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  BIP: 143
  Title: Transaction Signature Verification for Version 0 Witness Program
  Author: Johnson Lau <jl2012@xbt.hk>
          Pieter Wuille <pieter.wuille@gmail.com>
  Status: Draft
  Type: Standards Track
  Created: 2016-01-03

Table of Contents

Abstract

This proposal defines a new transaction digest algorithm for signature verification in version 0 witness program, in order to minimize redundant data hashing in verification, and to cover the input value by the signature.

Motivation

There are 4 ECDSA signature verification codes in the original Bitcoin script system: CHECKSIG, CHECKSIGVERIFY, CHECKMULTISIG, CHECKMULTISIGVERIFY (“sigops”). According to the sighash type (ALL, NONE, SINGLE, ANYONECANPAY), a transaction digest is generated with a double SHA256 of a serialized subset of the transaction, and the signature is verified against this digest with a given public key. The detailed procedure is described in a Bitcoin Wiki article. [1]

Unfortunately, there are at least 2 weaknesses in the original transaction digest algorithm:

  • For the verification of each signature, the amount of data hashing is proportional to the size of the transaction. Therefore, data hashing grows in O(n2) as the number of sigops in a transaction increases. While a 1 MB block would normally take 2 seconds to verify with an average computer in 2015, a 1MB transaction with 5569 sigops may take 25 seconds to verify. This could be fixed by optimizing the digest algorithm by introducing some reusable “midstate”, so the time complexity becomes O(n). [2][3][4]
  • The algorithm does not involve the amount of Bitcoin being spent by the input. This is usually not a problem for online network nodes as they could request for the specified transaction to acquire the output value. For an offline transaction signing device ("cold wallet"), however, the unknowing of input amount makes it impossible to calculate the exact amount being spent and the transaction fee. To cope with this problem a cold wallet must also acquire the full transaction being spent, which could be a big obstacle in the implementation of lightweight, air-gapped wallet. By including the input value of part of the transaction digest, a cold wallet may safely sign a transaction by learning the value from an untrusted source. In the case that a wrong value is provided and signed, the signature would be invalid and no funding might be lost. [5]
Deploying the aforementioned fixes in the original script system is not a simple task. That would be either a hardfork, or a softfork for new sigops without the ability to remove or insert stack items. However, the introduction of segregated witness softfork offers an opportunity to define a different set of script semantics without disrupting the original system, as the unupgraded nodes would always consider such a transaction output is spendable by arbitrary signature or no signature at all. [6]

Specification

A new transaction digest algorithm is defined, but only applicable to sigops in version 0 witness program: 

  Double SHA256 of the serialization of:
     1. nVersion of the transaction (4-byte little endian)
     2. hashPrevouts (32-byte hash)
     3. hashSequence (32-byte hash)
     4. outpoint (32-byte hash + 4-byte little endian) 
     5. scriptCode of the input (varInt for the length + script)
     6. value of the output spent by this input (8-byte little endian)
     7. nSequence of the input (4-byte little endian)
     8. hashOutputs (32-byte hash)
     9. nLocktime of the transaction (4-byte little endian)
    10. sighash type of the signature (4-byte little endian)

All components in the original algorithm, including the behavior OP_CODESEPERATOR, remains unchanged. The only difference is the way of serialization and the inclusion of amount being spent.

The items 1, 4, 7, 9, 10 have the same meaning as the original algorithm. [1]

The item 5:

  • For P2WPKH witness program, the scriptCode is 0x1976a914{20-byte-pubkey-hash}88ac.
  • For P2WSH witness program,
    • if the witnessScript does not contain any OP_CODESEPERATOR, the scriptCode is a varInt for the length of the witnessScript, followed by the witnessScript.
    • if the witnessScript contains any OP_CODESEPERATOR, the scriptCode is the evaluated script, with all OP_CODESEPARATOR and everything up to the last OP_CODESEPARATOR before the signature checking opcode being executed removed, and prepended by a varInt for the length of the truncated script.
The item 6 is a 8-byte value of the amount of bitcoin spent in this input.

hashPrevouts:

  • If the ANYONECANPAY flag is not set, hashPrevouts is the double SHA256 of the serialization of all input outpoints;
  • Otherwise, hashPrevouts is a uint256 of 0x0000......0000.
hashSequence:
  • If none of the ANYONECANPAY, SINGLE, NONE sighash type is set, hashSequence is the double SHA256 of the serialization of nSequence of all inputs;
  • Otherwise, hashSequence is a uint256 of 0x0000......0000.
hashOutputs:
  • If the sighash type is neither SINGLE nor NONE, hashOutputs is the double SHA256 of the serialization of all output value (8-byte little endian) with scriptPubKey (varInt for the length + script);
  • If sighash type is SINGLE and the input index is not greater than the number of outputs, hashOutputs is the double SHA256 of the output value with scriptPubKey of the same index as the input;
  • Otherwise, hashOutputs is a uint256 of 0x0000......0000.
The hashPrevouts, hashSequence, and hashOutputs calculated in an earlier verification may be reused in other inputs of the same transaction, so that the time complexity of the whole hashing process reduces from O(n2) to O(n).

Refer to the reference implementation, reproduced below, for the precise algorithm: 

  uint256 hashPrevouts;
  uint256 hashSequence;
  uint256 hashOutputs;
  
  if (!(nHashType & SIGHASH_ANYONECANPAY)) {
      CHashWriter ss(SER_GETHASH, 0);
      for (unsigned int n = 0; n < txTo.vin.size(); n++) {
          ss << txTo.vin[n].prevout;
      }
      hashPrevouts = ss.GetHash();
  }
  
  if (!(nHashType & SIGHASH_ANYONECANPAY) && (nHashType & 0x1f) != SIGHASH_SINGLE && (nHashType & 0x1f) != SIGHASH_NONE) {
      CHashWriter ss(SER_GETHASH, 0);
      for (unsigned int n = 0; n < txTo.vin.size(); n++) {
          ss << txTo.vin[n].nSequence;
      }
      hashSequence = ss.GetHash();
  }
  
  if ((nHashType & 0x1f) != SIGHASH_SINGLE && (nHashType & 0x1f) != SIGHASH_NONE) {
      CHashWriter ss(SER_GETHASH, 0);
      for (unsigned int n = 0; n < txTo.vout.size(); n++) {
          ss << txTo.vout[n];
      }
      hashOutputs = ss.GetHash();
  } else if ((nHashType & 0x1f) == SIGHASH_SINGLE && nIn < txTo.vout.size()) {
      CHashWriter ss(SER_GETHASH, 0);
      ss << txTo.vout[nIn];
      hashOutputs = ss.GetHash();
  }
  
  CHashWriter ss(SER_GETHASH, 0);
  // Version
  ss << txTo.nVersion;
  // Input prevouts/nSequence (none/all, depending on flags)
  ss << hashPrevouts;
  ss << hashSequence;
  // The input being signed (replacing the scriptSig with scriptCode + amount)
  // The prevout may already be contained in hashPrevout, and the nSequence
  // may already be contain in hashSequence.
  ss << txTo.vin[nIn].prevout;
  ss << static_cast<const CScriptBase&>(scriptCode);
  ss << amount;
  ss << txTo.vin[nIn].nSequence;
  // Outputs (none/one/all, depending on flags)
  ss << hashOutputs;
  // Locktime
  ss << txTo.nLockTime;
  // Sighash type
  ss << nHashType;
  
  return ss.GetHash();

Example

  The following is an unsigned transaction:
    0100000002fff7f7881a8099afa6940d42d1e7f6362bec38171ea3edf433541db4e4ad969f0000000000eeffffffef51e1b804cc89d182d279655c3aa89e815b1b309fe287d9b2b55d57b90ec68a0100000000ffffffff02202cb206000000001976a9148280b37df378db99f66f85c95a783a76ac7a6d5988ac9093510d000000001976a9143bde42dbee7e4dbe6a21b2d50ce2f0167faa815988ac11000000
    
    nVersion:  01000000
    txin:      02 fff7f7881a8099afa6940d42d1e7f6362bec38171ea3edf433541db4e4ad969f 00000000 00 eeffffff
                  ef51e1b804cc89d182d279655c3aa89e815b1b309fe287d9b2b55d57b90ec68a 01000000 00 ffffffff
    txout:     02 202cb20600000000 1976a9148280b37df378db99f66f85c95a783a76ac7a6d5988ac
                  9093510d00000000 1976a9143bde42dbee7e4dbe6a21b2d50ce2f0167faa815988ac
    nLockTime: 11000000
  
  The first input comes from an ordinary P2PK:
    scriptPubKey: 2103c9f4836b9a4f77fc0d81f7bcb01b7f1b35916864b9476c241ce9fc198bd25432ac value: 6.25
    
  The second input comes from a P2WPKH witness program:
    scriptPubKey: 00141d0f172a0ecb48aee1be1f2687d2963ae33f71a1, value: 6
    
  To sign it with a nHashType of 1 (SIGHASH_ALL):
  
  hashPrevouts:
    dSHA256(fff7f7881a8099afa6940d42d1e7f6362bec38171ea3edf433541db4e4ad969f00000000ef51e1b804cc89d182d279655c3aa89e815b1b309fe287d9b2b55d57b90ec68a01000000)
  = 96b827c8483d4e9b96712b6713a7b68d6e8003a781feba36c31143470b4efd37
  
  hashSequence:
    dSHA256(eeffffffffffffff)
  = 52b0a642eea2fb7ae638c36f6252b6750293dbe574a806984b8e4d8548339a3b
  
  hashOutputs:
    dSHA256(202cb206000000001976a9148280b37df378db99f66f85c95a783a76ac7a6d5988ac9093510d000000001976a9143bde42dbee7e4dbe6a21b2d50ce2f0167faa815988ac)
  = 863ef3e1a92afbfdb97f31ad0fc7683ee943e9abcf2501590ff8f6551f47e5e5
  
  hash preimage: 0100000096b827c8483d4e9b96712b6713a7b68d6e8003a781feba36c31143470b4efd3752b0a642eea2fb7ae638c36f6252b6750293dbe574a806984b8e4d8548339a3bef51e1b804cc89d182d279655c3aa89e815b1b309fe287d9b2b55d57b90ec68a010000001976a9141d0f172a0ecb48aee1be1f2687d2963ae33f71a188ac0046c32300000000ffffffff863ef3e1a92afbfdb97f31ad0fc7683ee943e9abcf2501590ff8f6551f47e5e51100000001000000
  
    nVersion:     01000000
    hashPrevouts: 96b827c8483d4e9b96712b6713a7b68d6e8003a781feba36c31143470b4efd37
    hashSequence: 52b0a642eea2fb7ae638c36f6252b6750293dbe574a806984b8e4d8548339a3b
    outpoint:     ef51e1b804cc89d182d279655c3aa89e815b1b309fe287d9b2b55d57b90ec68a01000000
    scriptCode:   1976a9141d0f172a0ecb48aee1be1f2687d2963ae33f71a188ac
    amount:       0046c32300000000
    nSequence:    ffffffff
    hashOutputs:  863ef3e1a92afbfdb97f31ad0fc7683ee943e9abcf2501590ff8f6551f47e5e5
    nLockTime:    11000000
    nHashType:    01000000
    
  sigHash:      c37af31116d1b27caf68aae9e3ac82f1477929014d5b917657d0eb49478cb670
  signature:    304402203609e17b84f6a7d30c80bfa610b5b4542f32a8a0d5447a12fb1366d7f01cc44a0220573a954c4518331561406f90300e8f3358f51928d43c212a8caed02de67eebee
    
  The serialized signed transaction is: 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
  
    nVersion:  01000000
    marker:    00
    flag:      01
    txin:      02 fff7f7881a8099afa6940d42d1e7f6362bec38171ea3edf433541db4e4ad969f 00000000 494830450221008b9d1dc26ba6a9cb62127b02742fa9d754cd3bebf337f7a55d114c8e5cdd30be022040529b194ba3f9281a99f2b1c0a19c0489bc22ede944ccf4ecbab4cc618ef3ed01 eeffffff
                  ef51e1b804cc89d182d279655c3aa89e815b1b309fe287d9b2b55d57b90ec68a 01000000 00 ffffffff
    txout:     02 202cb20600000000 1976a9148280b37df378db99f66f85c95a783a76ac7a6d5988ac
                  9093510d00000000 1976a9143bde42dbee7e4dbe6a21b2d50ce2f0167faa815988ac
    witness    00
               02 47304402203609e17b84f6a7d30c80bfa610b5b4542f32a8a0d5447a12fb1366d7f01cc44a0220573a954c4518331561406f90300e8f3358f51928d43c212a8caed02de67eebee01 21025476c2e83188368da1ff3e292e7acafcdb3566bb0ad253f62fc70f07aeee6357
    nLockTime: 11000000

The new serialization format is described in BIP144 [7]

Deployment

This proposal is deployed with Segregated Witness softfork (BIP 141)

Backward compatibility

As a soft fork, older software will continue to operate without modification. Non-upgraded nodes, however, will not see nor validate the witness data and will consider all witness programs, inculding the redefined sigops, as anyone-can-spend scripts.

Reference Implementation

https://github.com/sipa/bitcoin/commits/segwit

References

  1. a b https://en.bitcoin.it/wiki/OP_CHECKSIG
  2. ^ CVE-2013-2292
  3. ^ New Bitcoin vulnerability: A transaction that takes at least 3 minutes to verify
  4. ^ The Megatransaction: Why Does It Take 25 Seconds?
  5. ^ SIGHASH_WITHINPUTVALUE: Super-lightweight HW wallets and offline data
  6. ^ BIP141: Segregated Witness (Consensus layer)
  7. ^ BIP144: Segregated Witness (Peer Services)

Copyright

This document is placed in the public domain.