package security

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
	"slices"
)

func (s *Checker) VerifySignature(signature, path string) error {
	if len(s.config.Keys) == 0 || len(s.config.Salts) == 0 {
		return nil
	}

	if slices.Contains(s.config.TrustedSignatures, signature) {
		return nil
	}

	messageMAC, err := base64.RawURLEncoding.DecodeString(signature)
	if err != nil {
		return newSignatureError("Invalid signature encoding")
	}

	for i := 0; i < len(s.config.Keys); i++ {
		if hmac.Equal(messageMAC, signatureFor(path, s.config.Keys[i], s.config.Salts[i], s.config.SignatureSize)) {
			return nil
		}
	}

	return newSignatureError("Invalid signature")
}

func signatureFor(str string, key, salt []byte, signatureSize int) []byte {
	mac := hmac.New(sha256.New, key)
	mac.Write(salt)

	// It's supposed that path starts with '/'. However, if and input path comes with the
	// leading slash split, let's re-add it here.
	if str[0] != '/' {
		mac.Write([]byte{'/'})
	}

	mac.Write([]byte(str))
	expectedMAC := mac.Sum(nil)
	if signatureSize < 32 {
		return expectedMAC[:signatureSize]
	}
	return expectedMAC
}