CS 105 – Introduction to the World Wide Web  HTTP Request*  Domain Name Translation  Routing  HTTP Response*  Privacy and Cryptography  Adapted from Kurose & Ross, Computer Networking, Addison-Wesley, 2001.

An HTTP Request GET index.html HTTP/1.1 Host: Connection: close User-agent: Mozilla/4.0 Accept-language:en

Domain Name Translation  What is address???  Look it up! –Use a Domain Name Server –Translates a host name into an IP address

Routing  Routers –Internet ‘post offices’  Look at address, and send message ‘in the right direction’  Messages often take several ‘hops’ to get to their destination

HTTP Response HTTP/ OK Connection: close Date: Mon, 30 Sep 2002 Last-Modified: Mon, 23 Sep 2002 Content-Length: 6821 Content-Type: text/html …data data data …

Kinds of Messages  HTTP requests / responses   Instant Messages  Telnet sessions  Transactions (shopping, etc.)

Privacy  Any machine (router) along the path of the message can read it. –Message more like a postcard than a letter  Is this good??? –Of course not!  What can we do??? –Encrypt the messages

Cryptography  Need to figure out a way so the recipient can read the message, but nobody else.  Most common technique today: public key cryptography

Public Key Cryptography  Each person has a public key and a private key  The two keys ‘un-do’ each other –More on this in a minute  Public keys are publicly available on Key Servers (anyone can see / get them)

Sending a Message  Alice wants to send Bob a message  Alice gets Bob’s public key  Alice uses the key to encrypt her message  Bob gets the message, and uses his private key to decrypt it

Basic Idea  For a message encrypted using a given public key, the ONLY way to decrypt it is to use the corresponding private key  So, as long as Bob is the only one with his private key, he’s the only one that can read the message

Implementation  We want:  Efficient: –Encrypting with public key –Decrypting with private key  Intractable: –Decrypting without private key

RSA Algorithm  One way to do this:  Modular Exponentiation –Modular arithmetic – remainder stuff: 13 = 3 (mod 10) –Modular exponentiation: 4 3 = 64 = 4 (mod 10) This can be done efficiently

RSA Algorithm  Now, we need to pick numbers that ‘un-do’ each other’s exponentiation  For a mod of 10, 2 undoes itself: 0 3 = 0 (mod 10)5 3 = 125 = 5 (mod 10) 1 3 = 1 (mod 10)6 3 = 216 = 6 (mod 10) 2 3 = 8 (mod 10)7 3 = 343 = 3 (mod 10) 3 3 = 27 = 7 (mod 10)8 3 = 512 = 2 (mod 10) 4 3 = 64 = 4 (mod 10)9 3 = 729 = 9 (mod 10)

RSA Algorithm  So, our public key is {10, 3}  Private key is {2}  Encrypting a message: Let’s send 472 –Exponentiate each digit: 438  Decrypting a message: –Exponentiate again: 472

RSA Algorithm  Breaking RSA – know exponent, know modulus – just take the root –E.g. find cube root of 4 (mod 10)  Most public keys have very large numbers – ~150 digits.  Finding these roots is an intractable problem – bigger keys, harder problem!

Security and  Many products with built-in cryptographic protection  Outlook Express – S/MIME (Secure/Multipurpose Internet Mail Extension) Support  Other add-ons (choices for secret key methods, size of key, etc)  Pretty Good Privacy (PGP)

Encryption and Signatures  Encryption: encodes the message so that other users cannot read it  Signatures: let you emulate written signatures

To see more about getting a certificate:

Encrypting vs. Signing  Encrypt first, sign second: –Verify signatures before decrypting –Useful in automated verification systems  Sign first, encrypt second: –Decrypt before verifying signatures –Useful when plaintext is essence of the message.