From 52b9a78de2076ec8517b4290f7291678e609d251 Mon Sep 17 00:00:00 2001
From: Thomas Eizinger <thomas@eizinger.io>
Date: Wed, 24 Mar 2021 18:30:55 +1100
Subject: [PATCH] Alice to validate Bob's PSBT for correctness

In order for the re-construction of TxLock to be meaningful, we limit
`Message2` to the PSBT instead of the full struct. This is a breaking
change in the network layer.

The PSBT is valid if:

- It has at most two outputs (we allow a change output)
- One of the outputs pays the agreed upon amount to a shared output script

Resolves #260.
---
 CHANGELOG.md                               |   5 +
 Cargo.lock                                 | 355 +++++++++++++++++----
 swap/Cargo.toml                            |   1 +
 swap/src/bitcoin.rs                        |   1 +
 swap/src/bitcoin/lock.rs                   | 144 ++++++++-
 swap/src/bitcoin/wallet.rs                 |  33 ++
 swap/src/protocol.rs                       |   2 +-
 swap/src/protocol/alice/execution_setup.rs |   4 +-
 swap/src/protocol/alice/state.rs           |  11 +-
 swap/src/protocol/bob/state.rs             |   2 +-
 10 files changed, 485 insertions(+), 73 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8745f522..6561f042 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -17,4 +17,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   failing on non-english language systems preventing users from starting the swap-cli
   and asb.
 
+### Security
+
+- Fixed an issue where Alice would not verify if Bob's Bitcoin lock transaction is semantically correct, i.e. pays the agreed upon amount to an output owned by both of them.
+  Fixing this required a **breaking change** on the network layer and hence old versions are not compatible with this version.
+
 [Unreleased]: https://github.com/comit-network/xmr-btc-swap/compare/v0.3...HEAD
diff --git a/Cargo.lock b/Cargo.lock
index f4bbf4df..3ae505f2 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -221,6 +221,16 @@ dependencies = [
  "keccak-hash 0.1.2",
 ]
 
+[[package]]
+name = "base64"
+version = "0.9.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "489d6c0ed21b11d038c31b6ceccca973e65d73ba3bd8ecb9a2babf5546164643"
+dependencies = [
+ "byteorder",
+ "safemem",
+]
+
 [[package]]
 name = "base64"
 version = "0.10.1"
@@ -251,9 +261,9 @@ dependencies = [
  "async-trait",
  "bdk-macros",
  "bitcoin",
- "electrum-client",
+ "electrum-client 0.7.0",
  "js-sys",
- "log",
+ "log 0.4.14",
  "miniscript",
  "rand 0.7.3",
  "serde",
@@ -273,6 +283,22 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "bdk-testutils"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a6d9382c8dfda457f2be9b700ffd580f12babec5d34ee39343768f65724ddd64"
+dependencies = [
+ "bitcoin",
+ "bitcoincore-rpc",
+ "electrum-client 0.6.0",
+ "log 0.4.14",
+ "miniscript",
+ "serde",
+ "serde_json",
+ "serial_test",
+]
+
 [[package]]
 name = "bech32"
 version = "0.7.3"
@@ -326,7 +352,7 @@ dependencies = [
  "thiserror",
  "tokio",
  "tracing",
- "url",
+ "url 2.2.1",
 ]
 
 [[package]]
@@ -338,6 +364,19 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "bitcoincore-rpc"
+version = "0.13.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0d708433972bf78bd5f909d1d288f9ac1cceeab1460edb954e962f83e1f440a3"
+dependencies = [
+ "bitcoincore-rpc-json",
+ "jsonrpc",
+ "log 0.4.14",
+ "serde",
+ "serde_json",
+]
+
 [[package]]
 name = "bitcoincore-rpc-json"
 version = "0.13.0"
@@ -886,6 +925,22 @@ version = "1.6.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e78d4f1cc4ae33bbfc157ed5d5a5ef3bc29227303d595861deb238fcec4e9457"
 
+[[package]]
+name = "electrum-client"
+version = "0.6.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "21453800c95bb1aaa57490458c42d60c6277cb8a3e386030ec2381d5c2d4fa77"
+dependencies = [
+ "bitcoin",
+ "log 0.4.14",
+ "rustls 0.16.0",
+ "serde",
+ "serde_json",
+ "socks",
+ "webpki",
+ "webpki-roots 0.19.0",
+]
+
 [[package]]
 name = "electrum-client"
 version = "0.7.0"
@@ -893,7 +948,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "4cab4d90cc575a7daab4cfed9e315912a88071bc47462e6be57516a2f01ccc89"
 dependencies = [
  "bitcoin",
- "log",
+ "log 0.4.14",
  "rustls 0.16.0",
  "serde",
  "serde_json",
@@ -1050,7 +1105,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "5fc25a87fa4fd2094bffb06925852034d90a17f0d1e05197d4956d3555752191"
 dependencies = [
  "matches",
- "percent-encoding",
+ "percent-encoding 2.1.0",
 ]
 
 [[package]]
@@ -1194,7 +1249,7 @@ checksum = "501466ecc8a30d1d3b7fc9229b122b2ce8ed6e9d9223f1138d4babb253e51817"
 dependencies = [
  "serde",
  "typenum",
- "version_check",
+ "version_check 0.9.3",
 ]
 
 [[package]]
@@ -1395,6 +1450,25 @@ version = "0.3.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "494b4d60369511e7dea41cf646832512a94e542f68bb9c49e54518e0f468eb47"
 
+[[package]]
+name = "hyper"
+version = "0.10.16"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0a0652d9a2609a968c14be1a9ea00bf4b1d64e2e1f53a1b51b6fff3a6e829273"
+dependencies = [
+ "base64 0.9.3",
+ "httparse",
+ "language-tags",
+ "log 0.3.9",
+ "mime 0.2.6",
+ "num_cpus",
+ "time 0.1.43",
+ "traitobject",
+ "typeable",
+ "unicase",
+ "url 1.7.2",
+]
+
 [[package]]
 name = "hyper"
 version = "0.14.5"
@@ -1426,14 +1500,25 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "5f9f7a97316d44c0af9b0301e65010573a853a9fc97046d7331d7f6bc0fd5a64"
 dependencies = [
  "futures-util",
- "hyper",
- "log",
+ "hyper 0.14.4",
+ "log 0.4.14",
  "rustls 0.19.0",
  "tokio",
  "tokio-rustls",
  "webpki",
 ]
 
+[[package]]
+name = "idna"
+version = "0.1.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "38f09e0f0b1fb55fdee1f17470ad800da77af5186a1a76c026b679358b7e844e"
+dependencies = [
+ "matches",
+ "unicode-bidi",
+ "unicode-normalization",
+]
+
 [[package]]
 name = "idna"
 version = "0.2.2"
@@ -1545,6 +1630,18 @@ dependencies = [
  "wasm-bindgen",
 ]
 
+[[package]]
+name = "jsonrpc"
+version = "0.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "436f3455a8a4e9c7b14de9f1206198ee5d0bdc2db1b560339d2141093d7dd389"
+dependencies = [
+ "hyper 0.10.16",
+ "serde",
+ "serde_derive",
+ "serde_json",
+]
+
 [[package]]
 name = "jsonrpc_client"
 version = "0.5.1"
@@ -1556,7 +1653,7 @@ dependencies = [
  "reqwest",
  "serde",
  "serde_json",
- "url",
+ "url 2.2.1",
 ]
 
 [[package]]
@@ -1589,6 +1686,12 @@ dependencies = [
  "tiny-keccak",
 ]
 
+[[package]]
+name = "language-tags"
+version = "0.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a91d884b6667cd606bb5a69aa0c99ba811a115fc68915e7056ec08a46e93199a"
+
 [[package]]
 name = "lazy_static"
 version = "1.4.0"
@@ -1634,7 +1737,7 @@ dependencies = [
  "libp2p-tcp",
  "libp2p-yamux",
  "parity-multiaddr",
- "parking_lot",
+ "parking_lot 0.11.1",
  "pin-project 1.0.5",
  "smallvec",
  "wasm-timer",
@@ -1646,7 +1749,7 @@ version = "0.1.0"
 source = "git+https://github.com/comit-network/rust-libp2p-async-await#7a9006ceddd132ef5d40a597936cf15381a5cfe1"
 dependencies = [
  "libp2p",
- "log",
+ "log 0.4.14",
 ]
 
 [[package]]
@@ -1664,11 +1767,11 @@ dependencies = [
  "futures-timer",
  "lazy_static",
  "libsecp256k1",
- "log",
+ "log 0.4.14",
  "multihash",
  "multistream-select",
  "parity-multiaddr",
- "parking_lot",
+ "parking_lot 0.11.1",
  "pin-project 1.0.5",
  "prost",
  "prost-build",
@@ -1691,7 +1794,7 @@ checksum = "9712eb3e9f7dcc77cc5ca7d943b6a85ce4b1faaf91a67e003442412a26d6d6f8"
 dependencies = [
  "futures",
  "libp2p-core",
- "log",
+ "log 0.4.14",
  "smallvec",
  "trust-dns-resolver",
 ]
@@ -1706,9 +1809,9 @@ dependencies = [
  "bytes",
  "futures",
  "libp2p-core",
- "log",
+ "log 0.4.14",
  "nohash-hasher",
- "parking_lot",
+ "parking_lot 0.11.1",
  "rand 0.7.3",
  "smallvec",
  "unsigned-varint 0.7.0",
@@ -1725,7 +1828,7 @@ dependencies = [
  "futures",
  "lazy_static",
  "libp2p-core",
- "log",
+ "log 0.4.14",
  "prost",
  "prost-build",
  "rand 0.7.3",
@@ -1747,7 +1850,7 @@ dependencies = [
  "futures",
  "libp2p-core",
  "libp2p-swarm",
- "log",
+ "log 0.4.14",
  "lru",
  "minicbor",
  "rand 0.7.3",
@@ -1765,7 +1868,7 @@ dependencies = [
  "either",
  "futures",
  "libp2p-core",
- "log",
+ "log 0.4.14",
  "rand 0.7.3",
  "smallvec",
  "void",
@@ -1794,7 +1897,7 @@ dependencies = [
  "ipnet",
  "libc",
  "libp2p-core",
- "log",
+ "log 0.4.14",
  "socket2 0.4.0",
  "tokio",
 ]
@@ -1807,7 +1910,7 @@ checksum = "96d6144cc94143fb0a8dd1e7c2fbcc32a2808168bcd1d69920635424d5993b7b"
 dependencies = [
  "futures",
  "libp2p-core",
- "parking_lot",
+ "parking_lot 0.11.1",
  "thiserror",
  "yamux",
 ]
@@ -1834,6 +1937,15 @@ version = "0.5.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7fb9b38af92608140b86b693604b9ffcc5824240a484d1ecd4795bacb2fe88f3"
 
+[[package]]
+name = "lock_api"
+version = "0.3.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c4da24a77a3d8a6d4862d95f72e6fdb9c09a643ecdb402d754004a557f2bec75"
+dependencies = [
+ "scopeguard",
+]
+
 [[package]]
 name = "lock_api"
 version = "0.4.2"
@@ -1843,6 +1955,15 @@ dependencies = [
  "scopeguard",
 ]
 
+[[package]]
+name = "log"
+version = "0.3.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e19e8d5c34a3e0e2223db8e060f9e8264aeeb5c5fc64a4ee9965c062211c024b"
+dependencies = [
+ "log 0.4.14",
+]
+
 [[package]]
 name = "log"
 version = "0.4.14"
@@ -1906,6 +2027,15 @@ dependencies = [
  "autocfg 1.0.1",
 ]
 
+[[package]]
+name = "mime"
+version = "0.2.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ba626b8a6de5da682e1caa06bdb42a335aee5a84db8e5046a3e8ab17ba0a3ae0"
+dependencies = [
+ "log 0.3.9",
+]
+
 [[package]]
 name = "mime"
 version = "0.3.16"
@@ -1958,7 +2088,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2182a122f3b7f3f5329cb1972cee089ba2459a0a80a56935e6e674f096f8d839"
 dependencies = [
  "libc",
- "log",
+ "log 0.4.14",
  "miow",
  "ntapi",
  "winapi 0.3.9",
@@ -2059,7 +2189,7 @@ checksum = "7d91ec0a2440aaff5f78ec35631a7027d50386c6163aa975f7caa0d5da4b6ff8"
 dependencies = [
  "bytes",
  "futures",
- "log",
+ "log 0.4.14",
  "pin-project 1.0.5",
  "smallvec",
  "unsigned-varint 0.7.0",
@@ -2079,7 +2209,7 @@ checksum = "ffb4262d26ed83a1c0a33a38fe2bb15797329c85770da05e6b828ddb782627af"
 dependencies = [
  "lexical-core",
  "memchr",
- "version_check",
+ "version_check 0.9.3",
 ]
 
 [[package]]
@@ -2208,11 +2338,11 @@ dependencies = [
  "byteorder",
  "data-encoding",
  "multihash",
- "percent-encoding",
+ "percent-encoding 2.1.0",
  "serde",
  "static_assertions 1.1.0",
  "unsigned-varint 0.7.0",
- "url",
+ "url 2.2.1",
 ]
 
 [[package]]
@@ -2227,6 +2357,16 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "parking_lot"
+version = "0.10.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d3a704eb390aafdc107b0e392f56a82b668e3a71366993b5340f5833fd62505e"
+dependencies = [
+ "lock_api 0.3.4",
+ "parking_lot_core 0.7.2",
+]
+
 [[package]]
 name = "parking_lot"
 version = "0.11.1"
@@ -2234,8 +2374,22 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6d7744ac029df22dca6284efe4e898991d28e3085c706c972bcd7da4a27a15eb"
 dependencies = [
  "instant",
- "lock_api",
- "parking_lot_core",
+ "lock_api 0.4.2",
+ "parking_lot_core 0.8.3",
+]
+
+[[package]]
+name = "parking_lot_core"
+version = "0.7.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d58c7c768d4ba344e3e8d72518ac13e259d7c7ade24167003b8488e10b6740a3"
+dependencies = [
+ "cfg-if 0.1.10",
+ "cloudabi",
+ "libc",
+ "redox_syscall 0.1.57",
+ "smallvec",
+ "winapi 0.3.9",
 ]
 
 [[package]]
@@ -2263,6 +2417,12 @@ dependencies = [
  "regex",
 ]
 
+[[package]]
+name = "percent-encoding"
+version = "1.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "31010dd2e1ac33d5b46a5b413495239882813e0369f8ed8a5e266f173602f831"
+
 [[package]]
 name = "percent-encoding"
 version = "2.1.0"
@@ -2414,7 +2574,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "syn",
- "version_check",
+ "version_check 0.9.3",
 ]
 
 [[package]]
@@ -2425,7 +2585,7 @@ checksum = "a1be40180e52ecc98ad80b184934baf3d0d29f979574e439af5a55274b35f869"
 dependencies = [
  "proc-macro2",
  "quote",
- "version_check",
+ "version_check 0.9.3",
 ]
 
 [[package]]
@@ -2468,7 +2628,7 @@ dependencies = [
  "bytes",
  "heck",
  "itertools",
- "log",
+ "log 0.4.14",
  "multimap",
  "petgraph",
  "prost",
@@ -2826,14 +2986,14 @@ dependencies = [
  "futures-util",
  "http",
  "http-body",
- "hyper",
+ "hyper 0.14.4",
  "hyper-rustls",
  "ipnet",
  "js-sys",
  "lazy_static",
- "log",
- "mime",
- "percent-encoding",
+ "log 0.4.14",
+ "mime 0.3.16",
+ "percent-encoding 2.1.0",
  "pin-project-lite",
  "rustls 0.19.0",
  "serde",
@@ -2841,7 +3001,7 @@ dependencies = [
  "serde_urlencoded",
  "tokio",
  "tokio-rustls",
- "url",
+ "url 2.2.1",
  "wasm-bindgen",
  "wasm-bindgen-futures",
  "web-sys",
@@ -2925,7 +3085,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b25a18b1bf7387f0145e7f8324e700805aade3842dd3db2e74e4cdeb4677c09e"
 dependencies = [
  "base64 0.10.1",
- "log",
+ "log 0.4.14",
  "ring",
  "sct",
  "webpki",
@@ -2938,7 +3098,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "064fd21ff87c6e87ed4506e68beb42459caa4a0e2eb144932e6776768556980b"
 dependencies = [
  "base64 0.13.0",
- "log",
+ "log 0.4.14",
  "ring",
  "sct",
  "webpki",
@@ -2961,6 +3121,12 @@ version = "1.0.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "71d301d4193d031abdd79ff7e3dd721168a9572ef3fe51a1517aba235bd8f86e"
 
+[[package]]
+name = "safemem"
+version = "0.3.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ef703b7cb59335eae2eb93ceb664c0eb7ea6bf567079d843e09420219668e072"
+
 [[package]]
 name = "scopeguard"
 version = "1.1.0"
@@ -3097,6 +3263,28 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "serial_test"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fef5f7c7434b2f2c598adc6f9494648a1e41274a75c0ba4056f680ae0c117fd6"
+dependencies = [
+ "lazy_static",
+ "parking_lot 0.10.2",
+ "serial_test_derive",
+]
+
+[[package]]
+name = "serial_test_derive"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d08338d8024b227c62bd68a12c7c9883f5c66780abaef15c550dc56f46ee6515"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "sha-1"
 version = "0.9.4"
@@ -3196,8 +3384,8 @@ dependencies = [
  "fs2",
  "fxhash",
  "libc",
- "log",
- "parking_lot",
+ "log 0.4.14",
+ "parking_lot 0.11.1",
 ]
 
 [[package]]
@@ -3278,7 +3466,7 @@ version = "0.2.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a2beb4d1860a61f571530b3f855a1b538d0200f7871c63331ecd6f17b1f014f8"
 dependencies = [
- "version_check",
+ "version_check 0.9.3",
 ]
 
 [[package]]
@@ -3426,6 +3614,7 @@ dependencies = [
  "backoff",
  "base64 0.13.0",
  "bdk",
+ "bdk-testutils",
  "big-bytes",
  "bitcoin",
  "bitcoin-harness",
@@ -3437,7 +3626,7 @@ dependencies = [
  "ecdsa_fun",
  "futures",
  "get-port",
- "hyper",
+ "hyper 0.14.4",
  "libp2p",
  "libp2p-async-await",
  "miniscript",
@@ -3472,7 +3661,7 @@ dependencies = [
  "tracing",
  "tracing-futures",
  "tracing-subscriber",
- "url",
+ "url 2.2.1",
  "uuid",
  "void",
  "zip",
@@ -3545,7 +3734,7 @@ dependencies = [
  "derivative",
  "hex 0.4.3",
  "hmac 0.8.1",
- "log",
+ "log 0.4.14",
  "rand 0.7.3",
  "serde",
  "serde_json",
@@ -3560,7 +3749,7 @@ checksum = "d5e3ed6e3598dbf32cba8cb356b881c085e0adea57597f387723430dd94b4084"
 dependencies = [
  "hex 0.4.3",
  "hmac 0.10.1",
- "log",
+ "log 0.4.14",
  "rand 0.8.3",
  "serde",
  "serde_json",
@@ -3626,7 +3815,7 @@ dependencies = [
  "standback",
  "stdweb",
  "time-macros",
- "version_check",
+ "version_check 0.9.3",
  "winapi 0.3.9",
 ]
 
@@ -3690,7 +3879,7 @@ dependencies = [
  "mio",
  "num_cpus",
  "once_cell",
- "parking_lot",
+ "parking_lot 0.11.1",
  "pin-project-lite",
  "signal-hook-registry",
  "tokio-macros",
@@ -3751,7 +3940,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1e96bb520beab540ab664bd5a9cfeaa1fcd846fa68c830b42e2c8963071251d2"
 dependencies = [
  "futures-util",
- "log",
+ "log 0.4.14",
  "pin-project 1.0.5",
  "rustls 0.19.0",
  "tokio",
@@ -3770,7 +3959,7 @@ dependencies = [
  "bytes",
  "futures-core",
  "futures-sink",
- "log",
+ "log 0.4.14",
  "pin-project-lite",
  "tokio",
 ]
@@ -3841,7 +4030,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a6923477a48e41c1951f1999ef8bb5a3023eb723ceadafe78ffb65dc366761e3"
 dependencies = [
  "lazy_static",
- "log",
+ "log 0.4.14",
  "tracing-core",
 ]
 
@@ -3863,6 +4052,12 @@ dependencies = [
  "tracing-log",
 ]
 
+[[package]]
+name = "traitobject"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "efd1f82c56340fdf16f2a953d7bda4f8fdffba13d93b00844c25572110b26079"
+
 [[package]]
 name = "trust-dns-proto"
 version = "0.20.1"
@@ -3876,16 +4071,16 @@ dependencies = [
  "futures-channel",
  "futures-io",
  "futures-util",
- "idna",
+ "idna 0.2.2",
  "ipnet",
  "lazy_static",
- "log",
+ "log 0.4.14",
  "rand 0.8.3",
  "smallvec",
  "thiserror",
  "tinyvec",
  "tokio",
- "url",
+ "url 2.2.1",
 ]
 
 [[package]]
@@ -3898,9 +4093,9 @@ dependencies = [
  "futures-util",
  "ipconfig",
  "lazy_static",
- "log",
+ "log 0.4.14",
  "lru-cache",
- "parking_lot",
+ "parking_lot 0.11.1",
  "resolv-conf",
  "smallvec",
  "thiserror",
@@ -3926,17 +4121,23 @@ dependencies = [
  "http",
  "httparse",
  "input_buffer",
- "log",
+ "log 0.4.14",
  "rand 0.8.3",
  "rustls 0.19.0",
  "sha-1",
  "thiserror",
- "url",
+ "url 2.2.1",
  "utf-8",
  "webpki",
  "webpki-roots 0.21.0",
 ]
 
+[[package]]
+name = "typeable"
+version = "0.1.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1410f6f91f21d1612654e7cc69193b0334f909dcf2c790c4826254fbb86f8887"
+
 [[package]]
 name = "typenum"
 version = "1.13.0"
@@ -3967,6 +4168,15 @@ dependencies = [
  "static_assertions 1.1.0",
 ]
 
+[[package]]
+name = "unicase"
+version = "1.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7f4765f83163b74f957c797ad9253caf97f103fb064d3999aea9568d09fc8a33"
+dependencies = [
+ "version_check 0.1.5",
+]
+
 [[package]]
 name = "unicode-bidi"
 version = "0.3.4"
@@ -4037,6 +4247,17 @@ version = "0.7.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a156c684c91ea7d62626509bce3cb4e1d9ed5c4d978f7b4352658f96a4c26b4a"
 
+[[package]]
+name = "url"
+version = "1.7.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dd4e7c0d531266369519a4aa4f399d748bd37043b00bde1e4ff1f60a120b355a"
+dependencies = [
+ "idna 0.1.5",
+ "matches",
+ "percent-encoding 1.0.1",
+]
+
 [[package]]
 name = "url"
 version = "2.2.1"
@@ -4044,9 +4265,9 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9ccd964113622c8e9322cfac19eb1004a07e636c545f325da085d5cdde6f1f8b"
 dependencies = [
  "form_urlencoded",
- "idna",
+ "idna 0.2.2",
  "matches",
- "percent-encoding",
+ "percent-encoding 2.1.0",
  "serde",
 ]
 
@@ -4072,6 +4293,12 @@ version = "0.8.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f1bddf1187be692e79c5ffeab891132dfb0f236ed36a43c7ed39f1165ee20191"
 
+[[package]]
+name = "version_check"
+version = "0.1.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "914b1a6776c4c929a602fafd8bc742e06365d4bcbe48c30f9cca5824f70dc9dd"
+
 [[package]]
 name = "version_check"
 version = "0.9.3"
@@ -4090,7 +4317,7 @@ version = "0.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1ce8a968cb1cd110d136ff8b819a556d6fb6d919363c61534f6860c7eb172ba0"
 dependencies = [
- "log",
+ "log 0.4.14",
  "try-lock",
 ]
 
@@ -4126,7 +4353,7 @@ checksum = "5b7d8b6942b8bb3a9b0e73fc79b98095a27de6fa247615e59d096754a3bc2aa8"
 dependencies = [
  "bumpalo",
  "lazy_static",
- "log",
+ "log 0.4.14",
  "proc-macro2",
  "quote",
  "syn",
@@ -4182,7 +4409,7 @@ checksum = "be0ecb0db480561e9a7642b5d3e4187c128914e58aa84330b9493e3eb68c5e7f"
 dependencies = [
  "futures",
  "js-sys",
- "parking_lot",
+ "parking_lot 0.11.1",
  "pin-utils",
  "wasm-bindgen",
  "wasm-bindgen-futures",
@@ -4332,9 +4559,9 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1cc7bd8c983209ed5d527f44b01c41b7dc146fd960c61cf9e1d25399841dc271"
 dependencies = [
  "futures",
- "log",
+ "log 0.4.14",
  "nohash-hasher",
- "parking_lot",
+ "parking_lot 0.11.1",
  "rand 0.7.3",
  "static_assertions 1.1.0",
 ]
diff --git a/swap/Cargo.toml b/swap/Cargo.toml
index b66a4f3e..1ddaad4f 100644
--- a/swap/Cargo.toml
+++ b/swap/Cargo.toml
@@ -64,6 +64,7 @@ tokio-tar = { path = "../tokio-tar" }
 zip = "0.5"
 
 [dev-dependencies]
+bdk-testutils = { version = "0.3" }
 bitcoin-harness = { git = "https://github.com/coblox/bitcoin-harness-rs" }
 get-port = "3"
 hyper = "0.14"
diff --git a/swap/src/bitcoin.rs b/swap/src/bitcoin.rs
index fef2e252..84434ffe 100644
--- a/swap/src/bitcoin.rs
+++ b/swap/src/bitcoin.rs
@@ -14,6 +14,7 @@ pub use crate::bitcoin::redeem::TxRedeem;
 pub use crate::bitcoin::refund::TxRefund;
 pub use crate::bitcoin::timelocks::{BlockHeight, ExpiredTimelocks};
 pub use ::bitcoin::util::amount::Amount;
+pub use ::bitcoin::util::psbt::PartiallySignedTransaction;
 pub use ::bitcoin::{Address, Network, Transaction, Txid};
 pub use ecdsa_fun::adaptor::EncryptedSignature;
 pub use ecdsa_fun::fun::Scalar;
diff --git a/swap/src/bitcoin/lock.rs b/swap/src/bitcoin/lock.rs
index edbcbd85..63f2fb53 100644
--- a/swap/src/bitcoin/lock.rs
+++ b/swap/src/bitcoin/lock.rs
@@ -4,7 +4,8 @@ use crate::bitcoin::{
 };
 use ::bitcoin::util::psbt::PartiallySignedTransaction;
 use ::bitcoin::{OutPoint, TxIn, TxOut, Txid};
-use anyhow::Result;
+use anyhow::{bail, Result};
+use bdk::database::BatchDatabase;
 use bitcoin::Script;
 use ecdsa_fun::fun::Point;
 use miniscript::{Descriptor, DescriptorTrait};
@@ -18,7 +19,15 @@ pub struct TxLock {
 }
 
 impl TxLock {
-    pub async fn new(wallet: &Wallet, amount: Amount, A: PublicKey, B: PublicKey) -> Result<Self> {
+    pub async fn new<B, D, C>(
+        wallet: &Wallet<B, D, C>,
+        amount: Amount,
+        A: PublicKey,
+        B: PublicKey,
+    ) -> Result<Self>
+    where
+        D: BatchDatabase,
+    {
         let lock_output_descriptor = build_shared_output_descriptor(A.0, B.0);
         let address = lock_output_descriptor
             .address(wallet.get_network())
@@ -32,6 +41,56 @@ impl TxLock {
         })
     }
 
+    /// Creates an instance of `TxLock` from a PSBT, the public keys of the
+    /// parties and the specified amount.
+    ///
+    /// This function validates that the given PSBT does indeed pay that
+    /// specified amount to a shared output.
+    pub fn from_psbt(
+        psbt: PartiallySignedTransaction,
+        A: PublicKey,
+        B: PublicKey,
+        btc: Amount,
+    ) -> Result<Self> {
+        let shared_output_candidate = match psbt.global.unsigned_tx.output.as_slice() {
+            [shared_output_candidate, _] if shared_output_candidate.value == btc.as_sat() => {
+                shared_output_candidate
+            }
+            [_, shared_output_candidate] if shared_output_candidate.value == btc.as_sat() => {
+                shared_output_candidate
+            }
+            // A single output is possible if Bob funds without any change necessary
+            [shared_output_candidate] if shared_output_candidate.value == btc.as_sat() => {
+                shared_output_candidate
+            }
+            [_, _] => {
+                bail!("Neither of the two provided outputs pays the right amount!");
+            }
+            [_] => {
+                bail!("The provided output does not pay the right amount!");
+            }
+            other => {
+                let num_outputs = other.len();
+                bail!(
+                    "PSBT has {} outputs, expected one or two. Something is fishy!",
+                    num_outputs
+                );
+            }
+        };
+
+        let descriptor = build_shared_output_descriptor(A.0, B.0);
+        let legit_shared_output_script = descriptor.script_pubkey();
+
+        if shared_output_candidate.script_pubkey != legit_shared_output_script {
+            bail!("Output script is not a shared output")
+        }
+
+        Ok(TxLock {
+            inner: psbt,
+            output_descriptor: descriptor,
+        })
+    }
+
     pub fn lock_amount(&self) -> Amount {
         Amount::from_sat(self.inner.clone().extract_tx().output[self.lock_output_vout()].value)
     }
@@ -116,3 +175,84 @@ impl Watchable for TxLock {
         self.output_descriptor.script_pubkey()
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[tokio::test]
+    async fn given_bob_sends_good_psbt_when_reconstructing_then_succeeeds() {
+        let (A, B) = alice_and_bob();
+        let wallet = Wallet::new_funded(50000);
+        let agreed_amount = Amount::from_sat(10000);
+
+        let psbt = bob_make_psbt(A, B, &wallet, agreed_amount).await;
+        let result = TxLock::from_psbt(psbt, A, B, agreed_amount);
+
+        result.expect("PSBT to be valid");
+    }
+
+    #[tokio::test]
+    async fn bob_can_fund_without_a_change_output() {
+        let (A, B) = alice_and_bob();
+        let fees = 610;
+        let agreed_amount = Amount::from_sat(10000);
+        let wallet = Wallet::new_funded(agreed_amount.as_sat() + fees);
+
+        let psbt = bob_make_psbt(A, B, &wallet, agreed_amount).await;
+        assert_eq!(
+            psbt.global.unsigned_tx.output.len(),
+            1,
+            "psbt should only have a single output"
+        );
+        let result = TxLock::from_psbt(psbt, A, B, agreed_amount);
+
+        result.expect("PSBT to be valid");
+    }
+
+    #[tokio::test]
+    async fn given_bob_is_sending_less_than_agreed_when_reconstructing_txlock_then_fails() {
+        let (A, B) = alice_and_bob();
+        let wallet = Wallet::new_funded(50000);
+        let agreed_amount = Amount::from_sat(10000);
+
+        let bad_amount = Amount::from_sat(5000);
+        let psbt = bob_make_psbt(A, B, &wallet, bad_amount).await;
+        let result = TxLock::from_psbt(psbt, A, B, agreed_amount);
+
+        result.expect_err("PSBT to be invalid");
+    }
+
+    #[tokio::test]
+    async fn given_bob_is_sending_to_a_bad_output_reconstructing_txlock_then_fails() {
+        let (A, B) = alice_and_bob();
+        let wallet = Wallet::new_funded(50000);
+        let agreed_amount = Amount::from_sat(10000);
+
+        let E = eve();
+        let psbt = bob_make_psbt(E, B, &wallet, agreed_amount).await;
+        let result = TxLock::from_psbt(psbt, A, B, agreed_amount);
+
+        result.expect_err("PSBT to be invalid");
+    }
+
+    /// Helper function that represents Bob's action of constructing the PSBT.
+    ///
+    /// Extracting this allows us to keep the tests concise.
+    async fn bob_make_psbt(
+        A: PublicKey,
+        B: PublicKey,
+        wallet: &Wallet<(), bdk::database::MemoryDatabase, ()>,
+        amount: Amount,
+    ) -> PartiallySignedTransaction {
+        TxLock::new(&wallet, amount, A, B).await.unwrap().into()
+    }
+
+    fn alice_and_bob() -> (PublicKey, PublicKey) {
+        (PublicKey::random(), PublicKey::random())
+    }
+
+    fn eve() -> PublicKey {
+        PublicKey::random()
+    }
+}
diff --git a/swap/src/bitcoin/wallet.rs b/swap/src/bitcoin/wallet.rs
index 0bda71cb..c87c0f3e 100644
--- a/swap/src/bitcoin/wallet.rs
+++ b/swap/src/bitcoin/wallet.rs
@@ -348,6 +348,39 @@ impl<B, D, C> Wallet<B, D, C> {
     }
 }
 
+#[cfg(test)]
+impl Wallet<(), bdk::database::MemoryDatabase, ()> {
+    /// Creates a new, funded wallet to be used within tests.
+    pub fn new_funded(amount: u64) -> Self {
+        use bdk::database::MemoryDatabase;
+        use bdk::{LocalUtxo, TransactionDetails};
+        use bitcoin::OutPoint;
+        use std::str::FromStr;
+        use testutils::testutils;
+
+        let descriptors = testutils!(@descriptors ("wpkh(tpubEBr4i6yk5nf5DAaJpsi9N2pPYBeJ7fZ5Z9rmN4977iYLCGco1VyjB9tvvuvYtfZzjD5A8igzgw3HeWeeKFmanHYqksqZXYXGsw5zjnj7KM9/*)"));
+
+        let mut database = MemoryDatabase::new();
+        bdk::populate_test_db!(
+            &mut database,
+            testutils! {
+                @tx ( (@external descriptors, 0) => amount ) (@confirmations 1)
+            },
+            Some(100)
+        );
+
+        let wallet =
+            bdk::Wallet::new_offline(&descriptors.0, None, Network::Regtest, database).unwrap();
+
+        Self {
+            client: Arc::new(Mutex::new(())),
+            wallet: Arc::new(Mutex::new(wallet)),
+            finality_confirmations: 1,
+            network: Network::Regtest,
+        }
+    }
+}
+
 /// Defines a watchable transaction.
 ///
 /// For a transaction to be watchable, we need to know two things: Its
diff --git a/swap/src/protocol.rs b/swap/src/protocol.rs
index e446759c..fb3a35be 100644
--- a/swap/src/protocol.rs
+++ b/swap/src/protocol.rs
@@ -47,7 +47,7 @@ pub struct Message1 {
 
 #[derive(Clone, Debug, Serialize, Deserialize)]
 pub struct Message2 {
-    tx_lock: bitcoin::TxLock,
+    psbt: bitcoin::PartiallySignedTransaction,
 }
 
 #[derive(Clone, Debug, Serialize, Deserialize)]
diff --git a/swap/src/protocol/alice/execution_setup.rs b/swap/src/protocol/alice/execution_setup.rs
index d3a37519..a4b04aa0 100644
--- a/swap/src/protocol/alice/execution_setup.rs
+++ b/swap/src/protocol/alice/execution_setup.rs
@@ -57,7 +57,9 @@ impl Behaviour {
                 let message2 =
                     serde_cbor::from_slice::<Message2>(&substream.read_message(BUF_SIZE).await?)
                         .context("Failed to deserialize message2")?;
-                let state2 = state1.receive(message2);
+                let state2 = state1
+                    .receive(message2)
+                    .context("Failed to receive Message2")?;
 
                 substream
                     .write_message(
diff --git a/swap/src/protocol/alice/state.rs b/swap/src/protocol/alice/state.rs
index ecaee48f..fe0f5679 100644
--- a/swap/src/protocol/alice/state.rs
+++ b/swap/src/protocol/alice/state.rs
@@ -200,8 +200,11 @@ impl State1 {
         }
     }
 
-    pub fn receive(self, msg: Message2) -> State2 {
-        State2 {
+    pub fn receive(self, msg: Message2) -> Result<State2> {
+        let tx_lock = bitcoin::TxLock::from_psbt(msg.psbt, self.a.public(), self.B, self.btc)
+            .context("Failed to re-construct TxLock from received PSBT")?;
+
+        Ok(State2 {
             a: self.a,
             B: self.B,
             s_a: self.s_a,
@@ -215,8 +218,8 @@ impl State1 {
             refund_address: self.refund_address,
             redeem_address: self.redeem_address,
             punish_address: self.punish_address,
-            tx_lock: msg.tx_lock,
-        }
+            tx_lock,
+        })
     }
 }
 
diff --git a/swap/src/protocol/bob/state.rs b/swap/src/protocol/bob/state.rs
index c70b458e..512b6a25 100644
--- a/swap/src/protocol/bob/state.rs
+++ b/swap/src/protocol/bob/state.rs
@@ -188,7 +188,7 @@ pub struct State1 {
 impl State1 {
     pub fn next_message(&self) -> Message2 {
         Message2 {
-            tx_lock: self.tx_lock.clone(),
+            psbt: self.tx_lock.clone().into(),
         }
     }