2.1.8 Encryption Stream
The Encryption stream is an optional stream whose name MUST be "encryption". The format of this stream is specified in section 126.96.36.199. This stream MUST NOT be present unless both of the following conditions are met:
The document is encrypted with Office Binary Document RC4 CryptoAPI Encryption (section 188.8.131.52).
The fDocProps value is set in the EncryptionHeader.Flags.