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October 20, 1997, Section: Communications Design

Solid case for hardware-based encryption

By Richard S. Kagan, Vice President of Marketing, VPNet Technologies Inc., San Jose, Calif.

A virtual private network (VPN) is a communications system that uses public networks to transport private intra- and intercompany information. VPNs allow companies to reuse the same public facilities they've set up for e-mail or Internet connectivity to provide cost-effective remote access, extend intranet services across the WAN and improve communications with business partners through ad hoc extranet links.

Security is a primary consideration for businesses using public networks for private purposes. The Internet Engineering Task Force has defined a standard mechanism, the IP Layer Security Protocol (IPSec), for ensuring the privacy, integrity and authenticity of data traveling over IP nets.

Because of the business advantages they impart, VPNs have gained visibility and mind share very rapidly. Software and hardware vendors alike are scrambling to bring IPSec-compliant VPN products to market.

Today's VPN alternatives include add-on software for existing routers and access devices, most likely delivered by the hardware manufacturer; firewall software extended to support VPN capabilities; hardware accessories for routers and PCs, such as PCI or PCMCIA encryption cards; and dedicated, standalone VPN hardware devices, such as VPN service units or VPN servers. When deploying a VPN, the choice that's made among these alternatives can have a large impact on the security, performance, cost, manageability and scalability of the ultimate implementation.

Businesses will migrate their data to VPNs only when VPNs provide the required security features without compromising the quality and performance of the network. B ut delivering on the security requirements of VPNs requires complex mathematical transformations that can be quite processor-intensive.

Encryption works by converting data to an unreadable form and combining it mathematically with a special key; it is impossible to decipher the encrypted data without the correct key. The architecture for encrypting data under IPSec is called the Encapsulating Security P ayload. There are two defined types, each based on the Data Encryption Standard (DES). The DES-CBC algorithm is a 56-bit block cipher that's been in use for more than 20 years. The newer, stronger Triple DES-CBC algorithm passes data through DES three times, effectively increasing the key size to 112 bits. For data networking, the most robust type of encryption is Triple DES.

Routers and firewalls operate primarily by examining a few bytes in the packet header. But to perform encryption, a VPN device must touch every byte of data in a packet and perform heavy-duty mathematical transformations on each one. That is, it must do something computationally difficult many, many times over-at both ends of the connection. Triple DES encryption requires about 50 to 100 times more processing than straight IP routing, according to Infonetics Research. Data authentication also requires manipulating every byte of the packet at both ends.

For these reasons, VPN solutions that use dedicated high-performance ASICs for encryption offer higher performance than software implementations. The higher the data rate of the VPN connections, the slower the performance of software-based VPNs. End users accessing VPNs over dial-up lines, either analog or 128-kbit/second ISDN, might enjoy adequate performance from software-based VPNs. Businesses wanting to operate VPNs at T1 speeds and above should look to hardware-based solutions.

The VPN implementation also affects physical and logical security. A standalone hardware device can be designed to operate in a tamper-proof enclosure, offering physical protection. And a closed system, such as a standalone hardware device, allows no access to the operating system and thus provides more logical security than a software-based solution.

Both IPSec encryption and authentication incur overhead by adding headers to a packet. In many cases, this overhead can cause the packet to expand beyond the size limit imposed by the router. When confronted with an oversize packet, a router will fragment it into two or more packets of legal size, each with its own header. Thus, in addition to their computational burden, the price of standard encryption solutions is a reduction in network throughput due to the transmission of more and larger secure packets. In other words, standard solutions transmit fewer secure data packets than unsecured packets for a given data rate.

For example, implementing IPSec Triple DES encryption and authentication in tunnel mode adds 86 bytes of overhead per packet. The theoretical maximum for a normal 64-byte IP packets on a 10 Base T link is 14,800 packets per second. Adding the IPSec overhead to the same packet reduces the maximum throughput to 7,352 packets/s.

Real-time advantage

Compressing data before incurring the IPSec overhead can overcome the effects of this packet expansion. Only a hardware-based VPN solution can perform data compression in real-time.

For all of the reasons cited above, software-based VPNs face a trade-off: strong security or good performance. If performance is less of a concern, such as over dial-up access lines, a software-based VPN solution can offer good security at a substantially lower cost than a hardware solution, assuming companies already have the router or server needed to host the software. For this reason, software-based VPNs designed for PC implementations are especially attractive to mobile workers and telecommuters. End users can usually purchase these packages for $100 or less.

For companies that want both high performance and highly secure VPNs, standalone hardware offers the best bottom-line value.

Most tools are optimized around a particular application. For example, a management tool for a firewall is designed to be good at that one task, while the focus of router-management software is managing routers. Adding VPN management to these solutions is just that-an add on, not central to the original paradigm. Thus, instead of providing views of the network as a collection of VPNs, the router-management tool features routers doing VPNs, among many other things. In contrast, dedicated VPN hardware provides tools designed and optimized for creating, configuring and managing virtual private networks.

The true test of value of VPN solutions is their ability to deliver the performance and functionality that businesses and end users need over the long term. The difference with VPNs is that it's not simply the robustness of the platform, but the robustness of the security that ensures longevity.

The strength of a security algorithm is an exponential function of the key length. Many VPN solutions today support 56-bit DES encryption. To crack a message encoded with 56-bit DES encryption using brute force requires testing up to 256 keys, or roughly 72,000,000,000,000,000 (72 million billion) keys. For the more than 20 years that 56-bit DES has been in use, it has been considered "unbreakable" from a practical point of view. While many in the cryptographic community fretted over the security of 56-bit DES, many users remained convinced that the old 56-bit standard was still adequate.

That all changed in June, when an unfunded group of hackers, using spare cycles on processors scattered across the Internet, successfully completed a brute-force attack on a 56-bit DES encoded message in about four months. That has forced a reevaluation of 56-bit DES by many security officers. Based on papers written by the leading cryptography experts, a machine costing under $300,000 can crack 56-bit DES messages in just a few hours. As processing power continues to increase for ever lower costs, the routine cracking of 56-bit DES becomes a practical reality.

Triple DES encrypts a message using three independent 56-bit keys. The message is actually encrypted with one key, decrypted with a second key, then re-encrypted with the third. Thus, cracking a Triple DES message requires the equivalent of testing 72,000,000,000,000,000 times more keys than for a message encrypted with 56-bit DES. Cracking Triple DES messages will clearly remain computationally intractable for ears to come.

Hardware-based VPN solutions can deliver Triple DES encryption at wire speed for affordable prices.