Forum Discussion

Cumulonimbus

May 16, 2023

CBC ciphers in relation to RFC7366 Encrypt-then-MAC

Good morning, there is a recommendation from German federal security office to still allow CBC-mode ciphers as long as TLS extention "Encrypt-then-MAC" (RFC7366) is in use. But I can't find any info...

cbc

cipher

security

MegaZone
May 17, 2023
To my knowledge BIG-IP does not support/use the 'Encrypt-then-MAC' RFC7366 TLS extension. In fact, I can't find any mention of this RFC in our internal systems, so it is probably safe to say it is not supported. I think that, in general, the industry moved to AEAD ciphers instead.

As for AES-GCM - while it might be possible to configure a modern client NOT to use it, that'd very much be the exception and not the rule. Any browser old enough to lack AES-GCM support would be old enough to have many other issues (and probably wouldn't support TLSv1.2 anyway), so you're better off not allowing those to connect in the first place.

Especially has TLSv1.3 only has five supported cipher suites - and two of those are AES-GCM:

TLS_AES_256_GCM_SHA384

TLS_AES_128_GCM_SHA256

TLS_AES_128_CCM_8_SHA256

TLS_AES_128_CCM_SHA256

TLS_CHACHA20_POLY1305_SHA256

So AES-GCM support is basic table stakes for TLS these days.

Stefan_Klotz

Cumulonimbus

Thanks Paulius for sharing these links, especially the article from MegaZone (although I already know the summary PDF).

But I still can't find any hints regarding this "Encrypt-then-MAC" thing. So let's ask the other way round, as TLS1.0 and TLS1.1 shouldn't be used anyway, is there any other drawback of AES-GCM? I mean can ALL clients support GCM as long as they support TLS1.2 or might there be configurations out there, which can only support TLS1.2 with CBC-mode ciphers?

Thank you!

Regards Stefan 🙂

MegaZone

SIRT

May 17, 2023

To my knowledge BIG-IP does not support/use the 'Encrypt-then-MAC' RFC7366 TLS extension. In fact, I can't find any mention of this RFC in our internal systems, so it is probably safe to say it is not supported. I think that, in general, the industry moved to AEAD ciphers instead.

As for AES-GCM - while it might be possible to configure a modern client NOT to use it, that'd very much be the exception and not the rule. Any browser old enough to lack AES-GCM support would be old enough to have many other issues (and probably wouldn't support TLSv1.2 anyway), so you're better off not allowing those to connect in the first place.

Especially has TLSv1.3 only has five supported cipher suites - and two of those are AES-GCM:

TLS_AES_256_GCM_SHA384
TLS_AES_128_GCM_SHA256
TLS_AES_128_CCM_8_SHA256
TLS_AES_128_CCM_SHA256
TLS_CHACHA20_POLY1305_SHA256

So AES-GCM support is basic table stakes for TLS these days.

Nergalex
Employee
Sep 22, 2023
Apart from the reference of Encrypt-Then-Mac in this vuln K55462146, I found nothing too. It seems MegaZone is right when he says "I think that, in general, the industry moved to AEAD ciphers instead": regarding wikipedia "the Encrypt-and-MAC (E&M) approach has not been proved to be strongly unforgeable in itself".