package main

import (
	"crypto/rand"
	"crypto/rsa"
	"crypto/sha256"
	"crypto/x509"
	"encoding/base64"
	"encoding/pem"
	"errors"
	"fmt"
)

func decodeMessage(ciphertext []byte, stringKey string) ([]byte, error) {
	block, _ := pem.Decode([]byte(stringKey))
	if block == nil {
		return nil, errors.New("key is not found in given string")
	}

	key, err := x509.ParsePKCS1PrivateKey(block.Bytes)
	if err != nil {
		return nil, err
	}

	plaintext, err := rsa.DecryptPKCS1v15(rand.Reader, key, ciphertext)
	if err != nil {
		return nil, err
	}
	return plaintext, err
}

func checkSignature(payload string, signature string, key string) (bool, error) {
	h := sha256.Sum256([]byte(payload))
	requestHash := fmt.Sprintf("%x", h)

	decodedSign, err := base64.StdEncoding.DecodeString(signature)
	if err != nil {
		return false, err
	}
	signHash, err := decodeMessage(decodedSign, key)
	if err != nil {
		return false, err
	}

	return requestHash == string(signHash), nil
}