Verifiable Credentials

The foundation of Ontology's trust mechanism is based on a verifiable credential system.
Entities issue credentials and sell them to their customers, and this gives rise to a verification scenario. The closed loop of issuing requests, creation, and consumption of credentials is what makes up the trust mechanism. All entities can make and verify claims.
Credential Workflow
Here are the parties that are involved in the process.
Credential owner - has an ONT ID. This entity acquires a verifiable credential issued by another entity that is referred to as the credential issuer. This entity is able to manipulate credentials, with anonymous credential technology, and provide the credentials to the credential consumer. Thus, they play the role of a trust seller.
Credential issuer -  has an ONT ID. This entity issues credentials to endorse a target entity for certain qualifications or credentials. The category of credential Issuer includes trust anchors, i.e. the partners or entities that provide authentication services in the Ontology ecosystem. Trust anchors could be government agencies, universities, banks, third-party authentication services, bio-metric technology companies, etc. Credential issuers provide multi-dimensional authentication for entities that are part of the trust network. The authentication process and result are recorded on the Ontology blockchain with data privacy protection. Credential issuers provide a standardized and credible authentication method for credential consumers to verify the credentials. Credential issuers play the role of a trust endorser.
Credential consumers -  accept the user's verifiable credentials and initiate the credential verification process for the respective credentials. This includes many different scenarios, e.g., the employers who need to verify the interviewer's identity information/degree/industry skills. They the play the role of trust buyer.
Verifiable Credential Protocol
Credential Verification Process
It is clear from the workflow illustrated above that the process involves three major actions:
1.Credential request
2.Credential issue
3.Credential verification
The issuing process involves two parties, the credential issuer and the entity that owns the ONT ID.
A verifiable credential includes the contents of the credential (that would vary depending upon the system), the digital signatures, and blockchain attestation records. Some of the records are:
Credential ID: Unique identifier for credentials
Credential content: Specific credentials or information, for instance a degree certificate
Credential metadata-
Created time: Timestamp for when the credential was created
Issuer: ONT ID of the issuer
Recipient: ONT ID of the recipient party
Expiration time: UNIX timestamp for the credential automatically expires
Revocation mechanism: Use the revocation list or record the revocation information directly in the attestation contract
Blockchain proof
Signature-
Public key of the issuer
Signature value
A verifiable credential template for an employee's salary certificate is available here.
For centralized ONT ID systems, the first step in the workflow might differ in terms of the request that is sent to the credential issuer, since the owners will have delegated another body with access to their ONT ID and credentials. The request may not necessarily be sent by the owner themselves. The delegated body may also initiate and authorize credentials.
Issuance process
The issuance process involves four main steps:
1.The ONT ID owner initiates the process by sending a request to the credential issuer. 
2.The credential issuer generates a verifiable credential and transmits it to the recipient using a secure method. The credential is encrypted using the recipient's public key.
3.The owner (or the delegate) signs the credential and sends it back to the credential issuer.
4.The credential issuer finally completes the signing process and sets the status of the credential to attested. Next, the credential is transmitted to both the Ontology blockchain and the owner (or the delegate).
This whole process basically covers Steps 1~3 in the credential workflow illustrated above.
Credential verification
There are three major actions involved in verifying a credential and they correspond to the steps 4 ~ 5 in the workflow illustrated above.
Verifying whether the credential is in the blockchain
Verifying the signature and whether it has expired
Checking whether the credential has been revoked
Blockchain record verification
It is necessary to verify whether the record of the verifiable credential is present on the blockchain. In case the node is not fully synchronized with the Ontology blockchain, merkle proof can be used to verify the verifiable credential transaction.
Merkle proof consists of an array, and each element contains two data items, direction and hash.
Direction: Represents branch of the merkle tree the particular array element is in. There are two possible values.
Hash: Represents hash value of the element data.
The algorithm using which the merkle proof is verified is as follows:
1.Check if transaction is included in block indexed by proof.BlockHeight. If not, return false.
2.Execute p <- GetBlockHash(proof.BlockHeight).
3.For each element in proof.Nodes, update p as
if e.Direction == "Left", p <- H(e.TargetHash, p);
else, p <- H(p, e.TargetHash).
4.Return true if p equals to the proof.MerkleRoot. Otherwise, return false.
In addition, it is also necessary to verify the status of the credential attestation. This can be carried out by calling the inquiry interface GetStatus() of the attestation contract with the address proof.ContractAddr. If the status is not attested, an error would be returned.
Signature verification and Expiration time
When verifying the signature, the public key ID needs to be used to fetch the public key value and its current status. The verification algorithm is then called to carry out the verification process.
The format of the pubic key ID is <ONTID>#keys-<number>
The public key ID is used to call the ONT ID smart contract method that queries the status of the public key. GetPublicKeyStatus(byte[] ontId, byte[] pkId)
The response contains the following:
publicKey: Public key value (hex)
status: Two possible values: InUse, Revoked
Three possible results of signature verification:
Signature is invalid
Signature is valid
Signature is valid and the public key is revoked
Next, the expiration time can be verified by checking whether the timeout period has expired.
Revoking verification
Currently there are two revocation modes available- revocation list and revocation inquiry interface.
Here's an example of the revocation list credential request. It contains the URL of the list.
"clm-rev": {
    "type": "RevocationList",
    "url": "https://example.com/rev/1234"
}
Using the revocation inquiry interface as an example, if the revocation information is placed in the attestation contract, when calling the inquiry interface GetStatus of attest contract, revocation verification will return success if and only if the returned status field is attested. It will return fail if the status field is attest has been revoked.
"clm-rev": {
       "type": "AttestContract",
       "addr": "8055b362904715fd84536e754868f4c8d27ca3f6"
}
The revocation list mainly includes the unique identifier and the revocation time of the revoked verifiable credential.
Format of a verifiable credential
We will use an extension of the JSON Web Token format to build the structure of the credential which is transferred between the issuer and the recipient.
The fundamental structure of the token consists of three parts:
Header
Payload
Signature
The standard JWT attributes are used to a great extent while in certain special cases custom attributes are defined.
The standard JWT format is augmented by appending the blockchain proof at the end, a typical verifiable credential has the following layout: header.payload.signature.blockchain_proof
The blockchain_proof is not required in some cases, and is thus optional.
Header
The header defines the format, the signature scheme, and the ID of the public key used to verify the signature on the credential.
{
    "alg": "ES256",
    "typ": "JWT-X",
    "kid": "did:ont:TRAtosUZHNSiLhzBdHacyxMX4Bg3cjWy3r#keys-1"
}
Attribute
Description
alg
Specifies the signature scheme to use. A list of supported values can be found here​
typ
"JWT": blockchain proof is not contained in the credential
"JWT-X" : blockchain proof is a part of the credential
The credential ID, credential content and the metadata are packaged into a JSON object which then acts as the payload. It will use some of the registered credential names specified in the JWT specification, such as jti, iss, sub, iat, exp.
{
    "ver": "0.7.0",
    "iss": "did:ont:TRAtosUZHNSiLhzBdHacyxMX4Bg3cjWy3r",
    "sub": "did:ont:SI59Js0zpNSiPOzBdB5cyxu80BO3cjGT70",
    "iat": 1525465044,
    "exp": 1530735444,
    "jti": "4d9546fdf2eb94a364208fa65a9996b03ba0ca4ab2f56d106dac92e891b6f7fc",
    "@context": "https://example.com/template/v1",
    "clm": {
        "Name": "Bob Dylan",
        "Age": "22"
    },
    "clm-rev": { 
        "typ": "AttestContract",
        "addr": "8055b362904715fd84536e754868f4c8d27ca3f6"
    }
}
Attribute
Description
ver
Specifies the version of the credential specification being followed
iss
ONT ID of the issuer
sub
ONT ID of the recipient
iat
UNIX timestamp when the credential was created
exp
UNIX timestamp of when the credential expires automatically
jti
Unique identifier of the verifiable credential
@context
URI of the credential content definition document that defines each field and the respective values explicitly
clm
Object that contains the credential content
clm-rev
Object that defines the revocation mechanism the credential uses
A list of the supported revocation mechanisms has been listed here.
Next, to issue a credential, a JSON object needs to be constructed that would contain the credential ID, the content, and the metadata. The JSON object can then be serialized using the standard serialization method. One of the issuer's private keys is then used to sign the binary data of the payload and the header.
After serialization, the payload would look something like-
{
   "ver": "0.7.0",
   "iss": "did:ont:TRAtosUZHNSiLhzBdHacyxMX4Bg3cjWy3r",
   "sub": "did:ont:SI59Js0zpNSiPOzBdB5cyxu80BO3cjGT70",
   "iat": 1525465044,
   "exp": 1530735444,
   "jti": "4d9546fdf2eb94a364208fa65a9996b03ba0ca4ab2f56d106dac92e891b6f7fc",
   "@context": "https://example.com/template/v1",
   "clm": {
      "Name": "Bob Dylan",
      "Age": "22"
   },
   "clm-rev": {
      "Type": "Contract",
      "Addr": "8055b362904715fd84536e754868f4c8d27ca3f6"
   }
}
Signature
After the header and payload of the request are constructed the signature is computed according to the JWS standard. Full description of the standard can be found in the RF 7515 Section 5.1 of the IETF documentation.
The process is as follows:
1.Calculate the signing input as serialization of the header and payload according to JWS specification.
sig := sign(Base64URL(header) || . || Base64URL(payload))
1.Compute the JWS signature using the specified method for the particular signature scheme being used for the signing input.
2.Encode the signature.
signature := Base64URL(sig)
Blockchain Proof
The general format of a proof object is as follows:
{
    "Type": "MerkleProof",
    "TxnHash": "c89e76ee58ae6ad99cfab829d3bf5bd7e5b9af3e5b38713c9d76ef2dcba2c8e0",
    "ContractAddr": "8055b362904715fd84536e754868f4c8d27ca3f6",
    "BlockHeight": 10,
    "MerkleRoot": "bfc2ac895685fbb01e22c61462f15f2a6e3544835731a43ae0cba82255a9f904",
    "Nodes": [{
        "Direction": "Right",
        "TargetHash": "2fa49b6440104c2de900699d31506845d244cc0c8c36a2fffb019ee7c0c6e2f6"
    }, {
        "Direction": "Left",
        "TargetHash": "fc4990f9758a310e054d166da842dab1ecd15ad9f8f0122ec71946f20ae964a4"
    }]
}
Attribute
Description
Type
Fixed value 'Merkleproof'
TxnHash
Hash of the transaction that attests the credential ID in the attestation contract
ContractAddr
Address of the attestation contract
BlockHeight
Height of the block that contains the attestation contract
MerkleRoot
Root of the merkle tree when the tree size equals the BlockHeight
Nodes
Inclusion proof of block in the merkle tree
The MerkleProof is encoded in the following manner:
BASE64URL(MerkleProof)
Hence, a complete verifiable credential is created. The final structure is as follows:
BASE64URL(Header) || '.' || BASE64URL(Payload) || '.' || BASE64URL(Signature)  '.' || BASE64URL(MerkleProof)
Attestation Contract
The attestation contract of a verifiable credential provides attestation service and record availability information, that is, whether or not it has been revoked.
The available methods are described below:
Commit Attestation
bool Commit(byte[] claimId, byte[] committerOntId, byte[] ownerOntId);
In the attestation contract, claimID serves as the unique identifier for a credential. It is the first parameter; The committerOntId is the ONT ID of the attester. The ownerOntId is the ONT ID of the owner.
This method will return true if and only if the credential is not attested, and the method has been called by the committer; Otherwise, it will return false.
After the attestation is done, the status of the credential will be updated to attested.
Revoke credential
bool Revoke(byte[] claimId, byte[] revokerOntId);
This method will return true if and only if the credential is attested, and the revokerOntId is the same as the attester's ONT ID; Otherwise, it will return false.
Attestation Inquiry method
byte[] GetStatus(byte[] claimId);
This method returns the status of the credential. The response contains two parts of information:
Status: Not attested , Attested, Attest has been revoked;
ONT ID of the attester.
Signature schemes
Currently supported signature schemes are:
Scheme
Encryption Technology
ES224
ECDSA with SHA224
ES256
ECDSA with SHA256
ES384
ECDSA with SHA384
ES512
ECDSA with SHA512
ES3-224
ECDSA with SHA3 224
ES3-256
ECDSA with SHA3 256
ES3-384
ECDSA with SHA3 384
ES3-512
ECDSA with SHA3 512
ER160
ECDSA with RIPEMD160
SM
SM2 with SM3
EDS512
EDDSA with SHA256
Method Specification for BSC
Anonymous Credentials
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Verifiable Credential Protocol
Issuance process
Credential verification
Format of a verifiable credential
Attestation Contract
Signature schemes