# HF-20180820

    layout: specification
    title: OP_CHECKDATASIG and OP_CHECKDATASIGVERIFY Specification
    category: spec
    date: 2018-08-20
    activation: 1542300000
    version: 0.6

## Summary

OP_CHECKDATASIG and OP_CHECKDATASIGVERIFY check whether a signature is valid with respect to a message and a public key.

OP_CHECKDATASIG permits data to be imported into a script, and have its validity checked against some signing authority such as an "Oracle".

OP_CHECKDATASIG and OP_CHECKDATASIGVERIFY are designed to be implemented similarly to OP_CHECKSIG [1][1].
Conceptually, one could imagine OP_CHECKSIG functionality being replaced by OP_CHECKDATASIG, along with a separate Op Code to create a hash from the transaction based on the SigHash algorithm.

## OP_CHECKDATASIG Specification

### Semantics

OP_CHECKDATASIG fails immediately if the stack is not well formed.
To be well formed, the stack must contain at least three elements [`<sig>`, `<msg>`, `<pubKey>`] in this order where `<pubKey>` is the top element and

* `<pubKey>` must be a validly encoded public key
* `<msg>` can be any string
* `<sig>` must follow the strict DER encoding as described in [2][2] and the S-value of `<sig>` must be at most the curve order divided by 2 as described in [3][3]

If the stack is well formed, then OP_CHECKDATASIG pops the top three elements [`<sig>`, `<msg>`, `<pubKey>`] from the stack and pushes true onto the stack if `<sig>` is valid with respect to the raw single-SHA256 hash of `<msg>` and `<pubKey>` using the secp256k1 elliptic curve.
Otherwise, it pops three elements and pushes false onto the stack in the case that `<sig>` is the empty string and fails in all other cases.

Nullfail is enforced the same as for OP_CHECKSIG [3][3].
If the signature does not match the supplied public key and message hash, and the signature is not an empty byte array, the entire script fails.

### Opcode Number

OP_CHECKDATASIG uses the previously unused opcode number 186 (0xba in hex encoding)

### SigOps

Signature operations accounting for OP_CHECKDATASIG shall be calculated the same as OP_CHECKSIG.
This means that each OP_CHECKDATASIG shall be counted as one (1) SigOp.

### Activation

Use of OP_CHECKDATASIG, unless occuring in an unexecuted OP_IF branch, will make the transaction invalid if it is included in a block where the median timestamp of the prior 11 blocks is less than 1542300000.

### Unit Tests

 - `<sig> <msg> <pubKey> OP_CHECKDATASIG` fails if 15 November 2018 protocol upgrade is not yet activated.
 - `<sig> <msg> OP_CHECKDATASIG` fails if there are fewer than 3 items on stack.
 - `<sig> <msg> <pubKey> OP_CHECKDATASIG` fails if `<pubKey>` is not a validly encoded public key.
 - `<sig> <msg> <pubKey> OP_CHECKDATASIG` fails if `<sig>` is not a validly encoded signature with strict DER encoding.
 - `<sig> <msg> <pubKey> OP_CHECKDATASIG` fails if signature `<sig>` is not empty and does not pass the Low S check.
 - `<sig> <msg> <pubKey> OP_CHECKDATASIG` fails if signature `<sig>` is not empty and does not pass signature validation of `<msg>` and `<pubKey>`.
 - `<sig> <msg> <pubKey> OP_CHECKDATASIG` pops three elements and pushes false onto the stack if `<sig>` is an empty byte array.
 - `<sig> <msg> <pubKey> OP_CHECKDATASIG` pops three elements and pushes true onto the stack if `<sig>` is a valid signature of `<msg>` with respect to `<pubKey>`.

## OP_CHECKDATASIGVERIFY Specification

### Semantics

OP_CHECKDATASIGVERIFY is equivalent to OP_CHECKDATASIG followed by OP_VERIFY.
It leaves nothing on the stack, and will cause the script to fail immediately if the signature check does not pass.

### Opcode Number

OP_CHECKDATASIGVERIFY uses the previously unused opcode number 187 (0xbb in hex encoding)

### SigOps

Signature operations accounting for OP_CHECKDATASIGVERIFY shall be calculated the same as OP_CHECKSIGVERIFY.
This means that each OP_CHECKDATASIGVERIFY shall be counted as one (1) SigOp.

### Activation

Use of OP_CHECKDATASIGVERIFY, unless occuring in an unexecuted OP_IF branch, will make the transaction invalid if it is included in a block where the median timestamp of the prior 11 blocks is less than 1542300000.

### Unit Tests

 - `<sig> <msg> <pubKey> OP_CHECKDATASIGVERIFY` fails if 15 November 2018 protocol upgrade is not yet activated.
 - `<sig> <msg> OP_CHECKDATASIGVERIFY` fails if there are fewer than 3 item on stack.
 - `<sig> <msg> <pubKey> OP_CHECKDATASIGVERIFY`fails if `<pubKey>` is not a validly encoded public key.
 - `<sig> <msg> <pubKey> OP_CHECKDATASIGVERIFY` fails if `<sig>` is not a validly encoded signature with strict DER encoding.
 - `<sig> <msg> <pubKey> OP_CHECKDATASIGVERIFY` fails if signature `<sig>` is not empty and does not pass the Low S check.
 - `<sig> <msg> <pubKey> OP_CHECKDATASIGVERIFY` fails if `<sig>` is not a valid signature of `<msg>` with respect to `<pubKey>`.
 - `<sig> <msg> <pubKey> OP_CHECKDATASIGVERIFY` pops the top three stack elements if `<sig>` is a valid signature of `<msg>` with respect to `<pubKey>`.

## Sample Implementation [4][4], [5][5]

```
case OP_CHECKDATASIG:
case OP_CHECKDATASIGVERIFY: {
    // Make sure this remains an error before activation.
    if ((flags & SCRIPT_ENABLE_CHECKDATASIG) == 0) {
        return set_error(serror, SCRIPT_ERR_BAD_OPCODE);
    }

    // (sig message pubkey -- bool)
    if (stack.size() < 3) {
        return set_error(
            serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
    }

    valtype &vchSig = stacktop(-3);
    valtype &vchMessage = stacktop(-2);
    valtype &vchPubKey = stacktop(-1);

    if (!CheckDataSignatureEncoding(vchSig, flags,
                                    serror) ||
        !CheckPubKeyEncoding(vchPubKey, flags, serror)) {
        // serror is set
        return false;
    }

    bool fSuccess = false;
    if (vchSig.size()) {
        valtype vchHash(32);
        CSHA256()
            .Write(vchMessage.data(), vchMessage.size())
            .Finalize(vchHash.data());
        uint256 message(vchHash);
        CPubKey pubkey(vchPubKey);
        fSuccess = pubkey.Verify(message, vchSig);
    }

    if (!fSuccess && (flags & SCRIPT_VERIFY_NULLFAIL) &&
        vchSig.size()) {
        return set_error(serror, SCRIPT_ERR_SIG_NULLFAIL);
    }

    popstack(stack);
    popstack(stack);
    popstack(stack);
    stack.push_back(fSuccess ? vchTrue : vchFalse);
    if (opcode == OP_CHECKDATASIGVERIFY) {
        if (fSuccess) {
            popstack(stack);
        } else {
            return set_error(serror,
                                SCRIPT_ERR_CHECKDATASIGVERIFY);
        }
    }
} break;
```

## Sample Usage

The following example shows a spend and redeem script for a basic use of CHECKDATASIG.
This example validates the signature of some data, provides a placeholder where you would then process that data, and finally allows one of 2 signatures to spend based on the outcome of the data processing.

### spend script

```
push txsignature
push txpubkey
push msg
push sig
```

### redeem script

```
                                (txsig, txpubkey msg, sig)
OP_OVER                         (txsig, txpubkey, msg, sig, msg)
push data pubkey                (txsig, txpubkey, msg, sig, msg, pubkey)
OP_CHECKDATASIGVERIFY           (txsig, txpubkey, msg)
```

Now that msg is on the stack top, the script can write predicates on it, resulting in the message being consumed and a true/false condition left on the stack: `(txpubkey, txsig, boolean)`

```
OP_IF                           (txsig, txpubkey)
  OP_DUP                        (txsig, txpubkey, txpubkey)
  OP_HASH160                    (txsig, txpubkey, address)
  push <p2pkh spend address>    (txsig, txpubkey, address, p2pkh spend address)
  OP_EQUALVERIFY                (txsig, txpubkey)
  OP_CHECKSIG
OP_ELSE
  (same as if clause but a different <p2pkh spend address>)
OP_ENDIF
```

## History

This specification is based on Andrew Stone’s OP_DATASIGVERIFY proposal [6][6], [7][7].
It is modified from Stone's original proposal based on a synthesis of all the peer-review and feedback received [8][8].

## References

[1] [OP_CHECKSIG](https://en.bitcoin.it/wiki/OP_CHECKSIG)

[2] [Strict DER Encoding](https://github.com/bitcoin/bips/blob/master/bip-0066.mediawiki)

[3] [Low-S and Nullfail Specification](https://github.com/bitcoin/bips/blob/master/bip-0146.mediawiki)

[4] [Bitcoin ABC implementation](https://reviews.bitcoinabc.org/D1621)

[5] [Bitcoin ABC implementation update](https://reviews.bitcoinabc.org/D1646)

[6] [Andrew Stone’s OP_DATASIGVERIFY](https://github.com/BitcoinUnlimited/BitcoinUnlimited/blob/bucash1.3.0.0/doc/opdatasigverify.md)

[7] [Andrew Stone's article on Scripting](https://medium.com/@g.andrew.stone/bitcoin-scripting-applications-decision-based-spending-8e7b93d7bdb9)

[8] [Peer Review of Andrew Stone's Proposal](https://github.com/bitcoincashorg/bitcoincash.org/pull/10)