Elementary Cryptography Steven Bogaerts

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# Elementary Cryptography Steven Bogaerts - PowerPoint PPT Presentation

Elementary Cryptography Steven Bogaerts. Goal of Cryptography. Ensure security of communication over insecure medium Privacy (secrecy, confidentiality) Integrity Communicate even with possibility of adversaries. Approaches to Secure Communication. Steganography Hide message existence.

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## Elementary Cryptography Steven Bogaerts

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Presentation Transcript
Goal of Cryptography
• Ensure security of communication over insecure medium
• Privacy (secrecy, confidentiality)
• Integrity
• Communicate even with possibility of adversaries
Approaches to Secure Communication
• Steganography
• Hide message existence
• Cryptography
• Hide message meaning
Cryptosystem
• How does this help us accomplish our goals?
• Privacy
• Integrity
Security Failures
• Privacy failure
• Intercept
• Integrity failure
• Modify
• Fabricate
• Block
Encryption Algorithms
• Two categories:
• Keyless
• Keyed
• Symmetric
• Asymmetric
• Character representation
• A=0, B=1, ..., Z=25
• So A + 3 = D, etc.
Substitution Ciphers
• Replace each letter with another
• Caesar cipher
• ci = E(pi) = pi + 3
• Ex: THE EAGLE HAS LANDEDwkh hdjoh kdv odqghg
• Easy to memorize
• In general: ci = E(pi) = pi + n
Other Substitution Ciphers
• Key: "professional"ABCDEFGHIJKLMNOPQRSTUVWXYZprofesinalbcdghjkmqtuvwxyz
Cryptanalysis
• Brute force: 26! possibilities
• Hints:
• Frequencies:
• Letter (general)E, T, A, O, N, I, R, ...
• First letter
• Last letter
• Guesses at message content
• Double letter
• Digraphs, trigraphs
• Small words
Cryptanalysis
• Example:

wklv phvvdjh lv qrw wrr kdug wr euhdn

Cryptanalysis
• Example:

wklv phvvdjh lv qrw wrr kdug wr euhdn

• wrr --> see, too, add, odd, off...wr --> to, ofBest guess: w = T, r = O
Cryptanalysis
• Example:

wklv phvvdjh lv qrw wrr kdug wr euhdn

• wrr --> see, too, add, odd, off...wr --> to, ofBest guess: w = T, r = O
• lv --> so, is, in, ...T_SO very unlikely...T_IS likelyBest guess: l = I, v = S
Cryptanalysis
• Example:

wklv phvvdjh lv qrw wrr kdug wr euhdn

• wrr --> see, too, add, odd, off...wr --> to, ofBest guess: w = T, r = O
• lv --> so, is, in, ...T_SO very unlikely...T_IS likelyBest guess: l = I, v = S
• wklv phvvdjh lv qrw wrr kdug wr euhdnT-IS --SS--- IS -OT TOO ---- -- -----
Cryptanalysis

wklv phvvdjh lv qrw wrr kdug wr euhdn

THIS MESSAGE IS NOT TOO HARD TO BREAK

• Regularity
• Outside information
Cryptanalysis

wklv phvvdjh lv qrw wrr kdug wr euhdn

THIS MESSAGE IS NOT TOO HARD TO BREAK

• Regularity
• Outside information
• WWII: af to be target of a major assault
• "Damaged" water distillery
• Avoid regularity
• Vernam Cipher exampleKey: CIHJTFDJZMEPQLFHXYRLS...Plaintext: HELLOCIHJT 2 8 7 9 19HELLO + 7 4 11 11 14 9 12 18 20 7 7 = 33 mod 26Ciphertext: J M S U HRemaining key: FDJZMEPQLFHXYRLS...
• Hard to maintain keys
• Facilitate key distribution
• Novel
• Phone book
Transposition
• (Substitution - replace one letter with another)
• Transposition - mix up plaintext message
• Columnar
Transposition
• (Substitution - replace one letter with another)
• Transposition - mix up plaintext message
• Columnar: T A L L C OM P U T E RS C I E N TI S T S R OC K x k g j-->tmsicapcskluitxlteskcenrgortoj
Much More To Discuss...
• What makes a "good" encryption algorithm
• Data Encryption Standard (DES)
• Double and Triple DES
• Public Key Encryption
Summary So Far
• Maintain privacy, integrity despite adversaries
• Cryptosystems
• Substitution
• Transposition
• Cryptanalysis
Bible Codes?
• Equidistant Letter Sequences (ELS)
• Rips, Witztum in Statistical Science
• Reviewers not convinced, but nothing formally wrong
Bible Codes?
• Refutations
• McKay, Bar-Natan, Kalai, Bar-Hillel in Statistical Science
• Moby Dick "predicts" assassinations
Scripture
• "... Ask and it will be given to you; seek and you will find; knock and the door will be opened to you." (Luke 11:9)
• "...the counselor, the Holy Spirit, whom the Father will send in my name, will teach you all things and will remind you of everything I have said to you." (John 14:26)
• "The secret things belong to the LORD our God, but the things revealed belong to us and to our children forever, that we may follow all the words of this law." (Deuteronomy 29:29)