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Package cipher

import "crypto/cipher"
Overview
Index
Examples

Overview ▾

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

type AEAD 1.2

AEAD is a cipher mode providing authenticated encryption with associated data. For a description of the methodology, see https://en.wikipedia.org/wiki/Authenticated_encryption. 

type AEAD interface {
    // NonceSize returns the size of the nonce that must be passed to Seal
    // and Open.
    NonceSize() int

    // Overhead returns the maximum difference between the lengths of a
    // plaintext and its ciphertext.
    Overhead() int

    // Seal encrypts and authenticates plaintext, authenticates the
    // additional data and appends the result to dst, returning the updated
    // slice. The nonce must be NonceSize() bytes long and unique for all
    // time, for a given key.
    //
    // To reuse plaintext's storage for the encrypted output, use plaintext[:0]
    // as dst. Otherwise, the remaining capacity of dst must not overlap plaintext.
    Seal(dst, nonce, plaintext, additionalData []byte) []byte

    // Open decrypts and authenticates ciphertext, authenticates the
    // additional data and, if successful, appends the resulting plaintext
    // to dst, returning the updated slice. The nonce must be NonceSize()
    // bytes long and both it and the additional data must match the
    // value passed to Seal.
    //
    // To reuse ciphertext's storage for the decrypted output, use ciphertext[:0]
    // as dst. Otherwise, the remaining capacity of dst must not overlap plaintext.
    //
    // Even if the function fails, the contents of dst, up to its capacity,
    // may be overwritten.
    Open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error)
}

func NewGCM 1.2 

func NewGCM(cipher Block) (AEAD, error)

NewGCM returns the given 128-bit, block cipher wrapped in Galois Counter Mode with the standard nonce length.

In general, the GHASH operation performed by this implementation of GCM is not constant-time. An exception is when the underlying Block was created by aes.NewCipher on systems with hardware support for AES. See the crypto/aes package documentation for details.

▾ Example (Decrypt)

Code:

// Load your secret key from a safe place and reuse it across multiple
// Seal/Open calls. (Obviously don't use this example key for anything
// real.) If you want to convert a passphrase to a key, use a suitable
// package like bcrypt or scrypt.
// When decoded the key should be 16 bytes (AES-128) or 32 (AES-256).
key, _ := hex.DecodeString("6368616e676520746869732070617373776f726420746f206120736563726574")
ciphertext, _ := hex.DecodeString("c3aaa29f002ca75870806e44086700f62ce4d43e902b3888e23ceff797a7a471")
nonce, _ := hex.DecodeString("64a9433eae7ccceee2fc0eda")

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

aesgcm, err := cipher.NewGCM(block)
if err != nil {
    panic(err.Error())
}

plaintext, err := aesgcm.Open(nil, nonce, ciphertext, nil)
if err != nil {
    panic(err.Error())
}

fmt.Printf("%s\n", plaintext)

Output:

exampleplaintext

▾ Example (Encrypt)

Code:

// Load your secret key from a safe place and reuse it across multiple
// Seal/Open calls. (Obviously don't use this example key for anything
// real.) If you want to convert a passphrase to a key, use a suitable
// package like bcrypt or scrypt.
// When decoded the key should be 16 bytes (AES-128) or 32 (AES-256).
key, _ := hex.DecodeString("6368616e676520746869732070617373776f726420746f206120736563726574")
plaintext := []byte("exampleplaintext")

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

// Never use more than 2^32 random nonces with a given key because of the risk of a repeat.
nonce := make([]byte, 12)
if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
    panic(err.Error())
}

aesgcm, err := cipher.NewGCM(block)
if err != nil {
    panic(err.Error())
}

ciphertext := aesgcm.Seal(nil, nonce, plaintext, nil)
fmt.Printf("%x\n", ciphertext)

func NewGCMWithNonceSize 1.5 

func NewGCMWithNonceSize(cipher Block, size int) (AEAD, error)

NewGCMWithNonceSize returns the given 128-bit, block cipher wrapped in Galois Counter Mode, which accepts nonces of the given length. The length must not be zero.

Only use this function if you require compatibility with an existing cryptosystem that uses non-standard nonce lengths. All other users should use NewGCM, which is faster and more resistant to misuse.

func NewGCMWithTagSize 1.11 

func NewGCMWithTagSize(cipher Block, tagSize int) (AEAD, error)

NewGCMWithTagSize returns the given 128-bit, block cipher wrapped in Galois Counter Mode, which generates tags with the given length.

Tag sizes between 12 and 16 bytes are allowed.

Only use this function if you require compatibility with an existing cryptosystem that uses non-standard tag lengths. All other users should use NewGCM, which is more resistant to misuse.

type Block

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

    // Encrypt encrypts the first block in src into dst.
    // Dst and src must overlap entirely or not at all.
    Encrypt(dst, src []byte)

    // Decrypt decrypts the first block in src into dst.
    // Dst and src must overlap entirely or not at all.
    Decrypt(dst, src []byte)
}

type BlockMode

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

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 must overlap
    // entirely or not at all.
    //
    // If len(dst) < len(src), CryptBlocks should panic. It is acceptable
    // to pass a dst bigger than src, and in that case, CryptBlocks will
    // only update dst[:len(src)] and will not touch the rest of dst.
    //
    // Multiple calls to CryptBlocks behave as if the concatenation of
    // the src buffers was passed in a single run. That is, BlockMode
    // maintains state and does not reset at each CryptBlocks call.
    CryptBlocks(dst, src []byte)
}

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

Code:

// Load your secret key from a safe place and reuse it across multiple
// NewCipher calls. (Obviously don't use this example key for anything
// real.) If you want to convert a passphrase to a key, use a suitable
// package like bcrypt or scrypt.
key, _ := hex.DecodeString("6368616e676520746869732070617373")
ciphertext, _ := hex.DecodeString("73c86d43a9d700a253a96c85b0f6b03ac9792e0e757f869cca306bd3cba1c62b")

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

Code:

// Load your secret key from a safe place and reuse it across multiple
// NewCipher calls. (Obviously don't use this example key for anything
// real.) If you want to convert a passphrase to a key, use a suitable
// package like bcrypt or scrypt.
key, _ := hex.DecodeString("6368616e676520746869732070617373")
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)

type Stream

A Stream represents a stream cipher. 

type Stream interface {
    // XORKeyStream XORs each byte in the given slice with a byte from the
    // cipher's key stream. Dst and src must overlap entirely or not at all.
    //
    // If len(dst) < len(src), XORKeyStream should panic. It is acceptable
    // to pass a dst bigger than src, and in that case, XORKeyStream will
    // only update dst[:len(src)] and will not touch the rest of dst.
    //
    // Multiple calls to XORKeyStream behave as if the concatenation of
    // the src buffers was passed in a single run. That is, Stream
    // maintains state and does not reset at each XORKeyStream call.
    XORKeyStream(dst, src []byte)
}

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

Code:

// Load your secret key from a safe place and reuse it across multiple
// NewCipher calls. (Obviously don't use this example key for anything
// real.) If you want to convert a passphrase to a key, use a suitable
// package like bcrypt or scrypt.
key, _ := hex.DecodeString("6368616e676520746869732070617373")
ciphertext, _ := hex.DecodeString("7dd015f06bec7f1b8f6559dad89f4131da62261786845100056b353194ad")

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

Code:

// Load your secret key from a safe place and reuse it across multiple
// NewCipher calls. (Obviously don't use this example key for anything
// real.) If you want to convert a passphrase to a key, use a suitable
// package like bcrypt or scrypt.
key, _ := hex.DecodeString("6368616e676520746869732070617373")
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.
fmt.Printf("%x\n", ciphertext)

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

Code:

// Load your secret key from a safe place and reuse it across multiple
// NewCipher calls. (Obviously don't use this example key for anything
// real.) If you want to convert a passphrase to a key, use a suitable
// package like bcrypt or scrypt.
key, _ := hex.DecodeString("6368616e676520746869732070617373")
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

Code:

// Load your secret key from a safe place and reuse it across multiple
// NewCipher calls. (Obviously don't use this example key for anything
// real.) If you want to convert a passphrase to a key, use a suitable
// package like bcrypt or scrypt.
key, _ := hex.DecodeString("6368616e676520746869732070617373")
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

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

type StreamReader struct {
    S Stream
    R io.Reader
}

▾ Example

Code:

// Load your secret key from a safe place and reuse it across multiple
// NewCipher calls. (Obviously don't use this example key for anything
// real.) If you want to convert a passphrase to a key, use a suitable
// package like bcrypt or scrypt.
key, _ := hex.DecodeString("6368616e676520746869732070617373")

encrypted, _ := hex.DecodeString("cf0495cc6f75dafc23948538e79904a9")
bReader := bytes.NewReader(encrypted)

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[:])

reader := &cipher.StreamReader{S: stream, R: bReader}
// Copy the input to the output stream, decrypting as we go.
if _, err := io.Copy(os.Stdout, 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.

Output:

some secret text

func (StreamReader) Read 

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

type StreamWriter

StreamWriter wraps a Stream into an io.Writer. It calls XORKeyStream to process each slice of data which passes through. If any StreamWriter.Write call returns short then the StreamWriter is out of sync and must be discarded. A StreamWriter has no internal buffering; StreamWriter.Close does not need to be called to flush write data. 

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

▾ Example

Code:

// Load your secret key from a safe place and reuse it across multiple
// NewCipher calls. (Obviously don't use this example key for anything
// real.) If you want to convert a passphrase to a key, use a suitable
// package like bcrypt or scrypt.
key, _ := hex.DecodeString("6368616e676520746869732070617373")

bReader := bytes.NewReader([]byte("some secret text"))

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[:])

var out bytes.Buffer

writer := &cipher.StreamWriter{S: stream, W: &out}
// Copy the input to the output buffer, encrypting as we go.
if _, err := io.Copy(writer, bReader); 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.

fmt.Printf("%x\n", out.Bytes())

Output:

cf0495cc6f75dafc23948538e79904a9

func (StreamWriter) Close 

func (w StreamWriter) Close() error

Close closes the underlying Writer and returns its Close return value, if the Writer is also an io.Closer. Otherwise it returns nil.

func (StreamWriter) Write 

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