 struct PtyRequest {
     height_rows: u32,
@@ -94,12 +143,10 @@ struct ShellRequest {
 }
 
 #[derive(Debug, Serialize, Deserialize)]
-
 struct ShellRequestPty {
     term: String,
 }
 
-type SignResponse = Signature;
 type VerifySignatureResponse = bool;
 type CheckPublicKeyResponse = bool;
 type ShellResponse = ();
@@ -142,7 +189,9 @@ impl Server {
 
     pub async fn process(&mut self) -> Result<()> {
         loop {
-            let (recv, fds) = receive_with_fds::<Request>(&self.server).await?;
+            let (recv, fds) = receive_with_fds::<Request>(&self.server)
+                .await
+                .wrap_err("parsing request from client")?;
             ensure!(fds.is_empty(), "Client sent FDs in request");
             self.receive_message(recv).await?;
         }
@@ -152,20 +201,55 @@ impl Server {
         trace!(?req, "Received RPC message");
 
         match req {
-            Request::Sign { hash, public_key } => {
+            Request::KeyExchange(req) => {
                 let Some(private) = self
                     .host_keys
                     .iter()
-                    .find(|privkey| privkey.private_key.public_key() == public_key)
+                    .find(|privkey| privkey.private_key.public_key() == req.server_host_key)
                 else {
                     self.respond_err("missing private key".to_owned()).await?;
-
                     return Ok(());
                 };
 
-                let signature = private.private_key.sign(&hash);
+                let Some(kex_algorithm) =
+                    cluelessh_transport::crypto::kex_algorithm_by_name(&req.kex_algorithm)
+                else {
+                    self.respond_err("invalid kex algorithm".to_owned()).await?;
+                    return Ok(());
+                };
 
-                self.respond::<SignResponse>(Ok(signature)).await?;
+                let req = cluelessh_transport::server::KeyExchangeParameters {
+                    client_ident: req.client_ident,
+                    server_ident: req.server_ident,
+                    client_kexinit: req.client_kexinit,
+                    server_kexinit: req.server_kexinit,
+                    eph_client_public_key: req.eph_client_public_key,
+                    server_host_key_algorithm:
+                        cluelessh_transport::crypto::HostKeySigningAlgorithm::new(
+                            req.server_host_key,
+                        ),
+                    kex_algorithm,
+                };
+
+                let Ok(resp) = cluelessh_transport::server::do_key_exchange(
+                    req,
+                    private,
+                    &mut cluelessh_protocol::OsRng,
+                ) else {
+                    self.respond_err("key exchange failed".to_owned()).await?;
+                    return Ok(());
+                };
+
+                let resp = KeyExchangeResponse {
+                    hash: resp.hash,
+                    server_ephemeral_public_key: resp.server_ephemeral_public_key,
+                    shared_secret: Secret::new(SerializableSharedSecret(
+                        resp.shared_secret.expose_secret().0.to_vec(),
+                    )),
+                    signature: resp.signature,
+                };
+
+                self.respond::<KeyExchangeResponse>(Ok(resp)).await?;
             }
             Request::CheckPublicKey {
                 user,
@@ -367,7 +451,8 @@ impl Server {
         resp: ResponseResult<T>,
         fds: &[BorrowedFd<'_>],
     ) -> Result<()> {
-        send_with_fds(&self.server, &postcard::to_allocvec(&resp)?, fds).await?;
+        let data = Zeroizing::new(postcard::to_allocvec(&resp)?);
+        send_with_fds(&self.server, &data, fds).await?;
 
         Ok(())
     }
@@ -379,9 +464,32 @@ impl Client {
         Ok(Self { socket })
     }
 
-    pub async fn sign(&self, hash: [u8; 32], public_key: PublicKey) -> Result<Signature> {
-        self.request_response::<SignResponse>(&Request::Sign { hash, public_key })
-            .await
+    pub async fn kex_exchange(
+        &self,
+        params: cluelessh_transport::server::KeyExchangeParameters,
+    ) -> Result<cluelessh_transport::server::KeyExchangeResponse> {
+        let resp = self
+            .request_response::<KeyExchangeResponse>(&Request::KeyExchange(KeyExchangeRequest {
+                client_ident: params.client_ident,
+                server_ident: params.server_ident,
+                client_kexinit: params.client_kexinit,
+                server_kexinit: params.server_kexinit,
+                eph_client_public_key: params.eph_client_public_key,
+                server_host_key: params.server_host_key_algorithm.public_key(),
+                kex_algorithm: params.kex_algorithm.name().to_owned(),
+            }))
+            .await?;
+
+        Ok(cluelessh_transport::server::KeyExchangeResponse {
+            hash: resp.hash,
+            server_ephemeral_public_key: resp.server_ephemeral_public_key,
+            shared_secret: cluelessh_transport::crypto::SharedSecret::new(
+                cluelessh_transport::crypto::SharedSecretInner(
+                    resp.shared_secret.expose_secret().0.clone(),
+                ),
+            ),
+            signature: resp.signature,
+        })
     }
 
     pub async fn check_public_key(
@@ -478,6 +586,8 @@ impl Client {
     }
 
     async fn send_request(&self, req: &Request) -> Result<()> {
+        trace!(?req, "Sending RPC request");
+
         let data = postcard::to_allocvec(&req)?;
 
         send_with_fds(&self.socket, &data, &[]).await?;
@@ -489,7 +599,7 @@ impl Client {
     ) -> Result<(R, Vec<OwnedFd>)> {
         let (resp, fds) = receive_with_fds::<ResponseResult<R>>(&self.socket)
             .await
-            .wrap_err("failed to recv")?;
+            .wrap_err("parsing response from server")?;
 
         trace!(?resp, ?fds, "Received RPC response");
 
@@ -499,7 +609,15 @@ impl Client {
     }
 }
 
+const MAX_DATA_SIZE: usize = 4048;
+
 async fn send_with_fds(socket: &UnixDatagram, data: &[u8], fds: &[BorrowedFd<'_>]) -> Result<()> {
+    ensure!(
+        data.len() <= MAX_DATA_SIZE,
+        "Trying to send too much data: {} > {MAX_DATA_SIZE}",
+        data.len()
+    );
+
     socket
         .async_io(Interest::WRITABLE, || {
             let mut space = [0; rustix::cmsg_space!(ScmRights(3))]; //we send up to 3 fds at once
@@ -520,7 +638,7 @@ async fn send_with_fds(socket: &UnixDatagram, data: &[u8], fds: &[BorrowedFd<'_>
 }
 
 async fn receive_with_fds<R: DeserializeOwned>(socket: &UnixDatagram) -> Result<(R, Vec<OwnedFd>)> {
-    let mut data = [0; 1024];
+    let mut data = Zeroizing::new([0; MAX_DATA_SIZE]);
     let mut space = [0; rustix::cmsg_space!(ScmRights(3))]; // maximum size
     let mut cmesg_buf = RecvAncillaryBuffer::new(&mut space);
 
@@ -528,7 +646,7 @@ async fn receive_with_fds<R: DeserializeOwned>(socket: &UnixDatagram) -> Result<
         .async_io(Interest::READABLE, || {
             rustix::net::recvmsg(
                 socket,
-                &mut [IoSliceMut::new(&mut data)],
+                &mut [IoSliceMut::new(&mut *data)],
                 &mut cmesg_buf,
                 RecvFlags::empty(),
             )
@@ -538,7 +656,7 @@ async fn receive_with_fds<R: DeserializeOwned>(socket: &UnixDatagram) -> Result<
 
     let mut fds = Vec::new();
 
-    let data = postcard::from_bytes::<R>(&data[..read.bytes]).wrap_err("invalid request")?;
+    let data_parsed = postcard::from_bytes::<R>(&data[..read.bytes]).wrap_err("invalid request")?;
 
     for msg in cmesg_buf.drain() {
         match msg {
@@ -547,5 +665,5 @@ async fn receive_with_fds<R: DeserializeOwned>(socket: &UnixDatagram) -> Result<
         }
     }
 
-    Ok((data, fds))
+    Ok((data_parsed, fds))
 }
diff --git a/bin/cluelesshd/src/sandbox.rs b/bin/cluelesshd/src/sandbox.rs
index 2424d7d..7d90065 100644
--- a/bin/cluelesshd/src/sandbox.rs
+++ b/bin/cluelesshd/src/sandbox.rs
@@ -1,6 +1,7 @@
 use std::{
     fs::File,
     io::{Read, Write},
+    os::fd::RawFd,
     path::Path,
 };
 
@@ -10,9 +11,10 @@ use rustix::{
     process::WaitOptions,
     thread::{Pid, UnshareFlags},
 };
-use tracing::{debug, trace};
+use seccompiler::{BpfProgram, SeccompAction, SeccompFilter, SeccompRule, TargetArch};
+use tracing::{debug, trace, warn};
 
-use crate::SerializedConnectionState;
+use crate::{SerializedConnectionState, PRIVSEP_CONNECTION_RPC_CLIENT_FD, PRIVSEP_CONNECTION_STREAM_FD};
 
 #[tracing::instrument(skip(state), ret)]
 pub fn drop_privileges(state: &SerializedConnectionState) -> Result<()> {
@@ -40,6 +42,10 @@ pub fn drop_privileges(state: &SerializedConnectionState) -> Result<()> {
 
     rustix::thread::set_no_new_privs(true)?;
 
+    if state.config.security.experimental_seccomp {
+        seccomp().wrap_err("setting up seccomp")?;
+    }
+
     Ok(())
 }
 
@@ -72,7 +78,8 @@ fn pipe() -> Result<(File, File)> {
 /// If this fails, there might be zombie child processes.
 /// Therefore, the caller must exit if this function fails.
 #[tracing::instrument]
-pub fn unshare_namespaces() -> Result<()> {
+fn unshare_namespaces() -> Result<()> {
+    // TODO: respect unprivileged_uid config and stuff
     let (mut child_ready_read, mut child_ready_write) = pipe()?;
     let (mut uid_map_ready_read, mut uid_map_ready_write) = pipe()?;
 
@@ -189,3 +196,86 @@ pub fn unshare_namespaces() -> Result<()> {
 
     Ok(())
 }
+
+#[tracing::instrument]
+fn seccomp() -> Result<()> {
+    use seccompiler::{SeccompCmpArgLen as ArgLen, SeccompCmpOp as Op, SeccompCondition as Cond};
+
+    let arch = match std::env::consts::ARCH {
+        "x86_64" => TargetArch::x86_64,
+        "aarch64" => TargetArch::aarch64,
+        arch => {
+            warn!("Seccomp not supported for architecture ({arch})m skipping");
+            return Ok(());
+        }
+    };
+
+    let limit_fd = |fd: RawFd| {
+        SeccompRule::new(vec![Cond::new(
+            0, // fd
+            ArgLen::Dword,
+            Op::Eq,
+            fd as u64,
+        )
+        .unwrap()])
+        .unwrap()
+    };
+
+    let filter = SeccompFilter::new(
+        vec![
+            (libc::SYS_write, vec![]),
+            (libc::SYS_epoll_create1, vec![]),
+            (libc::SYS_eventfd2, vec![]),
+            (libc::SYS_epoll_wait, vec![]),
+            (libc::SYS_epoll_ctl, vec![]),
+            (libc::SYS_fcntl, vec![]), // todo: restrict (72)
+            (libc::SYS_socketpair, vec![]),
+            (libc::SYS_sendmsg, vec![limit_fd(PRIVSEP_CONNECTION_RPC_CLIENT_FD)],),
+            (libc::SYS_recvmsg, vec![limit_fd(PRIVSEP_CONNECTION_RPC_CLIENT_FD)]),
+            (libc::SYS_sendto, vec![limit_fd(PRIVSEP_CONNECTION_STREAM_FD)]),
+            (libc::SYS_recvfrom, vec![limit_fd(PRIVSEP_CONNECTION_STREAM_FD)]),
+            (libc::SYS_getrandom, vec![]),
+            (libc::SYS_rt_sigaction, vec![]),
+            (libc::SYS_rt_sigprocmask, vec![]),
+            (libc::SYS_mmap, vec![]),
+            (libc::SYS_munmap, vec![]),
+            (libc::SYS_sched_getaffinity, vec![]),
+            (libc::SYS_sigaltstack, vec![]),
+            (libc::SYS_futex, vec![]),
+            (libc::SYS_read, vec![]),
+            (libc::SYS_mprotect, vec![]),
+            (libc::SYS_rseq, vec![]),
+            (libc::SYS_set_robust_list, vec![]),
+            (libc::SYS_prctl, vec![]),
+            (libc::SYS_close, vec![]),
+            (libc::SYS_madvise, vec![]),
+            (libc::SYS_exit, vec![]),
+            (libc::SYS_exit_group, vec![]),
+            (libc::SYS_sched_yield, vec![]),
+            (
+                libc::SYS_ioctl,
+                vec![SeccompRule::new(vec![Cond::new(
+                    1, // op
+                    // dword for musl, qword for glibc :D.
+                    // but since FIONBIO is <u32::MAX, we can use dword.
+                    ArgLen::Dword,
+                    Op::Eq,
+                    libc::FIONBIO, // non-blocking
+                )?])?],
+            ),
+        ]
+        .into_iter()
+        .collect(),
+        SeccompAction::KillProcess,
+        SeccompAction::Allow,
+        arch,
+    )
+    .wrap_err("creating seccomp filter")?;
+
+    let program: BpfProgram = filter.try_into().wrap_err("compiling seccomp filter")?;
+
+    debug!("Installing seccomp filter");
+    seccompiler::apply_filter(&program).wrap_err("installing seccomp filter")?;
+
+    Ok(())
+}
diff --git a/lib/cluelessh-protocol/src/lib.rs b/lib/cluelessh-protocol/src/lib.rs
index 2b29565..64af42f 100644
--- a/lib/cluelessh-protocol/src/lib.rs
+++ b/lib/cluelessh-protocol/src/lib.rs
@@ -4,8 +4,6 @@ use std::mem;
 
 use auth::AuthOption;
 use cluelessh_connection::ChannelOperation;
-use cluelessh_keys::public::PublicKey;
-use cluelessh_keys::signature::Signature;
 use tracing::debug;
 
 // Re-exports
@@ -14,11 +12,11 @@ pub use cluelessh_connection::{ChannelUpdate, ChannelUpdateKind};
 pub use cluelessh_transport as transport;
 pub use cluelessh_transport::{Result, SshStatus};
 
-pub struct ThreadRngRand;
-impl transport::SshRng for ThreadRngRand {
+pub struct OsRng;
+impl transport::SshRng for OsRng {
     fn fill_bytes(&mut self, dest: &mut [u8]) {
         use rand::RngCore;
-        rand::thread_rng().fill_bytes(dest);
+        rand::rngs::OsRng.fill_bytes(dest);
     }
 }
 
@@ -78,12 +76,12 @@ impl ServerConnection {
         Ok(())
     }
 
-    pub fn is_waiting_on_signature(&self) -> Option<(&PublicKey, [u8; 32])> {
-        self.transport.is_waiting_on_signature()
+    pub fn is_waiting_on_key_exchange(&self) -> Option<transport::server::KeyExchangeParameters> {
+        self.transport.is_waiting_on_key_exchange()
     }
 
-    pub fn do_signature(&mut self, signature: Signature) {
-        self.transport.do_signature(signature);
+    pub fn do_key_exchange(&mut self, response: transport::server::KeyExchangeResponse) {
+        self.transport.do_key_exchange(response);
     }
 
     pub fn next_msg_to_send(&mut self) -> Option<cluelessh_transport::Msg> {
diff --git a/lib/cluelessh-tokio/src/client.rs b/lib/cluelessh-tokio/src/client.rs
index fea7f03..48c7787 100644
--- a/lib/cluelessh-tokio/src/client.rs
+++ b/lib/cluelessh-tokio/src/client.rs
@@ -60,7 +60,7 @@ impl<S: AsyncRead + AsyncWrite> ClientConnection<S> {
             channels: HashMap::new(),
             proto: cluelessh_protocol::ClientConnection::new(
                 cluelessh_transport::client::ClientConnection::new(
-                    cluelessh_protocol::ThreadRngRand,
+                    cluelessh_protocol::OsRng,
                 ),
                 cluelessh_protocol::auth::ClientAuth::new(auth.username.as_bytes().to_vec()),
             ),
diff --git a/lib/cluelessh-tokio/src/server.rs b/lib/cluelessh-tokio/src/server.rs
index 2c66132..5c50701 100644
--- a/lib/cluelessh-tokio/src/server.rs
+++ b/lib/cluelessh-tokio/src/server.rs
@@ -1,5 +1,6 @@
 use cluelessh_connection::{ChannelKind, ChannelNumber, ChannelOperation};
-use cluelessh_keys::{public::PublicKey, signature::Signature};
+use cluelessh_keys::public::PublicKey;
+use cluelessh_transport::server::{KeyExchangeParameters, KeyExchangeResponse};
 use futures::future::BoxFuture;
 use std::{
     collections::{HashMap, HashSet, VecDeque},
@@ -54,7 +55,7 @@ enum Operation {
     VerifyPassword(String, Result<bool>),
     CheckPubkey(Result<bool>, String, Vec<u8>),
     VerifySignature(String, Result<bool>),
-    SignatureReceived(Result<Signature>),
+    KeyExchangeResponseReceived(Result<KeyExchangeResponse>),
 }
 
 pub type AuthFn<A, R> = Arc<dyn Fn(A) -> BoxFuture<'static, R> + Send + Sync>;
@@ -64,7 +65,7 @@ pub struct ServerAuth {
     pub verify_password: Option<AuthFn<VerifyPassword, Result<bool>>>,
     pub verify_signature: Option<AuthFn<VerifySignature, Result<bool>>>,
     pub check_pubkey: Option<AuthFn<CheckPubkey, Result<bool>>>,
-    pub sign_with_hostkey: AuthFn<SignWithHostKey, Result<Signature>>,
+    pub do_key_exchange: AuthFn<KeyExchangeParameters, Result<KeyExchangeResponse>>,
     pub auth_banner: Option<String>,
 }
 fn _assert_send_sync() {
@@ -150,7 +151,7 @@ impl<S: AsyncRead + AsyncWrite> ServerConnection<S> {
             channels: HashMap::new(),
             proto: cluelessh_protocol::ServerConnection::new(
                 cluelessh_transport::server::ServerConnection::new(
-                    cluelessh_protocol::ThreadRngRand,
+                    cluelessh_protocol::OsRng,
                     transport_config,
                 ),
                 options,
@@ -169,16 +170,18 @@ impl<S: AsyncRead + AsyncWrite> ServerConnection<S> {
     /// Executes one loop iteration of the main loop.
     // IMPORTANT: no operations on this struct should ever block the main loop, except this one.
     pub async fn progress(&mut self) -> Result<(), Error> {
-        if let Some((public_key, hash)) = self.proto.is_waiting_on_signature() {
+        if let Some(params) = self.proto.is_waiting_on_key_exchange() {
             if !self.signature_in_progress {
                 self.signature_in_progress = true;
 
                 let send = self.operations_send.clone();
-                let public_key = public_key.clone();
-                let sign_with_hostkey = self.auth_verify.sign_with_hostkey.clone();
+
+                let do_key_exchange = self.auth_verify.do_key_exchange.clone();
                 tokio::spawn(async move {
-                    let result = sign_with_hostkey(SignWithHostKey { public_key, hash }).await;
-                    let _ = send.send(Operation::SignatureReceived(result)).await;
+                    let result = do_key_exchange(params).await;
+                    let _ = send
+                        .send(Operation::KeyExchangeResponseReceived(result))
+                        .await;
                 });
             }
         }
@@ -353,9 +356,9 @@ impl<S: AsyncRead + AsyncWrite> ServerConnection<S> {
                     Some(Operation::VerifyPassword(user, result)) => if let Some(auth) = self.proto.auth() {
                         auth.verification_result(result?, user);
                     },
-                    Some(Operation::SignatureReceived(signature)) => {
+                    Some(Operation::KeyExchangeResponseReceived(signature)) => {
                         let signature = signature?;
-                        self.proto.do_signature(signature);
+                        self.proto.do_key_exchange(signature);
                     }
                     None => {}
                 }
diff --git a/lib/cluelessh-transport/Cargo.toml b/lib/cluelessh-transport/Cargo.toml
index abfc524..7a31c05 100644
--- a/lib/cluelessh-transport/Cargo.toml
+++ b/lib/cluelessh-transport/Cargo.toml
@@ -21,6 +21,7 @@ x25519-dalek = "2.0.1"
 
 tracing.workspace = true
 base64 = "0.22.1"
+secrecy = "0.8.0"
 
 [dev-dependencies]
 hex-literal = "0.4.1"
diff --git a/lib/cluelessh-transport/src/client.rs b/lib/cluelessh-transport/src/client.rs
index 671ea54..9ff9bf4 100644
--- a/lib/cluelessh-transport/src/client.rs
+++ b/lib/cluelessh-transport/src/client.rs
@@ -4,8 +4,7 @@ use tracing::{debug, info, trace};
 
 use crate::{
     crypto::{
-        self, AlgorithmName, EncodedSshSignature, EncryptionAlgorithm, HostKeyVerifyAlgorithm,
-        KeyExchangeSecret, SupportedAlgorithms,
+        self, AlgorithmName, EncodedSshSignature, EncryptionAlgorithm, HostKeyVerifyAlgorithm, KeyExchangeSecret, SharedSecret, SupportedAlgorithms
     },
     packet::{Packet, PacketTransport, ProtocolIdentParser},
     peer_error, Msg, Result, SshRng, SshStatus,
@@ -46,7 +45,7 @@ enum ClientState {
     },
     NewKeys {
         h: [u8; 32],
-        k: Vec<u8>,
+        k: SharedSecret,
         encryption_client_to_server: EncryptionAlgorithm,
         encryption_server_to_client: EncryptionAlgorithm,
     },
diff --git a/lib/cluelessh-transport/src/crypto.rs b/lib/cluelessh-transport/src/crypto.rs
index 5635c20..36d08d9 100644
--- a/lib/cluelessh-transport/src/crypto.rs
+++ b/lib/cluelessh-transport/src/crypto.rs
@@ -2,6 +2,7 @@ pub mod encrypt;
 
 use cluelessh_keys::{public::PublicKey, signature::Signature};
 use p256::ecdsa::signature::Verifier;
+use secrecy::ExposeSecret;
 use sha2::Digest;
 
 use crate::{
@@ -9,6 +10,17 @@ use crate::{
     peer_error, Msg, Result, SshRng,
 };
 
+pub type SharedSecret = secrecy::Secret<SharedSecretInner>;
+
+#[derive(Clone)]
+pub struct SharedSecretInner(pub Vec<u8>);
+impl secrecy::Zeroize for SharedSecretInner {
+    fn zeroize(&mut self) {
+        secrecy::Zeroize::zeroize(&mut self.0);
+    }
+}
+impl secrecy::CloneableSecret for SharedSecretInner {}
+
 pub trait AlgorithmName {
     fn name(&self) -> &'static str;
 }
@@ -36,7 +48,15 @@ pub struct KeyExchangeSecret {
     /// Q_x
     pub pubkey: Vec<u8>,
     /// Does the exchange, returning the shared secret K.
-    pub exchange: Box<dyn FnOnce(&[u8]) -> Result<Vec<u8>> + Send + Sync>,
+    pub exchange: Box<dyn FnOnce(&[u8]) -> Result<SharedSecret> + Send + Sync>,
+}
+
+pub fn kex_algorithm_by_name(name: &str) -> Option<KexAlgorithm> {
+    match name {
+        "curve25519-sha256" => Some(KEX_CURVE_25519_SHA256),
+        "ecdh-sha2-nistp256" => Some(KEX_ECDH_SHA2_NISTP256),
+        _ => None,
+    }
 }
 
 /// <https://datatracker.ietf.org/doc/html/rfc8731>
@@ -58,7 +78,9 @@ pub const KEX_CURVE_25519_SHA256: KexAlgorithm = KexAlgorithm {
                 let peer_public_key = x25519_dalek::PublicKey::from(peer_public_key);
                 let shared_secret = secret.diffie_hellman(&peer_public_key); // K
 
-                Ok(shared_secret.as_bytes().to_vec())
+                Ok(secrecy::Secret::new(SharedSecretInner(
+                    shared_secret.as_bytes().to_vec(),
+                )))
             }),
         }
     },
@@ -83,7 +105,9 @@ pub const KEX_ECDH_SHA2_NISTP256: KexAlgorithm = KexAlgorithm {
 
                 let shared_secret = secret.diffie_hellman(&peer_public_key); // K
 
-                Ok(shared_secret.raw_secret_bytes().to_vec())
+                Ok(secrecy::Secret::new(SharedSecretInner(
+                    shared_secret.raw_secret_bytes().to_vec(),
+                )))
             }),
         }
     },
@@ -105,6 +129,7 @@ impl AlgorithmName for EncryptionAlgorithm {
 }
 pub struct EncodedSshSignature(pub Vec<u8>);
 
+#[derive(Clone)]
 pub struct HostKeySigningAlgorithm {
     public_key: PublicKey,
 }
@@ -304,7 +329,7 @@ pub(crate) trait Keys: Send + Sync + 'static {
     fn rekey(
         &mut self,
         h: [u8; 32],
-        k: &[u8],
+        k: &SharedSecret,
         encryption_client_to_server: EncryptionAlgorithm,
         encryption_server_to_client: EncryptionAlgorithm,
         is_server: bool,
@@ -326,7 +351,7 @@ impl Keys for Plaintext {
     fn rekey(
         &mut self,
         _: [u8; 32],
-        _: &[u8],
+        _: &SharedSecret,
         _: EncryptionAlgorithm,
         _: EncryptionAlgorithm,
         _: bool,
@@ -338,7 +363,7 @@ impl Keys for Plaintext {
 impl Session {
     pub(crate) fn new(
         h: [u8; 32],
-        k: &[u8],
+        k: &SharedSecret,
         encryption_client_to_server: EncryptionAlgorithm,
         encryption_server_to_client: EncryptionAlgorithm,
         is_server: bool,
@@ -357,7 +382,7 @@ impl Session {
     fn from_keys(
         session_id: [u8; 32],
         h: [u8; 32],
-        k: &[u8],
+        k: &SharedSecret,
         alg_c2s: EncryptionAlgorithm,
         alg_s2c: EncryptionAlgorithm,
         is_server: bool,
@@ -414,7 +439,7 @@ impl Keys for Session {
     fn rekey(
         &mut self,
         h: [u8; 32],
-        k: &[u8],
+        k: &SharedSecret,
         encryption_client_to_server: EncryptionAlgorithm,
         encryption_server_to_client: EncryptionAlgorithm,
         is_server: bool,
@@ -434,7 +459,7 @@ impl Keys for Session {
 /// Derive a key from the shared secret K and exchange hash H.
 /// <https://datatracker.ietf.org/doc/html/rfc4253#section-7.2>
 fn derive_key(
-    k: &[u8],
+    k: &SharedSecret,
     h: [u8; 32],
     letter: &str,
     session_id: [u8; 32],
@@ -446,7 +471,7 @@ fn derive_key(
 
     for i in 0..(padded_key_size / sha2len) {
         let mut hash = <sha2::Sha256 as sha2::Digest>::new();
-        encode_mpint_for_hash(k, |data| hash.update(data));
+        encode_mpint_for_hash(k.expose_secret().0.as_slice(), |data| hash.update(data));
         hash.update(h);
 
         if i == 0 {
@@ -480,7 +505,7 @@ pub fn key_exchange_hash(
     server_hostkey: &[u8],
     eph_client_public_key: &[u8],
     eph_server_public_key: &[u8],
-    shared_secret: &[u8],
+    shared_secret: &SharedSecret,
 ) -> [u8; 32] {
     let mut hash = sha2::Sha256::new();
     let add_hash = |hash: &mut sha2::Sha256, bytes: &[u8]| {
@@ -507,7 +532,7 @@ pub fn key_exchange_hash(
     // <https://datatracker.ietf.org/doc/html/rfc5656#section-4>
     hash_string(&mut hash, eph_client_public_key); // Q_C
     hash_string(&mut hash, eph_server_public_key); // Q_S
-    hash_mpint(&mut hash, shared_secret); // K
+    hash_mpint(&mut hash, shared_secret.expose_secret().0.as_slice()); // K
 
     let hash = hash.finalize();
     hash.into()
diff --git a/lib/cluelessh-transport/src/lib.rs b/lib/cluelessh-transport/src/lib.rs
index e73cca8..fb7617e 100644
--- a/lib/cluelessh-transport/src/lib.rs
+++ b/lib/cluelessh-transport/src/lib.rs
@@ -1,5 +1,5 @@
 pub mod client;
-mod crypto;
+pub mod crypto;
 pub mod packet;
 pub mod server;
 
@@ -25,7 +25,7 @@ impl From<ParseError> for SshStatus {
     }
 }
 
-pub trait SshRng {
+pub trait SshRng: Send + Sync {
     fn fill_bytes(&mut self, dest: &mut [u8]);
 }
 struct SshRngRandAdapter<'a>(&'a mut dyn SshRng);
diff --git a/lib/cluelessh-transport/src/packet.rs b/lib/cluelessh-transport/src/packet.rs
index bc60141..a9280a5 100644
--- a/lib/cluelessh-transport/src/packet.rs
+++ b/lib/cluelessh-transport/src/packet.rs
@@ -5,7 +5,7 @@ use std::mem;
 
 use tracing::{debug, trace};
 
-use crate::crypto::{self, EncryptionAlgorithm, Keys, Plaintext, Session};
+use crate::crypto::{self, EncryptionAlgorithm, Keys, Plaintext, Session, SharedSecret};
 use crate::peer_error;
 use crate::Result;
 use cluelessh_format::numbers;
@@ -112,7 +112,7 @@ impl PacketTransport {
     pub(crate) fn set_key(
         &mut self,
         h: [u8; 32],
-        k: &[u8],
+        k: &SharedSecret,
         encryption_client_to_server: EncryptionAlgorithm,
         encryption_server_to_client: EncryptionAlgorithm,
         is_server: bool,
diff --git a/lib/cluelessh-transport/src/server.rs b/lib/cluelessh-transport/src/server.rs
index 20b56de..8c30277 100644
--- a/lib/cluelessh-transport/src/server.rs
+++ b/lib/cluelessh-transport/src/server.rs
@@ -1,7 +1,8 @@
 use std::{collections::VecDeque, mem::take};
 
 use crate::crypto::{
-    self, AlgorithmName, EncryptionAlgorithm, HostKeySigningAlgorithm, SupportedAlgorithms,
+    self, AlgorithmName, EncryptionAlgorithm, HostKeySigningAlgorithm, KexAlgorithm, SharedSecret,
+    SupportedAlgorithms,
 };
 use crate::packet::{
     KeyExchangeEcDhInitPacket, KeyExchangeInitPacket, Packet, PacketTransport, ProtocolIdentParser,
@@ -10,7 +11,7 @@ use crate::Result;
 use crate::{peer_error, Msg, SshRng, SshStatus};
 use cluelessh_format::numbers;
 use cluelessh_format::{NameList, Reader, Writer};
-use cluelessh_keys::public::PublicKey;
+use cluelessh_keys::private::PlaintextPrivateKey;
 use cluelessh_keys::signature::Signature;
 use tracing::{debug, info, trace};
 
@@ -49,21 +50,21 @@ enum ServerState {
         encryption_client_to_server: EncryptionAlgorithm,
         encryption_server_to_client: EncryptionAlgorithm,
     },
-    WaitingForSignature {
-        /// h
-        hash: [u8; 32],
-        pub_hostkey: PublicKey,
-        /// k
-        shared_secret: Vec<u8>,
-        server_ephemeral_public_key: Vec<u8>,
+    WaitingForKeyExchange {
+        client_identification: Vec<u8>,
+        client_kexinit: Vec<u8>,
+        server_kexinit: Vec<u8>,
+        kex_algorithm: crypto::KexAlgorithm,
+        server_host_key_algorithm: HostKeySigningAlgorithm,
         encryption_client_to_server: EncryptionAlgorithm,
         encryption_server_to_client: EncryptionAlgorithm,
+        client_ephemeral_public_key: Vec<u8>,
     },
     NewKeys {
         /// h
         hash: [u8; 32],
         /// k
-        shared_secret: Vec<u8>,
+        shared_secret: SharedSecret,
         encryption_client_to_server: EncryptionAlgorithm,
         encryption_server_to_client: EncryptionAlgorithm,
     },
@@ -75,6 +76,23 @@ enum ServerState {
     },
 }
 
+pub struct KeyExchangeParameters {
+    pub client_ident: Vec<u8>,
+    pub server_ident: Vec<u8>,
+    pub client_kexinit: Vec<u8>,
+    pub server_kexinit: Vec<u8>,
+    pub eph_client_public_key: Vec<u8>,
+    pub server_host_key_algorithm: HostKeySigningAlgorithm,
+    pub kex_algorithm: KexAlgorithm,
+}
+
+pub struct KeyExchangeResponse {
+    pub hash: [u8; 32],
+    pub server_ephemeral_public_key: Vec<u8>,
+    pub shared_secret: SharedSecret,
+    pub signature: Signature,
+}
+
 impl ServerConnection {
     pub fn new(rng: impl SshRng + Send + Sync + 'static, config: ServerConfig) -> Self {
         Self {
@@ -258,37 +276,18 @@ impl ServerConnection {
 
                     let client_ephemeral_public_key = dh.qc;
 
-                    let server_secret = (kex_algorithm.generate_secret)(&mut *self.rng);
-                    let server_ephemeral_public_key = server_secret.pubkey;
-                    let shared_secret = (server_secret.exchange)(client_ephemeral_public_key)?;
-                    let pub_hostkey = server_host_key_algorithm.public_key();
-
-                    let hash = crypto::key_exchange_hash(
-                        client_identification,
-                        SERVER_IDENTIFICATION,
-                        client_kexinit,
-                        server_kexinit,
-                        &pub_hostkey.to_wire_encoding(),
-                        client_ephemeral_public_key,
-                        &server_ephemeral_public_key,
-                        &shared_secret,
-                    );
-
-                    // eprintln!("client_ephemeral_public_key: {:x?}", client_ephemeral_public_key);
-                    // eprintln!("server_ephemeral_public_key: {:x?}", server_ephemeral_public_key);
-                    // eprintln!("shared_secret:               {:x?}", shared_secret);
-                    // eprintln!("hash:                        {:x?}", hash);
-
-                    self.state = ServerState::WaitingForSignature {
-                        hash,
-                        pub_hostkey,
-                        shared_secret,
-                        server_ephemeral_public_key,
+                    self.state = ServerState::WaitingForKeyExchange {
+                        client_identification: client_identification.clone(),
+                        client_kexinit: client_kexinit.clone(),
+                        server_kexinit: server_kexinit.clone(),
+                        kex_algorithm: *kex_algorithm,
+                        server_host_key_algorithm: server_host_key_algorithm.clone(),
                         encryption_client_to_server: *encryption_client_to_server,
                         encryption_server_to_client: *encryption_server_to_client,
+                        client_ephemeral_public_key: client_ephemeral_public_key.to_vec(),
                     };
                 }
-                ServerState::WaitingForSignature { .. } => {
+                ServerState::WaitingForKeyExchange { .. } => {
                     return Err(peer_error!("unexpected packet"));
                 }
                 ServerState::NewKeys {
@@ -354,35 +353,47 @@ impl ServerConnection {
         }
     }
 
-    pub fn is_waiting_on_signature(&self) -> Option<(&PublicKey, [u8; 32])> {
+    pub fn is_waiting_on_key_exchange(&self) -> Option<KeyExchangeParameters> {
         match &self.state {
-            ServerState::WaitingForSignature {
-                pub_hostkey, hash, ..
-            } => Some((pub_hostkey, *hash)),
+            ServerState::WaitingForKeyExchange {
+                client_identification,
+                client_kexinit,
+                server_kexinit,
+                kex_algorithm,
+                server_host_key_algorithm,
+                client_ephemeral_public_key,
+                ..
+            } => Some(KeyExchangeParameters {
+                client_ident: client_identification.clone(),
+                server_ident: SERVER_IDENTIFICATION.to_vec(),
+                client_kexinit: client_kexinit.clone(),
+                server_kexinit: server_kexinit.clone(),
+                eph_client_public_key: client_ephemeral_public_key.clone(),
+                server_host_key_algorithm: server_host_key_algorithm.clone(),
+                kex_algorithm: *kex_algorithm,
+            }),
             _ => None,
         }
     }
 
-    pub fn do_signature(&mut self, signature: Signature) {
+    pub fn do_key_exchange(&mut self, response: KeyExchangeResponse) {
         match &self.state {
-            ServerState::WaitingForSignature {
-                hash,
-                pub_hostkey,
-                shared_secret,
-                server_ephemeral_public_key,
+            ServerState::WaitingForKeyExchange {
                 encryption_client_to_server,
                 encryption_server_to_client,
+                server_host_key_algorithm,
+                ..
             } => {
                 let packet = Packet::new_msg_kex_ecdh_reply(
-                    &pub_hostkey.to_wire_encoding(),
-                    &server_ephemeral_public_key,
-                    &signature.to_wire_encoding(),
+                    &server_host_key_algorithm.public_key().to_wire_encoding(),
+                    &response.server_ephemeral_public_key,
+                    &response.signature.to_wire_encoding(),
                 );
 
                 self.packet_transport.queue_packet(packet);
                 self.state = ServerState::NewKeys {
-                    hash: *hash,
-                    shared_secret: shared_secret.clone(),
+                    hash: response.hash,
+                    shared_secret: response.shared_secret.clone(),
                     encryption_client_to_server: *encryption_client_to_server,
                     encryption_server_to_client: *encryption_server_to_client,
                 };
@@ -404,6 +415,35 @@ impl ServerConnection {
     }
 }
 
+pub fn do_key_exchange(
+    msg: KeyExchangeParameters,
+    private: &PlaintextPrivateKey,
+    rng: &mut dyn SshRng,
+) -> Result<KeyExchangeResponse> {
+    let server_secret = (msg.kex_algorithm.generate_secret)(rng);
+    let server_ephemeral_public_key = server_secret.pubkey;
+    let shared_secret = (server_secret.exchange)(&msg.eph_client_public_key)?;
+    let pub_hostkey = msg.server_host_key_algorithm.public_key();
+
+    let hash = crypto::key_exchange_hash(
+        &msg.client_ident,
+        &msg.server_ident,
+        &msg.client_kexinit,
+        &msg.server_kexinit,
+        &pub_hostkey.to_wire_encoding(),
+        &msg.eph_client_public_key,
+        &server_ephemeral_public_key,
+        &shared_secret,
+    );
+
+    Ok(KeyExchangeResponse {
+        hash,
+        server_ephemeral_public_key,
+        shared_secret,
+        signature: private.private_key.sign(&hash),
+    })
+}
+
 #[cfg(test)]
 mod tests {
     use hex_literal::hex;