4.1 Encryption and Obfuscation (Password to Open)
When XOR obfuscation (section 184.108.40.206) is used, data can be easily extracted and the document password might be retrievable.
When XOR obfuscation (section 220.127.116.11) or Office binary document RC4 encryption (section 18.104.22.168) is used or when Office binary document RC4 CryptoAPI encryption (section 22.214.171.124) is used with fDocProps set to false in EncryptionHeader.Flags, the Document Summary Information stream and the Summary Information stream are not obfuscated or encrypted.
When Office binary document RC4 encryption (section 126.96.36.199) or Office binary document RC4 CryptoAPI encryption (section 188.8.131.52) is used, the same block numbers are reused in the WordDocument stream, the Table stream, and the entire Data stream. This reuse can occur potentially with known cleartext, implying that certain portions of encrypted data can be directly extracted or easily retrieved.
See [MS-OFFCRYPTO] section 4.1.3 for additional security considerations with encryption and obfuscation in Word binary files.