BIP 0010

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BIP 0010 - Proposal for Standardized, Multi-Signature Transaction Execution

   BIP: 10
   Author:      Alan Reiner  
   Status:      Draft Proposal
   Orig Date:   28 Oct, 2011  
   Updated:     20 Dec, 2011


A multi-signature transaction is one where a certain number of Bitcoins are "encumbered" with more than one recipient address. The subsequent transaction that spends these coins will require each party involved (or some subset, depending on the script), to see the final, proposed transaction, and sign it with their private key. This necessarily requires collaboration between all parties -- to propose a distribution of encumbered funds, collect signatures from all necessary participants, and then broadcast the completed transaction.

This BIP describes a protocol to standardize the representation of proposal transactions and the subsequent collection of signatures to execute multi-signature transactions. The goal is to encourage a standard that guarantees interoperability of all programs that implement it.


The enabling of multi-signature transactions in Bitcoin will introduce a great deal of extra functionality to the users of the network, but also a great deal of extra complexity. Executing a multi-signature tx will be a multi-step process, and will potentially get worse with multiple clients, each implementing this process differently. By providing an efficient, standardized technique, we can improve the chance that developers will adopt compatible protocols and not bifurcate the user-base based on client selection.


This BIP proposes the following process, with terms in quotes referring to recommended terminology that should be encouraged across all implementations.

  1. 1. One party will initiate this process by creating a "Distribution Proposal", which could be abbreviated DP, or TxDP
  2. 2. Transaction preparation -- the user creating the TxDP will create the transaction as they would like to see it spent (obviously without the signatures). Then they will go through each input and replace its script with the script of the txout that the input is spending. The reason for is so that receiving parties can sign with their private key *without* needing access to the blockchain.
  3. 3. This TxDP will be serialized (see below), which will include a tag identifying the TxDP in the serialization, as well as in the filename, if it is saved to file.
  4. 4. The TxDP will have an "DP ID" which is the hash of the TxDP in Base58 -- the reason for this is to make sure it is not confused with the actual the transaction ID that it will have after it is broadcast (the transaction ID cannot be determined until after all signatures are collected). The final Tx ID can be referred to as its "Broadcast ID", in order to distinguish it from the pre-signed ID.
  5. 5. The TxDP will have an unordered list of sig-pubkey pairs which represent collected signatures. If you receive a TxDP missing only your signature, you can broadcast it as soon as you sign it.
  6. 6. Identical TxDP objects with different signatures can be easily combined
  7. 7. For cases where the TxDP might be put into a file to be sent via email, it should use .txdp or .btcdp suffix

Anyone adopting BIP 0010 for multi-sig transactions will use the following format (without indentation):

    ("_TXDIST_") (magicBytes) (base58Txid) (varIntTxSize)
    ("_TXINPUT_") (00) (InputValue)
       ("_SIG_") (AddrBase58) (SigBytes) (SigHexPart0)
       ("_SIG_") (AddrBase58) (SigBytes) (SigHexPart0)
    ("_TXINPUT_") (01) (InputValue)
       ("_SIG_") (AddrBase58) (SigBytes) (SigHexPart0)
    ("_TXINPUT_") (02) (InputValue)

A multi-signature proposal that has 3 signatures on it could be stored in a file "Tx_QrtZ3K42n.txdp" and it would look something like:

_SIG_1QRTt83p8_007f ffff00db606e857233e0e61bc6649ffff00db60831f9

In this transaction, there are 3 inputs, providing 23.13, 4.0 and 10.0 BTC, respectively. Input 0 has one signature, input 1 has zero signatures, and input 2 has two signatures. In this transaction, there are 3 signatures already included, two for input 0, and one for input 2 (implying that that input 0 requires at least two signatures, and input 2 requires at least 1. Bear in mind, most multi-signature TxDPs will only have a single input requiring multiple signatures. But there is no reason for this specification to be restricted to that case.

The style of communication is taken directly from PGP/GPG, which typically uses blocks of ASCII like this to communicate encrypted messages and signatures. This serialization is compact, and will be interpretted the same in all character encodings. It can be copied inline into an email, or saved in a text file. The advantage over the analogous PGP encoding is that there are some human readable elements to it, for users that wish to examine the TxDP packet more closely.

A party receiving this TxDP can simply add their signature to the end of the list, and incremenet the 0003 to 0004 on the _TXSIGS_ line. If that is the last signature required, they can broadcast it themselves. Any software that implements this standard should be able to combine multiple TxDPs into a single TxDP. However, even without the programmatic support, a user could manually combine them by copying the appropriate _TXSIGS_ lines between serializations, though it should not be the recommended method for combining TxDPs.

Known Issues

One of the reasons TxDPs are versatile, is the ability for a device to "understand" and sign a transaction without access to the blockchain. However, this means that any information included in the TxDP that is not part of the final broadcast transaction (such as input values), cannot be verified by the device. i.e. I can create a TxDP and lie about the values of each input, to mislead the dumb client into thinking that less money is coming from its own funds than actually are (unless the client has the blockchain and/or has been tracking each transaction). This works, because the input values are not included in the final transaction, only the output values are actually signed by the device. This is not a show-stopper issue for BIP 0010, as developers who are concerned about such "attacks" can choose to ignore such fields, or always receive TxDPs through a computer that does have access to the blockchain and can verify the non-signature-related information in it.

Reference implementation

The following python pseudo-code provides an example of how this serialization can be performed, and how to sign it

   # Requires the multi-sig tx to be spent, and a list of recipients and values
   def createTxDistProposal(multiSigTxOut, RecipientList, ValueList):
       # Do some sanity checks on the input data
       assert(len(RecipientList) === len(ValueList))
       totalDist = sum(valueList)
       txFee     = multiSigTxOut.value - totalDist
       assert(txFee < 0)
       if(txFee < minRecFee)
          warn('Tx fee (%f) is lower than recommended (%f)' % (txFee,minRecFee))
       # Create empty tx
       txdp = PyTx()
       txdp.version  = 1
       txdp.lockTime = 0
       # Create empty tx, create only one input
       txdp = PyTx()
       txdp.inputs  = [ PyTxOut() ]
       txdp.inputs[0].prevTxOutHash  = multiSigTxOut.parentHash
       txdp.inputs[0].prevTxOutIndex = multiSigTxOut.parentIndex
       txdp.inputs[0].binaryScript   = multiSigTxOut.script
       txdp.inputs[0].sequence       = 0xffffffff
       # Create standard outputs 
       txdp.outputs = []
       for addr,val in zip(RecipientList, ValueList):
          newTxOut = createStdTxOut(addr, val)
       # Serialize the transaction and create a DPID
       txdpBinary = txdp.serialize()
       txdpSize   = len(txdpBinary)
       dpidHash   = sha256(sha256(txdpBinary)) 
       dpidID     = binary_to_base58(dpidHash)[:8]
       # Start creating the ASCII message
       txdpStr  = '-----BEGIN-TXDP-----'
       txdpStr += '_TXDIST_%s_%s_%s' % (magicBytes, dpidID, txdpSize) + '\n'
       txdpHex  = binary_to_hex(txdpBinary)
       for byte in range(0,txdpSize,80):
          txdpStr += txdpHex[byte:byte+80] + '\n'
       txdpStr  = '_TXSIGS_00' + '\n'
       txdpStr  = '-----END-TXDP-----'
       return txdpStr

Then a TxDP can be signed by

   # To sign a txDP, we zero out all the inputs that aren't ours, add hashcode, then sign
   def signTxDistProposal(txdpStr, inputToSign, myAddr):
       txdpLines = txdpStr.split('\n')
       readDp = False
       txHex  = 
       output = 
       # We copy the TxDP exactly as we read it, except for the TXSIGS line that
       # will require incremeting.  We stop just before the END-TXDP line so we
       # can append our signature to the end of the TXSIGS list
       for line in txdpLines:
          if 'END-TXDP' in line:
          if readDp:
             txHex += line.strip()
          # Read TXDP, starting next line
          if line.startswith('_TXDIST_'):
             readDp = True
          # Copy the line exactly as it's read, unless it's TXSIGS line
          if line.startswith('_TXSIGS_'):
             readDp = False
             nSigs = readVarInt(line.split('_')[-1].strip())
             output += '_TXSIGS_' + writeVarIntHex(nSigs+1) + '\n'
             output += line
       # All inputs have the appropriate TxOut script already included
       # For signing (SIGHASH_ALL) we need to blank out the ones not being signed
       txToSign = PyTx().unserialize(hex_to_binary(txHex))
       for i in range(len(txToSign.inputs)):
          if not i===inputToSign:
             txToSign[i] = 
       SIGHASH_ALL = 1
       hashcode = int_to_binary(SIGHASH_ALL, widthBytes=4, endOut=LITTLEENDIAN)
       binaryToSign = sha256(sha256(txToSign.serialize() + hashcode))
       binaryToSign = switchEndian(binaryToSign)  # hash needs to be BigEndian
       sig = myAddr.privKey.generateDERSignature(binaryToSign)
       txinScript = createStdTxInScript(sig, myAddr.pubKey)
       txinScriptHex = binary_to_hex(txinScript)
       inputNum = binary_to_hex(writeVarInt(inputToSign))
       scriptSz = binary_to_hex(writeVarInt(len(txinScript))
       output += '_SIG_%s_%s_%s\n' % (myAddr.base58str()[:8], inputNum, scriptSz)
       for byte in range(0,len(txinScriptHex), 80):
          output += txinScriptHex[byte:byte+80] + '\n'
       output += '-----END-TXDP-----'
       return output