Secretstore DKG protocol draft + distributed encryption/decryption tests (#4725)

* ECDKG protocol prototype * added test for enc/dec math * get rid of decryption_session * added licenses * fix after merge * get rid of unused serde dependency * doc * decryption session [without commutative enc] * failed_dec_session * fixed tests * added commen * added more decryption session tests * helper to localize an issue * more computations to localize error * decryption_session::SessionParams * added tests for EC math to localize problem
2017-03-13 14:54:56 +03:00 · 2017-03-13 14:54:56 +03:00 · fddbc9e5cb
commit fddbc9e5cb
parent b7862ac23a
10 changed files with 2677 additions and 0 deletions
--- a/ethkey/src/error.rs
+++ b/ethkey/src/error.rs
@ -51,6 +51,12 @@ impl fmt::Display for Error {
 	}
 }

+impl Into<String> for Error {
+	fn into(self) -> String {
+		format!("{}", self)
+	}
+}
+
 impl From<::secp256k1::Error> for Error {
 	fn from(e: ::secp256k1::Error) -> Error {
 		match e {
--- a/ethkey/src/math.rs
+++ b/ethkey/src/math.rs
@ -36,6 +36,17 @@ pub fn public_add(public: &mut Public, other: &Public) -> Result<(), Error> {
 	Ok(())
 }

+/// Inplace sub one public key from another (EC point - EC point)
+pub fn public_sub(public: &mut Public, other: &Public) -> Result<(), Error> {
+	let mut key_neg_other = to_secp256k1_public(other)?;
+	key_neg_other.mul_assign(&SECP256K1, &key::MINUS_ONE_KEY)?;
+
+	let mut key_public = to_secp256k1_public(public)?;
+	key_public.add_assign(&SECP256K1, &key_neg_other)?;
+	set_public(public, &key_public);
+	Ok(())
+}
+
 /// Return base point of secp256k1
 pub fn generation_point() -> Public {
 	let mut public_sec_raw = [0u8; 65];
@ -64,3 +75,35 @@ fn set_public(public: &mut Public, key_public: &key::PublicKey) {
 	let key_public_serialized = key_public.serialize_vec(&SECP256K1, false);
 	public.copy_from_slice(&key_public_serialized[1..65]);
 }
+
+#[cfg(test)]
+mod tests {
+	use super::super::{Random, Generator};
+	use super::{public_add, public_sub};
+
+	#[test]
+	fn public_addition_is_commutative() {
+		let public1 = Random.generate().unwrap().public().clone();
+		let public2 = Random.generate().unwrap().public().clone();
+
+		let mut left = public1.clone();
+		public_add(&mut left, &public2).unwrap();
+
+		let mut right = public2.clone();
+		public_add(&mut right, &public1).unwrap();
+
+		assert_eq!(left, right);
+	}
+
+	#[test]
+	fn public_addition_is_reversible_with_subtraction() {
+		let public1 = Random.generate().unwrap().public().clone();
+		let public2 = Random.generate().unwrap().public().clone();
+
+		let mut sum = public1.clone();
+		public_add(&mut sum, &public2).unwrap();
+		public_sub(&mut sum, &public2).unwrap();
+
+		assert_eq!(sum, public1);
+	}
+}
--- a/ethkey/src/secret.rs
+++ b/ethkey/src/secret.rs
@ -130,3 +130,54 @@ impl Deref for Secret {
 		&self.inner
 	}
 }
+
+#[cfg(test)]
+mod tests {
+	use std::str::FromStr;
+	use super::super::{Random, Generator};
+	use super::Secret;
+
+	#[test]
+	fn multiplicating_secret_inversion_with_secret_gives_one() {
+		let secret = Random.generate().unwrap().secret().clone();
+		let mut inversion = secret.clone();
+		inversion.inv().unwrap();
+		inversion.mul(&secret).unwrap();
+		assert_eq!(inversion, Secret::from_str("0000000000000000000000000000000000000000000000000000000000000001").unwrap());
+	}
+
+	#[test]
+	fn secret_inversion_is_reversible_with_inversion() {
+		let secret = Random.generate().unwrap().secret().clone();
+		let mut inversion = secret.clone();
+		inversion.inv().unwrap();
+		inversion.inv().unwrap();
+		assert_eq!(inversion, secret);
+	}
+
+	#[test]
+	fn secret_pow() {
+		let secret = Random.generate().unwrap().secret().clone();
+
+		let mut pow0 = secret.clone();
+		pow0.pow(0).unwrap();
+		assert_eq!(pow0, Secret::from_str("0000000000000000000000000000000000000000000000000000000000000001").unwrap());
+
+		let mut pow1 = secret.clone();
+		pow1.pow(1).unwrap();
+		assert_eq!(pow1, secret);
+
+		let mut pow2 = secret.clone();
+		pow2.pow(2).unwrap();
+		let mut pow2_expected = secret.clone();
+		pow2_expected.mul(&secret).unwrap();
+		assert_eq!(pow2, pow2_expected);
+
+		let mut pow3 = secret.clone();
+		pow3.pow(3).unwrap();
+		let mut pow3_expected = secret.clone();
+		pow3_expected.mul(&secret).unwrap();
+		pow3_expected.mul(&secret).unwrap();
+		assert_eq!(pow3, pow3_expected);
+	}
+}
--- a/secret_store/src/key_server_cluster/cluster.rs
+++ b/secret_store/src/key_server_cluster/cluster.rs
@ -0,0 +1,90 @@
+// Copyright 2015-2017 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+use key_server_cluster::{Error, NodeId};
+use key_server_cluster::message::Message;
+
+/// Cluster access for single encryption/decryption participant.
+pub trait Cluster {
+	/// Broadcast message to all other nodes.
+	fn broadcast(&self, message: Message) -> Result<(), Error>;
+	/// Send message to given node.
+	fn send(&self, to: &NodeId, message: Message) -> Result<(), Error>;
+	/// Blacklist node, close connection and remove all pending messages.
+	fn blacklist(&self, node: &NodeId);
+}
+
+#[cfg(test)]
+pub mod tests {
+	use std::collections::VecDeque;
+	use parking_lot::Mutex;
+	use key_server_cluster::{NodeId, Error};
+	use key_server_cluster::message::Message;
+	use key_server_cluster::cluster::Cluster;
+
+	#[derive(Debug)]
+	pub struct DummyCluster {
+		id: NodeId,
+		data: Mutex<DummyClusterData>,
+	}
+
+	#[derive(Debug, Default)]
+	struct DummyClusterData {
+		nodes: Vec<NodeId>,
+		messages: VecDeque<(NodeId, Message)>,
+	}
+
+	impl DummyCluster {
+		pub fn new(id: NodeId) -> Self {
+			DummyCluster {
+				id: id,
+				data: Mutex::new(DummyClusterData::default())
+			}
+		}
+
+		pub fn node(&self) -> NodeId {
+			self.id.clone()
+		}
+
+		pub fn add_node(&self, node: NodeId) {
+			self.data.lock().nodes.push(node);
+		}
+
+		pub fn take_message(&self) -> Option<(NodeId, Message)> {
+			self.data.lock().messages.pop_front()
+		}
+	}
+
+	impl Cluster for DummyCluster {
+		fn broadcast(&self, message: Message) -> Result<(), Error> {
+			let mut data = self.data.lock();
+			let all_nodes: Vec<_> = data.nodes.iter().cloned().filter(|n| n != &self.id).collect();
+			for node in all_nodes {
+				data.messages.push_back((node, message.clone()));
+			}
+			Ok(())
+		}
+
+		fn send(&self, to: &NodeId, message: Message) -> Result<(), Error> {
+			debug_assert!(&self.id != to);
+			self.data.lock().messages.push_back((to.clone(), message));
+			Ok(())
+		}
+
+		fn blacklist(&self, _node: &NodeId) {
+		}
+	}
+}
--- a/secret_store/src/key_server_cluster/decryption_session.rs
+++ b/secret_store/src/key_server_cluster/decryption_session.rs
@ -0,0 +1,707 @@
+// Copyright 2015-2017 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+use std::collections::{BTreeSet, BTreeMap};
+use std::sync::Arc;
+use parking_lot::Mutex;
+use ethkey::{self, Secret, Public, Signature};
+use key_server_cluster::{Error, AclStorage, EncryptedData, NodeId, SessionId};
+use key_server_cluster::cluster::Cluster;
+use key_server_cluster::math;
+use key_server_cluster::message::{Message, InitializeDecryptionSession, ConfirmDecryptionInitialization,
+	RequestPartialDecryption, PartialDecryption};
+
+/// Distributed decryption session.
+/// Based on "ECDKG: A Distributed Key Generation Protocol Based on Elliptic Curve Discrete Logarithm" paper:
+/// http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.124.4128&rep=rep1&type=pdf
+/// Brief overview:
+/// 1) initialization: master node (which has received request for decrypting the secret) requests all other nodes to decrypt the secret
+/// 2) ACL check: all nodes which have received the request are querying ACL-contract to check if requestor has access to the document
+/// 3) partial decryption: every node which has succussfully checked access for the requestor do a partial decryption
+/// 4) decryption: master node receives all partial decryptions of the secret and restores the secret
+pub struct Session {
+	/// Encryption session id.
+	id: SessionId,
+	/// Decryption session access key.
+	access_key: Secret,
+	/// Public identifier of this node.
+	self_node_id: NodeId,
+	/// Encrypted data.
+	encrypted_data: EncryptedData,
+	/// ACL storate to check access to the resource.
+	acl_storage: Arc<AclStorage>,
+	/// Cluster which allows this node to send messages to other nodes in the cluster.
+	cluster: Arc<Cluster>,
+	/// Mutable session data.
+	data: Mutex<SessionData>,
+}
+
+/// Session creation parameters
+pub struct SessionParams {
+	/// Session identifier.
+	pub id: SessionId,
+	/// Session access key.
+	pub access_key: Secret,
+	/// Id of node, on which this session is running.
+	pub self_node_id: Public,
+	/// Encrypted data (result of running encryption_session::Session).
+	pub encrypted_data: EncryptedData,
+	/// ACL storage.
+	pub acl_storage: Arc<AclStorage>,
+	/// Cluster
+	pub cluster: Arc<Cluster>,
+}
+
+#[derive(Debug)]
+/// Mutable data of encryption (distributed key generation) session.
+struct SessionData {
+	/// Current state of the session.
+	state: SessionState,
+
+	// === Values, filled when session initialization just starts ===
+	/// Reference to the node, which has started this session.
+	master: Option<NodeId>,
+	/// Public key of requestor.
+	requestor: Option<Public>,
+
+	// === Values, filled during session initialization ===
+	/// Nodes, which have been requested for decryption initialization.
+	requested_nodes: BTreeSet<NodeId>,
+	/// Nodes, which have responded with reject to initialization request.
+	rejected_nodes: BTreeSet<NodeId>,
+	/// Nodes, which have responded with confirm to initialization request.
+	confirmed_nodes: BTreeSet<NodeId>,
+	
+	// === Values, filled during partial decryption ===
+	/// Shadow points, received from nodes as a response to partial decryption request.
+	shadow_points: BTreeMap<NodeId, Public>,
+
+	/// === Values, filled during final decryption ===
+	/// Decrypted secret
+	decrypted_secret: Option<Public>,
+}
+
+#[derive(Debug)]
+struct NodeData {
+	/// Node-generated shadow point.
+	shadow_point: Option<Public>,
+}
+
+#[derive(Debug, Clone, PartialEq)]
+pub enum SessionState {
+	/// Every node starts in this state.
+	WaitingForInitialization,
+	/// Master node waits for other nodes to confirm decryption.
+	WaitingForInitializationConfirm,
+	/// Waiting for partial decrypion request.
+	WaitingForPartialDecryptionRequest,
+	/// Waiting for partial decryption responses.
+	WaitingForPartialDecryption,
+	/// Decryption session is finished for this node.
+	Finished,
+	/// Decryption session is failed for this node.
+	Failed,
+}
+
+impl Session {
+	/// Create new decryption session.
+	pub fn new(params: SessionParams) -> Result<Self, Error> {
+		check_encrypted_data(&params.self_node_id, &params.encrypted_data)?;
+
+		Ok(Session {
+			id: params.id,
+			access_key: params.access_key,
+			self_node_id: params.self_node_id,
+			encrypted_data: params.encrypted_data,
+			acl_storage: params.acl_storage,
+			cluster: params.cluster,
+			data: Mutex::new(SessionData {
+				state: SessionState::WaitingForInitialization,
+				master: None,
+				requestor: None,
+				requested_nodes: BTreeSet::new(),
+				rejected_nodes: BTreeSet::new(),
+				confirmed_nodes: BTreeSet::new(),
+				shadow_points: BTreeMap::new(),
+				decrypted_secret: None,
+			})
+		})
+	}
+
+	/// Get this node Id.
+	pub fn node(&self) -> &NodeId {
+		&self.self_node_id
+	}
+
+	/// Get this session access key.
+	pub fn access_key(&self) -> &Secret {
+		&self.access_key
+	}
+
+	/// Get current session state.
+	pub fn state(&self) -> SessionState {
+		self.data.lock().state.clone()
+	}
+
+	/// Get decrypted secret
+	pub fn decrypted_secret(&self) -> Option<Public> {
+		self.data.lock().decrypted_secret.clone()
+	}
+
+	/// Initialize decryption session.
+	pub fn initialize(&self, requestor_signature: Signature) -> Result<(), Error> {
+		let mut data = self.data.lock();
+
+		// check state
+		if data.state != SessionState::WaitingForInitialization {
+			return Err(Error::InvalidStateForRequest);
+		}
+
+		// recover requestor signature
+		let requestor_public = ethkey::recover(&requestor_signature, &self.id)?;
+
+		// update state
+		data.master = Some(self.node().clone());
+		data.state = SessionState::WaitingForInitializationConfirm;
+		data.requestor = Some(requestor_public.clone());
+		data.requested_nodes.extend(self.encrypted_data.id_numbers.keys().cloned());
+
+		// ..and finally check access on our's own
+		let is_requestor_allowed_to_read = self.acl_storage.check(&requestor_public, &self.id).unwrap_or(false);
+		process_initialization_response(&self.encrypted_data, &mut *data, self.node(), is_requestor_allowed_to_read)?;
+
+		// check if we have enough nodes to decrypt data
+		match data.state {
+			// not enough nodes => pass initialization message to all other nodes
+			SessionState::WaitingForInitializationConfirm => {
+				for node in self.encrypted_data.id_numbers.keys().filter(|n| *n != self.node()) {
+					self.cluster.send(node, Message::InitializeDecryptionSession(InitializeDecryptionSession {
+							session: self.id.clone(),
+							sub_session: self.access_key.clone(),
+							requestor_signature: requestor_signature.clone(),
+						}))?;
+				}
+			},
+			// we can decrypt data on our own
+			SessionState::WaitingForPartialDecryption => unimplemented!(),
+			// we can not decrypt data
+			SessionState::Failed => (),
+			// cannot reach other states
+			_ => unreachable!("process_initialization_response can change state to WaitingForPartialDecryption or Failed; checked that we are in WaitingForInitializationConfirm state above; qed"),
+		}
+
+		Ok(())
+	}
+
+	/// When session initialization message is received.
+	pub fn on_initialize_session(&self, sender: NodeId, message: InitializeDecryptionSession) -> Result<(), Error> {
+		debug_assert!(self.id == message.session);
+		debug_assert!(self.access_key == message.sub_session);
+		debug_assert!(&sender != self.node());
+
+		let mut data = self.data.lock();
+
+		// check state
+		if data.state != SessionState::WaitingForInitialization {
+			return Err(Error::InvalidStateForRequest);
+		}
+
+		// recover requestor signature
+		let requestor_public = ethkey::recover(&message.requestor_signature, &self.id)?;
+
+		// check access
+		let is_requestor_allowed_to_read = self.acl_storage.check(&requestor_public, &self.id).unwrap_or(false);
+		data.state = if is_requestor_allowed_to_read { SessionState::WaitingForPartialDecryptionRequest }
+			else { SessionState::Failed };
+		data.requestor = Some(requestor_public);
+
+		// respond to master node
+		data.master = Some(sender.clone());
+		self.cluster.send(&sender, Message::ConfirmDecryptionInitialization(ConfirmDecryptionInitialization {
+			session: self.id.clone(),
+			sub_session: self.access_key.clone(),
+			is_confirmed: is_requestor_allowed_to_read,
+		}))
+	}
+
+	/// When session initialization confirmation message is reeived.
+	pub fn on_confirm_initialization(&self, sender: NodeId, message: ConfirmDecryptionInitialization) -> Result<(), Error> {
+		debug_assert!(self.id == message.session);
+		debug_assert!(self.access_key == message.sub_session);
+		debug_assert!(&sender != self.node());
+
+		let mut data = self.data.lock();
+
+		// check state
+		if data.state != SessionState::WaitingForInitializationConfirm {
+			// if there were enough confirmations/rejections before this message
+			// we have already moved to the next state
+			return Ok(());
+		}
+
+		// update state
+		process_initialization_response(&self.encrypted_data, &mut *data, &sender, message.is_confirmed)?;
+
+		// check if we have enough nodes to decrypt data
+		match data.state {
+			// we do not yet have enough nodes for decryption
+			SessionState::WaitingForInitializationConfirm => Ok(()),
+			// we have enough nodes for decryption
+			SessionState::WaitingForPartialDecryption => {
+				let confirmed_nodes: BTreeSet<_> = data.confirmed_nodes.clone();
+				for node in data.confirmed_nodes.iter().filter(|n| n != &self.node()) {
+					self.cluster.send(node, Message::RequestPartialDecryption(RequestPartialDecryption {
+						session: self.id.clone(),
+						sub_session: self.access_key.clone(),
+						nodes: confirmed_nodes.clone(),
+					}))?;
+				}
+
+				assert!(data.confirmed_nodes.remove(self.node()));
+
+				let shadow_point = {
+					let requestor = data.requestor.as_ref().expect("requestor public is filled during initialization; WaitingForPartialDecryption follows initialization; qed");
+					do_partial_decryption(self.node(), &requestor, &data.confirmed_nodes, &self.access_key, &self.encrypted_data)?
+				};
+				data.shadow_points.insert(self.node().clone(), shadow_point);
+
+				Ok(())
+			},
+			// we can not have enough nodes for decryption
+			SessionState::Failed => Ok(()),
+			// cannot reach other states
+			_ => unreachable!("process_initialization_response can change state to WaitingForPartialDecryption or Failed; checked that we are in WaitingForInitializationConfirm state above; qed"),
+		}
+	}
+
+	/// When partial decryption is requested.
+	pub fn on_partial_decryption_requested(&self, sender: NodeId, message: RequestPartialDecryption) -> Result<(), Error> {
+		debug_assert!(self.id == message.session);
+		debug_assert!(self.access_key == message.sub_session);
+		debug_assert!(&sender != self.node());
+
+		// check message
+		if message.nodes.len() != self.encrypted_data.threshold + 1 {
+			return Err(Error::InvalidMessage);
+		}
+
+		let mut data = self.data.lock();
+
+		// check state
+		if data.master != Some(sender) {
+			return Err(Error::InvalidMessage);
+		}
+		if data.state != SessionState::WaitingForPartialDecryptionRequest {
+			return Err(Error::InvalidStateForRequest);
+		}
+
+		// calculate shadow point
+		let shadow_point = {
+			let requestor = data.requestor.as_ref().expect("requestor public is filled during initialization; WaitingForPartialDecryptionRequest follows initialization; qed");
+			do_partial_decryption(self.node(), &requestor, &message.nodes, &self.access_key, &self.encrypted_data)?
+		};
+		self.cluster.send(&sender, Message::PartialDecryption(PartialDecryption {
+			session: self.id.clone(),
+			sub_session: self.access_key.clone(),
+			shadow_point: shadow_point,
+		}))?;
+
+		// update sate
+		data.state = SessionState::Finished;
+
+		Ok(())
+	}
+
+	/// When partial decryption is received.
+	pub fn on_partial_decryption(&self, sender: NodeId, message: PartialDecryption) -> Result<(), Error> {
+		debug_assert!(self.id == message.session);
+		debug_assert!(self.access_key == message.sub_session);
+		debug_assert!(&sender != self.node());
+
+		let mut data = self.data.lock();
+
+		// check state
+		if data.state != SessionState::WaitingForPartialDecryption {
+			return Err(Error::InvalidStateForRequest);
+		}
+
+		if !data.confirmed_nodes.remove(&sender) {
+			return Err(Error::InvalidStateForRequest);
+		}
+		data.shadow_points.insert(sender, message.shadow_point);
+
+		// check if we have enough shadow points to decrypt the secret
+		if data.shadow_points.len() != self.encrypted_data.threshold + 1 {
+			return Ok(());
+		}
+
+		// decrypt the secret using shadow points
+		let joint_shadow_point = math::compute_joint_shadow_point(data.shadow_points.values())?;
+		let decrypted_secret = math::decrypt_with_joint_shadow(&self.access_key, &self.encrypted_data.encrypted_point, &joint_shadow_point)?;
+		data.decrypted_secret = Some(decrypted_secret);
+		data.state = SessionState::Finished;
+
+		Ok(())
+	}
+}
+
+fn check_encrypted_data(self_node_id: &Public, encrypted_data: &EncryptedData) -> Result<(), Error> {
+	use key_server_cluster::encryption_session::{check_cluster_nodes, check_threshold};
+
+	let nodes = encrypted_data.id_numbers.keys().cloned().collect();
+	check_cluster_nodes(self_node_id, &nodes)?;
+	check_threshold(encrypted_data.threshold, &nodes)?;
+
+	Ok(())
+}
+
+fn process_initialization_response(encrypted_data: &EncryptedData, data: &mut SessionData, node: &NodeId, check_result: bool) -> Result<(), Error> {
+	if !data.requested_nodes.remove(node) {
+		return Err(Error::InvalidMessage);
+	}
+
+	match check_result {
+		true => {
+			data.confirmed_nodes.insert(node.clone());
+
+			// check if we have enough nodes to do a decryption?
+			if data.confirmed_nodes.len() == encrypted_data.threshold + 1 {
+				data.state = SessionState::WaitingForPartialDecryption;
+			}
+		},
+		false => {
+			data.rejected_nodes.insert(node.clone());
+
+			// check if we still can receive enough confirmations to do a decryption?
+			if encrypted_data.id_numbers.len() - data.rejected_nodes.len() < encrypted_data.threshold + 1 {
+				data.state = SessionState::Failed;
+			}
+		},
+	}
+
+	Ok(())
+}
+
+fn do_partial_decryption(node: &NodeId, _requestor_public: &Public, participants: &BTreeSet<NodeId>, access_key: &Secret, encrypted_data: &EncryptedData) -> Result<Public, Error> {
+	let node_id_number = &encrypted_data.id_numbers[node];
+	let node_secret_share = &encrypted_data.secret_share;
+	let other_id_numbers = participants.iter()
+		.filter(|id| *id != node)
+		.map(|id| &encrypted_data.id_numbers[id]);
+	// TODO: commutative encryption using _requestor_public
+	let node_shadow = math::compute_node_shadow(node_id_number, node_secret_share, other_id_numbers)?;
+	math::compute_node_shadow_point(access_key, &encrypted_data.common_point, &node_shadow)
+}
+
+#[cfg(test)]
+mod tests {
+	use std::sync::Arc;
+	use std::str::FromStr;
+	use std::collections::BTreeMap;
+	use super::super::super::acl_storage::DummyAclStorage;
+	use ethkey::{self, Random, Generator, Public, Secret};
+	use key_server_cluster::{NodeId, EncryptedData, SessionId, Error};
+	use key_server_cluster::cluster::tests::DummyCluster;
+	use key_server_cluster::decryption_session::{Session, SessionParams, 	SessionState};
+	use key_server_cluster::message::{self, Message};
+
+	const SECRET_PLAIN: &'static str = "d2b57ae7619e070af0af6bc8c703c0cd27814c54d5d6a999cacac0da34ede279ca0d9216e85991029e54e2f0c92ee0bd30237725fa765cbdbfc4529489864c5f";
+
+	fn prepare_decryption_sessions() -> (Vec<Arc<DummyCluster>>, Vec<Arc<DummyAclStorage>>, Vec<Session>) {
+		// prepare encrypted data + cluster configuration for scheme 4-of-5
+		let session_id = SessionId::default();
+		let access_key = Random.generate().unwrap().secret().clone();
+		let secret_shares = vec![
+			Secret::from_str("834cb736f02d9c968dfaf0c37658a1d86ff140554fc8b59c9fdad5a8cf810eec").unwrap(),
+			Secret::from_str("5a3c1d90fafafa66bb808bcc464354a98b05e6b2c95b5f609d4511cdd1b17a0b").unwrap(),
+			Secret::from_str("71bf61e7848e08e3a8486c308ce521bdacfebcf9116a0151447eb301f3a2d0e9").unwrap(),
+			Secret::from_str("80c0e5e2bea66fa9b2e07f7ce09630a9563e8242446d5ee63221feb09c4338f4").unwrap(),
+			Secret::from_str("c06546b5669877ba579ca437a5602e89425c53808c708d44ccd6afcaa4610fad").unwrap(),
+		];
+		let id_numbers: Vec<(NodeId, Secret)> = vec![
+			("b486d3840218837b035c66196ecb15e6b067ca20101e11bd5e626288ab6806ecc70b8307012626bd512bad1559112d11d21025cef48cc7a1d2f3976da08f36c8".into(),
+				Secret::from_str("281b6bf43cb86d0dc7b98e1b7def4a80f3ce16d28d2308f934f116767306f06c").unwrap()),
+			("1395568277679f7f583ab7c0992da35f26cde57149ee70e524e49bdae62db3e18eb96122501e7cbb798b784395d7bb5a499edead0706638ad056d886e56cf8fb".into(),
+				Secret::from_str("00125d85a05e5e63e214cb60fe63f132eec8a103aa29266b7e6e6c5b7597230b").unwrap()),
+			("99e82b163b062d55a64085bacfd407bb55f194ba5fb7a1af9c34b84435455520f1372e0e650a4f91aed0058cb823f62146ccb5599c8d13372c300dea866b69fc".into(),
+				Secret::from_str("f43ac0fba42a5b6ed95707d2244659e89ba877b1c9b82c0d0a9dcf834e80fc62").unwrap()),
+			("7e05df9dd077ec21ed4bc45c9fe9e0a43d65fa4be540630de615ced5e95cf5c3003035eb713317237d7667feeeb64335525158f5f7411f67aca9645169ea554c".into(),
+				Secret::from_str("5a324938dfb2516800487d25ab7289ba8ec38811f77c3df602e4e65e3c9acd9f").unwrap()),
+			("321977760d1d8e15b047a309e4c7fe6f355c10bb5a06c68472b676926427f69f229024fa2692c10da167d14cdc77eb95d0fce68af0a0f704f0d3db36baa83bb2".into(),
+				Secret::from_str("12cf422d50002d04e52bd4906fd7f5f235f051ca36abfe37e061f8da248008d8").unwrap()),
+		];
+		let common_point: Public = "6962be696e1bcbba8e64cc7fddf140f854835354b5804f3bb95ae5a2799130371b589a131bd39699ac7174ccb35fc4342dab05331202209582fc8f3a40916ab0".into();
+		let encrypted_point: Public = "b07031982bde9890e12eff154765f03c56c3ab646ad47431db5dd2d742a9297679c4c65b998557f8008469afd0c43d40b6c5f6c6a1c7354875da4115237ed87a".into();
+		let encrypted_datas: Vec<_> = (0..5).map(|i| EncryptedData {
+			threshold: 3,
+			id_numbers: id_numbers.clone().into_iter().collect(),
+			secret_share: secret_shares[i].clone(),
+			common_point: common_point.clone(),
+			encrypted_point: encrypted_point.clone(),
+		}).collect();
+		let acl_storages: Vec<_> = (0..5).map(|_| Arc::new(DummyAclStorage::default())).collect();
+		let clusters: Vec<_> = (0..5).map(|i| Arc::new(DummyCluster::new(id_numbers.iter().nth(i).clone().unwrap().0))).collect();
+		let sessions: Vec<_> = (0..5).map(|i| Session::new(SessionParams {
+			id: session_id.clone(),
+			access_key: access_key.clone(),
+			self_node_id: id_numbers.iter().nth(i).clone().unwrap().0,
+			encrypted_data: encrypted_datas[i].clone(),
+			acl_storage: acl_storages[i].clone(),
+			cluster: clusters[i].clone()
+		}).unwrap()).collect();
+
+		(clusters, acl_storages, sessions)
+	}
+
+	fn do_messages_exchange(clusters: &[Arc<DummyCluster>], sessions: &[Session]) {
+		do_messages_exchange_until(clusters, sessions, |_, _, _| false);
+	}
+
+	fn do_messages_exchange_until<F>(clusters: &[Arc<DummyCluster>], sessions: &[Session], mut cond: F) where F: FnMut(&NodeId, &NodeId, &Message) -> bool {
+		while let Some((from, to, message)) = clusters.iter().filter_map(|c| c.take_message().map(|(to, msg)| (c.node(), to, msg))).next() {
+			let session = &sessions[sessions.iter().position(|s| s.node() == &to).unwrap()];
+			if cond(&from, &to, &message) {
+				break;
+			}
+
+			match message {
+				Message::InitializeDecryptionSession(message) => session.on_initialize_session(from, message).unwrap(),
+				Message::ConfirmDecryptionInitialization(message) => session.on_confirm_initialization(from, message).unwrap(),
+				Message::RequestPartialDecryption(message) => session.on_partial_decryption_requested(from, message).unwrap(),
+				Message::PartialDecryption(message) => session.on_partial_decryption(from, message).unwrap(),
+				_ => panic!("unexpected"),
+			}
+		}
+	}
+
+	#[test]
+	fn fails_to_construct_in_cluster_of_single_node() {
+		let mut nodes = BTreeMap::new();
+		let self_node_id = Random.generate().unwrap().public().clone();
+		nodes.insert(self_node_id, Random.generate().unwrap().secret().clone());
+		match Session::new(SessionParams {
+			id: SessionId::default(),
+			access_key: Random.generate().unwrap().secret().clone(),
+			self_node_id: self_node_id.clone(),
+			encrypted_data: EncryptedData {
+				threshold: 0,
+				id_numbers: nodes,
+				secret_share: Random.generate().unwrap().secret().clone(),
+				common_point: Random.generate().unwrap().public().clone(),
+				encrypted_point: Random.generate().unwrap().public().clone(),
+			},
+			acl_storage: Arc::new(DummyAclStorage::default()),
+			cluster: Arc::new(DummyCluster::new(self_node_id.clone())),
+		}) {
+			Err(Error::InvalidNodesCount) => (),
+			_ => panic!("unexpected"),
+		}
+	}
+
+	#[test]
+	fn fails_to_construct_if_not_a_part_of_cluster() {
+		let mut nodes = BTreeMap::new();
+		let self_node_id = Random.generate().unwrap().public().clone();
+		nodes.insert(Random.generate().unwrap().public().clone(), Random.generate().unwrap().secret().clone());
+		nodes.insert(Random.generate().unwrap().public().clone(), Random.generate().unwrap().secret().clone());
+		match Session::new(SessionParams {
+			id: SessionId::default(),
+			access_key: Random.generate().unwrap().secret().clone(),
+			self_node_id: self_node_id.clone(),
+			encrypted_data: EncryptedData {
+				threshold: 0,
+				id_numbers: nodes,
+				secret_share: Random.generate().unwrap().secret().clone(),
+				common_point: Random.generate().unwrap().public().clone(),
+				encrypted_point: Random.generate().unwrap().public().clone(),
+			},
+			acl_storage: Arc::new(DummyAclStorage::default()),
+			cluster: Arc::new(DummyCluster::new(self_node_id.clone())),
+		}) {
+			Err(Error::InvalidNodesConfiguration) => (),
+			_ => panic!("unexpected"),
+		}
+	}
+
+	#[test]
+	fn fails_to_construct_if_threshold_is_wrong() {
+		let mut nodes = BTreeMap::new();
+		let self_node_id = Random.generate().unwrap().public().clone();
+		nodes.insert(self_node_id.clone(), Random.generate().unwrap().secret().clone());
+		nodes.insert(Random.generate().unwrap().public().clone(), Random.generate().unwrap().secret().clone());
+		match Session::new(SessionParams {
+			id: SessionId::default(),
+			access_key: Random.generate().unwrap().secret().clone(),
+			self_node_id: self_node_id.clone(),
+			encrypted_data: EncryptedData {
+				threshold: 2,
+				id_numbers: nodes,
+				secret_share: Random.generate().unwrap().secret().clone(),
+				common_point: Random.generate().unwrap().public().clone(),
+				encrypted_point: Random.generate().unwrap().public().clone(),
+			},
+			acl_storage: Arc::new(DummyAclStorage::default()),
+			cluster: Arc::new(DummyCluster::new(self_node_id.clone())),
+		}) {
+			Err(Error::InvalidThreshold) => (),
+			_ => panic!("unexpected"),
+		}
+	}
+
+	#[test]
+	fn fails_to_initialize_when_already_initialized() {
+		let (_, _, sessions) = prepare_decryption_sessions();
+		assert_eq!(sessions[0].initialize(ethkey::sign(Random.generate().unwrap().secret(), &SessionId::default()).unwrap()).unwrap(), ());
+		assert_eq!(sessions[0].initialize(ethkey::sign(Random.generate().unwrap().secret(), &SessionId::default()).unwrap()).unwrap_err(), Error::InvalidStateForRequest);
+	}
+
+	#[test]
+	fn fails_to_accept_initialization_when_already_initialized() {
+		let (_, _, sessions) = prepare_decryption_sessions();
+		assert_eq!(sessions[0].initialize(ethkey::sign(Random.generate().unwrap().secret(), &SessionId::default()).unwrap()).unwrap(), ());
+		assert_eq!(sessions[0].on_initialize_session(sessions[1].node().clone(), message::InitializeDecryptionSession {
+			session: SessionId::default(),
+			sub_session: sessions[0].access_key().clone(),
+			requestor_signature: ethkey::sign(Random.generate().unwrap().secret(), &SessionId::default()).unwrap(),
+		}).unwrap_err(), Error::InvalidStateForRequest);
+	}
+
+	#[test]
+	fn fails_to_partial_decrypt_if_not_waiting() {
+		let (_, _, sessions) = prepare_decryption_sessions();
+		assert_eq!(sessions[1].on_initialize_session(sessions[0].node().clone(), message::InitializeDecryptionSession {
+			session: SessionId::default(),
+			sub_session: sessions[0].access_key().clone(),
+			requestor_signature: ethkey::sign(Random.generate().unwrap().secret(), &SessionId::default()).unwrap(),
+		}).unwrap(), ());
+		assert_eq!(sessions[1].on_partial_decryption_requested(sessions[0].node().clone(), message::RequestPartialDecryption {
+			session: SessionId::default(),
+			sub_session: sessions[0].access_key().clone(),
+			nodes: sessions.iter().map(|s| s.node().clone()).take(4).collect(),
+		}).unwrap(), ());
+		assert_eq!(sessions[1].on_partial_decryption_requested(sessions[0].node().clone(), message::RequestPartialDecryption {
+			session: SessionId::default(),
+			sub_session: sessions[0].access_key().clone(),
+			nodes: sessions.iter().map(|s| s.node().clone()).take(4).collect(),
+		}).unwrap_err(), Error::InvalidStateForRequest);
+	}
+
+	#[test]
+	fn fails_to_partial_decrypt_if_requested_by_slave() {
+		let (_, _, sessions) = prepare_decryption_sessions();
+		assert_eq!(sessions[1].on_initialize_session(sessions[0].node().clone(), message::InitializeDecryptionSession {
+			session: SessionId::default(),
+			sub_session: sessions[0].access_key().clone(),
+			requestor_signature: ethkey::sign(Random.generate().unwrap().secret(), &SessionId::default()).unwrap(),
+		}).unwrap(), ());
+		assert_eq!(sessions[1].on_partial_decryption_requested(sessions[2].node().clone(), message::RequestPartialDecryption {
+			session: SessionId::default(),
+			sub_session: sessions[0].access_key().clone(),
+			nodes: sessions.iter().map(|s| s.node().clone()).take(4).collect(),
+		}).unwrap_err(), Error::InvalidMessage);
+	}
+
+	#[test]
+	fn fails_to_partial_decrypt_if_wrong_number_of_nodes_participating() {
+		let (_, _, sessions) = prepare_decryption_sessions();
+		assert_eq!(sessions[1].on_initialize_session(sessions[0].node().clone(), message::InitializeDecryptionSession {
+			session: SessionId::default(),
+			sub_session: sessions[0].access_key().clone(),
+			requestor_signature: ethkey::sign(Random.generate().unwrap().secret(), &SessionId::default()).unwrap(),
+		}).unwrap(), ());
+		assert_eq!(sessions[1].on_partial_decryption_requested(sessions[0].node().clone(), message::RequestPartialDecryption {
+			session: SessionId::default(),
+			sub_session: sessions[0].access_key().clone(),
+			nodes: sessions.iter().map(|s| s.node().clone()).take(2).collect(),
+		}).unwrap_err(), Error::InvalidMessage);
+	}
+
+	#[test]
+	fn fails_to_accept_partial_decrypt_if_not_waiting() {
+		let (_, _, sessions) = prepare_decryption_sessions();
+		assert_eq!(sessions[0].on_partial_decryption(sessions[1].node().clone(), message::PartialDecryption {
+			session: SessionId::default(),
+			sub_session: sessions[0].access_key().clone(),
+			shadow_point: Random.generate().unwrap().public().clone(),
+		}).unwrap_err(), Error::InvalidStateForRequest);
+	}
+
+	#[test]
+	fn fails_to_accept_partial_decrypt_twice() {
+		let (clusters, _, sessions) = prepare_decryption_sessions();
+		sessions[0].initialize(ethkey::sign(Random.generate().unwrap().secret(), &SessionId::default()).unwrap()).unwrap();
+
+		let mut pd_from = None;
+		let mut pd_msg = None;
+		do_messages_exchange_until(&clusters, &sessions, |from, _, msg| match msg {
+			&Message::PartialDecryption(ref msg) => {
+				pd_from = Some(from.clone());
+				pd_msg = Some(msg.clone());
+				true
+			},
+			_ => false,
+		});
+
+		assert_eq!(sessions[0].on_partial_decryption(pd_from.clone().unwrap(), pd_msg.clone().unwrap()).unwrap(), ());
+		assert_eq!(sessions[0].on_partial_decryption(pd_from.unwrap(), pd_msg.unwrap()).unwrap_err(), Error::InvalidStateForRequest);
+	}
+
+	#[test]
+	fn complete_dec_session() {
+		let (clusters, _, sessions) = prepare_decryption_sessions();
+
+		// now let's try to do a decryption
+		let key_pair = Random.generate().unwrap();
+		let signature = ethkey::sign(key_pair.secret(), &SessionId::default()).unwrap();
+		sessions[0].initialize(signature).unwrap();
+
+		do_messages_exchange(&clusters, &sessions);
+
+		// now check that:
+		// 1) 4 of 5 sessions are in Finished state
+		assert_eq!(sessions.iter().filter(|s| s.state() == SessionState::Finished).count(), 4);
+		// 2) 1 session is in WaitingForPartialDecryptionRequest state
+		assert_eq!(sessions.iter().filter(|s| s.state() == SessionState::WaitingForPartialDecryptionRequest).count(), 1);
+		// 3) 1 session has decrypted key value
+		assert!(sessions.iter().skip(1).all(|s| s.decrypted_secret().is_none()));
+		assert_eq!(sessions[0].decrypted_secret(), Some(SECRET_PLAIN.into()));
+	}
+
+	#[test]
+	fn failed_dec_session() {
+		let (clusters, acl_storages, sessions) = prepare_decryption_sessions();
+
+		// now let's try to do a decryption
+		let key_pair = Random.generate().unwrap();
+		let signature = ethkey::sign(key_pair.secret(), &SessionId::default()).unwrap();
+		sessions[0].initialize(signature).unwrap();
+
+		// we need 4 out of 5 nodes to agree to do a decryption
+		// let's say that 2 of these nodes are disagree
+		acl_storages[1].prohibit(key_pair.public().clone(), SessionId::default());
+		acl_storages[2].prohibit(key_pair.public().clone(), SessionId::default());
+
+		do_messages_exchange(&clusters, &sessions);
+
+		// now check that:
+		// 1) 3 of 5 sessions are in Failed state
+		assert_eq!(sessions.iter().filter(|s| s.state() == SessionState::Failed).count(), 3);
+		// 2) 2 of 5 sessions are in WaitingForPartialDecryptionRequest state
+		assert_eq!(sessions.iter().filter(|s| s.state() == SessionState::WaitingForPartialDecryptionRequest).count(), 2);
+		// 3) 0 sessions have decrypted key value
+		assert!(sessions.iter().all(|s| s.decrypted_secret().is_none()));
+	}
+}
--- a/secret_store/src/key_server_cluster/encryption_session.rs
+++ b/secret_store/src/key_server_cluster/encryption_session.rs
--- a/secret_store/src/key_server_cluster/math.rs
+++ b/secret_store/src/key_server_cluster/math.rs
@ -0,0 +1,338 @@
+// Copyright 2015-2017 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+use ethkey::{Public, Secret, Random, Generator, math};
+use key_server_cluster::Error;
+
+#[derive(Debug)]
+/// Encryption result.
+pub struct EncryptedSecret {
+	/// Common encryption point.
+	pub common_point: Public,
+	/// Ecnrypted point.
+	pub encrypted_point: Public,
+}
+
+/// Generate random scalar
+pub fn generate_random_scalar() -> Result<Secret, Error> {
+	Ok(Random.generate()?.secret().clone())
+}
+
+/// Generate random point
+pub fn generate_random_point() -> Result<Public, Error> {
+	Ok(Random.generate()?.public().clone())
+}
+
+/// Update point by multiplying to random scalar
+pub fn update_random_point(point: &mut Public) -> Result<(), Error> {
+	Ok(math::public_mul_secret(point, &generate_random_scalar()?)?)
+}
+
+/// Generate random polynom of threshold degree
+pub fn generate_random_polynom(threshold: usize) -> Result<Vec<Secret>, Error> {
+	let mut polynom: Vec<_> = Vec::with_capacity(threshold + 1);
+	for _ in 0..threshold + 1 {
+		polynom.push(generate_random_scalar()?);
+	}
+	debug_assert_eq!(polynom.len(), threshold + 1);
+	Ok(polynom)
+}
+
+/// Compute value of polynom, using `node_number` as argument
+pub fn compute_polynom(polynom: &[Secret], node_number: &Secret) -> Result<Secret, Error> {
+	debug_assert!(!polynom.is_empty());
+
+	let mut result = polynom[0].clone();
+	for i in 1..polynom.len() {
+		// calculate pow(node_number, i)
+		let mut appendum = node_number.clone();
+		appendum.pow(i)?;
+
+		// calculate coeff * pow(point, i)
+		appendum.mul(&polynom[i])?;
+
+		// calculate result + coeff * pow(point, i)
+		result.add(&appendum)?;
+	}
+
+	Ok(result)
+}
+
+/// Generate public keys for other participants.
+pub fn public_values_generation(threshold: usize, derived_point: &Public, polynom1: &[Secret], polynom2: &[Secret]) -> Result<Vec<Public>, Error> {
+	debug_assert_eq!(polynom1.len(), threshold + 1);
+	debug_assert_eq!(polynom2.len(), threshold + 1);
+
+	// compute t+1 public values
+	let mut publics = Vec::with_capacity(threshold + 1);
+	for i in 0..threshold + 1 {
+		let coeff1 = &polynom1[i];
+
+		let mut multiplication1 = math::generation_point();
+		math::public_mul_secret(&mut multiplication1, &coeff1)?;
+
+		let coeff2 = &polynom2[i];
+		let mut multiplication2 = derived_point.clone();
+		math::public_mul_secret(&mut multiplication2, &coeff2)?;
+
+		math::public_add(&mut multiplication1, &multiplication2)?;
+
+		publics.push(multiplication1);
+	}
+	debug_assert_eq!(publics.len(), threshold + 1);
+
+	Ok(publics)
+}
+
+/// Check keys passed by other participants.
+pub fn keys_verification(threshold: usize, derived_point: &Public, number_id: &Secret, secret1: &Secret, secret2: &Secret, publics: &[Public]) -> Result<bool, Error> {
+	// calculate left part
+	let mut multiplication1 = math::generation_point();
+	math::public_mul_secret(&mut multiplication1, secret1)?;
+
+	let mut multiplication2 = derived_point.clone();
+	math::public_mul_secret(&mut multiplication2, secret2)?;
+
+	math::public_add(&mut multiplication1, &multiplication2)?;
+	let left = multiplication1;
+
+	// calculate right part
+	let mut right = publics[0].clone();
+	for i in 1..threshold + 1 {
+		let mut secret_pow = number_id.clone();
+		secret_pow.pow(i)?;
+
+		let mut public_k = publics[i].clone();
+		math::public_mul_secret(&mut public_k, &secret_pow)?;
+
+		math::public_add(&mut right, &public_k)?;
+	}
+
+	Ok(left == right)
+}
+
+/// Compute secret share.
+pub fn compute_secret_share<'a, I>(mut secret_values: I) -> Result<Secret, Error> where I: Iterator<Item=&'a Secret> {
+	let mut secret_share = secret_values.next().expect("compute_secret_share is called when cluster has at least one node; qed").clone();
+	while let Some(secret_value) = secret_values.next() {
+		secret_share.add(secret_value)?;
+	}
+	Ok(secret_share)
+}
+
+/// Compute public key share.
+pub fn compute_public_share(self_secret_value: &Secret) -> Result<Public, Error> {
+	let mut public_share = math::generation_point();
+	math::public_mul_secret(&mut public_share, self_secret_value)?;
+	Ok(public_share)
+}
+
+/// Compute joint public key.
+pub fn compute_joint_public<'a, I>(mut public_shares: I) -> Result<Public, Error> where I: Iterator<Item=&'a Public> {
+	let mut joint_public = public_shares.next().expect("compute_joint_public is called when cluster has at least one node; qed").clone();
+	while let Some(public_share) = public_shares.next() {
+		math::public_add(&mut joint_public, &public_share)?;
+	}
+	Ok(joint_public)
+}
+
+#[cfg(test)]
+/// Compute joint secret key.
+pub fn compute_joint_secret<'a, I>(mut secret_coeffs: I) -> Result<Secret, Error> where I: Iterator<Item=&'a Secret> {
+	let mut joint_secret = secret_coeffs.next().expect("compute_joint_private is called when cluster has at least one node; qed").clone();
+	while let Some(secret_coeff) = secret_coeffs.next() {
+		joint_secret.add(secret_coeff)?;
+	}
+	Ok(joint_secret)
+}
+
+/// Encrypt secret with joint public key.
+pub fn encrypt_secret(secret: Public, joint_public: &Public) -> Result<EncryptedSecret, Error> {
+	// this is performed by KS-cluster client (or KS master)
+	let key_pair = Random.generate()?;
+
+	// k * T
+	let mut common_point = math::generation_point();
+	math::public_mul_secret(&mut common_point, key_pair.secret())?;
+
+	// M + k * y
+	let mut encrypted_point = joint_public.clone();
+	math::public_mul_secret(&mut encrypted_point, key_pair.secret())?;
+	math::public_add(&mut encrypted_point, &secret)?;
+
+	Ok(EncryptedSecret {
+		common_point: common_point,
+		encrypted_point: encrypted_point,
+	})
+}
+
+/// Compute shadow for the node.
+pub fn compute_node_shadow<'a, I>(node_number: &Secret, node_secret_share: &Secret, mut other_nodes_numbers: I) -> Result<Secret, Error> where I: Iterator<Item=&'a Secret> {
+	let other_node_number = other_nodes_numbers.next().expect("compute_node_shadow is called when at least two nodes are required to decrypt secret; qed");
+	let mut shadow = node_number.clone();
+	shadow.sub(other_node_number)?;
+	shadow.inv()?;
+	shadow.mul(other_node_number)?;
+	while let Some(other_node_number) = other_nodes_numbers.next() {
+		let mut shadow_element = node_number.clone();
+		shadow_element.sub(other_node_number)?;
+		shadow_element.inv()?;
+		shadow_element.mul(other_node_number)?;
+		shadow.mul(&shadow_element)?;
+	}
+
+	shadow.mul(&node_secret_share)?;
+	Ok(shadow)
+}
+
+/// Compute shadow point for the node.
+pub fn compute_node_shadow_point(access_key: &Secret, common_point: &Public, node_shadow: &Secret) -> Result<Public, Error> {
+	let mut shadow_key = access_key.clone();
+	shadow_key.mul(node_shadow)?;
+	let mut node_shadow_point = common_point.clone();
+	math::public_mul_secret(&mut node_shadow_point, &shadow_key)?;
+	Ok(node_shadow_point)
+}
+
+/// Compute joint shadow point.
+pub fn compute_joint_shadow_point<'a, I>(mut nodes_shadow_points: I) -> Result<Public, Error> where I: Iterator<Item=&'a Public> {
+	let mut joint_shadow_point = nodes_shadow_points.next().expect("compute_joint_shadow_point is called when at least two nodes are required to decrypt secret; qed").clone();
+	while let Some(node_shadow_point) = nodes_shadow_points.next() {
+		math::public_add(&mut joint_shadow_point, &node_shadow_point)?;
+	}
+	Ok(joint_shadow_point)
+}
+
+#[cfg(test)]
+/// Compute joint shadow point (version for tests).
+pub fn compute_joint_shadow_point_test<'a, I>(access_key: &Secret, common_point: &Public, mut nodes_shadows: I) -> Result<Public, Error> where I: Iterator<Item=&'a Secret> {
+	let mut joint_shadow = nodes_shadows.next().expect("compute_joint_shadow_point_test is called when at least two nodes are required to decrypt secret; qed").clone();
+	while let Some(node_shadow) = nodes_shadows.next() {
+		joint_shadow.add(node_shadow)?;
+	}
+	joint_shadow.mul(access_key)?;
+
+	let mut joint_shadow_point = common_point.clone();
+	math::public_mul_secret(&mut joint_shadow_point, &joint_shadow)?;
+	Ok(joint_shadow_point)
+}
+
+/// Decrypt data using joint shadow point.
+pub fn decrypt_with_joint_shadow(access_key: &Secret, encrypted_point: &Public, joint_shadow_point: &Public) -> Result<Public, Error> {
+	let mut inv_access_key = access_key.clone();
+	inv_access_key.inv()?;
+	
+	let mut decrypted_point = joint_shadow_point.clone();
+	math::public_mul_secret(&mut decrypted_point, &inv_access_key)?;
+	math::public_add(&mut decrypted_point, encrypted_point)?;
+
+	Ok(decrypted_point)
+}
+
+/// Decrypt data using joint secret (version for tests).
+pub fn decrypt_with_joint_secret(encrypted_point: &Public, common_point: &Public, joint_secret: &Secret) -> Result<Public, Error> {
+	let mut common_point_mul = common_point.clone();
+	math::public_mul_secret(&mut common_point_mul, joint_secret)?;
+
+	let mut decrypted_point = encrypted_point.clone();
+	math::public_sub(&mut decrypted_point, &common_point_mul)?;
+
+	Ok(decrypted_point)
+}
+
+#[cfg(test)]
+pub mod tests {
+	use ethkey::KeyPair;
+	use super::*;
+
+	pub fn do_encryption_and_decryption(t: usize, joint_public: &Public, id_numbers: &[Secret], secret_shares: &[Secret], joint_secret: Option<&Secret>, document_secret_plain: Public) -> (Public, Public) {
+		// === PART2: encryption using joint public key ===
+
+		// the next line is executed on KeyServer-client
+		let encrypted_secret = encrypt_secret(document_secret_plain.clone(), &joint_public).unwrap();
+
+		// === PART3: decryption ===
+
+		// next line is executed on KeyServer client
+		let access_key = generate_random_scalar().unwrap();
+
+		// use t + 1 nodes to compute joint shadow point
+		let nodes_shadows: Vec<_> = (0..t + 1).map(|i|
+			compute_node_shadow(&id_numbers[i], &secret_shares[i], id_numbers.iter()
+				.enumerate()
+				.filter(|&(j, _)| j != i)
+				.take(t)
+				.map(|(_, id_number)| id_number)).unwrap()).collect();
+		let nodes_shadow_points: Vec<_> = nodes_shadows.iter().map(|s| compute_node_shadow_point(&access_key, &encrypted_secret.common_point, s).unwrap()).collect();
+		assert_eq!(nodes_shadows.len(), t + 1);
+		assert_eq!(nodes_shadow_points.len(), t + 1);
+
+		let joint_shadow_point = compute_joint_shadow_point(nodes_shadow_points.iter()).unwrap();
+		let joint_shadow_point_test = compute_joint_shadow_point_test(&access_key, &encrypted_secret.common_point, nodes_shadows.iter()).unwrap();
+		assert_eq!(joint_shadow_point, joint_shadow_point_test);
+
+		// decrypt encrypted secret using joint shadow point
+		let document_secret_decrypted = decrypt_with_joint_shadow(&access_key, &encrypted_secret.encrypted_point, &joint_shadow_point).unwrap();
+
+		// decrypt encrypted secret using joint secret [just for test]
+		let document_secret_decrypted_test = match joint_secret {
+			Some(joint_secret) => decrypt_with_joint_secret(&encrypted_secret.encrypted_point, &encrypted_secret.common_point, joint_secret).unwrap(),
+			None => document_secret_decrypted.clone(),
+		};
+
+		(document_secret_decrypted, document_secret_decrypted_test)
+	}
+
+	#[test]
+	fn full_encryption_math_session() {
+		let test_cases = [(1, 3)];
+		for &(t, n) in &test_cases {
+			// === PART1: DKG ===
+			
+			// data, gathered during initialization
+			let id_numbers: Vec<_> = (0..n).map(|_| generate_random_scalar().unwrap()).collect();
+
+			// data, generated during keys dissemination
+			let polynoms1: Vec<_> = (0..n).map(|_| generate_random_polynom(t).unwrap()).collect();
+			let secrets1: Vec<_> = (0..n).map(|i| (0..n).map(|j| compute_polynom(&polynoms1[i], &id_numbers[j]).unwrap()).collect::<Vec<_>>()).collect();
+
+			// data, generated during keys generation
+			let public_shares: Vec<_> = (0..n).map(|i| compute_public_share(&polynoms1[i][0]).unwrap()).collect();
+			let secret_shares: Vec<_> = (0..n).map(|i| compute_secret_share(secrets1.iter().map(|s| &s[i])).unwrap()).collect();
+
+			// joint public key, as a result of DKG
+			let joint_public = compute_joint_public(public_shares.iter()).unwrap();
+
+			// compute joint private key [just for test]
+			let joint_secret = compute_joint_secret(polynoms1.iter().map(|p| &p[0])).unwrap();
+			let joint_key_pair = KeyPair::from_secret(joint_secret.clone()).unwrap();
+			assert_eq!(&joint_public, joint_key_pair.public());
+
+			// check secret shares computation [just for test]
+			let secret_shares_polynom: Vec<_> = (0..t + 1).map(|k| compute_secret_share(polynoms1.iter().map(|p| &p[k])).unwrap()).collect();
+			let secret_shares_calculated_from_polynom: Vec<_> = id_numbers.iter().map(|id_number| compute_polynom(&*secret_shares_polynom, id_number).unwrap()).collect();
+			assert_eq!(secret_shares, secret_shares_calculated_from_polynom);
+
+			// now encrypt and decrypt data
+			let document_secret_plain = generate_random_point().unwrap();
+			let (document_secret_decrypted, document_secret_decrypted_test) =
+				do_encryption_and_decryption(t, &joint_public, &id_numbers, &secret_shares, Some(&joint_secret), document_secret_plain.clone());
+
+			assert_eq!(document_secret_plain, document_secret_decrypted_test);
+			assert_eq!(document_secret_plain, document_secret_decrypted);
+		}
+	}
+}
--- a/secret_store/src/key_server_cluster/message.rs
+++ b/secret_store/src/key_server_cluster/message.rs
@ -0,0 +1,168 @@
+// Copyright 2015-2017 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+use std::collections::{BTreeSet, BTreeMap};
+use ethkey::{Public, Secret, Signature};
+use key_server_cluster::{NodeId, SessionId};
+
+#[derive(Clone, Debug)]
+/// All possible messages that can be sent during DKG.
+pub enum Message {
+	/// Initialize new DKG session.
+	InitializeSession(InitializeSession),
+	/// Confirm DKG session initialization.
+	ConfirmInitialization(ConfirmInitialization),
+	/// Broadcast data, calculated during session initialization phase.
+	CompleteInitialization(CompleteInitialization),
+	/// Generated keys are sent to every node.
+	KeysDissemination(KeysDissemination),
+	/// Complaint against another node is broadcasted.
+	Complaint(Complaint),
+	/// Complaint response is broadcasted.
+	ComplaintResponse(ComplaintResponse),
+	/// Broadcast self public key portion.
+	PublicKeyShare(PublicKeyShare),
+
+	/// Initialize decryption session.
+	InitializeDecryptionSession(InitializeDecryptionSession),
+	/// Confirm/reject decryption session initialization.
+	ConfirmDecryptionInitialization(ConfirmDecryptionInitialization),
+	/// Request partial decryption from node.
+	RequestPartialDecryption(RequestPartialDecryption),
+	/// Partial decryption is completed
+	PartialDecryption(PartialDecryption),
+}
+
+#[derive(Clone, Debug)]
+/// Initialize new DKG session.
+pub struct InitializeSession {
+	/// Session Id.
+	pub session: SessionId,
+	/// Derived generation point. Starting from originator, every node must multiply this
+	/// point by random scalar (unknown by other nodes). At the end of initialization
+	/// `point` will be some (k1 * k2 * ... * kn) * G = `point` where `(k1 * k2 * ... * kn)`
+	/// is unknown for every node.
+	pub derived_point: Public,
+}
+
+#[derive(Clone, Debug)]
+/// Confirm DKG session initialization.
+pub struct ConfirmInitialization {
+	/// Session Id.
+	pub session: SessionId,
+	/// Derived generation point.
+	pub derived_point: Public,
+}
+
+#[derive(Clone, Debug)]
+/// Broadcast generated point to every other node.
+pub struct CompleteInitialization {
+	/// Session Id.
+	pub session: SessionId,
+	/// All session participants along with their identification numbers.
+	pub nodes: BTreeMap<NodeId, Secret>,
+	/// Decryption threshold. During decryption threshold-of-route.len() nodes must came to
+	/// consensus to successfully decrypt message.
+	pub threshold: usize,
+	/// Derived generation point.
+	pub derived_point: Public,
+}
+
+#[derive(Clone, Debug)]
+/// Generated keys are sent to every node.
+pub struct KeysDissemination {
+	/// Session Id.
+	pub session: SessionId,
+	/// Secret 1.
+	pub secret1: Secret,
+	/// Secret 2.
+	pub secret2: Secret,
+	/// Public values.
+	pub publics: Vec<Public>,
+}
+
+#[derive(Clone, Debug)]
+/// Complaint against node is broadcasted.
+pub struct Complaint {
+	/// Session Id.
+	pub session: SessionId,
+	/// Public values.
+	pub against: NodeId,
+}
+
+#[derive(Clone, Debug)]
+/// Node is responding to complaint.
+pub struct ComplaintResponse {
+	/// Session Id.
+	pub session: SessionId,
+	/// Secret 1.
+	pub secret1: Secret,
+	/// Secret 2.
+	pub secret2: Secret,
+}
+
+#[derive(Clone, Debug)]
+/// Node is sharing its public key share.
+pub struct PublicKeyShare {
+	/// Session Id.
+	pub session: SessionId,
+	/// Public key share.
+	pub public_share: Public,
+}
+
+#[derive(Clone, Debug)]
+/// Node is requested to decrypt data, encrypted in given session.
+pub struct InitializeDecryptionSession {
+	/// Encryption session Id.
+	pub session: SessionId,
+	/// Decryption session Id.
+	pub sub_session: Secret,
+	/// Requestor signature.
+	pub requestor_signature: Signature,
+}
+
+#[derive(Clone, Debug)]
+/// Node is responding to decryption request.
+pub struct ConfirmDecryptionInitialization {
+	/// Encryption session Id.
+	pub session: SessionId,
+	/// Decryption session Id.
+	pub sub_session: Secret,
+	/// Is node confirmed to make a decryption?.
+	pub is_confirmed: bool,
+}
+
+#[derive(Clone, Debug)]
+/// Node is requested to do a partial decryption.
+pub struct RequestPartialDecryption {
+	/// Encryption session Id.
+	pub session: SessionId,
+	/// Decryption session Id.
+	pub sub_session: Secret,
+	/// Nodes that are agreed to do a decryption.
+	pub nodes: BTreeSet<NodeId>,
+}
+
+#[derive(Clone, Debug)]
+/// Node has partially decrypted the secret.
+pub struct PartialDecryption {
+	/// Encryption session Id.
+	pub session: SessionId,
+	/// Decryption session Id.
+	pub sub_session: Secret,
+	/// Partially decrypted secret.
+	pub shadow_point: Public,
+}
--- a/secret_store/src/key_server_cluster/mod.rs
+++ b/secret_store/src/key_server_cluster/mod.rs
@ -0,0 +1,76 @@
+// Copyright 2015-2017 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+#![allow(dead_code)] // TODO: remove me
+
+use std::collections::BTreeMap;
+use ethkey::{self, Public, Secret, Signature};
+use super::types::all::DocumentAddress;
+
+pub use super::acl_storage::AclStorage;
+
+pub type NodeId = Public;
+pub type SessionId = DocumentAddress;
+pub type SessionIdSignature = Signature;
+
+#[derive(Debug, PartialEq)]
+/// Errors which can occur during encryption/decryption session
+pub enum Error {
+	/// Invalid number of nodes.
+	/// There must be at least two nodes participating in encryption.
+	/// There must be at least one node participating in decryption.
+	InvalidNodesCount,
+	/// Node which is required to start encryption/decryption session is not a part of cluster.
+	InvalidNodesConfiguration,
+	/// Invalid threshold value has been passed.
+	/// Threshold value must be in [0; n - 1], where n is a number of nodes participating in the encryption.
+	InvalidThreshold,
+	/// Current state of encryption/decryption session does not allow to proceed request.
+	/// This means that either there is some comm-failure or node is misbehaving/cheating.
+	InvalidStateForRequest,
+	/// Some data in passed message was recognized as invalid.
+	/// This means that node is misbehaving/cheating.
+	InvalidMessage,
+	/// Cryptographic error.
+	EthKey(String),
+}
+
+#[derive(Debug, Clone)]
+/// Data, which is stored on every node after DKG && encryption is completed.
+pub struct EncryptedData {
+	/// Decryption threshold (at least threshold + 1 nodes are required to decrypt data).
+	threshold: usize,
+	/// Nodes ids numbers.
+	id_numbers: BTreeMap<NodeId, Secret>,
+	/// Node secret share.
+	secret_share: Secret,
+	/// Common (shared) encryption point.
+	common_point: Public,
+	/// Encrypted point.
+	encrypted_point: Public,
+}
+
+impl From<ethkey::Error> for Error {
+	fn from(err: ethkey::Error) -> Self {
+		Error::EthKey(err.into())
+	}
+}
+
+mod cluster;
+mod decryption_session;
+mod encryption_session;
+mod math;
+mod message;
--- a/secret_store/src/lib.rs
+++ b/secret_store/src/lib.rs
@ -26,6 +26,7 @@ extern crate ethcore_ipc as ipc;
 extern crate ethcrypto;
 extern crate ethkey;

+mod key_server_cluster;
 mod types;

 mod traits {