Recap Of Week 6: Identity

• Identity means showing who you are without a central registry.
• Keys and signatures prove control of an account.
• Credentials are claims issued by one party and checked by another.
• Showing a full credential reveals more than the verifier needs.

What You Will Be Able To Do

• Explain what a zero-knowledge proof proves.
• Retell the cave analogy with the right roles.
• Build a hash commitment in Python.
• Name SNARKs and STARKs and one trade-off.
• Describe MACI-style private voting.

Three Kinds Of Privacy

• Confidentiality hides the content of data.
• Anonymity hides who performed an action.
• Unlinkability hides that two actions share an author.
• Public ledgers expose all three by default.

The Proof Problem

• A verifier needs to trust a claim.
• The prover owns data that supports the claim.
• Sharing the data creates a leak.
• A proof can carry the claim without the data.

Roles In A Proof

• Prover holds the secret witness.
• Verifier holds the public statement.
• Prover sends a proof message.
• Verifier answers accept or reject.

statement: hash(w) == H     (public)
witness:   w                (secret)
proof:     pi = Prove(w, H)
check:     Verify(H, pi) -> accept / reject

Three Properties

• Completeness: true statements are accepted.
• Soundness: false statements are rejected, almost always.
• Zero-knowledge: nothing beyond truth is revealed.
• All three must hold together.

The Cave Analogy

• A ring cave with a magic door at the back.
• Peggy enters and picks a path out of Victor's sight.
• Victor names the side she must exit from.
• Knowing the word lets her always comply.

      entrance
     /        \
  left        right
     \  door  /
      (secret word)

Why Repetition Convinces

• A cheater guesses the side half the time.
• Twenty rounds leave about one in a million.
• Victor never hears the secret word.
• He cannot replay the transcript to convince others.

Commitments

• A commitment is a sealed envelope.
• Hiding: the envelope reveals nothing.
• Binding: the value cannot change after sealing.
• A salt keeps small value sets from being guessed.

import hashlib, secrets
salt = secrets.token_hex(16)
c = hashlib.sha256((salt + 'yes').encode()).hexdigest()
print(c)

From Conversation To One Proof

• The cave needs many rounds of talk.
• Modern systems publish a single proof.
• Anyone can verify it at any time.
• Challenges come from a hash of the commitment.

SNARKs

• Succinct Non-interactive ARgument of Knowledge.
• Proofs are tiny and fast to check.
• Many need a trusted setup ceremony.
• One honest ceremony participant is enough.

STARKs

• Scalable Transparent ARgument of Knowledge.
• Transparent: no trusted setup needed.
• Built on hash functions.
• Proofs are larger than SNARK proofs.

From Statement To Circuit

• Write the claim as arithmetic constraints.
• Tools such as Circom and Noir help.
• A proving library turns a witness into a proof.
• A small verifier contract checks it on chain.

MACI: Private Voting

• Voters publish encrypted messages on chain.
• A coordinator decrypts and counts off chain.
• A zero-knowledge proof shows the count is correct.
• Voters may re-vote to override earlier messages.

voter -> encrypted vote -> chain
chain -> messages -> coordinator
coordinator -> tally + proof -> contract
contract verifies proof -> result

Recap

• A ZK proof shows truth and hides the secret.
• Commitments seal values and bind the sender.
• SNARKs are small with setup, STARKs are transparent.
• MACI makes bribery checks impossible.

Next Week: DAOs And On-Chain Governance

• What a DAO is and how it votes.
• Delegation and treasuries.
• How governance fails in practice.

Questions And Exercise

• Exercise: build and test a hash commitment.
• Bring one idea for a narrow fact worth proving.
• Read the Quisquater cave paper or Green's primer.