We use it every day and we rely on it. But what are the roots of cryptography? How were, for example, the ancient Greeks able to protect information from their enemies? In this talk we will go through 5500 years of developing encryption technologies and look at how these work. From the Un-Distinguished Lecture Series (http://ws.cs.ubc.ca/~udls/). The talk was given Mar. 23, 2007

1. A Brief History of
Cryptography
______________
Florian Deckert
23.03.2007

2. Encyclopedia Britannica:
“Cryptography: Practice of the enciphering and
deciphering of messages in secret code in order
to render them unintelligible to all but the intended
receiver.”
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3. Early Cryptography
• 3500 BC: Sumerians
– Cuneiform writings
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4. Early Cryptography
• 1900 BC: Egypt
– First known use of cryptography
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5. Early Cryptography
• 500 – 600 BC:
ATBASH Cipher
– Used by Hebrew
scribes
– Substitution cipher
(reversed alphabet)
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6. Early Cryptography
• 486 BC: Greece
– σκυτάλη – skytale
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7. Early Cryptography
• 60 – 50 BC: Julius Caesar
– substitution cipher
– Shift letters by X positions:
• E.g. X = 3: A D, B E,
C F, ...
– Weakness?
• Frequency analysis (1000 AD)
– 1466: Leon Albertini: cipher
disk
• Used until 16th century
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8. Medieval Cryptography
• 1587: Vigenère Cipher
– Polyalphabetic: one to
many relationship
– Example
• Encrypt: lamp
• Keyword: ubc
• Ciphertext: fboj
• Apart from that...
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9. Modern Cryptography
• 1845: Morse Code
– Represention by code
signal
– States (on and off)
composed into 5
symbols
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10. Modern Cryptography
• 1863: Kasiski breaks Vigenere:
– Find length of keyword
– Divide message into substitution cryptograms
– Use frequency analysis to solve these
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11. Modern Cryptography
• 1918: ADFGVX Cipher
– Used in the German army in WWI
A D F G X
B T A L P
A
D H O Z K
D
Q F V S N
F
G J C U X
G
M R E W Y
X
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12. Modern Cryptography
• 1918: The Enigma
– Arthur Scherbius
– Business: confidential
docs
– No codebooks
– Rotors multi
substitution
– Wireing changes as-you-
type
– German forces in WWII
– Room 40
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13. Modern Cryptography
• 1937 – 1945: Navajo Code Talkers
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14. Modern Cryptography
• 1949: Shannon:
– Communication
Theory of Secret
Systems
– Proved: One time pad
unbreakable
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15. Modern Cryptography
• Until mid 70‘s
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16. Modern Cryptography
• 1976: Diffie – Hellman Key Exchange
– Whitfield Diffie and Martin Hellman
– Discrete logarithm problem:
• G: finite cyclic group with n elements
• Modulo n multiplication
• b: generator of G: every element g of G can be
written as g = bk for some integer k
• Goal: find k given g and b and n!
• Very hard problem
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17. Modern Cryptography
• So how does it work?
• Exploits?
– Man in the middle
– Fix: additional authentication
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18. Modern Cryptography
• Public Key Crypto
– Key exchange problem
– Asymmetric key algorithm
– E.g: RSA, MIT, 1977
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19. Modern Cryptography
• 1991: PGP Pretty Good Privacy
– Protocol, uses RSA
– Encryption & decryption
– Digital signatures
• How does that work?
– Web of Trust
• Third party signs (public) key to attest association
between person and key
– Other Possibility: Hierarchical, CA based
• E.g.: X.509 Certificates in SSL
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20. Thank You!
Let’s go to Koerner’s.