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PGP - Pretty Good Privacy


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Introduction

The History and Philosophy of PGP

History before PGP

In the beginning, nobody needed encryption, because no one could write! Then, after many years of struggle, writing was invented! But there was still no problem, because only the rulers and their trusted servants could read or write. But, as time went on, more and more people learned to read, and there began to be a problem with written communications, in that if someone hostile intercepted a written document, they could read it!

This began a war between the people trying to find ever more complicated, neat ways of scrambling messages and the people trying to figure out ways of descrambling them. Sometimes the people scrambling messages would win, and sometimes the people descrambling messages would win. As time went on, this war over ciphers became intertwined with regular wars, and things got interesting! In spite of cryptography's use in wars, cryptography was invented by civilians. And it was used by civilians through out its history.

Anyhow, the first ciphers were single key or conventional ciphers. In single key ciphers, there is one key that must be used by both the sender to encrypt, and by the recipient to decrypt. The message can be decrypted by anyone who possess this key. This leads to the key distribution problem. There must be some way to insure that both the sender and the recipient have the same key. Care must be taken so that unwanted parties do not posses this key. It is necessary to have some kind of secure channel to accomplish this. The problem with secure channels is that they are difficult to establish and that people are always trying to compromise them.

Then Rivest, Shamir, and Aldeman invented Public Key Cryptography and the algorithm that bears their name, RSA. The RSA algorithm is based on the mathematics of exponentiation.

In this scheme, there are two keys, an encryption or public key and a decryption or secret key. The Recipient creates both keys. The encryption key is published. The decryption key is kept secret. Thus, anyone can encrypt and send a message to the recipient, but only the recipient can decrypt. This encryption scheme solves the key distribution problem, since the only key that the sender needs can be published without compromising the messages. It can be sent via any unsecured channel.

PGP is a hybrid program

Now, from your point of view of as a user of PGP, PGP behaves as if it were a Public Key Encryption program. A you create a secret/public key pair and publishes the public key. But internally, PGP is a hybrid encryption program. When PGP encrypts a file, it creates a random conventional encryption key. It sends that key to the recipient using a header block that is encrypted using RSA public key encryption. Thus RSA serves as the secure channel which is used to transmit a conventional key, solving the key distribution problem. The bulk of the message is encrypted using a conventional cipher, IDEA. PGP uses this scheme because RSA encryption is very slow, so that if the whole message were encrypted with RSA then PGP would be too slow. IDEA is thought to be a stronger encryption method than RSA so this scheme does not weaken PGP.

Now, PGP stores keys in keyrings. Usually, public keys are stored in a file called pubring.pgp and secret keys are stored in a file called secring.pgp.

PGP uses conventional encryption to protect secret keys

How does PGP help you keep your secret keys secret? RSA secret and public keys must be hundreds of bits long and they are must be chosen randomly. This presents a problem. If PGP were to ask you, a human being, to remember your public key, it would be incontinent, because most human beings do not remember long strings of binary information well. You might write it down. This would be bad, because someone might do a black bag job on you. A black bag job is when a practical cryptoanalyst comes to your house when your are not there with a bunch of locksmithing tools in his black bag. He comes away with your secret key in his black bag. The other alternative would be to store your secret key in a computer file. But this risks someone doing a black bag job on your computer. PGP solves this dilemma by storing both your public and secret keys in computer files called keyrings. But, it stores your secret key encrypted with conventional encryption. Your secret key will be protected with a pass phrase which you must specify every time you wish to use your secret key. (When you decrypt a message or sign a message.) Hopefully, you will choose your pass phrase so that it will be easy for you, as a human being, to remember it.

PGP scrunches your pass phrase to a 128 bit cryptographic checksum, using an algorithm called MD5. (English has a entropy of about 1 bit per character, so if you use a pass phrase 128 characters long, then all values for the checksum will be equally possible.) This checksum is used as a conventional key to encrypt or decrypt your secret key, using IDEA.

The upshot of all this is that your secret key is stored on a file on your disk. Thus your disk must be carefully backed up. The secret key is encrypted, so that you must specify a pass phrase anytime you wish to perform an action that uses your secret key. Thus you will be asked for your pass phrase when you wish decrypt a message, or when you wish to sign a message. You should choose your pass phrase so that you can remember it, but no one else can guess it.

PGP Can Authenticate Messages.

PGP can authenticate or sign messages. You can create digital signatures. A digital signature is additional information that tends to show that a given document really came from a given person. PGP can check digital signatures to check if a given document for tampering. How does PGP authenticate a message?

What do digital signatures prove?

A PGP digital signature shows that the person who signed the document had access to the PGP secret key and the pass phrase for the key indicated by the signature and that the document has not been modified since it was signed. This is different than hand writing signature authentication, which ultimately depends on the nervous system of the signer.

Overview of PGP

PGP is not an "user friendly" Program

PGP has no menus. there are no mouse buttons to click. PGP will not paint a fancy picture for you to look at and it will not cause sound to come out of your computer. PGP is a unix style command line program. The direct way to use PGP is to enter a PGP command. This command will typically include computer filenames, key identifiers and options. Options begin with the "-" character followed by a string of incomprehensible letters. (Except for the Mackintosh version of PGP. Hey, I do not know anything about the Macintrash or the Macintrash version of PGP, but you might try MacPGP and AppleScript FAQ.)

Read the PGP documentation

However, the command pgp -h displays all the information you need to accomplish most PGP tasks, provided you understand PGP terminology and philosophy.

Read the documentation that comes with PGP. There are subtle problems that you can get yourselves into if you do not. This document attempts to give you the background to understand the documentation.

Books on PGP

    There are six books on PGP.
  • PGP Pretty Good Privacy by Simson Garfinkel
  • Protect Your Privacy A Guide for PGP users by William Stallings
  • E-MAIL SECURITY How to keep Your Electronic Messages Private by Bruce Schneier
  • The Computer Privacy Handbook by André Bacard
  • The Official PGP User's Guide by Philip R. Zimmermann
  • PGP Companion for Windows The offical Guide to WinPGP by Peter Kent

PGP Web Pages

No one has to enter PGP commands directly.

There are a number of PGP frontend programs that give PGP a menu oriented interface.

Some mail programs such as ELM support PGP. Some editors support PGP extensions. There are PGP extensions to GNUEMACS . There is a second mode extension for GNUEMACS for PGP. OS/2's EPM editor can be extended to support PGP.. There are a number of shells for the MSDOS program loader: There is even a shell for WINDOZE or WPGP.

How PGP handles text.

There are two separate problems that encryption programs have in dealing with text. PGP has two separate solutions to these problems. It is important that these problems and their solutions not be confused. If the solution to one problem is used when the other solution is required, the results will be bad.

Making the output of PGP into a mailable file (-a).

Ordinarily, a well encrypted file is not mailable. The encrypted file usually uses characters that most mail programs will not accept. Since line terminators occur essentially randomly in the encrypted file, lines can easily be too long for most mail programs. PGP solves this problem with the -a option. This causes PGP to use the ascii radix-64 armor for output. The resulting output limits the characters used to a set of characters that mail programs will accept. It creates lines a reasonable size. Thus the output file should be mailable. PGP is programmed to accept such files, reversing the radix-64 format before decrypting. The -a option can be used when signing a file as well. This causes the signed file to be in the radix-64 format. Using -a options results in output that is about 30% larger.

The -a option can be specified by itself, without requesting encryption or authentication. In this case, PGP is being used as a superior uuencode/uudecode.

Different ways to represent plaintext. (-t)

Different Operating systems represent text in different ways. For example, different end of line characters are used. The UNIX operating system uses linefeed characters to end lines, but the MSDOS program loader uses the carriage return linefeed combination to end lines. The Macintrash operating system is said to use carriage returns to separate lines. These differences mean that text files must be converted, when they move between operating systems. If this conversion is not done, then the files will not display or print properly. This need for text file conversion, is why the FTP (file transfer program) needs to have binary and ascii modes, when it moves files between operating systems. Now when PGP is asked to encrypt a plaintext file on one operating system and decrypt it on another, then the PGP system as a whole is being asked to move a text file between operating systems. The same considerations apply when signing text files on one operating system and removing the signature on another. The PGP -t option should be specified when encrypting or signing a text file. This will ensure that the text file conversion is done correctly. The -t should not be used if the file contains binary data, such as executable, some word processor files, and many data files.

Using text examples.

  • Fred wishes to encrypt a spreadsheet file (.WKS) and put it on a floppy and mail it to his financial advisor.

    Since the file is not being sent through e-mail, the -a option is not needed. Since the file contains binary data, the -t option should not be used.

  • At the last minute, Fred finds that his financial advisor has an e-mail address and decides to e-mail the file.

    Since the file is going to be e-mailed, the -a option should be specified. The original file still contains binary data, so the -t option should not be specified.

  • A huge text file needs to be moved to the accounting department's computer. For security reasons, this computer is not connected to any networks, so the file will be moved on a floppy via sneakernet.

    The -t option should be used because the original file is text. The -a option is not needed, and should not be used so the resulting file has a better chance of fitting on one floppy.

  • Fred wishes to e-mail an encrypted love-letter to his girl friend.

    Since the plaintext is text, the -t should be used. Since the output from PGP must go through e-mail, the -a option should also be used.

Starting with PGP

The first thing to do is to create a directory for PGP and its files to live in. Unpack the distribution files to this directory. (If you are using PKUNZIP under the MSDOS program loader, be sure to use the "-d" switch.)

Modify the PGPPATH and PATH variables. PATH should point to the directory where the PGP executable exists. PGPPATH should point to the directory where the PGP data files are. The procedure for doing this depends on your operating system and should be documented in the documentation that comes with your PGP distribution. When using the MSDOS program loader, you would modify the file AUTOEXEC.BAT. when using OS/2 the file CONFIG.SYS should be modified.

Generate a Secret Key/Public Key Pair

The first thing you want to do after installing PGP is to generate keys using the -kg command. This will allow messages to encrypted for you, and it will allow you to sign messages.

Select a Key Size.

PGP will ask you for a key size. Unless your computer is a klunker, choose 1024 bits. (If your computer is fast, and you are extremely paranoid about the computers in the NSA's basement, you may want to create an additional key with 2047 bits, if you have PGP 2.6. But you would not want to make that key your primary key, because not all versions of PGP support keys with more than 1024 bits.)

Select a Key Identifier

Choose the key identifier for the key. From the point of view of PGP, the key identifier can be any string of characters, but to use the key identifier with mail programs, follow the convention: 

First Last <myaccount@mynode.mydomain>
Your internet mail address should be enclosed by the <> characters.

Select a pass phrase to protect your secret key

Choose your pass phrase according to the following rules:

  • Choose a pass phrase you can remember.
  • Choose a pass phrase that can not be guessed.
    • at least 128 characters.
    • can not be deduced from your personal history.
    • is not found in literature or popular culture.
  • Do not use your pass phrase for any other function.

PGP will ask you to type some text on your keyboard.

PGP needs to make itself unpredictable, so that a hostile cryptanalist can not predict what it is doing. To do this it is necessary for PGP to get some random numbers . It is difficult to get random numbers in a computer program, because computers and operating systems are designed to be predictable. To create random numbers, PGP will ask you to type some text on your computer. PGP creates the random numbers by measuring the timing of your keystrokes. It does not matter what you type, but do not use the autorepeat feature of your keyboard if it has one.

Sign your own key.

You should always sign your own public key. The following command will sign the key my-key-identifier: 


pgp -ks my-key-identifier -u my-key-identifier

Adjust the file CONFIG.TXT.

You may wish to adjust the file CONFIG.TXT to conform to your personal preferences. You can study the file PGPDOC2.TXT that comes with your PGP distribution to know how to do this. If you have more than one secret key, you may wish to set the configuration variable MYNAME to be your default secret key.

Extract your public key.

To allow others to send you encrypted messages, you must give them your public key. To do this, you must extract your public key to a file: 


pgp -kx my-identifier file
If you must send your public key thru e-mail, you will want to add the -a option. 

pgp -kxa my-identifier file

Add a key to your public key ring.

When someone sends you their public key, you can add it to your public key ring. 


pgp -ka file

Encrypt a file.

After you have added a public key to your keyring, you can encrypt a message using that key. 


pgp -e file key-identifier

You may need to add the -a and/or -t options to insure that text is handled properly.

Decrypt a file.

To decrypt a file, use the simple: 


pgp ciphertextfile
This command will also check digital signatures if any. To display a file a screenful at a time use the -m option.

Authentication

PGP can sign file creating digital signatures: 


pgp -s textfile -u my-key-identifier

You may need to add the -a and/or -t options to insure that text is handled properly.

Authentication can be combined with encryption, creating an encrypted, signed file. 


pgp -es textfile their-key-identifier -u my-key-identifier
This will create a file that the owner of their-key-identifier can decrypt and signed by my-key-identifier. The encryption is done after the signature, so that a person who can not decrypt the file can not tell who signed the file.

Create a clear signed message.

Suppose you wish to post a message to a USENET newsgroup. You want to sign the message, so that people can check its signature with PGP, but you want the text of the message to be readable by people that do not have PGP. You want to create a clear signed message: 


pgp +clearsig=on -sat textfile -u my-key-identifier

Be careful not to enter clear signed messages into FIDONET systems without the permission of the sysop. Most of FIDONET is extremely authoritarian, and does not allow encrypted or signed messages. Although we can laugh at the rigid orthodoxy of FIDONET, we should respect the property rights of the sysops.

Create detached signatures.

You can create a signature certificate in a separate file: 


pgp -sb textfile -u your-key-identifier
These certificates can be checked by specifying both the certificate and the original file to PGP. 

pgp certificate-file original-file
Detached certificates can be used to sign an executable file, without modifying the file so that it remains executable.

Miscellaneous PGP commands

Special PGP commands that do not really fit anywhere.

Wipe a file.

When a file is deleted under most operating systems, the data in the file is not necessarily destroyed. Usually, the directory entry that points to the blocks of the file is removed and the blocks of the file are returned to the disk's free space. It is possible that someone could recover the data of the file. There are the famous "undelete" programs that function under the MSDOS program loader. When PGP wipes a file, it overwrites the data so that the data is DEAD, DEAD, DEAD and can never be recovered. 


pgp -w datafile

The -w option can be used when encrypting a file to wipeout the plaintext after encrypting. 


pgp -esatw plaintextfile their-key-identifier -u my-key-identifier

PGP and pipes.

If your operating system supports pipes, you can cause PGP to take its input or output from a pipe by specifying the -f option. (If you do not know what a pipe is, do not worry about it. You can get along without pipes, and perhaps your operating system does not support them.)

Restoring the original plaintext file name.

To cause the decrypted file to have its original filename, use the -p option. 


pgp -p ciphertextfile

PGP and conventional encryption

If you are encrypting a file to your self, you can avoid fooling around with keyrings by using conventional encryption: 


pgp -c plaintextfile
PGP will ask for a pass phrase to use as a key. The pass phrase will be required to decrypt the file.

Publish your public key(s)

You need to publish your public keys, so that people who have not previously contacted you can send you encrypted messages and check your signatures. To publish your public keys, you should use a PGP public keyserver . These key servers are used by sending them e-mail commands. These commands are described by the file KEYSERV.DOC that comes with the PGP distribution.

To publish your public key, first extract the key as a radix-64 file. 


pgp -kxa my-key-identifier file
e-mail the resulting file to a public key server with subject "add". It is only necessary to send your public key to one public key server. They are like small town gossips. They all talk to each other. To tell one is to tell them all.

To get somebody else's public key from a public key server, send a null e-mail message to a server with subject get key-identifier. The mail server will mail you back a radix-64 encoded key file, which you can add to your public keyring. 


pgp -ka keyfile
The list of keyservers changes from time to time.

FLASH now you can access public key servers through the World Wide Web at MIT the University of Paderborn.

The public key servers make no checks to insure that a given keys in its database actually came from the person indicated by the key identifier. This determination is your responsibility.

Things to do with keys.

You can remove keys from your keyring with the -kr option. 

pgp -kr key-identifier

You can revoke keys, declaring that the keys have "gone bad" and should not be used: 


pgp -kd key-identifier
This will produce a revocation certificate, which should be sent to the public key servers to declare that the key has gone bad. Remember your pass phrase! You can not revoke a key without the pass phrase.

You can edit keys: 


pgp -ke key-identifier
This will allow you to edit the pass phrase or key-identifier of a secret key, and it will allow you to change the trust parameters of a public key. You might want to change your key identifier, when your name, or your internet mail address changes. Whenever you change a key identifier, you should always sign your new public key. 


pgp -ks my-key-identifier -u my-key-identifier

So how do I know this key is good?

How do we know that a given key actually belongs to the person indicated? A fraudulent key can be created to trick us into accepting bad signatures. It could be used to trick us into encrypting a message that the wrong person can decrypt. It is possible to forge an e-mail message, so we can not necessarily trust that the e-mail message that brought us a public key was not fraudulent. As we have seen, the public keys servers do not check that the keys stored in their databases are correct.

The first and most basic way of checking a public key is to contact the owner directly. You could call the owner on the phone, and ask the owner if the key you have really belongs to him. Then if you recognized him by voice, you could be sure you had the right key. But how would you know that you and the key owner were talking about the same key? He could have a key with key identifier and you could have a fraudulent key with the same key identifier. How would you check this? The answer is the -kvc option: 


pgp -kvc key-identifier

This will cause pgp to print a fingerprint that looks like this: 

Key ring: 'E:\PGPDATA\pubring.pgp', looking for user ID "paul.elliott".
Type bits/keyID     Date       User ID
pub  1024/D4849879 1994/01/29  Paul Elliott (standard 1024 bit key) 
          Key fingerprint =  F6 C7 33 D8 64 07 46 D7  FD 67 53 80 CE 7E 0B C8
                               Paul Elliott
The key fingerprint of a given key is designed to be unique. If two people get the same key fingerprint for a given key, then they know that they are dealing with the same key. Key fingerprints can be used to verify the validity of keys over the phone. We now pass on to more indirect methods of checking the validity of keys.

Key Signatures

Key signatures are used to transfer the knowledge of the "goodness" of a given key from one person to another. Let us do a thought experiment. Suppose that Judy has received a key from Fred through e-mail, and she needs to verify that it is really Fred's key. She does not know how to contact Fred directly, but she has a friend, Sally, who does. She already has Sally's valid key. She (Judy) could get Sally to send her a PGP signed message like this: 

-----BEGIN PGP SIGNED MESSAGE-----

Dear Judy,

I know of my own personal knowledge that the key
with key fingerprint:
F6 C7 33 D8 64 07 46 D7  FD 67 53 80 CE 7E 0B C8
Really does belong to Fred!

Sincerely, Sally

-----BEGIN PGP SIGNATURE-----
Version: 2.6

iQCVAgUBL3r/vfBUQYbUhJh5AQFvMQQAkE+UtvtKKNseHblNs5uccRTISXrEdkl9
QjeYtagh9Qs=
=VqdW
-----END PGP SIGNATURE-----

Judy can run this message through PGP to verify that it really came from Sally. She can check the fingerprint against the fingerprint she has. If the fingerprints match, and she trusts Sally, then she can be sure that the key really is Fred's key.

It is never necessary to actually send or create messages like the above. PGP supports the same functionally through the concept of "Key signatures". When one signs a key, one is essentially creating a message like the above, and attaching it to the key itself on the key ring! Then, when a key is extracted from a keyring, and sent to someone else, and added to another keyring, the "key signature" goes with it! Thus key signatures propagate from user to user in the same way that keys do. To sign a key: 


pgp -ks the_key_id_you_are_signing -u the_key_doing_the_signing

Whenever you sign a key, you should re-send the key to the public key servers so that other PGP users can use the signature to determine the validity of the key. You can view the signatures that are attached to a key using the -kvv option. 


pgp -kvv key_identifier

Who Do I trust?

When PGP uses a key for encryption or signing, it determines if in PGP's opinion, the key can be trusted. If PGP does not trust the key, it will print an message warning you that the key is not to be trusted. You can tell PGP to use the key anyway. PGP determines trust on the basis of signatures from trusted keys. When you add a key to your public keyring you are asked if the key can be trusted to introduce other keys. If a PGP notes a signature from a trusted key, it tends to trust the key bearing the signature. You can change the trust parameters on a key using the -ke option. 


pgp -ke key_identifier

Trust is not transitive.

If you trust a person, you do not necessarily trust everyone that that person trusts! You can believe that the person you trust is naive and that therefore the persons that he trusts is not necessarily trustworthy. Thus the PGP configuration option CERT_DEPTH should be set to 2.

Furthermore, you should not expect other people to trust your inferences. If you infer that a given key is good on the basis of key signatures, you should not sign that key. You should only sign a key when you know of your own personal knowledge that a key is valid.

When to use PGP.

Always, use PGP when the person you are corresponding with has it. This establishes that you are a regular PGP user. It asserts your right to use PGP. If you make it a habit to always use PGP then you will not draw attention to any sensitive messages you might wish to send.

Use conventional encryption.

Use conventional encryption, the -c option, when you are encrypting files to yourself. This avoids the RSA algorithm and is slightly stronger. It avoids the necessity to muck around with keyrings.

PGP encrypted files are identifiable.

Every PGP encrypted file contains header information that identifies it as a PGP encrypted file, even to someone who can not decrypt it. If you wish it to remain a open question if your encrypted file is in fact encrypted, you should use a raw conventional or symmetric cipher. You can remove the PGP headers from an conventionaly encrypted PGP using a program called stealth.

Protect your right to use encryption.

There have been recent rumblings about government plans to outlaw private strong encryption. I believe in the right to use encryption for a number of reasons:

  • As a member of EFH, I am for free speech in cyberspace and am for the advance in network culture that PGP represents.
  • As a member of the Libertarian Party, I oppose any unnecessary governmental interference in our lives.
  • As a supporter of the right To keep and bear arms, I wish the people to be armed with powerfull weapons, such as PGP.
Read my rant opposing the regulation of private cryptography. It is very well to support the right to private encryption, and everyone can do so for their own reasons, but what can be practically done to protect the right to use encryption?
  • Use a defensive formulation when discussing the uses of encryption. Choose examples that show encryption being used for good and against evil. Even though other more controversial uses of encryption are possible, the widespread availability of legal encryption is progressive and we need to be careful to talk that way.
  • Take political action. You can contact your political representatives and inform them of your views on the issue of strong encryption.
  • Spread the use of encryption. Make PGP commonly available. Upload it to BBSes. Teach other people how to use PGP. Get many powerful people routinely using PGP and other strong encryption, so that it becomes politically impossible to ban it.


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