Transaction Signing
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Transaction Signing

Transaction signatures are central to how Bitcoin Cash transactions are generally secured, preventing people other than the intended recipient of funds from spending them.
Bitcoin Cash signatures are created using asymmetric cryptography and involve generating a hash of the transaction and performing a signature operation using the sender’s private key.
Anyone with the corresponding public key can then verify the validity of the signature.
As described in Standard Scripts, the OP_CHECKSIG and related operations are used to validate signatures included in the unlocking script of a future transaction input.

However, there are a number of issues with signing a transaction that must be addressed:

  1. Transactions are identified by hashes of the full contents of the transaction
  2. The signatures are a part of the transaction data
  3. The signatures are created from a hash of the transaction’s data

Points (1) and (2) mean that if the signature is changed, the transaction’s hash will change.
Points (2) and (3) mean that the data that the signature hash preimage (i.e. the data that is hashed and signed) must not be the full transaction data.
In addition, because signatures relate only to a single input to a transaction (i.e. spending an unspent transaction output or UTXO) the may be multiple signatures in a transaction potentially created by different private keys, or even different people.

As a consequence of these factors, signatures have more parameters than may be immediately obvious, and the details of how signatures are generated can be, and have been, changed in a number of ways.
These parameters are encoded in the Hash Type.

In addition, as a part of BCH-UAHF (activated in block 478,559), the transaction signed format changed from the legacy Bitcoin Core (BTC) method to the Bitcoin Cash (BCH) Signatures. In both cases, there is a signature preimage format (input) and a signature format (output).

Hash Type

Parameters that change the way a signature hash is generated are encoded in the hash type field.
This field (which is always included in the preimage), is contained in 4 bytes.
The two least significant bits have the following collective meaning:

Value Meaning
0x01 SIGHASH_ALL. This is the default and indicates that all outputs are included in the signature preimage.
0x02 SIGHASH_NONE. Indicates that no outputs are included in the signature preimage.
0x03 SIGHASH_SINGLE. Indicates that only the output with the same index as the input the signature is being generated for will be included in the signature preimage.

In conjunction with the above values, the higher-order bits act as a bitmask with the following meaning:

Bit Meaning
0x00000040 SIGHASH_FORKID. If set, indicates that this signature is for a Bitcoin Cash transaction. Required following BCH-UAHF, to prevent transactions from being valid on both the BTC and BCH chains.
0x00000080 SIGHASH_ANYONECANPAY. Indicates that only information about the input the signature is for will be included, allowing other inputs to be added without impacting the signature for the current input.

Combining these, there are 6 valid signature hash types in Bitcoin Cash. Only the least significant byte (LSB) is shown in binary, since the rest of the bits are zero.

Signature hash type Value (hex) LSB (bin) Description
SIGHASH_ALL | SIGHASH_FORKID 0x00000041 0b01000001 Signature applies to all inputs and outputs.
SIGHASH_NONE | SIGHASH_FORKID 0x00000042 0b01000010 Signature applies to all inputs and none of the outputs.
SIGHASH_SINGLE | SIGHASH_FORKID 0x00000043 0b01000011 Signature applies to all inputs and the output with the same index.
SIGHASH_ALL | SIGHASH_ANYONECANPAY | SIGHASH_FORKID 0x000000C1 0b11000001 Signature applies to its own input and all outputs.
SIGHASH_NONE | SIGHASH_ANYONECANPAY | SIGHASH_FORKID 0x000000C2 0b11000010 Signature applies to its own input and none of the outputs.
SIGHASH_SINGLE | SIGHASH_ANYONECANPAY | SIGHASH_FORKID 0x000000C3 0b11000011 Signature applies to its own input and the output with the same index.

Bitcoin Cash Signatures

In Bitcoin Cash, transaction signatures use the transaction digest algorithm described in BIP143, in order to minimize redundant data hashing in verification and to cover the input value by the signature.

Preimage Format

Field Length Format Description
transaction version 4 bytes unsigned integer(LE) The value of transaction’s version field.
previous outputs hash 32 bytes hash(BE) A double SHA-256 hash of the set of previous outputs spent by the inputs of the transaction. See Previous Outputs for the hash preimage format.

If hash type is “ANYONE CAN PAY” then this is all 0x00 bytes.
sequence numbers hash 32 bytes hash(BE) A double SHA-256 hash of the set of sequence numbers of the inputs of the transaction. See Sequence Numbers for the hash preimage format.

If hash type is “ANYONE CAN PAY” then this is all 0x00 bytes.
previous output hash 32 bytes hash(LE) The transaction ID of the previous output being spent.
previous output index 4 bytes unsigned integer(LE) The index of the output to be spent.
modified locking script length variable variable length integer The number of bytes for modified_locking_script.
modified locking script modified_locking_script_length bytes bytes(BE) The subset of the locking script used for signing. See Modified Locking Script
previous output value 8 bytes unsigned integer(LE) The value of the transaction output being spent.
input sequence number 8 bytes unsigned integer(LE) The sequence number of the input this signature is for.
transaction outputs hash 32 bytes hash(BE) A double SHA-256 hash of the outputs of the transaction. See Transaction Outputs for the hash preimage format.
transaction lock time 4 bytes unsigned integer(LE) The lock time of the transaction.
hash type 4 bytes Hash Type(LE) Flags indicating the rules for how this signature was generated.

Previous Outputs Hash

The double-SHA256-hash of the following data is used.

For each transaction input in the transaction, append the following information:

Field Length Format Description
previous transaction hash 32 bytes bytes(LE) The hash of the transaction that generated the output to be spent.
output index 4 bytes unsigned integer(LE) The index of the output to be spent from the specified transaction.

Sequence Numbers Hash

The double-SHA256-hash of the following data is used.

For each transaction input in the transaction, append the following information:

Field Length Format Description
sequence number 4 bytes unsigned integer(LE) The sequence number field of the transaction input.

Modified Locking Script

The locking script included in the signature preimage is, first, dependent on the type of locking script included in the previous output.
For non-P2SH outputs, the locking script itself is used.
However, for P2SH outputs, the redeem script is used instead.

Second, the selected script (locking script or redeem script) is modified as follows.

  • Find the OP_CODESEPARATOR operation in the script preceding the expected signature-verification operation (e.g. OP_CHECKSIG).
  • Remove all operations before this point.
  • For Bitcoin Core signatures, remove any remaining OP_CODESEPARATOR operations. This requirement was dropped with BCH-UAHF (activated in block 478,559).
  • If creating this modified script for signature verification purposes, also remove any signatures that appeared in the unlocking script

The resulting script is what is included in the signature preimage.

Transaction Outputs Hash

If the hash type is SIGHASH_NONE then the hash should be all 0x00 bytes.

If hash type is SIGHASH_SINGLE then only the output with the same index as the input being signed is included.
If no such output exists (i.e. there are fewer outputs than the index of the input to be signed), this is again all 0x00 bytes.

Otherwise, all outputs of the transaction should be signed (i.e. SIGHASH_ALL).

For each transaction output to be signed (per the hash mode), append the following information:

Field Length Format Description
value 8 bytes unsigned integer(LE) The number of satoshis to be transferred.
locking script length variable variable length integer The size of the locking script in bytes.
locking script variable bytes(BE) The contents of the locking script.

Signature Format

Depending on the signature algorithm used, the representation of the signature itself can vary.
BCH has always supported DER-encoded ECDSA signatures.
Since HF-20190515 it also supports Schnorr signatures for the CHECKSIG/CHECKDATASIG[VERIFY] operations, and since HF-20191115, Schnorr signatures are also supported for the CHECKMULTISIG[VERIFY] operations.

The specific format of the signature depends on the operation to be executed and the algorithm being used to generate the signature.

ECDSA Signature Format

ECDSA signatures follow a strict DER encoding format, followed by the above hash type.
They are distinguished from Schnorr signatures by length, despite having a variable-length format (see Schorr signature format).

Field Length Format Description
magic number 1 byte byte The magic number value: 0x30.
signature length 1 byte unsigned integer The number of bytes to follow in the signature.
r variable DER-encoded integer The ECDSA “r” value.
s variable DER-encoded integer The ECDSA “s” value.
hash type 1 byte LSB of hash type Indicates the parameters used to generate the pre-image for this signature.
DER-Encoded Integer
Field Length Format Description
integer header-byte 1 byte byte The integer value indicator: 0x02.
value length 1 byte unsigned integer The number of bytes used to encode the integer value.
value value_length bytes unsigned integer(BE) The integer value being encoded.

This must be the smallest viable representation of the value being encoded. That is, the highest order byte may only be 0x00 if it is necessary to ensure the rest of the value is not interpreted as negative, in which case it is required.

For example, the value may start with 0x0080 or 0x7F but not 0x80 or 0x007F.

Schnorr Signature Format

Schnorr signatures have a less variable format, though the hash type field is removed for OP_CHECKDATASIG.
This allows them to be easily distinguished from ECDSA signatures on length alone.
In fact, ECDSA signatures that happen to be the length of a Schnorr signature in the same context (though they should be extremely rare, with probability 2-49) should be re-generated to avoid being errantly treated as an invalid Schnorr signature.

Field Length Format Description
r 32 bytes unsigned integer(BE) The Schorr “r” value.
s 32 bytes unsigned integer(BE) The Schorr “s” value.
hash type 0-1 bytes LSB of hash type Indicates the parameters used to generate the pre-image for this signature. Not included for OP_CHECKDATASIG signatures.

Bitcoin Core Signatures

Bitcoin Core signatures work very similarly to modern Bitcoin Cash signatures.
The primary difference is its different preimage format, as described in the following section.

Preimage Format

Bitcoin Core preimages are generated using the following steps:

  1. If SIGHASH_SINGLE is used without a corresponding output, due to a bug, the entire preimage used for that signature becomes 0x0100000000000000000000000000000000000000000000000000000000000000, and none of the following steps need performed.
  2. Take the Modified Locking Script
  3. Replace the current input’s scripts with this modified script
  4. Set all other input scripts to empty byte arrays
  5. Handle the Hash Type logic as in Bitcoin Cash, but SIGHASH_FORKID should NOT be set.

Signature Format

Bitcoin Core signatures follow the ECDSA signature format described above.