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Most encrypted data formats leak metadata via their plaintext headers, suchas format version, encryption schemes used, number of recipients who candecrypt the data, and even the recipients' identities. This leakage can posesecurity and privacy risks to users, e.g., by revealing the full membership ofa group of collaborators from a single encrypted e-mail, or by enabling aneavesdropper to fingerprint the precise encryption software version andconfiguration the sender used. We propose that future encrypted data formatsimprove security and privacy hygiene by producing $\textit{Padded UniformRandom Blobs}$ or PURBs: ciphertexts indistinguishable from random bit stringsto anyone without a decryption key. A PURB's content leaks $\textit{nothing atall}$, even the application that created it, and is padded such that even itslength leaks as little as possible. Encoding and decoding ciphertexts with$\textit{no}$ cleartext markers presents efficiency challenges, however. Wepresent cryptographically agile encodings enabling legitimate recipients todecrypt a PURB efficiently, even when encrypted for any number of recipients'public keys and/or passwords, and when these public keys are from differentcryptographic suites. PURBs employ Padm\'e, a~novel padding scheme that limitsinformation leakage via ciphertexts of maximum length $M$ to a practicaloptimum of $O(\log \log M)$ bits, comparable to padding to a power of two, butwith lower overhead of at most $12\%$ and decreasing with larger payloads.

Reducing Metadata Leakage from Encrypted Files and Communication with PURBs