crypto/cipher/example_test.go

// Copyright 2012 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package cipher_test

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"encoding/hex"
	"fmt"
	"io"
	"os"
)

func ExampleNewCBCDecrypter() {
	key := []byte("example key 1234")
	ciphertext, _ := hex.DecodeString("f363f3ccdcb12bb883abf484ba77d9cd7d32b5baecb3d4b1b3e0e4beffdb3ded")

	block, err := aes.NewCipher(key)
	if err != nil {
		panic(err)
	}

	// The IV needs to be unique, but not secure. Therefore it's common to
	// include it at the beginning of the ciphertext.
	if len(ciphertext) < aes.BlockSize {
		panic("ciphertext too short")
	}
	iv := ciphertext[:aes.BlockSize]
	ciphertext = ciphertext[aes.BlockSize:]

	// CBC mode always works in whole blocks.
	if len(ciphertext)%aes.BlockSize != 0 {
		panic("ciphertext is not a multiple of the block size")
	}

	mode := cipher.NewCBCDecrypter(block, iv)

	// CryptBlocks can work in-place if the two arguments are the same.
	mode.CryptBlocks(ciphertext, ciphertext)

	// If the original plaintext lengths are not a multiple of the block
	// size, padding would have to be added when encrypting, which would be
	// removed at this point. For an example, see
	// https://tools.ietf.org/html/rfc5246#section-6.2.3.2. However, it's
	// critical to note that ciphertexts must be authenticated (i.e. by
	// using crypto/hmac) before being decrypted in order to avoid creating
	// a padding oracle.

	fmt.Printf("%s\n", ciphertext)
	// Output: exampleplaintext
}

func ExampleNewCBCEncrypter() {
	key := []byte("example key 1234")
	plaintext := []byte("exampleplaintext")

	// CBC mode works on blocks so plaintexts may need to be padded to the
	// next whole block. For an example of such padding, see
	// https://tools.ietf.org/html/rfc5246#section-6.2.3.2. Here we'll
	// assume that the plaintext is already of the correct length.
	if len(plaintext)%aes.BlockSize != 0 {
		panic("plaintext is not a multiple of the block size")
	}

	block, err := aes.NewCipher(key)
	if err != nil {
		panic(err)
	}

	// The IV needs to be unique, but not secure. Therefore it's common to
	// include it at the beginning of the ciphertext.
	ciphertext := make([]byte, aes.BlockSize+len(plaintext))
	iv := ciphertext[:aes.BlockSize]
	if _, err := io.ReadFull(rand.Reader, iv); err != nil {
		panic(err)
	}

	mode := cipher.NewCBCEncrypter(block, iv)
	mode.CryptBlocks(ciphertext[aes.BlockSize:], plaintext)

	// It's important to remember that ciphertexts must be authenticated
	// (i.e. by using crypto/hmac) as well as being encrypted in order to
	// be secure.

	fmt.Printf("%x\n", ciphertext)
}

func ExampleNewCFBDecrypter() {
	key := []byte("example key 1234")
	ciphertext, _ := hex.DecodeString("22277966616d9bc47177bd02603d08c9a67d5380d0fe8cf3b44438dff7b9")

	block, err := aes.NewCipher(key)
	if err != nil {
		panic(err)
	}

	// The IV needs to be unique, but not secure. Therefore it's common to
	// include it at the beginning of the ciphertext.
	if len(ciphertext) < aes.BlockSize {
		panic("ciphertext too short")
	}
	iv := ciphertext[:aes.BlockSize]
	ciphertext = ciphertext[aes.BlockSize:]

	stream := cipher.NewCFBDecrypter(block, iv)

	// XORKeyStream can work in-place if the two arguments are the same.
	stream.XORKeyStream(ciphertext, ciphertext)
	fmt.Printf("%s", ciphertext)
	// Output: some plaintext
}

func ExampleNewCFBEncrypter() {
	key := []byte("example key 1234")
	plaintext := []byte("some plaintext")

	block, err := aes.NewCipher(key)
	if err != nil {
		panic(err)
	}

	// The IV needs to be unique, but not secure. Therefore it's common to
	// include it at the beginning of the ciphertext.
	ciphertext := make([]byte, aes.BlockSize+len(plaintext))
	iv := ciphertext[:aes.BlockSize]
	if _, err := io.ReadFull(rand.Reader, iv); err != nil {
		panic(err)
	}

	stream := cipher.NewCFBEncrypter(block, iv)
	stream.XORKeyStream(ciphertext[aes.BlockSize:], plaintext)

	// It's important to remember that ciphertexts must be authenticated
	// (i.e. by using crypto/hmac) as well as being encrypted in order to
	// be secure.
}

func ExampleNewCTR() {
	key := []byte("example key 1234")
	plaintext := []byte("some plaintext")

	block, err := aes.NewCipher(key)
	if err != nil {
		panic(err)
	}

	// The IV needs to be unique, but not secure. Therefore it's common to
	// include it at the beginning of the ciphertext.
	ciphertext := make([]byte, aes.BlockSize+len(plaintext))
	iv := ciphertext[:aes.BlockSize]
	if _, err := io.ReadFull(rand.Reader, iv); err != nil {
		panic(err)
	}

	stream := cipher.NewCTR(block, iv)
	stream.XORKeyStream(ciphertext[aes.BlockSize:], plaintext)

	// It's important to remember that ciphertexts must be authenticated
	// (i.e. by using crypto/hmac) as well as being encrypted in order to
	// be secure.

	// CTR mode is the same for both encryption and decryption, so we can
	// also decrypt that ciphertext with NewCTR.

	plaintext2 := make([]byte, len(plaintext))
	stream = cipher.NewCTR(block, iv)
	stream.XORKeyStream(plaintext2, ciphertext[aes.BlockSize:])

	fmt.Printf("%s\n", plaintext2)
	// Output: some plaintext
}

func ExampleNewOFB() {
	key := []byte("example key 1234")
	plaintext := []byte("some plaintext")

	block, err := aes.NewCipher(key)
	if err != nil {
		panic(err)
	}

	// The IV needs to be unique, but not secure. Therefore it's common to
	// include it at the beginning of the ciphertext.
	ciphertext := make([]byte, aes.BlockSize+len(plaintext))
	iv := ciphertext[:aes.BlockSize]
	if _, err := io.ReadFull(rand.Reader, iv); err != nil {
		panic(err)
	}

	stream := cipher.NewOFB(block, iv)
	stream.XORKeyStream(ciphertext[aes.BlockSize:], plaintext)

	// It's important to remember that ciphertexts must be authenticated
	// (i.e. by using crypto/hmac) as well as being encrypted in order to
	// be secure.

	// OFB mode is the same for both encryption and decryption, so we can
	// also decrypt that ciphertext with NewOFB.

	plaintext2 := make([]byte, len(plaintext))
	stream = cipher.NewOFB(block, iv)
	stream.XORKeyStream(plaintext2, ciphertext[aes.BlockSize:])

	fmt.Printf("%s\n", plaintext2)
	// Output: some plaintext
}

func ExampleStreamReader() {
	key := []byte("example key 1234")

	inFile, err := os.Open("encrypted-file")
	if err != nil {
		panic(err)
	}
	defer inFile.Close()

	block, err := aes.NewCipher(key)
	if err != nil {
		panic(err)
	}

	// If the key is unique for each ciphertext, then it's ok to use a zero
	// IV.
	var iv [aes.BlockSize]byte
	stream := cipher.NewOFB(block, iv[:])

	outFile, err := os.OpenFile("decrypted-file", os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0600)
	if err != nil {
		panic(err)
	}
	defer outFile.Close()

	reader := &cipher.StreamReader{S: stream, R: inFile}
	// Copy the input file to the output file, decrypting as we go.
	if _, err := io.Copy(outFile, reader); err != nil {
		panic(err)
	}

	// Note that this example is simplistic in that it omits any
	// authentication of the encrypted data. If you were actually to use
	// StreamReader in this manner, an attacker could flip arbitrary bits in
	// the output.
}

func ExampleStreamWriter() {
	key := []byte("example key 1234")

	inFile, err := os.Open("plaintext-file")
	if err != nil {
		panic(err)
	}
	defer inFile.Close()

	block, err := aes.NewCipher(key)
	if err != nil {
		panic(err)
	}

	// If the key is unique for each ciphertext, then it's ok to use a zero
	// IV.
	var iv [aes.BlockSize]byte
	stream := cipher.NewOFB(block, iv[:])

	outFile, err := os.OpenFile("encrypted-file", os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0600)
	if err != nil {
		panic(err)
	}
	defer outFile.Close()

	writer := &cipher.StreamWriter{S: stream, W: outFile}
	// Copy the input file to the output file, encrypting as we go.
	if _, err := io.Copy(writer, inFile); err != nil {
		panic(err)
	}

	// Note that this example is simplistic in that it omits any
	// authentication of the encrypted data. If you were actually to use
	// StreamReader in this manner, an attacker could flip arbitrary bits in
	// the decrypted result.
}