From 7f784e785252f2d79d9fc5935cf4960b1ca2c6b3 Mon Sep 17 00:00:00 2001
From: Andreas Fackler <AndreasFackler@gmx.de>
Date: Wed, 7 Nov 2018 14:26:14 +0100
Subject: [PATCH] Make Step independent of DistAlgorithm.

---
 examples/simulation.rs                    |  8 +-
 src/binary_agreement/binary_agreement.rs  | 46 +++++------
 src/binary_agreement/mod.rs               |  2 +-
 src/binary_agreement/sbv_broadcast.rs     | 76 +++++++----------
 src/broadcast/broadcast.rs                |  2 +-
 src/dynamic_honey_badger/mod.rs           |  2 +-
 src/honey_badger/epoch_state.rs           |  2 +-
 src/honey_badger/honey_badger.rs          |  2 +-
 src/lib.rs                                |  4 +-
 src/queueing_honey_badger/mod.rs          | 27 +++----
 src/sender_queue/dynamic_honey_badger.rs  | 10 ++-
 src/sender_queue/mod.rs                   | 36 ++++-----
 src/sender_queue/queueing_honey_badger.rs | 11 +--
 src/subset/proposal_state.rs              | 52 ++++--------
 src/subset/subset.rs                      | 14 ++--
 src/threshold_decrypt.rs                  |  2 +-
 src/threshold_sign.rs                     |  2 +-
 src/traits.rs                             | 99 +++++++++--------------
 tests/binary_agreement_mitm.rs            |  6 +-
 tests/net/adversary.rs                    |  6 +-
 tests/net/mod.rs                          | 24 +++---
 tests/network/mod.rs                      |  8 +-
 22 files changed, 187 insertions(+), 254 deletions(-)

diff --git a/examples/simulation.rs b/examples/simulation.rs
index 52a297f..bae54e4 100644
--- a/examples/simulation.rs
+++ b/examples/simulation.rs
@@ -27,7 +27,7 @@ use signifix::{metric, TryFrom};
 use hbbft::dynamic_honey_badger::DynamicHoneyBadger;
 use hbbft::queueing_honey_badger::{Batch, QueueingHoneyBadger};
 use hbbft::sender_queue::{Message, SenderQueue};
-use hbbft::{DistAlgorithm, NetworkInfo, Step, Target};
+use hbbft::{DaStep, DistAlgorithm, NetworkInfo, Step, Target};
 
 const VERSION: &str = env!("CARGO_PKG_VERSION");
 const USAGE: &str = "
@@ -137,14 +137,14 @@ pub struct TestNode<D: DistAlgorithm> {
     hw_quality: HwQuality,
 }
 
-type TestNodeStepResult<D> = Step<D>;
+type TestNodeStepResult<D> = DaStep<D>;
 
 impl<D: DistAlgorithm> TestNode<D>
 where
     D::Message: Serialize + DeserializeOwned,
 {
     /// Creates a new test node with the given broadcast instance.
-    fn new((algo, step): (D, Step<D>), hw_quality: HwQuality) -> TestNode<D> {
+    fn new((algo, step): (D, DaStep<D>), hw_quality: HwQuality) -> TestNode<D> {
         let out_queue = step
             .messages
             .into_iter()
@@ -264,7 +264,7 @@ where
         hw_quality: HwQuality,
     ) -> TestNetwork<D>
     where
-        F: Fn(NetworkInfo<NodeId>) -> (D, Step<D>),
+        F: Fn(NetworkInfo<NodeId>) -> (D, DaStep<D>),
     {
         let node_ids = (0..(good_num + adv_num)).map(NodeId);
         let netinfos = NetworkInfo::generate_map(node_ids, &mut rand::thread_rng())
diff --git a/src/binary_agreement/binary_agreement.rs b/src/binary_agreement/binary_agreement.rs
index 08ff3be..fcd781b 100644
--- a/src/binary_agreement/binary_agreement.rs
+++ b/src/binary_agreement/binary_agreement.rs
@@ -78,12 +78,12 @@ impl<N: NodeIdT, S: SessionIdT> DistAlgorithm for BinaryAgreement<N, S> {
     type Message = Message;
     type Error = Error;
 
-    fn handle_input(&mut self, input: Self::Input) -> Result<Step<N, S>> {
+    fn handle_input(&mut self, input: Self::Input) -> Result<Step<N>> {
         self.propose(input)
     }
 
     /// Receive input from a remote node.
-    fn handle_message(&mut self, sender_id: &Self::NodeId, msg: Message) -> Result<Step<N, S>> {
+    fn handle_message(&mut self, sender_id: &Self::NodeId, msg: Message) -> Result<Step<N>> {
         self.handle_message(sender_id, msg)
     }
 
@@ -122,20 +122,20 @@ impl<N: NodeIdT, S: SessionIdT> BinaryAgreement<N, S> {
     /// output. Otherwise either output is possible.
     ///
     /// Note that if `can_propose` returns `false`, it is already too late to affect the outcome.
-    pub fn propose(&mut self, input: bool) -> Result<Step<N, S>> {
+    pub fn propose(&mut self, input: bool) -> Result<Step<N>> {
         if !self.can_propose() {
             return Ok(Step::default());
         }
         // Set the initial estimated value to the input value.
         self.estimated = Some(input);
-        let sbvb_step = self.sbv_broadcast.handle_input(input)?;
+        let sbvb_step = self.sbv_broadcast.send_bval(input)?;
         self.handle_sbvb_step(sbvb_step)
     }
 
     /// Handles a message received from `sender_id`.
     ///
     /// This must be called with every message we receive from another node.
-    pub fn handle_message(&mut self, sender_id: &N, msg: Message) -> Result<Step<N, S>> {
+    pub fn handle_message(&mut self, sender_id: &N, msg: Message) -> Result<Step<N>> {
         let Message { epoch, content } = msg;
         if self.decision.is_some() || (epoch < self.epoch && content.can_expire()) {
             // Message is obsolete: We are already in a later epoch or terminated.
@@ -161,9 +161,9 @@ impl<N: NodeIdT, S: SessionIdT> BinaryAgreement<N, S> {
         &mut self,
         sender_id: &N,
         content: MessageContent,
-    ) -> Result<Step<N, S>> {
+    ) -> Result<Step<N>> {
         match content {
-            MessageContent::SbvBroadcast(msg) => self.handle_sbv_broadcast(sender_id, msg),
+            MessageContent::SbvBroadcast(msg) => self.handle_sbv_broadcast(sender_id, &msg),
             MessageContent::Conf(v) => self.handle_conf(sender_id, v),
             MessageContent::Term(v) => self.handle_term(sender_id, v),
             MessageContent::Coin(msg) => self.handle_coin(sender_id, *msg),
@@ -174,15 +174,15 @@ impl<N: NodeIdT, S: SessionIdT> BinaryAgreement<N, S> {
     fn handle_sbv_broadcast(
         &mut self,
         sender_id: &N,
-        msg: sbv_broadcast::Message,
-    ) -> Result<Step<N, S>> {
-        let sbvb_step = self.sbv_broadcast.handle_message(sender_id, msg)?;
+        msg: &sbv_broadcast::Message,
+    ) -> Result<Step<N>> {
+        let sbvb_step = self.sbv_broadcast.handle_message(sender_id, &msg)?;
         self.handle_sbvb_step(sbvb_step)
     }
 
     /// Handles a Synchronized Binary Value Broadcast step. On output, starts the `Conf` round or
     /// decides.
-    fn handle_sbvb_step(&mut self, sbvb_step: sbv_broadcast::Step<N>) -> Result<Step<N, S>> {
+    fn handle_sbvb_step(&mut self, sbvb_step: sbv_broadcast::Step<N>) -> Result<Step<N>> {
         let mut step = Step::default();
         let output = step.extend_with(sbvb_step, |msg| {
             MessageContent::SbvBroadcast(msg).with_epoch(self.epoch)
@@ -208,7 +208,7 @@ impl<N: NodeIdT, S: SessionIdT> BinaryAgreement<N, S> {
 
     /// Handles a `Conf` message. When _N - f_ `Conf` messages with values in `bin_values` have
     /// been received, updates the epoch or decides.
-    fn handle_conf(&mut self, sender_id: &N, v: BoolSet) -> Result<Step<N, S>> {
+    fn handle_conf(&mut self, sender_id: &N, v: BoolSet) -> Result<Step<N>> {
         self.received_conf.insert(sender_id.clone(), v);
         self.try_finish_conf_round()
     }
@@ -216,7 +216,7 @@ impl<N: NodeIdT, S: SessionIdT> BinaryAgreement<N, S> {
     /// Handles a `Term(v)` message. If we haven't yet decided on a value and there are more than
     /// _f_ such messages with the same value from different nodes, performs expedite termination:
     /// decides on `v`, broadcasts `Term(v)` and terminates the instance.
-    fn handle_term(&mut self, sender_id: &N, b: bool) -> Result<Step<N, S>> {
+    fn handle_term(&mut self, sender_id: &N, b: bool) -> Result<Step<N>> {
         self.received_term[b].insert(sender_id.clone());
         // Check for the expedite termination condition.
         if self.decision.is_some() {
@@ -234,7 +234,7 @@ impl<N: NodeIdT, S: SessionIdT> BinaryAgreement<N, S> {
 
     /// Handles a `ThresholdSign` message. If there is output, starts the next epoch. The function
     /// may output a decision value.
-    fn handle_coin(&mut self, sender_id: &N, msg: threshold_sign::Message) -> Result<Step<N, S>> {
+    fn handle_coin(&mut self, sender_id: &N, msg: threshold_sign::Message) -> Result<Step<N>> {
         let ts_step = match self.coin_state {
             CoinState::Decided(_) => return Ok(Step::default()), // Coin value is already decided.
             CoinState::InProgress(ref mut ts) => ts
@@ -245,7 +245,7 @@ impl<N: NodeIdT, S: SessionIdT> BinaryAgreement<N, S> {
     }
 
     /// Multicasts a `Conf(values)` message, and handles it.
-    fn send_conf(&mut self, values: BoolSet) -> Result<Step<N, S>> {
+    fn send_conf(&mut self, values: BoolSet) -> Result<Step<N>> {
         if self.conf_values.is_some() {
             // Only one `Conf` message is allowed in an epoch.
             return Ok(Step::default());
@@ -262,11 +262,11 @@ impl<N: NodeIdT, S: SessionIdT> BinaryAgreement<N, S> {
     }
 
     /// Multicasts and handles a message. Does nothing if we are only an observer.
-    fn send(&mut self, content: MessageContent) -> Result<Step<N, S>> {
+    fn send(&mut self, content: MessageContent) -> Result<Step<N>> {
         if !self.netinfo.is_validator() {
             return Ok(Step::default());
         }
-        let step: Step<N, S> = Target::All
+        let step: Step<N> = Target::All
             .message(content.clone().with_epoch(self.epoch))
             .into();
         let our_id = &self.our_id().clone();
@@ -274,7 +274,7 @@ impl<N: NodeIdT, S: SessionIdT> BinaryAgreement<N, S> {
     }
 
     /// Handles a step returned from the `ThresholdSign`.
-    fn on_coin_step(&mut self, ts_step: threshold_sign::Step<N>) -> Result<Step<N, S>> {
+    fn on_coin_step(&mut self, ts_step: threshold_sign::Step<N>) -> Result<Step<N>> {
         let mut step = Step::default();
         let epoch = self.epoch;
         let to_msg = |c_msg| MessageContent::Coin(Box::new(c_msg)).with_epoch(epoch);
@@ -293,7 +293,7 @@ impl<N: NodeIdT, S: SessionIdT> BinaryAgreement<N, S> {
     /// With two conf values, the next epoch's estimate is the coin value. If there is only one conf
     /// value and that disagrees with the coin, the conf value is the next epoch's estimate. If
     /// the unique conf value agrees with the coin, terminates and decides on that value.
-    fn try_update_epoch(&mut self) -> Result<Step<N, S>> {
+    fn try_update_epoch(&mut self) -> Result<Step<N>> {
         if self.decision.is_some() {
             // Avoid an infinite regression without making a Binary Agreement step.
             return Ok(Step::default());
@@ -330,7 +330,7 @@ impl<N: NodeIdT, S: SessionIdT> BinaryAgreement<N, S> {
     }
 
     /// Decides on a value and broadcasts a `Term` message with that value.
-    fn decide(&mut self, b: bool) -> Step<N, S> {
+    fn decide(&mut self, b: bool) -> Step<N> {
         if self.decision.is_some() {
             return Step::default();
         }
@@ -348,7 +348,7 @@ impl<N: NodeIdT, S: SessionIdT> BinaryAgreement<N, S> {
     }
 
     /// Checks whether the _N - f_ `Conf` messages have arrived, and if so, activates the coin.
-    fn try_finish_conf_round(&mut self) -> Result<Step<N, S>> {
+    fn try_finish_conf_round(&mut self) -> Result<Step<N>> {
         if self.conf_values.is_none() || self.count_conf() < self.netinfo.num_correct() {
             return Ok(Step::default());
         }
@@ -368,7 +368,7 @@ impl<N: NodeIdT, S: SessionIdT> BinaryAgreement<N, S> {
     }
 
     /// Increments the epoch, sets the new estimate and handles queued messages.
-    fn update_epoch(&mut self, b: bool) -> Result<Step<N, S>> {
+    fn update_epoch(&mut self, b: bool) -> Result<Step<N>> {
         self.sbv_broadcast.clear(&self.received_term);
         self.received_conf.clear();
         for (v, id) in &self.received_term {
@@ -384,7 +384,7 @@ impl<N: NodeIdT, S: SessionIdT> BinaryAgreement<N, S> {
         );
 
         self.estimated = Some(b);
-        let sbvb_step = self.sbv_broadcast.handle_input(b)?;
+        let sbvb_step = self.sbv_broadcast.send_bval(b)?;
         let mut step = self.handle_sbvb_step(sbvb_step)?;
         let queued_msgs = self
             .incoming_queue
diff --git a/src/binary_agreement/mod.rs b/src/binary_agreement/mod.rs
index 953c6b8..a504cfd 100644
--- a/src/binary_agreement/mod.rs
+++ b/src/binary_agreement/mod.rs
@@ -103,7 +103,7 @@ impl From<bincode::Error> for Error {
 /// An Binary Agreement result.
 pub type Result<T> = ::std::result::Result<T, Error>;
 
-pub type Step<N, T> = ::Step<BinaryAgreement<N, T>>;
+pub type Step<N> = ::Step<Message, bool, N>;
 
 #[derive(Serialize, Deserialize, Clone, Debug, PartialEq)]
 pub enum MessageContent {
diff --git a/src/binary_agreement/sbv_broadcast.rs b/src/binary_agreement/sbv_broadcast.rs
index 9ba1413..188bf43 100644
--- a/src/binary_agreement/sbv_broadcast.rs
+++ b/src/binary_agreement/sbv_broadcast.rs
@@ -16,11 +16,11 @@ use serde_derive::{Deserialize, Serialize};
 
 use super::bool_multimap::BoolMultimap;
 use super::bool_set::{self, BoolSet};
-use super::{Error, Result};
+use super::Result;
 use fault_log::{Fault, FaultKind};
-use {DistAlgorithm, NetworkInfo, NodeIdT, Target};
+use {NetworkInfo, NodeIdT, Target};
 
-pub type Step<N> = ::Step<SbvBroadcast<N>>;
+pub type Step<N> = ::Step<Message, BoolSet, N>;
 
 #[derive(Serialize, Deserialize, Clone, Debug, PartialEq)]
 pub enum Message {
@@ -59,33 +59,6 @@ pub struct SbvBroadcast<N> {
     terminated: bool,
 }
 
-impl<N: NodeIdT> DistAlgorithm for SbvBroadcast<N> {
-    type NodeId = N;
-    type Input = bool;
-    type Output = BoolSet;
-    type Message = Message;
-    type Error = Error;
-
-    fn handle_input(&mut self, input: Self::Input) -> Result<Step<N>> {
-        self.send_bval(input)
-    }
-
-    fn handle_message(&mut self, sender_id: &Self::NodeId, msg: Self::Message) -> Result<Step<N>> {
-        match msg {
-            Message::BVal(b) => self.handle_bval(sender_id, b),
-            Message::Aux(b) => self.handle_aux(sender_id, b),
-        }
-    }
-
-    fn terminated(&self) -> bool {
-        self.terminated
-    }
-
-    fn our_id(&self) -> &Self::NodeId {
-        self.netinfo.our_id()
-    }
-}
-
 impl<N: NodeIdT> SbvBroadcast<N> {
     pub fn new(netinfo: Arc<NetworkInfo<N>>) -> Self {
         SbvBroadcast {
@@ -108,6 +81,27 @@ impl<N: NodeIdT> SbvBroadcast<N> {
         self.terminated = false;
     }
 
+    pub fn handle_message(&mut self, sender_id: &N, msg: &Message) -> Result<Step<N>> {
+        match msg {
+            Message::BVal(b) => self.handle_bval(sender_id, *b),
+            Message::Aux(b) => self.handle_aux(sender_id, *b),
+        }
+    }
+
+    /// Returns the current `bin_values`: the set of `b` for which _2 f + 1_ `BVal`s were received.
+    pub fn bin_values(&self) -> BoolSet {
+        self.bin_values
+    }
+
+    /// Multicasts a `BVal(b)` message, and handles it.
+    pub fn send_bval(&mut self, b: bool) -> Result<Step<N>> {
+        // Record the value `b` as sent. If it was already there, don't send it again.
+        if !self.sent_bval.insert(b) {
+            return Ok(Step::default());
+        }
+        self.send(&Message::BVal(b))
+    }
+
     /// Handles a `BVal(b)` message.
     ///
     /// Upon receiving _f + 1_ `BVal(b)`, multicasts `BVal(b)`. Upon receiving _2 f + 1_ `BVal(b)`,
@@ -126,7 +120,7 @@ impl<N: NodeIdT> SbvBroadcast<N> {
             self.bin_values.insert(b);
 
             if self.bin_values != bool_set::BOTH {
-                step.extend(self.send(Message::Aux(b))?) // First entry: send `Aux(b)`.
+                step.extend(self.send(&Message::Aux(b))?) // First entry: send `Aux(b)`.
             } else {
                 step.extend(self.try_output()?); // Otherwise just check for `Conf` condition.
             }
@@ -139,28 +133,14 @@ impl<N: NodeIdT> SbvBroadcast<N> {
         Ok(step)
     }
 
-    /// Returns the current `bin_values`: the set of `b` for which _2 f + 1_ `BVal`s were received.
-    pub fn bin_values(&self) -> BoolSet {
-        self.bin_values
-    }
-
     /// Multicasts and handles a message. Does nothing if we are only an observer.
-    fn send(&mut self, msg: Message) -> Result<Step<N>> {
+    fn send(&mut self, msg: &Message) -> Result<Step<N>> {
         if !self.netinfo.is_validator() {
             return self.try_output();
         }
         let step: Step<_> = Target::All.message(msg.clone()).into();
-        let our_id = &self.our_id().clone();
-        Ok(step.join(self.handle_message(our_id, msg)?))
-    }
-
-    /// Multicasts a `BVal(b)` message, and handles it.
-    fn send_bval(&mut self, b: bool) -> Result<Step<N>> {
-        // Record the value `b` as sent. If it was already there, don't send it again.
-        if !self.sent_bval.insert(b) {
-            return Ok(Step::default());
-        }
-        self.send(Message::BVal(b))
+        let our_id = &self.netinfo.our_id().clone();
+        Ok(step.join(self.handle_message(our_id, &msg)?))
     }
 
     /// Handles an `Aux` message.
diff --git a/src/broadcast/broadcast.rs b/src/broadcast/broadcast.rs
index 3f658f0..4e90689 100644
--- a/src/broadcast/broadcast.rs
+++ b/src/broadcast/broadcast.rs
@@ -37,7 +37,7 @@ pub struct Broadcast<N> {
     readys: BTreeMap<N, Vec<u8>>,
 }
 
-pub type Step<N> = ::Step<Broadcast<N>>;
+pub type Step<N> = ::DaStep<Broadcast<N>>;
 
 impl<N: NodeIdT> DistAlgorithm for Broadcast<N> {
     type NodeId = N;
diff --git a/src/dynamic_honey_badger/mod.rs b/src/dynamic_honey_badger/mod.rs
index 28101b5..50c1bb4 100644
--- a/src/dynamic_honey_badger/mod.rs
+++ b/src/dynamic_honey_badger/mod.rs
@@ -91,7 +91,7 @@ pub use self::change::{Change, ChangeState, NodeChange};
 pub use self::dynamic_honey_badger::DynamicHoneyBadger;
 pub use self::error::{Error, ErrorKind, Result};
 
-pub type Step<C, N> = ::Step<DynamicHoneyBadger<C, N>>;
+pub type Step<C, N> = ::DaStep<DynamicHoneyBadger<C, N>>;
 
 /// The user input for `DynamicHoneyBadger`.
 #[derive(Clone, Debug)]
diff --git a/src/honey_badger/epoch_state.rs b/src/honey_badger/epoch_state.rs
index 768d3ed..469954f 100644
--- a/src/honey_badger/epoch_state.rs
+++ b/src/honey_badger/epoch_state.rs
@@ -19,7 +19,7 @@ use subset::{self as cs, Subset, SubsetOutput};
 use threshold_decrypt::{self as td, ThresholdDecrypt};
 use {Contribution, DistAlgorithm, NetworkInfo, NodeIdT};
 
-type CsStep<N> = cs::Step<N, EpochId>;
+type CsStep<N> = cs::Step<N>;
 
 /// The status of an encrypted contribution.
 #[derive(Debug)]
diff --git a/src/honey_badger/honey_badger.rs b/src/honey_badger/honey_badger.rs
index b1004bf..c513ca2 100644
--- a/src/honey_badger/honey_badger.rs
+++ b/src/honey_badger/honey_badger.rs
@@ -53,7 +53,7 @@ impl<C, N: Rand> Epoched for HoneyBadger<C, N> {
     }
 }
 
-pub type Step<C, N> = ::Step<HoneyBadger<C, N>>;
+pub type Step<C, N> = ::DaStep<HoneyBadger<C, N>>;
 
 impl<C, N> DistAlgorithm for HoneyBadger<C, N>
 where
diff --git a/src/lib.rs b/src/lib.rs
index 672b9b1..3fee244 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -153,4 +153,6 @@ pub use crypto::pairing;
 pub use fault_log::{Fault, FaultKind, FaultLog};
 pub use messaging::{SourcedMessage, Target, TargetedMessage};
 pub use network_info::NetworkInfo;
-pub use traits::{Contribution, DistAlgorithm, Epoched, Message, NodeIdT, SessionIdT, Step};
+pub use traits::{
+    Contribution, DaStep, DistAlgorithm, Epoched, Message, NodeIdT, SessionIdT, Step,
+};
diff --git a/src/queueing_honey_badger/mod.rs b/src/queueing_honey_badger/mod.rs
index 806a174..126e5f0 100644
--- a/src/queueing_honey_badger/mod.rs
+++ b/src/queueing_honey_badger/mod.rs
@@ -105,7 +105,7 @@ pub struct QueueingHoneyBadgerBuilder<T, N: Rand, Q> {
     _phantom: PhantomData<T>,
 }
 
-pub type QueueingHoneyBadgerWithStep<T, N, Q> = (QueueingHoneyBadger<T, N, Q>, Step<T, N, Q>);
+pub type QueueingHoneyBadgerWithStep<T, N, Q> = (QueueingHoneyBadger<T, N, Q>, Step<T, N>);
 
 impl<T, N, Q> QueueingHoneyBadgerBuilder<T, N, Q>
 where
@@ -187,7 +187,7 @@ pub struct QueueingHoneyBadger<T, N: Rand, Q> {
     rng: Box<dyn Rng + Send + Sync>,
 }
 
-pub type Step<T, N, Q> = ::Step<QueueingHoneyBadger<T, N, Q>>;
+pub type Step<T, N> = ::Step<Message<N>, Batch<T, N>, N>;
 
 impl<T, N, Q> DistAlgorithm for QueueingHoneyBadger<T, N, Q>
 where
@@ -201,7 +201,7 @@ where
     type Message = Message<N>;
     type Error = Error;
 
-    fn handle_input(&mut self, input: Self::Input) -> Result<Step<T, N, Q>> {
+    fn handle_input(&mut self, input: Self::Input) -> Result<Step<T, N>> {
         // User transactions are forwarded to `HoneyBadger` right away. Internal messages are
         // in addition signed and broadcast.
         match input {
@@ -210,7 +210,7 @@ where
         }
     }
 
-    fn handle_message(&mut self, sender_id: &N, message: Self::Message) -> Result<Step<T, N, Q>> {
+    fn handle_message(&mut self, sender_id: &N, message: Self::Message) -> Result<Step<T, N>> {
         self.handle_message(sender_id, message)
     }
 
@@ -243,7 +243,7 @@ where
     /// If no proposal has yet been made for the current epoch, this may trigger one. In this case,
     /// a nonempty step will returned, with the corresponding messages. (Or, if we are the only
     /// validator, even with the completed batch as an output.)
-    pub fn push_transaction(&mut self, tx: T) -> Result<Step<T, N, Q>> {
+    pub fn push_transaction(&mut self, tx: T) -> Result<Step<T, N>> {
         self.queue.extend(iter::once(tx));
         self.propose()
     }
@@ -252,12 +252,11 @@ where
     ///
     /// This stores a pending vote for the change. It will be included in some future batch, and
     /// once enough validators have been voted for the same change, it will take effect.
-    pub fn vote_for(&mut self, change: Change<N>) -> Result<Step<T, N, Q>> {
+    pub fn vote_for(&mut self, change: Change<N>) -> Result<Step<T, N>> {
         Ok(self
             .dyn_hb
             .handle_input(Input::Change(change))
             .map_err(ErrorKind::Input)?
-            .convert()
             .join(self.propose()?))
     }
 
@@ -265,7 +264,7 @@ where
     ///
     /// This stores a pending vote for the change. It will be included in some future batch, and
     /// once enough validators have been voted for the same change, it will take effect.
-    pub fn vote_to_add(&mut self, node_id: N, pub_key: PublicKey) -> Result<Step<T, N, Q>> {
+    pub fn vote_to_add(&mut self, node_id: N, pub_key: PublicKey) -> Result<Step<T, N>> {
         self.vote_for(Change::NodeChange(NodeChange::Add(node_id, pub_key)))
     }
 
@@ -273,19 +272,18 @@ where
     ///
     /// This stores a pending vote for the change. It will be included in some future batch, and
     /// once enough validators have been voted for the same change, it will take effect.
-    pub fn vote_to_remove(&mut self, node_id: N) -> Result<Step<T, N, Q>> {
+    pub fn vote_to_remove(&mut self, node_id: N) -> Result<Step<T, N>> {
         self.vote_for(Change::NodeChange(NodeChange::Remove(node_id)))
     }
 
     /// Handles a message received from `sender_id`.
     ///
     /// This must be called with every message we receive from another node.
-    pub fn handle_message(&mut self, sender_id: &N, message: Message<N>) -> Result<Step<T, N, Q>> {
+    pub fn handle_message(&mut self, sender_id: &N, message: Message<N>) -> Result<Step<T, N>> {
         let step = self
             .dyn_hb
             .handle_message(sender_id, message)
-            .map_err(ErrorKind::HandleMessage)?
-            .convert::<Self>();
+            .map_err(ErrorKind::HandleMessage)?;
         for batch in &step.output {
             self.queue.remove_multiple(batch.iter());
         }
@@ -308,7 +306,7 @@ where
     }
 
     /// Initiates the next epoch by proposing a batch from the queue.
-    fn propose(&mut self) -> Result<Step<T, N, Q>> {
+    fn propose(&mut self) -> Result<Step<T, N>> {
         let mut step = Step::default();
         while self.can_propose() {
             let amount = cmp::max(1, self.batch_size / self.dyn_hb.netinfo().num_nodes());
@@ -316,8 +314,7 @@ where
             step.extend(
                 self.dyn_hb
                     .handle_input(Input::User(proposal))
-                    .map_err(ErrorKind::Propose)?
-                    .convert(),
+                    .map_err(ErrorKind::Propose)?,
             );
         }
         Ok(step)
diff --git a/src/sender_queue/dynamic_honey_badger.rs b/src/sender_queue/dynamic_honey_badger.rs
index 67af96a..9fbc426 100644
--- a/src/sender_queue/dynamic_honey_badger.rs
+++ b/src/sender_queue/dynamic_honey_badger.rs
@@ -1,17 +1,19 @@
 //! Convenience methods for a `SenderQueue` wrapping a `DynamicHoneyBadger`.
 
+use std::result;
+
 use crypto::PublicKey;
 use rand::Rand;
 use serde::{de::DeserializeOwned, Serialize};
 
 use super::{
     SenderQueue, SenderQueueableDistAlgorithm, SenderQueueableEpoch, SenderQueueableMessage,
-    SenderQueueableOutput, Step,
+    SenderQueueableOutput,
 };
-use {Contribution, Epoched, NodeIdT};
+use {Contribution, DaStep, Epoched, NodeIdT};
 
 use dynamic_honey_badger::{
-    Batch, Change, ChangeState, DynamicHoneyBadger, Epoch, Message, NodeChange,
+    Batch, Change, ChangeState, DynamicHoneyBadger, Epoch, Error as DhbError, Message, NodeChange,
 };
 
 impl<C, N> SenderQueueableOutput<N, Message<N>> for Batch<C, N>
@@ -90,7 +92,7 @@ where
     }
 }
 
-type Result<C, N> = super::Result<Step<DynamicHoneyBadger<C, N>>, DynamicHoneyBadger<C, N>>;
+type Result<C, N> = result::Result<DaStep<SenderQueue<DynamicHoneyBadger<C, N>>>, DhbError>;
 
 impl<C, N> SenderQueue<DynamicHoneyBadger<C, N>>
 where
diff --git a/src/sender_queue/mod.rs b/src/sender_queue/mod.rs
index b2dd2bc..d1e9e0f 100644
--- a/src/sender_queue/mod.rs
+++ b/src/sender_queue/mod.rs
@@ -17,7 +17,7 @@ use std::fmt::Debug;
 use rand::Rand;
 use serde::{de::DeserializeOwned, Serialize};
 
-use {DistAlgorithm, Epoched, NodeIdT, Target};
+use {DaStep, DistAlgorithm, Epoched, NodeIdT, Target};
 
 pub use self::message::Message;
 
@@ -103,9 +103,7 @@ where
     peer_epochs: BTreeMap<D::NodeId, <D::Message as Epoched>::LinEpoch>,
 }
 
-pub type Step<D> = ::Step<SenderQueue<D>>;
-
-pub type Result<T, D> = ::std::result::Result<T, <D as DistAlgorithm>::Error>;
+pub type Step<D> = ::DaStep<SenderQueue<D>>;
 
 impl<D> DistAlgorithm for SenderQueue<D>
 where
@@ -121,7 +119,7 @@ where
     type Message = Message<D::Message>;
     type Error = D::Error;
 
-    fn handle_input(&mut self, input: Self::Input) -> Result<Step<D>, D> {
+    fn handle_input(&mut self, input: Self::Input) -> Result<DaStep<Self>, D::Error> {
         self.handle_input(input)
     }
 
@@ -129,7 +127,7 @@ where
         &mut self,
         sender_id: &D::NodeId,
         message: Self::Message,
-    ) -> Result<Step<D>, D> {
+    ) -> Result<DaStep<Self>, D::Error> {
         self.handle_message(sender_id, message)
     }
 
@@ -158,7 +156,7 @@ where
         SenderQueueBuilder::new(algo, peer_ids)
     }
 
-    pub fn handle_input(&mut self, input: D::Input) -> Result<Step<D>, D> {
+    pub fn handle_input(&mut self, input: D::Input) -> Result<DaStep<Self>, D::Error> {
         self.apply(|algo| algo.handle_input(input))
     }
 
@@ -166,7 +164,7 @@ where
         &mut self,
         sender_id: &D::NodeId,
         message: Message<D::Message>,
-    ) -> Result<Step<D>, D> {
+    ) -> Result<DaStep<Self>, D::Error> {
         match message {
             Message::EpochStarted(lin_epoch) => Ok(self.handle_epoch_started(sender_id, lin_epoch)),
             Message::Algo(msg) => self.handle_message_content(sender_id, msg),
@@ -175,9 +173,9 @@ where
 
     /// Applies `f` to the wrapped algorithm and converts the step in the result to a sender queue
     /// step, deferring or dropping messages, where necessary.
-    pub fn apply<F>(&mut self, f: F) -> Result<Step<D>, D>
+    pub fn apply<F>(&mut self, f: F) -> Result<DaStep<Self>, D::Error>
     where
-        F: FnOnce(&mut D) -> Result<::Step<D>, D>,
+        F: FnOnce(&mut D) -> Result<DaStep<D>, D::Error>,
     {
         let mut step = f(&mut self.algo)?;
         let mut sender_queue_step = self.update_lin_epoch(&step);
@@ -191,7 +189,7 @@ where
         &mut self,
         sender_id: &D::NodeId,
         lin_epoch: <D::Message as Epoched>::LinEpoch,
-    ) -> Step<D> {
+    ) -> DaStep<Self> {
         self.peer_epochs
             .entry(sender_id.clone())
             .and_modify(|e| {
@@ -225,7 +223,7 @@ where
         &mut self,
         sender_id: &D::NodeId,
         epoch: <D::Message as Epoched>::Epoch,
-    ) -> Step<D> {
+    ) -> DaStep<Self> {
         // Send any HB messages for the HB epoch.
         let mut ready_messages = self
             .outgoing_queue
@@ -239,7 +237,7 @@ where
                     .unwrap_or_default(),
             );
         }
-        Step::from(
+        Step::<D>::from(
             ready_messages
                 .into_iter()
                 .map(|msg| Target::Node(sender_id.clone()).message(Message::Algo(msg))),
@@ -251,12 +249,12 @@ where
         &mut self,
         sender_id: &D::NodeId,
         content: D::Message,
-    ) -> Result<Step<D>, D> {
+    ) -> Result<DaStep<Self>, D::Error> {
         self.apply(|algo| algo.handle_message(sender_id, content))
     }
 
     /// Updates the current Honey Badger epoch.
-    fn update_lin_epoch(&mut self, step: &::Step<D>) -> Step<D> {
+    fn update_lin_epoch(&mut self, step: &DaStep<D>) -> DaStep<Self> {
         // Look up `DynamicHoneyBadger` epoch updates and collect any added peers.
         let new_epoch = step.output.iter().fold(self.lin_epoch, |lin_epoch, batch| {
             let max_epoch = lin_epoch.max(batch.next_epoch());
@@ -276,7 +274,7 @@ where
                 .message(Message::EpochStarted(self.lin_epoch))
                 .into()
         } else {
-            Step::default()
+            Step::<D>::default()
         }
     }
 
@@ -284,7 +282,7 @@ where
     /// decomposing a `Target::All` message into `Target::Node` messages and sending some of the
     /// resulting messages while placing onto the queue those remaining messages whose recipient is
     /// currently at an earlier epoch.
-    fn defer_messages(&mut self, step: &mut ::Step<D>) {
+    fn defer_messages(&mut self, step: &mut DaStep<D>) {
         let max_future_epochs = self.algo.max_future_epochs();
         // Append the deferred messages onto the queues.
         for (id, message) in step.defer_messages(&self.peer_epochs, max_future_epochs) {
@@ -355,7 +353,7 @@ where
         self
     }
 
-    pub fn build(self, our_id: D::NodeId) -> (SenderQueue<D>, Step<D>) {
+    pub fn build(self, our_id: D::NodeId) -> (SenderQueue<D>, DaStep<SenderQueue<D>>) {
         let lin_epoch = <D::Message as Epoched>::LinEpoch::default();
         let sq = SenderQueue {
             algo: self.algo,
@@ -364,7 +362,7 @@ where
             outgoing_queue: self.outgoing_queue,
             peer_epochs: self.peer_epochs,
         };
-        let step: Step<D> = Target::All.message(Message::EpochStarted(lin_epoch)).into();
+        let step = Target::All.message(Message::EpochStarted(lin_epoch)).into();
         (sq, step)
     }
 }
diff --git a/src/sender_queue/queueing_honey_badger.rs b/src/sender_queue/queueing_honey_badger.rs
index 05e6203..0363ec8 100644
--- a/src/sender_queue/queueing_honey_badger.rs
+++ b/src/sender_queue/queueing_honey_badger.rs
@@ -1,13 +1,15 @@
 //! Convenience methods for a `SenderQueue` wrapping a `QueueingHoneyBadger`.
 
+use std::result;
+
 use crypto::PublicKey;
 use rand::Rand;
 use serde::{de::DeserializeOwned, Serialize};
 
-use super::{SenderQueue, SenderQueueableDistAlgorithm, Step};
-use queueing_honey_badger::{Change, QueueingHoneyBadger};
+use super::{SenderQueue, SenderQueueableDistAlgorithm};
+use queueing_honey_badger::{Change, Error as QhbError, QueueingHoneyBadger};
 use transaction_queue::TransactionQueue;
-use {Contribution, NodeIdT};
+use {Contribution, DaStep, NodeIdT};
 
 impl<T, N, Q> SenderQueueableDistAlgorithm for QueueingHoneyBadger<T, N, Q>
 where
@@ -20,8 +22,7 @@ where
     }
 }
 
-type Result<T, N, Q> =
-    super::Result<Step<QueueingHoneyBadger<T, N, Q>>, QueueingHoneyBadger<T, N, Q>>;
+type Result<T, N, Q> = result::Result<DaStep<SenderQueue<QueueingHoneyBadger<T, N, Q>>>, QhbError>;
 
 impl<T, N, Q> SenderQueue<QueueingHoneyBadger<T, N, Q>>
 where
diff --git a/src/subset/proposal_state.rs b/src/subset/proposal_state.rs
index 628f9cf..a5ebd7a 100644
--- a/src/subset/proposal_state.rs
+++ b/src/subset/proposal_state.rs
@@ -6,13 +6,13 @@ use super::{Error, MessageContent, Result};
 use binary_agreement;
 use broadcast::{self, Broadcast};
 use rand::Rand;
-use {DistAlgorithm, NetworkInfo, NodeIdT, SessionIdT};
+use {NetworkInfo, NodeIdT, SessionIdT};
 
 type BaInstance<N, S> = binary_agreement::BinaryAgreement<N, BaSessionId<S>>;
-type ValueAndStep<N, S> = (Option<Vec<u8>>, Step<N, S>);
-type BaResult<N, S> = binary_agreement::Result<binary_agreement::Step<N, BaSessionId<S>>>;
+type ValueAndStep<N> = (Option<Vec<u8>>, Step<N>);
+type BaResult<N> = binary_agreement::Result<binary_agreement::Step<N>>;
 
-pub type Step<N, S> = ::Step<ProposalState<N, S>>;
+pub type Step<N> = ::Step<MessageContent, Vec<u8>, N>;
 
 /// The state of a proposal's broadcast and agreement process.
 #[derive(Debug)]
@@ -28,30 +28,6 @@ pub enum ProposalState<N: Rand, S> {
     Complete(bool),
 }
 
-impl<N: NodeIdT + Rand, S: SessionIdT> DistAlgorithm for ProposalState<N, S> {
-    type NodeId = N;
-    type Input = Vec<u8>;
-    type Output = Vec<u8>;
-    type Message = MessageContent;
-    type Error = Error;
-
-    fn handle_input(&mut self, input: Self::Input) -> Result<Step<N, S>> {
-        self.propose(input)
-    }
-
-    fn handle_message(&mut self, sender_id: &N, message: MessageContent) -> Result<Step<N, S>> {
-        self.handle_message(sender_id, message)
-    }
-
-    fn terminated(&self) -> bool {
-        self.complete()
-    }
-
-    fn our_id(&self) -> &Self::NodeId {
-        unreachable!() // We don't actually need `DistAlgorithm`, just `Step`.
-    }
-}
-
 impl<N: NodeIdT + Rand, S: SessionIdT> ProposalState<N, S> {
     /// Creates a new `ProposalState::Ongoing`, with a fresh broadcast and agreement instance.
     pub fn new(netinfo: Arc<NetworkInfo<N>>, ba_id: BaSessionId<S>, prop_id: N) -> Result<Self> {
@@ -89,12 +65,12 @@ impl<N: NodeIdT + Rand, S: SessionIdT> ProposalState<N, S> {
     }
 
     /// Makes a proposal by broadcasting a value.
-    pub fn propose(&mut self, value: Vec<u8>) -> Result<Step<N, S>> {
+    pub fn propose(&mut self, value: Vec<u8>) -> Result<Step<N>> {
         self.transition(|state| state.handle_broadcast(|bc| bc.broadcast(value)))
     }
 
     /// Handles a message received from `sender_id`.
-    pub fn handle_message(&mut self, sender_id: &N, msg: MessageContent) -> Result<Step<N, S>> {
+    pub fn handle_message(&mut self, sender_id: &N, msg: MessageContent) -> Result<Step<N>> {
         self.transition(|state| match msg {
             MessageContent::Agreement(ba_msg) => {
                 state.handle_agreement(|ba| ba.handle_message(sender_id, ba_msg))
@@ -106,12 +82,12 @@ impl<N: NodeIdT + Rand, S: SessionIdT> ProposalState<N, S> {
     }
 
     /// Votes for rejecting the proposal, if still possible.
-    pub fn vote_false(&mut self) -> Result<Step<N, S>> {
+    pub fn vote_false(&mut self) -> Result<Step<N>> {
         self.transition(|state| state.handle_agreement(|ba| ba.propose(false)))
     }
 
     /// Applies `f` to the `Broadcast` instance, and updates the state according to the outcome.
-    fn handle_broadcast<F>(self, f: F) -> (Self, Result<Step<N, S>>)
+    fn handle_broadcast<F>(self, f: F) -> (Self, Result<Step<N>>)
     where
         F: FnOnce(&mut Broadcast<N>) -> broadcast::Result<broadcast::Step<N>>,
     {
@@ -137,9 +113,9 @@ impl<N: NodeIdT + Rand, S: SessionIdT> ProposalState<N, S> {
 
     /// Applies `f` to the `BinaryAgreement` instance, and updates the state according to the
     /// outcome.
-    fn handle_agreement<F>(self, f: F) -> (Self, Result<Step<N, S>>)
+    fn handle_agreement<F>(self, f: F) -> (Self, Result<Step<N>>)
     where
-        F: FnOnce(&mut BaInstance<N, S>) -> BaResult<N, S>,
+        F: FnOnce(&mut BaInstance<N, S>) -> BaResult<N>,
     {
         use self::ProposalState::*;
         match self {
@@ -160,7 +136,7 @@ impl<N: NodeIdT + Rand, S: SessionIdT> ProposalState<N, S> {
     }
 
     /// Converts a `Broadcast` result and returns the output, if there was one.
-    fn convert_bc(result: broadcast::Result<broadcast::Step<N>>) -> Result<ValueAndStep<N, S>> {
+    fn convert_bc(result: broadcast::Result<broadcast::Step<N>>) -> Result<ValueAndStep<N>> {
         let bc_step = result.map_err(Error::HandleBroadcast)?;
         let mut step = Step::default();
         let opt_value = step.extend_with(bc_step, MessageContent::Broadcast).pop();
@@ -168,7 +144,7 @@ impl<N: NodeIdT + Rand, S: SessionIdT> ProposalState<N, S> {
     }
 
     /// Converts a `BinaryAgreement` step and returns the output, if there was one.
-    fn convert_ba(result: BaResult<N, S>) -> Result<(Option<bool>, Step<N, S>)> {
+    fn convert_ba(result: BaResult<N>) -> Result<(Option<bool>, Step<N>)> {
         let ba_step = result.map_err(Error::HandleAgreement)?;
         let mut step = Step::default();
         let opt_decision = step.extend_with(ba_step, MessageContent::Agreement).pop();
@@ -176,9 +152,9 @@ impl<N: NodeIdT + Rand, S: SessionIdT> ProposalState<N, S> {
     }
 
     /// Applies the given transition to `self`.
-    fn transition<F>(&mut self, f: F) -> Result<Step<N, S>>
+    fn transition<F>(&mut self, f: F) -> Result<Step<N>>
     where
-        F: FnOnce(Self) -> (Self, Result<Step<N, S>>),
+        F: FnOnce(Self) -> (Self, Result<Step<N>>),
     {
         // Temporary value: We need to take ownership of the state to make it transition.
         let (new_state, result) = f(mem::replace(self, ProposalState::Complete(false)));
diff --git a/src/subset/subset.rs b/src/subset/subset.rs
index de2373a..fbcc24a 100644
--- a/src/subset/subset.rs
+++ b/src/subset/subset.rs
@@ -12,7 +12,7 @@ use super::{Error, Message, MessageContent, Result};
 use rand::Rand;
 use {util, DistAlgorithm, NetworkInfo, NodeIdT, SessionIdT};
 
-pub type Step<N, S> = ::Step<Subset<N, S>>;
+pub type Step<N> = ::Step<Message<N>, SubsetOutput<N>, N>;
 
 #[derive(Derivative, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[derivative(Debug)]
@@ -44,11 +44,11 @@ impl<N: NodeIdT + Rand, S: SessionIdT> DistAlgorithm for Subset<N, S> {
     type Message = Message<N>;
     type Error = Error;
 
-    fn handle_input(&mut self, input: Self::Input) -> Result<Step<N, S>> {
+    fn handle_input(&mut self, input: Self::Input) -> Result<Step<N>> {
         self.propose(input)
     }
 
-    fn handle_message(&mut self, sender_id: &N, message: Message<N>) -> Result<Step<N, S>> {
+    fn handle_message(&mut self, sender_id: &N, message: Message<N>) -> Result<Step<N>> {
         self.handle_message(sender_id, message)
     }
 
@@ -90,7 +90,7 @@ impl<N: NodeIdT + Rand, S: SessionIdT> Subset<N, S> {
     /// Proposes a value for the subset.
     ///
     /// Returns an error if we already made a proposal.
-    pub fn propose(&mut self, value: Vec<u8>) -> Result<Step<N, S>> {
+    pub fn propose(&mut self, value: Vec<u8>) -> Result<Step<N>> {
         if !self.netinfo.is_validator() {
             return Ok(Step::default());
         }
@@ -107,7 +107,7 @@ impl<N: NodeIdT + Rand, S: SessionIdT> Subset<N, S> {
     /// Handles a message received from `sender_id`.
     ///
     /// This must be called with every message we receive from another node.
-    pub fn handle_message(&mut self, sender_id: &N, msg: Message<N>) -> Result<Step<N, S>> {
+    pub fn handle_message(&mut self, sender_id: &N, msg: Message<N>) -> Result<Step<N>> {
         let prop_step = self
             .proposal_states
             .get_mut(&msg.proposer_id)
@@ -123,7 +123,7 @@ impl<N: NodeIdT + Rand, S: SessionIdT> Subset<N, S> {
         self.proposal_states.values().filter(received).count()
     }
 
-    fn convert_step(proposer_id: &N, prop_step: ProposalStep<N, S>) -> Step<N, S> {
+    fn convert_step(proposer_id: &N, prop_step: ProposalStep<N>) -> Step<N> {
         let from_p_msg = |p_msg: MessageContent| p_msg.with(proposer_id.clone());
         let mut step = Step::default();
         if let Some(value) = step.extend_with(prop_step, from_p_msg).pop() {
@@ -141,7 +141,7 @@ impl<N: NodeIdT + Rand, S: SessionIdT> Subset<N, S> {
 
     /// Checks the voting and termination conditions: If enough proposals have been accepted, votes
     /// "no" for the remaining ones. If all proposals have been decided, outputs `Done`.
-    fn try_output(&mut self) -> Result<Step<N, S>> {
+    fn try_output(&mut self) -> Result<Step<N>> {
         if self.decided || self.count_accepted() < self.netinfo.num_correct() {
             return Ok(Step::default());
         }
diff --git a/src/threshold_decrypt.rs b/src/threshold_decrypt.rs
index 372cd4a..7d218e5 100644
--- a/src/threshold_decrypt.rs
+++ b/src/threshold_decrypt.rs
@@ -80,7 +80,7 @@ pub struct ThresholdDecrypt<N> {
     terminated: bool,
 }
 
-pub type Step<N> = ::Step<ThresholdDecrypt<N>>;
+pub type Step<N> = ::DaStep<ThresholdDecrypt<N>>;
 
 impl<N: NodeIdT> DistAlgorithm for ThresholdDecrypt<N> {
     type NodeId = N;
diff --git a/src/threshold_sign.rs b/src/threshold_sign.rs
index 00d9c0e..4cefb33 100644
--- a/src/threshold_sign.rs
+++ b/src/threshold_sign.rs
@@ -75,7 +75,7 @@ pub struct ThresholdSign<N> {
     terminated: bool,
 }
 
-pub type Step<N> = ::Step<ThresholdSign<N>>;
+pub type Step<N> = ::DaStep<ThresholdSign<N>>;
 
 impl<N: NodeIdT> DistAlgorithm for ThresholdSign<N> {
     type NodeId = N;
diff --git a/src/traits.rs b/src/traits.rs
index f61ca84..f8884ac 100644
--- a/src/traits.rs
+++ b/src/traits.rs
@@ -57,22 +57,14 @@ impl<E> EpochT for E where E: Copy + Message + Default + Eq + Ord + Serialize +
 /// catch it, instead of potentially stalling the algorithm.
 #[must_use = "The algorithm step result must be used."]
 #[derive(Debug)]
-pub struct Step<D>
-where
-    D: DistAlgorithm,
-    <D as DistAlgorithm>::NodeId: NodeIdT,
-{
-    pub output: Vec<D::Output>,
-    pub fault_log: FaultLog<D::NodeId>,
-    pub messages: Vec<TargetedMessage<D::Message, D::NodeId>>,
+pub struct Step<M, O, N> {
+    pub output: Vec<O>,
+    pub fault_log: FaultLog<N>,
+    pub messages: Vec<TargetedMessage<M, N>>,
 }
 
-impl<D> Default for Step<D>
-where
-    D: DistAlgorithm,
-    <D as DistAlgorithm>::NodeId: NodeIdT,
-{
-    fn default() -> Step<D> {
+impl<M, O, N> Default for Step<M, O, N> {
+    fn default() -> Self {
         Step {
             output: Vec::default(),
             fault_log: FaultLog::default(),
@@ -81,15 +73,12 @@ where
     }
 }
 
-impl<D: DistAlgorithm> Step<D>
-where
-    <D as DistAlgorithm>::NodeId: NodeIdT,
-{
+impl<M, O, N> Step<M, O, N> {
     /// Creates a new `Step` from the given collections.
     pub fn new(
-        output: Vec<D::Output>,
-        fault_log: FaultLog<D::NodeId>,
-        messages: Vec<TargetedMessage<D::Message, D::NodeId>>,
+        output: Vec<O>,
+        fault_log: FaultLog<N>,
+        messages: Vec<TargetedMessage<M, N>>,
     ) -> Self {
         Step {
             output,
@@ -99,18 +88,17 @@ where
     }
 
     /// Returns the same step, with the given additional output.
-    pub fn with_output<T: Into<Option<D::Output>>>(mut self, output: T) -> Self {
+    pub fn with_output<T: Into<Option<O>>>(mut self, output: T) -> Self {
         self.output.extend(output.into());
         self
     }
 
     /// Converts `self` into a step of another type, given conversion methods for output and
     /// messages.
-    pub fn map<D2, FO, FM>(self, f_out: FO, f_msg: FM) -> Step<D2>
+    pub fn map<M2, O2, FO, FM>(self, f_out: FO, f_msg: FM) -> Step<M2, O2, N>
     where
-        D2: DistAlgorithm<NodeId = D::NodeId>,
-        FO: Fn(D::Output) -> D2::Output,
-        FM: Fn(D::Message) -> D2::Message,
+        FO: Fn(O) -> O2,
+        FM: Fn(M) -> M2,
     {
         Step {
             output: self.output.into_iter().map(f_out).collect(),
@@ -120,10 +108,9 @@ where
     }
 
     /// Extends `self` with `other`s messages and fault logs, and returns `other.output`.
-    pub fn extend_with<D2, FM>(&mut self, other: Step<D2>, f_msg: FM) -> Vec<D2::Output>
+    pub fn extend_with<M2, O2, FM>(&mut self, other: Step<M2, O2, N>, f_msg: FM) -> Vec<O2>
     where
-        D2: DistAlgorithm<NodeId = D::NodeId>,
-        FM: Fn(D2::Message) -> D::Message,
+        FM: Fn(M2) -> M,
     {
         self.fault_log.extend(other.fault_log);
         let msgs = other.messages.into_iter().map(|tm| tm.map(&f_msg));
@@ -144,27 +131,14 @@ where
         self
     }
 
-    /// Converts this step into an equivalent step for a different `DistAlgorithm`.
-    // This cannot be a `From` impl, because it would conflict with `impl From<T> for T`.
-    pub fn convert<D2>(self) -> Step<D2>
-    where
-        D2: DistAlgorithm<NodeId = D::NodeId, Output = D::Output, Message = D::Message>,
-    {
-        Step {
-            output: self.output,
-            fault_log: self.fault_log,
-            messages: self.messages,
-        }
-    }
-
     /// Returns `true` if there are no messages, faults or outputs.
     pub fn is_empty(&self) -> bool {
         self.output.is_empty() && self.fault_log.is_empty() && self.messages.is_empty()
     }
 }
 
-impl<D: DistAlgorithm> From<FaultLog<D::NodeId>> for Step<D> {
-    fn from(fault_log: FaultLog<D::NodeId>) -> Self {
+impl<M, O, N> From<FaultLog<N>> for Step<M, O, N> {
+    fn from(fault_log: FaultLog<N>) -> Self {
         Step {
             fault_log,
             ..Step::default()
@@ -172,8 +146,8 @@ impl<D: DistAlgorithm> From<FaultLog<D::NodeId>> for Step<D> {
     }
 }
 
-impl<D: DistAlgorithm> From<Fault<D::NodeId>> for Step<D> {
-    fn from(fault: Fault<D::NodeId>) -> Self {
+impl<M, O, N> From<Fault<N>> for Step<M, O, N> {
+    fn from(fault: Fault<N>) -> Self {
         Step {
             fault_log: fault.into(),
             ..Step::default()
@@ -181,8 +155,8 @@ impl<D: DistAlgorithm> From<Fault<D::NodeId>> for Step<D> {
     }
 }
 
-impl<D: DistAlgorithm> From<TargetedMessage<D::Message, D::NodeId>> for Step<D> {
-    fn from(msg: TargetedMessage<D::Message, D::NodeId>) -> Self {
+impl<M, O, N> From<TargetedMessage<M, N>> for Step<M, O, N> {
+    fn from(msg: TargetedMessage<M, N>) -> Self {
         Step {
             messages: once(msg).collect(),
             ..Step::default()
@@ -190,10 +164,9 @@ impl<D: DistAlgorithm> From<TargetedMessage<D::Message, D::NodeId>> for Step<D>
     }
 }
 
-impl<D, I> From<I> for Step<D>
+impl<I, M, O, N> From<I> for Step<M, O, N>
 where
-    D: DistAlgorithm,
-    I: IntoIterator<Item = TargetedMessage<D::Message, D::NodeId>>,
+    I: IntoIterator<Item = TargetedMessage<M, N>>,
 {
     fn from(msgs: I) -> Self {
         Step {
@@ -233,26 +206,28 @@ impl<M: Epoched, N> Epoched for TargetedMessage<M, N> {
     }
 }
 
-impl<'i, D> Step<D>
+/// An alias for the type of `Step` returned by `D`'s methods.
+pub type DaStep<D> =
+    Step<<D as DistAlgorithm>::Message, <D as DistAlgorithm>::Output, <D as DistAlgorithm>::NodeId>;
+
+impl<'i, M, O, N> Step<M, O, N>
 where
-    D: DistAlgorithm,
-    <D as DistAlgorithm>::NodeId: NodeIdT + Rand,
-    <D as DistAlgorithm>::Message:
-        'i + Clone + SenderQueueableMessage + Serialize + DeserializeOwned,
+    N: NodeIdT + Rand,
+    M: 'i + Clone + SenderQueueableMessage + Serialize + DeserializeOwned,
 {
     /// Removes and returns any messages that are not yet accepted by remote nodes according to the
     /// mapping `remote_epochs`. This way the returned messages are postponed until later, and the
     /// remaining messages can be sent to remote nodes without delay.
     pub fn defer_messages(
         &mut self,
-        peer_epochs: &'i BTreeMap<D::NodeId, <D::Message as Epoched>::LinEpoch>,
+        peer_epochs: &'i BTreeMap<N, <M as Epoched>::LinEpoch>,
         max_future_epochs: u64,
-    ) -> Vec<(D::NodeId, D::Message)>
+    ) -> Vec<(N, M)>
     where
-        <D as DistAlgorithm>::NodeId: 'i,
+        N: 'i,
     {
         let messages = &mut self.messages;
-        let mut deferred_msgs: Vec<(D::NodeId, D::Message)> = Vec::new();
+        let mut deferred_msgs: Vec<(N, M)> = Vec::new();
         let mut passed_msgs: Vec<_> = Vec::new();
         for msg in messages.drain(..) {
             match msg.target.clone() {
@@ -306,7 +281,7 @@ pub trait DistAlgorithm: Send + Sync {
     type Error: Fail;
 
     /// Handles an input provided by the user, and returns
-    fn handle_input(&mut self, input: Self::Input) -> Result<Step<Self>, Self::Error>
+    fn handle_input(&mut self, input: Self::Input) -> Result<DaStep<Self>, Self::Error>
     where
         Self: Sized;
 
@@ -315,7 +290,7 @@ pub trait DistAlgorithm: Send + Sync {
         &mut self,
         sender_id: &Self::NodeId,
         message: Self::Message,
-    ) -> Result<Step<Self>, Self::Error>
+    ) -> Result<DaStep<Self>, Self::Error>
     where
         Self: Sized;
 
diff --git a/tests/binary_agreement_mitm.rs b/tests/binary_agreement_mitm.rs
index 10caa34..7eb651c 100644
--- a/tests/binary_agreement_mitm.rs
+++ b/tests/binary_agreement_mitm.rs
@@ -16,7 +16,7 @@ use std::sync::{Arc, Mutex};
 
 use hbbft::binary_agreement::{BinaryAgreement, MessageContent, SbvMessage};
 use hbbft::threshold_sign::ThresholdSign;
-use hbbft::{DistAlgorithm, NetworkInfo, Step};
+use hbbft::{DaStep, DistAlgorithm, NetworkInfo};
 
 use net::adversary::{NetMutHandle, QueuePosition};
 use net::err::CrankError;
@@ -415,7 +415,7 @@ impl Adversary<Algo> for AbaCommonCoinAdversary {
         &mut self,
         _: NetMutHandle<Algo>,
         msg: NetMessage<Algo>,
-    ) -> Result<Step<Algo>, CrankError<Algo>> {
+    ) -> Result<DaStep<Algo>, CrankError<Algo>> {
         if let MessageContent::Coin(ref coin_msg) = msg.payload().content {
             let mut new_coin_state = None;
             if let CoinState::InProgress(ref mut coin) = self.coin_state {
@@ -430,7 +430,7 @@ impl Adversary<Algo> for AbaCommonCoinAdversary {
                 self.coin_state = new_coin_state;
             }
         }
-        Ok(Step::default())
+        Ok(DaStep::<Algo>::default())
     }
 }
 
diff --git a/tests/net/adversary.rs b/tests/net/adversary.rs
index cdf04de..39530d0 100644
--- a/tests/net/adversary.rs
+++ b/tests/net/adversary.rs
@@ -38,7 +38,7 @@ use std::{cmp, fmt};
 
 use rand::Rng;
 
-use hbbft::{DistAlgorithm, Step};
+use hbbft::{DaStep, DistAlgorithm};
 
 use net::{CrankError, NetMessage, Node, VirtualNet};
 
@@ -143,7 +143,7 @@ where
     }
 
     /// Normally dispatch a message
-    pub fn dispatch_message(&mut self, msg: NetMessage<D>) -> Result<Step<D>, CrankError<D>> {
+    pub fn dispatch_message(&mut self, msg: NetMessage<D>) -> Result<DaStep<D>, CrankError<D>> {
         self.0.dispatch_message(msg)
     }
 
@@ -336,7 +336,7 @@ where
         &mut self,
         mut net: NetMutHandle<D>,
         msg: NetMessage<D>,
-    ) -> Result<Step<D>, CrankError<D>> {
+    ) -> Result<DaStep<D>, CrankError<D>> {
         net.dispatch_message(msg)
     }
 }
diff --git a/tests/net/mod.rs b/tests/net/mod.rs
index 7ae54f2..7ac233c 100644
--- a/tests/net/mod.rs
+++ b/tests/net/mod.rs
@@ -26,7 +26,7 @@ use threshold_crypto as crypto;
 
 use hbbft::dynamic_honey_badger::Batch;
 use hbbft::util::SubRng;
-use hbbft::{self, Contribution, DistAlgorithm, NetworkInfo, NodeIdT, Step};
+use hbbft::{self, Contribution, DaStep, DistAlgorithm, NetworkInfo, NodeIdT, Step};
 
 use try_some;
 
@@ -197,7 +197,7 @@ pub type NetMessage<D> =
 fn process_step<'a, D>(
     nodes: &'a mut collections::BTreeMap<D::NodeId, Node<D>>,
     sender: D::NodeId,
-    step: &Step<D>,
+    step: &DaStep<D>,
     dest: &mut collections::VecDeque<NetMessage<D>>,
 ) -> usize
 where
@@ -307,7 +307,7 @@ where
     /// Number of faulty nodes in the network.
     num_faulty: usize,
     /// Dist-algorithm constructor function.
-    cons: Option<Box<Fn(NewNodeInfo<D>) -> (D, Step<D>)>>,
+    cons: Option<Box<Fn(NewNodeInfo<D>) -> (D, DaStep<D>)>>,
     /// Network adversary.
     adversary: Option<Box<dyn Adversary<D>>>,
     /// Trace-enabling flag. `None` means use environment.
@@ -463,7 +463,7 @@ where
     #[inline]
     pub fn using_step<F>(mut self, cons: F) -> Self
     where
-        F: Fn(NewNodeInfo<D>) -> (D, Step<D>) + 'static,
+        F: Fn(NewNodeInfo<D>) -> (D, DaStep<D>) + 'static,
     {
         self.cons = Some(Box::new(cons));
         self
@@ -729,7 +729,7 @@ where
         cons: F,
     ) -> Result<Self, crypto::error::Error>
     where
-        F: Fn(NewNodeInfo<D>) -> (D, Step<D>),
+        F: Fn(NewNodeInfo<D>) -> (D, DaStep<D>),
         I: IntoIterator<Item = D::NodeId>,
         R: rand::Rng,
     {
@@ -789,7 +789,7 @@ where
     ///
     /// Retrieves the receiving node for a `msg` and hands over the payload.
     #[inline]
-    pub fn dispatch_message(&mut self, msg: NetMessage<D>) -> Result<Step<D>, CrankError<D>> {
+    pub fn dispatch_message(&mut self, msg: NetMessage<D>) -> Result<DaStep<D>, CrankError<D>> {
         let node = self
             .nodes
             .get_mut(&msg.to)
@@ -816,7 +816,7 @@ where
     ///
     /// Panics if `id` does not name a valid node.
     #[inline]
-    pub fn send_input(&mut self, id: D::NodeId, input: D::Input) -> Result<Step<D>, D::Error> {
+    pub fn send_input(&mut self, id: D::NodeId, input: D::Input) -> Result<DaStep<D>, D::Error> {
         let step = self
             .nodes
             .get_mut(&id)
@@ -842,7 +842,7 @@ where
     /// If a successful `Step` was generated, all of its messages are queued on the network and the
     /// `Step` is returned.
     #[inline]
-    pub fn crank(&mut self) -> Option<Result<(D::NodeId, Step<D>), CrankError<D>>> {
+    pub fn crank(&mut self) -> Option<Result<(D::NodeId, DaStep<D>), CrankError<D>>> {
         // Check limits.
         if let Some(limit) = self.crank_limit {
             if self.crank_count >= limit {
@@ -893,7 +893,7 @@ where
                 .ok_or_else(|| CrankError::NodeDisappeared(msg.to.clone()))
         ).is_faulty();
 
-        let step: Step<_> = if is_faulty {
+        let step: Step<_, _, _> = if is_faulty {
             // The swap-dance is painful here, as we are creating an `opt_step` just to avoid
             // borrow issues.
             let mut adv = self.adversary.take();
@@ -932,7 +932,7 @@ where
     ///
     /// Shortcut for cranking the network, expecting both progress to be made as well as processing
     /// to proceed.
-    pub fn crank_expect(&mut self) -> (D::NodeId, Step<D>) {
+    pub fn crank_expect(&mut self) -> (D::NodeId, DaStep<D>) {
         self.crank()
             .expect("crank: network queue empty")
             .expect("crank: node failed to process step")
@@ -956,7 +956,7 @@ where
     pub fn broadcast_input<'a>(
         &'a mut self,
         input: &'a D::Input,
-    ) -> Result<Vec<(D::NodeId, Step<D>)>, D::Error> {
+    ) -> Result<Vec<(D::NodeId, DaStep<D>)>, D::Error> {
         // Note: The tricky lifetime annotation basically says that the input value given must
         //       live as long as the iterator returned lives (because it is cloned on every step,
         //       with steps only evaluated each time `next()` is called. For the same reason the
@@ -1050,7 +1050,7 @@ where
     D::Message: Clone,
     D::Output: Clone,
 {
-    type Item = Result<(D::NodeId, Step<D>), CrankError<D>>;
+    type Item = Result<(D::NodeId, DaStep<D>), CrankError<D>>;
 
     #[inline]
     fn next(&mut self) -> Option<Self::Item> {
diff --git a/tests/network/mod.rs b/tests/network/mod.rs
index bf1254f..bc2960a 100644
--- a/tests/network/mod.rs
+++ b/tests/network/mod.rs
@@ -10,7 +10,9 @@ use rand_derive::Rand;
 use serde_derive::{Deserialize, Serialize};
 
 use hbbft::dynamic_honey_badger::Batch;
-use hbbft::{Contribution, DistAlgorithm, Fault, NetworkInfo, Step, Target, TargetedMessage};
+use hbbft::{
+    Contribution, DaStep, DistAlgorithm, Fault, NetworkInfo, Step, Target, TargetedMessage,
+};
 
 /// A node identifier. In the tests, nodes are simply numbered.
 #[derive(Eq, PartialEq, Ord, PartialOrd, Hash, Debug, Clone, Copy, Serialize, Deserialize, Rand)]
@@ -59,7 +61,7 @@ impl<D: DistAlgorithm> TestNode<D> {
     }
 
     /// Creates a new test node with the given broadcast instance.
-    fn new((algo, step): (D, Step<D>)) -> TestNode<D> {
+    fn new((algo, step): (D, DaStep<D>)) -> TestNode<D> {
         TestNode {
             id: algo.our_id().clone(),
             algo,
@@ -403,7 +405,7 @@ where
         new_algo: F,
     ) -> TestNetwork<A, D>
     where
-        F: Fn(Arc<NetworkInfo<NodeId>>) -> (D, Step<D>),
+        F: Fn(Arc<NetworkInfo<NodeId>>) -> (D, DaStep<D>),
         G: Fn(BTreeMap<D::NodeId, Arc<NetworkInfo<D::NodeId>>>) -> A,
     {
         let mut rng = rand::thread_rng();