feat(daemon): TOTP secret generation + base32 + otpauth URI (feature-gated)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

daemon/src/totp/mod.rs

@@ -0,0 +1,99 @@
+//! Pure TOTP logic: generate 160-bit secret, base32-encode, render
+//! the otpauth:// URI for QR display. No verification — that's
+//! pam_google_authenticator's job at PAM time.
+
+#![allow(dead_code)] // wired through TotpStore (Task 3) and AppState (Task 4).
+
+use data_encoding::BASE32_NOPAD;
+use rand::RngCore;
+
+/// 160 bits per RFC 6238 §5.1.
+pub(crate) const SECRET_BYTES: usize = 20;
+
+pub(crate) fn generate_secret() -> [u8; SECRET_BYTES] {
+    let mut buf = [0u8; SECRET_BYTES];
+    rand::rng().fill_bytes(&mut buf);
+    buf
+}
+
+pub(crate) fn encode_secret(secret: &[u8]) -> String {
+    BASE32_NOPAD.encode(secret)
+}
+
+/// Build an `otpauth://` URI suitable for QR encoding.
+/// Format follows Google Authenticator's de-facto spec:
+/// otpauth://totp/<Issuer>:<account>?secret=<b32>&issuer=<Issuer>
+pub(crate) fn otpauth_uri(secret_b32: &str, account: &str, issuer: &str) -> String {
+    let acct = url_encode(account);
+    let iss = url_encode(issuer);
+    format!(
+        "otpauth://totp/{iss}:{acct}?secret={secret_b32}&issuer={iss}&algorithm=SHA1&digits=6&period=30"
+    )
+}
+
+/// Minimal RFC 3986 url-encoder for the small set of characters that show
+/// up in usernames + the literal "AuthForge" issuer. Avoids pulling in a
+/// percent-encoding crate just for this one call site.
+fn url_encode(s: &str) -> String {
+    let mut out = String::with_capacity(s.len());
+    for c in s.chars() {
+        match c {
+            'A'..='Z' | 'a'..='z' | '0'..='9' | '-' | '_' | '.' | '~' => out.push(c),
+            _ => {
+                let mut buf = [0u8; 4];
+                for b in c.encode_utf8(&mut buf).bytes() {
+                    out.push_str(&format!("%{b:02X}"));
+                }
+            }
+        }
+    }
+    out
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn generated_secret_is_160_bits() {
+        let s = generate_secret();
+        assert_eq!(s.len(), 20);
+        // Two consecutive draws should differ with overwhelming probability.
+        let s2 = generate_secret();
+        assert_ne!(s, s2);
+    }
+
+    #[test]
+    fn base32_round_trips() {
+        let s = generate_secret();
+        let enc = encode_secret(&s);
+        let dec = BASE32_NOPAD.decode(enc.as_bytes()).unwrap();
+        assert_eq!(dec, s);
+    }
+
+    #[test]
+    fn base32_uses_no_padding_uppercase() {
+        let s = [0u8; 20];
+        let enc = encode_secret(&s);
+        assert!(enc
+            .chars()
+            .all(|c| c.is_ascii_uppercase() || c.is_ascii_digit()));
+        assert!(!enc.contains('='));
+    }
+
+    #[test]
+    fn otpauth_uri_includes_account_and_issuer() {
+        let uri = otpauth_uri("ABCDEFGH", "alice", "AuthForge");
+        assert!(uri.starts_with("otpauth://totp/AuthForge:alice?"));
+        assert!(uri.contains("secret=ABCDEFGH"));
+        assert!(uri.contains("issuer=AuthForge"));
+    }
+
+    #[test]
+    fn url_encode_handles_special_chars() {
+        // Realistic-ish: a username with a dot and a space (rare on Linux,
+        // but the encoder must not silently drop bytes).
+        let uri = otpauth_uri("S", "user name", "AuthForge");
+        assert!(uri.contains("user%20name"));
+    }
+}