Notes - Shift Ciphers

The following are notes from Introduction to Cryptography with Coding Theory.

• Shift ciphers are one of the earliest cryptosystems and is attributed to Julius Caesar
• gaul is divided into three parts
• he then shifts each letter by 3 places
• JDXOLVGLYLGHGLQWRWKUHHSDUWV
• Encryption process is then x -> x + k where k = 3
• Attackers methodology
• Ciphertext only
• If the method is known then there are only 26 possible keys. However there is most likely more than one meaningful message that can be found when using these keys.
• Frequency analysis to see the shift, e is the most common letter in the English language, find the difference between the most common letter in the message and e
• Known Plaintext
• if you know one letter of plaintext the key is broken
• Chosen Plaintext
• again inputting even just one letter will give you the key
• Chosen Ciphertext
• seeing the output of an input will give you the key

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