Change PIN data path

This patch changes the path used to store PIN data inside the client
namespace from backend-auth/pin/<id> to backend-auth/pin.<id> to make
better use of the available space.

It also adds documentation for the filesystem layout.

Fixes https://github.com/trussed-dev/trussed-auth/issues/9

Cargo.toml

@@ -22,6 +22,7 @@ subtle = { version = "2.4.1", default-features = false }
 trussed = { version = "0.1.0", features = ["serde-extensions"] }
 
 [dev-dependencies]
+quickcheck = { version = "1.0.3", default-features = false }
 rand_core = { version = "0.6.4", default-features = false, features = ["getrandom"] }
 trussed = { version = "0.1.0", features = ["serde-extensions", "virt"] }
 

src/backend.rs

@@ -22,7 +22,7 @@ use trussed::{
 
 use crate::{
     extension::{reply, AuthExtension, AuthReply, AuthRequest},
-    PIN_PATH, SALT_PATH,
+    BACKEND_DIR,
 };
 use data::{Key, PinData, Salt, KEY_LEN, SALT_LEN};
 
@@ -50,6 +50,20 @@ impl fmt::Debug for HardwareKey {
 ///
 /// This implementation stores PINs together with their retry counters on the filesystem.  PINs are
 /// hashed with SHA-256 using a salt that is generated per PIN.
+///
+/// # Filesystem Layout
+///
+/// ```text
+/// trussed/
+///     backend-auth/
+///         salt            global salt for key derivation
+/// <client>/
+///     backend-auth/
+///         pin.<id>        PIN data, can be deleted with DeletePin or DeleteAllPins
+/// ```
+///
+/// The storage location can be set when creating the backend, see [`AuthBackend::new`][] and
+/// [`AuthBackend::with_hw_key`][].
 #[derive(Clone, Debug)]
 pub struct AuthBackend {
     location: Location,
@@ -80,7 +94,7 @@ impl AuthBackend {
         trussed_filestore: &mut impl Filestore,
         rng: &mut R,
     ) -> Result<Salt, Error> {
-        let path = PathBuf::from(SALT_PATH);
+        let path = PathBuf::from(BACKEND_DIR).join(&PathBuf::from("salt"));
         trussed_filestore
             .read(&path, self.location)
             .or_else(|_| {
@@ -263,7 +277,7 @@ impl ExtensionImpl<AuthExtension> for AuthBackend {
                 Ok(reply::DeletePin.into())
             }
             AuthRequest::DeleteAllPins(_) => {
-                fs.remove_dir_all(&PathBuf::from(PIN_PATH), self.location)
+                fs.remove_dir_all(&PathBuf::from(BACKEND_DIR), self.location)
                     .map_err(|_| Error::WriteFailed)?;
                 Ok(reply::DeleteAllPins.into())
             }

src/lib.rs

@@ -1,7 +1,7 @@
 // Copyright (C) Nitrokey GmbH
 // SPDX-License-Identifier: Apache-2.0 or MIT
 
-#![no_std]
+#![cfg_attr(not(test), no_std)]
 #![warn(
     missing_debug_implementations,
     missing_docs,
@@ -80,8 +80,7 @@ pub const MAX_PIN_LENGTH: usize = MAX_SHORT_DATA_LENGTH;
 /// A PIN.
 pub type Pin = Bytes<MAX_PIN_LENGTH>;
 
-const PIN_PATH: &str = "backend-auth/pin";
-const SALT_PATH: &str = "backend-auth/salt";
+const BACKEND_DIR: &str = "backend-auth";
 
 /// The ID of a PIN within the namespace of a client.
 ///
@@ -112,9 +111,18 @@ pub struct PinId(u8);
 
 impl PinId {
     fn path(&self) -> PathBuf {
-        let mut path = PathBuf::from(PIN_PATH);
-        path.push(&PathBuf::from(&self.hex()));
-        path
+        const PIN_PREFIX: &[u8] = b"/pin.";
+        const N: usize = BACKEND_DIR.len();
+        const M: usize = PIN_PREFIX.len();
+
+        let mut path = [0; N + M + 2];
+        let (backend_dir, rest) = path.split_at_mut(N);
+        let (pin, id) = rest.split_at_mut(M);
+        backend_dir.copy_from_slice(BACKEND_DIR.as_bytes());
+        pin.copy_from_slice(PIN_PREFIX);
+        id.copy_from_slice(&self.hex());
+
+        PathBuf::from(&path)
     }
 
     fn hex(&self) -> [u8; 2] {
@@ -137,3 +145,18 @@ impl From<PinId> for u8 {
         id.0
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::PinId;
+    use trussed::types::PathBuf;
+
+    quickcheck::quickcheck! {
+        fn test_pin_path(id: u8) -> bool {
+            let actual = PinId(id).path();
+            let expected = PathBuf::from(format!("backend-auth/pin.{id:02x}").as_str());
+            println!("id: {id}, actual: {actual}, expected: {expected}");
+            actual == expected
+        }
+    }
+}