Millennium Time Capsule/AC2000.BIN/disks/ac8_disk/pgp_long/pgp.asc

Text File | 1998-01-01 | 15.2 KB | 305 lines

Stop reading my mail!               (c) Thomas Binder/Atari Computing 1998
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Have you ever thought about how many people may be able to read your
electronic mail? Thomas Binder takes a look at "Pretty Good Privacy", a
tool which keeps your private mail private, and a lot more besides...

If you need to send confidential text or sensitive data via email, you
should seriously consider encrypting your mail, so only the intended
recipient can read it. Conventionally, you would agree on a key and use
some cryptographic algorithm on the source material before you send it.
The recipient uses the same key to decrypt the material.

There are a many such algorithms - called "symmetric ciphers" because they
use the same key to encrypt and decrypt data. However there are some
serious problems which make them less than ideal for exchanging email.

- The key has to be agreed in advance - which obviously can't be done by
  email as it's likely to be compromised before it gets used!
- If there's more than one recipient and you don't want them to be able to
  read each other's mail you have to agree on a different key with each of
  them.

A solution to these problems are "asymmetric" or "public key" algorithms,
which use different keys for encryption and decryption. Everyone involved
has two keys, a public key which can be passed on to anyone, and a secret
key which is never given to anyone else.

Now, when A wants to send private email to B, s/he uses B's public key to
encrypt the text. Subsequently only B can decrypt the message, using
his/her secret key. There's no longer a problem exchanging keys, because
the public key is used to encrypt material and only the intended recipient
has the secret key required to decrypt the material again.

Nothing is perfect and there are some minor disadvantages. There has to be
some correlation between the public and secret key so the keys need to be
much larger than those used for symmetric ciphers for the same level of
security. The algorithms are considerably slower.

"Pretty Good Privacy" (PGP) combines the advantages of both cipher types.
Email and other data is encrypted using the "International Data Encryption
Algorithm" (IDEA) which was developed at the ETH in Zurich. IDEA is very
secure using keys with a length of 128 bits (by comparison the alternative
"Data Encryption Standard" (DES) uses 56 bits and is considered insecure
these days). IDEA keys do not have to be exchanged; instead, PGP creates a
truly random session key, encrypts the message using IDEA with this key,
and then uses the public key scheme RSA (see boxout) to encrypt the
session key. After that, the crypted message and the crypted key are put
together and can be passed to the recipient.

For example, when A sends a message to B, PGP does the following:

- Creates a random 128-bit session key
- Packs the message using the ZIP algorithm (which makes it shorter and
  less vulnerable to cryptoanalysis)
- IDEA-encrypts the message with the session key
- RSA-encrypts the session key with B's public key
- Concatenates both the crypted message and the crypted key into an output
  file

When B receives the message, PGP performs the following steps to
recover the data:

- Separates the crypted message and session key
- Restores the original session key using RSA with B's secret key
- Decrypts the message using the original session key
- Unpacks the message

If you have more than one intended recipient, PGP does not create multiple
session keys, instead it puts RSA-encrypted "versions" of the session key
for each recipient into the output file.
Thus, if you send the same message to ten people, the PGP output file
would contain the IDEA-encrypted message once and ten session keys, each
RSA-encrypted with the public key of the individual recipient.

PGP also enables messages to be "digitally signed" which makes it possible
to be sure a message written by you has not been altered by anyone. To do
this a "digest" is created which contains a sequence of numbers which
exactly matches the message. No other message could have the same digest.
This digest is then encrypted with the secret key of the writer, resulting
in the signature. To check it, PGP simply uses the public key of the
author to decrypt the digest, then compares it to the digest of the
message which the signature belongs to. As only the owner of the secret
key is able to create a correct signature, a match is proof that the
message is unaltered and was written by him.

The nice thing about digital signatures is that PGP supports both plain
text and binary data which means it's possible to digitally sign file
archives so anybody with access to the author's public key can check
whether their copy is an original!

Key management

If you want to exchange email with someone using PGP, you must first
exchange your public keys. PGP offers lots of useful key management
functions. It manages two keyrings, one contains your secret key/s and
should never be accessible to others, the other contains all public keys,
including the counterpart to your own secret key. You should maintain
backups of both keyrings, because the loss of your own keys could be very
embarrassing! The only thing worse than losing your secret keyring is if
someone else gets hold of your private key so PGP insists this is password
protected - make sure you don't choose an obvious one!

Unless you obtain a public key directly from the owner (which means s/he
is physically present) how can you be sure it's not someone else's?
This may seems unlikely but public keys are vulnerable to
"man-in-the-middle attacks". For example, if someone intercepted email en
route to you and replaces the senders public key with his/her own they
would be able to intercept and decrypt your reply! Even worse, after
having read the contents s/he could encrypt the message again with the
senders original public key and neither you nor the sender would be aware
of the intrusion! Naturally such an attack requires a considerable
technical effort, but is possible in most networks.

To try to prevent this intrusion PGP offers a couple of possibilities:

- Every key has a "fingerprint", which could be checked by phone.
- Keys could be digitally signed. When you're absolutely sure a key
  belongs to the person you think it does, you can sign the key, using
  your secret key, and send it back to the owner.

If you subsequently receive a new key, you can check if it has the
signature of someone you trust (and naturally whose public keys you
already have), before deciding whether the new key is likely to be
genuine.

Risks

Even though PGP offers excellent security it still needs to be used
responsibly:

- Don't trust public keys without at least checking their fingerprint, and
  don't check them by email.
- Never ever sign a key unless you are absolutely sure it belongs to the
  person you think it does because once you sign a key, you signal to
  others the key is genuine.

Of course, it's also possible someone will try to crack PGP-encrypted
messages but the effort needed to calculate the secret key to a public key
or break the IDEA cipher is so high it would take years to do so, even
using very powerful computer networks - they're more likely to get results
using "conventional" extortion or violence.

Why use PGP?

Having read the last paragraph, you may wonder whether to bother using PGP
at all, especially if you don't have any confidential stuff to send.
You're not alone, some experts suggest sending encrypted data simply
attracts the attention of hackers. On the other hand if more people use
PGP (or other cryptographic tools) it wouldn't attract the same degree of
attention, a typical chicken and egg scenario!

Another reason to use PGP is simply prevent casual intrusion. Think of PGP
as envelope for your email. When you send a postcard you have to accept
anyone can read the contents, email is analogous to a postcard the same.
Bear in mind your messages travel across the internet via lots of
computers and it's possible someone, somewhere could be reading your mail
and almost every tiny bit of private information can be abused in some
form so it's a sensible precaution to pop it in a PGP "envelope".

PGP and Atari

PGP is a command line utility written with portability in mind which has
the advantage that versions are available for most platforms, including
ours. Unfortunately these are not very user friendly so but happily there
are various GEM shells which provide a convenient graphical user interface
to use PGP. One of the best known is Easy PGP, programmed by Manfred
Ssykor, which is multitasking aware and offers access to nearly all PGP
features.

Easy PGP can help you manage different keyrings, which is useful if you
have both personal and professional email correspondence but don't wish to
keep the keys together to avoid confusion.

Of course, it is still inconvenient when you receive an encrypted email,
to have to call PGP (or shell) to read it and the same applies for sending
PGP encrypted mail. However there are mail readers which offer built-in
PGP support or allow the use of PGP as an external filter. This means,
you can optionally encrypt/sign email before sending it, and incoming
encrypted mail can be automatically decrypted when you want to read it.

For example Okami, an offline mail and newsreader, includes PGP support,
as well as mutt, which is a mail program ported from the Unix world,
primarily designed for online use.

Getting PGP

Almost every BBS or ftp server carrying Atari software will carry a copy
of PGP. The URL below is an excellent jumping off point because it allows
you to select your platform then displays a list of hosts in various
countries:

URL: http://www.pgpi.com/download/#2.6.3i

The current version is 2.6.3. Be sure to get the international release if
you live outside the US (usually denoted by the "i" suffix, for example,
PGP 2.6.3i). After unpacking the archive do take the time to read the
documentation. They are easy to read and provide the necessary knowledge
to use and handle PGP correctly (and that does not only refer to technical
usage). It's not a good idea to use PGP without reading them and be wary
of any PGP version which came without documentation!

You should also make sure your copy has not been hacked, rendering the
encryption useless (for example, by always using the same IDEA-key instead
of generating a random one for each new message). A quick way to check
this is to encrypt the same text twice for the same recipient (create some
test keys) and if the resulting encrypted files are different your copy is
probably OK (though this isn't an absolutely reliable test). The best way
to check your copy of PGP is to check its signature, but this only
possible if you're already using an older version of PGP, and have and
trust the public key of the person who signed the archive.

Finally, here's the fingerprint of my public key, which you can verify
when you get it. Naturally if you use this you're already assuming the
editor, publisher and everyone else involved in the production process
haven't tampered with the fingerprint!

92 6D 22 37 8F 8D 76 FC  2F FB DD 33 50 2C ED 21

If you'd like my public key my email address is:

[e-mail]


RSA
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RSA is a public key cryptography scheme named by its developers Rivest,
Shamir, and Adleman, uses prime numbers. Today, only one way is known to
break a non-trivial RSA-key: the factorisation of the product of the two
primes used to create the key. In other words: The product is known and
you have to search for the two primes used to create it. The security of
RSA is based on the fact that there's currently no easy, or more
accurately, no fast, way to do this for large primes. Consequently the
larger the primes, the more secure the resulting key.

Experts reckon it's safe to use keys with 768 bits and onwards bearing in
mind the speed of today's computers and the known factorisation
algorithms. That means, a key with 768 bits will be safe for years,
provided no one comes up with much faster algorithm to factorise large
numbers or an alternative strategy to break RSA.

I'd recommend anyone creating a new key to opt for at least 1024 bits but
if you're currently using one with 768 bits you don't need to worry unless
you need to protect data which has to remain secure in ten years.


PGP 5.0
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Even though the article statest PGP 2.6.3 is the current version, PGP 5.0
has been available for months. However the new version has not yet been
ported to the Atari platform and is incompatible with the old one. This
doesn't mean you can't exchange messages with the other version or that
you have to create new keys, merely that PGP 5.0 offers completely
different command line options, so shell programs designed to work with
2.6.x will simply fail. So, unless you intend use PGP directly from the
command line, use v2.6.3 or wait until the new version has been ported and
the programs you want to use have been adapted.

PGP 5.0 also supports a new public key scheme in addition to RSA, which is
incompatible. Thus, you can't exchange protected messages with someone who
has such a new key, and vice versa.


Recognising PGP messages
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As PGP encrypted text is no longer readable and can't be sent via email as
it is, PGP offers "ASCII armour". This means the resulting ciphertext is
converted so it consists of characters which can transmitted by email -
similar to the uuencode process you may already be familiar with.

To inform the recipient (or mailing program used) the email contains a PGP
message, it is surrounded by the following two lines:

-----BEGIN PGP MESSAGE-----
-----END PGP MESSAGE-----

In discussion forums such as UseNet you'll also often spot
messages beginning with:

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

That means, the following text has been signed by its author, the
signature is attached at the end of the text, in the section
marked with:

-----BEGIN PGP SIGNATURE-----

Sometimes, people also send or post their public keys. These are
surrounded by these two lines:

-----BEGIN PGP PUBLIC KEY BLOCK-----
-----END PGP PUBLIC KEY BLOCK-----

In all cases, you can simply drag the file containing the message
on the PGP executable. PGP automatically detects the type of the
message and takes the appropriate action.

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