Known-plaintext Attack - Present Day

Present Day

Modern ciphers such as Advanced Encryption Standard are not currently susceptible to known-plaintext attacks.

The PKZIP stream cipher used by older versions of the zip format specification is prone to this attack. For example, an attacker with an encrypted ZIP file needs only (part of) one unencrypted file from the archive which forms the "known-plaintext". Then using some publicly available software they can quickly calculate the key required to decrypt the entire archive. To obtain this unencrypted file the attacker could search the website for a suitable file, find it from another archive they can open, or manually try to reconstruct a plaintext file armed with the knowledge of the filename from the encrypted archive. However, the attack does not work on AES-encrypted zip files.