SUNY Geneseo Department of Computer Science
Substitution Ciphers
Thursday, January 23
Intd 105 13, Spring 2014
Prof. Doug Baldwin
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Misc
Syllabus: dates give time intervals over which we’ll be focusing on certain things, not definite due dates.
You can read and edit these notes if you want
Questions?
“The Adventure of the Dancing Men”
Comments (interesting points, plot summary, etc.)?
- Steganography = hiding existence of message
- Cryptography/cryptanalysis = writing and breaking secret codes
The cipher
- (Terminology: cipher = secret code that works by encrypting letters; plaintext = unencrypted message, ciphertext = encrypted one)
- Each dancing figure stands for a letter
- Substitution cipher (substitute each letter with some other symbol)
How Holmes broke it
- (More terminology: cryptanalysis = breaking ciphers, cryptography = inventing and using them)
- (Letter) frequency analysis
- First described by al-Kindi, 9th century AD
Try it
- Black Chamber web site provides tools for breaking substitution ciphers and puzzles to try them on
Essay 1
Describe your thinking as you break another substitution cipher
Handout
Ethics and Cryptography
When is it OK to keep a secret, to try to discover someone else’s secrets, etc.
For example
- Is it OK for Elsie to keep her past secret from Hilton?
- Isn’t Hilton ignoring the intent of his promise to respect Elsie’s desire to keep her past secret if he refuses to ask her directly about it but hires Holmes to decipher her messages?
- Is Holmes “lying” if he sends Slaney a message written in the secret code, knowing that Slaney will believe, since the message is in code, that it comes from Elsie?
We’ll look more deeply at such questions after we critique the code-breaking essays. But first…
Next
Peer critiques of draft cipher-breaking essays
Bring 2 printed copies to class (not necessarily final or polished)
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