From da3a531e53ecbcbc8f4105daea74ee6987104ab7 Mon Sep 17 00:00:00 2001
From: Russ Magee <rmagee@gmail.com>
Date: Tue, 23 Oct 2018 23:25:31 -0700
Subject: [PATCH] Initial commit

---
 cryptmt.go | 57 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)
 create mode 100644 cryptmt.go

diff --git a/cryptmt.go b/cryptmt.go
new file mode 100644
index 0000000..d0c0d4d
--- /dev/null
+++ b/cryptmt.go
@@ -0,0 +1,57 @@
+// CryptMT
+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.
+
+import (
+	"errors"
+
+	mtwist "blitter.com/go/mtwist"
+)
+
+type Cipher struct {
+	accum uint64
+	m     *mtwist.MT19937_64
+}
+
+func (c *Cipher) yield8() (r byte) {
+	c.accum = c.accum * (c.m.Int63() | 1)
+	r = byte(c.accum>>56) & 0xFF
+	return
+}
+
+// NewCipher creates and returns a Cipher. The key argument should be the
+// CryptMT key, 64 bytes.
+func NewCipher(key []byte) (c *Cipher) {
+	c = &Cipher{m: mtwist.New()}
+	c.m.SeedFullState(key)
+	c.accum = 1
+	// from paper, discard first 64 bytes of output
+	for idx := 0; idx < 64; idx++ {
+		_ = c.yield8()
+	}
+	return c
+}
+
+// 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.yield8()
+	}
+}