Editing Encryption (section)

=== Ciphertext length and padding ===

{{main| Padding (cryptography)}}

Even when encryption correctly hides a message's content and it cannot be tampered with at rest or in transit, a message's ''length'' is a form of [[metadata]] that can still leak sensitive information about the message. For example, the well-known [[CRIME]] and [[BREACH]] attacks against [[HTTPS]] were [[side-channel attack]]s that relied on information leakage via the length of encrypted content.<ref>{{cite report|url=https://tools.ietf.org/html/rfc7457|title= Summarizing Known Attacks on Transport Layer Security (TLS) and Datagram TLS (DTLS)|first1=Y.|last1=Sheffer|first2=R.|last2=Holz|first3=P.|last3=Saint-Andre|date= February 2015}}</ref> [[Traffic analysis]] is a broad class of techniques that often employs message lengths to infer sensitive implementation about traffic flows by aggregating information about a large number of messages.

[[Padding (cryptography)|Padding]] a message's payload before encrypting it can help obscure the cleartext's true length, at the cost of increasing the ciphertext's size and introducing or increasing [[Overhead (computing)|bandwidth overhead]]. Messages may be padded [[Padding (cryptography)#Randomized padding|randomly]] or [[Padding (cryptography)#Deterministic padding|deterministically]], with each approach having different tradeoffs. Encrypting and padding messages to form [[PURB (cryptography)|padded uniform random blobs or PURBs]] is a practice guaranteeing that the cipher text leaks no [[metadata]] about its cleartext's content, and leaks asymptotically minimal <math>O(\log\log M)</math> [[Entropy (information theory)|information]] via its length.<ref>{{cite journal|url=https://petsymposium.org/2019/files/papers/issue4/popets-2019-0056.pdf|title=Reducing Metadata Leakage from Encrypted Files and Communication with PURBs|first1=Kirill|last1=Nikitin|first2=Ludovic|last2=Barman|first3=Wouter|last3=Lueks|first4=Matthew|last4=Underwood|first5=Jean-Pierre|last5=Hubaux|first6=Bryan|last6=Ford|journal=Proceedings on Privacy Enhancing Technologies (PoPETS)|volume=2019|issue=4|pages=6–33|doi=10.2478/popets-2019-0056|year=2019|arxiv=1806.03160 |s2cid=47011059|doi-access=free}}</ref>