Improve ErrInvalidKeyType error messages (#361)

* Improve ErrInvalidKeyType error message

* add specific expected type to error message

* fix ErrInvalidKey error to ErrInvalidKeyType in rsa and rsapss

* format

* revert changes from example_test.go remove the comments

* fix: udpate the signing names to uppercase

ecdsa.go

@@ -62,7 +62,7 @@ func (m *SigningMethodECDSA) Verify(signingString string, sig []byte, key interf
 	case *ecdsa.PublicKey:
 		ecdsaKey = k
 	default:
-		return ErrInvalidKeyType
+		return newError("ECDSA verify expects *ecsda.PublicKey", ErrInvalidKeyType)
 	}
 
 	if len(sig) != 2*m.KeySize {
@@ -96,7 +96,7 @@ func (m *SigningMethodECDSA) Sign(signingString string, key interface{}) ([]byte
 	case *ecdsa.PrivateKey:
 		ecdsaKey = k
 	default:
-		return nil, ErrInvalidKeyType
+		return nil, newError("ECDSA sign expects *ecsda.PrivateKey", ErrInvalidKeyType)
 	}
 
 	// Create the hasher

ed25519.go

@@ -38,7 +38,7 @@ func (m *SigningMethodEd25519) Verify(signingString string, sig []byte, key inte
 	var ok bool
 
 	if ed25519Key, ok = key.(ed25519.PublicKey); !ok {
-		return ErrInvalidKeyType
+		return newError("Ed25519 verify expects ed25519.PublicKey", ErrInvalidKeyType)
 	}
 
 	if len(ed25519Key) != ed25519.PublicKeySize {
@@ -60,7 +60,7 @@ func (m *SigningMethodEd25519) Sign(signingString string, key interface{}) ([]by
 	var ok bool
 
 	if ed25519Key, ok = key.(crypto.Signer); !ok {
-		return nil, ErrInvalidKeyType
+		return nil, newError("Ed25519 sign expects crypto.Signer", ErrInvalidKeyType)
 	}
 
 	if _, ok := ed25519Key.Public().(ed25519.PublicKey); !ok {

hmac.go

@@ -59,7 +59,7 @@ func (m *SigningMethodHMAC) Verify(signingString string, sig []byte, key interfa
 	// Verify the key is the right type
 	keyBytes, ok := key.([]byte)
 	if !ok {
-		return ErrInvalidKeyType
+		return newError("HMAC verify expects []byte", ErrInvalidKeyType)
 	}
 
 	// Can we use the specified hashing method?
@@ -91,7 +91,7 @@ func (m *SigningMethodHMAC) Verify(signingString string, sig []byte, key interfa
 func (m *SigningMethodHMAC) Sign(signingString string, key interface{}) ([]byte, error) {
 	if keyBytes, ok := key.([]byte); ok {
 		if !m.Hash.Available() {
-			return nil, ErrHashUnavailable
+			return nil, newError("HMAC sign expects []byte", ErrInvalidKeyType)
 		}
 
 		hasher := hmac.New(m.Hash.New, keyBytes)

rsa.go

@@ -51,7 +51,7 @@ func (m *SigningMethodRSA) Verify(signingString string, sig []byte, key interfac
 	var ok bool
 
 	if rsaKey, ok = key.(*rsa.PublicKey); !ok {
-		return ErrInvalidKeyType
+		return newError("RSA verify expects *rsa.PublicKey", ErrInvalidKeyType)
 	}
 
 	// Create hasher
@@ -73,7 +73,7 @@ func (m *SigningMethodRSA) Sign(signingString string, key interface{}) ([]byte,
 
 	// Validate type of key
 	if rsaKey, ok = key.(*rsa.PrivateKey); !ok {
-		return nil, ErrInvalidKey
+		return nil, newError("RSA sign expects *rsa.PrivateKey", ErrInvalidKeyType)
 	}
 
 	// Create the hasher

rsa_pss.go

@@ -88,7 +88,7 @@ func (m *SigningMethodRSAPSS) Verify(signingString string, sig []byte, key inter
 	case *rsa.PublicKey:
 		rsaKey = k
 	default:
-		return ErrInvalidKey
+		return newError("RSA-PSS verify expects *rsa.PublicKey", ErrInvalidKeyType)
 	}
 
 	// Create hasher
@@ -115,7 +115,7 @@ func (m *SigningMethodRSAPSS) Sign(signingString string, key interface{}) ([]byt
 	case *rsa.PrivateKey:
 		rsaKey = k
 	default:
-		return nil, ErrInvalidKeyType
+		return nil, newError("RSA-PSS sign expects *rsa.PrivateKey", ErrInvalidKeyType)
 	}
 
 	// Create the hasher