CREATE TABLE IF NOT EXISTS key_signatures (
    master_key_id INTEGER NOT NULL,
    signer_key_id INTEGER NOT NULL,
    PRIMARY KEY(master_key_id, signer_key_id),
    FOREIGN KEY(master_key_id) REFERENCES keyrings_public(master_key_id) ON DELETE CASCADE
);

insertKeySignature:
INSERT INTO key_signatures (master_key_id, signer_key_id) VALUES (?, ?);

selectMasterKeyIdsBySigner:
SELECT master_key_id
   FROM key_signatures WHERE signer_key_id IN ?;