Source: ctr.go in package crypto/cipher

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 cipher

import "crypto/internal/subtle"

type ctr struct {
	b       Block
	ctr     []byte
	out     []byte
	outUsed int
}

const streamBufferSize = 512

ctrAble is an interface implemented by ciphers that have a specific optimized implementation of CTR, like crypto/aes. NewCTR will check for this interface and return the specific Stream if found.

type ctrAble interface {
	NewCTR(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.

func NewCTR(block Block, iv []byte) Stream {
	if ctr, ok := block.(ctrAble); ok {
		return ctr.NewCTR(iv)
	}
	if len(iv) != block.BlockSize() {
		panic("cipher.NewCTR: IV length must equal block size")
	}
	bufSize := streamBufferSize
	if bufSize < block.BlockSize() {
		bufSize = block.BlockSize()
	}
	return &ctr{
		b:       block,
		ctr:     dup(iv),
		out:     make([]byte, 0, bufSize),
		outUsed: 0,
	}
}

func (x *ctr) refill() {
	remain := len(x.out) - x.outUsed
	copy(x.out, x.out[x.outUsed:])
	x.out = x.out[:cap(x.out)]
	bs := x.b.BlockSize()
	for remain <= len(x.out)-bs {
		x.b.Encrypt(x.out[remain:], x.ctr)
		remain += bs

Increment counter

		for i := len(x.ctr) - 1; i >= 0; i-- {
			x.ctr[i]++
			if x.ctr[i] != 0 {
				break
			}
		}
	}
	x.out = x.out[:remain]
	x.outUsed = 0
}

func (x *ctr) XORKeyStream(dst, src []byte) {
	if len(dst) < len(src) {
		panic("crypto/cipher: output smaller than input")
	}
	if subtle.InexactOverlap(dst[:len(src)], src) {
		panic("crypto/cipher: invalid buffer overlap")
	}
	for len(src) > 0 {
		if x.outUsed >= len(x.out)-x.b.BlockSize() {
			x.refill()
		}
		n := xorBytes(dst, src, x.out[x.outUsed:])
		dst = dst[n:]
		src = src[n:]
		x.outUsed += n
	}