I am using OpenSSL 1.1.1 to implement a TLS 1.3 communication module so I tried to learn how data integrity and authentication is ensured. I read a lot about AEAD, GCM, CBC, Poly1305, EtM, MtE and E&M to the point that I know what they are, but I do not know how they are related to each other. My Questions are

  1. I know that TLS 1.3 uses AEAD. Does that mean that it is EtM, MtE or E&M?
  2. Is GCM an EtM algorithm for block ciphers?
  3. What does OpenSSL use by default? EtM, MtE or E&M? Which are supported?

I do not know how they are related to each other.

  • EtM, MtE and E&M are generic constructions that take pairs of schemes that satisfy weaker security notions (chosen-plaintext security and unforgeability) and turn them into AEAD-secure schemes.
  • GCM is a construction that takes a 128-bit block cipher and turns it into - an AEAD-secure scheme and internally uses something like EtM for that.
  • CBC is one construction that takes a block cipher and turns it into something that is suitable for the encryption part of the EtM, MtE and E&M schemes (though it is admittedly not easy to do this right in face of padding / timing attacks).
  • Poly1305 is a one-time MAC (that is a MAC that is only secure when used at most once for a given key) which can be paired up with e.g. a block cipher (to generate one-time keys from nonces) to become a regular MAC suitable for the MAC part in EtM, MtE and E&M schemes.

What does OpenSSL use by default?

Apparently OpenSSL will default to using either AES-GCM or ChaCha20-Poly1305 for symmetric encryption

Which are supported?

TLS 1.3 abandoned the concept of these generic compositions so you will only find complete AEAD schemes to be specified as ciphersuite.

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  • $\begingroup$ It appears that the OpenSSL wiki is down (due to some configuration error), so I couldn't look into the TLS 1.3 page there to see the current supported list. $\endgroup$ – SEJPM Sep 27 '19 at 11:03
  • $\begingroup$ 'use' is ambiguous. Yes, OpenSSL 1.1.1 by default offers or accepts AES-128/256-GCM and ChaCha-Poly, but what is negotiated on a particular connection also depends on the peer, and whether the server uses client or server preference. $\endgroup$ – dave_thompson_085 Sep 28 '19 at 1:54

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