Onion Routing

Updated 11 Aug 2026

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Definition
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  a technique (also called anonymous routing) for sending a message so that no single relay knows both who sent it and where it's ultimately going — used to post anonymously (e.g. to an online forum) without revealing identity
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How it works
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  Alice nests the message in layers of encryption, one per relay ("mix"): $c_2 := E(k_c, \langle \text{David}, E(k_d, \langle \text{Bob}, m \rangle) \rangle)$ for relays Carol and David
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  each relay decrypts one layer with its own key, learns only "who to forward to next," and passes the remaining ciphertext along — like peeling an onion
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  a single relay (even if malicious) never learns both the original sender and the final content/destination; anonymity breaks only if enough relays collude
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Mixing service
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  a relay that batches and shuffles many users' messages together, so an outside observer watching traffic in and out can't correlate which incoming message maps to which outgoing one; this batch is called the anonymity set
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Security requirement: nested encryption
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  for onion routing to work, each relay holding its own key must learn nothing about the original message $m$ from the nested ciphertext
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  key result — if the underlying cipher is semantically secure, then $n$ -way nested encryption remains semantically secure even if the adversary is given all but one of the $n$ layer keys
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Related
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  Semantic Security
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  Security Reduction

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