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Public Key Systems
DES works well, but relies on both parties having a copy of the
same secret key. This can be a big problem. In
1976, Diffie and, Hellman[1] proposed
a 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.
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