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Algorithm Agility?

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Re: Algorithm Agility?

#21

Post by emteycz »

QuinnWilton wrote: post #4
tptacek wrote: post #2

Algorithm agility is bad. You see it happening at the beginning of every TLS session; what you don't see are the long trail of vulnerabilities that have resulted from TLS's overly complicated handshake, and, worse, the essentially nonexistent vulnerabilities algorithm agility has mitigated over TLS's lifetime; in general, when TLS constructions have failed, they've failed in the joinery (block cipher modes, MAC arran…

And all of this is to say nothing of the frankly embarrassing problems that have plagued JWT as a result of algorithm agility (alg=none). Removing agility from JWT wouldn't make it a good specification, but it would certainly make it a better specification.

What is a good alternative?

Re: Algorithm Agility?

#22

Post by QuinnWilton »

emteycz wrote: post #21
QuinnWilton wrote: post #4

Earlier quoted context omitted.

And all of this is to say nothing of the frankly embarrassing problems that have plagued JWT as a result of algorithm agility (alg=none). Removing agility from JWT wouldn't make it a good specification, but it would certainly make it a better specification.

What is a good alternative?

This is a really tough question to answer, because the answer depends on what you're using JWT for. JWT crams as much functionality into the format as possible, and most of that functionality isn't needed for most use cases. This means that offering an alternative requires knowing some context about what you need out of JWT in the first place.

That being said, for most purposes, you can do worse than using either mutual TLS or Macaroons [0]. As always with cryptography though, the devil is in the details, so for a more thorough discussion, check out @tptacek's "A Child's Garden of Inter-Service Authentication Schemes" [1]. It's one of my favourite treatments of the topic, and discusses the tradeoffs of a few different techniques for different use-cases.

[0] https://en.wikipedia.org/wiki/Macaroons_(computer_science)

[1] https://latacora.micro.blog/a-childs-garden/

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