Create method SetIntString in the Scalar interface to allow the creat…

…ion of a large Scalar from a string.

Previously, to create arbitrary numbers the methods SetInt64 or SetBytes had to be used.
SetInt64 only allows the creation of 64 bit integers, while SetBytes requires the byte array representation of the number, which is inconvenient.

group.go

@@ -35,6 +35,9 @@ type Scalar interface {
 	// SetInt64 sets the receiver to a small integer value.
 	SetInt64(v int64) Scalar
 
+	// SetIntString sets the receiver to a string encoded integer value.
+	SetIntString(v string) (Scalar, error)
+
 	// Set to the additive identity (0).
 	Zero() Scalar
 

group/edwards25519/scalar.go

@@ -59,6 +59,14 @@ func (s *scalar) SetInt64(v int64) kyber.Scalar {
 	return s.setInt(mod.NewInt64(v, primeOrder))
 }
 
+func (s *scalar) SetIntString(v string) (kyber.Scalar, error) {
+	i, ok := new(big.Int).SetString(v, 0)
+	if !ok {
+		return nil, errors.New("invalid scalar string")
+	}
+	return s.setInt(mod.NewInt(i, primeOrder)), nil
+}
+
 func (s *scalar) toInt() *mod.Int {
 	return mod.NewIntBytes(s.v[:], primeOrder, defaultEndianess)
 }

group/mod/int.go

@@ -6,6 +6,7 @@ import (
 	"crypto/cipher"
 	"encoding/hex"
 	"errors"
+	"fmt"
 	"io"
 	"math/big"
 
@@ -173,6 +174,15 @@ func (i *Int) SetInt64(v int64) kyber.Scalar {
 	return i
 }
 
+func (i *Int) SetIntString(v string) (kyber.Scalar, error) {
+	bigV := new(big.Int)
+	bigV, ok := bigV.SetString(v, 0)
+	if !ok {
+		return nil, fmt.Errorf("unable to set string number: %v", v)
+	}
+	return i.Init(bigV, i.M), nil
+}
+
 // Int64 returns the int64 representation of the value.
 // If the value is not representable in an int64 the result is undefined.
 func (i *Int) Int64() int64 {

pairing/bls12381/circl/scalar.go

@@ -2,6 +2,7 @@ package circl
 
 import (
 	"crypto/cipher"
+	"fmt"
 	"io"
 	"math/big"
 
@@ -62,6 +63,14 @@ func (s *Scalar) SetInt64(v int64) kyber.Scalar {
 	return s
 }
 
+func (s *Scalar) SetIntString(v string) (kyber.Scalar, error) {
+	err := s.inner.SetString(v)
+	if err != nil {
+		return nil, fmt.Errorf("unable to set string number: %v", err)
+	}
+	return s, nil
+}
+
 func (s *Scalar) Zero() kyber.Scalar { s.inner.SetUint64(0); return s }
 
 func (s *Scalar) Add(a, b kyber.Scalar) kyber.Scalar {

share/vss/pedersen/vss_test.go

@@ -64,7 +64,10 @@ func TestMinimumT(t *testing.T) {
 
 func TestVSSWhole(t *testing.T) {
 	dealer, verifiers := genAll()
+	vssWhole(t, dealer, verifiers, secret)
+}
 
+func vssWhole(t *testing.T, dealer *Dealer, verifiers []*Verifier, secret kyber.Scalar) {
 	// 1. dispatch deal
 	resps := make([]*Response, nbVerifiers)
 	encDeals, err := dealer.EncryptedDeals()
@@ -591,6 +594,15 @@ func TestVSSContext(t *testing.T) {
 	assert.Len(t, c, suite.Hash().Size())
 }
 
+func TestDeterministicStringSecret(t *testing.T) {
+	deterministicSec, err := suite.Scalar().SetIntString("0x123456789abcdef")
+	require.NoError(t, err)
+	dealer, err := NewDealer(suite, dealerSec, deterministicSec, verifiersPub, vssThreshold)
+	require.NoError(t, err)
+	verifiers := genVerifiers()
+	vssWhole(t, dealer, verifiers, deterministicSec)
+}
+
 func genPair() (kyber.Scalar, kyber.Point) {
 	secret := suite.Scalar().Pick(suite.RandomStream())
 	public := suite.Point().Mul(secret, nil)
@@ -612,13 +624,16 @@ func genDealer() *Dealer {
 }
 
 func genAll() (*Dealer, []*Verifier) {
-	dealer := genDealer()
+	return genDealer(), genVerifiers()
+}
+
+func genVerifiers() []*Verifier {
 	var verifiers = make([]*Verifier, nbVerifiers)
 	for i := 0; i < nbVerifiers; i++ {
 		v, _ := NewVerifier(suite, verifiersSec[i], dealerPub, verifiersPub)
 		verifiers[i] = v
 	}
-	return dealer, verifiers
+	return verifiers
 }
 
 func randomBytes(n int) []byte {