cipher

package
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 Go to latest Published: Jun 1, 2013 License: BSD-3-Clause Imports: 1 Imported by: 0

Documentation

Overview

Package cipher implements standard block cipher modes that can be wrapped around low-level block cipher implementations. See http://csrc.nist.gov/groups/ST/toolkit/BCM/current_modes.html and NIST Special Publication 800-38A.

Index

Examples

Constants

This section is empty.

Variables

This section is empty.

Functions

This section is empty.

Types

type Block 

type Block interface {
	// BlockSize returns the cipher's block size.
	BlockSize() int

	// Encrypt encrypts the first block in src into dst.
	// Dst and src may point at the same memory.
	Encrypt(dst, src []byte)

	// Decrypt decrypts the first block in src into dst.
	// Dst and src may point at the same memory.
	Decrypt(dst, src []byte)
}

A Block represents an implementation of block cipher using a given key. It provides the capability to encrypt or decrypt individual blocks. The mode implementations extend that capability to streams of blocks.

type BlockMode 

type BlockMode interface {
	// BlockSize returns the mode's block size.
	BlockSize() int

	// CryptBlocks encrypts or decrypts a number of blocks. The length of
	// src must be a multiple of the block size. Dst and src may point to
	// the same memory.
	CryptBlocks(dst, src []byte)
}

A BlockMode represents a block cipher running in a block-based mode (CBC, ECB etc).

func NewCBCDecrypter 

func NewCBCDecrypter(b Block, iv []byte) BlockMode

NewCBCDecrypter returns a BlockMode which decrypts in cipher block chaining mode, using the given Block. The length of iv must be the same as the Block's block size and must match the iv used to encrypt the data.

Example 
package main

import (
	"crypto/aes"
	"crypto/cipher"
	"encoding/hex"
	"fmt"
)

func main() {
	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 NewCBCEncrypter 

func NewCBCEncrypter(b Block, iv []byte) BlockMode

NewCBCEncrypter returns a BlockMode which encrypts in cipher block chaining mode, using the given Block. The length of iv must be the same as the Block's block size.

Example 
package main

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"fmt"
	"io"
)

func main() {
	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)
}

Output:

type Stream 

type Stream interface {
	// XORKeyStream XORs each byte in the given slice with a byte from the
	// cipher's key stream. Dst and src may point to the same memory.
	XORKeyStream(dst, src []byte)
}

A Stream represents a stream cipher.

func NewCFBDecrypter 

func NewCFBDecrypter(block Block, iv []byte) Stream

NewCFBDecrypter returns a Stream which decrypts with cipher feedback mode, using the given Block. The iv must be the same length as the Block's block size.

Example 
package main

import (
	"crypto/aes"
	"crypto/cipher"
	"encoding/hex"
	"fmt"
)

func main() {
	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 NewCFBEncrypter 

func NewCFBEncrypter(block Block, iv []byte) Stream

NewCFBEncrypter returns a Stream which encrypts with cipher feedback mode, using the given Block. The iv must be the same length as the Block's block size.

Example 
package main

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"io"
)

func main() {
	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.
}

Output:

func NewCTR 

func NewCTR(block Block, iv []byte) Stream

NewCTR returns a Stream which encrypts/decrypts using the given Block in counter mode. The length of iv must be the same as the Block's block size.

Example 
package main

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"fmt"
	"io"
)

func main() {
	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 NewOFB 

func NewOFB(b Block, iv []byte) Stream

NewOFB returns a Stream that encrypts or decrypts using the block cipher b in output feedback mode. The initialization vector iv's length must be equal to b's block size.

Example 
package main

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"fmt"
	"io"
)

func main() {
	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

type StreamReader 

type StreamReader struct {
	S Stream
	R io.Reader
}

StreamReader wraps a Stream into an io.Reader. It calls XORKeyStream to process each slice of data which passes through.

Example 
package main

import (
	"crypto/aes"
	"crypto/cipher"
	"io"
	"os"
)

func main() {
	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. It you were actually to use
	// StreamReader in this manner, an attacker could flip arbitrary bits in
	// the output.
}

Output:

func (StreamReader) Read 

func (r StreamReader) Read(dst []byte) (n int, err error)

type StreamWriter 

type StreamWriter struct {
	S   Stream
	W   io.Writer
	Err error // unused
}

StreamWriter wraps a Stream into an io.Writer. It calls XORKeyStream to process each slice of data which passes through. If any Write call returns short then the StreamWriter is out of sync and must be discarded.

Example 
package main

import (
	"crypto/aes"
	"crypto/cipher"
	"io"
	"os"
)

func main() {
	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. It you were actually to use
	// StreamReader in this manner, an attacker could flip arbitrary bits in
	// the decrypted result.
}

Output:

func (StreamWriter) Close 

func (w StreamWriter) Close() error

func (StreamWriter) Write 

func (w StreamWriter) Write(src []byte) (n int, err error)

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