diff --git a/rosenpass/src/app_server.rs b/rosenpass/src/app_server.rs
index 90a4b11..ecdfdab 100644
--- a/rosenpass/src/app_server.rs
+++ b/rosenpass/src/app_server.rs
@@ -8,6 +8,7 @@ use mio::Interest;
 use mio::Token;
 use rosenpass_secret_memory::Public;
 use rosenpass_secret_memory::Secret;
+use rosenpass_util::build::ConstructionSite;
 use rosenpass_util::file::StoreValueB64;
 use rosenpass_wireguard_broker::WireguardBrokerMio;
 use rosenpass_wireguard_broker::{WireguardBrokerCfg, WG_KEY_LEN};
@@ -31,6 +32,7 @@ use std::slice;
 use std::time::Duration;
 use std::time::Instant;
 
+use crate::protocol::BuildCryptoServer;
 use crate::protocol::HostIdentification;
 use crate::{
     config::Verbosity,
@@ -147,7 +149,7 @@ pub struct AppServerTest {
 // TODO add user control via unix domain socket and stdin/stdout
 #[derive(Debug)]
 pub struct AppServer {
-    pub crypt: Option<CryptoServer>,
+    pub crypto_site: ConstructionSite<BuildCryptoServer, CryptoServer>,
     pub sockets: Vec<mio::net::UdpSocket>,
     pub events: mio::Events,
     pub mio_poll: mio::Poll,
@@ -606,7 +608,7 @@ impl AppServer {
         // TODO use mio::net::UnixStream together with std::os::unix::net::UnixStream for Linux
 
         Ok(Self {
-            crypt: Some(CryptoServer::new(sk, pk)),
+            crypto_site: ConstructionSite::from_product(CryptoServer::new(sk, pk)),
             peers: Vec::new(),
             verbosity,
             sockets,
@@ -627,14 +629,14 @@ impl AppServer {
     }
 
     pub fn crypto_server(&self) -> anyhow::Result<&CryptoServer> {
-        self.crypt
-            .as_ref()
+        self.crypto_site
+            .product_ref()
             .context("Cryptography handler not initialized")
     }
 
     pub fn crypto_server_mut(&mut self) -> anyhow::Result<&mut CryptoServer> {
-        self.crypt
-            .as_mut()
+        self.crypto_site
+            .product_mut()
             .context("Cryptography handler not initialized")
     }
 
@@ -688,8 +690,13 @@ impl AppServer {
         broker_peer: Option<BrokerPeer>,
         hostname: Option<String>,
     ) -> anyhow::Result<AppPeerPtr> {
-        let PeerPtr(pn) = self.crypto_server_mut()?.add_peer(psk, pk)?;
+        let PeerPtr(pn) = match &mut self.crypto_site {
+            ConstructionSite::Void => bail!("Crypto server construction site is void"),
+            ConstructionSite::Builder(builder) => builder.add_peer(psk, pk),
+            ConstructionSite::Product(srv) => srv.add_peer(psk, pk)?,
+        };
         assert!(pn == self.peers.len());
+
         let initial_endpoint = hostname
             .map(Endpoint::discovery_from_hostname)
             .transpose()?;
@@ -774,16 +781,31 @@ impl AppServer {
                 }
             }
 
-            match self.poll(&mut *rx)? {
-                #[allow(clippy::redundant_closure_call)]
-                SendInitiation(peer) => tx_maybe_with!(peer, || self
+            enum CryptoSrv {
+                Avail,
+                Missing,
+            }
+
+            let poll_result = self.poll(&mut *rx)?;
+            let have_crypto = match self.crypto_site.is_available() {
+                true => CryptoSrv::Avail,
+                false => CryptoSrv::Missing,
+            };
+
+            #[allow(clippy::redundant_closure_call)]
+            match (have_crypto, poll_result) {
+                (CryptoSrv::Missing, SendInitiation(_)) => {}
+                (CryptoSrv::Avail, SendInitiation(peer)) => tx_maybe_with!(peer, || self
                     .crypto_server_mut()?
                     .initiate_handshake(peer.lower(), &mut *tx))?,
-                #[allow(clippy::redundant_closure_call)]
-                SendRetransmission(peer) => tx_maybe_with!(peer, || self
+
+                (CryptoSrv::Missing, SendRetransmission(_)) => {}
+                (CryptoSrv::Avail, SendRetransmission(peer)) => tx_maybe_with!(peer, || self
                     .crypto_server_mut()?
                     .retransmit_handshake(peer.lower(), &mut *tx))?,
-                DeleteKey(peer) => {
+
+                (CryptoSrv::Missing, DeleteKey(_)) => {}
+                (CryptoSrv::Avail, DeleteKey(peer)) => {
                     self.output_key(peer, Stale, &SymKey::random())?;
 
                     // There was a loss of connection apparently; restart host discovery
@@ -797,7 +819,8 @@ impl AppServer {
                     );
                 }
 
-                ReceivedMessage(len, endpoint) => {
+                (CryptoSrv::Missing, ReceivedMessage(_, _)) => {}
+                (CryptoSrv::Avail, ReceivedMessage(len, endpoint)) => {
                     let msg_result = match self.under_load {
                         DoSOperation::UnderLoad => {
                             self.handle_msg_under_load(&endpoint, &rx[..len], &mut *tx)
@@ -910,17 +933,32 @@ impl AppServer {
     pub fn poll(&mut self, rx_buf: &mut [u8]) -> anyhow::Result<AppPollResult> {
         use crate::protocol::PollResult as C;
         use AppPollResult as A;
-        loop {
-            return Ok(match self.crypto_server_mut()?.poll()? {
-                C::DeleteKey(PeerPtr(no)) => A::DeleteKey(AppPeerPtr(no)),
-                C::SendInitiation(PeerPtr(no)) => A::SendInitiation(AppPeerPtr(no)),
-                C::SendRetransmission(PeerPtr(no)) => A::SendRetransmission(AppPeerPtr(no)),
-                C::Sleep(timeout) => match self.try_recv(rx_buf, timeout)? {
-                    Some((len, addr)) => A::ReceivedMessage(len, addr),
-                    None => continue,
-                },
-            });
-        }
+        let res = loop {
+            // Call CryptoServer's poll (if available)
+            let crypto_poll = self
+                .crypto_site
+                .product_mut()
+                .map(|crypto| crypto.poll())
+                .transpose()?;
+
+            // Map crypto server's poll result to our poll result
+            let io_poll_timeout = match crypto_poll {
+                Some(C::DeleteKey(PeerPtr(no))) => break A::DeleteKey(AppPeerPtr(no)),
+                Some(C::SendInitiation(PeerPtr(no))) => break A::SendInitiation(AppPeerPtr(no)),
+                Some(C::SendRetransmission(PeerPtr(no))) => {
+                    break A::SendRetransmission(AppPeerPtr(no))
+                }
+                Some(C::Sleep(timeout)) => timeout, // No event from crypto-server, do IO
+                None => crate::protocol::UNENDING,  // Crypto server is uninitialized, do IO
+            };
+
+            // Perform IO (look for a message)
+            if let Some((len, addr)) = self.try_recv(rx_buf, io_poll_timeout)? {
+                break A::ReceivedMessage(len, addr);
+            }
+        };
+
+        Ok(res)
     }
 
     /// Tries to receive a new message
diff --git a/rosenpass/src/protocol/build_crypto_server.rs b/rosenpass/src/protocol/build_crypto_server.rs
new file mode 100644
index 0000000..224fbf0
--- /dev/null
+++ b/rosenpass/src/protocol/build_crypto_server.rs
@@ -0,0 +1,127 @@
+use rosenpass_util::{
+    build::Build,
+    mem::{DiscardResultExt, SwapWithDefaultExt},
+    result::ensure_or,
+};
+use thiserror::Error;
+
+use super::{CryptoServer, PeerPtr, SPk, SSk, SymKey};
+
+#[derive(Debug, Clone)]
+pub struct Keypair {
+    pub sk: SSk,
+    pub pk: SPk,
+}
+
+// TODO: We need a named tuple derive
+impl Keypair {
+    pub fn new(sk: SSk, pk: SPk) -> Self {
+        Self { sk, pk }
+    }
+
+    pub fn zero() -> Self {
+        Self::new(SSk::zero(), SPk::zero())
+    }
+
+    pub fn random() -> Self {
+        Self::new(SSk::random(), SPk::random())
+    }
+
+    pub fn from_parts(parts: (SSk, SPk)) -> Self {
+        Self::new(parts.0, parts.1)
+    }
+
+    pub fn into_parts(self) -> (SSk, SPk) {
+        (self.sk, self.pk)
+    }
+}
+
+#[derive(Error, Debug)]
+#[error("PSK already set in BuildCryptoServer")]
+pub struct PskAlreadySet;
+
+#[derive(Error, Debug)]
+#[error("Keypair already set in BuildCryptoServer")]
+pub struct KeypairAlreadySet;
+
+#[derive(Error, Debug)]
+#[error("Can not construct CryptoServer: Missing keypair")]
+pub struct MissingKeypair;
+
+#[derive(Debug, Default)]
+pub struct BuildCryptoServer {
+    pub keypair: Option<Keypair>,
+    pub peers: Vec<PeerParams>,
+}
+
+impl Build<CryptoServer> for BuildCryptoServer {
+    type Error = anyhow::Error;
+
+    fn build(self) -> Result<CryptoServer, Self::Error> {
+        let Some(Keypair { sk, pk }) = self.keypair else {
+            return Err(MissingKeypair)?;
+        };
+
+        let mut srv = CryptoServer::new(sk, pk);
+
+        for (idx, PeerParams { psk, pk }) in self.peers.into_iter().enumerate() {
+            let PeerPtr(idx2) = srv.add_peer(psk, pk)?;
+            assert!(idx == idx2, "Peer id changed during CryptoServer construction from {idx} to {idx2}. This is a developer error.")
+        }
+
+        Ok(srv)
+    }
+}
+
+#[derive(Debug)]
+pub struct PeerParams {
+    pub psk: Option<SymKey>,
+    pub pk: SPk,
+}
+
+impl BuildCryptoServer {
+    pub fn new(keypair: Option<Keypair>, peers: Vec<PeerParams>) -> Self {
+        Self { keypair, peers }
+    }
+
+    pub fn empty() -> Self {
+        Self::new(None, Vec::new())
+    }
+
+    pub fn from_parts(parts: (Option<Keypair>, Vec<PeerParams>)) -> Self {
+        Self {
+            keypair: parts.0,
+            peers: parts.1,
+        }
+    }
+
+    pub fn take_parts(&mut self) -> (Option<Keypair>, Vec<PeerParams>) {
+        (self.keypair.take(), self.peers.swap_with_default())
+    }
+
+    pub fn into_parts(mut self) -> (Option<Keypair>, Vec<PeerParams>) {
+        self.take_parts()
+    }
+
+    pub fn with_keypair(&mut self, keypair: Keypair) -> Result<&mut Self, KeypairAlreadySet> {
+        ensure_or(self.keypair.is_none(), KeypairAlreadySet)?;
+        self.keypair.insert(keypair).discard_result();
+        Ok(self)
+    }
+
+    pub fn with_added_peer(&mut self, psk: Option<SymKey>, pk: SPk) -> &mut Self {
+        // TODO: Check here already whether peer was already added
+        self.peers.push(PeerParams { psk, pk });
+        self
+    }
+
+    pub fn add_peer(&mut self, psk: Option<SymKey>, pk: SPk) -> PeerPtr {
+        let id = PeerPtr(self.peers.len());
+        self.with_added_peer(psk, pk);
+        id
+    }
+
+    pub fn emancipate(&mut self) -> Self {
+        Self::from_parts(self.take_parts())
+    }
+}
diff --git a/rosenpass/src/protocol/mod.rs b/rosenpass/src/protocol/mod.rs
new file mode 100644
index 0000000..68d48bb
--- /dev/null
+++ b/rosenpass/src/protocol/mod.rs
@@ -0,0 +1,6 @@
+mod build_crypto_server;
+#[allow(clippy::module_inception)]
+mod protocol;
+
+pub use build_crypto_server::*;
+pub use protocol::*;
diff --git a/rosenpass/src/protocol.rs b/rosenpass/src/protocol/protocol.rs
similarity index 100%
rename from rosenpass/src/protocol.rs
rename to rosenpass/src/protocol/protocol.rs
diff --git a/util/src/build.rs b/util/src/build.rs
new file mode 100644
index 0000000..b268c4c
--- /dev/null
+++ b/util/src/build.rs
@@ -0,0 +1,169 @@
+use crate::{
+    functional::ApplyExt,
+    mem::{SwapWithDefaultExt, SwapWithExt},
+};
+
+#[derive(thiserror::Error, Debug)]
+pub enum ConstructionSiteErectError<E> {
+    #[error("Construction site is void")]
+    IsVoid,
+    #[error("Construction is already built")]
+    AlreadyBuilt,
+    #[error("Other construction site error {0:?}")]
+    Other(#[from] E),
+}
+
+pub trait Build<T>: Sized {
+    type Error;
+    fn build(self) -> Result<T, Self::Error>;
+}
+
+#[derive(Debug)]
+pub enum ConstructionSite<Builder, T>
+where
+    Builder: Build<T>,
+{
+    Void,
+    Builder(Builder),
+    Product(T),
+}
+
+impl<Builder, T> Default for ConstructionSite<Builder, T>
+where
+    Builder: Build<T>,
+{
+    fn default() -> Self {
+        Self::Void
+    }
+}
+
+impl<Builder, T> ConstructionSite<Builder, T>
+where
+    Builder: Build<T>,
+{
+    pub fn void() -> Self {
+        Self::Void
+    }
+
+    pub fn new(builder: Builder) -> Self {
+        Self::Builder(builder)
+    }
+
+    pub fn from_product(value: T) -> Self {
+        Self::Product(value)
+    }
+
+    pub fn take(&mut self) -> Self {
+        self.swap_with_default()
+    }
+
+    pub fn modify_taken_with_return<R, F>(&mut self, f: F) -> R
+    where
+        F: FnOnce(Self) -> (Self, R),
+    {
+        let (site, res) = self.take().apply(f);
+        self.swap_with(site);
+        res
+    }
+
+    pub fn modify_taken<F>(&mut self, f: F)
+    where
+        F: FnOnce(Self) -> Self,
+    {
+        self.take().apply(f).swap_with_mut(self)
+    }
+
+    #[allow(clippy::result_unit_err)]
+    pub fn erect(&mut self) -> Result<(), ConstructionSiteErectError<Builder::Error>> {
+        self.modify_taken_with_return(|site| {
+            let builder = match site {
+                site @ Self::Void => return (site, Err(ConstructionSiteErectError::IsVoid)),
+                site @ Self::Product(_) => {
+                    return (site, Err(ConstructionSiteErectError::AlreadyBuilt))
+                }
+                Self::Builder(builder) => builder,
+            };
+
+            let product = match builder.build() {
+                Err(e) => {
+                    return (Self::void(), Err(ConstructionSiteErectError::Other(e)));
+                }
+                Ok(p) => p,
+            };
+
+            (Self::from_product(product), Ok(()))
+        })
+    }
+
+    /// Returns `true` if the construction site is [`Void`].
+    ///
+    /// [`Void`]: ConstructionSite::Void
+    #[must_use]
+    pub fn is_void(&self) -> bool {
+        matches!(self, Self::Void)
+    }
+
+    /// Returns `true` if the construction site is [`InProgress`].
+    ///
+    /// [`InProgress`]: ConstructionSite::InProgress
+    #[must_use]
+    pub fn in_progess(&self) -> bool {
+        matches!(self, Self::Builder(..))
+    }
+
+    /// Returns `true` if the construction site is [`Done`].
+    ///
+    /// [`Done`]: ConstructionSite::Done
+    #[must_use]
+    pub fn is_available(&self) -> bool {
+        matches!(self, Self::Product(..))
+    }
+
+    pub fn into_builder(self) -> Option<Builder> {
+        use ConstructionSite as S;
+        match self {
+            S::Builder(v) => Some(v),
+            _ => None,
+        }
+    }
+
+    pub fn builder_ref(&self) -> Option<&Builder> {
+        use ConstructionSite as S;
+        match self {
+            S::Builder(v) => Some(v),
+            _ => None,
+        }
+    }
+
+    pub fn builder_mut(&mut self) -> Option<&mut Builder> {
+        use ConstructionSite as S;
+        match self {
+            S::Builder(v) => Some(v),
+            _ => None,
+        }
+    }
+
+    pub fn into_product(self) -> Option<T> {
+        use ConstructionSite as S;
+        match self {
+            S::Product(v) => Some(v),
+            _ => None,
+        }
+    }
+
+    pub fn product_ref(&self) -> Option<&T> {
+        use ConstructionSite as S;
+        match self {
+            S::Product(v) => Some(v),
+            _ => None,
+        }
+    }
+
+    pub fn product_mut(&mut self) -> Option<&mut T> {
+        use ConstructionSite as S;
+        match self {
+            S::Product(v) => Some(v),
+            _ => None,
+        }
+    }
+}
diff --git a/util/src/functional.rs b/util/src/functional.rs
index 10c5f94..54ebe8c 100644
--- a/util/src/functional.rs
+++ b/util/src/functional.rs
@@ -6,6 +6,32 @@ where
     v
 }
 
+pub trait MutatingExt {
+    fn mutating<F>(self, f: F) -> Self
+    where
+        F: Fn(&mut Self);
+    fn mutating_mut<F>(&mut self, f: F) -> &mut Self
+    where
+        F: Fn(&mut Self);
+}
+
+impl<T> MutatingExt for T {
+    fn mutating<F>(self, f: F) -> Self
+    where
+        F: Fn(&mut Self),
+    {
+        mutating(self, f)
+    }
+
+    fn mutating_mut<F>(&mut self, f: F) -> &mut Self
+    where
+        F: Fn(&mut Self),
+    {
+        f(self);
+        self
+    }
+}
+
 pub fn sideeffect<T, F>(v: T, f: F) -> T
 where
     F: Fn(&T),
@@ -14,6 +40,58 @@ where
     v
 }
 
+pub trait SideffectExt {
+    fn sideeffect<F>(self, f: F) -> Self
+    where
+        F: Fn(&Self);
+    fn sideeffect_ref<F>(&self, f: F) -> &Self
+    where
+        F: Fn(&Self);
+    fn sideeffect_mut<F>(&mut self, f: F) -> &mut Self
+    where
+        F: Fn(&Self);
+}
+
+impl<T> SideffectExt for T {
+    fn sideeffect<F>(self, f: F) -> Self
+    where
+        F: Fn(&Self),
+    {
+        sideeffect(self, f)
+    }
+
+    fn sideeffect_ref<F>(&self, f: F) -> &Self
+    where
+        F: Fn(&Self),
+    {
+        f(self);
+        self
+    }
+
+    fn sideeffect_mut<F>(&mut self, f: F) -> &mut Self
+    where
+        F: Fn(&Self),
+    {
+        f(self);
+        self
+    }
+}
+
 pub fn run<R, F: FnOnce() -> R>(f: F) -> R {
     f()
 }
+
+pub trait ApplyExt: Sized {
+    fn apply<R, F>(self, f: F) -> R
+    where
+        F: FnOnce(Self) -> R;
+}
+
+impl<T: Sized> ApplyExt for T {
+    fn apply<R, F>(self, f: F) -> R
+    where
+        F: FnOnce(Self) -> R,
+    {
+        f(self)
+    }
+}
diff --git a/util/src/lib.rs b/util/src/lib.rs
index c4d3f31..eeaa920 100644
--- a/util/src/lib.rs
+++ b/util/src/lib.rs
@@ -1,6 +1,7 @@
 #![recursion_limit = "256"]
 
 pub mod b64;
+pub mod build;
 pub mod fd;
 pub mod file;
 pub mod functional;
diff --git a/util/src/mem.rs b/util/src/mem.rs
index 620e3d8..2cc5785 100644
--- a/util/src/mem.rs
+++ b/util/src/mem.rs
@@ -92,3 +92,47 @@ impl<T> Drop for Forgetting<T> {
         forget(value)
     }
 }
+
+pub trait DiscardResultExt {
+    fn discard_result(self);
+}
+
+impl<T> DiscardResultExt for T {
+    fn discard_result(self) {}
+}
+
+pub trait ForgetExt {
+    fn forget(self);
+}
+
+impl<T> ForgetExt for T {
+    fn forget(self) {
+        std::mem::forget(self)
+    }
+}
+
+pub trait SwapWithExt {
+    fn swap_with(&mut self, other: Self) -> Self;
+    fn swap_with_mut(&mut self, other: &mut Self);
+}
+
+impl<T> SwapWithExt for T {
+    fn swap_with(&mut self, mut other: Self) -> Self {
+        self.swap_with_mut(&mut other);
+        other
+    }
+
+    fn swap_with_mut(&mut self, other: &mut Self) {
+        std::mem::swap(self, other)
+    }
+}
+
+pub trait SwapWithDefaultExt {
+    fn swap_with_default(&mut self) -> Self;
+}
+
+impl<T: Default> SwapWithDefaultExt for T {
+    fn swap_with_default(&mut self) -> Self {
+        self.swap_with(Self::default())
+    }
+}
diff --git a/util/src/result.rs b/util/src/result.rs
index 8493857..041adbd 100644
--- a/util/src/result.rs
+++ b/util/src/result.rs
@@ -8,6 +8,16 @@ macro_rules! attempt {
     };
 }
 
+pub trait OkExt<E>: Sized {
+    fn ok(self) -> Result<Self, E>;
+}
+
+impl<T, E> OkExt<E> for T {
+    fn ok(self) -> Result<Self, E> {
+        Ok(self)
+    }
+}
+
 /// Trait for container types that guarantee successful unwrapping.
 ///
 /// The `.guaranteed()` function can be used over unwrap to show that