mirror of https://gogs.blitter.com/RLabs/xs
61 lines
1.7 KiB
Go
61 lines
1.7 KiB
Go
// Package CryptMT - implementation of cryptMTv1 stream cipher
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// (but with mtwist64 as base accum)
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// https://eprint.iacr.org/2005/165.pdf
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package cryptmt
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// TODO rlm: according to go docs, stream ciphers do not implement the
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// cipher.Block interface at all (thus do not support Encrypt() or
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// Decrypt() .. cipher.StreamReader/StreamWriter() only call
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// XORKeyStream() anyhow and for my own purposes this is all that is
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// required.
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import (
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"errors"
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mtwist "blitter.com/go/mtwist"
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)
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type Cipher struct {
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accum uint64
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m *mtwist.MT19937_64
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}
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func (c *Cipher) yield8() (r byte) {
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c.accum = c.accum * (c.m.Int63() | 1)
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r = byte(c.accum>>56) & 0xFF
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return
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}
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// NewCipher creates and returns a Cipher. The key argument should be the
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// CryptMT key, 64 bytes.
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func NewCipher(key []byte) (c *Cipher) {
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c = &Cipher{m: mtwist.New()}
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c.m.SeedFullState(key)
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c.accum = 1
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// from paper, discard first 64 bytes of output
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for idx := 0; idx < 64; idx++ {
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_ = c.yield8()
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}
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return c
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}
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// XORKeyStream XORs each byte in the given slice with a byte from the
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// cipher's key stream. Dst and src must overlap entirely or not at all.
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//
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// If len(dst) < len(src), XORKeyStream should panic. It is acceptable
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// to pass a dst bigger than src, and in that case, XORKeyStream will
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// only update dst[:len(src)] and will not touch the rest of dst.
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//
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// Multiple calls to XORKeyStream behave as if the concatenation of
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// the src buffers was passed in a single run. That is, Stream
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// maintains state and does not reset at each XORKeyStream call.
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func (c *Cipher) XORKeyStream(dst, src []byte) {
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if len(dst) < len(src) {
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panic(errors.New("len(dst) < len(src)"))
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}
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for i, b := range src {
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dst[i] = b ^ c.yield8()
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}
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}
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