Calculating ciphertext sizes

Aaron Toponce aaron.toponce at
Tue Jul 12 00:52:31 CEST 2011

On Mon, Jul 11, 2011 at 06:29:42PM -0400, Robert J. Hansen wrote:
> > The reason for asking (which actually isn't realted to GnuPG) was I wanted
> > to know the amount of data transferred over the wire with SCP.
> Then this isn't a question related to encipherment: this is a protocol question.  Once you start looking at the protocol layer, other things have enormously more impact than just encryption operations.  For instance, if your wire protocol requires data be 7-bit clean binary data will expand out significantly.  If your wire protocol supports compression, the transmitted data might substantially decrease.
> In the case of SCP, the OpenSSH geeks do their best to obfuscate the size of the transmitted data.  They do this in order to make traffic analysis more difficult, but also makes predicting the amount of data sent more difficult.

Understood, however I disabled compression on the wire. I wanted raw data
with raw packets, and because the encryption algorithm is the primary data
manipulator, and I can only measure the data segment of the packets,
ignoring headers, I would think this works fairly well, unless I'm missing

At any rate, the mathematics table of predicting the output of each input,
without compression or signing, would be very handy. Curious how you got
the numbers from before.


. o .   o . o   . . o   o . .   . o .
. . o   . o o   o . o   . o o   . . o
o o o   . o .   . o o   o o .   o o o
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