It would be beyond the goal of this FAQ to discuss all possible attacks against or possible flaws in PGP. If you want to know more than what is available in here, see infiNity's PGP Attack FAQ.
The only type of attack that might succeed is one that tries to solve the problem from a mathematical standpoint by analyzing the transformations that take place between plain text blocks, and their cipher text equivalents. IDEA is still a fairly new algorithm, and work still needs to be done on it as it relates to complexity theory, but so far, it appears that there is no algorithm much better suited to solving an IDEA cipher than the brute force attack, which we have already shown is unworkable. The nonlinear transformation that takes place in IDEA puts it in a class of extremely difficult to solve mathmatical problems.
This option is especially useful if you want to back up sensitive files, or want to take an encrypted file to another system where you will decrypt it. Now you don't have to take your secret key with you. It will also be useful when you lose your secret key. And you can even pick a different passphrase for each file you encrypt, so that an attacker who manages to get one file decrypted can't decrypt all the other files as well now.
For this reason, when you read messages on USENET saying that "someone told them" that the NSA is able to break pgp, take it with a grain of salt and ask for some documentation on exactly where the information is coming from. In particular, the message at http://www.quadralay.com/www/Crypt/NSA/break-pgp.html is a joke.
First, there is the RSA-129 key. The inventors of RSA published a message encrypted with a 129-digits (430 bits) RSA public key, and offered $100 to the first person who could decrypt the message. In 1994, an international team coordinated by Paul Leyland, Derek Atkins, Arjen Lenstra, and Michael Graff successfully factored this public key and recovered the plaintext. The message read:
THE MAGIC WORDS ARE SQUEAMISH OSSIFRAGE
They headed a huge volunteer effort in which work was distributed via E-mail, fax, and regular mail to workers on the Internet, who processed their portion and sent the results back. About 1600 machines took part, with computing power ranging from a fax machine to Cray supercomputers. They used the best known factoring algorithm of the time; better methods have been discovered since then, but the results are still instructive in the amount of work required to crack a RSA-encrypted message.
The coordinators have estimated that the project took about eight months of real time and used approximately 5000 MIPS-years of computing time.
What does all this have to do with PGP? The RSA-129 key is approximately equal in security to a 426-bit PGP key. This has been shown to be easily crackable by this project. PGP used to recommend 384-bit keys as "casual grade" security; recent versions offer 512 bits as a recommended minimum security level.
Note that this effort cracked only a single RSA key. Nothing was discovered during the course of the experiment to cause any other keys to become less secure than they had been.
For more information on the RSA-129 project, see: ftp://ftp.ox.ac.uk/pub/math/rsa129/rsa129.ps.gz
A year later, the first real PGP key was cracked. It was the infamous Blacknet key, a 384-bits key for the anonymous entity known as "Blacknet". A team consisting of Alec Muffett, Paul Leyland, Arjen Lenstra and Jim Gillogly managed to use enough computation power (approximately 1300 MIPS) to factor the key in three months. It was then used to decrypt a publicly-available message encrypted with that key.
The most important thing in this attack is that it was done in almost complete secrecy. Unlike with the RSA-129 attack, there was no publicity on the crack until it was complete. Most of the computers only worked on it in spare time, and the total power is well within reach of a large, perhaps even a medium sized organization.
pgp [filename] > [diskfile]
The -m option was not intended as a fail-safe option to prevent plain text files from being generated, but to serve simply as a warning to the person decrypting the file that he probably shouldn't keep a copy of the plain text on his system.
This is why picking a strong pass phrase is so important. Many of these cracker programs are very sophisticated and can take advantage of language idioms, popular phrases, and rules of grammar in building their guesses. Single-word "phrases", proper names (especially famous ones), or famous quotes are almost always crackable by a program with any "smarts" in it at all.
There is a program available which can "crack" conventionally encrypted files by guessing the passphrase. It does not do any cryptanalysis, so if you pick a strong passphrase your files will still be safe. See http://www.voicenet.com/~markm/pgpcrack.html for more information and the program itself.
There are also other methods to get at the contents of an encrypted message, such as bribery, snooping of electronic emanation from the computers processing the message (often called a TEMPEST attack), blackmail, or "rubber-hose cryptography" - beating you on the head with a rubber hose until you give the passphrase.
A pass phrase which is composed of ordinary words without punctuation or special characters is susceptible to a dictionary attack. Transposing characters or mis-spelling words makes your pass phrase less vulnerable, but a professional dictionary attack will cater for this sort of thing.
See Randall T. Williams' Passphrase FAQ for a more detailed analysis.
It may be a good idea to periodically try out all the passphrases, or to iterate them in your mind. Repeating them often enough will help keep them from being completely blanked out when the time comes that you need them.
If you use long passphrases, it may be possible to write down the initial portion without risking compromising it, so that you can read the "hint" and remember the rest of the passphrase. For a simple way to pick provably strong passphrases that are easy to remember, please see Arnold Reinhold's Diceware website.
If you already own a trusted version of PGP, it is easy to check the validity of any future version. Newer binary versions of MIT PGP are distributed in popular archive formats; the archive file you receive will contain only another archive file, a file with the same name as the archive file with the extension .ASC, and a "setup.doc" file. The .ASC file is a stand-alone signature file for the inner archive file that was created by the developer in charge of that particular PGP distribution. Since nobody except the developer has access to his/her secret key, nobody can tamper with the archive file without it being detected. Of course, the inner archive file contains the newer PGP distribution.
A quick note: If you upgrade to MIT PGP from an older copy (2.3a or before), you may have problems verifying the signature. See question 3.14 for a more detailed treatment of this problem.
To check the signature, you must use your old version of PGP to check
the archive file containing the new version. If your old version of
PGP is in a directory called C:\PGP and your new archive file and
signature is in C:\NEW (and you have retrieved MIT PGP 2.6.2), you may
execute the following command:
c:\pgp\pgp c:\new\pgp262i.asc c:\new\pgp262i.zip
If you retrieve the source distribution of MIT PGP, you will find two more files in your distribution: an archive file for the RSAREF library and a signature file for RSAREF. You can verify the RSAREF library in the same way as you verify the main PGP source archive.
Non-MIT versions typically include a signature file for the PGP.EXE program file only. This file will usually be called PGPSIG.ASC. You can check the integrity of the program itself this way by running your older version of PGP on the new version's signature file and program file.
Phil Zimmermann himself signed all versions of PGP up to 2.3a. Since then, the primary developers for each of the different versions of PGP have signed their distributions. As of this writing, the developers whose signatures appear on the distributions are:
MIT PGP 2.6.2 Jeff Schiller <email@example.com> ViaCrypt PGP 2.7.1 ViaCrypt PGP 2.6.2i Stale Schumacher <firstname.lastname@example.org> PGP 2.6ui mathew <email@example.com>
You may, first of all, not verify the signature and follow other methods for making sure you aren't getting a bad copy. This isn't as secure, though; if you're not careful, you could get passed a bad copy of PGP.
If you're intent on checking the signature, you may do an intermediate upgrade to MIT PGP 2.6. This older version was signed before the "time bomb" took effect, so its signature is readable by the older versions of PGP. Once you have validated the signature on the intermediate version, you can then use that version to check the current version.
As another alternative, you may upgrade to PGP 2.6.2i or 2.6ui, checking their signatures with 2.3a, and use them to check the signature on the newer version. People living in the USA who do this may be violating the RSA patent in doing so; then again, you may have been violating it anyway by using 2.3a, so you're not in much worse shape.
Second, all the freeware versions of PGP are released with full source code to both PGP and to the RSAREF library they use (just as every other freeware version before them were). Thus, it is subject to the same peer review mentioned in the question above. If there were an intentional hole, it would probably be spotted. If you're really paranoid, you can read the code yourself and look for holes!
There are no intentional backdoors of any kind in the international version, nor is the encryption strength reduced in any way.
You should be very careful, however. Your pass phrase may be passed over the network in the clear where it could be intercepted by network monitoring equipment, or the operator on a multi-user machine may install "keyboard sniffers" to record your pass phrase as you type it in. Also, while it is being used by PGP on the host system, it could be caught by some Trojan Horse program. Also, even though your secret key ring is encrypted, it would not be good practice to leave it lying around for anyone else to look at.
So why distribute PGP with directions for making it on Unix and VMS machines at all? The simple answer is that not all Unix and VMS machines are network servers or "mainframes." If you use your machine only from the console (or if you use some network encryption package such as Kerberos), you are the only user, you take reasonable system security measures to prevent unauthorized access, and you are aware of the risks above, you can securely use PGP on one of these systems.
You can still use PGP on multi-user systems or networks without a secret key for checking signatures and encrypting. As long as you don't process a private key or type a pass phrase on the multiuser system, you can use PGP securely there.
Of course, it all comes down to how important you consider your secret key. If it's only used to sign posts to Usenet, and not for important private correspondence, you don't have to be as paranoid about guarding it. If you trust your system administrators, then you can protect yourself against malicious users by making the directory in which the keyrings are only accessible by you.
The problem with using PGP on a system that swaps is that the system will often swap PGP out to disk while it is processing your pass phrase. If this happens at the right time, your pass phrase could end up in cleartext in your swap file. How easy it is to swap "at the right time" depends on the operating system; Windows reportedly swaps the pass phrase to disk quite regularly, though it is also one of the most inefficient systems. PGP does make every attempt to not keep the pass phrase in memory by "wiping" memory used to hold the pass phrase before freeing it, but this solution isn't perfect.
Because swapfiles shrink, and many applications (eg: MsWord) grab disk space (and unused memory) and don't always fill it all out, you will regularly get fragments of other work embedded in files unrelated to it.
Disabling swapping (after getting more memory) will help, but you should also be cautious about sending binary attachments (like Word DOCs). If you wish to keep your hard-drive more secure, you should consider a sector-level encryptor (such as SFS or SecureDisk or CryptDisk)
If you have reason to be concerned about this, you might consider getting a swapfile wiping utility to securely erase any trace of the pass phrase once you are done with the system. Several such utilities exist for Windows and Linux at least. Not all of them perform as well as claimed in the documentation, especially when it comes to erasing leftover bits in the last sector and removing traces from the file allocation table.
In addition, don't forget that private keys are useful for more than decrypting. Someone with your private key can also sign items that could later prove to be difficult to deny. Keeping your private key secure can prevent, at the least, a bit of embarassment, and at most could prevent charges of fraud or breach of contract.
Besides, many of the above procedures are also effective against some common indirect attacks. As an example, the digital signature also serves as an effective integrity check of the file signed; thus, checking the signature on new copies of PGP ensures that your computer will not get a virus through PGP (unless, of course, the PGP version developer contracts a virus and infects PGP before signing).
The following information applies only to citizens of the United States in U.S. Courts. The laws in other countries may vary.
There have been several threads on Internet concerning the question of whether or not the fifth amendment right about not being forced to give testimony against yourself can be applied to the subject of being forced to reveal your pass phrase. Not wanting to settle for the many conflicting opinions of armchair lawyers on usenet, I asked for input from individuals who were more qualified in the area. The results were somewhat mixed. There apparently has NOT been much case history to set precedents in this area. So if you find yourself in this situation, you should be prepared for a long and costly legal fight on the matter. Do you have the time and money for such a fight? Also remember that judges have great freedom in the use of "Contempt of Court". They might choose to lock you up until you decide to reveal the pass phrase and it could take your lawyer some time to get you out. (If only you just had a poor memory!)
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