sframe 2.0.0

use crate::header;
use std::collections::HashMap;

use super::{
    FrameValidation, ReplayAttackProtection, ReplayAttackProtectionError, ReplayToken, Tolerance,
    UnvalidatedFrame,
};

/// Tracks replay protection per key id, keeping a [`ReplayAttackProtection`] for
/// each of them, see [RFC 9605 9.3](https://www.rfc-editor.org/rfc/rfc9605.html#name-anti-replay).
///
/// Key ids are tracked until they are [`remove`](Self::remove)d, thus a key id
/// should be dropped together with its key. Otherwise the store keeps growing,
/// note that a sender may use a set of key ids, e.g. when ratcheting.
pub struct ReplayAttackProtectionStore {
    validators: HashMap<header::KeyId, ReplayAttackProtection>,
    tolerance: Tolerance,
}

impl ReplayAttackProtectionStore {
    /// Creates a store, tracking each key id with the given [`Tolerance`] for the
    /// frame count.
    pub fn new(tolerance: Tolerance) -> Self {
        ReplayAttackProtectionStore {
            validators: HashMap::new(),
            tolerance,
        }
    }

    /// Stops tracking the given key id, returning whether it was tracked at all.
    pub fn remove<K>(&mut self, key_id: K) -> bool
    where
        K: Into<header::KeyId>,
    {
        self.validators.remove(&key_id.into()).is_some()
    }

    /// Stops tracking every key id which does not match the predicate
    pub fn retain<F>(&mut self, mut keep: F)
    where
        F: FnMut(header::KeyId) -> bool,
    {
        self.validators.retain(|key_id, _| keep(*key_id));
    }
}

/// Evidence that a frame passed the store's replay check, recordable only by a
/// [`ReplayAttackProtectionStore`]. Drop it to discard the screened frame.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct ReplayStoreToken(Screened);

/// What screening found: a key id either has a window which accepted the frame,
/// or none to screen it against.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Screened {
    /// Accepted by the window of a tracked key id.
    Tracked(ReplayToken),
    /// The key id has no window yet, so nothing was screened.
    Untracked {
        key_id: header::KeyId,
        counter: header::Counter,
    },
}

impl FrameValidation for ReplayAttackProtectionStore {
    type Token = ReplayStoreToken;

    type Error = ReplayAttackProtectionError;

    /// Screens a frame against the window of its key id. An unknown key id
    /// passes: it is only tracked once a frame of it was decrypted, otherwise
    /// anyone could grow the store with forged headers.
    fn screen(&self, unvalidated: UnvalidatedFrame<'_>) -> Result<Self::Token, Self::Error> {
        let header = unvalidated.header();
        let screened = match self.validators.get(&header.key_id()) {
            Some(validator) => Screened::Tracked(validator.screen(unvalidated)?),
            None => Screened::Untracked {
                key_id: header.key_id(),
                counter: header.counter(),
            },
        };

        Ok(ReplayStoreToken(screened))
    }

    // TODO(perf): avoid allocs in the hot path
    fn record(&mut self, ReplayStoreToken(screened): Self::Token) {
        let tolerance = self.tolerance;
        let (key_id, token) = match screened {
            Screened::Tracked(token) => (token.key_id, token),
            Screened::Untracked { key_id, counter } => (key_id, ReplayToken { key_id, counter }),
        };

        self.validators
            .entry(key_id)
            .or_insert_with(|| ReplayAttackProtection::new(key_id, tolerance))
            .record(token);
    }
}

#[cfg(test)]
mod test {
    use super::*;
    use crate::frame::validation::util::test::{screen, screen_and_record};
    use ReplayAttackProtectionError::DuplicatedFrame;

    const TOLERANCE: usize = 128;
    const KID_A: header::KeyId = 1;
    const KID_B: header::KeyId = 2;
    const COUNTER: header::Counter = 2480;

    fn store() -> ReplayAttackProtectionStore {
        ReplayAttackProtectionStore::new(Tolerance::new(TOLERANCE))
    }

    #[test]
    fn tracks_key_ids_independently() {
        let mut store = store();

        // The same counter from two key ids is not a replay.
        assert!(screen_and_record(&mut store, KID_A, COUNTER).is_ok());
        assert!(screen_and_record(&mut store, KID_B, COUNTER).is_ok());
    }

    #[test]
    fn reuses_the_window_of_a_key_id() {
        let mut store = store();

        // A duplicate is only detected if the key id keeps its window,
        // instead of getting a new one per frame.
        assert!(screen_and_record(&mut store, KID_A, COUNTER).is_ok());
        assert!(matches!(
            screen(&store, KID_A, COUNTER),
            Err(DuplicatedFrame { .. })
        ));
    }

    #[test]
    fn screens_a_known_key_id() {
        let mut store = store();
        screen_and_record(&mut store, KID_A, COUNTER).unwrap();

        assert!(matches!(
            screen(&store, KID_A, COUNTER),
            Err(DuplicatedFrame { .. })
        ));
    }

    #[test]
    fn screening_accepts_an_unknown_key_id() {
        let store = store();

        assert!(screen(&store, KID_A, COUNTER).is_ok());
    }

    #[test]
    fn screening_does_not_track_a_key_id() {
        let store = store();

        screen(&store, KID_A, COUNTER).unwrap();

        // Key ids are unauthenticated, tracking them on screening would let
        // anyone grow the store.
        assert!(store.validators.is_empty());
    }

    #[test]
    fn removes_a_tracked_key_id() {
        let mut store = store();
        screen_and_record(&mut store, KID_A, COUNTER).unwrap();

        assert!(store.remove(KID_A));

        // Its window is gone, so the counter is not known anymore.
        assert!(screen(&store, KID_A, COUNTER).is_ok());
    }

    #[test]
    fn retains_only_the_matching_key_ids() {
        let mut store = store();
        screen_and_record(&mut store, KID_A, COUNTER).unwrap();
        screen_and_record(&mut store, KID_B, COUNTER).unwrap();

        store.retain(|key_id| key_id == KID_B);

        // The window of KID_A is gone, KID_B keeps its counter.
        assert!(screen(&store, KID_A, COUNTER).is_ok());
        assert!(matches!(
            screen(&store, KID_B, COUNTER),
            Err(DuplicatedFrame { .. })
        ));
    }

    #[test]
    fn reports_removal_of_an_untracked_key_id() {
        let mut store = store();

        assert!(!store.remove(KID_A));
    }

    #[test]
    fn keeps_other_key_ids_on_removal() {
        let mut store = store();
        screen_and_record(&mut store, KID_A, COUNTER).unwrap();
        screen_and_record(&mut store, KID_B, COUNTER).unwrap();

        store.remove(KID_A);

        assert!(matches!(
            screen(&store, KID_B, COUNTER),
            Err(DuplicatedFrame { .. })
        ));
    }
}