added notes about bits of entropy for each generator

src/main.rs

@@ -10,7 +10,7 @@ const NUMBER_CHARS: &[u8] = b"0123456789";
 // still exclude symbols that can trip up URLs and CLIs
 // excludes: backtick(`), quote('), dquote("), slash(/), bslash(\)
 //           pipe (|), arrows (<), (>), brackets ([), (]), ({), (})
-const SYMBOL_CHARS: &[u8] = b"!@#$%^&*()?,.-_=+~";
+const SYMBOL_CHARS: &[u8] = b"!@#$%^&*()?,.-_=+~"; // 18 chars
 
 // saves some boilerplate manual bitflags and less clunky to use
 bitflags! {
@@ -73,6 +73,12 @@ fn random_ascii(rng: &mut OsRng, char_set: &[u8]) -> u8 {
 }
 
 // todo: protect from small len
+//
+// How many bits of entropy?
+// max_chars_available -> 80
+// log2(possible_symbols^len) yields bits of entropy
+// so say len is 20 and we use all 80 chars:
+// that's log2(80^20) -> 126.4 bits of entropy per password
 fn generate_random_chars(config: PasswordConfig, rng: &mut OsRng) -> String {
     // String is a thin wrapper around Vec<u8> so we can expect re-allocation
     // and copy only when we exceed the capacity of the vector
@@ -80,18 +86,23 @@ fn generate_random_chars(config: PasswordConfig, rng: &mut OsRng) -> String {
     let mut res = String::with_capacity(config.len);
     let mut chars : Vec<u8> = Vec::new();
     if config.charset.contains(CharSet::UPPERCASE) {
-        chars.extend_from_slice(UPPERCASE_CHARS);
+        chars.extend_from_slice(UPPERCASE_CHARS); // 26 chars
     }
     if config.charset.contains(CharSet::LOWERCASE) {
-        chars.extend_from_slice(LOWERCASE_CHARS);
+        chars.extend_from_slice(LOWERCASE_CHARS); // 26 chars
     }
     if config.charset.contains(CharSet::NUMBERS) {
-        chars.extend_from_slice(NUMBER_CHARS);
+        chars.extend_from_slice(NUMBER_CHARS); // 10 chars
     }
     if config.charset.contains(CharSet::SYMBOLS) {
-        chars.extend_from_slice(SYMBOL_CHARS);
+        chars.extend_from_slice(SYMBOL_CHARS); // 18 chars
     }
 
+    // How many bits of entropy?
+    // max_chars_available -> 80
+    // log2(possible_symbols^len) yields bits of entropy
+    // so say len is 20 and we use all 80 chars
+    // that's log2(80^20) -> 126.4 bits of entropy per password
     for _ in 0..config.len {
         res.push(random_ascii(rng, &chars) as char);
     }
@@ -102,6 +113,11 @@ fn generate_random_chars(config: PasswordConfig, rng: &mut OsRng) -> String {
 
 // we can take a slice of "words" because we don't care about:
 // capacity, mutation (push/pop), allocation strategy
+//
+// How many bits of entropy?
+// log2(104078) -> 16.6 bits per word derived from word_list size 
+// log2(18) -> 4.1 bits for separator assuming seporator is one of our SYMBOLs
+// total 70.5 bits of entropy
 fn generate_passphrase(phrase_count: usize,
         word_list: &[&str], separator: char, rng: &mut OsRng) -> String {
     let mut res = String::new(); // don't bother saving allocations with a bad guess 
@@ -119,7 +135,6 @@ fn random_word<'a>(word_list: &[&'a str], rng: &mut OsRng) -> &'a str {
 }
 
 // Some questions:
-// todo: “How many bits of entropy does your generator produce?”
 // todo: “What if I need reproducibility?”
 fn main() {
     // todo: not cross-platform
@@ -128,6 +143,7 @@ fn main() {
         .expect("could not read dictionary");
 
     // filter words here for only ascii
+    // word_list length is 104078
     let word_list: Vec<&str> = content.lines().filter(|w|
         w.is_ascii()).collect();