package lnwallet import ( "bytes" "crypto/sha256" "fmt" "math/big" "golang.org/x/crypto/hkdf" "github.com/btcsuite/fastsha256" "github.com/roasbeef/btcd/btcec" "github.com/roasbeef/btcd/txscript" "github.com/roasbeef/btcd/wire" "github.com/roasbeef/btcutil" ) var ( // TODO(roasbeef): remove these and use the one's defined in txscript // within testnet-L. SequenceLockTimeSeconds = uint32(1 << 22) SequenceLockTimeMask = uint32(0x0000ffff) OP_CHECKSEQUENCEVERIFY byte = txscript.OP_NOP3 ) // witnessScriptHash generates a pay-to-witness-script-hash public key script // paying to a version 0 witness program paying to the passed redeem script. func witnessScriptHash(redeemScript []byte) ([]byte, error) { bldr := txscript.NewScriptBuilder() bldr.AddOp(txscript.OP_0) scriptHash := fastsha256.Sum256(redeemScript) bldr.AddData(scriptHash[:]) return bldr.Script() } // genMultiSigScript generates the non-p2sh'd multisig script for 2 of 2 // pubkeys. func genMultiSigScript(aPub, bPub []byte) ([]byte, error) { if len(aPub) != 33 || len(bPub) != 33 { return nil, fmt.Errorf("Pubkey size error. Compressed pubkeys only") } // Swap to sort pubkeys if needed. Keys are sorted in lexicographical // order. The signatures within the scriptSig must also adhere to the // order, ensuring that the signatures for each public key appears // in the proper order on the stack. if bytes.Compare(aPub, bPub) == -1 { aPub, bPub = bPub, aPub } bldr := txscript.NewScriptBuilder() bldr.AddOp(txscript.OP_2) bldr.AddData(aPub) // Add both pubkeys (sorted). bldr.AddData(bPub) bldr.AddOp(txscript.OP_2) bldr.AddOp(txscript.OP_CHECKMULTISIG) return bldr.Script() } // genFundingPkScript creates a redeem script, and its matching p2wsh // output for the funding transaction. func genFundingPkScript(aPub, bPub []byte, amt int64) ([]byte, *wire.TxOut, error) { // As a sanity check, ensure that the passed amount is above zero. if amt <= 0 { return nil, nil, fmt.Errorf("can't create FundTx script with " + "zero, or negative coins") } // First, create the 2-of-2 multi-sig script itself. redeemScript, err := genMultiSigScript(aPub, bPub) if err != nil { return nil, nil, err } // With the 2-of-2 script in had, generate a p2wsh script which pays // to the funding script. pkScript, err := witnessScriptHash(redeemScript) if err != nil { return nil, nil, err } return redeemScript, wire.NewTxOut(amt, pkScript), nil } // spendMultiSig generates the witness stack required to redeem the 2-of-2 p2wsh // multi-sig output. func spendMultiSig(redeemScript, pubA, sigA, pubB, sigB []byte) [][]byte { witness := make([][]byte, 4) // When spending a p2wsh multi-sig script, rather than an OP_0, we add // a nil stack element to eat the extra pop. witness[0] = nil // When initially generating the redeemScript, we sorted the serialized // public keys in descending order. So we do a quick comparison in order // ensure the signatures appear on the Script Virual Machine stack in // the correct order. if bytes.Compare(pubA, pubB) == -1 { witness[1] = sigB witness[2] = sigA } else { witness[1] = sigA witness[2] = sigB } // Finally, add the pre-image as the last witness element. witness[3] = redeemScript return witness } // findScriptOutputIndex finds the index of the public key script output // matching 'script'. Additionally, a boolean is returned indicating if // a matching output was found at all. // NOTE: The search stops after the first matching script is found. func findScriptOutputIndex(tx *wire.MsgTx, script []byte) (bool, uint32) { found := false index := uint32(0) for i, txOut := range tx.TxOut { if bytes.Equal(txOut.PkScript, script) { found = true index = uint32(i) break } } return found, index } // senderHTLCScript constructs the public key script for an outgoing HTLC // output payment for the sender's version of the commitment transaction: // // Possible Input Scripts: // SENDR: 0 // RECVR: 0 1 // REVOK: 1 1 // * receiver revoke // // OP_IF // //Receiver // OP_IF // //Revoke // // OP_ELSE // //Receive // OP_SIZE 32 OP_EQUALVERIFY // // OP_ENDIF // OP_SWAP // OP_SHA256 OP_EQUALVERIFY // OP_CHECKSIG // OP_ELSE // //Sender // OP_CHECKLOCKTIMEVERIFY // OP_CHECKSEQUENCEVERIFY // OP_2DROP // OP_CHECKSIG // OP_ENDIF func senderHTLCScript(absoluteTimeout, relativeTimeout uint32, senderKey, receiverKey *btcec.PublicKey, revokeHash, paymentHash []byte) ([]byte, error) { builder := txscript.NewScriptBuilder() // The receiver of the HTLC places a 1 as the first item in the witness // stack, forcing Script execution to enter the "if" clause within the // main body of the script. builder.AddOp(txscript.OP_IF) // The receiver will place a 1 as the second item of the witness stack // in the case the sender broadcasts a revoked commitment transaction. // Executing this branch allows the receiver to claim the sender's // funds as a result of their contract violation. builder.AddOp(txscript.OP_IF) builder.AddData(revokeHash) // Alternatively, the receiver can place a 0 as the second item of the // witness stack if they wish to claim the HTLC with the proper // pre-image as normal. In order to prevent an over-sized pre-image // attack (which can create undesirable redemption asymmerties, we // strongly require that all HTLC pre-images are exactly 32 bytes. builder.AddOp(txscript.OP_ELSE) builder.AddOp(txscript.OP_SIZE) builder.AddInt64(32) builder.AddOp(txscript.OP_EQUALVERIFY) builder.AddData(paymentHash) builder.AddOp(txscript.OP_ENDIF) builder.AddOp(txscript.OP_SWAP) builder.AddOp(txscript.OP_SHA256) builder.AddOp(txscript.OP_EQUALVERIFY) // In either case, we require a valid signature by the receiver. builder.AddData(receiverKey.SerializeCompressed()) builder.AddOp(txscript.OP_CHECKSIG) // Otherwise, the sender of the HTLC will place a 0 as the first item // of the witness stack in order to sweep the funds back after the HTLC // times out. builder.AddOp(txscript.OP_ELSE) // In this case, the sender will need to wait for an absolute HTLC // timeout, then afterwards a relative timeout before we claim re-claim // the unsettled funds. This delay gives the other party a chance to // present the pre-image to the revocation hash in the event that the // sender (at this time) broadcasts this commitment transaction after // it has been revoked. builder.AddInt64(int64(absoluteTimeout)) builder.AddOp(txscript.OP_CHECKLOCKTIMEVERIFY) builder.AddInt64(int64(relativeTimeout)) builder.AddOp(OP_CHECKSEQUENCEVERIFY) builder.AddOp(txscript.OP_2DROP) builder.AddData(senderKey.SerializeCompressed()) builder.AddOp(txscript.OP_CHECKSIG) builder.AddOp(txscript.OP_ENDIF) return builder.Script() } // senderHtlcSpendRevoke constructs a valid witness allowing the reciever of an // HTLC to claim the output with knowledge of the revocation preimage in the // scenario that the sender of the HTLC broadcasts a previously revoked // commitment transaction. A valid spend requires knowledge of the pre-image to // the commitment transaction's revocation hash, and a valid signature under // the receiver's public key. func senderHtlcSpendRevoke(commitScript []byte, outputAmt btcutil.Amount, reciverKey *btcec.PrivateKey, sweepTx *wire.MsgTx, revokePreimage []byte) (wire.TxWitness, error) { hashCache := txscript.NewTxSigHashes(sweepTx) sweepSig, err := txscript.RawTxInWitnessSignature( sweepTx, hashCache, 0, int64(outputAmt), commitScript, txscript.SigHashAll, reciverKey) if err != nil { return nil, err } // In order to force script execution to enter the revocation clause, // we place two one's as the first items in the final evalulated // witness stack. witnessStack := wire.TxWitness(make([][]byte, 5)) witnessStack[0] = sweepSig witnessStack[1] = revokePreimage witnessStack[2] = []byte{1} witnessStack[3] = []byte{1} witnessStack[4] = commitScript return witnessStack, nil } // senderHtlcSpendRedeem constructs a valid witness allowing the receiver of an // HTLC to redeem the pending output in the scenario that the sender broadcasts // their version of the commitment transaction. A valid spend requires // knowledge of the payment pre-image, and a valid signature under the // receivers public key. func senderHtlcSpendRedeem(commitScript []byte, outputAmt btcutil.Amount, reciverKey *btcec.PrivateKey, sweepTx *wire.MsgTx, paymentPreimage []byte) (wire.TxWitness, error) { hashCache := txscript.NewTxSigHashes(sweepTx) sweepSig, err := txscript.RawTxInWitnessSignature( sweepTx, hashCache, 0, int64(outputAmt), commitScript, txscript.SigHashAll, reciverKey) if err != nil { return nil, err } // We force script execution into the HTLC redemption clause by placing // a one, then a zero as the first items in the final evalulated // witness stack. witnessStack := wire.TxWitness(make([][]byte, 5)) witnessStack[0] = sweepSig witnessStack[1] = paymentPreimage witnessStack[2] = []byte{0} witnessStack[3] = []byte{1} witnessStack[4] = commitScript return witnessStack, nil } // htlcSpendTimeout constructs a valid witness allowing the sender of an HTLC // to recover the pending funds after an absolute, then relative locktime // period. func senderHtlcSpendTimeout(commitScript []byte, outputAmt btcutil.Amount, senderKey *btcec.PrivateKey, sweepTx *wire.MsgTx, absoluteTimeout, relativeTimeout uint32) (wire.TxWitness, error) { // Since the HTLC output has an absolute timeout before we're permitted // to sweep the output, we need to set the locktime of this sweepign // transaction to that aboslute value in order to pass Script // verification. sweepTx.LockTime = absoluteTimeout // Additionally, we're required to wait a relative period of time // before we can sweep the output in order to allow the other party to // contest our claim of validity to this version of the commitment // transaction. sweepTx.TxIn[0].Sequence = lockTimeToSequence(false, relativeTimeout) // Finally, OP_CSV requires that the version of the transaction // spending a pkscript with OP_CSV within it *must* be >= 2. sweepTx.Version = 2 hashCache := txscript.NewTxSigHashes(sweepTx) sweepSig, err := txscript.RawTxInWitnessSignature( sweepTx, hashCache, 0, int64(outputAmt), commitScript, txscript.SigHashAll, senderKey) if err != nil { return nil, err } // We place a zero as the first item of the evaluated witness stack in // order to force Script execution to the HTLC timeout clause. witnessStack := wire.TxWitness(make([][]byte, 3)) witnessStack[0] = sweepSig witnessStack[1] = []byte{0} witnessStack[2] = commitScript return witnessStack, nil } // receiverHTLCScript constructs the public key script for an incoming HTLC // output payment for the receiver's version of the commitment transaction: // // Possible Input Scripts: // RECVR: 1 // REVOK: 1 0 // SENDR: 0 0 // // OP_IF // //Receiver // OP_SIZE 32 OP_EQUALVERIFY // OP_SHA256 // OP_EQUALVERIFY // OP_CHECKSEQUENCEVERIFY OP_DROP // OP_CHECKSIG // OP_ELSE // //Sender // OP_IF // //Revocation // OP_SHA256 // OP_EQUALVERIFY // OP_ELSE // //Refund // OP_CHECKLOCKTIMEVERIFY OP_DROP // OP_ENDIF // OP_CHECKSIG // OP_ENDIF // TODO(roasbeef): go back to revocation keys in the HTLC outputs? // * also could combine pre-image with their key? func receiverHTLCScript(absoluteTimeout, relativeTimeout uint32, senderKey, receiverKey *btcec.PublicKey, revokeHash, paymentHash []byte) ([]byte, error) { builder := txscript.NewScriptBuilder() // The receiver of the script will place a 1 as the first item of the // witness stack forcing Script execution to enter the "if" clause of // the main body of the script. builder.AddOp(txscript.OP_IF) // In this clause, the receiver can redeem the HTLC after a relative timeout. // This added delay gives the sender (at this time) an opportunity to // re-claim the pending HTLC in the event that the receiver // (at this time) broadcasts this old commitment transaction after it // has been revoked. Additionally, we require that the pre-image is // exactly 32-bytes in order to avoid undesirable redemption // asymmerties in the multi-hop scenario. builder.AddOp(txscript.OP_SIZE) builder.AddInt64(32) builder.AddOp(txscript.OP_EQUALVERIFY) builder.AddOp(txscript.OP_SHA256) builder.AddData(paymentHash) builder.AddOp(txscript.OP_EQUALVERIFY) builder.AddInt64(int64(relativeTimeout)) builder.AddOp(OP_CHECKSEQUENCEVERIFY) builder.AddOp(txscript.OP_DROP) builder.AddData(receiverKey.SerializeCompressed()) builder.AddOp(txscript.OP_CHECKSIG) // Otherwise, the sender will place a 0 as the first item of the // witness stack forcing exeuction to enter the "else" clause of the // main body of the script. builder.AddOp(txscript.OP_ELSE) // The sender will place a 1 as the second item of the witness stack // in the scenario that the receiver broadcasts an invalidated // commitment transaction, allowing the sender to sweep all the // receiver's funds. builder.AddOp(txscript.OP_IF) builder.AddOp(txscript.OP_SHA256) builder.AddData(revokeHash) builder.AddOp(txscript.OP_EQUALVERIFY) // If not, then the sender needs to wait for the HTLC timeout. This // clause may be executed if the receiver fails to present the r-value // in time. This prevents the pending funds from being locked up // indefinately. // The sender will place a 0 as the second item of the witness stack if // they wish to sweep the HTLC after an absolute refund timeout. This // time out clause prevents the pending funds from being locked up // indefinately. builder.AddOp(txscript.OP_ELSE) builder.AddInt64(int64(absoluteTimeout)) builder.AddOp(txscript.OP_CHECKLOCKTIMEVERIFY) builder.AddOp(txscript.OP_DROP) builder.AddOp(txscript.OP_ENDIF) // In either case, we also require a valid signature with the sender's // commitment private key. builder.AddData(senderKey.SerializeCompressed()) builder.AddOp(txscript.OP_CHECKSIG) builder.AddOp(txscript.OP_ENDIF) return builder.Script() } // receiverHtlcSpendRedeem constructs a valid witness allowing the receiver of // an HTLC to redeem the conditional payment in the event that their commitment // transaction is broadcast. Since this is a pay out to the receiving party as // an output on their commitment transaction, a relative time delay is required // before the output can be spent. func receiverHtlcSpendRedeem(commitScript []byte, outputAmt btcutil.Amount, reciverKey *btcec.PrivateKey, sweepTx *wire.MsgTx, paymentPreimage []byte, relativeTimeout uint32) (wire.TxWitness, error) { // In order to properly spend the transaction, we need to set the // sequence number. We do this by convering the relative block delay // into a sequence number value able to be interpeted by // OP_CHECKSEQUENCEVERIFY. sweepTx.TxIn[0].Sequence = lockTimeToSequence(false, relativeTimeout) // Additionally, OP_CSV requires that the version of the transaction // spending a pkscript with OP_CSV within it *must* be >= 2. sweepTx.Version = 2 hashCache := txscript.NewTxSigHashes(sweepTx) sweepSig, err := txscript.RawTxInWitnessSignature( sweepTx, hashCache, 0, int64(outputAmt), commitScript, txscript.SigHashAll, reciverKey) if err != nil { return nil, err } // Place a one as the first item in the evaluated witness stack to // force script execution to the HTLC redemption clause. witnessStack := wire.TxWitness(make([][]byte, 4)) witnessStack[0] = sweepSig witnessStack[1] = paymentPreimage witnessStack[2] = []byte{1} witnessStack[3] = commitScript return witnessStack, nil } // receiverHtlcSpendRevoke constructs a valid witness allowing the sender of an // HTLC within a previously revoked commitment transaction to re-claim the // pending funds in the case that the receiver broadcasts this revoked // commitment transaction. func receiverHtlcSpendRevoke(commitScript []byte, outputAmt btcutil.Amount, senderKey *btcec.PrivateKey, sweepTx *wire.MsgTx, revokePreimage []byte) (wire.TxWitness, error) { // TODO(roasbeef): move sig generate outside func, or just factor out? hashCache := txscript.NewTxSigHashes(sweepTx) sweepSig, err := txscript.RawTxInWitnessSignature( sweepTx, hashCache, 0, int64(outputAmt), commitScript, txscript.SigHashAll, senderKey) if err != nil { return nil, err } // We place a zero, then one as the first items in the evaluated // witness stack in order to force script execution to the HTLC // revocation clause. witnessStack := wire.TxWitness(make([][]byte, 5)) witnessStack[0] = sweepSig witnessStack[1] = revokePreimage witnessStack[2] = []byte{1} witnessStack[3] = []byte{0} witnessStack[4] = commitScript return witnessStack, nil } // receiverHtlcSpendTimeout constructs a valid witness allowing the sender of // an HTLC to recover the pending funds after an absolute timeout in the // scenario that the receiver of the HTLC broadcasts their version of the // commitment transaction. func receiverHtlcSpendTimeout(commitScript []byte, outputAmt btcutil.Amount, senderKey *btcec.PrivateKey, sweepTx *wire.MsgTx, absoluteTimeout uint32) (wire.TxWitness, error) { // The HTLC output has an absolute time period before we are permitted // to recover the pending funds. Therefore we need to set the locktime // on this sweeping transaction in order to pass Script verification. sweepTx.LockTime = absoluteTimeout hashCache := txscript.NewTxSigHashes(sweepTx) sweepSig, err := txscript.RawTxInWitnessSignature( sweepTx, hashCache, 0, int64(outputAmt), commitScript, txscript.SigHashAll, senderKey) if err != nil { return nil, err } witnessStack := wire.TxWitness(make([][]byte, 4)) witnessStack[0] = sweepSig witnessStack[1] = []byte{0} witnessStack[2] = []byte{0} witnessStack[3] = commitScript return witnessStack, nil } // lockTimeToSequence converts the passed relative locktime to a sequence // number in accordance to BIP-68. // See: https://github.com/bitcoin/bips/blob/master/bip-0068.mediawiki // * (Compatibility) func lockTimeToSequence(isSeconds bool, locktime uint32) uint32 { if !isSeconds { // The locktime is to be expressed in confirmations. Apply the // mask to restrict the number of confirmations to 65,535 or // 1.25 years. return SequenceLockTimeMask & locktime } // Set the 22nd bit which indicates the lock time is in seconds, then // shift the locktime over by 9 since the time granularity is in // 512-second intervals (2^9). This results in a max lock-time of // 33,554,431 seconds, or 1.06 years. return SequenceLockTimeSeconds | (locktime >> 9) } // commitScriptToSelf constructs the public key script for the output on the // commitment transaction paying to the "owner" of said commitment transaction. // If the other party learns of the pre-image to the revocation hash, then they // can claim all the settled funds in the channel, plus the unsettled funds. // // Possible Input Scripts: // REVOKE: 1 // SENDRSWEEP: 0 // // Output Script: // OP_IF // OP_CHECKSIG // OP_ELSE // OP_CHECKSIGVERIFY // OP_CHECKSEQUENCEVERIFY // OP_ENDIF func commitScriptToSelf(csvTimeout uint32, selfKey, revokeKey *btcec.PublicKey) ([]byte, error) { // This script is spendable under two conditions: either the 'csvTimeout' // has passed and we can redeem our funds, or they can produce a valid // signature with the revocation public key. The revocation public key // will *only* be known to the other party if we have divulged the // revocation hash, allowing them to homomorphically derive the proper // private key which coresponds to the revoke public key. builder := txscript.NewScriptBuilder() builder.AddOp(txscript.OP_IF) // If a valid signature using the revocation key is presented, then // allow an immediate spend provided the proper signature. builder.AddData(revokeKey.SerializeCompressed()) builder.AddOp(txscript.OP_CHECKSIG) builder.AddOp(txscript.OP_ELSE) // Otherwise, we can re-claim our funds after a CSV delay of // 'csvTimeout' timeout blocks, and a valid signature. builder.AddData(selfKey.SerializeCompressed()) builder.AddOp(txscript.OP_CHECKSIGVERIFY) builder.AddInt64(int64(csvTimeout)) builder.AddOp(OP_CHECKSEQUENCEVERIFY) builder.AddOp(txscript.OP_ENDIF) return builder.Script() } // commitScriptUnencumbered constructs the public key script on the commitment // transaction paying to the "other" party. The constructed output is a normal // p2wkh output spendable immediately, requiring no contestation period. func commitScriptUnencumbered(key *btcec.PublicKey) ([]byte, error) { // This script goes to the "other" party, and it spendable immediately. builder := txscript.NewScriptBuilder() builder.AddOp(txscript.OP_0) builder.AddData(btcutil.Hash160(key.SerializeCompressed())) return builder.Script() } // commitSpendTimeout constructs a valid witness allowing the owner of a // particular commitment transaction to spend the output returning settled // funds back to themselves after an absolute block timeout. func commitSpendTimeout(commitScript []byte, outputAmt btcutil.Amount, blockTimeout uint32, selfKey *btcec.PrivateKey, sweepTx *wire.MsgTx) (wire.TxWitness, error) { // In order to properly spend the transaction, we need to set the // sequence number. We do this by convering the relative block delay // into a sequence number value able to be interpeted by // OP_CHECKSEQUENCEVERIFY. sweepTx.TxIn[0].Sequence = lockTimeToSequence(false, blockTimeout) // Additionally, OP_CSV requires that the version of the transaction // spending a pkscript with OP_CSV within it *must* be >= 2. sweepTx.Version = 2 // With the sequence number in place, we're now able to properly sign // off on the sweep transaction. hashCache := txscript.NewTxSigHashes(sweepTx) sweepSig, err := txscript.RawTxInWitnessSignature( sweepTx, hashCache, 0, int64(outputAmt), commitScript, txscript.SigHashAll, selfKey) if err != nil { return nil, err } // Place a zero as the first item in the evaluated witness stack to // force script execution to the timeout spend clause. witnessStack := wire.TxWitness(make([][]byte, 3)) witnessStack[0] = sweepSig witnessStack[1] = []byte{0} witnessStack[2] = commitScript return witnessStack, nil } // commitSpendRevoke constructs a valid witness allowing a node to sweep the // settled output of a malicious counter-party who broadcasts a revoked // commitment trransaction. func commitSpendRevoke(commitScript []byte, outputAmt btcutil.Amount, revocationPriv *btcec.PrivateKey, sweepTx *wire.MsgTx) (wire.TxWitness, error) { hashCache := txscript.NewTxSigHashes(sweepTx) sweepSig, err := txscript.RawTxInWitnessSignature( sweepTx, hashCache, 0, int64(outputAmt), commitScript, txscript.SigHashAll, revocationPriv) if err != nil { return nil, err } // Place a 1 as the first item in the evaluated witness stack to // force script execution to the revocation clause. witnessStack := wire.TxWitness(make([][]byte, 3)) witnessStack[0] = sweepSig witnessStack[1] = []byte{1} witnessStack[2] = commitScript return witnessStack, nil } // commitSpendNoDelay constructs a valid witness allowing a node to spend their // settled no-delay output on the counter-party's commitment transaction. func commitSpendNoDelay(commitScript []byte, outputAmt btcutil.Amount, commitPriv *btcec.PrivateKey, sweepTx *wire.MsgTx) (wire.TxWitness, error) { // This is just a regular p2wkh spend which looks something like: // * witness: hashCache := txscript.NewTxSigHashes(sweepTx) witness, err := txscript.WitnessScript(sweepTx, hashCache, 0, int64(outputAmt), commitScript, txscript.SigHashAll, commitPriv, true) if err != nil { return nil, err } return wire.TxWitness(witness), nil } // deriveRevocationPubkey derives the revocation public key given the // counter-party's commitment key, and revocation pre-image derived via a // pseudo-random-function. In the event that we (for some reason) broadcast a // revoked commitment transaction, then if the other party knows the revocation // pre-image, then they'll be able to derive the corresponding private key to // this private key by exploting the homomorphism in the elliptic curve group: // * https://en.wikipedia.org/wiki/Group_homomorphism#Homomorphisms_of_abelian_groups // // The derivation is performed as follows: // // revokeKey := commitKey + revokePoint // := G*k + G*h // := G * (k+h) // // Therefore, once we divulge the revocation pre-image, the remote peer is able to // compute the proper private key for the revokeKey by computing: // revokePriv := commitPriv + revokePreimge mod N // // Where N is the order of the sub-group. func deriveRevocationPubkey(commitPubKey *btcec.PublicKey, revokePreimage []byte) *btcec.PublicKey { // First we need to convert the revocation hash into a point on the // elliptic curve. revokePointX, revokePointY := btcec.S256().ScalarBaseMult(revokePreimage) // Now that we have the revocation point, we add this to their commitment // public key in order to obtain the revocation public key. revokeX, revokeY := btcec.S256().Add(commitPubKey.X, commitPubKey.Y, revokePointX, revokePointY) return &btcec.PublicKey{X: revokeX, Y: revokeY} } // deriveRevocationPrivKey derives the revocation private key given a node's // commitment private key, and the pre-image to a previously seen revocation // hash. Using this derived private key, a node is able to claim the output // within the commitment transaction of a node in the case that they broadcast // a previously revoked commitment transaction. // // The private key is derived as follwos: // revokePriv := commitPriv + revokePreimage mod N // // Where N is the order of the sub-group. func deriveRevocationPrivKey(commitPrivKey *btcec.PrivateKey, revokePreimage []byte) *btcec.PrivateKey { // Convert the revocation pre-image into a scalar value so we can // manipulate it within the curve's defined finite field. revokeScalar := new(big.Int).SetBytes(revokePreimage) // To derive the revocation private key, we simply add the revocation // pre-image to the commitment private key. // // This works since: // P = G*a + G*b // = G*(a+b) // = G*p revokePriv := revokeScalar.Add(revokeScalar, commitPrivKey.D) revokePriv = revokePriv.Mod(revokePriv, btcec.S256().N) privRevoke, _ := btcec.PrivKeyFromBytes(btcec.S256(), revokePriv.Bytes()) return privRevoke } // deriveElkremRoot derives an elkrem root unique to a channel given the // private key for our public key in the 2-of-2 multi-sig, and the remote // node's multi-sig public key. The root is derived using the HKDF[1][2] // instantiated with sha-256. The secret data used is our multi-sig private // key, with the salt being the remote node's public key. // // [1]: https://eprint.iacr.org/2010/264.pdf // [2]: https://tools.ietf.org/html/rfc5869 func deriveElkremRoot(localMultiSigKey *btcec.PrivateKey, remoteMultiSigKey *btcec.PublicKey) wire.ShaHash { secret := localMultiSigKey.Serialize() salt := remoteMultiSigKey.SerializeCompressed() info := []byte("elkrem") rootReader := hkdf.New(sha256.New, secret, salt, info) // It's safe to ignore the error her as we know for sure that we won't // be draining the HKDF past its available entropy horizon. // TODO(roasbeef): revisit... var elkremRoot wire.ShaHash rootReader.Read(elkremRoot[:]) return elkremRoot }