algorithm.go

package rtu

import (
	"crypto/sha256"
	"fmt"

	"github.com/lestrrat-go/jwx/v3/jwa"
)

type SignatureAlgorithm string

// ParseJwa takes a jwa.SignatureAlgorithm and parses a SignatureAlgorithm. (example "ES256" -> "ecdsa-p256")
func ParseJwa(alg jwa.SignatureAlgorithm) (SignatureAlgorithm, error) {
	switch alg {
	case jwa.EmptySignatureAlgorithm():
		return AlgorithmNone, nil
	case jwa.ES256():
		return AlgorithmEcdsaP256, nil
	default:
		return AlgorithmNone, fmt.Errorf("%w: unknown jwa algorithm: %s", ErrUnknownSignatureAlgorithm, alg)
	}
}

const (
	// AlgorithmNone is used when there is no algorithm given in the RTU
	AlgorithmNone SignatureAlgorithm = ""
	// AlgorithmEcdsaP256 is the signature algorithm for ECDSA with P-256 curve, signing a SHA256 hash.
	AlgorithmEcdsaP256 SignatureAlgorithm = "ecdsa-p256"
)

// Digest returns the hash version of payload based on the SignatureAlgorithm given.
// if returned value is nil, it should be treated the same as an ErrSignatureAlgorithmInvalid.
func (s SignatureAlgorithm) Digest(payload []byte) []byte {
	switch s {
	case AlgorithmEcdsaP256:
		hash := sha256.New()
		hash.Write(payload)
		return hash.Sum(nil)
	default:
		return nil
	}
}

// Validate validates the SignatureAlgorithm, to ensure this library supports it fully
func (s SignatureAlgorithm) Validate() error {
	switch s {
	case AlgorithmNone:
		return ErrNoSignatureAlgorithm
	case AlgorithmEcdsaP256:
		return nil
	default:
		return fmt.Errorf("%w: %s", ErrUnknownSignatureAlgorithm, s)
	}
}

// ToJWA returns the JSON Web Token Algorithm string for the given SignatureAlgorithm
func (s SignatureAlgorithm) ToJWA() (jwa.SignatureAlgorithm, error) {
	switch s {
	case AlgorithmNone:
		return jwa.EmptySignatureAlgorithm(), ErrNoSignatureAlgorithm
	case AlgorithmEcdsaP256:
		return jwa.ES256(), nil
	default:
		return jwa.EmptySignatureAlgorithm(), fmt.Errorf("%w: %s", ErrUnknownSignatureAlgorithm, s)
	}
}