fix(v0): restores validateSig behavior

eddsa.go

@@ -79,11 +79,7 @@ func NewEdDSAVarsig(curve EdDSACurve, hashAlgorithm HashAlgorithm, payloadEncodi
 		hashAlg: hashAlgorithm,
 	}
 
-	err := v.validateSig(ed25519.SignatureSize)
-	if err != nil {
-		return EdDSAVarsig{}, err
-	}
-	return v, nil
+	return validateSig(v, ed25519.SignatureSize)
 }
 
 // Curve returns the Edwards curve used to generate the EdDSA signature.
@@ -142,10 +138,5 @@ func decodeEd25519(r BytesReader, vers Version, disc Discriminator) (Varsig, err
 		return nil, err
 	}
 
-	err = v.validateSig(ed25519.SignatureSize)
-	if err != nil {
-		return RSAVarsig{}, err
-	}
-
-	return v, nil
+	return validateSig(v, ed25519.SignatureSize)
 }

rsa.go

@@ -45,12 +45,7 @@ func NewRSAVarsig(hashAlgorithm HashAlgorithm, keyLength uint64, payloadEncoding
 		sigLen:  keyLength,
 	}
 
-	err := v.validateSig(v.sigLen)
-	if err != nil {
-		return RSAVarsig{}, err
-	}
-
-	return v, nil
+	return validateSig(v, v.sigLen)
 }
 
 // Encode returns the encoded byte format of the RSAVarsig.
@@ -100,10 +95,5 @@ func decodeRSA(r BytesReader, vers Version, disc Discriminator) (Varsig, error)
 		return nil, err
 	}
 
-	err = vs.validateSig(vs.sigLen)
-	if err != nil {
-		return RSAVarsig{}, err
-	}
-
-	return vs, nil
+	return validateSig(vs, vs.sigLen)
 }

rsa_test.go

@@ -69,7 +69,7 @@ func TestUCANExample(t *testing.T) {
 		vs, err := varsig.Decode(example)
 		require.ErrorIs(t, err, varsig.ErrMissingSignature)
 
-		rsaVs, ok := vs.(*varsig.RSAVarsig)
+		rsaVs, ok := vs.(varsig.RSAVarsig)
 		require.True(t, ok)
 
 		assert.Equal(t, varsig.Version0, rsaVs.Version())

varsig.go

@@ -20,6 +20,7 @@ package varsig
 
 import (
 	"encoding/binary"
+	"errors"
 	"io"
 )
 
@@ -100,30 +101,40 @@ func (v varsig) decodePayEncAndSig(r BytesReader) (PayloadEncoding, []byte, erro
 	}
 
 	var signature []byte
-	if v.Version() == Version0 {
+	switch v.Version() {
+	case Version0:
 		signature, err = io.ReadAll(r)
 		if err != nil {
 			return 0, nil, err
 		}
+	case Version1:
+		_, err := r.ReadByte()
+		if err != nil && !errors.Is(err, io.EOF) {
+			return 0, nil, err
+		}
+		if err == nil {
+			return 0, nil, ErrUnexpectedSignaturePresent
+		}
+
 	}
 
 	return payEnc, signature, nil
 }
 
-func (v varsig) validateSig(expectedLength uint64) error {
-	if v.Version() == Version0 && len(v.sig) == 0 {
-		return ErrMissingSignature
+func validateSig[T Varsig](v T, expectedLength uint64) (T, error) {
+	if v.Version() == Version0 && len(v.Signature()) == 0 {
+		return v, ErrMissingSignature
 	}
 
-	if v.Version() == Version0 && uint64(len(v.sig)) != expectedLength {
-		return ErrUnexpectedSignatureSize
+	if v.Version() == Version0 && uint64(len(v.Signature())) != expectedLength {
+		return *new(T), ErrUnexpectedSignatureSize
 	}
 
-	if v.Version() == Version1 && len(v.sig) != 0 {
-		return ErrUnexpectedSignaturePresent
+	if v.Version() == Version1 && len(v.Signature()) != 0 {
+		return *new(T), ErrUnexpectedSignaturePresent
 	}
 
-	return nil
+	return v, nil
 }
 
 type BytesReader interface {

varsig_test.go

@@ -133,7 +133,7 @@ func TestDecode(t *testing.T) {
 
 		vs, err := varsig.Decode(data)
 		require.ErrorIs(t, err, varsig.ErrUnexpectedSignatureSize)
-		assert.Nil(t, vs)
+		assert.Zero(t, vs)
 	})
 
 	t.Run("fails - unexpected signature present - v1", func(t *testing.T) {