ecvrf

README

go-ecvrf

Golang implementation of Elliptic Curve Verifiable Random Function (VRF) follows draft-irtf-cfrg-vrf-06 and RFC 6979.

What's VRF

A Verifiable Random Function (VRF) is the public-key version of a keyed cryptographic hash. Only the holder of the private key can compute the hash, but anyone with public key can verify the correctness of the hash.

A key application of the VRF is to provide privacy against offline enumeration (e.g. dictionary attacks) on data stored in a hash-based data structure. In this application, a Prover holds the VRF private key and uses the VRF hashing to construct a hash-based data structure on the input data. Due to the nature of the VRF, only the Prover can answer queries about whether or not some data is stored in the data structure. Anyone who knows the public VRF key can verify that the Prover has answered the queries correctly. However no offline inferences (i.e. inferences without querying the Prover) can be made about the data stored in the data strucuture.

Installation

go get -u github.com/vechain/go-ecvrf

Examples

Using SECP256K1_SHA256_TAI cipher suite:

• VRF Proving

  // the private key
  var sk *ecdsa.PrivateKey
  // code to load sk
  // ... 
  
  // the input to be hashed by the VRF
  alpha := "Hello VeChain"
  
  // `beta`: the VRF hash output
  // `pi`: the VRF proof
  beta, pi, err := ecvrf.Secp256k1Sha256Tai.Prove(sk, []byte(alpha))
  if err != nil {
      // something wrong.
      // most likely sk is not properly loaded.
      return
  }
  

• VRF Verifying

  // the public key
  var pk *ecdsa.PublicKey
  // code to load pk
  // ...
  
  // the input to be hashed by the VRF
  alpha := "Hello VeChain"
  
  // `pi` is the VRF proof
  beta, err := ecvrf.Secp256k1Sha256Tai.Verify(pk, []byte(alpha), pi)
  if err != nil {
      // invalid proof
      return
  }
  
  // got correct beta
  

Supported Cipher Suites

• P256_SHA256_TAI
• SECP256K1_SHA256_TAI

It's easy to extends this library to use different Weierstrass curves and Hash algorithms, by providing cooked Config like:

// the following codes build a new P256_SHA256_TAI VRF object.
vrf := ecvrf.New(&ecvrf.Config{
    Curve:       elliptic.P256(),
    SuiteString: 0x01,
    Cofactor:    0x01,
    NewHasher:   sha256.New,
    Decompress: elliptic.UnmarshalCompressed,
})

Benchmark

$ go test -benchmem -run=^$ -bench ^BenchmarkVRF$ github.com/vechain/go-ecvrf -benchtime=5s
goos: linux
goarch: amd64
pkg: github.com/vechain/go-ecvrf
cpu: Intel(R) Core(TM) i7-6700 CPU @ 3.40GHz
BenchmarkVRF/secp256k1sha256tai-proving-8         	   22207	    279600 ns/op	    4881 B/op	      95 allocs/op
BenchmarkVRF/secp256k1sha256tai-verifying-8       	   15150	    399938 ns/op	    5009 B/op	     114 allocs/op
BenchmarkVRF/p256sha256tai-proving-8              	   31328	    193911 ns/op	    9083 B/op	     294 allocs/op
BenchmarkVRF/p256sha256tai-verifying-8            	   19875	    300613 ns/op	   19472 B/op	     515 allocs/op
PASS
ok  	github.com/vechain/go-ecvrf	36.060s

References

• draft-irtf-cfrg-vrf-06
• RFC 6979
• witnet/vrf-rs
• google/keytransparency

License

Copyright (c) 2020 vechain.org. Licensed under the MIT license.

Documentation

Overview

Package ecvrf is the Elliptic Curve Verifiable Random Function (VRF) library.

Index

• Variables
• type Config
• type VRF
  • func New(cfg *Config) VRF

Variables

var (
	// Secp256k1Sha256Tai is the pre-configured VRF object with secp256k1/SHA256 and hash_to_curve_try_and_increment algorithm.
	Secp256k1Sha256Tai = New(&Config{
		Curve:       secp256k1.S256(),
		SuiteString: 0xfe,
		Cofactor:    0x01,
		NewHasher:   sha256.New,
		Decompress: func(c elliptic.Curve, pk []byte) (x, y *big.Int) {

			if len(pk) != 33 {
				return
			}

			var fx, fy secp256k1.FieldVal
			format := pk[0]
			switch format {
			case secp256k1.PubKeyFormatCompressedEven, secp256k1.PubKeyFormatCompressedOdd:
			default:
				return
			}

			if overflow := fx.SetByteSlice(pk[1:33]); overflow {
				return
			}

			wantOddY := format == secp256k1.PubKeyFormatCompressedOdd
			if !secp256k1.DecompressY(&fx, wantOddY, &fy) {
				return
			}
			fy.Normalize()
			return new(big.Int).SetBytes(fx.Bytes()[:]), new(big.Int).SetBytes(fy.Bytes()[:])
		},
	})
	// P256Sha256Tai is the pre-configured VRF object with P256/SHA256 and hash_to_curve_try_and_increment algorithm.
	P256Sha256Tai = New(&Config{
		Curve:       elliptic.P256(),
		SuiteString: 0x01,
		Cofactor:    0x01,
		NewHasher:   sha256.New,
		Decompress:  elliptic.UnmarshalCompressed,
	})
)

Types

type Config

type Config struct {
	// the elliptic curve.
	Curve elliptic.Curve
	// a single nonzero octet specifying the ECVRF ciphersuite.
	SuiteString byte
	// number of points on curve divided by group order.
	Cofactor byte
	// create cryptographic hash function.
	NewHasher func() hash.Hash
	// decompress the compressed public key into x and y coordinate.
	Decompress func(c elliptic.Curve, pk []byte) (x, y *big.Int)
}

Config contains VRF parameters.

type VRF

type VRF interface {
	// Prove constructs a VRF proof `pi` for the given input `alpha`,
	// using the private key `sk`. The hash output is returned as `beta`.
	Prove(sk *ecdsa.PrivateKey, alpha []byte) (beta, pi []byte, err error)

	// Verify checks the proof `pi` of the message `alpha` against the given
	// public key `pk`. The hash output is returned as `beta`.
	Verify(pk *ecdsa.PublicKey, alpha, pi []byte) (beta []byte, err error)
}

VRF is the interface that wraps VRF methods.

func New

func New(cfg *Config) VRF

New creates and initializes a VRF object using customized config.