Stealth Key Exchange and Confined Access to the Record Protocol Data in TLS 1.3

We show how to embed a covert key exchange sub protocol within a regular TLS 1.3 execution, generating a stealth key in addition to the regular session keys. The idea, which has appeared in the literature before, is to use the exchanged nonces to transport another key value. Our contribution is to give a rigorous model and analysis of the security of such embedded key exchanges, requiring that the stealth key remains secure even if the regular key is under adversarial control. Specifically for our stealth version of the TLS 1.3 protocol we show that this extra key is secure in this setting under the common assumptions about the TLS protocol.