The Test Credential Manager

Block Qualified implements native support for the Test Credential, which operates as a Credential Manager under the Credentials Registry. Each Test Credential has a multiple choice question component and an open answer component, with a minimum grade needed to obtain it. Users can gain these credentials by providing proofs of knowledge of their solution. The actual solutions are encoded as part of the proof and thus are kept private, preventing other users from cheating by looking at public on-chain data.

Creating a Test Credential

To create a Test Credential, the createCredential function of the Credential Registry is called, providing:

• credentialId: unique identifier of the test credential to be created.

• treeDepth: tree depth of the different groups that will be created -- must be set to 16.

• credentialType: the credential manager type that enforces the test credential logic -- by default it is 0.

• merkleTreeDuration: maximum time that an expired Merkle root can still be used to generate proofs of membership for this credential.

• credentialData: encoded data bytes that will define the test credential.

• credentialURI: external resource containing more information about the credential.

The encoded data bytes credentialData for the Test Credential can be generated using the library function encodeTestCredential, and providing:

• testHeight: the height of the trees that define the test, as defined in Block Qualified tests.

• minimumGrade: out of 100, minimum total grade the user must get to obtain the credential.

• multipleChoiceWeight: out of 100, contribution of the multiple choice component towards the total grade: 100 for pure multiple choice tests, 0 for pure open answer tests.

• nQuestions: number of open answer questions the test has -- must be set to 1 for pure multiple choice tests.

• timeLimit: unix time limit after which it is not possible to obtain this credential -- must be set to 0 for unlimited.

• admin: address that controls this credential.

• requiredCredential: the credentialId of the credential that needs to be obtained before this one -- set 0 for unrestricted. If set, a Semaphore credential ownership proof will have to be provided alongside the test proof.

• requiredCredentialGradeThreshold: minimum grade that must be obtained for the required credential -- set 0 for unrestricted. If set, a grade claim proof will have to be provided alongside the test proof.

• multipleChoiceRoot: root of the multiple choice Merkle tree, where each leaf is the correct choice out of the given ones, as specified in the test object.

• openAnswersHashesRoot: root of the open answers Merkle tree, where each leaf is the hash of the corresponding correct answer, as specified in the test object.

The Credential Registry will finish initializing this credential by defining the corresponding groups: grade, credentials, and no-credentials.

Updating a Test Credential

To update a Test Credential, the updateCredential function of the Credential Registry is called, providing:

• credentialId: unique identifier of the test credential to be updated.

• credentialUpdate: encoded data bytes that defines the credential update.

This function cannot be called directly, as it only supports calls made from the Credential Registry.

The encoded data bytes credentialUpdate will contain a valid proof for the Test circuit that verifies their proof of knowledge of their solution. Users can generate these proofs via the library, as described in Block Qualified test proof.

To obtain restricted test credentials, users will also have to include a proof that they meets the imposed requirements.

Unrestricted Tests

The encoded data bytes credentialUpdate for unrestricted tests can be generated using the library function encodeTestFullProof, and providing:

• testFullProof: the proof generated for the Block Qualified test.

• testPassed: a boolean parameter indicating if their solution achieves a grade over the minimumGrade or not

Verifying that users cannot cheat by claiming that they passed a test when they did not is enforced is by setting the testParameters public signal of the proof:

• If the user sets testPassed to true, the public input testParameters set when verifying the proof will make it invalid if the grade obtained is below minimumGrade.

• If the user sets testPassed to false, the public input testParamters will set the minimumGrade to 0, so the grade check inside the proof will clear.

This means that a user can potentially provide a passing solution and still decide to add themselves to the no-credentials group.

Depending on the value for testPassed, the user will get their Semaphore identity commitment added to the credentials group or to the no-credentials group, respectively. Their grade commitment will be added to the grade group either way. These groups are managed by the Credentials Registry.

Credential Restricted Tests

Users must provide aan additional Semaphore proof that verifies that they own the requiredCredential. Users can generate these proofs via the library, as described in grade restricted tests.

The external nullifier being used to prevent double-signaling is the string bq-credential-restricted-test.

The encoded data bytes credentialUpdate for credentail restricted tests can be generated using the library function encodeCredentialRestrictedTestFullProof, and providing:

• credentialRestrictedTestFullProof: the credential restricted test proof generated with the library.

• testPassed: a boolean parameter indicating if their solution achieves a grade over the minimumGrade or not.

Grade Restricted Tests

Users must provide aan additional grade claim proof that verifies that they obtained over a grade over a certain threshold for the requiredCredential. Users can generate these proofs via the library, as described in grade restricted tests.

The external nullifier being used to prevent double-signaling is the string bq-grade-restricted-test.

The encoded data bytes credentialUpdate for grade restricted tests can be generated using the library function encodeGradeRestrictedTestFullProof, and providing:

• gradeRestrictedTestFullProof: the grade restricted test proof generated with the library.

• testPassed: a boolean parameter indicating if their solution achieves a grade over the minimumGrade or not.