CVE-2026-12803: KCCMBlockCipher MAC does not bind nonce when AAD is absent (cross-nonce AEAD forgery)

Affects Bouncy Castle for Java LTS before 2.73.12. DSTU 7624 CCM mode folds the nonce into the MAC block only inside processAAD(), which is skipped when no associated data is present. Without AAD the MAC is a zero-IV CBC-MAC over the plaintext alone, so tags do not depend on the nonce. A chosen-plaintext attacker can derive the keystreams for two nonces in three oracle queries and forge a valid ciphertext and tag for any unqueried plaintext and nonce, defeating authenticity.