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Updated on February 15, 2022

Serialize

gnark objects

gnark objects implement io.WriterTo and io.ReaderFrom interfaces.

To serialize a gnark object:

// compile a circuit
cs, err := frontend.Compile(ecc.BN254, backend.GROTH16, &circuit)

// cs implements io.WriterTo
var buf bytes.Buffer
cs.WriteTo(&buf)

To deserialize, first instantiate a curve-typed object, as per gnark API design choices, these objects are not directly accessible (under internal/).

// instantiate a curve-typed object
cs := groth16.NewCS(ecc.BN254)
// cs implements io.ReaderFrom
cs.ReadFrom(&buf)

Important

Constraint systems (R1CS and SparseR1CS, for Groth16 and PLONK) currently use the cbor serialization protocol.

Other gnark objects, like ProvingKey, VerifyingKey or Proof contains elliptic curve points, and use a binary serialization protocol, allowing point compression.

We strongly discourage to using another protocol, because for security reasons, deserialization must also perform curve and subgroup checks.

Compression

Elliptic curve points, which are the main citizens of ProvingKey, VerifyingKey or Proof objects can be compressed by storing only the X coordinate, and a parity bit. This divides the required bytes that represent these objects by two, but comes at a significant CPU cost on the deserialization side.

These objects implement io.WriterRawTo, which doesn’t use point compression.

provingKey.WriteRawTo(&buf) // alternatively, provingKey.WriteTo(&buf)
...
pk := groth16.NewProvingKey(ecc.BN254)
pk.ReadFrom(&buf) // reader will detect if points are compressed or not.

Tip

Use WriteRawTo when deserialization speed >>> storage cost, otherwise use WriteTo with point compression.

Witness

Witnesses (inputs to the Prove or Verify functions) may be constructed outside of gnark, in a non-Go codebase.

Two types of witnesses exist:

  • Full witness: contains public and secret inputs, needed by Prove
  • Public witness: contains public inputs only, needed by Verify

For performance reason (witnesses can be large), witnesses should be encoded using a binary protocol. For convenience, gnark also support JSON encoding.

Binary protocol

While there is no standard yet, we followed similar patterns used by other zk-SNARK libraries.

// Full witness     ->  [uint32(nbElements) | publicVariables | secretVariables]
// Public witness   ->  [uint32(nbElements) | publicVariables ]

Where:

  • nbElements == len(publicVariables) + len(secretVariables).
  • each variable (a field element) is encoded as a big-endian byte array, where len(bytes(variable)) == len(bytes(modulus))

Ordering

The ordering sequence is first, publicVariables, then secretVariables. Each subset is ordered from the order of definition in the circuit structure.

For example, with this circuit on ecc.BN254:

type Circuit struct {
    X frontend.Variable
    Y frontend.Variable `gnark:",public"`
    Z frontend.Variable
}

A valid witness would be:

  • [uint32(3)|bytes(Y)|bytes(X)|bytes(Z)]
  • Hex representation with values Y = 35, X = 3, Z = 2 00000003000000000000000000000000000000000000000000000000000000000000002300000000000000000000000000000000000000000000000000000000000000030000000000000000000000000000000000000000000000000000000000000002

Example

This example is intended for a multi-process usage of gnark where you need to construct the witness in one process and deserialize it in another.

Tip

If the witness creation and proof creation live in the same process, refer to Construct the witness.

Full witness in Go

// witness
var assignment cubic.Circuit
assignment.X = 3 
assignment.Y = 35
witness, _ := frontend.NewWitness(&assignment, ecc.BN254)

// Binary marshalling
data, err := witness.MarshalBinary()

// JSON marshalling
json, err := witness.MarshalJSON()

...
// recreate a witness
witness, err := witness.New(ecc.BN254, ccs.GetSchema()) // note that schema is optional for binary encoding

// Binary unmarshalling
err := witness.UnmarshalBinary(data)

// JSON unmarshalling
err := witness.UnmarshalJSON(json)

// extract the public part only
publicWitness, _ := witness.Public()
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