GCM Implementation and TLSCompressed.Length
nmav at gnutls.org
Tue Oct 18 00:01:03 CEST 2011
On 10/17/2011 05:59 PM, Alfredo Pironti wrote:
> I'm a post-doc researcher at INRIA, France, and I'm developing a TLS
> implementation (with the goal of formal verification), and I would
> like to include support for AEAD ciphers (e.g. AEAD_AES_128_GCM).
> However, I got stuck because of the following problem.
> According to RFC 5246, sec 220.127.116.11, the additional data (AD) for AEAD
> consist of "seq_num + TLSCompressed.type + TLSCompressed.version +
> Computing such AD, and in particular TLSCompressed.length, is feasible
> when encrypting. However, when decrypting it seems impossible to me to
> retrieve that value (indeed it should be secret, and the AEAD
> ciphertext should not reveal the size of the plaintext, right?
The supported AEAD ciphers in TLS are stream thus the length of the
ciphertext is the length of the plaintext. The spec prevents block AEAD
ciphers or any kind of random padding to be added in the future.
> all, in the Mac-then-encrypt mode of TLS, random padding is added for
> this exact purpose -- and TLSCompressed.length becomes available only
> after decryption, and before mac verification).
> Can you please explain me where am I wrong?
You are not. The spec was published in haste. However it works because
the currently supported AEAD ciphers are stream.
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