Another method is the AES encryption first introduced by WinZIP and available in some software such as WinZIP and 7zip. This is not very secure as shown in the chart below and can be cracked using standard tools.
The standard format for an encrypted zip file uses the ZIP2.0 format. This is a similar methodology to that used in cracking other hashes such as MD5 password recovery, SHA1 password recovery or even Microsoft Office document password recovery (Word doc / Excel xls). Recovery of the password can be achieved by retrieving the hash from the document and matching the hash against calculated hashes of known strings. So a strong password should be used to ensure security of the file. Using a tool such as John the Ripper you can break out the password by matching the computed hash at a rate of millions of attempts per second. Removal of a password from an encrypted zip file can be easy or hard depending on the complexity of the password.
