Copyright 2010 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.
CFB (Cipher Feedback) Mode.

package cipher

import 

type cfb struct {
	b       Block
	next    []byte
	out     []byte
	outUsed int

	decrypt bool
}

func ( *cfb) (,  []byte) {
	if len() < len() {
		panic("crypto/cipher: output smaller than input")
	}
	if subtle.InexactOverlap([:len()], ) {
		panic("crypto/cipher: invalid buffer overlap")
	}
	for len() > 0 {
		if .outUsed == len(.out) {
			.b.Encrypt(.out, .next)
			.outUsed = 0
		}
We can precompute a larger segment of the keystream on decryption. This will allow larger batches for xor, and we should be able to match CTR/OFB performance.
			copy(.next[.outUsed:], )
		}
		 := xorBytes(, , .out[.outUsed:])
		if !.decrypt {
			copy(.next[.outUsed:], )
		}
		 = [:]
		 = [:]
		.outUsed += 
	}
}
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.
func ( Block,  []byte) Stream {
	return newCFB(, , false)
}
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.
func ( Block,  []byte) Stream {
	return newCFB(, , true)
}

func ( Block,  []byte,  bool) Stream {
	 := .BlockSize()
stack trace will indicate whether it was de or encryption
		panic("cipher.newCFB: IV length must equal block size")
	}
	 := &cfb{
		b:       ,
		out:     make([]byte, ),
		next:    make([]byte, ),
		outUsed: ,
		decrypt: ,
	}
	copy(.next, )

	return