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Signing Bitcoin Cash / SV / ABC Transactionswith (SIGHASH_FORKID

I have a good deal of experience with the bitcoin protocol, but was recently tasked with developing a system for Bitcoin SV, which changes the way transactions are signed. There unfortunately isn’t much in the way of documentation on this except this page on Github, and needless to say, it took longer than I would have liked to get signing and sending to work. I hope this guide serves useful for anyone developing wallet software that sends via the Bitcoin Cash / SV / ABC networks.

This guide assumes you have working knowledge of the bitcoin protocol already. To keep things simple, all examples will be in hex format, so if you’re working with binary just use your favorite languages functions to translate hex into binary. A huge shout out to the developers of the Bitcash Python project, as their codebase is how I managed to learn this.

The Short and Skinny

To sign a BTC transaction, you do the following:

Create the tx with blank inputs, and append ‘01000000’ to it for signing purposes.
Iterate through the inputs, add the sig script (redeem for multisig, public key otherwise) to the input.
Create double SHA256 hash of the tx, sign it. Use the resulting signature to create the DER signature, and place within the appropriate place in the tx for that input.
Done, and broadcast tx.

This changes with BCH as follows:

The end of the DER signature within BTC transactions is ‘01’. This needs to be changed to ‘41’, so it has the 0x40 SIGHASH_FORKID.
The way the hash you need to sign is generated is completely different, so totally disregard how it’s generated within BTC transactions. You no longer use the “blank” tx, nor do you even use public keys / redeem scripts. See next section for details.
Everything else stays the same. Sign the hash, encode the DER signature, place it within the tx at the same location for each input, and broadcast the tx.

Generating the SHA256 Hash

Although I can’t claim to know why, the hash that needs to be signed for BCH transactions is a double SHA256 hash of a concatenated string with the following elements:

Element	Type	Notes
Version	int32_t	Will be “02000000”.
Prev Outputs Hash	Double SHA256 hash	Create a concatenated string that consists of the out points of all inputs within the tx being signed. Each out point is (reversed txid (char32) + vin (uint32_t). Double hash the resulting string via SHA256.
Sequences Hash	Double SHA256 hash	A concatenated string of the sequences (generally ‘ffffffff’) of all inputs within the tx being signed. Double hash the resulting string via SHA256.
TxID	char(32)	Reversed txid of the input being signed.
Vin	uint32_t	The vin of the input being signed.
Sig Script Length	var_int	The length of the signature script of the input being signed.
Sig Script	uchar()	The signature script of the input being signed.
Input Amount	int64_t	The amount of the input being signed.
Input Sequence	uint32_t	The sequence of the input being signed, generally ‘ffffffff’.
Outputs Hash	Double SHA256 hash	A concatenated string of all outputs within the tx being signed. Each output consists of (amount (int64_t) + scriptsig_length (var_int) + scriptsig (uchar)). Double hash the resulting string via SHA256.
Locktime	uint32_t	The locktime of the tx being signed.
Hash Type	uint32_t	The hash type. For BCH, this will always be “41000000” (0x01 + 0x40).

Create a concatenated string with the above elements, then double hash the string via SHA256. This is the hash that must be signed with the private key, which is then placed within the input of the tx at the same location as always.

DER Signature

This guide assumes you already know how to encode the DER signature, and will not go into detail on it. As you know, a DER signature looks like:

Element	Note
	0x30 to signify a DER signature.
length	The length of the entire DER signature.
	0x02 to signify beginning of R.
R Length	The length of the R value.
R	The R value of the signature.
	0x02 to signify the beginning of S.
S Length	The length of the S value.
S	The S value of the signature.
Hash Type	The hash type, for BTC transactions this will be 0x01. For BCH transactions though, this changes to 0x41.

The DER signature is still generated the exact same way as with BTC transactions, the only difference being the end of the DER signature changes from 0x01 to 0x41. That’s it.

Example

For an example, we will be signing a transaction with the following inputs and outputs:

{
    "inputs": [
        {
            "txid": "66829a9aba4d6548893af42ad81d445fab035dc5b123fc199da7d8d487da8b1d",
            "vin": 0,
            "amount": 0.001,
            "address": "mphVs2gGCHhZ1eW24CtLkYVLmj3yZtBA6p",
            "sequence": "ffffffff"
        },
        {
            "txid": "1be1a6f5873ca1e656b999b07ee0e74c7e1efbc4d6a973174e0953692445a16e",
            "vin": 1,
            "amount": 0.001,
            "address": "mvcKh1GYs234FqvQK23MvQ6FRd6gwBQHao",
            "sequence": "ffffffff"
        }
    ],
    "outputs": [
        {
            "address": "n18WhospQpmfNnKZYQe8PM7sRhM3Xm2mk9",
            "amount": 0.0015,
            "sigscript": "76a914d72400b0f606548e1a618b150388cfac371f243c88ac"
        },
        {
            "address": "mgDt5TD4yaC3nAexFC45D5gp4mxKKrPNG8",
            "amount": 0.000203,
            "sigscript": "76a91407bd88b6baf048869b5a8f41e479e470271f776988ac"
        }
    ]
}

We will generate the necessary SHA256 hash that needs to be signed for the second input. The values of the elements from the above table in hex format will become:

Element	Hex Value
Version	02000000
Prev Outputs Hash	5717eac77c25c5bdb9372361cecbc1b53a5d57c046722073cddc51ba6a9caa36
Sequences Hash	752adad0a7b9ceca853768aebb6965eca126a62965f698a0c1bc43d83db632ad
TxID	6ea145246953094e1773a9d6c4fb1e7e4ce7e07eb099b956e6a13c87f5a6e11b
Vin	01000000
Sig Script Length	19
Sig Script	76a914a58dd3a9b590600967b6b9523e3adfca8744523a88ac
Input Amount	a086010000000000
Input Sequence	ffffffff
Outputs Hash	96c5528b809d169f4b136505909f4571b709415d5361d14912dd93ada29e7625
Locktime	00000000
Hash Type	41000000

Some notes regarding the above values.

The ‘Prev Outputs Hash’ is a double SHA256 hash of the string (txid1 + vin1 + txid2 + vin2): 1d8bda87d4d8a79d19fc23b1c55d03ab5f441dd82af43a8948654dba9a9a8266000000006ea145246953094e1773a9d6c4fb1e7e4ce7e07eb099b956e6a13c87f5a6e11b01000000
The ‘Sequences Hash’ is a double SHA256 hash of the string: ffffffffffffffff.
The ‘Outputs Hash’ is a double SHA256 hash of all outputs within the tx, which is: f0490200000000001976a914d72400b0f606548e1a618b150388cfac371f243c88ac4c4f0000000000001976a91407bd88b6baf048869b5a8f41e479e470271f776988ac

You then generate a double SHA256 hash of the above large string, and as an end result will have:

92550cb93bc172642437be5c4ef234edddcee133b74dbe83cd00a462f6e71b49

This is the hash that needs to be signed with the private key, and included within the tx for the input.

About the Author

Matt Dizak is a software entrepreneur with 19 years of high paced experience within the online software industry, and is the creator of Apex, an open source PHP based software platform at https://apexpl.io/. Please feel free to follow me on Twitter, Youtube, or feel free to e-mail me directly anytime at matt.dizak@gmail.com.