crypto/cipher: add examples
Fixes #1390.
R=golang-dev, minux.ma, adg, agl
CC=golang-dev
https://golang.org/cl/6631044
diff --git a/src/pkg/crypto/cipher/example_test.go b/src/pkg/crypto/cipher/example_test.go
new file mode 100644
index 0000000..c888eb2
--- /dev/null
+++ b/src/pkg/crypto/cipher/example_test.go
@@ -0,0 +1,283 @@
+// 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{stream, 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. It you were actually to use
+ // StreamReader in this manner, an attacker could flip arbitary 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{stream, outFile, nil}
+ // 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. It you were actually to use
+ // StreamReader in this manner, an attacker could flip arbitary bits in
+ // the decrypted result.
+}
