diff --git a/src/algo/feistel0.rs b/src/algo/feistel0.rs
index 7901704..2a33ab5 100644
--- a/src/algo/feistel0.rs
+++ b/src/algo/feistel0.rs
@@ -10,13 +10,13 @@
 
 const SBOX: [u8; 0x10] = [0x4, 3, 9, 0xa, 0xb, 2, 0xe, 1, 0xd, 0xc, 8, 6, 7, 5, 0, 0xf];
 
-fn swap_bits(n: u8, p1: u8, p2: u8) -> u8 {
-    let bit1 = (n >> p1) & 1;
-    let bit2 = (n >> p2) & 1;
-    let mut x = (bit1 ^ bit2);
-    x = (x << p1) | (x << p2);
- 
-    n ^ x
+#[test]
+/// test inner against the given values
+fn test_inner() {
+    assert!(inner(0x1234, 0x0000) == 0x29a8);
+    assert!(inner(0x1234, 0x2345) == 0x0aed);
+    assert!(inner(0xabcd, 0xbeef) == 0x089a);
+    assert!(inner(0x9876, 0xfedc) == 0x93c5);
 }
 
 pub fn inner(input: u16, key: u16) -> u16 {
@@ -24,11 +24,12 @@ pub fn inner(input: u16, key: u16) -> u16 {
     let mut blocks: [u8; 4] = [
         ((input & 0xf000) >> 12).to_be_bytes()[1],
         ((input & 0x0f00) >> 08).to_be_bytes()[1],
-        ((input & 0x000f) >> 00).to_be_bytes()[1],
         ((input & 0x00f0) >> 04).to_be_bytes()[1],
+        ((input & 0x000f) >> 00).to_be_bytes()[1],
     ];
 
-    dbg!(&blocks);
+    println!("{:04b} {:04b} {:04b} {:04b}", blocks[0], blocks[1], blocks[2], blocks[3]);
+    println!("{:x}{:x}{:x}{:x}", blocks[0], blocks[1], blocks[2], blocks[3]);
 
     // each block must not be more than 0xf
     for block in blocks {
@@ -41,17 +42,43 @@ pub fn inner(input: u16, key: u16) -> u16 {
     blocks[1] = SBOX[blocks[1] as usize];
     blocks[2] = SBOX[blocks[2] as usize];
 
-    dbg!(&blocks);
+    println!("{:04b} {:04b} {:04b} {:04b}", blocks[0], blocks[1], blocks[2], blocks[3]);
+    println!("{:x}{:x}{:x}{:x}", blocks[0], blocks[1], blocks[2], blocks[3]);
 
     // swap position and endianess for outer blocks
-    let tmp = blocks[0];
-    blocks[0] = u8::swap_bytes(blocks[3]);
-    blocks[3] = u8::swap_bytes(tmp);
+    let mut b0 = 0;
+    let mut b3 = 0;
+    b0 |= (blocks[0] & 0b1000) >> 3;
+    b0 |= (blocks[0] & 0b0100) >> 1;
+    b0 |= (blocks[0] & 0b0010) << 1;
+    b0 |= (blocks[0] & 0b0001) << 3;
+    b3 |= (blocks[3] & 0b1000) >> 3;
+    b3 |= (blocks[3] & 0b0100) >> 1;
+    b3 |= (blocks[3] & 0b0010) << 1;
+    b3 |= (blocks[3] & 0b0001) << 3;
 
-    dbg!(&blocks);
+    blocks[0] = b3;
+    blocks[3] = b0;
     
     println!("{:04b} {:04b} {:04b} {:04b}", blocks[0], blocks[1], blocks[2], blocks[3]);
     println!("{:x}{:x}{:x}{:x}", blocks[0], blocks[1], blocks[2], blocks[3]);
 
-    panic!("Not implemented");
+    for block in blocks {
+        if block > 0xf {
+            panic!("Block is more than 0xf: {block}");
+        }
+    }
+
+    let mut result: u16 = 0;
+    result |= (blocks[0] as u16) << 12;
+    result |= (blocks[1] as u16) << 08;
+    result |= (blocks[2] as u16) << 04;
+    result |= (blocks[3] as u16) << 00;
+
+    return result ^ key
+}
+
+pub fn sbox(index: u8) -> u8 {
+    assert!(index < 0xf);
+    return SBOX[index as usize];
 }
diff --git a/src/main.rs b/src/main.rs
index da52e0b..4437897 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -111,8 +111,10 @@ struct XorArgs {
 
 #[derive(Subcommand, Clone, Debug, PartialEq, Eq)]
 enum AlgoActions {
-    #[command(name="feistel-i")]
-    Feistel0Inner(Feistel0InnerArgs)
+    #[command(name="feistel0-i")]
+    Feistel0Inner(Feistel0InnerArgs),
+    #[command(name="feistel0-sbox")]
+    Feistel0SBOX(Feistel0SBOXArgs)
 }
 
 #[derive(Args, Clone, Debug, PartialEq, Eq)]
@@ -123,6 +125,12 @@ struct Feistel0InnerArgs {
     key: u16,
 }
 
+#[derive(Args, Clone, Debug, PartialEq, Eq)]
+struct Feistel0SBOXArgs {
+    #[clap(value_parser=maybe_hex::<u8>)]
+    index: u8,
+}
+
 /*************************************************************************************************/
 /// main function of plexcryptool.
 ///
@@ -183,7 +191,16 @@ pub fn main() {
                         println!("{}", result)
                     }
                     else {
-                        println!("result is {}", result)
+                        println!("result is {} ({:04x})", result, result)
+                    }
+                }
+                AlgoActions::Feistel0SBOX(alg_fsb_args) => {
+                    let result: u8 = algo::feistel0::sbox(alg_fsb_args.index);
+                    if args.machine {
+                        println!("{}", result)
+                    }
+                    else {
+                        println!("result is {} ({:x})", result, result)
                     }
                 }
             }