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Cryptography for dummies 1 / 14 Cryptography for Dummies From Classical to Modern Laszlo Csirmaz Central European University March 26–30, 2012

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Page 1: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 1 / 14

Cryptography for DummiesFrom Classical to Modern

Laszlo Csirmaz

Central European University

March 26–30, 2012

Page 2: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 2 / 14

Classic Definition of Cryptography

Cryptography, or the art of “hidden writing,” comes from Greekhiding the contents or existence of messages from and adversary

Encryption makes a message unintelligible to anyone notpossessing some secret information

Decryption undoes the scrambling

Steganography, or “covered writing,” is concerned with hidingthe existence of a message – often in plain sight.

Page 3: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 3 / 14

Scytale transposition cypher

Page 4: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 4 / 14

Caesar Substitution Cipher

Page 5: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 5 / 14

Caesar Substitution Cipher

E T T U B R U T E

↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓H W W X E U X W H

Page 6: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 6 / 14

Vigenere Polyalphabetic Substitution

A B C D E F G H I J K L MN O P Q R S T U VWX Y Z

A A B C D E F G H I J K L MN O P Q R S T U VWX Y ZB B C D E F G H I J K L MN O P Q R S T U VWX Y Z AC C D E F G H I J K L MN O P Q R S T U VWX Y Z A BD D E F G H I J K L MN O P Q R S T U VWX Y Z A B CE E F G H I J K L MN O P Q R S T U VWX Y Z A B C DF F G H I J K L MN O P Q R S T U VWX Y Z A B C D EG G H I J K L MN O P Q R S T U VWX Y Z A B C D E FH H I J K L MN O P Q R S T U VWX Y Z A B C D E F GI I J K L MN O P Q R S T U VWX Y Z A B C D E F G HJ J K L MN O P Q R S T U VWX Y Z A B C D E F G H IK K L MN O P Q R S T U VWX Y Z A B C D E F G H I JL L MN O P Q R S T U VWX Y Z A B C D E F G H I J kM MN O P Q R S T U VWX Y Z A B C D E F G H I J K LN N O P Q R S T U VWX Y Z A B C D E F G H I J K L MO O P Q R S T U VWX Y Z A B C D E F G H I J K L MNP P Q R S T U VWX Y Z A B C D E F G H I J K L MN OQ Q R S T U VWX Y Z A B C D E F G H I J K L MN O PR R S T U VWX Y Z A B C D E F G H I J K L MN O P QS S T U VWX Y Z A B C D E F G H I J K L MN O P Q RT T U VWX Y Z A B C D E F G H I J K L MN O P Q R SU U VWX Y Z A B C D E F G H I J K L MN O P Q R S TV VWX Y Z A B C D E F G H I J K L MN O P Q R S R UW WX Y Z A B C D E F G H I J K L MN O P Q R S T U VX X Y Z A B C D E F G H I J K L MN O P Q R S T U VWY Y Z A B C D E F G H I J K L MN O P Q R S T U VWXZ Z A B C D E F G H I J K L MN O P Q R S T U VWX Y

Key:SECRET

Plaintext:PARTY TODAY

Ciphertext:HETKC MGHAP

Page 7: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 7 / 14

Enigma machine

Page 8: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 8 / 14

Steganography

Herodotus tattoo and wax tablets

invisible ink

microdots

“The Finger”

Low order bits

Page 9: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 9 / 14

Code book

Codes replace a specific piece of plaintext with a certain codeword. They can also replace several letters at once.

“Yes” if by land, “no” if by sea

Beale code

number stations

electronic code book (ECB) mode

Page 10: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 10 / 14

Kerckhoff’s Principle

A cryptosystem should be secure even if everything aboutit is public knowledge except the secret key.

It contrasts to “security through obscurity.”

KeeLoq R© used by: Chrysler, Daewoo, Fiat, GM, Honda, Jaguar,Toyota, Volvo, WVAttack: Listen in a parking lot for about 60 minutes, then useyour computer for two days. Scary, isn’t is?

Page 11: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 11 / 14

One-time pads

Invented several times before and afterthe II Word War

used even through the cold war

one time means one time

needs separate keys for each message,and for each recipient

keys are as big as messages (disposablekeypads)

Page 12: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 12 / 14

Problems with Classical Crypto

weak: pen-and-paper, and mechanical systems became weakin face of modern computers.

informal: construction was ad hoc. They were not publiclyavailable, there were no “proof” for fit, no definition for whatthey were good.

closed: knowledge and technology available to military andintelligence agencies.

key distribution: the number of keys grow enormously withthe number of participants.

Page 13: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 12 / 14

Problems with Classical Crypto

weak: pen-and-paper, and mechanical systems became weakin face of modern computers.

informal: construction was ad hoc. They were not publiclyavailable, there were no “proof” for fit, no definition for whatthey were good.

closed: knowledge and technology available to military andintelligence agencies.

key distribution: the number of keys grow enormously withthe number of participants.

Page 14: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 12 / 14

Problems with Classical Crypto

weak: pen-and-paper, and mechanical systems became weakin face of modern computers.

informal: construction was ad hoc. They were not publiclyavailable, there were no “proof” for fit, no definition for whatthey were good.

closed: knowledge and technology available to military andintelligence agencies.

key distribution: the number of keys grow enormously withthe number of participants.

Page 15: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 12 / 14

Problems with Classical Crypto

weak: pen-and-paper, and mechanical systems became weakin face of modern computers.

informal: construction was ad hoc. They were not publiclyavailable, there were no “proof” for fit, no definition for whatthey were good.

closed: knowledge and technology available to military andintelligence agencies.

key distribution: the number of keys grow enormously withthe number of participants.

Page 16: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 13 / 14

Modern Cryptographic Era

cryptography now is an everyday commodity

new cryptographic primitives

asymmetric and public cryptosystem

standardization, formalization

internet and wireless communication

liberalization of cryptographic restrictions

Page 17: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 14 / 14

Cryptography Topics

Key Distribution ProblemHow Alice and Bob first agree on a shared key?

MIM attack (man in the middle)

Public Key EncryptionDiffie–Hellman, RSAHow do you get the public keys?Why should you trust the encrypted message?How will you know that is was not modified?

AuthenticationHow do you know that a message was sent by your bank?How do you know that it has not been altered?

Page 18: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 14 / 14

Cryptography Topics

Key Distribution ProblemHow Alice and Bob first agree on a shared key?

MIM attack (man in the middle)

Public Key EncryptionDiffie–Hellman, RSAHow do you get the public keys?Why should you trust the encrypted message?How will you know that is was not modified?

AuthenticationHow do you know that a message was sent by your bank?How do you know that it has not been altered?

Page 19: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 14 / 14

Cryptography Topics

Key Distribution ProblemHow Alice and Bob first agree on a shared key?

MIM attack (man in the middle)

Public Key EncryptionDiffie–Hellman, RSAHow do you get the public keys?Why should you trust the encrypted message?How will you know that is was not modified?

AuthenticationHow do you know that a message was sent by your bank?How do you know that it has not been altered?

Page 20: From Classical to Modern - Central European University · Code book Codes replace a speci c piece of plaintext with a certain code word. They can also replace several letters at once

Cryptography for dummies 14 / 14

Cryptography Topics

Key Distribution ProblemHow Alice and Bob first agree on a shared key?

MIM attack (man in the middle)

Public Key EncryptionDiffie–Hellman, RSAHow do you get the public keys?Why should you trust the encrypted message?How will you know that is was not modified?

AuthenticationHow do you know that a message was sent by your bank?How do you know that it has not been altered?


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