diff --git a/Cargo.toml b/Cargo.toml
index 098fd0f..1c9609a 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "plexcryptool"
-version = "0.2.1"
+version = "0.2.3"
 edition = "2021"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
diff --git a/src/main.rs b/src/main.rs
index ab0cdc6..6c55dd9 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -78,6 +78,7 @@ struct AlgoCommand {
 enum MathActions {
     #[command(name="modexp")]
     Modexp(ModexpArgs),
+    Modred(ModredArgs),
     Pm1(PM1Args),
 }
 
@@ -88,9 +89,19 @@ struct ModexpArgs {
     field: String
 }
 
+#[derive(Args, Clone, Debug, PartialEq, Eq)]
+struct ModredArgs {
+    #[clap(value_parser=maybe_hex::<u64>)]
+    polynomial: u64,
+    #[clap(value_parser=maybe_hex::<u64>)]
+    relation: u64,
+}
+
 #[derive(Args, Clone, Debug, PartialEq, Eq)]
 struct PM1Args {
+    #[clap(value_parser=maybe_hex::<u128>)]
     n: u128,
+    #[clap(value_parser=maybe_hex::<u128>)]
     max_prime: u128,
 }
 
@@ -106,13 +117,17 @@ enum BinaryActions {
 struct RotateArgs {
     #[arg(short, long, default_value_t = false)]
     left: bool,
+    #[clap(value_parser=maybe_hex::<u32>)]
     base: u32,
+    #[clap(value_parser=maybe_hex::<u32>)]
     shift_width: u32,
 }
 
 #[derive(Args, Clone, Debug, PartialEq, Eq)]
 struct XorArgs {
+    #[clap(value_parser=maybe_hex::<u128>)]
     a: u128,
+    #[clap(value_parser=maybe_hex::<u128>)]
     b: u128,
 }
 
@@ -173,6 +188,29 @@ pub fn main() {
                         println!("result is {}", result)
                     }
                 }
+                MathActions::Modred(mod_red_args) => {
+                    let result = math::modred::modred(mod_red_args.polynomial, mod_red_args.relation, args.verbose);
+                    match result {
+                        Ok(res) => {
+                            if args.machine {
+                                println!("0x{:x}", res)
+                            }
+                            else {
+                                println!("=======================================================================");
+                                println!("result is 0x{:x}", res)
+                            }
+                        }
+                        Err(e) => {
+                            if args.machine {
+                                println!("{:?}", e)
+                            }
+                            else {
+                                println!("=======================================================================");
+                                println!("could not compute: {:?}", e)
+                            }
+                        }
+                    }
+                }
                 MathActions::Pm1(pm1_args) => {
                     let result: Result<Vec<u128>, String> = math::pm1::p_minus_one(
                         pm1_args.n, 
diff --git a/src/math/mod.rs b/src/math/mod.rs
index 65c4dca..6beca9c 100644
--- a/src/math/mod.rs
+++ b/src/math/mod.rs
@@ -8,3 +8,4 @@
 /// Source:     <https://git.cscherr.de/PlexSheep/plexcryptool/>
 pub mod modexp;
 pub mod pm1;
+pub mod modred;
diff --git a/src/math/modred.rs b/src/math/modred.rs
new file mode 100644
index 0000000..f496b95
--- /dev/null
+++ b/src/math/modred.rs
@@ -0,0 +1,46 @@
+#![allow(dead_code)]
+/// modular reduction
+///
+/// Implements automatic modular reduction in a field specified by a given relation.
+///
+/// Basically, any binary number can be written as a polynomial. This polynomial can be reduced by
+/// the relation that defines a field. In that field. This is what we call modular reduction.
+///
+/// Author:     Christoph J. Scherr <software@cscherr.de>
+/// License:    MIT
+/// Source:     <https://git.cscherr.de/PlexSheep/plexcryptool/>
+
+#[test]
+fn test_modred() {
+    let rel: u64 = 0x1053;
+    let pol0: u64 = 0x100001;
+    assert_eq!(modred(pol0, rel, false).unwrap(), 0x21e);
+}
+
+pub fn modred(mut poly: u64, relation: u64, verbose: bool) -> Result<u64, String> {
+
+    let mut diffrence: u32;
+    let mut index: usize = 0;
+    if verbose {
+        println!("relation:\t0x{:x}\t", relation);
+        println!("polynomial:\t0x{:x}\t", poly);
+        println!("=======================================================================");
+    }
+    while relation.leading_zeros() - poly.leading_zeros() != 0 {
+        diffrence = relation.leading_zeros() - poly.leading_zeros();
+        poly = poly ^ (relation << diffrence);
+        if verbose {
+            println!("{index}:\tpoly: 0x{:x}\t", poly);
+            //println!("{index}:\tpoly: 0b{:b}\t", poly);
+        }
+        index += 1;
+    }
+    // one more time!
+    diffrence = relation.leading_zeros() - poly.leading_zeros();
+    poly = poly ^ (relation << diffrence);
+    if verbose {
+        println!("{index}:\tpoly: 0x{:x}\t", poly);
+        //println!("{index}:\tpoly: 0b{:b}\t", poly);
+    }
+    return Ok(poly);
+}