Space Shuttle Wireless Part 3
Space Shuttle Wireless Part 3
By: David Geer
Jul. 28, 2003 11:07 AM
In this concluding part of a three-part series on the space shuttle, WBT's David Geer looks at spread spectrum technology and its role in keeping encrypted data secure.
The communications link to the orbiter is a spread spectrum link, a technology that was invented by Hedy Lamar (believe it or not!) during WWII for secure radio transmissions. Spread spectrum makes signal jamming and eavesdropping much more challenging, an important issue for a radio signal coming from orbit, which could be intercepted from anywhere within its footprint. Eavesdropping can also be used to interfere with satellite transmissions via availability attacks, wherein interference from the other channel makes the signal unavailable.
The spread spectrum link uses a modulation scheme in which there are only 72KB of actual data sent. The data is convolutionally encoded (a complex form of modulation, which provides for error correction). This high data rate is like "taking an 11MHz signal and modulating that on top of the 72KB of coded data," says Phil Liebrecht, associate director and program manager for Mission Services, NASA Goddard Space Flight Center.
The process modulates 72KB of data with a much higher pseudo-noise code, which gives the orbiter link protection against interference from other RF signals. It also provides protection for those other signals to keep the orbiter signal from causing interference to the other S-band systems that are in use on earth. This interference could come from signals such as microwave links.
The DES (Data Encryption Standard) is an older and popularly accepted, encryption standard, which met FIPS requirements. It used a 56-bit key. Then came the Triple DES with a 112-bit key, offering enhanced security. A new standard adopted just a few years ago is the AES (Advanced Encryption Standard), based on the Rijndael algorithm. This standard is the latest direction for cryptography in commercial applications. For everything classified (secret, top secret) the NSA (National Security Agency) at Fort Mead, Maryland is responsible for the security algorithms.
One aspect of DES and AES is what is called symmetric encryption. "You apply the encryption algorithm to your data and then you apply it again to decrypt the data. The symmetric algorithms are very fast. People have demonstrated Rijndael chips that work on gigabit Ethernet connections," says Mark A. Sturza, senior advisor to Wavestream Corporation, 3CsysCo.com. (Wavestream manufactures amplifiers for the Ku and Ka bands, which will replace the current tube amplifiers used by NASA. These amplifiers are used on the ground and in space for transmissions to the shuttle.)
To do symmetric encryption you need to have a single key, which is used at both ends of the communication. Key exchange is done utilizing public key cryptography, making use of the well-known RSA algorithm created by Ron Rivest, Adi Shamir, and Leonard Adelman. There are other methods of key exchange than RSA, but RSA has shown its strength over many years and is known to offer high security.
How Is Shuttle Mission Data Encrypted?
For something as important as the space shuttle program, the algorithms and/or keys for encryption are probably changed regularly. "I would strongly suspect that they change it on a mission-by-mission basis," says Sturza. It's possible that this is done through hard coding using cryptoloaders such as the NSA uses. These devices are loaded with the keys at a secure location and then taken to the shuttle and plugged in so that the keys can be loaded into the embedded equipment.
Differences in Voice and Data Handling
The NSP digitizes analog voice received from the ACCU. The signal is multiplexed again with the telemetry data from the PCMMU (the pulse code modulation master unit); this multiplexed data is then ready for real-time transmission to the ground. The data is also routed to the operational recorders for archiving. Data is always encrypted at the ComSec before going to the NSP and back to the ground.
While this series was in production, the shuttle Columbia was destroyed while re-entering the atmosphere, a tragic reminder of just how complex space technology is, and how easily it can fail. But as I look out my window into the night sky, I'm reminded that somewhere among the stars, one of those grand lights is an International Space Station, a wireless one. It's no longer current to say that we live in a Wireless World. With GPS for Mars coming so quickly, and a space-based Internet already in design, it's time to start talking about our Wireless Solar System.
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