Add Scalar/SetBigInt function (#377)

* Add Scalar/SetBigInt function This change enables one to set Scalars to integer values encapsulated in big.Int types. Previously, only uint64 inputs were admitted. * scalar: add bigint warning Co-authored-by: Bas Westerbaan <[email protected]>
cloudflare · Oct 10, 2022 · 03a6a6e · 03a6a6e
1 parent 3a03536
commit 03a6a6e
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Showing 3 changed files with 16 additions and 8 deletions.
diff --git a/group/group.go b/group/group.go
@@ -5,6 +5,7 @@ import (
 	"encoding"
 	"errors"
 	"io"
+	"math/big"
 )
 
 // Params stores the size in bytes of elements and scalars.
@@ -105,8 +106,12 @@ type Scalar interface {
 	IsZero() bool
 	// IsEqual returns true if the receiver is equal to x.
 	IsEqual(x Scalar) bool
-	// SetUint64 set the receiver to x, and returns the receiver.
+	// SetUint64 sets the receiver to x, and returns the receiver.
 	SetUint64(x uint64) Scalar
+	// SetBigInt sets the receiver to x, and returns the receiver.
+	// Warning: operations on big.Int are not constant time. Do not use them
+	// for cryptography unless you're sure it's safe in your use-case.
+	SetBigInt(b *big.Int) Scalar
 	// CMov sets the receiver to x if b=1; the receiver is unmodified if b=0;
 	// otherwise panics if b is not 0 or 1. In all the cases, it returns the
 	// receiver.

diff --git a/group/ristretto255.go b/group/ristretto255.go
@@ -5,6 +5,7 @@ import (
 	_ "crypto/sha512"
 	"fmt"
 	"io"
+	"math/big"
 
 	r255 "github.com/bwesterb/go-ristretto"
 	"github.com/cloudflare/circl/expander"
@@ -203,10 +204,11 @@ func (e *ristrettoElement) UnmarshalBinary(data []byte) error {
 	return e.p.UnmarshalBinary(data)
 }
 
-func (s *ristrettoScalar) Group() Group              { return Ristretto255 }
-func (s *ristrettoScalar) String() string            { return conv.BytesLe2Hex(s.s.Bytes()) }
-func (s *ristrettoScalar) SetUint64(n uint64) Scalar { s.s.SetUint64(n); return s }
-func (s *ristrettoScalar) IsZero() bool              { return s.s.IsNonZeroI() == 0 }
+func (s *ristrettoScalar) Group() Group                { return Ristretto255 }
+func (s *ristrettoScalar) String() string              { return conv.BytesLe2Hex(s.s.Bytes()) }
+func (s *ristrettoScalar) SetUint64(n uint64) Scalar   { s.s.SetUint64(n); return s }
+func (s *ristrettoScalar) SetBigInt(x *big.Int) Scalar { s.s.SetBigInt(x); return s }
+func (s *ristrettoScalar) IsZero() bool                { return s.s.IsNonZeroI() == 0 }
 func (s *ristrettoScalar) IsEqual(x Scalar) bool {
 	return s.s.Equals(&x.(*ristrettoScalar).s)
 }

diff --git a/group/short.go b/group/short.go
@@ -271,9 +271,10 @@ type wScl struct {
 	k []byte
 }
 
-func (s *wScl) Group() Group              { return s.wG }
-func (s *wScl) String() string            { return fmt.Sprintf("0x%x", s.k) }
-func (s *wScl) SetUint64(n uint64) Scalar { s.fromBig(new(big.Int).SetUint64(n)); return s }
+func (s *wScl) Group() Group                { return s.wG }
+func (s *wScl) String() string              { return fmt.Sprintf("0x%x", s.k) }
+func (s *wScl) SetUint64(n uint64) Scalar   { s.fromBig(new(big.Int).SetUint64(n)); return s }
+func (s *wScl) SetBigInt(x *big.Int) Scalar { s.fromBig(x); return s }
 func (s *wScl) IsZero() bool {
 	return subtle.ConstantTimeCompare(s.k, make([]byte, (s.wG.c.Params().BitSize+7)/8)) == 1
 }