wip: milestone 2

2025-02-01 08:51:09 +03:00 · 2022-02-10 20:33:35 -05:00 · 2022-02-10 20:33:35 -05:00 · 68929fd493
commit 68929fd493
parent a7c811fdd6
12 changed files with 3952 additions and 197 deletions
--- a/Cargo.lock
+++ b/Cargo.lock
--- a/Cargo.toml
+++ b/Cargo.toml
@ -10,19 +10,31 @@ blake2 = { package = "blake2-rfc", version = "0.2"}
 byteorder = "1"
 bytes = "0.5.6"
 chacha20 = "0.8.1"
+clap = { version = "2.33", features = ["yaml"] }
 failure = "0.1.8"
 futures = "0.3"
-hmac = { version = "0.11.0", features = ["std"]}
+hmac = { version = "0.12.0", features = ["std"]}
 hyper = { version = "0.14", features = ["full"] }
 jsonrpc-core = "18.0"
 jsonrpc-derive = "18.0"
 jsonrpc-http-server = "18.0"
 lazy_static = "1"
+pbkdf2 = "0.8.0"
 rand = "0.8.4"
+ring = "0.16"
 serde = { version = "1", features= ["derive"]}
 serde_derive = "1"
 serde_json = "1"
-sha2 = "0.9.8"
+sha2 = "0.10.0"
 tokio = { version = "1", features = ["full"] }
+toml = "0.5"
 grin_secp256k1zkp = { version = "0.7.11", features = ["bullet-proof-sizing"]}
 grin_util = "5"
+grin_api = { git = "https://github.com/mimblewimble/grin", version = "5.2.0-alpha.1" }
+grin_core = { git = "https://github.com/mimblewimble/grin", version = "5.2.0-alpha.1" }
+grin_chain = { git = "https://github.com/mimblewimble/grin", version = "5.2.0-alpha.1" }
+grin_keychain = { git = "https://github.com/mimblewimble/grin", version = "5.2.0-alpha.1" }
+grin_servers = { git = "https://github.com/mimblewimble/grin", version = "5.2.0-alpha.1" }
+grin_wallet_api = { git = "https://github.com/mimblewimble/grin-wallet", version = "5.1.0-alpha.1" }
+grin_wallet_impls = { git = "https://github.com/mimblewimble/grin-wallet", version = "5.1.0-alpha.1" }
+grin_wallet_libwallet = { git = "https://github.com/mimblewimble/grin-wallet", version = "5.1.0-alpha.1" }
--- a/mwixnet.yml
+++ b/mwixnet.yml
@ -0,0 +1,37 @@
+name: mwixnet
+about: MWixnet CoinSwap Server
+author: scilio
+
+args:
+  - config_file:
+      help: Path to load/save the mwixnet-config.toml configuration file
+      short: c
+      long: config_file
+      takes_value: true
+  - pass:
+      help: Password to encrypt/decrypt the server key in the configuration file
+      short: p
+      long: pass
+      takes_value: true
+      required: true
+  - grin_node_url:
+      help: Api address of running GRIN node on which to check inputs and post transactions
+      short: n
+      long: grin_node_url
+      takes_value: true
+  - wallet_owner_url:
+      help: Api address of running wallet owner listener
+      short: l
+      long: wallet_owner_url
+      takes_value: true
+  - wallet_pass:
+      help: The wallet's password
+      long: wallet_pass
+      takes_value: true
+  - bind_addr:
+      help: Address to bind the rpc server to (e.g. 0.0.0.0:3000)
+      long: bind_addr
+      takes_value: true
+subcommands:
+  - init-config:
+      about: Writes a new configuration file
--- a/src/config.rs
+++ b/src/config.rs
@ -0,0 +1,177 @@
+use crate::error::{self, Result};
+use crate::secp::SecretKey;
+
+use core::num::NonZeroU32;
+use grin_util::{ToHex, ZeroingString};
+use rand::{thread_rng, Rng};
+use ring::{aead, pbkdf2};
+use serde_derive::{Deserialize, Serialize};
+use std::fs::File;
+use std::io::prelude::*;
+use std::net::SocketAddr;
+use std::path::PathBuf;
+
+// The decrypted server config to be passed around and used by the rest of the mwixnet code
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
+pub struct ServerConfig {
+	pub key: SecretKey,
+    pub addr: SocketAddr,
+	pub grin_node_url: SocketAddr,
+	pub wallet_owner_url: SocketAddr,
+}
+
+/// Encrypted server key, for storing on disk and decrypting with provided password
+#[derive(Serialize, Deserialize, Clone, Debug, PartialEq)]
+struct EncryptedServerKey {
+	encrypted_key: String,
+	pub salt: String,
+	pub nonce: String,
+}
+
+impl EncryptedServerKey {
+	pub fn from_secret_key(
+		server_key: &SecretKey,
+		password: &ZeroingString,
+	) -> Result<EncryptedServerKey> {
+		let salt: [u8; 8] = thread_rng().gen();
+		let nonce: [u8; 12] = thread_rng().gen();
+		let password = password.as_bytes();
+		let mut key = [0; 32];
+		ring::pbkdf2::derive(
+			ring::pbkdf2::PBKDF2_HMAC_SHA512,
+			NonZeroU32::new(100).unwrap(),
+			&salt,
+			password,
+			&mut key,
+		);
+		let content = server_key.0.to_vec();
+		let mut enc_bytes = content;
+
+		let unbound_key = aead::UnboundKey::new(&aead::CHACHA20_POLY1305, &key).unwrap();
+		let sealing_key: aead::LessSafeKey = aead::LessSafeKey::new(unbound_key);
+		let aad = aead::Aad::from(&[]);
+		let _ = sealing_key.seal_in_place_append_tag(
+			aead::Nonce::assume_unique_for_key(nonce),
+			aad,
+			&mut enc_bytes,
+		).map_err(|_| error::ErrorKind::SaveConfigError)?;
+
+		Ok(EncryptedServerKey {
+			encrypted_key: enc_bytes.to_hex(),
+			salt: salt.to_hex(),
+			nonce: nonce.to_hex(),
+		})
+	}
+
+	pub fn decrypt(&self, password: &str) -> Result<SecretKey> {
+		let mut encrypted_seed = grin_util::from_hex(&self.encrypted_key.clone())
+        	.map_err(|_| error::ErrorKind::LoadConfigError)?;
+		let salt = grin_util::from_hex(&self.salt.clone())
+        	.map_err(|_| error::ErrorKind::LoadConfigError)?;
+		let nonce = grin_util::from_hex(&self.nonce.clone())
+        	.map_err(|_| error::ErrorKind::LoadConfigError)?;
+		let password = password.as_bytes();
+		let mut key = [0; 32];
+		pbkdf2::derive(
+			ring::pbkdf2::PBKDF2_HMAC_SHA512,
+			NonZeroU32::new(100).unwrap(),
+			&salt,
+			password,
+			&mut key,
+		);
+
+		let mut n = [0u8; 12];
+		n.copy_from_slice(&nonce[0..12]);
+		let unbound_key = aead::UnboundKey::new(&aead::CHACHA20_POLY1305, &key).unwrap();
+		let opening_key: aead::LessSafeKey = aead::LessSafeKey::new(unbound_key);
+		let aad = aead::Aad::from(&[]);
+		let _ = opening_key.open_in_place(
+			aead::Nonce::assume_unique_for_key(n),
+			aad,
+			&mut encrypted_seed,
+		).map_err(|_| error::ErrorKind::LoadConfigError)?;
+
+		for _ in 0..aead::AES_256_GCM.tag_len() {
+			encrypted_seed.pop();
+		}
+
+		let secp = secp256k1zkp::Secp256k1::new();
+		Ok(SecretKey::from_slice(&secp, &encrypted_seed).unwrap())
+	}
+}
+
+/// The config attributes saved to disk
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
+struct RawConfig {
+	pub encrypted_key: String,
+	pub salt: String,
+	pub nonce: String,
+    pub addr: SocketAddr,
+	pub grin_node_url: SocketAddr,
+	pub wallet_owner_url: SocketAddr,
+}
+
+/// Writes the server config to the config_path given, encrypting the server_key first.
+pub fn write_config(config_path: &PathBuf, server_config: &ServerConfig,  password: &ZeroingString) -> Result<()> {
+	let encrypted = EncryptedServerKey::from_secret_key(&server_config.key, &password)?;
+
+	let raw_config = RawConfig{
+		encrypted_key: encrypted.encrypted_key,
+		salt: encrypted.salt,
+		nonce: encrypted.nonce,
+		addr: server_config.addr,
+		grin_node_url: server_config.grin_node_url,
+		wallet_owner_url: server_config.wallet_owner_url,
+	};
+	let encoded: String = toml::to_string(&raw_config).map_err(|_| error::ErrorKind::SaveConfigError)?;
+	
+	let mut file = File::create(config_path)?;
+	file.write_all(encoded.as_bytes()).map_err(|_| error::ErrorKind::SaveConfigError)?;
+
+	Ok(())
+}
+
+/// Reads the server config from the config_path given and decrypts it with the provided password.
+pub fn load_config(config_path: &PathBuf, password: &ZeroingString) -> Result<ServerConfig> {
+    let contents = std::fs::read_to_string(config_path)?;
+    let raw_config: RawConfig = toml::from_str(&contents)
+        .map_err(|_| error::ErrorKind::LoadConfigError)?;
+	
+    let encrypted_key = EncryptedServerKey{
+		encrypted_key: raw_config.encrypted_key,
+		salt: raw_config.salt,
+		nonce: raw_config.nonce
+	};
+	let secret_key = encrypted_key.decrypt(&password)?;
+    
+	Ok(ServerConfig {
+		key: secret_key,
+		addr: raw_config.addr,
+		grin_node_url: raw_config.grin_node_url,
+		wallet_owner_url: raw_config.wallet_owner_url
+	})
+}
+
+#[cfg(test)]
+mod tests {
+	use super::*;
+	use crate::secp;
+
+	#[test]
+	fn server_key_encrypt() {
+		let password = ZeroingString::from("password");
+		let server_key = secp::random_secret();
+		let mut enc_key = EncryptedServerKey::from_secret_key(&server_key, &password).unwrap();
+		let decrypted_key = enc_key.decrypt(&password).unwrap();
+		assert_eq!(server_key, decrypted_key);
+
+		// Wrong password
+		let decrypted_key = enc_key.decrypt("wrongpass");
+		assert!(decrypted_key.is_err());
+
+		// Wrong nonce
+		enc_key.nonce = "wrongnonce".to_owned();
+		let decrypted_key = enc_key.decrypt(&password);
+		assert!(decrypted_key.is_err());
+	}
+}
--- a/src/error.rs
+++ b/src/error.rs
@ -1,6 +1,7 @@
 use failure::{self, Context, Fail};
 use std::fmt::{self, Display};
 use std::io;
+use grin_wallet_libwallet as libwallet;

 /// MWixnet error definition
 #[derive(Debug)]
@ -45,6 +46,27 @@ pub enum ErrorKind {
    /// When asked to read too much data
    #[fail(display = "TooLargeReadErr")]
    TooLargeReadErr,
+    /// Error from grin's api crate
+    #[fail(display = "GRIN API Error")]
+    GrinApiError,
+    /// Error from grin core
+    #[fail(display = "grincore Error")]
+    GrinCoreError,
+    /// Error from grin-wallet's libwallet
+    #[fail(display = "libwallet Error")]
+    LibWalletError,
+    /// Error from serde-json
+    #[fail(display = "serde json Error")]
+    SerdeJsonError,
+    /// Error from invalid signature
+    #[fail(display = "invalid signature Error")]
+    InvalidSigError,
+    /// Error while saving config
+    #[fail(display = "save config Error")]
+    SaveConfigError,
+    /// Error while loading config
+    #[fail(display = "load config Error")]
+    LoadConfigError,
 }

 impl std::error::Error for Error {
@ -70,6 +92,12 @@ impl Display for Error {
 }

 impl Error {
+    pub fn new(kind: ErrorKind) -> Error {
+        Error {
+            inner: Context::new(kind),
+        }
+    }
+
    pub fn kind(&self) -> ErrorKind {
        self.inner.get_context().clone()
    }
@ -101,10 +129,50 @@ impl From<secp256k1zkp::Error> for Error {
    }
 }

-impl From<hmac::crypto_mac::InvalidKeyLength> for Error {
-    fn from(_error: hmac::crypto_mac::InvalidKeyLength) -> Error {
+impl From<hmac::digest::InvalidLength> for Error {
+    fn from(_error: hmac::digest::InvalidLength) -> Error {
        Error {
            inner: Context::new(ErrorKind::InvalidKeyLength),
        }
    }
 }
+
+impl From<grin_api::Error> for Error {
+    fn from(_error: grin_api::Error) -> Error {
+        Error {
+            inner: Context::new(ErrorKind::GrinApiError),
+        }
+    }
+}
+
+impl From<grin_api::json_rpc::Error> for Error {
+    fn from(_error: grin_api::json_rpc::Error) -> Error {
+        Error {
+            inner: Context::new(ErrorKind::GrinApiError),
+        }
+    }
+}
+
+impl From<grin_core::core::transaction::Error> for Error {
+    fn from(_error: grin_core::core::transaction::Error) -> Error {
+        Error {
+            inner: Context::new(ErrorKind::GrinCoreError),
+        }
+    }
+}
+
+impl From<libwallet::Error> for Error {
+    fn from(_error: libwallet::Error) -> Error {
+        Error {
+            inner: Context::new(ErrorKind::LibWalletError),
+        }
+    }
+}
+
+impl From<serde_json::Error> for Error {
+    fn from(_error: serde_json::Error) -> Error {
+        Error {
+            inner: Context::new(ErrorKind::SerdeJsonError),
+        }
+    }
+}
--- a/src/main.rs
+++ b/src/main.rs
@ -1,22 +1,85 @@
-use server::ServerConfig;
+use config::ServerConfig;
+use error::{Error, ErrorKind};
+use wallet::Wallet;
+
+use clap::App;
+use grin_util::ZeroingString;
+use std::env;
+use std::path::PathBuf;

 #[macro_use]
 extern crate lazy_static;

+#[macro_use]
+extern crate clap;
+
+mod config;
 mod error;
+mod node;
 mod onion;
 mod secp;
 mod ser;
 mod server;
 mod types;
+mod wallet;

-fn main() -> Result<(), Box<dyn std::error::Error>> {
-    let secret_key = secp::insecure_rand_secret()?; // todo - load from encrypted key file
-    let server_config = ServerConfig {
-        key: secret_key,
-        addr: "127.0.0.1:3000".parse().unwrap(),
-        is_first: true
+fn main() -> std::result::Result<(), Box<dyn std::error::Error>> {
+	let yml = load_yaml!("../mwixnet.yml");
+	let args = App::from_yaml(yml).get_matches();
+    
+    let config_path = match args.value_of("config_file") {
+        Some(path) => PathBuf::from(path),
+        None => {
+            let mut current_dir = env::current_dir()?;
+            current_dir.push("mwixnet-config.toml");
+            current_dir
+        }
    };
+    let password = args.value_of("pass").ok_or(Error::new(ErrorKind::LoadConfigError))?;
+    let password = ZeroingString::from(password);
+
+    let bind_addr = args.value_of("bind_addr");
+    let grin_node_url = args.value_of("grin_node_url");
+    let wallet_owner_url = args.value_of("wallet_owner_url");
+    
+    // Write a new config file if init-config command is supplied
+    if let ("init-config", Some(_)) = args.subcommand() {
+        if config_path.exists() {
+            panic!("Config file already exists at {}", config_path.to_string_lossy());
+        }
+
+        let server_config = ServerConfig {
+            key: secp::random_secret(),
+            addr: bind_addr.unwrap_or("0.0.0.0:3000").parse()?,
+            grin_node_url: grin_node_url.unwrap_or("127.0.0.1:3413").parse()?,
+            wallet_owner_url: wallet_owner_url.unwrap_or("127.0.0.1:3420").parse()?,
+        };
+
+        config::write_config(&config_path, &server_config, &password)?;
+        return Ok(());
+    }
+
+    let mut server_config = config::load_config(&config_path, &password)?;
+
+    // Override bind_addr, if supplied
+    if let Some(bind_addr) = bind_addr {
+        server_config.addr = bind_addr.parse()?;
+    }
+
+    // Override grin_node_url, if supplied
+    if let Some(grin_node_url) = grin_node_url {
+        server_config.grin_node_url = grin_node_url.parse()?;
+    }
+
+    // Override wallet_owner_url, if supplied
+    if let Some(wallet_owner_url) = wallet_owner_url {
+        server_config.wallet_owner_url = wallet_owner_url.parse()?;
+    }
+
+    // Open wallet
+    let wallet_pass = args.value_of("wallet_pass").ok_or(error::Error::new(error::ErrorKind::LoadConfigError))?;
+    let wallet_pass = grin_util::ZeroingString::from(wallet_pass);
+    let wallet = Wallet::open_wallet(&server_config.wallet_owner_url, &wallet_pass)?;

    let shutdown_signal = async move {
        // Wait for the CTRL+C signal
@ -24,6 +87,5 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
            .await
            .expect("failed to install CTRL+C signal handler");
    };
-
-    server::listen(&server_config, shutdown_signal)
+    server::listen(&server_config, &wallet, shutdown_signal)
 }
--- a/src/node.rs
+++ b/src/node.rs
@ -0,0 +1,169 @@
+use crate::config::ServerConfig;
+use crate::error::{ErrorKind, Result};
+use crate::secp::Commitment;
+
+use grin_api::client;
+use grin_api::json_rpc::{build_request, Request, Response};
+use grin_api::{BlockPrintable, LocatedTxKernel, OutputPrintable, OutputType, Tip};
+use grin_core::consensus::COINBASE_MATURITY;
+use grin_core::core::{Input, OutputFeatures, Transaction};
+use grin_util::ToHex;
+
+use serde_json::json;
+use std::net::SocketAddr;
+
+const ENDPOINT: &str = "/v2/foreign";
+
+#[derive(Clone)]
+pub struct HTTPNodeClient {
+    node_url: SocketAddr,
+    node_api_secret: Option<String>,
+}
+
+impl HTTPNodeClient {
+    /// Create a new client that will communicate with the given grin node
+    pub fn new(node_url: &SocketAddr, node_api_secret: Option<String>) -> HTTPNodeClient {
+        HTTPNodeClient {
+            node_url: node_url.to_owned(),
+            node_api_secret: node_api_secret,
+        }
+    }
+
+    fn send_json_request<D: serde::de::DeserializeOwned>(
+        &self,
+        method: &str,
+        params: &serde_json::Value,
+    ) -> Result<D> {
+        let url = format!("http://{}{}", self.node_url, ENDPOINT);
+        let req = build_request(method, params);
+        let res =
+            client::post::<Request, Response>(url.as_str(), self.node_api_secret.clone(), &req)?;
+        let parsed = res.clone().into_result()?;
+        Ok(parsed)
+    }
+}
+
+#[derive(Clone)]
+pub struct GrinNode {
+    client: HTTPNodeClient,
+}
+
+impl GrinNode {
+    pub fn new(server_config: &ServerConfig) -> GrinNode {
+        GrinNode { 
+            client: HTTPNodeClient::new(&server_config.grin_node_url, None),
+        }
+    }
+
+    // Checks whether a commitment is spendable at the block height provided
+    pub fn is_spendable(&self, output_commit: &Commitment, next_block_height: u64) -> Result<bool> {
+        let output = self.get_output(&output_commit)?;
+        if let Some(out) = output {
+            let is_coinbase = match out.output_type {
+                OutputType::Coinbase => true,
+                OutputType::Transaction => false,
+            };
+
+            if is_coinbase {
+                if let Some(block_height) = out.block_height {
+                    if block_height + COINBASE_MATURITY < next_block_height {
+                        return Ok(false);
+                    }
+                } else {
+                    return Ok(false);
+                }
+            }
+
+            return Ok(true);
+        }
+
+        Ok(false)
+    }
+
+    /// Builds an input for an unspent output commitment
+    pub fn build_input(&self, output_commit: &Commitment) -> Result<Option<Input>> {
+        let output = self.get_output(&output_commit)?;
+
+        if let Some(out) = output {
+            let features = match out.output_type {
+                OutputType::Coinbase => OutputFeatures::Coinbase,
+                OutputType::Transaction => OutputFeatures::Plain,
+            };
+
+            let input = Input::new(features, out.commit);
+            return Ok(Some(input));
+        }
+
+        Ok(None)
+    }
+
+    fn get_output(&self, output_commit: &Commitment) -> Result<Option<OutputPrintable>> {
+        let commits : Vec<String> = vec![output_commit.to_hex()];
+        let start_height : Option<u64> = None;
+        let end_height : Option<u64> = None;
+        let include_proof : Option<bool> = Some(false);
+        let include_merkle_proof : Option<bool> = Some(false);
+
+        let params = json!([Some(commits), start_height, end_height, include_proof, include_merkle_proof]);
+        let outputs = self.client.send_json_request::<Vec<OutputPrintable>>("get_outputs", &params)?;
+        if outputs.is_empty() {
+            return Ok(None);
+        }
+
+        Ok(Some(outputs[0].clone()))
+    }
+
+    /// Gets the height of the chain tip
+    pub fn get_chain_height(&self) -> Result<u64> {
+        let params = json!([]);
+        let tip_json = self.client.send_json_request::<serde_json::Value>("get_tip", &params)?;
+
+        let tip: Result<Tip> = serde_json::from_value(tip_json["Ok"].clone())
+            .map_err(|_| ErrorKind::SerdeJsonError.into());
+        
+        Ok(tip?.height)
+    }
+
+    /// Posts a transaction to the grin node
+    pub fn post_tx(&self, tx: &Transaction) -> Result<()> {
+        let params = json!([tx, true]);
+        self.client.send_json_request::<serde_json::Value>("push_transaction", &params)?;
+        Ok(())
+    }
+
+    // milestone 3: needed to handle chain reorgs
+    pub fn chain_has_kernel(
+        &self,
+        kernel_excess: &Commitment,
+        start_height: Option<u64>,
+        end_height: Option<u64>,
+    ) -> Result<bool> {
+        let params = json!([kernel_excess, start_height, end_height]);
+        let located_kernel_json = self.client.send_json_request::<serde_json::Value>("get_kernel", &params)?;
+
+        let located_kernel : Result<LocatedTxKernel> = serde_json::from_value(located_kernel_json["Ok"].clone())
+            .map_err(|_| ErrorKind::SerdeJsonError.into());
+
+        Ok(located_kernel.is_ok())
+    }
+
+    // milestone 3: needed to handle chain reorgs
+    pub fn block_has_kernel(&self, block_hash: &String, kernel_excess: &Commitment) -> Result<bool> {
+        let block = self.get_block(None, Some(block_hash.clone()), None)?;
+
+        let found = block.kernels.into_iter()
+            .any(|kern| kern.excess == kernel_excess.to_hex());
+        Ok(found)
+    }
+
+    // milestone 3: needed to handle chain reorgs
+    fn get_block(&self,
+        height: Option<u64>,
+        hash: Option<String>,
+        commit: Option<String>,
+    ) -> Result<BlockPrintable> {
+        let params = json!([height, hash, commit]);
+        let block_printable = self.client.send_json_request::<BlockPrintable>("get_block", &params)?;
+        Ok(block_printable)
+    }
+}
--- a/src/onion.rs
+++ b/src/onion.rs
@ -5,12 +5,13 @@ use crate::ser;

 use chacha20::{ChaCha20, Key, Nonce};
 use chacha20::cipher::{NewCipher, StreamCipher};
-use hmac::{Hmac, Mac, NewMac};
+use hmac::{Hmac, Mac};
 use sha2::{Digest, Sha256};

 type HmacSha256 = Hmac<Sha256>;

 /// Create an Onion for the Commitment, encrypting the payload for each hop
+#[allow(dead_code)] // used by component tests
 pub fn create_onion(commitment: &Commitment, session_key: &SecretKey, hops: &Vec<Hop>) -> Result<Onion> {
    let secp = Secp256k1::new();
    let mut ephemeral_key = session_key.clone();
@ -71,6 +72,7 @@ pub fn peel_layer(onion: &Onion, secret_key: &SecretKey) -> Result<(Payload, Oni

    let mut commitment = onion.commit.clone();
    commitment = secp::add_excess(&commitment, &decrypted_payload.excess)?;
+    commitment = secp::sub_value(&commitment, decrypted_payload.fee.into())?;

    let peeled_onion = Onion{
        ephemeral_pubkey: ephemeral_pubkey,
@ -109,30 +111,35 @@ mod tests {
    use super::super::types;
    use super::super::onion;

+    use grin_core::core::FeeFields;
+    
    /// Test end-to-end Onion creation and unwrapping logic.
    #[test]
    fn onion() {
-        let value : u64 = 1000;
-        let blind = secp::insecure_rand_secret().unwrap(); 
-        let commitment = secp::commit(value, &blind).unwrap();
+        let total_fee : u64 = 10;
+        let fee_per_hop : u64 = 2;
+        let in_value : u64 = 1000;
+        let out_value : u64 = in_value - total_fee;
+        let blind = secp::random_secret(); 
+        let commitment = secp::commit(in_value, &blind).unwrap();
    
-        let session_key = secp::insecure_rand_secret().unwrap();
+        let session_key = secp::random_secret();
        let mut hops : Vec<types::Hop> = Vec::new();
    
        let mut keys : Vec<secp::SecretKey> = Vec::new();
-        let mut final_commit = commitment.clone();
+        let mut final_commit = secp::commit(out_value, &blind).unwrap();
        let mut final_blind = blind.clone();
        for i in 0..5 {
-            keys.push(secp::insecure_rand_secret().unwrap());
+            keys.push(secp::random_secret());
    
-            let excess = secp::insecure_rand_secret().unwrap();
+            let excess = secp::random_secret();
    
            let secp = secp256k1zkp::Secp256k1::with_caps(secp256k1zkp::ContextFlag::Commit);
            final_blind.add_assign(&secp, &excess).unwrap();
            final_commit = secp::add_excess(&final_commit, &excess).unwrap();
            let proof = if i == 4 {
-                let n1 = secp::insecure_rand_secret().unwrap();
-                let rp = secp.bullet_proof(value, final_blind.clone(), n1.clone(), n1.clone(), None, None);
+                let n1 = secp::random_secret();
+                let rp = secp.bullet_proof(out_value, final_blind.clone(), n1.clone(), n1.clone(), None, None);
                assert!(secp.verify_bullet_proof(final_commit, rp, None).is_ok());
                Some(rp)
            } else {
@ -143,6 +150,7 @@ mod tests {
                pubkey: secp::PublicKey::from_secret_key(&secp, &keys[i]).unwrap(),
                payload: types::Payload{
                    excess: excess,
+                    fee: FeeFields::from(fee_per_hop as u32),
                    rangeproof: proof,
                }
            });
@ -151,7 +159,8 @@ mod tests {
        let mut onion_packet = onion::create_onion(&commitment, &session_key, &hops).unwrap();
    
        let mut payload = types::Payload{
-            excess: secp::insecure_rand_secret().unwrap(),
+            excess: secp::random_secret(),
+            fee: FeeFields::from(fee_per_hop as u32),
            rangeproof: None
        };
        for i in 0..5 {
@ -162,6 +171,7 @@ mod tests {
    
        assert!(payload.rangeproof.is_some());
        assert_eq!(payload.rangeproof.unwrap(), hops[4].payload.rangeproof.unwrap());
-        assert_eq!(secp::commit(value, &final_blind).unwrap(), final_commit);
+        assert_eq!(secp::commit(out_value, &final_blind).unwrap(), final_commit);
+        assert_eq!(payload.fee, FeeFields::from(fee_per_hop as u32));
    }
 }
--- a/src/secp.rs
+++ b/src/secp.rs
@ -1,56 +1,102 @@
 pub use secp256k1zkp::{ContextFlag, Message, Secp256k1, Signature};
+pub use secp256k1zkp::aggsig;
 pub use secp256k1zkp::ecdh::SharedSecret;
 pub use secp256k1zkp::pedersen::{Commitment, RangeProof};
-pub use secp256k1zkp::key::{PublicKey, SecretKey};
+pub use secp256k1zkp::key::{PublicKey, SecretKey, ZERO_KEY};
 pub use secp256k1zkp::constants::{AGG_SIGNATURE_SIZE, COMPRESSED_PUBLIC_KEY_SIZE, MAX_PROOF_SIZE, PEDERSEN_COMMITMENT_SIZE, SECRET_KEY_SIZE};

-use crate::ser::{Readable, Reader, Writeable, Writer};
-use crate::error::{ErrorKind, Result};
+use crate::ser::{self, Readable, Reader, Writeable, Writer};
+use crate::error::{Error, ErrorKind, Result};

-use rand::RngCore;
+use blake2::blake2b::Blake2b;
+use byteorder::{BigEndian, ByteOrder};
+use secp256k1zkp::rand::thread_rng;
 use std::cmp;

 /// A generalized Schnorr signature with a pedersen commitment value & blinding factors as the keys
-pub const COM_SIGNATURE_SIZE : usize = 96;
-
-pub struct ComSignature(pub [u8; COM_SIGNATURE_SIZE]);
-impl ComSignature {
-    /// Builds a ComSignature from a byte vector. If the vector is too short, it will be
-    /// completed by zeroes. If it's too long, it will be truncated.
-    pub fn from_vec(v: Vec<u8>) -> ComSignature {
-        let mut h = [0; COM_SIGNATURE_SIZE];
-        for i in 0..cmp::min(v.len(), COM_SIGNATURE_SIZE) {
-            h[i] = v[i];
+pub struct ComSignature {
+    pub pub_nonce: Commitment,
+    pub s: SecretKey,
+    pub t: SecretKey,
+}
+
+impl ComSignature {
+    pub fn new(pub_nonce: &Commitment, s: &SecretKey, t: &SecretKey) -> ComSignature {
+        ComSignature {
+            pub_nonce: pub_nonce.clone(),
+            s: s.clone(),
+            t: t.clone(),
        }
-        ComSignature(h)
    }

    #[allow(dead_code)]
-    pub fn sign(_value: u64, _blind: &SecretKey, _msg: &Vec<u8>) -> Result<ComSignature> {
-        // milestone 2 - todo
-        let mut h = [0u8; COM_SIGNATURE_SIZE];
-        for i in 0..COM_SIGNATURE_SIZE {
-            h[i] = i as u8;
-        }
-        Ok(ComSignature(h))
+    pub fn sign(amount: u64, blind: &SecretKey, msg: &Vec<u8>) -> Result<ComSignature> {
+        let secp = Secp256k1::with_caps(ContextFlag::Commit);
+
+		let mut amt_bytes = [0; 32];
+		BigEndian::write_u64(&mut amt_bytes[24..31], amount);
+        let k_amt = SecretKey::from_slice(&secp, &amt_bytes)?;
+        
+        let k_1 = SecretKey::new(&secp, &mut thread_rng());
+        let k_2 = SecretKey::new(&secp, &mut thread_rng());
+
+        let commitment = secp.commit(amount, blind.clone())?;
+        let nonce_commitment = secp.commit_blind(k_1.clone(), k_2.clone())?;
+
+        let e = ComSignature::calc_challenge(&secp, &commitment, &nonce_commitment, &msg)?;
+
+        // s = k_1 + (e * amount)
+        let mut s = k_amt.clone();
+        s.mul_assign(&secp, &e)?;
+        s.add_assign(&secp, &k_1)?;
+
+        // t = k_2 + (e * blind)
+        let mut t = blind.clone();
+        t.mul_assign(&secp, &e)?;
+        t.add_assign(&secp, &k_2)?;
+
+        Ok(ComSignature::new(&nonce_commitment, &s, &t))
+    }
+
+    #[allow(non_snake_case)]
+    pub fn verify(&self, commit: &Commitment, msg: &Vec<u8>) -> Result<()> {
+        let secp = Secp256k1::with_caps(ContextFlag::Commit);
+
+        let S1 = secp.commit_blind(self.s.clone(), self.t.clone())?;
+
+        let mut Ce = commit.to_pubkey(&secp)?;
+        let e = ComSignature::calc_challenge(&secp, &commit, &self.pub_nonce, &msg)?;
+        Ce.mul_assign(&secp, &e)?;
+
+        let commits = vec![Commitment::from_pubkey(&secp, &Ce)?, self.pub_nonce.clone()];
+        let S2 = secp.commit_sum(commits, Vec::new())?;
+
+        if S1 == S2 {
+            return Err(Error::new(ErrorKind::InvalidSigError));
        }

-    pub fn verify(self, _commit: &Commitment, _msg: &Vec<u8>) -> Result<()> {
-        // milestone 2 - todo
        Ok(())
    }
-}

-impl AsRef<[u8]> for ComSignature {
-    fn as_ref(&self) -> &[u8] {
-        &self.0
+    fn calc_challenge(secp: &Secp256k1, commit: &Commitment, nonce_commit: &Commitment, msg: &Vec<u8>) -> Result<SecretKey> {
+        let mut challenge_hasher = Blake2b::new(32);
+        challenge_hasher.update(&commit.0);
+        challenge_hasher.update(&nonce_commit.0);
+        challenge_hasher.update(msg);
+
+		let mut challenge = [0; 32];
+		challenge.copy_from_slice(challenge_hasher.finalize().as_bytes());
+
+        Ok(SecretKey::from_slice(&secp, &challenge)?)
    }
 }

 /// Serializes a ComSignature to and from hex
 pub mod comsig_serde {
    use super::ComSignature;
+    use super::ser::{self, BinReader, Readable};
    use serde::{Deserialize, Serializer};
+    use std::io::Cursor;
    use grin_util::ToHex;

    /// Serializes a ComSignature as a hex string
@ -58,7 +104,9 @@ pub mod comsig_serde {
    where
        S: Serializer,
    {
-        serializer.serialize_str(&comsig.to_hex())
+        use serde::ser::Error;
+        let bytes = ser::ser_vec(&comsig).map_err(Error::custom)?;
+        serializer.serialize_str(&bytes.to_hex())
    }

    /// Creates a ComSignature from a hex string
@ -67,9 +115,12 @@ pub mod comsig_serde {
        D: serde::Deserializer<'de>,
    {
        use serde::de::Error;
-        String::deserialize(deserializer)
-            .and_then(|string| grin_util::from_hex(&string).map_err(Error::custom))
-            .and_then(|bytes: Vec<u8>| Ok(ComSignature::from_vec(bytes.to_vec())))
+        let bytes = String::deserialize(deserializer)
+            .and_then(|string| grin_util::from_hex(&string).map_err(Error::custom))?;
+        
+        let mut cursor = Cursor::new(&bytes);
+        let mut reader = BinReader::new(&mut cursor);
+        ComSignature::read(&mut reader).map_err(Error::custom)
    }
 }

@ -80,13 +131,10 @@ pub fn to_public_key(secret_key: &SecretKey) -> Result<PublicKey> {
    Ok(pubkey)
 }

-/// Generate a random SecretKey. Not for production use
-pub fn insecure_rand_secret() -> Result<SecretKey> {
+/// Generate a random SecretKey.
+pub fn random_secret() -> SecretKey {
    let secp = Secp256k1::new();
-    let mut seed = [0u8; 32];
-    rand::thread_rng().fill_bytes(&mut seed);
-    let secret = SecretKey::from_slice(&secp, &seed)?;
-    Ok(secret)
+    SecretKey::new(&secp, &mut thread_rng())
 }

 /// Build a Pedersen Commitment using the provided value and blinding factor
@ -106,6 +154,32 @@ pub fn add_excess(commitment: &Commitment, excess: &SecretKey) -> Result<Commitm
    Ok(sum)
 }

+pub fn sub_value(commitment: &Commitment, value: u64) -> Result<Commitment> {
+    let secp = Secp256k1::with_caps(ContextFlag::Commit);
+    let neg_commit : Commitment = secp.commit(value, ZERO_KEY)?;
+    let sum = secp.commit_sum(vec![commitment.clone()], vec![neg_commit.clone()])?;
+    Ok(sum)
+}
+
+pub fn add_blinds(excesses: &Vec<SecretKey>) -> Result<SecretKey> {
+    let secp = Secp256k1::with_caps(ContextFlag::Commit);
+    let sum = secp.blind_sum(excesses.clone(), Vec::new())?;
+    Ok(sum)
+}
+
+pub fn sub_blinds(minuend: &SecretKey, subtrahend: &SecretKey) -> Result<SecretKey> {
+    let secp = Secp256k1::with_caps(ContextFlag::Commit);
+    let result = secp.blind_sum(vec![minuend.clone()], vec![subtrahend.clone()])?;
+    Ok(result)
+}
+
+pub fn sign(sk: &SecretKey, msg: &Message) -> Result<Signature> {
+    let secp = Secp256k1::with_caps(ContextFlag::Full);
+    let pubkey = PublicKey::from_secret_key(&secp, &sk)?;
+    let sig = aggsig::sign_single(&secp, &msg, &sk, None, None, None, Some(&pubkey), None)?;
+    Ok(sig)
+}
+
 /// secp256k1-zkp object serialization

 impl Readable for Commitment {
@ -192,3 +266,21 @@ impl Writeable for SecretKey {
        Ok(())
    }
 }
+
+#[allow(non_snake_case)]
+impl Readable for ComSignature {
+    fn read<R: Reader>(reader: &mut R) -> Result<ComSignature> {
+        let R = Commitment::read(reader)?;
+        let s = SecretKey::read(reader)?;
+        let t = SecretKey::read(reader)?;
+        Ok(ComSignature::new(&R, &s, &t))
+    }
+}
+
+impl Writeable for ComSignature {
+    fn write<W: Writer>(&self, writer: &mut W) -> Result<()> {
+        writer.write_fixed_bytes(self.pub_nonce.0)?;
+        writer.write_fixed_bytes(self.s.0)?;
+        writer.write_fixed_bytes(self.t.0)
+    }
+}
--- a/src/server.rs
+++ b/src/server.rs
@ -1,26 +1,26 @@
+use crate::config::ServerConfig;
+use crate::node::GrinNode;
 use crate::onion;
-use crate::secp::{self, Commitment, ComSignature, SecretKey};
+use crate::secp::{self, ComSignature, SecretKey};
 use crate::ser;
 use crate::types::Onion;
+use crate::wallet::Wallet;

+use grin_core::core::{Input, Output, OutputFeatures, TransactionBody};
+use grin_core::global::DEFAULT_ACCEPT_FEE_BASE;
 use jsonrpc_derive::rpc;
 use jsonrpc_http_server::*;
 use jsonrpc_http_server::jsonrpc_core::*;
 use jsonrpc_core::{Result, Value};
 use serde::{Deserialize, Serialize};
-use std::net::SocketAddr;
-use std::sync::Mutex;
+use std::sync::{Arc, Mutex};

 #[derive(Clone, Debug, PartialEq)]
-pub struct ServerConfig {
-    pub key: SecretKey,
-    pub addr: SocketAddr,
-    pub is_first: bool,
-}
-
 pub struct Submission {
    pub excess: SecretKey,
-    pub input_commit: Commitment,
+    pub output: Option<Output>,
+    pub input: Input,
+    pub fee: u64,
    pub onion: Onion,
 }

@ -42,48 +42,130 @@ pub trait Server {
    #[rpc(name = "swap")]
    fn swap(&self, swap: SwapReq) -> Result<Value>;

-    // milestone 3:
+    // milestone 3: Used by mwixnet coinswap servers to communicate with each other
    // fn derive_outputs(&self, entries: Vec<Onion>) -> Result<Value>;
    // fn derive_kernel(&self, tx: Tx) -> Result<Value>;
 }

+#[derive(Clone)]
 pub struct ServerImpl {
-    server_key: SecretKey,
+    server_config: ServerConfig,
+
+    wallet: Wallet,
+
+    node: GrinNode,
 }

 impl ServerImpl {
-    pub fn new(server_key: SecretKey) -> Self {
-        ServerImpl { server_key }
+    pub fn new(server_config: ServerConfig, wallet: Wallet, node: GrinNode) -> Self {
+        ServerImpl { server_config, wallet, node }
+    }
+
+    /// The fee base to use. For now, just using the default.
+    fn get_fee_base(&self) -> u64 {
+        DEFAULT_ACCEPT_FEE_BASE
+    }
+
+    /// Minimum fee to perform a swap.
+    /// Requires enough fee for the mwixnet server's kernel, 1 input and its output to swap.
+    pub fn get_minimum_swap_fee(&self) -> u64 {
+        TransactionBody::weight_by_iok(1, 1, 1) * self.get_fee_base()
+    }
+
+    /// Iterate through all saved submissions, filter out any inputs that are no longer spendable,
+    /// and assemble the coinswap transaction, posting the transaction to the configured node.
+    /// 
+    /// Currently only a single mix node is used. Milestone 3 will include support for multiple mix nodes.
+    pub fn execute_round(&self) -> crate::error::Result<()> {
+        let locked_state = SERVER_STATE.lock().unwrap();
+        let next_block_height = self.node.get_chain_height()? + 1;
+
+        let spendable : Vec<Submission> = locked_state
+            .iter()
+            .filter(|s| self.node.is_spendable(&s.input.commit, next_block_height).unwrap_or(false))
+            .cloned()
+            .collect();
+
+        let total_fee : u64 = spendable
+            .iter()
+            .enumerate()
+            .map(|(_, s)| s.fee)
+            .sum();
+        
+        let inputs : Vec<Input> = spendable
+            .iter()
+            .enumerate()
+            .map(|(_, s)| s.input)
+            .collect();
+        
+        let outputs : Vec<Output> = spendable
+            .iter()
+            .enumerate()
+            .filter_map(|(_, s)| s.output)
+            .collect();
+        
+        let excesses : Vec<SecretKey> = spendable
+            .iter()
+            .enumerate()
+            .map(|(_, s)| s.excess.clone())
+            .collect();
+        
+		let excess_sum = secp::add_blinds(&excesses)?;
+
+        let tx = self.wallet.assemble_tx(&inputs, &outputs, total_fee, &excess_sum)?;
+        
+        self.node.post_tx(&tx)?;
+        
+        Ok(())
    }
 }

 impl Server for ServerImpl {
+    /// Implements the 'swap' API
    fn swap(&self, swap: SwapReq) -> Result<Value> {
-        // milestone 2 - check that commitment is unspent
-    
        // Verify commitment signature to ensure caller owns the output
        let _ = swap.comsig.verify(&swap.onion.commit, &swap.msg)
            .map_err(|_| jsonrpc_core::Error::invalid_params("ComSignature invalid"))?;

-        let peeled = onion::peel_layer(&swap.onion, &self.server_key)
+        // Verify that commitment is unspent
+        let input = self.node.build_input(&swap.onion.commit)
+            .map_err(|_| jsonrpc_core::Error::internal_error())?;
+        let input = input.ok_or(jsonrpc_core::Error::invalid_params("Commitment not found"))?;
+    
+        let peeled = onion::peel_layer(&swap.onion, &self.server_config.key)
            .map_err(|e| jsonrpc_core::Error::invalid_params(e.message()))?;
+
+        let fee: u64 = peeled.0.fee.into();
+        if fee < self.get_minimum_swap_fee() {
+            return Err(jsonrpc_core::Error::invalid_params("Fee does not meet minimum"));
+        }
+
+        let output_commit = secp::add_excess(&swap.onion.commit, &peeled.0.excess)
+            .map_err(|_| jsonrpc_core::Error::internal_error())?;
+        let output = match peeled.0.rangeproof {
+            Some(r) => Some(Output::new(OutputFeatures::Plain, output_commit, r)),
+            None => None
+        };
        SERVER_STATE.lock().unwrap().push(Submission{
            excess: peeled.0.excess,
-            input_commit: swap.onion.commit,
+            output: output,
+            input: input,
+            fee: fee,
            onion: peeled.1
        });
        Ok(Value::String("success".into()))
    }
-    
 }

 /// Spin up the JSON-RPC web server
-pub fn listen<F>(server_config: &ServerConfig, shutdown_signal: F) -> std::result::Result<(), Box<dyn std::error::Error>>
+pub fn listen<F>(server_config: &ServerConfig, wallet: &Wallet, shutdown_signal: F) -> std::result::Result<(), Box<dyn std::error::Error>>
 where
    F: futures::future::Future<Output = ()> + Send + 'static,
 {
+    let server_impl = Arc::new(ServerImpl::new(server_config.clone(), wallet.clone(), GrinNode::new(&server_config)));
+
    let mut io = IoHandler::new();
-    io.extend_with(ServerImpl::to_delegate(ServerImpl::new(server_config.key.clone())));
+    io.extend_with(ServerImpl::to_delegate(server_impl.as_ref().clone()));

    let server = ServerBuilder::new(io)
        .cors(DomainsValidation::Disabled)
@ -97,6 +179,13 @@ where
        .start_http(&server_config.addr)
        .expect("Unable to start RPC server");

+    std::thread::spawn(move || {
+        loop {
+            std::thread::sleep(std::time::Duration::from_secs(30)); // todo: Graceful shutdown
+            let _ = server_impl.as_ref().execute_round();
+        }
+    });
+
    let close_handle = server.close_handle();
    std::thread::spawn(move || {
        futures::executor::block_on(shutdown_signal);
@ -109,7 +198,8 @@ where

 #[cfg(test)]
 mod tests {
-    use crate::{onion, secp, server, types};
+    use crate::{config, onion, secp, server, types, wallet};
+    use grin_core::core::FeeFields;
    use std::net::TcpListener;
    use std::time::Duration;
    use std::thread;
@ -124,19 +214,22 @@ mod tests {

    /// Spin up a temporary web service, query the API, then cleanup and return response
    fn make_request(server_key: secp::SecretKey, req: String) -> Result<String, Box<dyn std::error::Error>> {
-        let server_config = server::ServerConfig { 
+        let server_config = config::ServerConfig { 
            key: server_key,
            addr: TcpListener::bind("127.0.0.1:0")?.local_addr()?,
-            is_first: true
+            grin_node_url: "127.0.0.1:3413".parse()?,
+            wallet_owner_url: "127.0.0.1:3420".parse()?
        };

        let threaded_rt = runtime::Runtime::new()?;
        let (shutdown_sender, shutdown_receiver) = futures::channel::oneshot::channel();
        let uri = format!("http://{}/v1", server_config.addr);

+        let wallet = wallet::Wallet::open_wallet(&server_config.wallet_owner_url, &grin_util::ZeroingString::from("wallet_pass"))?;
+
        // Spawn the server task
        threaded_rt.spawn(async move {
-            server::listen(&server_config, async { shutdown_receiver.await.ok(); }).unwrap()
+            server::listen(&server_config, &wallet, async { shutdown_receiver.await.ok(); }).unwrap()
        });

        // Wait for listener
@ -167,18 +260,20 @@ mod tests {
    /// UTXO creation and bulletproof generation reserved for milestones 2 & 3.
    #[test]
    fn swap_lifecycle() -> Result<(), Box<dyn std::error::Error>> {
-        let server_key = secp::insecure_rand_secret()?;
+        let server_key = secp::random_secret();
        
        let secp = secp::Secp256k1::new();
        let value: u64 = 100;
-        let blind = secp::insecure_rand_secret()?;
+        let fee: u64= 10;
+        let blind = secp::random_secret();
        let commitment = secp::commit(value, &blind)?;
-        let session_key = secp::insecure_rand_secret()?;
+        let session_key = secp::random_secret();

        let hop = types::Hop {
            pubkey: secp::PublicKey::from_secret_key(&secp, &server_key)?,
            payload: types::Payload{
-                excess: secp::insecure_rand_secret()?,
+                excess: secp::random_secret(),
+                fee: FeeFields::from(fee as u32),
                rangeproof: None,
            }
        };
@ -203,7 +298,7 @@ mod tests {
    fn swap_bad_request() -> Result<(), Box<dyn std::error::Error>> {
        let params = "{ \"param\": \"Not a valid Swap request\" }";
        let req = format!("{{\"jsonrpc\": \"2.0\", \"method\": \"swap\", \"params\": [{}], \"id\": \"1\"}}", params);
-        let response = make_request(secp::insecure_rand_secret()?, req)?;
+        let response = make_request(secp::random_secret(), req)?;
        let expected = "{\"jsonrpc\":\"2.0\",\"error\":{\"code\":-32602,\"message\":\"Invalid params: missing field `onion`.\"},\"id\":\"1\"}\n";
        assert_eq!(response, expected);
        Ok(())
--- a/src/types.rs
+++ b/src/types.rs
@ -1,6 +1,8 @@
 use crate::error::{ErrorKind, Result};
 use crate::secp::{Commitment, PublicKey, RangeProof, SecretKey, Secp256k1};
 use crate::ser::{self, BinReader, Readable, Reader, Writeable, Writer};
+
+use grin_core::core::FeeFields;
 use grin_util::{self, ToHex};
 use serde::{Deserialize, Serialize};
 use serde::ser::SerializeStruct;
@ -14,6 +16,7 @@ const CURRENT_VERSION : u8 = 0;
 #[derive(Serialize, Deserialize, Debug, Clone)]
 pub struct Payload {
    pub excess: SecretKey,
+    pub fee: FeeFields,
    pub rangeproof: Option<RangeProof>,
 }

@ -25,6 +28,7 @@ impl Readable for Payload {
        }

        let excess = SecretKey::read(reader)?;
+        let fee = FeeFields::try_from(reader.read_u64()?)?;
        let rangeproof = if reader.read_u8()? == 0 {
            None
        } else {
@ -33,6 +37,7 @@ impl Readable for Payload {

        let payload = Payload {
            excess: excess,
+            fee: fee,
            rangeproof: rangeproof
        };
        Ok(payload)
@ -43,6 +48,7 @@ impl Writeable for Payload {
    fn write<W: Writer>(&self, writer: &mut W) -> Result<()> {
        writer.write_u8(CURRENT_VERSION)?;
        writer.write_fixed_bytes(&self.excess)?;
+        writer.write_u64(self.fee.into())?;

        match &self.rangeproof {
            Some(proof) => {
@ -71,6 +77,7 @@ pub struct Hop {
    pub payload: Payload,
 }

+#[derive(Clone, Debug, PartialEq)]
 pub struct Onion {
    pub ephemeral_pubkey: PublicKey,
    pub commit: Commitment,
--- a/src/wallet.rs
+++ b/src/wallet.rs
@ -0,0 +1,91 @@
+use crate::error::Result;
+use crate::secp::{self, SecretKey};
+
+use grin_api::client;
+use grin_api::json_rpc::{build_request, Request, Response};
+use grin_core::core::{FeeFields, Input, Inputs, KernelFeatures, Output, Transaction, TxKernel};
+use grin_keychain::BlindingFactor;
+use grin_util::ZeroingString;
+use grin_wallet_api::Token;
+use serde_json::json;
+use std::net::SocketAddr;
+
+const ENDPOINT: &str = "/v3/owner";
+
+#[derive(Clone)]
+pub struct HTTPWalletClient {
+    wallet_owner_url: SocketAddr,
+}
+
+impl HTTPWalletClient {
+    /// Create a new client that will communicate with the given grin node
+    pub fn new(wallet_owner_url: &SocketAddr) -> HTTPWalletClient {
+        HTTPWalletClient {
+            wallet_owner_url: wallet_owner_url.to_owned(),
+        }
+    }
+
+    fn send_json_request<D: serde::de::DeserializeOwned>(
+        &self,
+        method: &str,
+        params: &serde_json::Value,
+    ) -> Result<D> {
+        let url = format!("http://{}{}", self.wallet_owner_url, ENDPOINT);
+        let req = build_request(method, params);
+        let res = client::post::<Request, Response>(url.as_str(), None, &req)?;
+        let parsed = res.clone().into_result()?;
+        Ok(parsed)
+    }
+}
+
+#[derive(Clone)]
+pub struct Wallet {
+    pub client: HTTPWalletClient,
+
+    pub token: Token,
+}
+
+impl Wallet {
+    pub fn open_wallet(wallet_owner_url: &SocketAddr, wallet_pass: &ZeroingString) -> Result<Wallet> {
+        let client = HTTPWalletClient::new(&wallet_owner_url);
+
+        let open_wallet_params = json!({
+            "name": null,
+            "password": wallet_pass.to_string()
+        });
+        let token: Token = client.send_json_request("open_wallet", &open_wallet_params)?;
+
+        Ok(Wallet { 
+            client: client,
+            token: token,
+        })
+    }
+
+    /// Builds and verifies a <code>Transaction</code> using the provided components.
+    pub fn assemble_tx(&self, inputs: &Vec<Input>, outputs: &Vec<Output>, total_fee: u64, total_excess: &SecretKey) -> Result<Transaction> {
+        // generate random transaction offset
+        let offset = secp::random_secret();
+
+        // build and verify kernel
+        let kern_excess = secp::sub_blinds(&total_excess, &offset)?;
+        let kern = Wallet::build_kernel(total_fee, &kern_excess)?;
+
+        // assemble the transaction
+        let tx = Transaction::new(Inputs::from(inputs.as_slice()), &outputs, &[kern])
+            .with_offset(BlindingFactor::from_secret_key(offset));
+        Ok(tx)
+    }
+
+    /// Builds and verifies a <code>TxKernel</code> from the provided fee and excess.
+    fn build_kernel(total_fee: u64, kern_excess: &SecretKey) -> Result<TxKernel> {
+        let mut kernel = TxKernel::with_features(KernelFeatures::Plain {
+            fee: FeeFields::new(0, total_fee).unwrap(),
+        });
+        let msg = kernel.msg_to_sign()?;
+        kernel.excess = secp::commit(0, &kern_excess)?;
+        kernel.excess_sig = secp::sign(&kern_excess, &msg)?;
+        kernel.verify()?;
+
+        Ok(kernel)
+    }
+}