Adobe PDF Security Specification

Adobe's Portable Document Format (PDF) includes various security mechanisms designed to protect document integrity, restrict access, and prevent unauthorized modifications. These security features are defined within the PDF specification and implemented across different versions of the format.

Encryption and Password Protection

PDF files can be encrypted to prevent unauthorized access. Encryption is implemented using the following key mechanisms:

User and Owner Passwords

  • User Password: Restricts document opening by unauthorized users. The user must enter a password to view the document.

  • Owner Password: Grants permissions to modify the document, including changing security settings.

Encryption Algorithms

  • RC4 (40-bit, 128-bit): Used in earlier PDF versions.

  • AES (128-bit, 256-bit): Introduced in later versions for stronger encryption. (BoardCloud uses AES-256 encryption)

  • Public Key Encryption: Uses digital certificates instead of passwords for enhanced security.

Digital Signatures

PDF supports digital signatures to verify document authenticity and integrity. This is implemented through:

Certificate-Based Signatures

  • Uses X.509 certificates for identity verification.

  • Supports multiple signature fields for multi-party signing.

  • Ensures that any modifications after signing are detectable.

Timestamping

  • Provides a trusted timestamp from an external authority.

  • Ensures that the document existed at a specific time and has not been altered since.

Permissions and Restrictions

PDF documents can include restrictions to control what users can do with the file. These include:

Permission Flags

  • Printing: Allows or disallows document printing.

  • Content Copying: Prevents copying of text or images.

  • Editing: Restricts document modification.

  • Annotation: Controls adding or modifying comments.

PDF/A Compliance

  • PDF/A is an archival format that enforces security restrictions to ensure long-term preservation.

Redaction

PDF allows permanent removal of sensitive content using redaction tools:

  • Removes selected text, images, or metadata.

  • Ensures redacted content is irrecoverable.

  • Used for legal and confidential document handling.

JavaScript Security Restrictions

PDFs support embedded JavaScript, which can introduce security risks. To mitigate threats:

  • Execution is sandboxed.

  • User permission is required for certain operations.

  • Restricted access to system resources.

Metadata and Document Security

PDFs store metadata, which can contain sensitive information. Security measures include:

  • Metadata removal tools to erase hidden information.

  • Watermarking for tracking document distribution.

References

Adobe PDF Reference Documentation

ISO 32000-1: PDF 1.7 Specification