AES Cipher

AES is a symmetric cipher, so the same secret key is used for both encryption and decryption. Key length defines the protection level: 128-bit covers most everyday use cases, 192-bit is chosen for higher assurance environments, and 256-bit is the strongest common option used in finance and other security-sensitive systems.

The secret key must remain confidential. Anyone who gets the key can decrypt the data, so avoid sharing it over unsafe channels and prefer randomly generated keys instead of manual input.

The IV (Initialization Vector) or nonce makes sure that encrypting the same content with the same key still produces different ciphertext each time, which helps prevent pattern-based attacks.

CBC and CTR use a 16-byte IV (32 hex characters), while GCM uses a 12-byte nonce (24 hex characters). The IV does not need to stay secret, but it must be fresh for every encryption run and should never be reused.

AES works on fixed 16-byte blocks, and the selected mode controls how those blocks are chained together for data of any length.

CBC (Cipher Block Chaining): a classic and widely used mode for files and communication workflows.

CTR (Counter): turns AES into a stream-like mode that can be processed in parallel, making it well suited for large files.

GCM (Galois/Counter Mode): adds authentication on top of CTR, so it protects both confidentiality and integrity and is usually the best modern default.

Longer keys raise the cost of brute-force attacks dramatically, although they also add a small performance overhead.

128-bit: strong enough for the vast majority of workloads and usually the fastest option.

192-bit: chosen when a higher security margin is required.

256-bit: the strongest standard option and a solid default when performance constraints are not critical.

AES encrypts 16-byte blocks. If the input length is not a multiple of 16, the last block must be padded before encryption.

PKCS#7: fills the final block with bytes whose value equals the number of padding bytes, which makes removal precise during decryption.

Zero Padding: fills the remaining bytes with 00. It is simple, but ambiguous when the original data can naturally end with zero bytes.

Raw AES ciphertext is binary data, so it is often converted into a printable text format before transport or storage.

Base64: compact and well suited to APIs, JSON, and copy-paste workflows.

Hex: larger in size, but easier to inspect byte by byte during debugging and logging.