World War II is full of stories where the Allies retrieved daily
code booklets from German submarines and Japanese troops,
using them for decrypting intercepted secret communications.
The solution to the distribution problem emerged few years ago,
with the introduction of asymmetric key algorithms (or public
key cryptography) in the 1970s. The new class of algorithms
uses functions that cannot be reversed. In other words, the key
that is used for encrypting the plaintext cannot be used for decrypting
the ciphertext. As a result, a plaintext-ciphertext-plaintext
full cycle needs two keys: one for encrypting and one for
decrypting. The two keys are naturally related, but one cannot
be derived from the other. Note that the roles of the two keys
CLEAR
TEXT
AEBFFA
CBCBAE
Symmetric Encryption
Algorithm
ENCRYPTION
DECRYPTION
KEY
Figure 1-4 Symmetric key encryption: the same key can scramble and
reveal the clear text.
In asymmetric key
algorithms, you
encrypt with one
key but decrypt
with another; hence
you don??™t need to
distribute the
decryption key
The Babel of Cryptography 41
can be freely exchanged: If I will encrypt with A, I will have to
decrypt with B, and vice versa.
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