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Thursday, November 15, 2007 5:00 AM

Controlling for Clock Skew

Message replay is an attack where a message is presented to a processor more than once in the hopes of fooling the processor into taking some action. One protection against message replay and other timing-based attacks is to have the sender timestamp
Posted by Nicholas Allen | 3 Comments
Friday, September 22, 2006 5:00 AM

Math Behind the RSA Algorithm

This post is to tie up some loose ends in regards to actually performing the RSA computations. I've avoided including too much math in the earlier posts to make them easier to read. Here are some references that help explain the individual steps. The
Posted by Nicholas Allen | 2 Comments
Thursday, September 21, 2006 5:00 AM

A More Recent RSA Attack

One of the interesting things about writing articles ahead of time is that the plan sometimes changes when it's time to publish the articles. It turns out that in the last few weeks someone has found an interesting forgery attack on RSA signatures . This
Posted by Nicholas Allen | 3 Comments
Wednesday, September 20, 2006 5:00 AM

Attacks on RSA

RSA has several weaknesses called protocol failures. Protocol failures are not actually an exploit in the RSA algorithm. Instead, a protocol failure occurs when you perform inadvisable actions that give the attacker more information than they would otherwise
Posted by Nicholas Allen | 5 Comments
Tuesday, September 19, 2006 5:00 AM

Using RSA for Signing Messages

A nice property of RSA is that if we swap the role of the encryption and decryption keys, it's still possible to transmit messages . That's because the computation (M e ) d mod n is the same as (M d ) e mod n. Typically, messages are encrypted with your
Posted by Nicholas Allen | 6 Comments
Monday, September 18, 2006 5:00 AM

Splitting Messages for RSA

For your particular pair of RSA primes, there is a fixed size to the messages that can be encrypted with the product, n, of those primes. During decryption, you will always end up with the smallest positive integer message that satisfies the algorithm.
Posted by Nicholas Allen | 7 Comments
Thursday, September 14, 2006 5:00 AM

Using RSA for Sending Messages

One of the key points made about the Diffie-Hellman algorithm is that it doesn't actually allow you to send a message from one party to another. DH is useful for constructing a new shared secret value but can't directly be used to exchange an existing
Posted by Nicholas Allen | 4 Comments
Wednesday, September 13, 2006 5:00 AM

Attacks on Diffie-Hellman

We're going to continue looking at the Diffie-Hellman algorithm today by examining how to configure the algorithm to be more resistant to attacks. DH is small enough that I'm not going to summarize the algorithm here. You can go back to the description
Posted by Nicholas Allen | 3 Comments
Tuesday, September 12, 2006 5:00 AM

Diffie-Hellman Key Exchange

If you've been reading the previous posts on network security , then you've seen several instances where two parties need a shared secret. We've just been assuming that a shared secret is magically known. How can two parties share a secret without having
Posted by Nicholas Allen | 10 Comments
Tuesday, August 01, 2006 5:00 AM

Advanced Encryption Standard

The last cipher I'm going to talk about is the Advanced Encryption Standard (AES). With this, we'll have covered about half of the important algorithms needed for a transport security implementation, such as SSL. AES started out as a contest to replace
Posted by Nicholas Allen | 3 Comments
Monday, July 31, 2006 5:00 AM

More Symmetric Cipher Suites

Block ciphers are more popular than stream ciphers , with several either in active use or recent enough to require supporting for legacy interoperability. I'll talk about the RC2 and DES algorithms today and cover the newer AES algorithm tomorrow. DES
Posted by Nicholas Allen | 4 Comments
Friday, July 28, 2006 5:00 AM

Symmetric Cipher Suites

The list of commonly used stream ciphers is very short because there's really only one. RC4, developed by Ron Rivest, is essentially the only stream cipher that has been widely deployed. RC4 is very fast and found in wireless networking devices, as part
Posted by Nicholas Allen | 1 Comments
Tuesday, July 25, 2006 5:00 AM

Symmetric Encryption Algorithm Design Issues

When using symmetric encryption, repetition is the enemy of security. For the basic stream cipher and block cipher algorithms, an attacker can exploit repetition in either the input or key to gain information about the protected message. Stream ciphers
Posted by Nicholas Allen | 1 Comments
Friday, July 21, 2006 5:00 AM

How Stream Ciphers Work

Yesterday I kicked off the topic of symmetric encryption by talking about block ciphers . Stream ciphers are another common pattern for symmetric encryption algorithm. Unlike block ciphers that operate on chunks of input text, a stream cipher operates
Posted by Nicholas Allen | 8 Comments
Thursday, July 20, 2006 5:00 AM

How Block Ciphers Work

Back in May I gave a brief introduction to encryption and decryption . The next few posts are a short series on symmetric encryption algorithms, which use a shared secret key for both encryption and decryption. I've got a little bit of coverage about
Posted by Nicholas Allen | 11 Comments
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