Re: [TLS] Question regarding TLS 1.3 session resumption

"Martin Thomson" <mt@lowentropy.net> Mon, 06 May 2019 03:32 UTC

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Date: Sun, 05 May 2019 23:31:57 -0400
From: "Martin Thomson" <mt@lowentropy.net>
To: tls@ietf.org
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Archived-At: <https://mailarchive.ietf.org/arch/msg/tls/Roe4vWnRrCPCzsEEBCWk883FAa0>
Subject: Re: [TLS] Question regarding TLS 1.3 session resumption
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Ilari answered your question, but this jumped out:

On Sun, May 5, 2019, at 21:11, geyer.lukas@gmail.com wrote:
> [...] both sides calculate,
>     PSK = [...]
> and the server sends back to the client the PSK (unencrypted [...])

The PSK never gets sent.  As you say, both sides can calculate the same value.

> My question is what key is encrypting the session ticket (is it a 
> symmetric key that is generated by the webserver/SSL library for each 
> session? 

If you want a bad design choice, NSS uses RSA encryption using a key from one of its certificates to encrypt a symmetric key that we use with CBC.  The symmetric key is used for all tickets once it is recovered, so the asymmetric encryption/decryption costs are paid just once when the system runs.

The ways in which this design is bad are lengthy enough that I won't bother to list them.  A better scheme would use a straight symmetric key with an AEAD, but there are historical deployment reasons for that design.