The Advanced Encryption Standard (AES) is arguably the most important encryption algorithm in use today, protecting everything from classified government communications to your personal web browsing sessions. AES was selected through a public competition organized by the U.S. National Institute of Standards and Technology (NIST) between 1997 and 2000, replacing the aging Data Encryption Standard (DES) which had become vulnerable to brute-force attacks due to its short 56-bit key length. The winning algorithm, originally named Rijndael after its Belgian creators Joan Daemen and Vincent Rijmen, was chosen from fifteen competing designs based on its security strength, computational efficiency, and elegant mathematical structure.

AES operates on fixed-size blocks of 128 bits (16 bytes) using keys of 128, 192, or 256 bits. The algorithm performs multiple rounds of mathematical transformations—10 rounds for 128-bit keys, 12 for 192-bit, and 14 for 256-bit. Each round consists of four operations: SubBytes (a non-linear byte substitution using a lookup table), ShiftRows (a cyclic shift of rows in the state matrix), MixColumns (a mathematical mixing operation on columns), and AddRoundKey (XORing with a round-specific key derived from the main key). This combination of substitution, permutation, and mixing provides the "confusion and diffusion" properties that make the cipher resistant to cryptanalysis.

The mode of operation determines how AES handles data larger than a single 128-bit block. GCM (Galois/Counter Mode) is the preferred mode for modern applications because it provides both confidentiality (encryption) and authenticity (tamper detection) simultaneously—a property called authenticated encryption. GCM generates a random initialization vector (IV) for each encryption operation, ensuring that encrypting the same file with the same key twice produces completely different ciphertext. It also produces an authentication tag that the decryptor verifies, immediately detecting any tampering with the encrypted data. This is crucial because encryption alone does not prevent an attacker from modifying the ciphertext in ways that produce predictably altered plaintext.

Password-Based Key Derivation Function 2 (PBKDF2), used by this tool to convert your password into an AES key, addresses the fundamental mismatch between human-memorable passwords and cryptographic keys. A truly random 256-bit key has enormous entropy, but human passwords are far weaker—most passwords have only 20–40 bits of effective entropy. PBKDF2 strengthens weak passwords by applying a pseudorandom function (typically HMAC-SHA256) thousands or hundreds of thousands of times iteratively, combined with a random salt. This computational cost means that even if your password is relatively simple, an attacker must spend significant computation per guess, making brute-force attacks impractical.

The security guarantee of AES-256 is extraordinarily strong. A brute-force attack against a 256-bit key would require trying 2²⁵⁶ possible keys—a number so vast that even if every atom in the observable universe were a computer performing billions of operations per second, success would remain impossible within the lifetime of the universe. No practical attack against properly-implemented AES has ever been demonstrated, and the algorithm is approved for protecting information classified at the Top Secret level by the U.S. government.