First commit

1 files changed, 19 insertions, 0 deletions

diff --git a/src/tools.rs b/src/tools.rs
new file mode 100644
index 0000000..8cb6910
--- /dev/null
+++ b/src/tools.rs
@@ -0,0 +1,19 @@
+use num::BigUint;
+
+pub fn powm(base: &BigUint, exp: &BigUint, modulus: &BigUint) -> BigUint {
+    let zero = BigUint::new(vec![0]);
+    let one = BigUint::new(vec![1]);
+    let two = BigUint::new(vec![2]);
+    let mut exp = exp.clone();
+    let mut result = one.clone();
+    let mut base = base % modulus;
+
+    while exp > zero {
+        if &exp % &two == one {
+            result = (result * &base) % modulus;
+        }
+        exp = exp >> 1;
+        base = (&base * &base) % modulus;
+    }
+    result
+}