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Version: 1.3

Zero Knowledge Selective Disclosure (ZK-SD-VCs)

ZK-SD-VCs allow holders to verify their VCs without having to disclose the entire VC's claim set to verifiers. This is done through the creation of a Zero Knowledge Proof (ZKP) that guarantees the integrity and authenticity of the VC, even when only partially disclosed to the verifier.

note

Although ZK-SD-VCs offer similar functionalities to SD-JWT VCs - at least on a high level - they rely on completely different concepts and security concerns. For a user, the most notable difference is the shifted capability of choosing which fields can be concealed from a verifier. For ZK-SD-VCs it's the holder that has total control over which parts of the credential can be undisclosed, whereas for SD-JWT VCs it's the issuer that decides which fields may be concealed by the holder.

Concepts​

Issuance​

The issuer of a ZK-SD-VC creates the credential, signs it using the BBS+ signature scheme and sends both the credential and the signature to the holder. To facilitate this process, the credential is first encoded as a JSON Proof Token (JPT), which is then used as the payload of a JSON Web Proof (JWP) and sent to the holder as JPT.

note

JWPs and JPTs can be reasoned about as the Zero Knowledge (ZK) based counterparts of JWSs and JWTs.

In code, this process would look like the following snippet:

examples/1_advanced/9_zkp.rs
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Note how the VC issuer makes no prescription whatsoever regarding the disclosability of the VC's fields.

Holder presentation​

Once the holder receives a presentation challenge from a verifier, they construct a selective disclosure presentation for the requested credential and send it back for verification. For this process the JWP in possession of the holder undergoes a transformation that allows the holder to conceal any fields from the credentials claims through the creation of a Zero Knowledge Proof (ZKP) of the issuer's signature and becomes a presented JWP. The proof value depends on the selected JSON Proof Algorithm (JPA).

examples/1_advanced/9_zkp.rs
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Here's an example presented JWP in its JPT JSON serialization format where the undisclosed values are replaced by null:

{
  "payloads": [
    null,
    "IkpheSI",
    null,
    "NDI"
  ],
  "issuer": "eyJpc3MiOiJodHRwczovL2lzc3Vlci50bGQiLCJjbGFpbXMiOlsiZmFt
  aWx5X25hbWUiLCJnaXZlbl9uYW1lIiwiZW1haWwiLCJhZ2UiXSwidHlwIjoiSlBUIiw
  icHJvb2ZfandrIjp7ImNydiI6IlAtMjU2Iiwia3R5IjoiRUMiLCJ4IjoiYWNiSVFpdU
  1zM2k4X3VzekVqSjJ0cFR0Uk00RVUzeXo5MVBINkNkSDJWMCIsInkiOiJfS2N5TGo5d
  ldNcHRubUt0bTQ2R3FEejh3Zjc0STVMS2dybDJHekgzblNFIn0sInByZXNlbnRhdGlv
  bl9qd2siOnsiY3J2IjoiUC0yNTYiLCJrdHkiOiJFQyIsIngiOiJvQjFUUHJFX1FKSUw
  2MWZVT09LNURwS2dkOGoyemJaSnRxcElMRFRKWDZJIiwieSI6IjNKcW5ya3VjTG9ia2
  RSdU9xWlhPUDlNTWxiRnllbkZPTHlHbEctRlBBQ00ifSwiYWxnIjoiU1UtRVMyNTYif
  Q",
  "proof": "LJMiN6caEqShMJ5jPNts8OescqNq5vKSqkfAdSuGJA1GyJyyrfjkpAG0c
  DJKZoUgomHu5MzYhTUsa0YRXVBnMB91RjonrnWVsakfXtfm2h7gHxA_8G1wkB09x09k
  on2eK9gTv4iKw4GP6Rh02PEIAVAvnhtuiShMnPqVw1tCBdhweWzjyxJbG86J7Y8MDt2
  H9f5hhHIwmSLwXYzCbD37WmvUEQ2_6whgAYB5ugSQN3BjXEviCA__VX3lbhH1RVc27E
  YkRHdRgGQwWNtuExKz7OmwH8oWizplEtjWJ5WIlJpee79gQ9HTa2QIOT9bUDvjjkkO-
  jK_zuDjZwh5MkrcaQ",
  "presentation": "eyJub25jZSI6InVURUIzNzFsMXB6V0psN2FmQjB3aTBIV1VOaz
  FMZS1iQ29tRkx4YThLLXMifQ"
}

Verification​

The verifier decodes the received JPT presentation and asserts the validity of the ZKP it contains, thus proving the authenticity and integrity of the presented credential, without knowledge of any of the undisclosed fields and of the issuer signature.

examples/1_advanced/9_zkp.rs
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Full Example Code​

examples/1_advanced/9_zkp.rs
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