Cryptography, Essay - Computer Science, Essays (high school) of Software Engineering

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Cryptography

Email

 Suppose I send an email from [email protected] to [email protected]  Who has access to that email?  What if I want the message to be private?

Privacy

"The question is whether people should be able to conduct private conversations, immune from government surveillance, even when that surveillance is fully authorized by a Court order." --Ron Rivest

sketchystore.com

Suppose I'm making a purchase. I click on a link, and it takes me to http://www.sketchystore.com/checkout.jsp

What comes back

Enter your credit card number:
Enter your expiration date:

How it looks in my browser

Privacy

 If this information is sent unencrypted, who has access to my credit card number?  Other people who can connect to my wireless ethernet?  Other people physically connected to my wired ethernet?

Privacy

 When I send a letter through the mail, it passes through the hands of many mail carriers. What keeps them from reading my mail?  What if I send a postcard?

Encryption Scheme

 Shift every letter forward by 1  A → B, B → C, ..., Z → A  MESSAGE → NFTTBHF  Can you decrypt TFDSFU?

Encryption Scheme

 Caesar Cipher. Shift forward n letters.  For example, shift forward 3 letters: A → D, B → E, ..., Z → C  MESSAGE → PHVVDJH  Decode this (Hint: n ≠ 3): DEEDUSEKBTFEIIYRBOTUSETUJXYI

Encryption Scheme

 Shift different amount for each letter.  Vigenère cipher  Pick a secret key: TEACH  Message: ATTACKATDAWN  Add Key: TEACHTEACHTE  Encrypted: TXTCJDETFHPR

Vigenère Cipher

 If you don't know the key, how could you decrypt the message?  What makes a good key?

Exchanging the Key

IERUH KDJKE HCIUE QZXJW QZXJW

Exchanging the Key

IERUH KDJKE HCIUE QZXJW QZXJW Why is this a problem?