Pretty-Good-Privacy under Linux

Introduction

PGP is the tool appropriate to ensure the privacy and authentication of information in insecure communication networks such as the Internet. 

The privacy ensures that only the recipient of the information can make use of it. If this information were to make it in other hands, it would be totally useless because it could not be decoded. 

The authentication ensures that certain information generated by person "A"  effectively comes from "A" and has not being falsified or manipulated by anyone along the way. 

PGP is based on a cryptographic system known as public key, which can be used through insecure channels. This makes it ideal for ensuring the privacy of information transmitted through networks such as the Internet. 

It is not necessary to be involved in a thick industrial spy plot :) in order to be interested in the privacy of  your communications and  therefore having the need of a tool for cryptography. Something as simple as E-mail can be a perfect reason for beginning to use PGP.  Let's look at why: 

We can compare E-Mail to a postcards. Anyone who puts his hands on it can read it because there is no physical obstacle to prevent it. On the other hand, a letter inside an envelope is more private. People can handle the envelop but cannot read the letter. If they want to read it they must break the envelop.

We could make an analogy between our envelops and PGP, which acts as the support for our E-Mail. PGP does not allow anybody except the legitimate recipient to read it the message; this is one of the many advantages of  using PGP. 

How Public Key Cryptography Works 

The public key can and must be divulged freely, since it will be with that key that the rest of the world will be able to send information to you privately.  However, the public key one does not endangered the privacy of the private key. 

Let us consider the case of two friends, Juan and Pedro. Juan will be able to send information to Pedro privately if he knows Pedro's public key. On the other hand Pedro, using his own private key, will be able to decode the message Juan just sent. Let us suppose there is another person, Marcos, who intercepts the message that Juan has sent to Pedro. Marcos will not be able to do anything with the message because he does not have the private key of Pedro. Even Juan himself, the sender and creator of the message, is unable to decode it, only Pedro can, with the private key. 

The security of a system is based on every user storing their private key safely, even while the public key is widely known. If someone would try to break the system, not knowing the private key of the recipient, it would take so many years that it would be useless. 

As mentioned before in the introduction, besides the privacy, PGP also offers the possibility of authenticating the information. Let's see why: 

Our private key not only encodes messages but it can also "sign" the information sent; quite analogous to the signatures often added to paper documents. 

A document digitally signed without a private key can be authenticated by anyone having a public key. This authentication provides a means to check that the document effectively originated with the person it claims it comes from and that it has not been altered or falsified. 

Both processes, encoding and signing, can be performed to offer privacy and authentication. First the document is signed with our private key and then it is encoded using the public key of the recipient. 

When the recipient gets the message he inverts the steps, first decoding the document with his private key and then checking our signature with our public key. 

All these processes can be automated as we will see later on. 

A public key is stored in what is known as a key certificate , this is simply the public key itself together with the name of the owner and the date when it was generated. 

The private key is protected with a password that forbids its usage by unauthorized persons. 

Both keys are stored together in a file known as key ring which stores also various key certificates. Normally there is a ring for public keys and another one for private ones. 

The keys are internally reference by a key ID which is made up of  the last 64 bits of the key. When displaying information about the key, what actually gets shown are the last 32 bits of the key. These key IDs will be used by PGP, for example, to locate a key at the moment of decoding a message. 

When signing a document, PGP generates 128 bits that represent the document. This signature is a sort of checksum or CRC that enables it to detect changes in the document. In contrast to usual CRC or checksum, no impostor can regenerate this signature to legitimize any changes made to the original document.  The signature is made using the private key of the sender and the impostor normally will not have access to it. 

PGP Versions

At this point it must be emphasized that there is considerable confusion surrounding the various versions of PGP. Due to the politics existing in the United States with regards to the export of cryptographic material, several versions of PGP have emerged, along with several laws regarding their use. In order to make as clear as possible all this mess I will enumerate the various versions available for PGP to date. 
Freeware PGP Versions: 

• PGP 2.3a 

This is the "classic" PGP. It can still be used although it may experience incompatibility problems while processing keys and messages generated with versions 2.6.x and later or when using keys bigger than 1280 bits. Supposedly PGP 2.3a can not be used outside the United State due to restrictions in the patent. 
 

• PGP 2.6ui

This is the non-official version of PGP 2.3a that corrects for the incompatibility problems previously mentioned. This version is not a version 2.6.x because it is based on the sources for 2.3a 
 

• PGP 2.62ui

It is based on the sources for 2.6ui, is a modification that attempts to be compatible with the latest innovations introduce in versions 2.6.x. 
 

• MIT PGP 2.6.2

This is the last official version of PGP. Its messages can be read by versions previous to 2.5 and it uses the RSAREF encoding library. It is illegal to export this version outside the United States but curiously, once exported can be used freely. 
 

• PGP 2.6.3i

Based on the sources for MIT PGP 2.6.2 which have been modified for its international use. One of the changes introduced is the removal of the RSAREF encoding library mentioned above. The use of this version in the United States is illegal. 
 

• PGP 5.0

PGP 5.0 (previously known as PGP 3.0) is a completely new version of PGP. Its sources have been written from scratch. It adds new options, including support for other cryptographic algorithms besides RSA and IDEA. It will include a GUI interface to simplify its use. This version will be available mid summer. 

• ViaCrypt PGP 2.7.1 y 4.0 

Being commercial includes manual and individual usage license. It does not include the sources. 
 

• PGP 4.5 y 5.0

In June 1996 PGP Inc. buys ViaCrypt and starts developing commercial versions of PGP for the United States and Canada. The most recent version is PGPMail 4.5.

It must be taken into account that in some countries like France, Iran, Iraq, Russia and China the use of cryptography is regulated or prohibited. 

Some interesting links to get version 2.6.3i for various Linux distributions: 

• Binaries ELF:

tonelli.sns.it/pub/Linux/pgp/pgp263.is.bin.tgz 
• Binaries and Sources for Red Hat:

ftp.replay.com/pub/replay/ub/redhat/i386/pgp-2.6.3i-1.i386.rpm 
ftp.replay.com/pub/replay/ub/redhat/SRPMS/pgp-2.6.3i-1.src.rpm 
• Sources:

ftp.dit.upm.es/mirror/ftp.ifi.uio.no/pub/pgp/src/pgp263is.tar.gz 

Installation of PGP

The first step is to create a directory for the sources: 

mkdir pgp

tar -C ./pgp -xzvf pgp263is.tar.gz

cd pgp

tar -xvf pgp263ii.tar

cd src

make linux

Similarly, one can copy the manual page pgp.1 to /usr/man/man1 in case of a system wide installation. 

Basic Configuration

cd

mkdir .pgp

Optionally, instead of ~/.pgp/config.txt you can rename it .pgprc and save it in your home directory, that is ~/.pgprc 

Among other things, this file can specify the language to use through the parameter Languaje, the possibilities are: 

Languaje = en (English)

Languaje = es (Spanish)

Languaje = ja (Japanese)

Another recommended step, is to copy the personalized help file for your language to ~/.pgp.  In the case of spanish speaking users one would copy the file es.hlp 

Creating the Pair of Keys

pgp -kg

After selecting the size of the key, you will be asked to type the identifier to be given to the public key. People usually write their names here, or their E-Mail address.  In my case I have written: 

Angel Lopez Gonzalez <alogo@mx2.redestb.es> 

Next comes the password to protect your private key. Choose a phrase that is easy for you to remember. This is necessary to protect the private key, so that if some one steals it, it will not be usable without the password. 

Finally the program asks to randomly touch multiple times keys on your keyboard in order to generate a sequence of random numbers. The program will based the sequence of bits on the interval taken between strokes on the keyboard. 

PGP will generate the keys after a few seconds and will notify you with a message. After proper generation of the keys they should be saved in the directory ~/.pgp as the following files: pubring.pgp and secring.pgp 

The first one, pubring.pgp, is the ring with the public keys. At the moment it contains only our key. 

The second, secring.pgp is, as you can imagine, the ring of private keys which only contains at the moment your own private key. 

It should be remembered that all the security of the public key methods is based on the secrecy of the private key; therefore, be sure to keep it in a save place and that no one can access it from the ring of private keys. Check the permissions of secring.pgp and make sure that only you have access to read and write, with the rest of people no access at all. 

Finally it should be mentioned that both the identification of the keys and the passwords for the private keys can be edited and modified using the command: 

pgp -ke identifier [ring]

Adding keys to a Ring

If the file Somekey.pgp contains a key and you would like to add it to one of your rings the procedure is quite simple, execute: 

pgp -ka Somekey [ring]

When a key is added, PGP could notify you that the key being added is not completely certified; what means that the key in hand may authentically belong to its claimed owner or not. 

If there is "certainty" that the key truly belongs to its claimed owner, either because he or she gave it to us in person or via a secure channel, then it can be certified by the user him or herself.  This implies that we are putting our signature on that key as a prove of certification. 

This facilitates passing our key to a person that trusts us and who has the complete certainty that we are passing him a good and authentic key. 

There is even a name for this process, a web trust. in the United States there are even gatherings of PGP users where they interchange public keys and sign them :) 
 
Let us consider an example with name to make clear this concept. Consider again two friends, Juan and  Pedro. Juan gives his public key to Pedro. Pedro is sure that the key Juan has given to him is correct since there is trust between them. When he arrives home and adds it to his ring of public keys he can then certified that the key truly belongs to Juan, so he signs it with his own private key. 

Now two more people enter the scene: Luis and Maria. Luis receives from Pedro, Juan's key and later on Luis sends it to Maria. Maria does not trust Luis but she sees that Juan's key comes with Pedro's certification. Maria can check the public key of Juan thanks to Pedro's signature. She has Pedro's public key because he gave it to her in person, so now she can trust the veracity of Luis' key by checking the authenticity of Pedro's signature. Here we see how Maria can now use and trust the key given to her by an untrustworthy character like Luis. 

It is messy :) but necessary in order to protect against the only weak point of this type of cryptography: the fact that the public keys could be falsified. 

Removing Keys from a Ring

pgp -kr identifier [ring]

Extracting a Key

pgp -kx identifier file [ring]

The file generated mykey.pgp is not in ASCII format (try using cat to see it). However,  if one wishes to get an ASCII formatted key file to send by E-Mail, for example, or to add it as additional information to a finger database, one would type: 

pgp -kxa identifies file [ring]

Together with the key are also extracted all the certifications that endorse it. 

Content of a Ring

pgp -kv [identifier] [ring]

In order to see all the certificates for every key, use: 

pgp -kvv [identifier] [ring]

Encoding a Message 

pgp -e file identifier

pgp -e exam.doc marcos

Remember that the file generated, exam.pgp, is not an ASCII file, therefore for E-Mail purposes it may be better to add one more option -a, so that the output encoded file is in ASCII format, like this: 

pgp -ea exam.doc marcos

pgp -eaw exam.doc marcos

Encoding a Message for Several Recipients 

pgp -ea exam.doc marcos juan alicia

How to Sign a Message

To sign a document it is necessary to use our private key: 

pgp -s file [-u identifier]

When our teacher in the example decides to sign the exam so that his colleagues know the message did not come from a funny student :) he types the following: 

pgp -s exam.doc

pgp -sta exam.doc

It is also possible to sign a document and then encode it using the following command: 

pgp -es file recipient_identifier

[-u my_identifier]

pgp -es exam.doc marcos -u angel

Even in this case an ASCII file could be generated using the option -a. 

Another possibility of interest would be to generate a signature for a file separate from the data.  To achieve this, use the -b option: 

pgp -sb exam.doc

Decoding

pgp input_file [-o output_file]

However, after decoding a file we can also specify the standard output for the decoded file.  This is achieved using the -m option: 

pgp -m file

pgp -fs identifier < input_file >

output_file

pgp -d exam

Dealing with Text Files

pgp -sta text_file identifier

"Fingerprints"

To display the fingerprint of a key, use the command: 

pgp -kvc identifier [ring]

Using PGP in Shells

+batchmode 

With this option PGP will not ask anything not strictly necessary. Use this option to check a signature automatically.  When the file has no signature the error code returned is 1; if the file has a signature and it was correct it returns 0. 

pgp +batchmode file

This option forces any operation of overwriting a file or deleting a key. 

pgp +force +kr marcos

Here is an example: 

PGPPASS="password"

export PGPPASS

pgp -s file.txt marcos

As in here: 

pgp -sta exams.txt angel -z "password"

pgp file.pgp +verbose=0

Integration with Mail Readers

As an example, I will discuss the integration of PGP with Pine. Hopefully this is the mail reader used by the reader. 

Although I will describe working with PGP for Pine, the principles behind these steps can be applied to other readers as well.  The configuration options will, of course, be different for different mail programs. 

In order to decode mail automatically before reading it, one needs a filter that processes the message in order to display it to the screen.  Another possibility would be to write a macro that combines decoding and displaying to the window. 

In the case of  Pine there is an option for defining filters that are executed before visualizing a message.  The option is called 'display-filters' and is found in Pine's configuration menu.  In this option we add a new filter that looks like this: 

_BEGINNING("-----BEGIN PGP MESSAGE-----")

_ /usr/local/bin/pgp

Then, if there is, in fact, an encoded message in he body PGP gets executed.  It will ask you for the password and then you can read the message. If you wish to automate this process even more, by saving the time required to type your password each time, then you can define the environment variable PGPPASS or use the option -z  as described above.

Now we only have to define a filter that will encode our messages with the public keys of the recipients available in our public ring before the message is actually sent. Pine again helps us, with the configuration option 'sending-filters.'  Here is the filter to include for this option:

/usr/local/bin/pgp -etaf _RECIPIENTS_

This is a very simple and basic setup which allows you to use PGP under Pine, using only two filters. To obtain more information on how to integrate PGP further, with Pine and other systems, consult these links:

elm | GNU Emacs | tin | mailx | MH | Pine | sendmail | vi | zmail