feat(core/totp): emit Steam Guard alphabet for kind=Steam

crates/relicario-core/src/item_types/totp.rs

@@ -8,6 +8,10 @@ use zeroize::Zeroizing;
 
 use crate::error::{RelicarioError, Result};
 
+/// Steam Mobile Authenticator's 5-character output alphabet.
+/// Deliberately excludes ambiguous glyphs (0/O, 1/I/L, S/5, A/Z).
+const STEAM_ALPHABET: &[u8] = b"23456789BCDFGHJKMNPQRTVWXY";
+
 #[derive(Debug, Clone, Serialize, Deserialize, Default)]
 pub struct TotpCore {
     pub config: TotpConfig,
@@ -96,6 +100,16 @@ pub fn compute_totp_code(config: &TotpConfig, now_unix_seconds: u64) -> Result<S
                   | ((hmac_out[offset + 1] as u32)        << 16)
                   | ((hmac_out[offset + 2] as u32)        << 8)
                   |  (hmac_out[offset + 3] as u32);
+    if matches!(config.kind, TotpKind::Steam) {
+        let mut t = truncated;
+        let mut out = String::with_capacity(5);
+        for _ in 0..5 {
+            out.push(STEAM_ALPHABET[(t % 26) as usize] as char);
+            t /= 26;
+        }
+        return Ok(out);
+    }
+
     let modulus = 10u32.pow(config.digits as u32);
     Ok(format!("{:0width$}", truncated % modulus, width = config.digits as usize))
 }
@@ -168,3 +182,103 @@ mod tests {
         assert!(json.contains("steam"));
     }
 }
+
+#[cfg(test)]
+mod steam_tests {
+    use super::*;
+
+    /// Reference implementation of the Steam 5-character output, per the
+    /// Steam Mobile Authenticator (and WinAuth's Steam-Guard adapter).
+    /// Used by tests below to cross-check the production impl without
+    /// requiring a third-party vector. The algorithm is short enough to
+    /// be reproduced here in isolation.
+    fn steam_output_reference(truncated: u32) -> String {
+        const ALPHA: &[u8] = b"23456789BCDFGHJKMNPQRTVWXY";
+        let mut t = truncated;
+        let mut out = String::with_capacity(5);
+        for _ in 0..5 {
+            out.push(ALPHA[(t % 26) as usize] as char);
+            t /= 26;
+        }
+        out
+    }
+
+    /// Compute the dynamic-truncated u32 the same way `compute_totp_code`
+    /// does internally — used to drive the reference impl.
+    fn truncated_for(secret: &[u8], counter: u64) -> u32 {
+        use hmac::{Hmac, Mac};
+        use sha1::Sha1;
+        let mut mac = Hmac::<Sha1>::new_from_slice(secret).unwrap();
+        mac.update(&counter.to_be_bytes());
+        let bytes = mac.finalize().into_bytes();
+        let offset = (bytes[bytes.len() - 1] & 0x0F) as usize;
+        ((bytes[offset]     as u32 & 0x7F) << 24)
+        | ((bytes[offset + 1] as u32)        << 16)
+        | ((bytes[offset + 2] as u32)        << 8)
+        |  (bytes[offset + 3] as u32)
+    }
+
+    #[test]
+    fn steam_output_matches_reference_impl() {
+        let secret = b"12345678901234567890".to_vec();
+        let cfg = TotpConfig {
+            secret: Zeroizing::new(secret.clone()),
+            algorithm: TotpAlgorithm::Sha1,
+            digits: 5,
+            period_seconds: 30,
+            kind: TotpKind::Steam,
+        };
+        let code_at_30  = compute_totp_code(&cfg, 30).unwrap();
+        let code_at_60  = compute_totp_code(&cfg, 60).unwrap();
+        let code_at_120 = compute_totp_code(&cfg, 120).unwrap();
+        assert_eq!(code_at_30,  steam_output_reference(truncated_for(&secret, 1)));
+        assert_eq!(code_at_60,  steam_output_reference(truncated_for(&secret, 2)));
+        assert_eq!(code_at_120, steam_output_reference(truncated_for(&secret, 4)));
+    }
+
+    #[test]
+    fn steam_output_is_exactly_5_chars_regardless_of_digits() {
+        let secret = b"hello world!".to_vec();
+        for digits in [4u8, 5, 6, 7, 8] {
+            let cfg = TotpConfig {
+                secret: Zeroizing::new(secret.clone()),
+                algorithm: TotpAlgorithm::Sha1,
+                digits,
+                period_seconds: 30,
+                kind: TotpKind::Steam,
+            };
+            let code = compute_totp_code(&cfg, 0).unwrap();
+            assert_eq!(code.len(), 5, "Steam output must be 5 chars (digits={})", digits);
+        }
+    }
+
+    #[test]
+    fn steam_output_uses_only_alphabet_chars() {
+        const ALPHA: &str = "23456789BCDFGHJKMNPQRTVWXY";
+        let secret = b"hello world!".to_vec();
+        let cfg = TotpConfig {
+            secret: Zeroizing::new(secret),
+            algorithm: TotpAlgorithm::Sha1,
+            digits: 5,
+            period_seconds: 30,
+            kind: TotpKind::Steam,
+        };
+        for t in 0u64..1000 {
+            let code = compute_totp_code(&cfg, t * 30).unwrap();
+            for ch in code.chars() {
+                assert!(ALPHA.contains(ch), "char {ch:?} not in Steam alphabet (t={t})");
+            }
+        }
+    }
+
+    #[test]
+    fn steam_alphabet_excludes_ambiguous_glyphs() {
+        // Authoritative Steam Guard alphabet from Valve's Steam Mobile
+        // Authenticator: 26 chars, excludes 0/O, 1/I/L, S, A, E, U, Z.
+        // (Note: '5' IS in the alphabet — S is excluded, so 5 is unambiguous.)
+        const ALPHA: &str = "23456789BCDFGHJKMNPQRTVWXY";
+        for ch in ['0', 'O', '1', 'I', 'L', 'S', 'A', 'Z'] {
+            assert!(!ALPHA.contains(ch), "ambiguous glyph {ch:?} must not be in alphabet");
+        }
+    }
+}