server.go

package main

import (
	"crypto/rand"
	"crypto/rsa"
	"crypto/sha512"
	"crypto/x509"
	"encoding/pem"
	"fmt"
	"net/http"

	"github.com/labstack/echo"
	"github.com/labstack/echo/middleware"
)

type Buffer struct {
	Type string `json:"type"`
	Data []byte `json:"data"`
}

type JsonOut struct {
	Key Buffer `json:"key"`
	Iv  Buffer `json:"iv"`
}

func main() {
	e := echo.New()
	cors := middleware.CORSWithConfig(middleware.DefaultCORSConfig)
	e.Use(cors)

	e.POST("/download-text", func(c echo.Context) error {

		body := echo.Map{}
		if err := c.Bind(&body); err != nil {
			return err
		}
		publicKey := body["public"].(string)
		key, iv := generateKey(publicKey)

		out := &JsonOut{
			Key: Buffer{Type: "Buffer", Data: []byte(key)},
			Iv:  Buffer{Type: "Buffer", Data: []byte(iv)},
		}
		return c.JSON(http.StatusOK, out)
	})
	e.Logger.Fatal(e.Start(":3001"))
}

func generateKey(publicKey string) ([]byte, []byte) {
	key := string(randomBytes(32))
	iv := randomBytes(16)
	encrypted := EncryptWithPublicKey([]byte(key), BytesToPublicKey([]byte(publicKey)))

	return encrypted, iv
}

func EncryptWithPublicKey(msg []byte, pub *rsa.PublicKey) []byte {
	hash := sha512.New()
	ciphertext, err := rsa.EncryptOAEP(hash, rand.Reader, pub, msg, nil)
	if err != nil {
		fmt.Println(err)
	}
	return ciphertext
}

func BytesToPublicKey(pub []byte) *rsa.PublicKey {
	block, _ := pem.Decode(pub)
	enc := x509.IsEncryptedPEMBlock(block)
	b := block.Bytes
	var err error
	if enc {
		fmt.Println("is encrypted pem block")
		b, err = x509.DecryptPEMBlock(block, nil)
		if err != nil {
			fmt.Println(err)
		}
	}
	ifc, err := x509.ParsePKIXPublicKey(b)
	if err != nil {
		fmt.Println(err)
	}
	key, ok := ifc.(*rsa.PublicKey)
	if !ok {
		fmt.Println("not ok")
	}
	return key
}

func randomBytes(size int) []byte {
	b := make([]byte, size)
	_, err := rand.Read(b)
	if err != nil {
		fmt.Println("error:", err)
		return make([]byte, 0)
	}
	return b
}