Re: [TLS] AAED ciphers: AES-GCM vs AES-EAX/AES-CCM: a meta-analysis

"Paterson, Kenny" <Kenny.Paterson@rhul.ac.uk> Tue, 13 January 2015 18:28 UTC

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From: "Paterson, Kenny" <Kenny.Paterson@rhul.ac.uk>
To: Watson Ladd <watsonbladd@gmail.com>
Thread-Topic: [TLS] AAED ciphers: AES-GCM vs AES-EAX/AES-CCM: a meta-analysis
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Date: Tue, 13 Jan 2015 18:15:15 +0000
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Subject: Re: [TLS] AAED ciphers: AES-GCM vs AES-EAX/AES-CCM: a meta-analysis
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Hi Watson,

On 13/01/2015 18:09, "Watson Ladd" <watsonbladd@gmail.com>; wrote:

>From what I understand, when AES-GCM is used there isn't any padding,
>and the length of the encrypted record is equal to the unencrypted one
>plus the tag, 

True.

>so this attack still works.

Actually, it's an even simpler, passive attack for AES-GCM - you just
observe the ciphertext length and you are done. The attack on CBC mode
requires an activity adversary and leads to closure of the TLS session
half of the time.

>So if we accept this attack
>(and I think we should), then the way AEAD ciphers are used in TLS are
>also insecure. I believe this attack got used to determine autofill
>entries in the Google search bar via passive observation, but I've not
>dug up the paper, so my memory may be wrong.

That would be an interesting reference to have to hand. Please dig!

>To fix this we need to add padding in TLS 1.3 and TLS 1.2 for AEAD modes.

https://tools.ietf.org/html/draft-pironti-tls-length-hiding-01 would be a
good starting point, no?


Cheers

Kenny