Ilari answered your question, but this jumped out:

On Sun, May 5, 2019, at 21:11, [email protected] 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.

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