diff --git a/src/main.rs b/src/main.rs
index 1b6bc6e..fbad598 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,4 +1,5 @@
 use rand::Rng;
+use rand::RngCore;
 use rand::rngs::OsRng;
 use rand::SeedableRng;
 use rand::rngs::StdRng;
@@ -147,7 +148,7 @@ impl Default for PasswordConfig {
     }
 }
 
-fn random_ascii(rng: &mut impl Rng, char_set: &[u8]) -> u8 {
+fn random_ascii(rng: &mut MyRng, char_set: &[u8]) -> u8 {
     // NOT IDEAL FOR CRYPTO/password generation 
     // let range_max = 122; // 125 - 33
     // let n = rng.next_u32() % range_max; // slightly modulo biased
@@ -156,13 +157,13 @@ fn random_ascii(rng: &mut impl Rng, char_set: &[u8]) -> u8 {
     // todo: validate this fact
     // this function under the hood creates a zone that divides evenly
     // into the range so we don't encounter modulo bias
-    *char_set.choose(rng).expect("char_set should not be empty")
+    *char_set.choose(rng.as_rng()).expect("char_set should not be empty")
 }
 
 // returns error if generated password doesn't contain the necessary chars
 // to satisfy the policy
 // let the caller decide what to do
-fn generate_password(config: &PasswordConfig, rng: &mut impl Rng) -> Result<String,String> {
+fn generate_password(config: &PasswordConfig, rng: &mut MyRng) -> Result<String,String> {
     if config.len < 8 {
         return Err(String::from("Password length not long enough"));
     }
@@ -209,7 +210,7 @@ fn generate_password(config: &PasswordConfig, rng: &mut impl Rng) -> Result<Stri
 // we can take a slice of "words" because we don't care about:
 // capacity, mutation (push/pop), allocation strategy
 fn generate_passphrase(config: &PassphraseConfig,
-        word_list: &[&str], rng: &mut impl Rng) -> Result<String,String> {
+        word_list: &[&str], rng: &mut MyRng) -> Result<String,String> {
     if config.phrases < 2 {
         return Err("Phrase count too low".to_string());
     }
@@ -237,8 +238,8 @@ fn generate_passphrase(config: &PassphraseConfig,
     Ok(res)
 }
 
-fn random_word<'a>(word_list: &[&'a str], rng: &mut impl Rng) -> &'a str {
-    word_list[rng.gen_range(0..word_list.len())]
+fn random_word<'a>(word_list: &[&'a str], rng: &mut MyRng) -> &'a str {
+    word_list[rng.as_rng().gen_range(0..word_list.len())]
 }
 
 fn get_word_list() -> &'static [&'static str] {
@@ -255,20 +256,43 @@ fn get_word_list() -> &'static [&'static str] {
     Wordlist::get_list(&Wordlist::EffLong)
 }
 
+enum Determinism {
+    Predictable,
+    #[allow(dead_code)]
+    Random,
+}
+
+enum MyRng { Std(StdRng), Os(OsRng) }
+
+impl MyRng {
+    // todo: understand RngCore
+    fn as_rng(&mut self) -> &mut dyn RngCore {
+        match self {
+            MyRng::Std(r) => r,
+            MyRng::Os(r) => r,
+        }
+    }
+}
+
+fn get_rng(d: &Determinism) -> MyRng {
+    match d {
+        // when we are looking for Reproducability we can use the same seed value
+        // to create our Rng. Because both StdRng and OsRng implement Rng
+        // we make our methods slightly more generic and this works just fine
+        // comment out when we need random version!
+        Determinism::Predictable => MyRng::Std(StdRng::seed_from_u64(42)),
+        // we definitely want to use the OS for randomness
+        // doesn't require seeding because it is backed by CSPRNG, backed by OS
+        // the OS maintains it's own entropy from mouse movement, hardware noise,etc
+        // we pass it through the functions to avoid a potential syscall
+        Determinism::Random => MyRng::Os(OsRng),
+    }
+}
+
 fn main() {
     let word_list = get_word_list();
 
-    // we definitely want to use the OS for randomness
-    // doesn't require seeding because it is backed by CSPRNG, backed by OS
-    // the OS maintains it's own entropy from mouse movement, hardware noise,etc
-    // we pass it through the functions to avoid a potential syscall
-    let mut _rng = OsRng;
-
-    // when we are looking for Reproducability we can use the same seed value
-    // to create our Rng. Because both StdRng and OsRng implement Rng
-    // we make our methods slightly more generic and this works just fine
-    // comment out when we need random version!
-    let mut rng = StdRng::seed_from_u64(42);
+    let mut rng = get_rng(&Determinism::Predictable);
 
     // todo: add actual tests
     // password with alphnumeric