add aes_xts, which was in the mkfile

Reference: /n/atom/patch/applied/aesxts
Date: Thu Nov  5 05:35:59 CET 2015
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libsec/port/aes_xts.c	Thu Jan  1 00:00:00 1970
+++ /sys/src/libsec/port/aes_xts.c	Thu Nov  5 05:35:49 2015
@@ -0,0 +1,72 @@
+/*
+ * Author Taru Karttunen <taruti@taruti.net>
+ * This file can be used as both Public Domain or Creative Commons CC0.
+ */
+#include <u.h>
+#include <libsec.h>
+
+static void
+xor128(uchar *uo, uchar *ui0, uchar *ui1)
+{
+	u64int *o, *i0, *i1;
+
+	o = (u64int*)uo;
+	i0 = (u64int*)ui0;
+	i1 = (u64int*)ui1;
+
+	o[0] = i0[0] ^ i1[0];
+	o[1] = i0[1] ^ i1[1];
+}
+
+static void
+gf_mulx(u64int *x)
+{
+	u64int t;
+
+	t = (x[1]>>63)*0x87;
+	x[1] = x[1]<<1 | x[0]>>63;
+	x[0] = (x[0] << 1) ^ t;
+}
+
+int
+aes_xts_encrypt(u32int tweak[], u32int ecb[], vlong secno, uchar *input, uchar *output, usize len)
+{
+	uchar T[AESbsize], x[AESbsize];
+	int i;
+
+	if(len % AESbsize != 0)
+		return -1;
+	for(i=0; i<AESbsize; i++)
+		T[i] = secno >> i*8;
+	aes_encrypt((ulong*)tweak, 10, T, T);
+
+	for (i=0; i<len; i+=AESbsize) {
+		xor128(x, &input[i], T);
+		aes_encrypt((ulong*)ecb, 10, x, x);
+		xor128(&output[i], x, T);
+		gf_mulx((u64int*)T);
+	}
+
+	return 0;
+}
+
+int
+aes_xts_decrypt(u32int tweak[], u32int ecb[], vlong secno, uchar *input, uchar *output, usize len)
+{
+	uchar T[AESbsize], x[AESbsize];
+	int i;
+
+	if(len % AESbsize != 0)
+		return -1;
+	for(i=0; i<AESbsize; i++)
+		T[i] = secno >> i*8;
+	aes_encrypt((ulong*)tweak, 10, T, T);
+
+	for (i=0; i<len; i+=AESbsize){
+		xor128(x, &input[i], T);
+		aes_decrypt((ulong*)ecb, 10, x, x);
+		xor128(&output[i], x, T);
+		gf_mulx((u64int*)T);
+	}
+	return 0;
+}