2 stable releases
1.0.1 | Jun 19, 2024 |
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1.0.0 | Jun 18, 2024 |
#697 in Cryptography
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31KB
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ms-offcrypto-writer
This crate allows encrypting ECMA376/OOXML (so newer MS-Office files such as XLSX) using the agile encryption method as described in MS-OFFCRYPTO.
How to use
Use the Ecma376AgileWriter
wrapper around a File
or Cursor
(or whatever writer you're using) for other crates such as simple-xlsx-writer
.
If you find security flaws
If you find any security flaws beyond the lack of zeroing out data structures, please send me an email at 42triangles@tutanota.com. I will try to answer within two days.
Compatibility notes
The created files are NOT byte-equivalent to the ones created by the used reference implementation of "Microsoft Excel für Microsoft 365 MSO (16.0.13001.20508) 64-Bit", but the streams embedded in the CFB files are, and the CFB metadata beyond access dates (specifically the CFB version, and per storage & stream the state bits & CLSID) are equivalent as well.
This does also include one deviation from the standard, which specifies that the HMAC key should have a length equal to the salt length in <keyData>
. However, the reference implementation uses an HMAC key length of 64.
The used values are:
- A salt size of 16 for both
<keyData>
and<p:encryptedKey>
- for encryption in both
<keyData>
and<p:encryptedKey>
: AES256 with CBC, and its derived values - for hashing in both
<keyData>
and<p:encryptedKey>
: SHA512, and its derived values - An HMAC key length of 64, contrary to the standard, but following the reference implementation
- A spin count of
100_000
Contributing
PRs are open; this is specifically aims to be a very simple and easy to audit implementation. A more complete implementation that includes reading as well, plus a lot more features & configurability, is in the works though - if you're interested in working on that instead, please send me an email at 42triangles@tutanota.com!
The src/encryption_info.xml
file
If you need to edit this, you may want to do it in a binary editor. It includes both binary data in the beginning, uses CRLF and should NOT include a trailing newline.
Roadmap
- A more complete implementation (though that will be a second crate, this crate will continue to receive updates)
- Make the XML templating saner. Mostly such that the file is fully UTF8 & trailing newlines (such as the ones added by vim) aren't an issue.
- Add examples & tests
- Maybe byte equivalent files including the CFB container file (though that will require a reimplementation for CFBs).
- Maybe on-the-fly encryption that never actually commits unencrypted data to the underlying writer. While I'm very interested in this, there's some design decisions to hash out & benchmarks to be done.
- Maybe optimising it for speed. It should be plenty fast, and either the
cfb
crate or thewrite!
usage directly into the stream is a likely culprit if things aren't, but nothing has been verified or even tested regarding performance. - Maybe implement zeroing of data structures.
The following things are NOT the goal of this crate, and will only be found in the more complete crate:
- Any Information Rights Management features
- Encryption other than agile
- Configurable agile encryption
- Certificates
- Supporting the old office binary file format (XLS for example, as opposed to XLSX)
- Reading encrypted files
- Write protection
- Signatures
- Things in MS-OFFCRYPTO that I don't know what they do yet: Sensitivity labels, MsoDataStore, EncryptedSIHash, EncryptedDSIHash, EncryptedPropertyStreamInfo
Dependencies
~2.5MB
~45K SLoC