Implicit conversion of the minimum difficulty threshold to the compact representation

[teor2345]

Issue

zcashd converts the PoWLimit to the compact representation before using it to perform the alternate difficulty check for testnet minimum difficulty blocks.

But this conversion is not explicitly specified in the Zcash Specification, ZIP-205, or ZIP-208.

This means that a strict reading of the specification might allow some block hashes that zcashd rejects.

Details

zcashd implements the proof of work limit as a compact value, truncating some of its least significant bits:

unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHeader *pblock, const Consensus::Params& params)
{
    unsigned int nProofOfWorkLimit = UintToArith256(params.powLimit).GetCompact();

https://github.com/zcash/zcash/blob/514d86817990a1bf9578ee5fa2d01c34c6ca6035/src/pow.cpp#L18

But the Zcash specification says:

PoWLimit :=
    2^243 − 1 for Mainnet
    2^251 − 1 for Testnet

https://zips.z.cash/protocol/protocol.pdf#constants

And ZIP-205 says:

If the block time of a block from this height onward is at least 15 minutes after that of the preceding block, then the block is a minimum-difficulty block, and its target threshold is set to the value of PoWLimit for testnet ...
https://zips.z.cash/zip-0205#change-to-difficulty-adjustment-on-testnet

And ZIP-208 says:

... if the block time of a block at height height ≥ 299188 is at least 6 · PoWTargetSpacing(height) seconds after that of the preceding block, then the block is a minimum-difficulty block, and its target threshold is set to the value of PoWLimit for testnet ...

https://zips.z.cash/zip-0208#minimum-difficulty-blocks-on-the-test-network

Suggested Resolution

@dconnolly or @daira, I'm not sure which of the following updates you'll want to make:

• update the Zcash Specification so the PoWLimit constants are always converted to the compact representation
• update the Zcash Specification so the difficulty filter specifies the precise comparison for testnet minimum difficulty blocks
• update ZIP 205 and ZIP 208 to convert the PoWLimit constant to the compact representation before comparison

[daira]

Thanks for reporting this. Please review zcash/zips#417 :-)

[daira]

Fixed in zcash/zips#417.

[mpguerra]

@teor2345 anything else to be done here?

[teor2345]

It's fixed in the ZIPs.