package main

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

func main() {
	// The GenerateKey method takes in a reader that returns random bits, and
	// the number of bits
	privateKey, err := rsa.GenerateKey(rand.Reader, 2048)
	if err != nil {
		panic(err)
	}

	// The public key is a part of the *rsa.PrivateKey struct
	publicKey := privateKey.PublicKey

	// use the public and private keys
	// ...
	// https://play.golang.org/p/tldFUt2c4nx
	modulusBytes := base64.StdEncoding.EncodeToString(privateKey.N.Bytes())
	privateExponentBytes := base64.StdEncoding.EncodeToString(privateKey.D.Bytes())
	fmt.Println(modulusBytes)
	fmt.Println(privateExponentBytes)
	fmt.Println(publicKey.E)

	encryptedBytes, err := rsa.EncryptOAEP(
		sha256.New(),
		rand.Reader,
		&publicKey,
		[]byte("super secret message"),
		nil)
	if err != nil {
		panic(err)
	}

	fmt.Println("encrypted bytes: ", encryptedBytes)

	// The first argument is an optional random data generator (the rand.Reader we used before)
	// we can set this value as nil
	// The OEAPOptions in the end signify that we encrypted the data using OEAP, and that we used
	// SHA256 to hash the input.
	decryptedBytes, err := privateKey.Decrypt(nil, encryptedBytes, &rsa.OAEPOptions{Hash: crypto.SHA256})
	if err != nil {
		panic(err)
	}

	// We get back the original information in the form of bytes, which we
	// the cast to a string and print
	fmt.Println("decrypted message: ", string(decryptedBytes))

msg := []byte("verifiable message")

// Before signing, we need to hash our message
// The hash is what we actually sign
msgHash := sha256.New()
_, err = msgHash.Write(msg)
if err != nil {
	panic(err)
}
msgHashSum := msgHash.Sum(nil)

// In order to generate the signature, we provide a random number generator,
// our private key, the hashing algorithm that we used, and the hash sum
// of our message
signature, err := rsa.SignPSS(rand.Reader, privateKey, crypto.SHA256, msgHashSum, nil)
if err != nil {
	panic(err)
}

// To verify the signature, we provide the public key, the hashing algorithm
// the hash sum of our message and the signature we generated previously
// there is an optional "options" parameter which can omit for now
err = rsa.VerifyPSS(&publicKey, crypto.SHA256, msgHashSum, signature, nil)
if err != nil {
	fmt.Println("could not verify signature: ", err)
	return
}
// If we don't get any error from the `VerifyPSS` method, that means our
// signature is valid
fmt.Println("signature verified")
}