Circuits
Circuits implement the base logic of the protocol, and are used to obtain credentials and prove ownership of them.
The Test Circuit
The Test circuit implements the base logic of the natively supported Test Credential. It allows users to prove that they have the necessary knowledge to obtain a given test credential. It consists of three parts:
Proof of Knowledge
To solve a test credential, users provide as private inputs to the proof their multipleChoiceAnswers and openAnswers to the multiple choice and open answer components of the test, respectively. The circuit then computes the resulting grade ⋅ nQuestions as specified by the Block Qualified Test. This is the value that is later commited to the grade tree, alongside the user's identity secret. The circuit outputs the value for the testRoot and the testParameters, as specified by the Block Qualified Test. These values are then verified inside the Test Credential smart contract.
Identity Tree Inclusion
As part of the proof, the user updates an empty leaf (identityTreeEmptyLeaf) inside a Semaphore Group by including their identity commitment. Depending on whether the user passed the test or not, this Semaphore group will be the credentials group or the no-credentials group, respectively. This is enforced inside of the Test Credential smart contract.
The circuit outputs the old Merkle root of the Group (oldIdentityTreeRoot), the new Merkle root of the group (newIdentityTreeRoot), the user's identity commitment (identityCommitment), and its index within the tree (identityCommitmentIndex).
Grade Tree Inclusion
Similarly to the identity tree inclusion, as part of the proof the user updates an empty leaf gradeTreeEmptyLeaf inside a Semaphore-like Group by including their grade commitment. Unlike a Semaphore identity commitment, which is computed as the Poseidon hash of the identity secret, a Block Qualified grade commitment is computed as:
The circuit outputs the old Merkle root of the grade Semaphore-like Group (oldGradeTreeRoot), the new Merkle root of the Semaphore-like Group (newGradeTreeRoot), the user's grade commitment (gradeCommitment), and its inde within the tree (gradeCommitmentIndex).
Proof of Ownership With Semaphore
Once a user has attempted to solve a credential, their identity commitment will be added to either the credentials group or the no-credentials group. This will depend on the logic being implemented by the Credential Manager. For the natively supported Test Credential, when users pass the mixed test, their identity commitment is added to the credentials tree. Otherwise, they are added to the no-credentials tree.
Using the Semaphore circuit, users can signal anonymously with a zero-knowledge proof that they are a part of the credentials/no-credentials group for any Block Qualified defined credential.
The Grade Claim Circuit
Similarly to the Semaphore circuit, the Grade Claim circuit allows users to signal anonymously with a zero-knowledge proof that they obtained a grade that is greater than or equal to a certain threshold. This circuit works similarly to the Semaphore circuit, with two main differences:
Instead of proving that their identity commitment is part of the credentials/no-credentials tree, the user proves that their grade commitment is part of the grade tree. This grade commitment is linked to both their identity and the grade they obtained, as it is computed via:
The user then proves that the grade linked to their grade commitment is above a certain threshold of their choosing.
The public signals for this circuit include those inherited from the Semaphore proof of ownership: gradeTreeRoot (the Merkle root of the grade tree), nullifierHash, signalHash, and externalNullifier. The circuit also outputs the grade threshold that is claimed, gradeThreshold.
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