Subject: Re: [bitcoin-dev] CHECKSIGFROMSTACK/{Verify} BIP for Bitcoin

From: "Russell O'Connor" <[email-masked]>
Date: Sat, 3 Jul 2021 16:12:51 -0400

There is one line written at
https://github.com/ElementsProject/elements/pull/949/files#r660130155. I
suppose we need to decide on which variants of *VERIFY and *ADD we want to
include (presumably all of them) and choose which opcodes they will be
assigned to.  And I guess for CHECKSIGFROMSTACKADD will want to place the n
value between the signature and the message on the stack.  ... So I suppose
we will need more than one sentence.

The semantics would be basically to call secp256k1_schnorrsig_verify <
https://github.com/bitcoin-core/secp256k1/blob/0440945fb5ce69d335fed32827b5166e84b02e05/include/secp256k1_schnorrsig.h#L158>,
treating pubkeys and signatures the same way the other CHECKSIG operations
do, and in passing the (variable length) message from the stack.
CHECKSIGFROMSTACK would also be subject to the same sigops budget that
CHECKSIG has in tapscript.

On Sat, Jul 3, 2021 at 2:30 PM Jeremy <[email-masked]> wrote:

> Awesome to hear that!
>
> Actually I don't think I did know (or I forgot/didn't catch it) that there
> was an updated spec for elements, I searched around for what I could find
> and came up empty handed. Do you have any links for that? That sounds
> perfect to me.
>
>
> On Sat, Jul 3, 2021, 10:50 AM Russell O'Connor <[email-masked]>
> wrote:
>
>> Hi Jermy,
>>
>> As you are aware, we, and by we I mean mostly Sanket, are developing an
>> updated OP_CHECKSIGFROMSTACK implementation for tapscript on elements.  The
>> plan here would be to effectively support the an interface to the
>> variable-length extension of BIP-0340 schnorr signatures.
>>
>> BIP-0340 would dispense with DER encoding (good riddance).
>> BIP-0340 signatures are batch verifiable along with other BIP-0340
>> transaction signatures and taproot tweak verification.
>> Support for variable length messages in BIP-0340 has been discussed in <
>> https://github.com/sipa/bips/issues/207> and an implementation has
>> recently been merged in <
>> https://github.com/bitcoin-core/secp256k1/pull/844>.  The BIP has not
>> yet been updated but the difference is that the message m does not have to
>> be 32-bytes (it is recommended that the message be a 32-bit tagged hash or
>> a message with a 64-bit application specific prefix). The CHECKSIGFROMSTACK
>> operation (in tapscript) would use a stack item for this m value to
>> BIP-0340 signature verification and would not necessarily have to be 32
>> bytes.
>>
>> I think this design we are aiming for would be perfectly suited for
>> Bitcoin as well.
>>
>> On Sat, Jul 3, 2021 at 12:32 PM Jeremy via bitcoin-dev <
>> [email-masked]> wrote:
>>
>>> Reproduced below is the BIP text from Bitcoin Cash's (MIT-Licensed)
>>> specification for "CheckDataSig", more or less the same thing as
>>> CHECKSIGFROMSTACK
>>> https://github.com/bitcoincashorg/bitcoincash.org/blob/master/spec/op_checkdatasig.md.
>>> In contrast to Element's implementation, it does not have Element's bugs
>>> around verify semantics and uses the nullfail rule, and there is a
>>> specification document so it seemed like the easiest starting point for
>>> discussion v.s. drafting something from scratch.
>>>
>>> Does anyone have any issue with adapting this exact text and
>>> implementation to a BIP for Bitcoin using 2 OP_SUCCESSX opcodes?
>>>
>>> Note that with *just* CheckSigFromStack, while you can do some very
>>> valuable use cases, but without OP_CAT it does not enable sophisticated
>>> covenants (and as per
>>> https://www.wpsoftware.net/andrew/blog/cat-and-schnorr-tricks-i.html
>>> just CAT alone enables such uses).
>>>
>>> Design questions worth considering as modifications:
>>>
>>> 1. Should CSFS require some sort of tagged hash? Very likely answer is
>>> no – tags interfere with certain use cases
>>> 2. Should CSFS split the signature’s R & S value stack items for some
>>> applications that otherwise may require OP_CAT? E.g. using a pinned R value
>>> allows you to extract a private key if ever double signed, using 2 R values
>>> allows pay-to-reveal-key contracts. Most likely answer is no, if that is
>>> desired then OP_CAT can be introduced
>>> 3. Should CSFS support a cheap way to reference the taproot internal or
>>> external key? Perhaps, can be handled with undefined upgradeable keytypes.
>>> One might want to use the internal key, if the signed data should be valid
>>> independent of the tapscript tree. One might want to use the external key,
>>> if the data should only be valid for a single tapscript key + tree.
>>> 4. Should invalid public keys types be a NOP to support future extended
>>> pubkey types?
>>>
>>>
>>>
>>> Best,
>>>
>>>
>>> Jeremy
>>>
>>>
>>> ---
>>> layout: specification
>>> title: OP_CHECKDATASIG and OP_CHECKDATASIGVERIFY Specification
>>> category: spec
>>> date: 2018-08-20
>>> activation: 1542300000
>>> version: 0.6
>>> ---
>>>
>>> OP_CHECKDATASIG
>>> ===============
>>>
>>> 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]. 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] and the S-value of `<sig>` must be at most the curve order divided by 2 as described in [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]. 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, 5]
>>> ----------------------------
>>>
>>> ```c++
>>>                     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, 7]. It is modified from Stone's original proposal based on a synthesis of all the peer-review and feedback received [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)
>>>
>>>
>>> --
>>> @JeremyRubin <https://twitter.com/JeremyRubin>
>>> <https://twitter.com/JeremyRubin>
>>> _______________________________________________
>>> bitcoin-dev mailing list
>>> [email-masked]
>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>
>>