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// Copyright (c) 2015-2016 The btcsuite developers
// Use of this source code is governed by an ISC
// license that can be found in the LICENSE file.
package btcec
import (
"bytes"
"crypto/aes"
"crypto/cipher"
"crypto/hmac"
"crypto/rand"
"crypto/sha256"
"crypto/sha512"
"errors"
"io"
)
var (
// ErrInvalidMAC occurs when Message Authentication Check (MAC) fails
// during decryption. This happens because of either invalid private key or
// corrupt ciphertext.
ErrInvalidMAC = errors.New("invalid mac hash")
// errInputTooShort occurs when the input ciphertext to the Decrypt
// function is less than 134 bytes long.
errInputTooShort = errors.New("ciphertext too short")
// errUnsupportedCurve occurs when the first two bytes of the encrypted
// text aren't 0x02CA (= 712 = secp256k1, from OpenSSL).
errUnsupportedCurve = errors.New("unsupported curve")
errInvalidXLength = errors.New("invalid X length, must be 32")
errInvalidYLength = errors.New("invalid Y length, must be 32")
errInvalidPadding = errors.New("invalid PKCS#7 padding")
// 0x02CA = 714
ciphCurveBytes = [2]byte{0x02, 0xCA}
// 0x20 = 32
ciphCoordLength = [2]byte{0x00, 0x20}
)
// GenerateSharedSecret generates a shared secret based on a private key and a
// public key using Diffie-Hellman key exchange (ECDH) (RFC 4753).
// RFC5903 Section 9 states we should only return x.
func GenerateSharedSecret(privkey *PrivateKey, pubkey *PublicKey) []byte {
x, _ := pubkey.Curve.ScalarMult(pubkey.X, pubkey.Y, privkey.D.Bytes())
return x.Bytes()
}
// Encrypt encrypts data for the target public key using AES-256-CBC. It also
// generates a private key (the pubkey of which is also in the output). The only
// supported curve is secp256k1. The `structure' that it encodes everything into
// is:
//
// struct {
// // Initialization Vector used for AES-256-CBC
// IV [16]byte
// // Public Key: curve(2) + len_of_pubkeyX(2) + pubkeyX +
// // len_of_pubkeyY(2) + pubkeyY (curve = 714)
// PublicKey [70]byte
// // Cipher text
// Data []byte
// // HMAC-SHA-256 Message Authentication Code
// HMAC [32]byte
// }
//
// The primary aim is to ensure byte compatibility with Pyelliptic. Also, refer
// to section 5.8.1 of ANSI X9.63 for rationale on this format.
func Encrypt(pubkey *PublicKey, in []byte) ([]byte, error) {
ephemeral, err := NewPrivateKey(S256())
if err != nil {
return nil, err
}
ecdhKey := GenerateSharedSecret(ephemeral, pubkey)
derivedKey := sha512.Sum512(ecdhKey)
keyE := derivedKey[:32]
keyM := derivedKey[32:]
paddedIn := addPKCSPadding(in)
// IV + Curve params/X/Y + padded plaintext/ciphertext + HMAC-256
out := make([]byte, aes.BlockSize+70+len(paddedIn)+sha256.Size)
iv := out[:aes.BlockSize]
if _, err = io.ReadFull(rand.Reader, iv); err != nil {
return nil, err
}
// start writing public key
pb := ephemeral.PubKey().SerializeUncompressed()
offset := aes.BlockSize
// curve and X length
copy(out[offset:offset+4], append(ciphCurveBytes[:], ciphCoordLength[:]...))
offset += 4
// X
copy(out[offset:offset+32], pb[1:33])
offset += 32
// Y length
copy(out[offset:offset+2], ciphCoordLength[:])
offset += 2
// Y
copy(out[offset:offset+32], pb[33:])
offset += 32
// start encryption
block, err := aes.NewCipher(keyE)
if err != nil {
return nil, err
}
mode := cipher.NewCBCEncrypter(block, iv)
mode.CryptBlocks(out[offset:len(out)-sha256.Size], paddedIn)
// start HMAC-SHA-256
hm := hmac.New(sha256.New, keyM)
hm.Write(out[:len(out)-sha256.Size]) // everything is hashed
copy(out[len(out)-sha256.Size:], hm.Sum(nil)) // write checksum
return out, nil
}
// Decrypt decrypts data that was encrypted using the Encrypt function.
func Decrypt(priv *PrivateKey, in []byte) ([]byte, error) {
// IV + Curve params/X/Y + 1 block + HMAC-256
if len(in) < aes.BlockSize+70+aes.BlockSize+sha256.Size {
return nil, errInputTooShort
}
// read iv
iv := in[:aes.BlockSize]
offset := aes.BlockSize
// start reading pubkey
if !bytes.Equal(in[offset:offset+2], ciphCurveBytes[:]) {
return nil, errUnsupportedCurve
}
offset += 2
if !bytes.Equal(in[offset:offset+2], ciphCoordLength[:]) {
return nil, errInvalidXLength
}
offset += 2
xBytes := in[offset : offset+32]
offset += 32
if !bytes.Equal(in[offset:offset+2], ciphCoordLength[:]) {
return nil, errInvalidYLength
}
offset += 2
yBytes := in[offset : offset+32]
offset += 32
pb := make([]byte, 65)
pb[0] = byte(0x04) // uncompressed
copy(pb[1:33], xBytes)
copy(pb[33:], yBytes)
// check if (X, Y) lies on the curve and create a Pubkey if it does
pubkey, err := ParsePubKey(pb, S256())
if err != nil {
return nil, err
}
// check for cipher text length
if (len(in)-aes.BlockSize-offset-sha256.Size)%aes.BlockSize != 0 {
return nil, errInvalidPadding // not padded to 16 bytes
}
// read hmac
messageMAC := in[len(in)-sha256.Size:]
// generate shared secret
ecdhKey := GenerateSharedSecret(priv, pubkey)
derivedKey := sha512.Sum512(ecdhKey)
keyE := derivedKey[:32]
keyM := derivedKey[32:]
// verify mac
hm := hmac.New(sha256.New, keyM)
hm.Write(in[:len(in)-sha256.Size]) // everything is hashed
expectedMAC := hm.Sum(nil)
if !hmac.Equal(messageMAC, expectedMAC) {
return nil, ErrInvalidMAC
}
// start decryption
block, err := aes.NewCipher(keyE)
if err != nil {
return nil, err
}
mode := cipher.NewCBCDecrypter(block, iv)
// same length as ciphertext
plaintext := make([]byte, len(in)-offset-sha256.Size)
mode.CryptBlocks(plaintext, in[offset:len(in)-sha256.Size])
return removePKCSPadding(plaintext)
}
// Implement PKCS#7 padding with block size of 16 (AES block size).
// addPKCSPadding adds padding to a block of data
func addPKCSPadding(src []byte) []byte {
padding := aes.BlockSize - len(src)%aes.BlockSize
padtext := bytes.Repeat([]byte{byte(padding)}, padding)
return append(src, padtext...)
}
// removePKCSPadding removes padding from data that was added with addPKCSPadding
func removePKCSPadding(src []byte) ([]byte, error) {
length := len(src)
padLength := int(src[length-1])
if padLength > aes.BlockSize || length < aes.BlockSize {
return nil, errInvalidPadding
}
return src[:length-padLength], nil
}
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