From Cryptography Decrypted

Part I Overview:

Secret Key Cryptography

The good news is more and more “good” people are connected to the web and the bad news is more and more “bad” people are connected to the web. Unfortunately, most of us understand through personal experience that our computer, our identity and perhaps even our livelihood is vulnerable to attack and our computer controlled national infrastructure is at risk. What most of us don’t quite ‘get’ yet is how cryptography can help and why it isn’t yet a tool everyone consciously uses to protect themselves.

Cryptography is by its very nature designed to be confusing, and computer cryptography does a good job of making such digital hiding places even more circuitous. So let’s start by looking at one of computer cryptography’s simpler essential components – secret keys – before we explain why public key cryptography had to be invented to usher in the possibility of secure communications in the digital age.

The Basics

Cryptography is both the lock and combination or “key” that can help protect your data/identity. There are a variety of cryptographic methods and keys. The method and key together determine cryptographic security. If the cryptographic algorithm or method is secure and there are great quantities of potential secret keys, the method is said to be strong.

Strong methods are made more secure by being published since they can be scrutinized by cryptanalysts, mathematical and linguistic analysts who attempt to remove the disguise to meaning that cryptographers create.

Openly publishing a cryptographic method is a good way to assure its security. Digital Encryption Standard (DES), the published cryptographic standard from 1977 - 2000 withstood attack over the years. The DES algorithm was strong; cryptanalysts had no choice but to attack the keys. This means trying, on average, half of all possible keys — some number of trillion keys.

However, computer hardware advances compromised the strength and security of DES because it’s easier to search through all those keys now than in 1977. Rijndael (pronounced “rain doll”) was selected by the National Institute of Standards and Technology (NIST) to replace DES in 2000 and is known as the Advanced Encryption Standard (AES). Rijndael, an algorithm created by European cryptographers, was deemed the strongest candidate submitted for consideration. But just because a cryptographic method is considered strong doesn’t mean it gives us all the assurances we want and need.

Assurances We Need

We want our digital communications to provide us with all the security assurances we have historically enjoyed from our face-to-face communications. We want to know that only those we intend can receive our communications (confidentiality), that we know who we are talking to (authentication), that our message hasn’t been changed (integrity) and that we can be assured the person with whom we communicated can’t deny having received our message (non-repudiation). Secret keys provide us with most but not all of these assurances.

In digital terms these assurances can be described as follows:

1. Confidentiality is assurance that only owners of a shared secret key can decrypt a computer file that has been encrypted with the identical shared secret key.

2. Authentication is assurance of the identity of the person at the other end of the line. This is digitally accomplished by Challenge Response. Since Bob can't send the shared secret to prove who he is, Alice challenges Bob to correctly encrypt a previously unused random number with their shared secret key. Only the shared secret key will correctly encrypt the random number.

3. Integrity, or message authentication, is assurance that a file was not changed during transit. A message and shared secret key make a unique Message Authentication Code (MAC), message "fingerprint”. Only someone with a copy of the shared secret key can correctly reproduce the fingerprint.

4. Non-repudiation is assurance that the sender cannot deny a file was sent. This cannot be done with secret key alone; we need a mutually trusted third party or public key technology.

The Big Problem: Key Distribution

Secret communications with secret keys implies that only trusted parties should have copies of the secret key. That is, although secret keys can assure us of confidentiality, authentication of users and message integrity, in a global world we must be able to securely distribute keys at a distance in a timely manner.

If security is to be maintained, key distribution must be as solid as the cryptographic method and be able to assure that only trusted parties have copies of the keys. Obviously, key distribution is a very big problem.

Traditional methods of key distribution use trusted couriers to place the initial secret key. If the key is shared with a trusted third party (TTP), additional keys can be shared because secret keys can encrypt other secret keys. When the TTP encrypts any additional keys with the shared secret key, the TTP is often referred to as a Key Distribution Center (KDC).

The more users the more keys, the more key management and the bigger the potential bottleneck at the KDC. Additionally, if the KDC also acts as a key escrow agent, the KDC, itself, is an attractive target.

Secret keys get very close to giving us all the digital security we want, but because you and I have a hard time sharing and maintaining such keys, secret keys are a necessary but not sufficient component to usher in a more secure digital age. Public key encryption makes (secret) key distribution and authentication much easier. The phrase Public Key Infrastructure (PKI) is used to refer to public key management/distribution systems that many of us read about but have yet to get a chance to use.

Part II: Public Key

(c) H. X. Mel & Doris Baker all rights reserved

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