feat(ecdsa): implement varsig for EdDSA

ecdsa.go

@@ -1,5 +1,10 @@
 package varsig
 
+import (
+	"encoding/binary"
+	"fmt"
+)
+
 // DiscriminatorECDSA is the value specifying an ECDSA signature.
 const DiscriminatorECDSA = Discriminator(0xec)
 
@@ -15,3 +20,136 @@ const (
 	CurveP384      = ECDSACurve(0x1201)
 	CurveP521      = ECDSACurve(0x1202)
 )
+
+func decodeECDSACurve(r BytesReader) (ECDSACurve, error) {
+	u, err := binary.ReadUvarint(r)
+	if err != nil {
+		return 0, err
+	}
+
+	switch curve := ECDSACurve(u); curve {
+	case CurveSecp256k1, CurveP256, CurveP384, CurveP521:
+		return curve, nil
+	default:
+		return 0, fmt.Errorf("%w: %x", ErrUnknownECDSACurve, u)
+	}
+}
+
+var _ Varsig = ECDSAVarsig{}
+
+// ECDSAVarsig is a varsig that encodes the parameters required to describe
+// an ECDSA signature.
+type ECDSAVarsig struct {
+	varsig
+
+	curve   ECDSACurve
+	hashAlg Hash
+}
+
+// NewECDSAVarsig creates and validates an ECDSA varsig with the provided
+// curve, hash algorithm and payload encoding.
+func NewECDSAVarsig(curve ECDSACurve, hashAlgorithm Hash, payloadEncoding PayloadEncoding, opts ...Option) (ECDSAVarsig, error) {
+	options := newOptions(opts...)
+
+	var (
+		vers = Version1
+		disc = DiscriminatorECDSA
+		sig  []byte
+	)
+
+	if options.ForceVersion0() {
+		vers = Version0
+		disc = Discriminator(curve)
+		sig = options.Signature()
+	}
+
+	v := ECDSAVarsig{
+		varsig: varsig{
+			vers:   vers,
+			disc:   disc,
+			payEnc: payloadEncoding,
+			sig:    sig,
+		},
+		curve:   curve,
+		hashAlg: hashAlgorithm,
+	}
+
+	switch curve {
+	case CurveSecp256k1, CurveP256:
+		return validateSig(v, 64)
+	case CurveP384:
+		return validateSig(v, 96)
+	case CurveP521:
+		return validateSig(v, 132)
+	default:
+		return ECDSAVarsig{}, fmt.Errorf("%w: %x", ErrUnknownECDSACurve, curve)
+	}
+}
+
+// Curve returns the elliptic curve used to generate the ECDSA signature.
+func (v ECDSAVarsig) Curve() ECDSACurve {
+	return v.curve
+}
+
+// Hash returns the value describing the hash algorithm used to hash
+// the payload content before the signature is generated.
+func (v ECDSAVarsig) Hash() Hash {
+	return v.hashAlg
+}
+
+// Encode returns the encoded byte format of the ECDSAVarsig.
+func (v ECDSAVarsig) Encode() []byte {
+	buf := v.encode()
+
+	if v.vers != Version0 {
+		buf = binary.AppendUvarint(buf, uint64(v.curve))
+	}
+
+	buf = binary.AppendUvarint(buf, uint64(v.hashAlg))
+	buf = binary.AppendUvarint(buf, uint64(v.payEnc))
+	buf = append(buf, v.Signature()...)
+
+	return buf
+}
+
+func decodeECDSA(r BytesReader, vers Version, disc Discriminator) (Varsig, error) {
+	curve := ECDSACurve(disc)
+	if vers != Version0 {
+		var err error
+
+		curve, err = decodeECDSACurve(r)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	hashAlg, err := DecodeHashAlgorithm(r)
+	if err != nil {
+		return nil, err
+	}
+
+	v := ECDSAVarsig{
+		varsig: varsig{
+			vers: vers,
+			disc: disc,
+		},
+		curve:   curve,
+		hashAlg: hashAlg,
+	}
+
+	v.payEnc, v.sig, err = v.decodePayEncAndSig(r)
+	if err != nil {
+		return nil, err
+	}
+
+	switch curve {
+	case CurveSecp256k1, CurveP256:
+		return validateSig(v, 64)
+	case CurveP384:
+		return validateSig(v, 96)
+	case CurveP521:
+		return validateSig(v, 132)
+	default:
+		return ECDSAVarsig{}, fmt.Errorf("%w: %x", ErrUnknownECDSACurve, curve)
+	}
+}