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processing power that can be leveled against it. The greater the MIPs rating, the greater the threat and the faster the data falls to a simple brute force attack.
DES, in particular, has been the focus of some discussion concerning its security into the 21st century. One of the reasons for its waning protection is that it uses only 56 of the 64 bits of its
key. Although it cannot be considered academic for another few years yet, the possibility of creating a simple and widely distributed software brute force crack of DES is looking quite
real just on the horizon. As of July 1998, a cheap hardware DES cracker was demonstrated by the Electronic Frontier Foundation, constructed of off-the-shelf components. One way to
combat these threats is to multiply encrypt the inputs. The triple-DES standard is such an implementation, and given that it increases the number of possible keys, it effectively
removes any threat for the foreseeable future.
RSAs strength is derived from the inability to factor a huge number quickly. As we discussed in the explanation of the RSA algorithm earlier, the sheer size of the numbers that are used is
enormous, large enough that it surpasses that number of bits that even large computers use to store numbers internally. Think about it in terms of how many digits you could put into a
calculator. The bigger problem is that even if you could represent the numbers in a way that a computer can handle, the act of factoring is a long and tedious process without shortcuts and
without easy, simplifiable steps. Because of this, you could spend a great deal of time just looking through billions of numbers and never finding a factor of n. Remember that if you
wanted to crack RSA, the surefire way of doing it would be to recreate the original inputs used to calculate d and e. That means factoring n.
2.2.6 Use of Cryptosystems and Authentication in a VPN
As with all secure communications, all protection systems have three important functions in common. Secure communications first protects the data in transit so that hostile or curious
third parties in the middle are not able to intercept and read the transmission this is the concept of encryption. Popular encryption techniques used in VPNs include DES, Triple
DES, RC2, and RC4.
Second, both parties must know with confidence that they are speaking to one another. In other words, Alice is certain she is speaking with Bob, and vice versa, even though they can
not see each other this is the idea of authentication.
Lastly, both Alice and Bob need to be able to detect if any third party is trying to tamper with the messages, either with a destructive goal like the insertion of many messages to prevent
Alice from reaching Bob or even a benign one like infrequently inserting garbage to test a hypothesis. This is the concept of message integrity, and is sometimes referred to as a
message digest for example: MD5 or SHA.
How all of this ties together can be illustrated by a short scenario. When Bob wants to send a message to Alice, he would first run his clear text through a message digest function that
would produce a hash code or a message authentication code MAC code, which would be encrypted with his own private key, then included with the message itself, and then the whole
thing would be encrypted with the receivers public key. After transmission, the receiver would decrypt the whole package with her private key, compute her own MAC code from the
now clear text document, decrypt the senders MAC code with his public key, then compare
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the MAC code she locally created to the one sent in transit. If they differ, tampering has occurred.
Suns SKIP protocol is a fairly popular implementation of encryption and authentication, comparable to the IPSec standard discussed later in this chapter. SKIP is available from
various vendors on a number of Unix operating systems as well as Windows. Each host using SKIP maintains an access control list specifying which hosts its willing to receive traffic from
and what type of encryption to use for each one. When an IP packet is sent from one SKIP host to another, the sender encrypts the packet with the SKIP protocol and then wraps another
IP packet around it so it can go over the general Internet.
The keys to erecting a private data exchange or a secured data store rely on being able to fence out unwanted people and place locking boxes around what you wish to protect. Without
firewalls, a VPN could exist, albeit without the same security philosophy. But, without encryption, a VPN most certainly could not exist. Cipher routines solve the fundamental
problem of secure communication over an insecure channel in a hostile environment.
Using the components of encryption, authentication, and integrity, we will next explore the different protocols that are used to build VPNs. After that, we will briefly delve into the
different compromise methodologies that can be leveled against these security schemes.
2.3 VPN Protocols
Coming from different directions and supporting different products and services, several security protocols have been in development over the last few years. We will start with one
that has firmed up only recently, but will probably become nearly universal—the IPSec standard.
2.3.1 IPSec
Over the years as vendor after vendor labored over reinventing wheels, trying to hide IP packets in a secure protocol, people began to wonder why the TCPIP protocol itself wasnt
updated to support authentication and encryption. That way, the network itself is secure and everything built upon it must also be secure. IPSec is the answer to this question.
The Internet Security Protocol IPSec is a generic structure initiated and maintained by a working group of the Internet Engineering Task Force IETF to provide various security
services for the Internet Protocol IP, for both IPv4 the current standard and IPv6 the upcoming one. IPSec presents design goals for a top-level component-oriented structure,
rather than detailing specific encryption algorithms or key-exchange methodologies.
Conceptually, IPSec was created to secure the network itself, presenting no real changes to the applications that run above it. Since the TCPIP protocol is so ubiquitous, it is a natural
evolution to produce a secure network system developed almost in parallel to the existing system. Upgrading to IPSec products and services will only enhance security, as current
network-oriented applications can still be used to transport data.
The IPSec documents produced by the IETF are predominantly concerned with three basic areas of securing the IP protocol: encryption algorithms, authentication algorithms, and key
management. These components help define the entire architecture of a security scheme,
