_ Git - rust-lightning/blob - lightning/src/ln/chan_utils.rs

   1 //! Various utilities for building scripts and deriving keys related to channels. These are
   2 //! largely of interest for those implementing chain::keysinterface::ChannelKeys message signing
   3 //! by hand.
   4
   5 use bitcoin::blockdata::script::{Script,Builder};
   6 use bitcoin::blockdata::opcodes;
   7 use bitcoin::blockdata::transaction::{TxIn,TxOut,OutPoint,Transaction, SigHashType};
   8 use bitcoin::consensus::encode::{self, Decodable, Encodable};
   9 use bitcoin::util::bip143;
  10
  11 use bitcoin_hashes::{Hash, HashEngine};
  12 use bitcoin_hashes::sha256::Hash as Sha256;
  13 use bitcoin_hashes::ripemd160::Hash as Ripemd160;
  14 use bitcoin_hashes::hash160::Hash as Hash160;
  15 use bitcoin_hashes::sha256d::Hash as Sha256dHash;
  16
  17 use ln::channelmanager::{PaymentHash, PaymentPreimage};
  18 use ln::msgs::DecodeError;
  19 use util::ser::{Readable, Writeable, Writer, WriterWriteAdaptor};
  20
  21 use secp256k1::key::{SecretKey,PublicKey};
  22 use secp256k1::{Secp256k1, Signature};
  23 use secp256k1;
  24
  25 pub(super) const HTLC_SUCCESS_TX_WEIGHT: u64 = 703;
  26 pub(super) const HTLC_TIMEOUT_TX_WEIGHT: u64 = 663;
  27
  28 // Various functions for key derivation and transaction creation for use within channels. Primarily
  29 // used in Channel and ChannelMonitor.
  30
  31 pub(super) fn build_commitment_secret(commitment_seed: &[u8; 32], idx: u64) -> [u8; 32] {
  32         let mut res: [u8; 32] = commitment_seed.clone();
  33         for i in 0..48 {
  34                 let bitpos = 47 - i;
  35                 if idx & (1 << bitpos) == (1 << bitpos) {
  36                         res[bitpos / 8] ^= 1 << (bitpos & 7);
  37                         res = Sha256::hash(&res).into_inner();
  38                 }
  39         }
  40         res
  41 }
  42
  43 /// Derives a per-commitment-transaction private key (eg an htlc key or payment key) from the base
  44 /// private key for that type of key and the per_commitment_point (available in TxCreationKeys)
  45 pub fn derive_private_key<T: secp256k1::Signing>(secp_ctx: &Secp256k1<T>, per_commitment_point: &PublicKey, base_secret: &SecretKey) -> Result<SecretKey, secp256k1::Error> {
  46         let mut sha = Sha256::engine();
  47         sha.input(&per_commitment_point.serialize());
  48         sha.input(&PublicKey::from_secret_key(&secp_ctx, &base_secret).serialize());
  49         let res = Sha256::from_engine(sha).into_inner();
  50
  51         let mut key = base_secret.clone();
  52         key.add_assign(&res)?;
  53         Ok(key)
  54 }
  55
  56 pub(super) fn derive_public_key<T: secp256k1::Signing>(secp_ctx: &Secp256k1<T>, per_commitment_point: &PublicKey, base_point: &PublicKey) -> Result<PublicKey, secp256k1::Error> {
  57         let mut sha = Sha256::engine();
  58         sha.input(&per_commitment_point.serialize());
  59         sha.input(&base_point.serialize());
  60         let res = Sha256::from_engine(sha).into_inner();
  61
  62         let hashkey = PublicKey::from_secret_key(&secp_ctx, &SecretKey::from_slice(&res)?);
  63         base_point.combine(&hashkey)
  64 }
  65
  66 /// Derives a revocation key from its constituent parts.
  67 /// Note that this is infallible iff we trust that at least one of the two input keys are randomly
  68 /// generated (ie our own).
  69 pub(super) fn derive_private_revocation_key<T: secp256k1::Signing>(secp_ctx: &Secp256k1<T>, per_commitment_secret: &SecretKey, revocation_base_secret: &SecretKey) -> Result<SecretKey, secp256k1::Error> {
  70         let revocation_base_point = PublicKey::from_secret_key(&secp_ctx, &revocation_base_secret);
  71         let per_commitment_point = PublicKey::from_secret_key(&secp_ctx, &per_commitment_secret);
  72
  73         let rev_append_commit_hash_key = {
  74                 let mut sha = Sha256::engine();
  75                 sha.input(&revocation_base_point.serialize());
  76                 sha.input(&per_commitment_point.serialize());
  77
  78                 Sha256::from_engine(sha).into_inner()
  79         };
  80         let commit_append_rev_hash_key = {
  81                 let mut sha = Sha256::engine();
  82                 sha.input(&per_commitment_point.serialize());
  83                 sha.input(&revocation_base_point.serialize());
  84
  85                 Sha256::from_engine(sha).into_inner()
  86         };
  87
  88         let mut part_a = revocation_base_secret.clone();
  89         part_a.mul_assign(&rev_append_commit_hash_key)?;
  90         let mut part_b = per_commitment_secret.clone();
  91         part_b.mul_assign(&commit_append_rev_hash_key)?;
  92         part_a.add_assign(&part_b[..])?;
  93         Ok(part_a)
  94 }
  95
  96 pub(super) fn derive_public_revocation_key<T: secp256k1::Verification>(secp_ctx: &Secp256k1<T>, per_commitment_point: &PublicKey, revocation_base_point: &PublicKey) -> Result<PublicKey, secp256k1::Error> {
  97         let rev_append_commit_hash_key = {
  98                 let mut sha = Sha256::engine();
  99                 sha.input(&revocation_base_point.serialize());
 100                 sha.input(&per_commitment_point.serialize());
 101
 102                 Sha256::from_engine(sha).into_inner()
 103         };
 104         let commit_append_rev_hash_key = {
 105                 let mut sha = Sha256::engine();
 106                 sha.input(&per_commitment_point.serialize());
 107                 sha.input(&revocation_base_point.serialize());
 108
 109                 Sha256::from_engine(sha).into_inner()
 110         };
 111
 112         let mut part_a = revocation_base_point.clone();
 113         part_a.mul_assign(&secp_ctx, &rev_append_commit_hash_key)?;
 114         let mut part_b = per_commitment_point.clone();
 115         part_b.mul_assign(&secp_ctx, &commit_append_rev_hash_key)?;
 116         part_a.combine(&part_b)
 117 }
 118
 119 /// The set of public keys which are used in the creation of one commitment transaction.
 120 /// These are derived from the channel base keys and per-commitment data.
 121 pub struct TxCreationKeys {
 122         /// The per-commitment public key which was used to derive the other keys.
 123         pub per_commitment_point: PublicKey,
 124         /// The revocation key which is used to allow the owner of the commitment transaction to
 125         /// provide their counterparty the ability to punish them if they broadcast an old state.
 126         pub revocation_key: PublicKey,
 127         /// A's HTLC Key
 128         pub a_htlc_key: PublicKey,
 129         /// B's HTLC Key
 130         pub b_htlc_key: PublicKey,
 131         /// A's Payment Key (which isn't allowed to be spent from for some delay)
 132         pub a_delayed_payment_key: PublicKey,
 133         /// B's Payment Key
 134         pub b_payment_key: PublicKey,
 135 }
 136
 137 impl TxCreationKeys {
 138         pub(super) fn new<T: secp256k1::Signing + secp256k1::Verification>(secp_ctx: &Secp256k1<T>, per_commitment_point: &PublicKey, a_delayed_payment_base: &PublicKey, a_htlc_base: &PublicKey, b_revocation_base: &PublicKey, b_payment_base: &PublicKey, b_htlc_base: &PublicKey) -> Result<TxCreationKeys, secp256k1::Error> {
 139                 Ok(TxCreationKeys {
 140                         per_commitment_point: per_commitment_point.clone(),
 141                         revocation_key: derive_public_revocation_key(&secp_ctx, &per_commitment_point, &b_revocation_base)?,
 142                         a_htlc_key: derive_public_key(&secp_ctx, &per_commitment_point, &a_htlc_base)?,
 143                         b_htlc_key: derive_public_key(&secp_ctx, &per_commitment_point, &b_htlc_base)?,
 144                         a_delayed_payment_key: derive_public_key(&secp_ctx, &per_commitment_point, &a_delayed_payment_base)?,
 145                         b_payment_key: derive_public_key(&secp_ctx, &per_commitment_point, &b_payment_base)?,
 146                 })
 147         }
 148 }
 149
 150 /// Gets the "to_local" output redeemscript, ie the script which is time-locked or spendable by
 151 /// the revocation key
 152 pub(super) fn get_revokeable_redeemscript(revocation_key: &PublicKey, to_self_delay: u16, delayed_payment_key: &PublicKey) -> Script {
 153         Builder::new().push_opcode(opcodes::all::OP_IF)
 154                       .push_slice(&revocation_key.serialize())
 155                       .push_opcode(opcodes::all::OP_ELSE)
 156                       .push_int(to_self_delay as i64)
 157                       .push_opcode(opcodes::all::OP_CSV)
 158                       .push_opcode(opcodes::all::OP_DROP)
 159                       .push_slice(&delayed_payment_key.serialize())
 160                       .push_opcode(opcodes::all::OP_ENDIF)
 161                       .push_opcode(opcodes::all::OP_CHECKSIG)
 162                       .into_script()
 163 }
 164
 165 #[derive(Clone, PartialEq)]
 166 /// Information about an HTLC as it appears in a commitment transaction
 167 pub struct HTLCOutputInCommitment {
 168         /// Whether the HTLC was "offered" (ie outbound in relation to this commitment transaction).
 169         /// Note that this is not the same as whether it is ountbound *from us*. To determine that you
 170         /// need to compare this value to whether the commitment transaction in question is that of
 171         /// the remote party or our own.
 172         pub offered: bool,
 173         /// The value, in msat, of the HTLC. The value as it appears in the commitment transaction is
 174         /// this divided by 1000.
 175         pub amount_msat: u64,
 176         /// The CLTV lock-time at which this HTLC expires.
 177         pub cltv_expiry: u32,
 178         /// The hash of the preimage which unlocks this HTLC.
 179         pub payment_hash: PaymentHash,
 180         /// The position within the commitment transactions' outputs. This may be None if the value is
 181         /// below the dust limit (in which case no output appears in the commitment transaction and the
 182         /// value is spent to additional transaction fees).
 183         pub transaction_output_index: Option<u32>,
 184 }
 185
 186 #[inline]
 187 pub(super) fn get_htlc_redeemscript_with_explicit_keys(htlc: &HTLCOutputInCommitment, a_htlc_key: &PublicKey, b_htlc_key: &PublicKey, revocation_key: &PublicKey) -> Script {
 188         let payment_hash160 = Ripemd160::hash(&htlc.payment_hash.0[..]).into_inner();
 189         if htlc.offered {
 190                 Builder::new().push_opcode(opcodes::all::OP_DUP)
 191                               .push_opcode(opcodes::all::OP_HASH160)
 192                               .push_slice(&Hash160::hash(&revocation_key.serialize())[..])
 193                               .push_opcode(opcodes::all::OP_EQUAL)
 194                               .push_opcode(opcodes::all::OP_IF)
 195                               .push_opcode(opcodes::all::OP_CHECKSIG)
 196                               .push_opcode(opcodes::all::OP_ELSE)
 197                               .push_slice(&b_htlc_key.serialize()[..])
 198                               .push_opcode(opcodes::all::OP_SWAP)
 199                               .push_opcode(opcodes::all::OP_SIZE)
 200                               .push_int(32)
 201                               .push_opcode(opcodes::all::OP_EQUAL)
 202                               .push_opcode(opcodes::all::OP_NOTIF)
 203                               .push_opcode(opcodes::all::OP_DROP)
 204                               .push_int(2)
 205                               .push_opcode(opcodes::all::OP_SWAP)
 206                               .push_slice(&a_htlc_key.serialize()[..])
 207                               .push_int(2)
 208                               .push_opcode(opcodes::all::OP_CHECKMULTISIG)
 209                               .push_opcode(opcodes::all::OP_ELSE)
 210                               .push_opcode(opcodes::all::OP_HASH160)
 211                               .push_slice(&payment_hash160)
 212                               .push_opcode(opcodes::all::OP_EQUALVERIFY)
 213                               .push_opcode(opcodes::all::OP_CHECKSIG)
 214                               .push_opcode(opcodes::all::OP_ENDIF)
 215                               .push_opcode(opcodes::all::OP_ENDIF)
 216                               .into_script()
 217         } else {
 218                 Builder::new().push_opcode(opcodes::all::OP_DUP)
 219                               .push_opcode(opcodes::all::OP_HASH160)
 220                               .push_slice(&Hash160::hash(&revocation_key.serialize())[..])
 221                               .push_opcode(opcodes::all::OP_EQUAL)
 222                               .push_opcode(opcodes::all::OP_IF)
 223                               .push_opcode(opcodes::all::OP_CHECKSIG)
 224                               .push_opcode(opcodes::all::OP_ELSE)
 225                               .push_slice(&b_htlc_key.serialize()[..])
 226                               .push_opcode(opcodes::all::OP_SWAP)
 227                               .push_opcode(opcodes::all::OP_SIZE)
 228                               .push_int(32)
 229                               .push_opcode(opcodes::all::OP_EQUAL)
 230                               .push_opcode(opcodes::all::OP_IF)
 231                               .push_opcode(opcodes::all::OP_HASH160)
 232                               .push_slice(&payment_hash160)
 233                               .push_opcode(opcodes::all::OP_EQUALVERIFY)
 234                               .push_int(2)
 235                               .push_opcode(opcodes::all::OP_SWAP)
 236                               .push_slice(&a_htlc_key.serialize()[..])
 237                               .push_int(2)
 238                               .push_opcode(opcodes::all::OP_CHECKMULTISIG)
 239                               .push_opcode(opcodes::all::OP_ELSE)
 240                               .push_opcode(opcodes::all::OP_DROP)
 241                               .push_int(htlc.cltv_expiry as i64)
 242                               .push_opcode(opcodes::all::OP_CLTV)
 243                               .push_opcode(opcodes::all::OP_DROP)
 244                               .push_opcode(opcodes::all::OP_CHECKSIG)
 245                               .push_opcode(opcodes::all::OP_ENDIF)
 246                               .push_opcode(opcodes::all::OP_ENDIF)
 247                               .into_script()
 248         }
 249 }
 250
 251 /// note here that 'a_revocation_key' is generated using b_revocation_basepoint and a's
 252 /// commitment secret. 'htlc' does *not* need to have its previous_output_index filled.
 253 #[inline]
 254 pub fn get_htlc_redeemscript(htlc: &HTLCOutputInCommitment, keys: &TxCreationKeys) -> Script {
 255         get_htlc_redeemscript_with_explicit_keys(htlc, &keys.a_htlc_key, &keys.b_htlc_key, &keys.revocation_key)
 256 }
 257
 258 /// Gets the redeemscript for a funding output from the two funding public keys.
 259 /// Note that the order of funding public keys does not matter.
 260 pub fn make_funding_redeemscript(a: &PublicKey, b: &PublicKey) -> Script {
 261         let our_funding_key = a.serialize();
 262         let their_funding_key = b.serialize();
 263
 264         let builder = Builder::new().push_opcode(opcodes::all::OP_PUSHNUM_2);
 265         if our_funding_key[..] < their_funding_key[..] {
 266                 builder.push_slice(&our_funding_key)
 267                         .push_slice(&their_funding_key)
 268         } else {
 269                 builder.push_slice(&their_funding_key)
 270                         .push_slice(&our_funding_key)
 271         }.push_opcode(opcodes::all::OP_PUSHNUM_2).push_opcode(opcodes::all::OP_CHECKMULTISIG).into_script()
 272 }
 273
 274 /// panics if htlc.transaction_output_index.is_none()!
 275 pub fn build_htlc_transaction(prev_hash: &Sha256dHash, feerate_per_kw: u64, to_self_delay: u16, htlc: &HTLCOutputInCommitment, a_delayed_payment_key: &PublicKey, revocation_key: &PublicKey) -> Transaction {
 276         let mut txins: Vec<TxIn> = Vec::new();
 277         txins.push(TxIn {
 278                 previous_output: OutPoint {
 279                         txid: prev_hash.clone(),
 280                         vout: htlc.transaction_output_index.expect("Can't build an HTLC transaction for a dust output"),
 281                 },
 282                 script_sig: Script::new(),
 283                 sequence: 0,
 284                 witness: Vec::new(),
 285         });
 286
 287         let total_fee = if htlc.offered {
 288                         feerate_per_kw * HTLC_TIMEOUT_TX_WEIGHT / 1000
 289                 } else {
 290                         feerate_per_kw * HTLC_SUCCESS_TX_WEIGHT / 1000
 291                 };
 292
 293         let mut txouts: Vec<TxOut> = Vec::new();
 294         txouts.push(TxOut {
 295                 script_pubkey: get_revokeable_redeemscript(revocation_key, to_self_delay, a_delayed_payment_key).to_v0_p2wsh(),
 296                 value: htlc.amount_msat / 1000 - total_fee //TODO: BOLT 3 does not specify if we should add amount_msat before dividing or if we should divide by 1000 before subtracting (as we do here)
 297         });
 298
 299         Transaction {
 300                 version: 2,
 301                 lock_time: if htlc.offered { htlc.cltv_expiry } else { 0 },
 302                 input: txins,
 303                 output: txouts,
 304         }
 305 }
 306
 307 /// Signs a transaction created by build_htlc_transaction. If the transaction is an
 308 /// HTLC-Success transaction (ie htlc.offered is false), preimage must be set!
 309 pub(crate) fn sign_htlc_transaction<T: secp256k1::Signing>(tx: &mut Transaction, their_sig: &Signature, preimage: &Option<PaymentPreimage>, htlc: &HTLCOutputInCommitment, a_htlc_key: &PublicKey, b_htlc_key: &PublicKey, revocation_key: &PublicKey, per_commitment_point: &PublicKey, htlc_base_key: &SecretKey, secp_ctx: &Secp256k1<T>) -> Result<(Signature, Script), ()> {
 310         if tx.input.len() != 1 { return Err(()); }
 311         if tx.input[0].witness.len() != 0 { return Err(()); }
 312
 313         let htlc_redeemscript = get_htlc_redeemscript_with_explicit_keys(&htlc, a_htlc_key, b_htlc_key, revocation_key);
 314
 315         let our_htlc_key = derive_private_key(secp_ctx, per_commitment_point, htlc_base_key).map_err(|_| ())?;
 316         let sighash = hash_to_message!(&bip143::SighashComponents::new(&tx).sighash_all(&tx.input[0], &htlc_redeemscript, htlc.amount_msat / 1000)[..]);
 317         let local_tx = PublicKey::from_secret_key(&secp_ctx, &our_htlc_key) == *a_htlc_key;
 318         let our_sig = secp_ctx.sign(&sighash, &our_htlc_key);
 319
 320         tx.input[0].witness.push(Vec::new()); // First is the multisig dummy
 321
 322         if local_tx { // b, then a
 323                 tx.input[0].witness.push(their_sig.serialize_der().to_vec());
 324                 tx.input[0].witness.push(our_sig.serialize_der().to_vec());
 325         } else {
 326                 tx.input[0].witness.push(our_sig.serialize_der().to_vec());
 327                 tx.input[0].witness.push(their_sig.serialize_der().to_vec());
 328         }
 329         tx.input[0].witness[1].push(SigHashType::All as u8);
 330         tx.input[0].witness[2].push(SigHashType::All as u8);
 331
 332         if htlc.offered {
 333                 tx.input[0].witness.push(Vec::new());
 334                 assert!(preimage.is_none());
 335         } else {
 336                 tx.input[0].witness.push(preimage.unwrap().0.to_vec());
 337         }
 338
 339         tx.input[0].witness.push(htlc_redeemscript.as_bytes().to_vec());
 340
 341         Ok((our_sig, htlc_redeemscript))
 342 }
 343
 344 #[derive(Clone)]
 345 /// We use this to track local commitment transactions and put off signing them until we are ready
 346 /// to broadcast. Eventually this will require a signer which is possibly external, but for now we
 347 /// just pass in the SecretKeys required.
 348 pub(crate) struct LocalCommitmentTransaction {
 349         tx: Transaction
 350 }
 351 impl LocalCommitmentTransaction {
 352         #[cfg(test)]
 353         pub fn dummy() -> Self {
 354                 Self { tx: Transaction {
 355                         version: 2,
 356                         input: Vec::new(),
 357                         output: Vec::new(),
 358                         lock_time: 0,
 359                 } }
 360         }
 361
 362         pub fn new_missing_local_sig(mut tx: Transaction, their_sig: &Signature, our_funding_key: &PublicKey, their_funding_key: &PublicKey) -> LocalCommitmentTransaction {
 363                 if tx.input.len() != 1 { panic!("Tried to store a commitment transaction that had input count != 1!"); }
 364                 if tx.input[0].witness.len() != 0 { panic!("Tried to store a signed commitment transaction?"); }
 365
 366                 tx.input[0].witness.push(Vec::new()); // First is the multisig dummy
 367
 368                 if our_funding_key.serialize()[..] < their_funding_key.serialize()[..] {
 369                         tx.input[0].witness.push(Vec::new());
 370                         tx.input[0].witness.push(their_sig.serialize_der().to_vec());
 371                         tx.input[0].witness[2].push(SigHashType::All as u8);
 372                 } else {
 373                         tx.input[0].witness.push(their_sig.serialize_der().to_vec());
 374                         tx.input[0].witness[1].push(SigHashType::All as u8);
 375                         tx.input[0].witness.push(Vec::new());
 376                 }
 377
 378                 Self { tx }
 379         }
 380
 381         pub fn txid(&self) -> Sha256dHash {
 382                 self.tx.txid()
 383         }
 384
 385         pub fn has_local_sig(&self) -> bool {
 386                 if self.tx.input.len() != 1 { panic!("Commitment transactions must have input count == 1!"); }
 387                 if self.tx.input[0].witness.len() == 4 {
 388                         assert!(!self.tx.input[0].witness[1].is_empty());
 389                         assert!(!self.tx.input[0].witness[2].is_empty());
 390                         true
 391                 } else {
 392                         assert_eq!(self.tx.input[0].witness.len(), 3);
 393                         assert!(self.tx.input[0].witness[0].is_empty());
 394                         assert!(self.tx.input[0].witness[1].is_empty() || self.tx.input[0].witness[2].is_empty());
 395                         false
 396                 }
 397         }
 398
 399         pub fn add_local_sig<T: secp256k1::Signing>(&mut self, funding_key: &SecretKey, funding_redeemscript: &Script, channel_value_satoshis: u64, secp_ctx: &Secp256k1<T>) {
 400                 if self.has_local_sig() { return; }
 401                 let sighash = hash_to_message!(&bip143::SighashComponents::new(&self.tx)
 402                         .sighash_all(&self.tx.input[0], funding_redeemscript, channel_value_satoshis)[..]);
 403                 let our_sig = secp_ctx.sign(&sighash, funding_key);
 404
 405                 if self.tx.input[0].witness[1].is_empty() {
 406                         self.tx.input[0].witness[1] = our_sig.serialize_der().to_vec();
 407                         self.tx.input[0].witness[1].push(SigHashType::All as u8);
 408                 } else {
 409                         self.tx.input[0].witness[2] = our_sig.serialize_der().to_vec();
 410                         self.tx.input[0].witness[2].push(SigHashType::All as u8);
 411                 }
 412
 413                 self.tx.input[0].witness.push(funding_redeemscript.as_bytes().to_vec());
 414         }
 415
 416         pub fn without_valid_witness(&self) -> &Transaction { &self.tx }
 417         pub fn with_valid_witness(&self) -> &Transaction {
 418                 assert!(self.has_local_sig());
 419                 &self.tx
 420         }
 421 }
 422 impl PartialEq for LocalCommitmentTransaction {
 423         // We dont care whether we are signed in equality comparison
 424         fn eq(&self, o: &Self) -> bool {
 425                 self.txid() == o.txid()
 426         }
 427 }
 428 impl Writeable for LocalCommitmentTransaction {
 429         fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> {
 430                 if let Err(e) = self.tx.consensus_encode(&mut WriterWriteAdaptor(writer)) {
 431                         match e {
 432                                 encode::Error::Io(e) => return Err(e),
 433                                 _ => panic!("local tx must have been well-formed!"),
 434                         }
 435                 }
 436                 Ok(())
 437         }
 438 }
 439 impl<R: ::std::io::Read> Readable<R> for LocalCommitmentTransaction {
 440         fn read(reader: &mut R) -> Result<Self, DecodeError> {
 441                 let tx = match Transaction::consensus_decode(reader.by_ref()) {
 442                         Ok(tx) => tx,
 443                         Err(e) => match e {
 444                                 encode::Error::Io(ioe) => return Err(DecodeError::Io(ioe)),
 445                                 _ => return Err(DecodeError::InvalidValue),
 446                         },
 447                 };
 448
 449                 if tx.input.len() != 1 {
 450                         // Ensure tx didn't hit the 0-input ambiguity case.
 451                         return Err(DecodeError::InvalidValue);
 452                 }
 453                 Ok(Self { tx })
 454         }
 455 }