Unpadded encrypted messages leak the size of the message being sent. This is useful for traffic analysis. For example, if several people forward a large message to each other, an eavesdropper can guess that is happening. Fixed sized messages close this leak.
This Python code pads out a message with "meaningless garbage" like this:
-----BEGIN MEANINGLESS GARBAGE----- m9ufRv/AXymM+haOzBCmJ5I6gHtqJUMjH3wiYnBK2MgGJIDnxEQ9xxQMGXIiKPYtriND8ipX m/4MU2rx/gVAFjFsPJcDAcQRMKVdhpgDSxhT2qlbMY1Ji4tRojx9dp56q6rWsLHjNslQU8Pl gZmsGpbyaWx2tywAy4ncpihmc7YCK5RkW/sQ61L+uKkW/NCy0Yc/6XEMFMfeSYD+/HmrUyLR ALPxTZsc3EyhURxZjid0rfDu298nlCuN6D5yH90C9C1oeJ/J4LS85TDAbLaIbUTwKGi/BDma 9HQAOr7lXnCEZtiPrLPlyHvmv1dlK+BjFMlD/gtgKDL+RGVn3lPVCaw8sZz2+1YAAAjqRi5S slXs2BSjD6aZvYZenXexrr7z+jDoOOqkDhT8uNoz+duYm1m1ef0n4286FLBUKew+g0Je3W9b pm -----END MEANINGLESS GARBAGE-----
Users should turn off compression. Compressing a message padded to a known fixed size may reveal significant information, particularly if the eavesdropper is able to make guesses about the contents. (GnuPG users can turn off compression with "-z 0".)
The current release is padding-0.1.tar.gz.