/* SPDX-License-Identifier: GPL-3.0-or-later

 * Copyright © 2022 The TokTok team.

 */

#include "shared_key_cache.h"

#include <stdint.h>

#include <string.h>     // memcpy(...)

#include "ccompat.h"

#include "crypto_core.h"

#include "logger.h"

#include "mem.h"

#include "mono_time.h"

typedef struct Shared_Key {

    uint8_t public_key[CRYPTO_PUBLIC_KEY_SIZE];

    uint8_t shared_key[CRYPTO_SHARED_KEY_SIZE];

    uint64_t time_last_requested;

} Shared_Key;

struct Shared_Key_Cache {

    Shared_Key *keys;

    const uint8_t* self_secret_key;

    uint64_t timeout; /** After this time (in seconds), a key is erased on the next housekeeping cycle */

    const Mono_Time *mono_time;

    const Memory *mem;

    const Logger *log;

    uint8_t keys_per_slot;

};

non_null()

static bool shared_key_is_empty(const Logger *log, const Shared_Key *k) {

    LOGGER_ASSERT(log, k != nullptr, "shared key must not be NULL");

    /*

     * Since time can never be 0, we use that to determine if a key slot is empty.

     * Additionally this allows us to use crypto_memzero and leave the slot in a valid state.

     */

    return k->time_last_requested == 0;

}

non_null()

static void shared_key_set_empty(const Logger *log, Shared_Key *k) {

    crypto_memzero(k, sizeof (Shared_Key));

    LOGGER_ASSERT(log, shared_key_is_empty(log, k), "shared key must be empty after clearing it");

}

Shared_Key_Cache *shared_key_cache_new(const Logger *log, const Mono_Time *mono_time, const Memory *mem, const uint8_t *self_secret_key, uint64_t timeout, uint8_t keys_per_slot)

{

    if (mono_time == nullptr || self_secret_key == nullptr || timeout == 0 || keys_per_slot == 0) {

        return nullptr;

    }

    // Time must not be zero, since we use that as special value for empty slots

    if (mono_time_get(mono_time) == 0) {

        // Fail loudly in debug environments

        LOGGER_FATAL(log, "time must not be zero (mono_time not initialised?)");

        return nullptr;

    }

    Shared_Key_Cache *res = (Shared_Key_Cache *)mem_alloc(mem, sizeof(Shared_Key_Cache));

    if (res == nullptr) {

        return nullptr;

    }

    res->self_secret_key = self_secret_key;

    res->mono_time = mono_time;

    res->mem = mem;

    res->log = log;

    res->keys_per_slot = keys_per_slot;

    // We take one byte from the public key for each bucket and store keys_per_slot elements there

    const size_t cache_size = 256 * keys_per_slot;

    Shared_Key *keys = (Shared_Key *)mem_valloc(mem, cache_size, sizeof(Shared_Key));

    if (keys == nullptr) {

        mem_delete(mem, res);

        return nullptr;

    }

    crypto_memlock(keys, cache_size * sizeof(Shared_Key));

    res->keys = keys;

    return res;

}

void shared_key_cache_free(Shared_Key_Cache *cache)

{

    if (cache == nullptr) {

        return;

    }

    const size_t cache_size = 256 * cache->keys_per_slot;

    // Don't leave key material in memory

    crypto_memzero(cache->keys, cache_size * sizeof (Shared_Key));

    crypto_memunlock(cache->keys, cache_size * sizeof (Shared_Key));

    mem_delete(cache->mem, cache->keys);

    mem_delete(cache->mem, cache);

}

/* NOTE: On each lookup housekeeping is performed to evict keys that did timeout. */

const uint8_t *shared_key_cache_lookup(Shared_Key_Cache *cache, const uint8_t public_key[CRYPTO_PUBLIC_KEY_SIZE])

{

    // caching the time is not necessary, but calls to mono_time_get(...) are not free

    const uint64_t cur_time = mono_time_get(cache->mono_time);

    // We can't use the first and last bytes because they are masked in curve25519. Selected 8 for good alignment.

    const uint8_t bucket_idx = public_key[8];

    Shared_Key* bucket_start = &cache->keys[bucket_idx*cache->keys_per_slot];

    const uint8_t* found = nullptr;

    // Perform lookup

    for(size_t i = 0; i < cache->keys_per_slot; ++i) {

        if (shared_key_is_empty(cache->log, &bucket_start[i])) {

            continue;

        }

        if (pk_equal(public_key, bucket_start[i].public_key)) {

            found = bucket_start[i].shared_key;

            bucket_start[i].time_last_requested = cur_time;

            break;

        }

    }

    // Perform housekeeping for this bucket

    for (size_t i = 0; i < cache->keys_per_slot; ++i) {

        if (shared_key_is_empty(cache->log, &bucket_start[i])) {

            continue;

        }

        const bool timed_out = (bucket_start[i].time_last_requested + cache->timeout) < cur_time;

        if (timed_out) {

            shared_key_set_empty(cache->log, &bucket_start[i]);

        }

    }

    if (found == nullptr) {

        // Insert into cache

        uint64_t oldest_timestamp = UINT64_MAX;

        size_t oldest_index = 0;

        /*

         *  Find least recently used entry, unused entries are prioritised,

         *  because their time_last_requested field is zeroed.

         */

        for (size_t i = 0; i < cache->keys_per_slot; ++i) {

            if (bucket_start[i].time_last_requested < oldest_timestamp) {

                oldest_timestamp = bucket_start[i].time_last_requested;

                oldest_index = i;

            }

        }

        // Compute the shared key for the cache

        if (encrypt_precompute(public_key, cache->self_secret_key, bucket_start[oldest_index].shared_key) != 0) {

            // Don't put anything in the cache on error

            return nullptr;

        }

        // update cache entry

        memcpy(bucket_start[oldest_index].public_key, public_key, CRYPTO_PUBLIC_KEY_SIZE);

        bucket_start[oldest_index].time_last_requested = cur_time;

        found = bucket_start[oldest_index].shared_key;

    }

    return found;

}