diff --git a/Makefile b/Makefile
index 13ba925..471e7ac 100644
--- a/Makefile
+++ b/Makefile
@@ -6,7 +6,7 @@ OBJS     	= sha3.o main.o
 DIST            = tiny_sha3
 
 CC              = gcc
-CFLAGS		= -Wall -O3
+CFLAGS		= -Wall -O3 -DBIT_INTERLEAVING
 LIBS            =
 LDFLAGS         =
 INCLUDES	=
@@ -18,7 +18,7 @@ $(BINARY):      $(OBJS)
 		$(CC) $(CFLAGS) $(INCLUDES) -c $< -o $@
 
 clean:
-		rm -rf $(DIST)-*.txz $(OBJS) $(BINARY) *~ 
+		rm -rf $(DIST)-*.txz $(OBJS) $(BINARY) *~
 
 dist:		clean
 		cd ..; \
diff --git a/sha3.c b/sha3.c
index e359707..f5b66cf 100644
--- a/sha3.c
+++ b/sha3.c
@@ -18,6 +18,9 @@ uint64_t unshuffle(uint32_t even, uint32_t odd) {
     return result;
 }
 
+/* Get 32 bits from 'x' located on even possitions.
+ * Example: Assuming x={1,0,1,0,1,0} and index of first
+ * bit start from 0. This function returns x={0,0,0}. */
 uint32_t shuffle_even(uint64_t x) {
     x &= 0x5555555555555555ULL;
     x = (x | (x >> 1)) & 0x3333333333333333ULL;
@@ -28,6 +31,9 @@ uint32_t shuffle_even(uint64_t x) {
     return (uint32_t)x;
 }
 
+/* Get 32 bits from 'x' located on even possitions.
+ * Example: Assuming x={1,0,1,0,1,0} and index of first
+ * bit start from 0. This function returns x={1,1,1}. */
 uint32_t shuffle_odd(uint64_t x) {
     return shuffle_even(x >> 1);
 }
@@ -54,6 +60,96 @@ void sha3_keccakf(uint64_t st[25])
         15, 23, 19, 13, 12, 2, 20, 14, 22, 9,  6,  1
     };
 
+    // variables
+    int i, j, r;
+    uint64_t t, bc[5];
+
+#if __BYTE_ORDER__ != __ORDER_LITTLE_ENDIAN__
+    uint8_t *v;
+
+    // endianess conversion. this is redundant on little-endian targets
+    for (i = 0; i < 25; i++) {
+        v = (uint8_t *) &st[i];
+        st[i] = ((uint64_t) v[0])     | (((uint64_t) v[1]) << 8) |
+            (((uint64_t) v[2]) << 16) | (((uint64_t) v[3]) << 24) |
+            (((uint64_t) v[4]) << 32) | (((uint64_t) v[5]) << 40) |
+            (((uint64_t) v[6]) << 48) | (((uint64_t) v[7]) << 56);
+    }
+#endif
+
+    // actual iteration
+    for (r = 0; r < KECCAKF_ROUNDS; r++) {
+
+        // Theta
+        for (i = 0; i < 5; i++)
+            bc[i] = st[i] ^ st[i + 5] ^ st[i + 10] ^ st[i + 15] ^ st[i + 20];
+
+        for (i = 0; i < 5; i++) {
+            t = bc[(i + 4) % 5] ^ ROTL64(bc[(i + 1) % 5], 1);
+            for (j = 0; j < 25; j += 5)
+                st[j + i] ^= t;
+        }
+
+        // Rho Pi
+        t = st[1];
+        for (i = 0; i < 24; i++) {
+            j = keccakf_piln[i];
+            bc[0] = st[j];
+            st[j] = ROTL64(t, keccakf_rotc[i]);
+            t = bc[0];
+        }
+
+        //  Chi
+        for (j = 0; j < 25; j += 5) {
+            for (i = 0; i < 5; i++)
+                bc[i] = st[j + i];
+            for (i = 0; i < 5; i++)
+                st[j + i] ^= (~bc[(i + 1) % 5]) & bc[(i + 2) % 5];
+        }
+
+        //  Iota
+        st[0] ^= keccakf_rndc[r];
+    }
+
+#if __BYTE_ORDER__ != __ORDER_LITTLE_ENDIAN__
+    // endianess conversion. this is redundant on little-endian targets
+    for (i = 0; i < 25; i++) {
+        v = (uint8_t *) &st[i];
+        t = st[i];
+        v[0] = t & 0xFF;
+        v[1] = (t >> 8) & 0xFF;
+        v[2] = (t >> 16) & 0xFF;
+        v[3] = (t >> 24) & 0xFF;
+        v[4] = (t >> 32) & 0xFF;
+        v[5] = (t >> 40) & 0xFF;
+        v[6] = (t >> 48) & 0xFF;
+        v[7] = (t >> 56) & 0xFF;
+    }
+#endif
+}
+
+void sha3_keccakf_bi(uint64_t st[25])
+{
+    // constants
+    const uint64_t keccakf_rndc[24] = {
+        0x0000000000000001, 0x0000000000008082, 0x800000000000808a,
+        0x8000000080008000, 0x000000000000808b, 0x0000000080000001,
+        0x8000000080008081, 0x8000000000008009, 0x000000000000008a,
+        0x0000000000000088, 0x0000000080008009, 0x000000008000000a,
+        0x000000008000808b, 0x800000000000008b, 0x8000000000008089,
+        0x8000000000008003, 0x8000000000008002, 0x8000000000000080,
+        0x000000000000800a, 0x800000008000000a, 0x8000000080008081,
+        0x8000000000008080, 0x0000000080000001, 0x8000000080008008
+    };
+    const int keccakf_rotc[24] = {
+        1,  3,  6,  10, 15, 21, 28, 36, 45, 55, 2,  14,
+        27, 41, 56, 8,  25, 43, 62, 18, 39, 61, 20, 44
+    };
+    const int keccakf_piln[24] = {
+        10, 7,  11, 17, 18, 3, 5,  16, 8,  21, 24, 4,
+        15, 23, 19, 13, 12, 2, 20, 14, 22, 9,  6,  1
+    };
+
     // variables
     int i, j, r;
     uint32_t t1, t2;
@@ -89,6 +185,8 @@ void sha3_keccakf(uint64_t st[25])
         // Chi
         for (i = 0; i < 5; i++) {
 
+            /* Note that we are rotating by 1. In this case we only care about
+             * "odd" bits. */
             uint32_t rot32 = ROTL32(bc_odd[(i + 1) % 5], 1);
             t1 = bc_even[(i + 4) % 5] ^ rot32;
             t2 = bc_odd[(i + 4) % 5] ^ bc_even[(i + 1) % 5];
@@ -159,6 +257,12 @@ void sha3_keccakf(uint64_t st[25])
 #endif
 }
 
+#ifdef BIT_INTERLEAVING
+#define KECCAK_F sha3_keccakf_bi
+#else
+#define KECCAK_F sha3_keccakf
+#endif
+
 // Initialize the context for SHA3
 
 int sha3_init(sha3_ctx_t *c, int mdlen)