An introduction to secure communication

Shift cipher

Caesar’s cipher substitution for key k=3

Better security by increasing the key space

Leaking information from ciphertexts

Perfect secrecy and the one time pad

An encryption scheme is perfectly secret if the probability of a message does not change by the observation of a ciphertext.
XOR operation for the one time pad for messages of 1 bit.

Improving the one time pad: stream ciphers

Improving the stream cipher using quantum physics

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