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package nostr
import (
"crypto/aes"
"crypto/cipher"
"crypto/rand"
"crypto/sha256"
"encoding/base64"
"encoding/hex"
"fmt"
"strings"
"github.com/btcsuite/btcd/btcec/v2"
)
// NIP04Encrypt encrypts a plaintext message for a recipient using NIP-04
// (ECDH shared secret + AES-256-CBC). The sender's private key and
// recipient's public key (64-char hex) are required.
// Returns the ciphertext in NIP-04 format: "<base64>?iv=<base64>"
func (k *Key) NIP04Encrypt(recipientPubHex, plaintext string) (string, error) {
if k.priv == nil {
return "", fmt.Errorf("cannot encrypt: public-only key")
}
shared, err := nip04SharedSecret(k.priv, recipientPubHex)
if err != nil {
return "", err
}
iv := make([]byte, 16)
if _, err := rand.Read(iv); err != nil {
return "", fmt.Errorf("generating iv: %w", err)
}
block, err := aes.NewCipher(shared)
if err != nil {
return "", fmt.Errorf("aes cipher: %w", err)
}
padded := pkcs7Pad([]byte(plaintext), aes.BlockSize)
ct := make([]byte, len(padded))
cipher.NewCBCEncrypter(block, iv).CryptBlocks(ct, padded)
return base64.StdEncoding.EncodeToString(ct) + "?iv=" + base64.StdEncoding.EncodeToString(iv), nil
}
// NIP04Decrypt decrypts a NIP-04 ciphertext using the receiver's private key
// and sender's public key (64-char hex).
func (k *Key) NIP04Decrypt(senderPubHex, ciphertext string) (string, error) {
if k.priv == nil {
return "", fmt.Errorf("cannot decrypt: public-only key")
}
parts := strings.SplitN(ciphertext, "?iv=", 2)
if len(parts) != 2 {
return "", fmt.Errorf("invalid NIP-04 ciphertext format")
}
ct, err := base64.StdEncoding.DecodeString(parts[0])
if err != nil {
return "", fmt.Errorf("decode ciphertext: %w", err)
}
iv, err := base64.StdEncoding.DecodeString(parts[1])
if err != nil {
return "", fmt.Errorf("decode iv: %w", err)
}
shared, err := nip04SharedSecret(k.priv, senderPubHex)
if err != nil {
return "", err
}
block, err := aes.NewCipher(shared)
if err != nil {
return "", fmt.Errorf("aes cipher: %w", err)
}
if len(ct)%aes.BlockSize != 0 {
return "", fmt.Errorf("ciphertext is not a multiple of block size")
}
plain := make([]byte, len(ct))
cipher.NewCBCDecrypter(block, iv).CryptBlocks(plain, ct)
unpadded, err := pkcs7Unpad(plain)
if err != nil {
return "", fmt.Errorf("unpad: %w", err)
}
return string(unpadded), nil
}
// nip04SharedSecret computes the ECDH shared secret per NIP-04:
// SHA256(privKey * recipientPubKey).x
func nip04SharedSecret(priv *btcec.PrivateKey, recipientPubHex string) ([]byte, error) {
pubBytes, err := hex.DecodeString(recipientPubHex)
if err != nil {
return nil, fmt.Errorf("decode pubkey: %w", err)
}
// NIP-04 pubkeys are x-only (32 bytes, Schnorr). Prefix with 0x02 for
// compressed SEC1 parsing.
if len(pubBytes) == 32 {
pubBytes = append([]byte{0x02}, pubBytes...)
}
pub, err := btcec.ParsePubKey(pubBytes)
if err != nil {
return nil, fmt.Errorf("parse pubkey: %w", err)
}
// ECDH: scalar multiply
var point btcec.JacobianPoint
pub.AsJacobian(&point)
priv.Key.SetByteSlice(priv.Serialize())
btcec.ScalarMultNonConst(&priv.Key, &point, &point)
point.ToAffine()
// Shared secret is SHA256 of the x-coordinate
xBytes := point.X.Bytes()
shared := sha256.Sum256(xBytes[:])
return shared[:], nil
}
func pkcs7Pad(data []byte, blockSize int) []byte {
pad := blockSize - len(data)%blockSize
padded := make([]byte, len(data)+pad)
copy(padded, data)
for i := len(data); i < len(padded); i++ {
padded[i] = byte(pad)
}
return padded
}
func pkcs7Unpad(data []byte) ([]byte, error) {
if len(data) == 0 {
return nil, fmt.Errorf("empty data")
}
pad := int(data[len(data)-1])
if pad == 0 || pad > aes.BlockSize {
return nil, fmt.Errorf("invalid padding size: %d", pad)
}
if len(data) < pad {
return nil, fmt.Errorf("data shorter than padding")
}
return data[:len(data)-pad], nil
}
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