Make maximum resident credential count configurable

CHANGELOG.md

@@ -4,12 +4,15 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+- Add config option for setting a maximum number of resident credentials.
+
 ## [0.1.1] - 2022-08-22
 - Fix bug that treated U2F payloads as APDU over APDU in NFC transport @conorpp
 - Add config option to skip UP when device was just booted,
   as insertion is a kind of UP check @robin-nitrokey
 
-## [Unreleased]
+## [0.1.0] - 2022-03-17
 
 - use 2021 edition
 - use @szszszsz's credential ID shortening

src/constants.rs

@@ -7,3 +7,5 @@ pub const U2F_UP_TIMEOUT: u32 = 250;
 
 pub const ATTESTATION_CERT_ID: CertId = CertId::from_special(0);
 pub const ATTESTATION_KEY_ID: KeyId = KeyId::from_special(0);
+
+pub const MAX_RESIDENT_CREDENTIALS_GUESSTIMATE: u32 = 100;

src/ctap2.rs

@@ -375,6 +375,16 @@ impl<UP: UserPresence, T: TrussedRequirements> Authenticator for crate::Authenti
             self.delete_resident_key_by_user_id(&rp_id_hash, &credential.user.id)
                 .ok();
 
+            // then check the maximum number of RK credentials
+            if let Some(max_count) = self.config.max_resident_credential_count {
+                let mut cm = credential_management::CredentialManagement::new(self);
+                let metadata = cm.get_creds_metadata()?;
+                let count = metadata.existing_resident_credentials_count.unwrap_or(max_count);
+                if count >= max_count {
+                    return Err(Error::KeyStoreFull);
+                }
+            }
+
             // then store key, making it resident
             let credential_id_hash = self.hash(credential_id.0.as_ref());
             try_syscall!(self.trussed.write_file(

src/ctap2/credential_management.rs

@@ -18,6 +18,7 @@ use ctap_types::{
 use littlefs2::path::{Path, PathBuf};
 
 use crate::{
+    constants::MAX_RESIDENT_CREDENTIALS_GUESSTIMATE,
     credential::Credential,
     state::{CredentialManagementEnumerateCredentials, CredentialManagementEnumerateRps},
     Authenticator, Result, TrussedRequirements, UserPresence,
@@ -67,9 +68,12 @@ where
         info!("get metadata");
         let mut response: Response = Default::default();
 
-        let guesstimate = self.state.persistent.max_resident_credentials_guesstimate();
+        let max_resident_credentials = self
+            .config
+            .max_resident_credential_count
+            .unwrap_or(MAX_RESIDENT_CREDENTIALS_GUESSTIMATE);
         response.existing_resident_credentials_count = Some(0);
-        response.max_possible_remaining_residential_credentials_count = Some(guesstimate);
+        response.max_possible_remaining_residential_credentials_count = Some(max_resident_credentials);
 
         let dir = PathBuf::from(b"rk");
         let maybe_first_rp =
@@ -98,11 +102,7 @@ where
                 None => {
                     response.existing_resident_credentials_count = Some(num_rks);
                     response.max_possible_remaining_residential_credentials_count =
-                        Some(if num_rks >= guesstimate {
-                            0
-                        } else {
-                            guesstimate - num_rks
-                        });
+                        Some(max_resident_credentials.saturating_sub(num_rks));
                     return Ok(response);
                 }
                 Some(rp) => {

src/lib.rs

@@ -76,6 +76,8 @@ pub struct Config {
     /// If set, the first Get Assertion or Authenticate request within the specified time after
     /// boot is accepted without additional user presence verification.
     pub skip_up_timeout: Option<Duration>,
+    /// The maximum number of resident credentials.
+    pub max_resident_credential_count: Option<u32>,
 }
 
 // impl Default for Config {

src/state.rs

@@ -273,11 +273,6 @@ pub struct PersistentState {
 impl PersistentState {
     const RESET_RETRIES: u8 = 8;
     const FILENAME: &'static [u8] = b"persistent-state.cbor";
-    const MAX_RESIDENT_CREDENTIALS_GUESSTIMATE: u32 = 100;
-
-    pub fn max_resident_credentials_guesstimate(&self) -> u32 {
-        Self::MAX_RESIDENT_CREDENTIALS_GUESSTIMATE
-    }
 
     pub fn load<T: client::Client + client::Chacha8Poly1305>(trussed: &mut T) -> Result<Self> {
         // TODO: add "exists_file" method instead?