Calculating ciphertext sizes

[David Shaw]

On Jul 11, 2011, at 3:26 PM, Aaron Toponce wrote:

> When encrypting a plaintext source, is there a way to predict the size of
> the ciphertext output? I'm sure this depends on the cipher used, as well if
> compression or hashing algos are used.

The single largest thing that affects your output is the compression used, and how well your input compresses.  For example, if you are encrypting straight text, you will get much better compression than if you are encrypting a movie file (which is generally already compressed, so can't be compressed much more, if at all).  On top of that there is a bunch of general OpenPGP overhead (encrypted session key, etc).

The cipher does make a difference here, but it's small and dwarfed by other factors.

David