secure_gate/
fixed.rs

1//! Stack-allocated wrapper for fixed-size secrets.
2//!
3//! Provides [`Fixed<T>`], a zero-cost wrapper enforcing explicit access to sensitive data.
4//! Treat secrets as radioactive — minimize exposure surface.
5//!
6//! Inner type **must implement `Zeroize`** for automatic zeroization on drop.
7//!
8//! # Examples
9//!
10//! ```rust
11//! use secure_gate::{Fixed, RevealSecret};
12//!
13//! let secret = Fixed::new([1u8, 2, 3, 4]);
14//! let sum = secret.with_secret(|arr| arr.iter().sum::<u8>());
15//! assert_eq!(sum, 10);
16//! ```
17
18use crate::RevealSecret;
19use crate::RevealSecretMut;
20
21#[cfg(feature = "encoding-base64")]
22use crate::traits::encoding::base64_url::ToBase64Url;
23#[cfg(feature = "encoding-hex")]
24use crate::traits::encoding::hex::ToHex;
25
26#[cfg(feature = "rand")]
27use rand::{rngs::OsRng, TryRngCore};
28use zeroize::Zeroize;
29
30#[cfg(feature = "encoding-base64")]
31use crate::traits::decoding::base64_url::FromBase64UrlStr;
32#[cfg(feature = "encoding-bech32")]
33use crate::traits::decoding::bech32::FromBech32Str;
34#[cfg(feature = "encoding-bech32m")]
35use crate::traits::decoding::bech32m::FromBech32mStr;
36#[cfg(feature = "encoding-hex")]
37use crate::traits::decoding::hex::FromHexStr;
38
39/// Zero-cost stack-allocated wrapper for fixed-size secrets.
40///
41/// Always available. Inner type **must implement `Zeroize`** for automatic zeroization on drop.
42///
43/// No `Deref`, `AsRef`, or `Copy` by default — all access requires
44/// [`expose_secret()`](RevealSecret::expose_secret) or
45/// [`with_secret()`](RevealSecret::with_secret) (scoped, recommended).
46/// `Debug` always prints `[REDACTED]`. Performance indistinguishable from raw arrays.
47pub struct Fixed<T: zeroize::Zeroize> {
48    inner: T,
49}
50
51impl<T: zeroize::Zeroize> Fixed<T> {
52    /// Creates a new [`Fixed<T>`] by wrapping a value.
53    #[inline(always)]
54    pub const fn new(value: T) -> Self {
55        Fixed { inner: value }
56    }
57}
58
59impl<const N: usize> From<[u8; N]> for Fixed<[u8; N]> {
60    #[inline(always)]
61    fn from(arr: [u8; N]) -> Self {
62        Self::new(arr)
63    }
64}
65
66impl<const N: usize> core::convert::TryFrom<&[u8]> for Fixed<[u8; N]> {
67    type Error = crate::error::FromSliceError;
68
69    fn try_from(slice: &[u8]) -> Result<Self, Self::Error> {
70        if slice.len() != N {
71            #[cfg(debug_assertions)]
72            return Err(crate::error::FromSliceError::InvalidLength {
73                actual: slice.len(),
74                expected: N,
75            });
76            #[cfg(not(debug_assertions))]
77            return Err(crate::error::FromSliceError::InvalidLength);
78        }
79        let mut arr = [0u8; N];
80        arr.copy_from_slice(slice);
81        Ok(Self::new(arr))
82    }
83}
84
85/// Ergonomic encoding helpers for `Fixed<[u8; N]>`.
86impl<const N: usize> Fixed<[u8; N]> {
87    #[cfg(feature = "encoding-hex")]
88    #[inline]
89    pub fn to_hex(&self) -> alloc::string::String {
90        self.with_secret(|s: &[u8; N]| s.to_hex())
91    }
92
93    #[cfg(feature = "encoding-hex")]
94    #[inline]
95    pub fn to_hex_upper(&self) -> alloc::string::String {
96        self.with_secret(|s: &[u8; N]| s.to_hex_upper())
97    }
98
99    #[cfg(feature = "encoding-base64")]
100    #[inline]
101    pub fn to_base64url(&self) -> alloc::string::String {
102        self.with_secret(|s: &[u8; N]| s.to_base64url())
103    }
104}
105
106/// Explicit access to immutable [`Fixed<[T; N]>`] contents.
107impl<const N: usize, T: zeroize::Zeroize> RevealSecret for Fixed<[T; N]> {
108    type Inner = [T; N];
109
110    #[inline(always)]
111    fn with_secret<F, R>(&self, f: F) -> R
112    where
113        F: FnOnce(&[T; N]) -> R,
114    {
115        f(&self.inner)
116    }
117
118    #[inline(always)]
119    fn expose_secret(&self) -> &[T; N] {
120        &self.inner
121    }
122
123    #[inline(always)]
124    fn len(&self) -> usize {
125        N * core::mem::size_of::<T>()
126    }
127}
128
129/// Explicit access to mutable [`Fixed<[T; N]>`] contents.
130impl<const N: usize, T: zeroize::Zeroize> RevealSecretMut for Fixed<[T; N]> {
131    #[inline(always)]
132    fn with_secret_mut<F, R>(&mut self, f: F) -> R
133    where
134        F: FnOnce(&mut [T; N]) -> R,
135    {
136        f(&mut self.inner)
137    }
138
139    #[inline(always)]
140    fn expose_secret_mut(&mut self) -> &mut [T; N] {
141        &mut self.inner
142    }
143}
144
145#[cfg(feature = "rand")]
146impl<const N: usize> Fixed<[u8; N]> {
147    #[inline]
148    pub fn from_random() -> Self {
149        let mut bytes = [0u8; N];
150        OsRng
151            .try_fill_bytes(&mut bytes)
152            .expect("OsRng failure is a program error");
153        Self::from(bytes)
154    }
155}
156
157#[cfg(feature = "encoding-hex")]
158impl<const N: usize> Fixed<[u8; N]> {
159    pub fn try_from_hex(hex: &str) -> Result<Self, crate::error::HexError> {
160        let bytes = zeroize::Zeroizing::new(hex.try_from_hex()?);
161        if bytes.len() != N {
162            #[cfg(debug_assertions)]
163            return Err(crate::error::HexError::InvalidLength {
164                expected: N,
165                got: bytes.len(),
166            });
167            #[cfg(not(debug_assertions))]
168            return Err(crate::error::HexError::InvalidLength);
169        }
170        let mut arr = [0u8; N];
171        arr.copy_from_slice(&bytes);
172        Ok(Self::new(arr))
173    }
174}
175
176#[cfg(feature = "encoding-base64")]
177impl<const N: usize> Fixed<[u8; N]> {
178    pub fn try_from_base64url(s: &str) -> Result<Self, crate::error::Base64Error> {
179        let bytes = zeroize::Zeroizing::new(s.try_from_base64url()?);
180        if bytes.len() != N {
181            #[cfg(debug_assertions)]
182            return Err(crate::error::Base64Error::InvalidLength {
183                expected: N,
184                got: bytes.len(),
185            });
186            #[cfg(not(debug_assertions))]
187            return Err(crate::error::Base64Error::InvalidLength);
188        }
189        let mut arr = [0u8; N];
190        arr.copy_from_slice(&bytes);
191        Ok(Self::new(arr))
192    }
193}
194
195#[cfg(feature = "encoding-bech32")]
196impl<const N: usize> Fixed<[u8; N]> {
197    /// Decodes a Bech32 (BIP-173) string into `Fixed<[u8; N]>`.
198    ///
199    /// # Warning
200    ///
201    /// The HRP is **not validated** — any HRP will be accepted as long as the checksum
202    /// is valid and the payload length equals `N`. For security-critical code where
203    /// cross-protocol confusion must be prevented, use [`try_from_bech32`](Self::try_from_bech32).
204    pub fn try_from_bech32_unchecked(s: &str) -> Result<Self, crate::error::Bech32Error> {
205        let (_hrp, bytes_raw) = s.try_from_bech32_unchecked()?;
206        let bytes = zeroize::Zeroizing::new(bytes_raw);
207        if bytes.len() != N {
208            #[cfg(debug_assertions)]
209            return Err(crate::error::Bech32Error::InvalidLength {
210                expected: N,
211                got: bytes.len(),
212            });
213            #[cfg(not(debug_assertions))]
214            return Err(crate::error::Bech32Error::InvalidLength);
215        }
216        let mut arr = [0u8; N];
217        arr.copy_from_slice(&bytes);
218        Ok(Self::new(arr))
219    }
220
221    /// Decodes a Bech32 (BIP-173) string into `Fixed<[u8; N]>`, validating that the HRP
222    /// matches `expected_hrp` (case-insensitive).
223    ///
224    /// Prefer this over [`try_from_bech32_unchecked`](Self::try_from_bech32_unchecked) in
225    /// security-critical code to prevent cross-protocol confusion attacks.
226    pub fn try_from_bech32(s: &str, expected_hrp: &str) -> Result<Self, crate::error::Bech32Error> {
227        let bytes_raw = s.try_from_bech32(expected_hrp)?;
228        let bytes = zeroize::Zeroizing::new(bytes_raw);
229        if bytes.len() != N {
230            #[cfg(debug_assertions)]
231            return Err(crate::error::Bech32Error::InvalidLength {
232                expected: N,
233                got: bytes.len(),
234            });
235            #[cfg(not(debug_assertions))]
236            return Err(crate::error::Bech32Error::InvalidLength);
237        }
238        let mut arr = [0u8; N];
239        arr.copy_from_slice(&bytes);
240        Ok(Self::new(arr))
241    }
242}
243
244#[cfg(feature = "encoding-bech32m")]
245impl<const N: usize> Fixed<[u8; N]> {
246    /// Decodes a Bech32m (BIP-350) string into `Fixed<[u8; N]>`.
247    ///
248    /// # Warning
249    ///
250    /// The HRP is **not validated** — any HRP will be accepted as long as the checksum
251    /// is valid and the payload length equals `N`. For security-critical code where
252    /// cross-protocol confusion must be prevented, use [`try_from_bech32m`](Self::try_from_bech32m).
253    pub fn try_from_bech32m_unchecked(s: &str) -> Result<Self, crate::error::Bech32Error> {
254        let (_hrp, bytes_raw) = s.try_from_bech32m_unchecked()?;
255        let bytes = zeroize::Zeroizing::new(bytes_raw);
256        if bytes.len() != N {
257            #[cfg(debug_assertions)]
258            return Err(crate::error::Bech32Error::InvalidLength {
259                expected: N,
260                got: bytes.len(),
261            });
262            #[cfg(not(debug_assertions))]
263            return Err(crate::error::Bech32Error::InvalidLength);
264        }
265        let mut arr = [0u8; N];
266        arr.copy_from_slice(&bytes);
267        Ok(Self::new(arr))
268    }
269
270    /// Decodes a Bech32m (BIP-350) string into `Fixed<[u8; N]>`, validating that the HRP
271    /// matches `expected_hrp` (case-insensitive).
272    ///
273    /// Prefer this over [`try_from_bech32m_unchecked`](Self::try_from_bech32m_unchecked) in
274    /// security-critical code to prevent cross-protocol confusion attacks.
275    pub fn try_from_bech32m(s: &str, expected_hrp: &str) -> Result<Self, crate::error::Bech32Error> {
276        let bytes_raw = s.try_from_bech32m(expected_hrp)?;
277        let bytes = zeroize::Zeroizing::new(bytes_raw);
278        if bytes.len() != N {
279            #[cfg(debug_assertions)]
280            return Err(crate::error::Bech32Error::InvalidLength {
281                expected: N,
282                got: bytes.len(),
283            });
284            #[cfg(not(debug_assertions))]
285            return Err(crate::error::Bech32Error::InvalidLength);
286        }
287        let mut arr = [0u8; N];
288        arr.copy_from_slice(&bytes);
289        Ok(Self::new(arr))
290    }
291}
292
293#[cfg(feature = "ct-eq")]
294impl<T: zeroize::Zeroize> crate::ConstantTimeEq for Fixed<T>
295where
296    T: crate::ConstantTimeEq,
297{
298    fn ct_eq(&self, other: &Self) -> bool {
299        self.inner.ct_eq(&other.inner)
300    }
301}
302
303impl<T: zeroize::Zeroize> core::fmt::Debug for Fixed<T> {
304    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
305        f.write_str("[REDACTED]")
306    }
307}
308
309#[cfg(feature = "cloneable")]
310impl<T: zeroize::Zeroize + crate::CloneableSecret> Clone for Fixed<T> {
311    fn clone(&self) -> Self {
312        Self::new(self.inner.clone())
313    }
314}
315
316#[cfg(feature = "serde-serialize")]
317impl<T: zeroize::Zeroize + crate::SerializableSecret> serde::Serialize for Fixed<T> {
318    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
319    where
320        S: serde::Serializer,
321    {
322        self.inner.serialize(serializer)
323    }
324}
325
326#[cfg(feature = "serde-deserialize")]
327impl<'de, const N: usize> serde::Deserialize<'de> for Fixed<[u8; N]> {
328    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
329    where
330        D: serde::Deserializer<'de>,
331    {
332        use core::fmt;
333        use serde::de::Visitor;
334        struct FixedVisitor<const M: usize>;
335        impl<'de, const M: usize> Visitor<'de> for FixedVisitor<M> {
336            type Value = Fixed<[u8; M]>;
337            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
338                write!(formatter, "a byte array of length {}", M)
339            }
340            fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
341            where
342                A: serde::de::SeqAccess<'de>,
343            {
344                let mut vec: zeroize::Zeroizing<alloc::vec::Vec<u8>> =
345                    zeroize::Zeroizing::new(alloc::vec::Vec::with_capacity(M));
346                while let Some(value) = seq.next_element()? {
347                    vec.push(value);
348                }
349                if vec.len() != M {
350                    #[cfg(debug_assertions)]
351                    return Err(serde::de::Error::invalid_length(
352                        vec.len(),
353                        &M.to_string().as_str(),
354                    ));
355                    #[cfg(not(debug_assertions))]
356                    return Err(serde::de::Error::custom("decoded length mismatch"));
357                }
358                let mut arr = [0u8; M];
359                arr.copy_from_slice(&vec);
360                Ok(Fixed::new(arr))
361            }
362        }
363        deserializer.deserialize_seq(FixedVisitor::<N>)
364    }
365}
366
367// Zeroize integration — now always present
368impl<T: zeroize::Zeroize> zeroize::Zeroize for Fixed<T> {
369    fn zeroize(&mut self) {
370        self.inner.zeroize();
371    }
372}
373
374impl<T: zeroize::Zeroize> Drop for Fixed<T> {
375    fn drop(&mut self) {
376        self.zeroize();
377    }
378}
379
380impl<T: zeroize::Zeroize> zeroize::ZeroizeOnDrop for Fixed<T> {}
secure_gate/fixed.rs

secure_gate/
fixed.rs
