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crypto/cipher: add examples
Fixes #1390. R=golang-dev, minux.ma, adg, agl CC=golang-dev https://golang.org/cl/6631044
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Adam Langley
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0cbca268d8
commit
5176481f16
283
src/pkg/crypto/cipher/example_test.go
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283
src/pkg/crypto/cipher/example_test.go
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// Copyright 2012 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package cipher_test
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import (
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"crypto/aes"
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"crypto/cipher"
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"crypto/rand"
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"encoding/hex"
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"fmt"
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"io"
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"os"
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)
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func ExampleNewCBCDecrypter() {
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key := []byte("example key 1234")
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ciphertext, _ := hex.DecodeString("f363f3ccdcb12bb883abf484ba77d9cd7d32b5baecb3d4b1b3e0e4beffdb3ded")
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block, err := aes.NewCipher(key)
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if err != nil {
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panic(err)
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}
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// The IV needs to be unique, but not secure. Therefore it's common to
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// include it at the beginning of the ciphertext.
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if len(ciphertext) < aes.BlockSize {
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panic("ciphertext too short")
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}
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iv := ciphertext[:aes.BlockSize]
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ciphertext = ciphertext[aes.BlockSize:]
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// CBC mode always works in whole blocks.
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if len(ciphertext)%aes.BlockSize != 0 {
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panic("ciphertext is not a multiple of the block size")
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}
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mode := cipher.NewCBCDecrypter(block, iv)
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// CryptBlocks can work in-place if the two arguments are the same.
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mode.CryptBlocks(ciphertext, ciphertext)
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// If the original plaintext lengths are not a multiple of the block
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// size, padding would have to be added when encrypting, which would be
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// removed at this point. For an example, see
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// https://tools.ietf.org/html/rfc5246#section-6.2.3.2. However, it's
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// critical to note that ciphertexts must be authenticated (i.e. by
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// using crypto/hmac) before being decrypted in order to avoid creating
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// a padding oracle.
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fmt.Printf("%s\n", ciphertext)
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// Output: exampleplaintext
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}
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func ExampleNewCBCEncrypter() {
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key := []byte("example key 1234")
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plaintext := []byte("exampleplaintext")
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// CBC mode works on blocks so plaintexts may need to be padded to the
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// next whole block. For an example of such padding, see
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// https://tools.ietf.org/html/rfc5246#section-6.2.3.2. Here we'll
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// assume that the plaintext is already of the correct length.
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if len(plaintext)%aes.BlockSize != 0 {
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panic("plaintext is not a multiple of the block size")
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}
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block, err := aes.NewCipher(key)
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if err != nil {
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panic(err)
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}
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// The IV needs to be unique, but not secure. Therefore it's common to
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// include it at the beginning of the ciphertext.
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ciphertext := make([]byte, aes.BlockSize+len(plaintext))
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iv := ciphertext[:aes.BlockSize]
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if _, err := io.ReadFull(rand.Reader, iv); err != nil {
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panic(err)
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}
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mode := cipher.NewCBCEncrypter(block, iv)
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mode.CryptBlocks(ciphertext[aes.BlockSize:], plaintext)
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// It's important to remember that ciphertexts must be authenticated
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// (i.e. by using crypto/hmac) as well as being encrypted in order to
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// be secure.
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fmt.Printf("%x\n", ciphertext)
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}
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func ExampleNewCFBDecrypter() {
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key := []byte("example key 1234")
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ciphertext, _ := hex.DecodeString("22277966616d9bc47177bd02603d08c9a67d5380d0fe8cf3b44438dff7b9")
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block, err := aes.NewCipher(key)
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if err != nil {
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panic(err)
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}
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// The IV needs to be unique, but not secure. Therefore it's common to
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// include it at the beginning of the ciphertext.
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if len(ciphertext) < aes.BlockSize {
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panic("ciphertext too short")
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}
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iv := ciphertext[:aes.BlockSize]
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ciphertext = ciphertext[aes.BlockSize:]
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stream := cipher.NewCFBDecrypter(block, iv)
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// XORKeyStream can work in-place if the two arguments are the same.
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stream.XORKeyStream(ciphertext, ciphertext)
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fmt.Printf("%s", ciphertext)
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// Output: some plaintext
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}
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func ExampleNewCFBEncrypter() {
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key := []byte("example key 1234")
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plaintext := []byte("some plaintext")
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block, err := aes.NewCipher(key)
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if err != nil {
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panic(err)
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}
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// The IV needs to be unique, but not secure. Therefore it's common to
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// include it at the beginning of the ciphertext.
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ciphertext := make([]byte, aes.BlockSize+len(plaintext))
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iv := ciphertext[:aes.BlockSize]
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if _, err := io.ReadFull(rand.Reader, iv); err != nil {
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panic(err)
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}
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stream := cipher.NewCFBEncrypter(block, iv)
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stream.XORKeyStream(ciphertext[aes.BlockSize:], plaintext)
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// It's important to remember that ciphertexts must be authenticated
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// (i.e. by using crypto/hmac) as well as being encrypted in order to
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// be secure.
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}
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func ExampleNewCTR() {
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key := []byte("example key 1234")
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plaintext := []byte("some plaintext")
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block, err := aes.NewCipher(key)
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if err != nil {
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panic(err)
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}
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// The IV needs to be unique, but not secure. Therefore it's common to
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// include it at the beginning of the ciphertext.
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ciphertext := make([]byte, aes.BlockSize+len(plaintext))
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iv := ciphertext[:aes.BlockSize]
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if _, err := io.ReadFull(rand.Reader, iv); err != nil {
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panic(err)
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}
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stream := cipher.NewCTR(block, iv)
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stream.XORKeyStream(ciphertext[aes.BlockSize:], plaintext)
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// It's important to remember that ciphertexts must be authenticated
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// (i.e. by using crypto/hmac) as well as being encrypted in order to
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// be secure.
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// CTR mode is the same for both encryption and decryption, so we can
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// also decrypt that ciphertext with NewCTR.
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plaintext2 := make([]byte, len(plaintext))
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stream = cipher.NewCTR(block, iv)
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stream.XORKeyStream(plaintext2, ciphertext[aes.BlockSize:])
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fmt.Printf("%s\n", plaintext2)
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// Output: some plaintext
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}
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func ExampleNewOFB() {
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key := []byte("example key 1234")
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plaintext := []byte("some plaintext")
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block, err := aes.NewCipher(key)
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if err != nil {
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panic(err)
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}
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// The IV needs to be unique, but not secure. Therefore it's common to
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// include it at the beginning of the ciphertext.
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ciphertext := make([]byte, aes.BlockSize+len(plaintext))
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iv := ciphertext[:aes.BlockSize]
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if _, err := io.ReadFull(rand.Reader, iv); err != nil {
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panic(err)
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}
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stream := cipher.NewOFB(block, iv)
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stream.XORKeyStream(ciphertext[aes.BlockSize:], plaintext)
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// It's important to remember that ciphertexts must be authenticated
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// (i.e. by using crypto/hmac) as well as being encrypted in order to
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// be secure.
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// OFB mode is the same for both encryption and decryption, so we can
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// also decrypt that ciphertext with NewOFB.
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plaintext2 := make([]byte, len(plaintext))
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stream = cipher.NewOFB(block, iv)
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stream.XORKeyStream(plaintext2, ciphertext[aes.BlockSize:])
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fmt.Printf("%s\n", plaintext2)
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// Output: some plaintext
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}
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func ExampleStreamReader() {
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key := []byte("example key 1234")
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inFile, err := os.Open("encrypted-file")
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if err != nil {
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panic(err)
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}
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defer inFile.Close()
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block, err := aes.NewCipher(key)
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if err != nil {
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panic(err)
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}
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// If the key is unique for each ciphertext, then it's ok to use a zero
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// IV.
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var iv [aes.BlockSize]byte
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stream := cipher.NewOFB(block, iv[:])
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outFile, err := os.OpenFile("decrypted-file", os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0600)
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if err != nil {
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panic(err)
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}
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defer outFile.Close()
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reader := &cipher.StreamReader{stream, inFile}
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// Copy the input file to the output file, decrypting as we go.
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if _, err := io.Copy(outFile, reader); err != nil {
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panic(err)
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}
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// Note that this example is simplistic in that it omits any
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// authentication of the encrypted data. It you were actually to use
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// StreamReader in this manner, an attacker could flip arbitary bits in
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// the output.
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}
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func ExampleStreamWriter() {
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key := []byte("example key 1234")
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inFile, err := os.Open("plaintext-file")
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if err != nil {
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panic(err)
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}
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defer inFile.Close()
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block, err := aes.NewCipher(key)
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if err != nil {
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panic(err)
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}
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// If the key is unique for each ciphertext, then it's ok to use a zero
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// IV.
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var iv [aes.BlockSize]byte
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stream := cipher.NewOFB(block, iv[:])
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outFile, err := os.OpenFile("encrypted-file", os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0600)
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if err != nil {
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panic(err)
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}
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defer outFile.Close()
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writer := &cipher.StreamWriter{stream, outFile, nil}
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// Copy the input file to the output file, encrypting as we go.
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if _, err := io.Copy(writer, inFile); err != nil {
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panic(err)
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}
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// Note that this example is simplistic in that it omits any
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// authentication of the encrypted data. It you were actually to use
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// StreamReader in this manner, an attacker could flip arbitary bits in
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// the decrypted result.
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}
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