ODF format security: encryption, signatures and metadata management

Open Document Format (ODF) is an open standard for office documents – texts, spreadsheets, presentations and more – that is flexible and interoperable. As with any other digital format, its security is a key concern, as ODF files often contain sensitive information that, without adequate protection measures, can be exposed, tampered with or tracked.

This post analyses how ODF handles security, focusing on encryption, digital signatures and metadata management: three features that protect documents from prying eyes and tampering.

Encryption: content locking

ODF supports file-level encryption using standard algorithms. When you save an ODF document with a password, the content is compressed and then encrypted using AES (Advanced Encryption Standard), typically with a 256-bit key.

Here’s what happens behind the scenes:

1. The document content (the XML file) is compressed.
2. A random salt (a sequence of bits used together with a password as input to a one-way function) and an initialisation vector (IV), which is a block of bits of a predefined length, are generated.
3. A key is derived from the password using PBKDF2 (Password-Based Key Derivation Function 2). The key is a string of data used with an algorithm to encode (encrypt) and decode (decrypt) the text, transforming it from plain text to unreadable text.
4. AES encrypts the content using this key.

This is encryption based on open and verified algorithms, sufficiently strong when implemented correctly, whose security depends largely on the strength of the password. Users should therefore always use long, unique passwords, preferably created by a password generator.

Unfortunately, not all applications that support the ODF format implement encryption in the same way, with possible repercussions on interoperability.

Digital signatures: who modified the document?

Digital signatures guarantee authenticity and integrity, and show who created or modified the ODF file, and whether it has been modified by another user since its creation.

How it works:

1. ODF uses XML digital signatures.
2. A cryptographic hash of the document’s content is created (a fixed-size digital fingerprint of data, created by a one-way algorithm that is almost impossible to reverse).
3. This hash is signed with the signer’s private key (a secret code, similar to a password, used to encrypt and decrypt data and digitally sign transactions).
4. The signature is stored in “META-INF/documentsignatures.xml”.

This makes it possible to verify the origin of the document, but verifying signatures requires access to the signer’s public key or certificate. If the workflow involves multiple people, multiple signatures are supported. Any changes to the file after signing invalidate the signature.

Unfortunately, not all office suites that support ODF consistently display or validate signatures.

Metadata management: a potential information leak

Metadata can unintentionally disclose various information, including sensitive information such as usernames, file paths, software versions, timestamps (creation and save dates), and even content revision history.

What does metadata contain?

1. Author name, change history, and save times.
2. Custom properties, such as project codes or internal comments.
3. Application-specific data embedded in namespaces.

Malicious actors can extract metadata for social engineering, document tracking, or profiling purposes. To prevent this, use metadata cleaning tools—such as those in LibreOffice—or document sanitisation tools, such as manually inspecting the “meta.xml” file within the ODF archive. Of course, it is always best to avoid including unnecessary comments or tracked changes before sharing.

Final considerations

ODF is a robust format, and its security features are solid when used correctly. Encrypt sensitive files with strong passwords. Sign important documents to assert ownership and integrity. And don’t forget to clean up metadata before hitting “send”.

Security is not just about features, but also about habits. Make these protections an integral part of your standard workflow, and your ODF files will be able to withstand most threats.