Calculating ciphertext sizes

Robert J. Hansen rjh at
Tue Jul 12 00:29:42 CEST 2011

> 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.

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