Credentials

Human ID Standard Credential Format

A leaf is an element of a Merkle tree. Merkle trees allow for efficient and anonymous proofs of set membership in zero-knowledge. Leaves are Poseidon hashes of 6 254-bit field elements:

Note that while credentials can only have 2 issuer-defined fields, nothing prevents these fields from representing more: they can be set to the hash of unlimited custom fields.

Nullifier Secret

The nullifier secret is a value that only the user knows. Because it is pseudorandom, the preimage of the leaf cannot be guessed without at least brute-forcing the secret, which is infeasible at 16 bytes. Thus, even if the other fields are predictable, the credential will still be anonymous without knowing the secret. In this way, it acts as a pepper, obscuring the preimage from the publicly known digest.

It also has a particular property allowing for sybil resistance: credentials can be spent for a specific use case by publishing a salted hash of the secret pepper. This is referred to as a “hashbrown” because it is a mixture of salt & pepper. Accurate computation of the hashbrown can be verified in a ZKP without revealing the pepper. Some use cases involve anonymous reputation, anonymous voting, bot prevention, and fair airdrops. For example:

Whenever a user votes in an election with actionID abc123, they must publish hash(abc123, secret). And they generate a proof that the result came from abc123 and secret. This hash then gets added on-chain, so the user can never vote again without people seeing they’re trying to “double-spend” their secret.

Leaf Formats of Human Tech Issuers

Human Tech provides two issuers, though others are encouraged to create more issuers.

Government ID Issuer

Phone Issuer

Biometrics Issuer