diff --git a/flake.nix b/flake.nix
index cd2c196..89bfdb9 100644
--- a/flake.nix
+++ b/flake.nix
@@ -28,15 +28,17 @@
           fd
           #(rust-bin.stable.latest.default.override {
           # go with nightly to have async fn in traits
-          (rust-bin.nightly."2023-02-01".default.override {
-            #extensions = [ "rust-src" ];
-            #targets = [ "arm-unknown-linux-gnueabihf" ];
-          })
+          #(rust-bin.nightly."2023-02-01".default.override {
+          #  #extensions = [ "rust-src" ];
+          #  #targets = [ "arm-unknown-linux-gnueabihf" ];
+          #})
           clippy
-          lld
           cargo-watch
-          rustfmt
           cargo-license
+          lld
+          rust-bin.stable.latest.default
+          rustfmt
+          rust-analyzer
         ];
         shellHook = ''
           # use zsh or other custom shell
diff --git a/src/auth/mod.rs b/src/auth/mod.rs
new file mode 100644
index 0000000..f7fd3ec
--- /dev/null
+++ b/src/auth/mod.rs
@@ -0,0 +1,44 @@
+//! Authentication  reslated struct definitions
+
+/// User identifier. 16 bytes for easy uuid conversion
+#[derive(Debug, Copy, Clone)]
+pub struct UserID([u8; 16]);
+
+impl From<[u8; 16]> for UserID {
+    fn from(raw: [u8; 16]) -> Self {
+        UserID(raw)
+    }
+}
+
+impl UserID {
+    /// length of the User ID in bytes
+    pub const fn len() -> usize {
+        16
+    }
+}
+/// Authentication Token, basically just 32 random bytes
+#[derive(Copy, Clone)]
+pub struct Token([u8; 32]);
+
+impl Token {
+    /// length of the token in bytes
+    pub const fn len() -> usize {
+        32
+    }
+}
+
+impl From<[u8; 32]> for Token {
+    fn from(raw: [u8; 32]) -> Self {
+        Token(raw)
+    }
+}
+
+// Fake debug implementation to avoid leaking tokens
+impl ::core::fmt::Debug for Token {
+    fn fmt(
+        &self,
+        f: &mut core::fmt::Formatter<'_>,
+    ) -> Result<(), ::std::fmt::Error> {
+        ::core::fmt::Debug::fmt("[hidden token]", f)
+    }
+}
diff --git a/src/connection/handshake/dirsync.rs b/src/connection/handshake/dirsync.rs
index 95fc4f2..788a231 100644
--- a/src/connection/handshake/dirsync.rs
+++ b/src/connection/handshake/dirsync.rs
@@ -10,13 +10,16 @@
 
 use super::{Error, HandshakeData};
 use crate::{
+    auth,
     connection::ID,
     enc::{
         asym::{ExchangePubKey, KeyExchange, KeyID},
         sym::CipherKind,
     },
 };
-use ::std::vec::Vec;
+use ::std::{collections::VecDeque, num::NonZeroU64, vec::Vec};
+
+type Nonce = [u8; 16];
 
 /// Parsed handshake
 #[derive(Debug, Clone)]
@@ -39,7 +42,14 @@ pub struct Req {
     /// Client ephemeral public key used for key exchanges
     pub exchange_key: ExchangePubKey,
     /// encrypted data
-    pub enc: Vec<u8>,
+    pub data: ReqInner,
+}
+
+impl Req {
+    /// Set the cleartext data after it was parsed
+    pub fn set_data(&mut self, data: ReqData) {
+        self.data = ReqInner::Data(data);
+    }
 }
 
 impl super::HandshakeParsing for Req {
@@ -63,30 +73,102 @@ impl super::HandshakeParsing for Req {
             Ok(exchange_key) => exchange_key,
             Err(e) => return Err(e.into()),
         };
-        let enc = raw[(4 + len)..].to_vec();
+        let mut vec = VecDeque::with_capacity(raw.len() - (4 + len));
+        vec.extend(raw[(4 + len)..].iter().copied());
+        let _ = vec.make_contiguous();
+        let data = ReqInner::Ciphertext(vec);
         Ok(HandshakeData::DirSync(DirSync::Req(Self {
             key_id,
             exchange,
             cipher,
             exchange_key,
-            enc,
+            data,
         })))
     }
 }
 
+/// Quick way to avoid mixing cipher and clear text
+#[derive(Debug, Clone)]
+pub enum ReqInner {
+    /// Client data, still in ciphertext
+    Ciphertext(VecDeque<u8>),
+    /// Client data, decrypted but unprocessed
+    Cleartext(VecDeque<u8>),
+    /// Parsed client data
+    Data(ReqData),
+}
+impl ReqInner {
+    /// Get the ciptertext, or panic
+    pub fn ciphertext<'a>(&'a mut self) -> &'a mut VecDeque<u8> {
+        match self {
+            ReqInner::Ciphertext(data) => data,
+            _ => panic!(),
+        }
+    }
+    /// switch from ciphertext to cleartext
+    pub fn mark_as_cleartext(&mut self) {
+        let mut newdata: VecDeque<u8>;
+        match self {
+            ReqInner::Ciphertext(data) => {
+                newdata = VecDeque::new();
+                ::core::mem::swap(&mut newdata, data);
+            }
+            _ => return,
+        }
+        *self = ReqInner::Cleartext(newdata);
+    }
+}
+
 /// Decrypted request data
 #[derive(Debug, Clone, Copy)]
 pub struct ReqData {
     /// Random nonce, the client can use this to track multiple key exchanges
-    pub nonce: [u8; 16],
+    pub nonce: Nonce,
     /// Client key id so the client can use and rotate keys
     pub client_key_id: KeyID,
+    /// User of the domain
+    pub user: auth::UserID,
     /// Authentication token
-    pub token: [u8; 32],
+    pub token: auth::Token,
     /// Receiving connection id for the client
     pub id: ID,
     // TODO: service info
 }
+impl ReqData {
+    /// Parse the cleartext raw data
+    pub fn parse(raw: &ReqInner) -> Result<Self, Error> {
+        const MIN_PKT_LEN: usize = 16
+            + KeyID::len()
+            + auth::UserID::len()
+            + auth::Token::len()
+            + ID::len();
+        let raw = match raw {
+            ReqInner::Cleartext(raw) => raw.as_slices().0,
+            _ => return Err(Error::Parsing),
+        };
+        if raw.len() < MIN_PKT_LEN {
+            return Err(Error::NotEnoughData);
+        }
+        let nonce: Nonce = raw.try_into().unwrap();
+        let client_key_id =
+            KeyID(u16::from_le_bytes(raw[16..17].try_into().unwrap()));
+        let raw_user: [u8; 16] = raw[18..34].try_into().unwrap();
+        let user: auth::UserID = raw_user.into();
+        let raw_token: [u8; 32] = raw[34..66].try_into().unwrap();
+        let token: auth::Token = raw_token.into();
+        let id: ID = u64::from_le_bytes(raw[66..74].try_into().unwrap()).into();
+        if id.is_handshake() {
+            return Err(Error::Parsing);
+        }
+        Ok(Self {
+            nonce,
+            client_key_id,
+            user,
+            token,
+            id,
+        })
+    }
+}
 
 /// Server response in a directory synchronized handshake
 #[derive(Debug, Clone)]
@@ -107,7 +189,7 @@ impl super::HandshakeParsing for Resp {
 #[derive(Debug, Clone, Copy)]
 pub struct RespData {
     /// Client nonce, copied from the request
-    client_nonce: [u8; 16],
+    client_nonce: Nonce,
     /// Server Connection ID
     id: ID,
     /// Service Connection ID
@@ -115,3 +197,27 @@ pub struct RespData {
     /// Service encryption key
     service_key: [u8; 32],
 }
+
+impl RespData {
+    const NONCE_LEN: usize = ::core::mem::size_of::<Nonce>();
+    /// Return the expected length for buffer allocation
+    pub fn len() -> usize {
+        Self::NONCE_LEN + ID::len() + ID::len() + 32
+    }
+    /// Serialize the data into a buffer
+    pub fn serialize(&self, out: &mut [u8]) {
+        assert!(out.len() == Self::len(), "wrong buffer size");
+        let mut start = 0;
+        let mut end = Self::NONCE_LEN;
+        out[start..end].copy_from_slice(&self.client_nonce);
+        start = end;
+        end = end + Self::NONCE_LEN;
+        self.id.serialize(&mut out[start..end]);
+        start = end;
+        end = end + Self::NONCE_LEN;
+        self.service_id.serialize(&mut out[start..end]);
+        start = end;
+        end = end + Self::NONCE_LEN;
+        out[start..end].copy_from_slice(&self.service_key);
+    }
+}
diff --git a/src/connection/packet.rs b/src/connection/packet.rs
index a185838..db97c8b 100644
--- a/src/connection/packet.rs
+++ b/src/connection/packet.rs
@@ -17,6 +17,20 @@ impl ConnectionID {
     pub fn is_handshake(&self) -> bool {
         *self == ConnectionID::Handshake
     }
+    /// length if the connection ID in bytes
+    pub const fn len() -> usize {
+        8
+    }
+    /// write the ID to a buffer
+    pub fn serialize(&self, out: &mut [u8]) {
+        assert!(out.len() == 8, "Insufficient buffer");
+        match self {
+            ConnectionID::Handshake => out[..].copy_from_slice(&[0; 8]),
+            ConnectionID::ID(id) => {
+                out[..].copy_from_slice(&id.get().to_le_bytes())
+            }
+        }
+    }
 }
 
 impl From<u64> for ConnectionID {
diff --git a/src/enc/asym.rs b/src/enc/asym.rs
index 7eccda4..f7a79fa 100644
--- a/src/enc/asym.rs
+++ b/src/enc/asym.rs
@@ -10,6 +10,13 @@ use crate::enc::sym::Secret;
 #[derive(Debug, Copy, Clone, PartialEq)]
 pub struct KeyID(pub u16);
 
+impl KeyID {
+    /// Length of the Key ID in bytes
+    pub const fn len() -> usize {
+        2
+    }
+}
+
 /// Kind of key used in the handshake
 #[derive(Debug, Copy, Clone, PartialEq, ::num_derive::FromPrimitive)]
 #[repr(u8)]
diff --git a/src/enc/errors.rs b/src/enc/errors.rs
index 86af252..ae1961b 100644
--- a/src/enc/errors.rs
+++ b/src/enc/errors.rs
@@ -19,4 +19,7 @@ pub enum Error {
     /// Unsupported cipher
     #[error("unsupported cipher")]
     UnsupportedCipher,
+    /// Can not decrypt. Either corrupted or malicious data
+    #[error("decrypt: corrupted data")]
+    Decrypt,
 }
diff --git a/src/enc/sym.rs b/src/enc/sym.rs
index 4e819d0..a7e1c80 100644
--- a/src/enc/sym.rs
+++ b/src/enc/sym.rs
@@ -1,5 +1,7 @@
 //! Symmetric cypher stuff
 
+use super::Error;
+use ::std::collections::VecDeque;
 use ::zeroize::Zeroize;
 
 /// Secret, used for keys.
@@ -106,31 +108,41 @@ impl Cipher {
             }
         }
     }
-    fn decrypt(&self, aad: AAD, data: &mut [u8]) -> Result<(), ()> {
+    fn decrypt<'a>(
+        &self,
+        aad: AAD,
+        data: &mut VecDeque<u8>,
+    ) -> Result<(), Error> {
         match self {
             Cipher::XChaCha20Poly1305(cipher) => {
                 use ::chacha20poly1305::{
                     aead::generic_array::GenericArray, AeadInPlace,
                 };
-                // FIXME: check min data length
-                let (nonce_bytes, data_and_tag) = data.split_at_mut(13);
-                let (data_notag, tag_bytes) = data_and_tag.split_at_mut(
-                    data_and_tag.len() + 1
-                        - ::ring::aead::CHACHA20_POLY1305.tag_len(),
-                );
-                let nonce = GenericArray::from_slice(nonce_bytes);
-                let tag = GenericArray::from_slice(tag_bytes);
-                let maybe = cipher.cipher.decrypt_in_place_detached(
-                    nonce.into(),
-                    aad.0,
-                    data_notag,
-                    tag,
-                );
-                if maybe.is_err() {
-                    Err(())
-                } else {
-                    Ok(())
+                let final_len: usize;
+                {
+                    let raw_data = data.as_mut_slices().0;
+                    // FIXME: check min data length
+                    let (nonce_bytes, data_and_tag) = raw_data.split_at_mut(13);
+                    let (data_notag, tag_bytes) = data_and_tag.split_at_mut(
+                        data_and_tag.len() + 1
+                            - ::ring::aead::CHACHA20_POLY1305.tag_len(),
+                    );
+                    let nonce = GenericArray::from_slice(nonce_bytes);
+                    let tag = GenericArray::from_slice(tag_bytes);
+                    let maybe = cipher.cipher.decrypt_in_place_detached(
+                        nonce.into(),
+                        aad.0,
+                        data_notag,
+                        tag,
+                    );
+                    if maybe.is_err() {
+                        return Err(Error::Decrypt);
+                    }
+                    final_len = data_notag.len();
                 }
+                data.drain(..Nonce::len());
+                data.truncate(final_len);
+                Ok(())
             }
         }
     }
@@ -151,7 +163,11 @@ impl CipherRecv {
     }
     /// Decrypt a paket. Nonce and Tag are taken from the packet,
     /// while you need to provide AAD (Additional Authenticated Data)
-    pub fn decrypt(&self, aad: AAD, data: &mut [u8]) -> Result<(), ()> {
+    pub fn decrypt<'a>(
+        &self,
+        aad: AAD,
+        data: &mut VecDeque<u8>,
+    ) -> Result<(), Error> {
         self.0.decrypt(aad, data)
     }
 }
@@ -236,6 +252,10 @@ impl Nonce {
             }
         }
     }
+    /// Length of this nonce in bytes
+    pub fn len() -> usize {
+        return 12;
+    }
     /// Get reference to the nonce bytes
     pub fn as_bytes(&self) -> &[u8] {
         #[allow(unsafe_code)]
diff --git a/src/lib.rs b/src/lib.rs
index c97fc38..fa19e94 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -13,13 +13,15 @@
 //!
 //! libFenrir is the official rust library implementing the Fenrir protocol
 
+pub mod auth;
 mod config;
 pub mod connection;
 pub mod dnssec;
 pub mod enc;
 
-use ::arc_swap::{ArcSwap, ArcSwapAny};
-use ::std::{net::SocketAddr, sync::Arc};
+use ::arc_swap::{ArcSwap, ArcSwapAny, ArcSwapOption};
+use ::std::{net::SocketAddr, pin::Pin, sync::Arc};
+use ::tokio::macros::support::Future;
 use ::tokio::{net::UdpSocket, task::JoinHandle};
 
 use crate::enc::{
@@ -50,18 +52,35 @@ pub enum Error {
     Key(#[from] crate::enc::Error),
 }
 
+// No async here
 struct FenrirInner {
     key_exchanges: ArcSwapAny<Arc<Vec<(asym::Key, asym::KeyExchange)>>>,
     ciphers: ArcSwapAny<Arc<Vec<CipherKind>>>,
     keys: ArcSwapAny<Arc<Vec<HandshakeKey>>>,
 }
 
+/// Intermediate actions to be taken while parsing the handshake
+#[derive(Debug, Clone)]
+pub enum HandshakeAction {
+    /// Parsing finished, all ok, nothing to do
+    None,
+    /// Packet parsed, now go perform authentication
+    AuthNeeded(Handshake),
+}
+
+// No async here
 impl FenrirInner {
-    fn recv_handshake(&self, handshake: Handshake) -> Result<(), Error> {
-        use connection::handshake::{dirsync::DirSync, HandshakeData};
+    fn recv_handshake(
+        &self,
+        mut handshake: Handshake,
+    ) -> Result<HandshakeAction, Error> {
+        use connection::handshake::{
+            dirsync::{self, DirSync},
+            HandshakeData,
+        };
         match handshake.data {
-            HandshakeData::DirSync(ds) => match ds {
-                DirSync::Req(mut req) => {
+            HandshakeData::DirSync(ref mut ds) => match ds {
+                DirSync::Req(ref mut req) => {
                     let ephemeral_key = {
                         // Keep this block short to avoid contention
                         // on self.keys
@@ -70,7 +89,7 @@ impl FenrirInner {
                             keys.iter().find(|k| k.id == req.key_id)
                         {
                             use enc::asym::PrivKey;
-                            // Directory synchronized can only used keys
+                            // Directory synchronized can only use keys
                             // for key exchange, not signing keys
                             if let PrivKey::Exchange(k) = &h_k.key {
                                 Some(k.clone())
@@ -115,9 +134,15 @@ impl FenrirInner {
                     let cipher_recv = CipherRecv::new(req.cipher, secret_recv);
                     use crate::enc::sym::AAD;
                     let aad = AAD(&mut []); // no aad for now
-                    let _ = cipher_recv.decrypt(aad, &mut req.enc);
+                    match cipher_recv.decrypt(aad, &mut req.data.ciphertext()) {
+                        Ok(()) => req.data.mark_as_cleartext(),
+                        Err(e) => {
+                            return Err(handshake::Error::Key(e).into());
+                        }
+                    }
+                    req.set_data(dirsync::ReqData::parse(&req.data)?);
 
-                    todo!();
+                    return Ok(HandshakeAction::AuthNeeded(handshake));
                 }
                 DirSync::Resp(resp) => {
                     todo!();
@@ -127,6 +152,12 @@ impl FenrirInner {
     }
 }
 
+type TokenChecker =
+    fn(
+        user: auth::UserID,
+        token: auth::Token,
+    ) -> ::futures::future::BoxFuture<'static, Result<bool, ()>>;
+
 /// Instance of a fenrir endpoint
 #[allow(missing_copy_implementations, missing_debug_implementations)]
 pub struct Fenrir {
@@ -140,6 +171,8 @@ pub struct Fenrir {
     stop_working: ::tokio::sync::broadcast::Sender<bool>,
     /// Private keys used in the handshake
     _inner: Arc<FenrirInner>,
+    /// where to ask for token check
+    token_check: Arc<ArcSwapOption<TokenChecker>>,
 }
 
 // TODO: graceful vs immediate stop
@@ -165,6 +198,7 @@ impl Fenrir {
                 key_exchanges: ArcSwapAny::new(Arc::new(Vec::new())),
                 keys: ArcSwapAny::new(Arc::new(Vec::new())),
             }),
+            token_check: Arc::new(ArcSwapOption::from(None)),
         };
         Ok(endpoint)
     }
@@ -259,6 +293,7 @@ impl Fenrir {
     async fn listen_udp(
         mut stop_working: ::tokio::sync::broadcast::Receiver<bool>,
         fenrir: Arc<FenrirInner>,
+        token_check: Arc<ArcSwapOption<TokenChecker>>,
         socket: Arc<UdpSocket>,
     ) -> ::std::io::Result<()> {
         // jumbo frames are 9K max
@@ -272,7 +307,13 @@ impl Fenrir {
                     result?
                 }
             };
-            Self::recv(fenrir.clone(), &buffer[0..bytes], sock_from).await;
+            Self::recv(
+                fenrir.clone(),
+                token_check.clone(),
+                &buffer[0..bytes],
+                sock_from,
+            )
+            .await;
         }
         Ok(())
     }
@@ -293,6 +334,7 @@ impl Fenrir {
                     let join = ::tokio::spawn(Self::listen_udp(
                         stop_working,
                         self._inner.clone(),
+                        self.token_check.clone(),
                         s.clone(),
                     ));
                     self.sockets.push((s, join));
@@ -323,6 +365,7 @@ impl Fenrir {
     /// Read and do stuff with the udp packet
     async fn recv(
         fenrir: Arc<FenrirInner>,
+        token_check: Arc<ArcSwapOption<TokenChecker>>,
         buffer: &[u8],
         _sock_from: SocketAddr,
     ) {
@@ -340,10 +383,52 @@ impl Fenrir {
                     return;
                 }
             };
-            if let Err(err) = fenrir.recv_handshake(handshake) {
-                ::tracing::debug!("Handshake recv error {}", err);
-                return;
-            }
+            let action = match fenrir.recv_handshake(handshake) {
+                Ok(action) => action,
+                Err(err) => {
+                    ::tracing::debug!("Handshake recv error {}", err);
+                    return;
+                }
+            };
+            match action {
+                HandshakeAction::AuthNeeded(hshake) => {
+                    let tk_check = match token_check.load_full() {
+                        Some(tokenchecker) => tokenchecker,
+                        None => {
+                            ::tracing::error!(
+                                "Handshake received, but no tocken_checker"
+                            );
+                            return;
+                        }
+                    };
+                    use handshake::{
+                        dirsync::{self, DirSync},
+                        HandshakeData,
+                    };
+                    match hshake.data {
+                        HandshakeData::DirSync(ds) => match ds {
+                            DirSync::Req(req) => {
+                                use dirsync::ReqInner;
+                                let req_data = match req.data {
+                                    ReqInner::Data(req_data) => req_data,
+                                    _ => {
+                                        ::tracing::error!(
+                                            "token_check: expected Data"
+                                        );
+                                        return;
+                                    }
+                                };
+                                tk_check(req_data.user, req_data.token).await;
+                                todo!()
+                            }
+                            _ => {
+                                todo!()
+                            }
+                        },
+                    }
+                }
+                _ => {}
+            };
         }
         // copy packet, spawn
         todo!();