TLS Client Puzzles Extension

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Document Type Expired Internet-Draft (individual)
Author Erik Nygren 
Last updated 2016-01-03 (latest revision 2015-07-02)
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Client puzzles allow a TLS server to defend itself against asymmetric DDoS attacks. In particular, it allows a server to request clients perform a selected amount of computation prior to the server performing expensive cryptographic operations. This allows servers to employ a layered defense that represents an improvement over pure rate-limiting strategies. Client puzzles are implemented as an extension to TLS 1.3 [I-D.ietf-tls-tls13] wherein a server can issue a HelloRetryRequest containing the puzzle as an extension. The client must then resend its ClientHello with the puzzle results in the extension.


Erik Nygren (

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