Slides · 16 slides
The history and purpose of cryptography
The same module told visually, ready to present. Speaker notes carry the full explanation.
Foundations
The history and purpose of cryptography
2,500 years of codemakers against codebreakers, and what it was all for
Purpose
Four things cryptography delivers
Pick the tool for the property you need: encryption alone proves nothing about who sent it
The big picture
2,500 years in one line
c. 500 BCE · Transposition
The scytale: rearrange the letters
c. 50 BCE · Substitution
Caesar: replace every letter
The lesson
Hide the key, not the method
c. 850 · The first cryptanalysis
Al-Kindi counts the letters
A substitution cipher changes the letters but keeps their frequencies
1553 · Polyalphabetic
Vigenère: a different shift for every letter
1854–1863 · The break
Kasiski: repeats give the key away
1918 · Machines
Enigma: a new substitution every key press
1932–1945 · Codebreakers
Enigma was broken in Poland first
1949 · Mathematics
Shannon: perfect secrecy is possible
Every plaintext of the same length is equally likely, so the ciphertext reveals nothing
The 2,500-year problem
First, share a key
1976 · Public-key cryptography
Agree on a secret in public
Mixing paint is easy, separating it isn't: Diffie–Hellman does the same with modular arithmetic
1976–77 · The ancestors
Everything on this site descends from here
Where this leaves us
The classical era, and what comes next
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