xs/vendor/blitter.com/go/cryptmt/cryptmt.go

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// Package CryptMT - implementation of cryptMTv1 stream cipher
// (but with mtwist64 as base accum)
// https://eprint.iacr.org/2005/165.pdf
package cryptmt
// TODO rlm: according to go docs, stream ciphers do not implement the
// cipher.Block interface at all (thus do not support Encrypt() or
// Decrypt() .. cipher.StreamReader/StreamWriter() only call
// XORKeyStream() anyhow and for my own purposes this is all that is
// required.
import (
"errors"
"io"
mtwist "blitter.com/go/mtwist"
)
type Cipher struct {
r io.Reader
w io.Writer
accum uint64
m *mtwist.MT19937_64
}
func (c *Cipher) yield() (r byte) {
c.accum = c.accum * (c.m.Int63() | 1)
r = byte(c.accum>>56) & 0xFF
return
}
// New creates and returns a Cipher. The key argument should be the
// CryptMT key, 64 bytes.
func New(r io.Reader, w io.Writer, key []byte) (c *Cipher) {
c = &Cipher{m: mtwist.New(), r: r, w: w}
c.m.SeedFullState(key)
c.accum = 1
// from paper, discard first 64 bytes of output
for idx := 0; idx < 64; idx++ {
_ = c.yield()
}
return c
}
func (c *Cipher) Read(p []byte) (n int, err error) {
n, err = c.r.Read(p)
if err == nil {
for idx := 0; idx < n; idx++ {
p[idx] = p[idx] ^ c.yield()
}
}
return n, err
}
func (c *Cipher) Write(p []byte) (n int, err error) {
n, err = c.w.Write(p)
return n, err
}
// 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.
func (c *Cipher) XORKeyStream(dst, src []byte) {
if len(dst) < len(src) {
panic(errors.New("len(dst) < len(src)"))
}
for i, b := range src {
dst[i] = b ^ c.yield()
}
}