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http://eprint.iacr.org/2001/012

http://csrc.nist.gov/groups/ST/toolkit/BCM/documents/proposedmodes/ffsem/ffsem-spec.pdf

but the basic idea is this.

If you try re-encrypting the output of an otherwise ordinary algo (such as aes) enough times, eventually you end up with one the same size or smaller than the one you entered. (I.E. there are enough trailing zeros you can truncate to original size and that the remainder is zeros can be read as implicit)

Conversely, if you decrypt, and the output is larger than the input, you re-decrypt until you get a result with enough trailing zeros that you can truncate it to input size, and end up with the value you started with.

Note that these will be arbitrary non-ascii strings though - raw binary - and probably not printable.