Don't use self-decrypting files.

You've come to this page because you've said something similar to the following:

Self-decrypting files (a.k.a. self-decrypting archives, or SDAs) are cool.

This is the Frequently Given Answer to such statements.

Self-decrypting files are not cool at all. As far as security goes self-decrypting files are in fact a very a bad idea. They undermine the very reasons that one is using encryption in the first place. If you think about why one encrypts files in the first place, and what parts of an encryption/decryption system need to be trustworthy, you should see why self-decrypting files are yet another Half-Baked Idea from those industrious chaps in the Half-Baked Ideas Brigade.

Self-decrypting files are bad for portability reasons, as well. Self-decrypting files only work if the sender and recipient have computers with the same CPU architecture and the same operating system. They are also not future-proof.

(History teaches a lesson here. Look at all of the self-extracting archives created for MS/PC-DOS systems in the 1980s and 1990s — which people then thought were "nifty" and "cool", too — and think about how many such DOS softwares now cannot run on modern PCs because they only work on old versions of MS/PC-DOS, or only work on MS/PC-DOS, or cannot cope with large volume sizes, or don't work right on modern CPUs, or … )

Don't use self-decrypting files. If someone sends a self-decrypting file to you, ask for an file that can be decrypted with an ordinary standalone decryption tool, instead. If you want to send someone an encrypted file, and they don't have the decryption tool, then either

In an encryption/decryption system, the tool used to perform decryption must be trusted by the recipient. Self-decrypting files do not provide that trust, because any man-in-the-middle attacker who provides a fake encrypted file to the recipient, pretending to be the sender, gets to provide the decryption tool, that processes the file on the recipient's behalf, as well.

Self-decrypting files are thus a man-in-the-middle attacker's dream come true. The attacker can make the decryption tool do whatever xe wants. Always yielding a positive response, and assuring the recipient that the file created by the man-in-the-middle was digitally signed/created by the sender, is in fact not even the most major of the recipient's problems as far as man-in-the-middle attacks are concerned.

Recipients who have been sent self-decrypting files should bear in mind three important points:

People who publish decryption tools expend effort to ensure that recipients can trust the actual decryption tools themselves before running them. (Witness the integrity check instructions for GNU Privacy Guard (GPG), for example.) There are reasons for this.

© Copyright 2006–2006 Jonathan de Boyne Pollard. "Moral" rights asserted.
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