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Cellular Networking Perspectives is proud to be able to link you to the anonymous Crypto Answer Man. He will attempt to answer questions of general interest related to wireless security. Due to personal security requirements (and a few unpaid bills) he cannot reveal his identity. Questions that are accepted by our man, will be posted here, along with the answers. Particularly interesting questions and answers will be published in Wireless Security Perspectives.
This question was submitted by Alan Turing :)
The response is shown in more detail, and with figures, in the July 1999 issue of Dr. Jon's Wireless Security (now called Wireless Security Perspectives).
A very good question! This is especially important today in the world of ubiquitous wireless communications. Well, one way to securely transmit a secret key is to not actually transmit it at all. Instead have the communicants derive it on both sides. That is, let our well-known friends Alice and Bob (or two radio devices) solve the key management problem without sharing any secret information using a key agreement protocol. The first and best known of these is the Diffie-Hellman Key Exchange (DHKE, also called exponential key exchange).
Simple put, for the DHKE, Alice selects a large prime number (p) at least 512 bits in length. She also chooses another number that is less than the first (alpha). She then sends these two values to Bob. Next both Alice and Bob independently select their own secret random numbers (a and b), also at least 512-bits long. They then commence to perform modular exponentiation with their secret numbers and the public numbers. That is,
alphaa modulo p (Alice)
alphab modulo p (Bob)
They then exchange the results of these calculations and perform another calculation using the other’s numbers. Upon completing this second computation, Alice and Bob have identical results and a shared key, K = alphaab mod p. In doing this, Alice and Bob have generated a key that they both know and no one else can figure out based on the information that was sent between them. The security of the system rests on the intractability of computing discrete logarithms.
DHKE predates the RSA public key cryptosystem. However, it does not provide encryption or digital signature capabilities. The DHKE, the oldest PKC still in use today, is used simply for pair-wise key establishment.
July, 1999 issue of Dr. Jon's Wireless Security
Alan Mathison Turing (1912-1954) was a British mathematician, who after the breakout of WWII, worked for the Code and Cipher School at Bletchley Park, and was instrumental in breaking the German Enigma Codes. His brilliant career came to a sad end after he was jailed for his homosexuality, which was still a crime in England in the 1950's. He was forced to take hormone therapy that eliminated his sex drive and disfigured his body. In shame and despair, he ended his life by eating a poisoned apple, based on his fascination with the Snow White movie.
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