SNARK-friendly STM: Revise Generic Digest to support multiple hash functions

[curiecrypt]

Content

This PR includes the changes required to support multiple hash functions for different proof systems.

Pre-submit checklist

• Branch
  • Crates versions are updated (if relevant)
  • CHANGELOG file is updated (if relevant)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
• PR
  • All check jobs of the CI have succeeded
  • Self-reviewed the diff
  • Useful pull request description
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• Documentation
  • No new TODOs introduced

Comments

Issue(s)

Relates to #2794

[github-actions]

Test Results

    4 files  ±0    169 suites  ±0   22m 15s ⏱️ - 2m 4s
2 275 tests ±0  2 275 ✅ ±0  0 💤 ±0  0 ❌ ±0 
7 136 runs  ±0  7 136 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit fad50b2. ± Comparison against base commit ecad03d.

♻️ This comment has been updated with latest results.

[Copilot]

Pull request overview

This PR introduces a generic MembershipDigest trait to abstract hash function usage in the SNARK-friendly STM protocol, replacing direct dependencies on Blake2 hash types throughout the codebase. This enables support for multiple hash function implementations while maintaining backward compatibility.

Key Changes

• Introduces MembershipDigest trait with ConcatenationHash and SnarkHash associated types
• Provides CustomMembershipDigest as a default Blake2-based implementation
• Updates all STM protocol structs and functions to use the new trait abstraction instead of concrete Blake2 types

Reviewed changes

Copilot reviewed 24 out of 25 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
mithril-stm/src/membership_commitment/mod.rs	Defines the core MembershipDigest trait and CustomMembershipDigest implementation
mithril-stm/src/lib.rs	Exports new trait and implementation to public API
mithril-stm/src/protocol/key_registration.rs	Updates ClosedKeyRegistration to use MembershipDigest with associated type for merkle tree
mithril-stm/src/protocol/single_signature/*.rs	Replaces Digest + FixedOutput bounds with MembershipDigest in signature operations
mithril-stm/src/protocol/participant/*.rs	Updates Signer and Initializer to use new trait abstraction
mithril-stm/src/protocol/aggregate_signature/*.rs	Updates aggregation components (Clerk, AggregateSignature, etc.) to use trait
mithril-stm/src/proof_system/concatenation.rs	Updates ConcatenationProof to use MembershipDigest::ConcatenationHash
mithril-stm/tests/*.rs	Updates tests to use CustomMembershipDigest instead of direct Blake2 types
mithril-stm/examples/key_registration.rs	Defines local CustomMembershipDigest for example usage
mithril-stm/benches/*.rs	Updates benchmarks to use new abstraction
mithril-stm/README.md	Updates example code to use CustomMembershipDigest
mithril-common/src/crypto_helper/*.rs	Updates to import and use CustomMembershipDigest from mithril-stm
demo/protocol-demo/src/types.rs	Updates to import and use CustomMembershipDigest
demo/protocol-demo/Cargo.toml	Removes direct blake2 dependency (now transitively available)

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[jpraynaud]

Can you add a comment to mention that this Blake2b<U34> will be replaced by Poseidon when implementing the SNARK primitives?

[curiecrypt]

There is already a comment about it in the doc of impl (line 162).
Should we add one more on top of type SnarkHash = Blake2b<U64>;?

[jpraynaud]

LGTM 👍

[Copilot]

Pull request overview

Copilot reviewed 25 out of 26 changed files in this pull request and generated 4 comments.

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