Adding nip04 (encrypted direct messages) support (#8)

* implementing encrypted direct messages

* added encrypted direct message support

Co-authored-by: Raphael <raphjaph@protonmail.com>

nip04/nip04.go

@@ -0,0 +1,91 @@
+package nip04
+
+import (
+	"bytes"
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"encoding/base64"
+	"encoding/hex"
+	"fmt"
+	"strings"
+
+	"github.com/btcsuite/btcd/btcec"
+)
+
+// ECDH
+func ComputeSharedSecret(senderPrivKey string, receiverPubKey string) (sharedSecret []byte, err error) {
+	privKeyBytes, err := hex.DecodeString(senderPrivKey)
+	if err != nil {
+		return nil, fmt.Errorf("Error decoding sender private key: %s. \n", err)
+	}
+	privKey, _ := btcec.PrivKeyFromBytes(btcec.S256(), privKeyBytes)
+
+	// adding 02 to signal that this is a compressed public key (33 bytes)
+	pubKeyBytes, err := hex.DecodeString("02" + receiverPubKey)
+	if err != nil {
+		return nil, fmt.Errorf("Error decoding hex string of receiver public key: %s. \n", err)
+	}
+	pubKey, err := btcec.ParsePubKey(pubKeyBytes, btcec.S256())
+	if err != nil {
+		return nil, fmt.Errorf("Error parsing receiver public key: %s. \n", err)
+	}
+
+	return btcec.GenerateSharedSecret(privKey, pubKey), nil
+}
+
+// aes-256-cbc
+func Encrypt(message string, key []byte) (string, error) {
+	// block size is 16 bytes
+	iv := make([]byte, 16)
+	// can probably use a less expensive lib since IV has to only be unique; not perfectly random; math/rand?
+	_, err := rand.Read(iv)
+	if err != nil {
+		return "", fmt.Errorf("Error creating initization vector: %s. \n", err.Error())
+	}
+
+	// automatically picks aes-256 based on key length (32 bytes)
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return "", fmt.Errorf("Error creating block cipher: %s. \n", err.Error())
+	}
+	mode := cipher.NewCBCEncrypter(block, iv)
+
+	// PKCS5 padding
+	padding := block.BlockSize() - len([]byte(message))%block.BlockSize()
+	padtext := bytes.Repeat([]byte{byte(padding)}, padding)
+	paddedMsgBytes := append([]byte(message), padtext...)
+
+	ciphertext := make([]byte, len(paddedMsgBytes))
+	mode.CryptBlocks(ciphertext, paddedMsgBytes)
+
+	return base64.StdEncoding.EncodeToString(ciphertext) + "?iv=" + base64.StdEncoding.EncodeToString(iv), nil
+}
+
+// aes-256-cbc
+func Decrypt(content string, key []byte) (string, error) {
+	parts := strings.Split(content, "?iv=")
+	if len(parts) < 2 {
+		return "", fmt.Errorf("Error parsing encrypted message: no initilization vector. \n")
+	}
+
+	ciphertext, err := base64.StdEncoding.DecodeString(parts[0])
+	if err != nil {
+		return "", fmt.Errorf("Error decoding ciphertext from base64: %s. \n", err)
+	}
+
+	iv, err := base64.StdEncoding.DecodeString(parts[1])
+	if err != nil {
+		return "", fmt.Errorf("Error decoding iv from base64: %s. \n", err)
+	}
+
+	block, err := aes.NewCipher(key)
+	if err != nil {
+		return "", fmt.Errorf("Error creating block cipher: %s. \n", err.Error())
+	}
+	mode := cipher.NewCBCDecrypter(block, iv)
+	plaintext := make([]byte, len(ciphertext))
+	mode.CryptBlocks(plaintext, ciphertext)
+
+	return string(plaintext[:]), nil
+}