zcashd/doc/release-notes.md

(note: this is a temporary file, to be added-to by anybody, and moved to release-notes at release time)

Notable changes

Fake chain detection during initial block download

One of the mechanisms that zcashd uses to detect whether it is in "initial block download" (IBD) mode is to compare the active chain's cumulative work against a hard-coded "minimum chain work" value. This mechanism (inherited from Bitcoin Core) means that once a node exits IBD mode, it is either on the main chain, or a fake alternate chain with similar amounts of work. In the latter case, the node has most likely become the victim of a 50% + 1 adversary.

Starting from this release, zcashd additionally hard-codes the block hashes for the activation blocks of each past network upgrade (NU). During initial chain synchronization, and after the active chain has reached "minimum chain work", the node checks the blocks at each NU activation height against the hard-coded hashes. If any of them do not match, the node will immediately alert the user and shut down for safety.

Disabling old Sprout proofs

As part of our ongoing work to clean up the codebase and minimise the security surface of zcashd, we are removing libsnark from the codebase, and dropping support for creating and verifying old Sprout proofs. Funds stored in Sprout addresses are not affected, as they are spent using the hybrid Sprout circuit (built using bellman) that was deployed during the Sapling network upgrade.

This change has several implications:

• zcashd no longer verifies old Sprout proofs, and will instead assume they are valid. This has a minor implication for nodes: during initial block download, an adversary could feed the node fake blocks containing invalid old Sprout proofs, and the node would accept the fake chain as valid. However, as soon as the active chain contains at least as much work as the hard-coded "minimum chain work" value, the node will detect this situation and shut down.

• Shielded transactions can no longer be created before Sapling has activated. This does not affect Zcash itself, but will affect downstream codebases that have not yet activated Sapling (or that start a new chain after this point and do not activate Sapling from launch). Note that the old Sprout circuit is vulnerable to counterfeiting and should not be used in current deployments.

• Starting from this release, the circuit parameters from the original Sprout MPC are no longer required to start zcashd, and will not be downloaded by fetch-params.sh. They are not being automatically deleted at this time.

We would like to take a moment to thank the libsnark authors and contributors. It was vital to the success of Zcash, and the development of zero-knowledge proofs in general, to have this code available and usable.