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ofb8.go
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ofb8.go
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package cipher
import "crypto/cipher"
const streamBufferSize = 512
type ofb8 struct {
b cipher.Block
cipher []byte
out []byte
outUsed int
}
func NewOFB8(b cipher.Block, iv []byte) cipher.Stream {
blockSize := b.BlockSize()
if len(iv) != blockSize {
panic("crypto/ofb8: IV length must equal block size")
}
bufSize := streamBufferSize
if bufSize < blockSize {
bufSize = blockSize
}
x := &ofb8{
b: b,
cipher: make([]byte, blockSize),
out: make([]byte, 0, bufSize),
outUsed: 0,
}
copy(x.cipher, iv)
return x
}
func (x *ofb8) refill() {
remain := len(x.out) - x.outUsed
if remain > x.outUsed {
return
}
copy(x.out, x.out[x.outUsed:])
x.out = x.out[:cap(x.out)]
dst := make([]byte, len(x.cipher))
for remain <= len(x.out)-1 {
x.b.Encrypt(dst, x.cipher)
x.out[remain] = dst[0]
copy(x.cipher, x.cipher[1:])
x.cipher[len(x.cipher)-1] = dst[0]
remain++
}
x.out = x.out[:remain]
x.outUsed = 0
}
func (x *ofb8) XORKeyStream(dst, src []byte) {
if len(dst) < len(src) {
panic("crypto/ofb8: output smaller than input")
}
for len(src) > 0 {
if x.outUsed >= len(x.out)-x.b.BlockSize() {
x.refill()
}
dst[0] = src[0] ^ x.out[x.outUsed]
dst = dst[1:]
src = src[1:]
x.outUsed++
}
}