2.2.11 Encryption (Password to Open)
Workbook files can contain sensitive information that needs to be protected. A file can be protected by encrypting it using a password. Once a file is encrypted, the data can only be accessed by decrypting the file using a password.
File encryption for this format is specified in [MS-OFFCRYPTO] section 2.3.4.
See section 4 for information about security concerns relating to file encryption for this file format.