When encrypting data using a counter (CTR) derived block cipher mode of operation, it is essential not to reuse the same initialization vector (IV) for a given key. An IV that complies with this requirement is called a "nonce" (**n**umber used **once**). Galois/Counter (GCM) and Counter with Cipher Block Chaining-Message Authentication Code (CCM) are both derived from counter mode. 

When using AES-GCM or AES-CCM, a given key and IV pair will create a "keystream" that is used to encrypt a plaintext (original content) into a ciphertext (encrypted content.) For any key and IV pair, this keystream is always deterministic. Because of this property, encrypting several plaintexts with one key and IV pair can be catastrophic. If an attacker has access to one plaintext and its associated ciphertext, they are able to decrypt everything that was created using the same pair.

Additionally, IV reuse also drastically decreases the key recovery computational complexity by downgrading it to a simpler polynomial root-finding problem. This means that even without access to a plaintext/ciphertext pair, an attacker may still be able to decrypt all the sensitive data.