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DTTF/NB479: Jouspevdujpo up Dszquphsbqiz. Nbuu Cpvufmm G-222 y8534 cpvufmm@sptf-ivmnbo.fev. (It should now be obvious whether or not you are in the right classroom…). CSSE/MA479: Introduction to Cryptography. Matt Boutell F-222 x8534 boutell@rose-hulman.edu.

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dttf nb479 jouspevdujpo up dszquphsbqiz

DTTF/NB479: Jouspevdujpo up Dszquphsbqiz

NbuuCpvufmmG-222 y8534 cpvufmm@sptf-ivmnbo.fev

(It should now be obvious whether or not you are in the right classroom…)

csse ma479 introduction to cryptography

CSSE/MA479: Introduction to Cryptography

Matt BoutellF-222 x8534 boutell@rose-hulman.edu

agenda introductions to

And intro to daily quizzes, worth 5% of grade: Q1

Agenda: Introductions to…
  • The players
  • The topic
  • The course structure
  • The course material
introductions
Introductions
  • Roll call:
    • Pronunciations and nicknames
    • Help me learn your names quickly
    • You’ll share with classmates on discussion forum
  • Me:
    • Since 2005 (but in Zambia last year)
    • Taught CSSE120, 120 Robotics, 220, 221, 230, Image Recognition, Android, Cryptography, Fractals, Mechatronics, Robotics senior design
what is cryptography
What is Cryptography?
  • Designing systems to communicate over non-secure channels

Trappe and Washington, p. 3

sherlock holmes the adventure of the dancing men 1898
Sherlock Holmes, The Adventure of the Dancing Men (1898)

In a letter:

2 weeks later:

2 mornings later:

3 days later:

4 days later:

non secure channels

2

Non-secure channels

Encryption

Key (+1)

Decryption

Key (-1)

Alice

plaintext

Encrypt

CIPHERTEXT

Decrypt

Bob

DSZQUPHSBQIZ

cryptography

cryptography

Objectives:

1. Confidentiality

2. Integrity

3. Authentication

4. Non-repudiation

Eve

Trappe and Washington, p. 3

agenda
Agenda
  • The players
  • The topic
  • The course structure
  • The course material
what will we do
What will we do?

Learn theory (lecture, text, written problems)

What would happen if you used composite numbers as factors in RSA?

Make and break codes (programming)

DES Block cipher, classic crypto

Research something new (term project)

Quantum cryptography, TwoFish, PGP

admin

3

Admin
  • Syllabus
    • Text: highly recommended by students
    • Grading, attendance, academic integrity
    • Angel: Please use the merged course:
      • CSSE/MA479 Cryptography (Spring 12-13)
      • The original csse479-01 and ma479-01 are empty
  • Schedule
    • Contains links to homeworks (first due Monday)
    • Easy first week…
    • Bookmark in browser:
      • http://www.rose-hulman.edu/class/csse/csse479/201330/
  • Post to piazza for questions
agenda1
Agenda
  • The players
  • The topic
  • The course structure
  • The course material
shift ciphers
Shift ciphers
  • Attributed to Julius Caesar
  • Letters represented as 0-25.
  • x  x + k (mod 26)
  • Cryptography  ETARVQITCRJA
  • Weak cryptosystem.
    • We learn it to show that “encryption” isn’t useful if it’s not secure.
    • We also use it to study 4 typical attacks to find the decryption key:
      • Ciphertext only (the discussion forums)
      • Known plaintext
      • Chosen plaintext
      • Chosen ciphertext
1 ciphertext only
Consider dszquphsbqiz

dszquphsbqiz

etarvqitcrja

fubswrjudskb

gvctxskvetlc

hwduytlwfumd

ixevzumxgvne

jyfwavnyhwof

kzgxbwozixpg

lahycxpajyqh

mbizdyqbkzri

ncjaezrclasj

odkbfasdmbtk

pelcgbtencul

qfmdhcufodvm

rgneidvgpewn

shofjewhqfxo

tipgkfxirgyp

ujqhlgyjshzq

vkrimhzktiar

wlsjnialujbs

xmtkojbmvkct

ynulpkcnwldu

zovmqldoxmev

apwnrmepynfw

bqxosnfqzogx

cryptography

How did you attack the cipher?

Another trick for long ciphers…

4

1. Ciphertext only
2 known plaintext

5

2. Known plaintext

Say I know sample of plaintext and corresponding ciphertext.

How long does the sample need to be to find the key?

3 chosen plaintext

6-7

3. Chosen plaintext

Say I have access to the encryption machine and can choose a sample of plaintext to encode. How can I deduce the key?

Just encode a. That gives the encryption key

4. Chosen ciphertext

Say I can choose a sample of ciphertext to decode.

Just decode A. Howdoes this give the encryption and decryption keys?

homework due monday
Homework due Monday

See the schedule page

where did you sit today
Where did you sit today?

http://www.phdcomics.com/comics/archive.php?comicid=1017

affine ciphers
Affine ciphers

Somewhat stronger since scale, then shift:

x  ax + b (mod 26)

Say y = 5x + 3; x = ‘hellothere’;

Then y = ‘mxggv…’

(Hint: my table mapping the alphabet to 0-25 is really handy)

affine ciphers x a x b mod 26
Affine ciphers: x  ax + b (mod 26)

Consider the 4 attacks:

  • How many possibilities must we consider in brute force attack?
a c an t be just anything
a can’t be just anything!

Consider y= 2x, y = 4x, or y = 13x

Is mapping unique?

The problem is that gcd(a, 26) != 1.

The function has no inverse.

finding the decryption key
Finding the decryption key
  • What’s the inverse of y = 5x + 3?
    • a = 5 is OK.
  • In Integer (mod 26) World, of course…
affine ciphers x a x b mod 261
Affine ciphers: x  ax + b (mod 26)
  • Consider the 4 attacks:
    • Ciphertext only:
      • How long is brute force?
    • Known plaintext
      • How many characters do we need?
    • Chosen plaintext
      • Wow, this is easy. Which plaintext easiest?
    • Chosen ciphertext
      • Also easy: which ciphertext?