blockchain_db: add k-anonymity to txid fetching

Read more about k-anonymity [here](https://en.wikipedia.org/wiki/K-anonymity). We implement this feature in the monero daemon for transactions
by providing a "Txid Template", which is simply a txid with all but `num_matching_bits` bits zeroed out, and the number `num_matching_bits`. We add an operation to `BlockchainLMDB` called
`get_txids_loose` which takes a txid template and returns all txids in the database (chain and mempool) that satisfy that template. Thus, a client can
ask about a specific transaction from a daemon without revealing the exact transaction they are inquiring about. The client can control the statistical
chance that other TXIDs (besides the one in question) match the txid template sent to the daemon up to a power of 2. For example, if a client sets their `num_matching_bits`
to 5, then statistically any txid has a 1/(2^5) chance to match. With `num_matching_bits`=10, there is a 1/(2^10) chance, so on and so forth.

Co-authored-by: ACK-J <60232273+ACK-J@users.noreply.github.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/src/blockchain_db/blockchain_db.h b/src/blockchain_db/blockchain_db.h
index 835d6dcad..f3e962c81 100644
--- a/src/blockchain_db/blockchain_db.h
+++ b/src/blockchain_db/blockchain_db.h
@@ -1306,6 +1306,21 @@ public:
   virtual bool get_pruned_tx_blobs_from(const crypto::hash& h, size_t count, std::vector<cryptonote::blobdata> &bd) const = 0;
 
   /**
+   * @brief Get all txids in the database (chain and pool) that match a certain nbits txid template
+   *
+   * To be more specific, for all `dbtxid` txids in the database, return `dbtxid` if
+   * `0 == cryptonote::compare_hash32_reversed_nbits(txid_template, dbtxid, nbits)`.
+   *
+   * @param txid_template the transaction id template
+   * @param nbits number of bits to compare against in the template
+   * @param max_num_txs The maximum number of txids to match, if we hit this limit, throw early
+   * @return std::vector<crypto::hash> the list of all matching txids
+   *
+   * @throw TX_EXISTS if the number of txids that match exceed `max_num_txs`
+   */
+  virtual std::vector<crypto::hash> get_txids_loose(const crypto::hash& txid_template, std::uint32_t nbits, uint64_t max_num_txs = 0) = 0;
+
+  /**
    * @brief fetches a variable number of blocks and transactions from the given height, in canonical blockchain order
    *
    * The subclass should return the blocks and transactions stored from the one with the given