{
    _id: <int>,
    purpose: 'signLogicalTime',
    key: <20 byte key generated with secure PRNG in BinData>,
    expiresAt: <LogicalTime, currently Timestamp format>
}

class KeysCollectionDocument {
public:
    KeysCollectionDocument(long long keyId, std::string purpose, TimeProofService::Key key, LogicalTime expiresAt);
    static StatusWith<TypeKeys> fromBSON(const BSONObj& doc);
    BSONObj toBSON() const;

    // getter methods
};