ZipCrypto

ZipCrypto is the legacy encryption method used by the ZIP file format. It is a symmetric stream cipher that encrypts the contents of ZIP entries using a password-derived key. Introduced with early ZIP implementations, ZipCrypto relies on a 96-bit key formed from three 32-bit keys that are initialized from the password through a CRC-32 based transform.

Mechanism details are simple by modern standards: for each plaintext byte, a keystream byte produced from the current state of the keys is XORed with the plaintext to produce the ciphertext. After processing each byte, the three keys are updated using the value of the plaintext byte and a CRC operation, causing the keystream to evolve over the course of the file. In practice, ZIP encryption with ZipCrypto also includes a small verification value to help determine if the correct password was used, but this does not provide strong authentication.

Security and limitations are well documented: ZipCrypto offers no cryptographic authentication or integrity protection, and its keystream is predictable under certain conditions. It is vulnerable to brute-force and known-plaintext attacks, particularly when weak passwords are used. As cryptographic standards progressed, ZipCrypto was superseded by stronger options, such as AES-based encryption in ZIP archives.

Usage and compatibility: ZipCrypto was once widely supported by legacy ZIP tools and operating systems. Today, many modern tools support both ZipCrypto and AES-based encryption, but ZipCrypto is often discouraged for protecting sensitive data. Some software defaults to stronger encryption and may warn or disable ZipCrypto when available.

See also: AES encryption in ZIP, ZIP file format, PKZIP.