Few things in digital preservation are as frustrating as finding out you can’t open an encrypted archive. It must have been something important, and now you can’t get at it at all! You might not even have a clue what the file is.
The number one cause of this situation is stupidity. Back when I was doing consulting work, I’d sometimes send my project on a disk. (This was before the Internet and high-bandwidth data connections.) As a safety measure I encrypted it and sent my client the password separately, with a strong reminder to decrypt the disk immediately on receipt.
In one of those cases, I got back a reply months later saying, “We got this disk from you a few months ago and we can’t figure out what’s on it.” I hadn’t kept the password around, so I couldn’t help them. All I could do was send them another disk. They didn’t get around to decrypting that one either.
If you’ve received an encrypted archive and a password for it, do one of two things right away: Either decrypt it and store the extracted data in a safe place, or store the password where you’re sure you won’t lose it.
Another way to lose encrypted data is to find that you have the archive and the decryption key, but no working software to do the decryption. Perhaps software using some obscure homemade scheme created the archive. The encryption is doubtless second-rate and has serious theoretical weaknesses, but that’s not much help unless you’re a topnotch cryptanalyst. Or you may just not be able to tell what encryption scheme was used; one collection of random-seeming bits looks a lot like another.
Finally, encrypted files are fragile. Accidentally changing one bit will usually make the whole file or a large chunk of it undecipherable.
Sometimes the first challenge is to find out what kind of software to use. FI Tools from Forensic Innovations claims to be able to identify encrypted file types. The file extension may give a clue; File-extensions.org lists over 300 of these.
Why so many? For one thing, in the nineties the US government put severe restrictions on the strength of encryption that published software, especially for export, could use. This did little to keep terrorists from using strong encryption, but it encouraged people to roll their own encryption methods. Finding software for some of these could be very tough today.
On the positive side, you may be able to break old archives created with those feeble algorithms just by throwing computational power at them. The once popular DES encryption used 56-bit keys; it’s possible to crack a DES archive on a modern computer in a matter of hours.
You have to be careful when looking for encryption-breaking software. A large part of the market is for espionage and data theft, and the people who sell to this segment aren’t the most trustworthy.
As always, preventing the problem is far easier than curing it. Keep good track of encrypted archives, store the decryption keys securely, don’t let the only person who knows the password quit, and make sure decryption software is still available.