CCT275H5 Lecture Notes - Lecture 10: Ciphertext, Plaintext, Pretty Good Privacy
11 Jan 2017
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Tuesday, November 10, 2015
1
CCT275
Lecture 10
- CRYPTOGRAPHY
• What is it good for?
1. Ensuring Privacy:
- Techniques to encrypt the contents of your message so that only the recipient can
read them
2. Ensure Authentication:
- Techniques to ensure that message shave not been altered en route; signatures
3. Ensure Anonymity/Pseudonymity:
- Techniques that hide or mask the route or origin of a message
>> I need all this information about you so that I can authenticate that
• How it works?
>> Remember that digital data is numeric data; everything is a number in 1’s and 0’s
** Crypto: is using a key to alter a message so that it can only be recovered with
another key
ex.
** choose a really big number as the key
To encrypt, multiply the message (the plaintext) by that really big number.
>> This produced the Cipher-text
To decrypt, divide the cipher text by the really big number.
>> This leaves you with the original Plaintext.
find more resources at oneclass.com
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Tuesday, November 10, 2015
2
• Why is this secure?
>> To break the encryption without knowing the key, you now have to factor the cipher-
text number and decide which factors are the key and the who are the message. This is
relatively difficult. Much more difficult, anyway, than just dividing, when you know the
key.
- Symmetric Key
>> Use the same key encrypts and decrypts message, as in above example
This is how crypto has worked for millennia
But, it requires secure way of sharing keys (ex. Codebooks)
- Asymmetric (Public Key)
The key pair
>> Generate mathematically related key pair such that:
( Anything encrypted with one ca be decrypted only with the other )
ex.
Generate PubK and PrivK s.t. (M^PrivK)^PubK = M
>> this also implies (M^PubK)^PrivK = M
(exponentiation is modulo exponentiation, exponentiation in clock arithmetic)
>> So any message that is encrypted with PubK can be decrypted with PrivK,
** Using Asymmetric Keys (Public Key Cryptography)
Publish PubK, Keep PrivK secret.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Cryptography: what is it good for, ensuring privacy: Techniques to encrypt the contents of your message so that only the recipient can read them: ensure authentication: Techniques to ensure that message shave not been altered en route; (cid:1688)signatures(cid:1689: ensure anonymity/pseudonymity: Techniques that hide or mask the route or origin of a message. >> remember that digital data is numeric data; everything is a number in 1"s and 0"s. ** crypto: is using a (cid:1688)key(cid:1689) to alter a message so that it can only be recovered with another (cid:1688)key(cid:1689) ex. ** choose a really big number as the key. To encrypt, multiply the message (the (cid:1688)plaintext(cid:1689)) by that really big number. To decrypt, divide the cipher text by the really big number. >> this leaves you with the original plaintext. >> to break the encryption without knowing the key, you now have to factor the cipher- text number and decide which factors are the key and the who are the message.
