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// Copyright 2009 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.
// Counter (CTR) mode.
// CTR converts a block cipher into a stream cipher by
// repeatedly encrypting an incrementing counter and
// xoring the resulting stream of data with the input.
// See NIST SP 800-38A, pp 13-15
package block
import (
"io"
)
type ctrStream struct {
c Cipher
ctr []byte
out []byte
}
func newCTRStream(c Cipher, ctr []byte) *ctrStream {
x := new(ctrStream)
x.c = c
x.ctr = dup(ctr)
x.out = make([]byte, len(ctr))
return x
}
func (x *ctrStream) Next() []byte {
// Next block is encryption of counter.
x.c.Encrypt(x.out, x.ctr)
// Increment counter
for i := len(x.ctr) - 1; i >= 0; i-- {
x.ctr[i]++
if x.ctr[i] != 0 {
break
}
}
return x.out
}
// NewCTRReader returns a reader that reads data from r, decrypts (or encrypts)
// it using c in counter (CTR) mode with the initialization vector iv.
// The returned Reader does not buffer and has no block size.
// In CTR mode, encryption and decryption are the same operation:
// a CTR reader applied to an encrypted stream produces a decrypted
// stream and vice versa.
func NewCTRReader(c Cipher, iv []byte, r io.Reader) io.Reader {
return newXorReader(newCTRStream(c, iv), r)
}
// NewCTRWriter returns a writer that encrypts (or decrypts) data using c
// in counter (CTR) mode with the initialization vector iv
// and writes the encrypted data to w.
// The returned Writer does not buffer and has no block size.
// In CTR mode, encryption and decryption are the same operation:
// a CTR writer applied to an decrypted stream produces an encrypted
// stream and vice versa.
func NewCTRWriter(c Cipher, iv []byte, w io.Writer) io.Writer {
return newXorWriter(newCTRStream(c, iv), w)
}
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