Week 2 Lecture Notes

Week 2 Lecture Notes - January 14: Lecture 4 Announcements...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
January 14: Lecture 4 Announcements You should be done reading Chapter 1 of the Code Book by this lecture Start reading Chapter 2 and Chapter 3 (pp. 115-124 only) WebCT Assignment should be up by noon and due Wednesday January To decipher a code, one uses frequency analysis Most common English letter: E Things are NOT random in languages; there are tendencies and commonalities Frequency Analysis 1) Letter Frequencies (The shorter the text, the less likely the letter percentages match) a) Positional information b) “bigram” letter frequencies (“th,” “wh,” “he”) 2) Word Frequencies 3) Grammar, collocations (collocations – “end to end”; fixed phrase) Variants of Monoalphabetic Substitution Cipher 1) Nulls : ciphertext symbols that do not map to anything (Ex: All 2 digit numbers but not using all the numbers therefore leaving some 2 digit numbers as nothing) 2) Dowbleths : double the last letter (Ex: put numbers, then 99 to double previous letter) 3) Non-standard spelling : when original message is poor grammar to throw off decoder
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 3

Week 2 Lecture Notes - January 14: Lecture 4 Announcements...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online