ecencryption.go

package main

import (
    "crypto/aes"
    "crypto/cipher"
    "crypto/ecdsa"
    "crypto/elliptic"
    "crypto/rand"
    "crypto/sha256"
    "encoding/hex"
    "encoding/binary"
    "fmt"
)

func main() {
    curve := elliptic.P256()
    
    // ephemeral key
    
    ephemeraPrivateKey, err := ecdsa.GenerateKey(curve, rand.Reader)
    
    if err != nil {
        fmt.Println(err)
        return
    }
    
    ephemeralPublicKey := elliptic.Marshal(curve, ephemeraPrivateKey.X, ephemeraPrivateKey.Y)
    
    otherPublicKeyBytes, _ := hex.DecodeString("0460292C66AC34D5E7EACAA90F750EF0930567172CED2A0EAAB8CB51C025BBA8DB82D0187A67361DDB742EDD9D65103B7FCED10A6A3372B7CB6CF1F001D97A690B")
    otherPublicKeyX, otherPublicKeyY := elliptic.Unmarshal(curve, otherPublicKeyBytes)
    
    // ECDH
    
    x, _ := curve.ScalarMult(otherPublicKeyX, otherPublicKeyY, ephemeraPrivateKey.D.Bytes())
    shared_key := x.Bytes()
    
    // X963 KDF
    
    length := 32
    output := make([]byte, 0)
    outlen := 0
    counter := uint32(1)
    
    for outlen < length {
        h := sha256.New()
        h.Write(shared_key) // Key Material: ECDH Key
        
        counterBuf := make([]byte, 4)
        binary.BigEndian.PutUint32(counterBuf, counter)
        h.Write(counterBuf)
        
        h.Write(ephemeralPublicKey) // Shared Info: Our public key
        
        output = h.Sum(output)
        outlen += h.Size()
        counter += 1
    }
    
    // Key
    
    encryptionKey := output[0:16]
    iv := output[16:]
    
    fmt.Println(hex.EncodeToString(encryptionKey))
    fmt.Println(hex.EncodeToString(iv))
    
    // AES
    
    block, _ := aes.NewCipher(encryptionKey)
    aesgcm, _ := cipher.NewGCMWithNonceSize(block, 16)
    
    ct := aesgcm.Seal(nil, iv, []byte("Hello World"), nil)
    
    fmt.Println(hex.EncodeToString(ephemeralPublicKey) + hex.EncodeToString(ct))
}