Reducing Metadata Leakage from Encrypted Files and Communication with PURBs

TL;DR

This paper introduces PURBs, a new encrypted data format that minimizes metadata leakage by producing ciphertexts indistinguishable from random data, enhancing privacy and security.

Contribution

The paper presents PURBs, a cryptographically secure, padding-efficient encrypted format that prevents metadata leakage even from the format's own creation process.

Findings

PURBs produce ciphertexts indistinguishable from random data.

The Padmé padding scheme minimizes information leakage to O(log log M) bits.

Efficient decryption is possible for multiple recipients with diverse cryptographic suites.

Abstract

Most encrypted data formats leak metadata via their plaintext headers, such as format version, encryption schemes used, number of recipients who can decrypt the data, and even the recipients' identities. This leakage can pose security and privacy risks to users, e.g., by revealing the full membership of a group of collaborators from a single encrypted e-mail, or by enabling an eavesdropper to fingerprint the precise encryption software version and configuration the sender used. We propose that future encrypted data formats improve security and privacy hygiene by producing $\textit{Padded Uniform Random Blobs}$ or PURBs: ciphertexts indistinguishable from random bit strings to anyone without a decryption key. A PURB's content leaks $\textit{nothing at all}$, even the application that created it, and is padded such that even its length leaks as little as possible. Encoding and decoding ciphertexts with $\textit{no}$ cleartext markers presents efficiency challenges, however. We present cryptographically agile encodings enabling legitimate recipients to decrypt a PURB efficiently, even when encrypted for any number of recipients' public keys and/or passwords, and when these public keys are from different cryptographic suites. PURBs employ Padm\'e, a~novel padding scheme that limits information leakage via ciphertexts of maximum length $M$ to a practical optimum of $O(\log \log M)$ bits, comparable to padding to a power of two, but with lower overhead of at most $12\%$ and decreasing with larger payloads.