[VOIPSEC] RTP or SRTP inside UDP - how understand?

[Simon Horne]

At 05:39 AM 28/03/2006, Hadriel Kaplan wrote:


> > -----Original Message-----
> > From: Voipsec-bounces at voipsa.org [mailto:Voipsec-bounces at voipsa.org] On
> > Behalf Of Simon Horne
> > Sent: Monday, March 27, 2006 3:32 PM
> > To: Sergey Vointsev; voipsec at voipsa.org
> > Subject: Re: [VOIPSEC] RTP or SRTP inside UDP - how understand?
> >
> > You can tell (and it is not definitive) if the payload is encrypted by
> > examining the payload length and seeing if it is not exactly the same as
> > what would be expected for that particular codec.. Usually the output
> > length from the cipher (due to the cipher key length) does not match
> > exactly the normal unencrypted payload size. It may only be a couple of
> > bytes but it is detectable.
>
>I don't think so - at least not for the default AES cipher in the spec.
>It's in counter mode.

AFAIK AES has a fixed block size of 128 bits or 16 bytes, A RTP payload may 
be any length (depending on the codec) but must be padded out to a multiple 
of 16kb for the cipher. So I have a payload of 28 bytes, this has to be 
padded out to 32 bytes (2x16) to put into the cipher which results in a 32 
byte encrypted block. There is a 4 byte variance.
Reference
http://en.wikipedia.org/wiki/Advanced_Encryption_Standard#Description_of_the_cipher

Have I missed something?

Simon