Lecture 18: Encryption #2 - Public Key Systems

Public Key Systems

DES and its derivatives works well, but relies on both parties having a copy of the same secret key. The problem of Key Distribution in such systems can be a very difficult to overcome. In 1976, Diffie and, Hellman^[1] proposed an enitrely new system called Public Key Cryptography. The basic ideas are:

An encryption method E is applied to the plaintext P giving E(P), the ciphertext.
A decryption method D can be applied to the ciphertext to yield the plaintext, thus:
```
P = D ( E ( P ) )
   
```
It is exceedingly difficult to deduce D from E.
E cannot be broken by any standard attack.

The original algorithm proposed by Diffie and Hellman for implementing Public Key Cryptography turned out to be insufficiently strong to implement a secure system. However, the idea of this type of cryptosystem was considered worthy of further work.

^[1] This is the standard story on the origins of Public Key Cryptography. It's now apparent that a group of British researchers had come up with the same idea somewhat earlier: see, for example: The Open Secret.

Lecture 18: Encryption #2 -- Public Key Systems

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