One-way encryption

Follow-up on distributed processing. Follow-up on encryption.

Client-side versus server sideHomework: postings, talks, comments

Distributed processing

Strategies for doing

• sorting of known set of values (e.g., card deck)

• sorting of unknown set of values

• encrypting message– Is this embarrassingly parallel?

Status

• You can make your weekly posting on encryption.

• More guest lecturers coming.

• Readings: – Check out moodle for chapter 10 on

databases. This will be useful for today!– There will be a paper on AI.

Follow-up from talk

• Summarize?

• Comment?

mod

• JavaScript and Processing each use % for the mod (aka modulo) operator

• You can think of it as remainder• 10 % 5 => 0• 100 % 10 => 0• 101 % 10 => 1• 5 % 10 => 5• ???

Uses of mod

• Slide show

• Wrap around screen

• Computing change

• Checking if one number is divisible by another

• ?

My summary• Pick 2 very big primes (each at least 300

digits) p and q• Compute n = p*q• Compute (n) = (p-1)* (q-1)• Choose e such that gcd(e, (n) ) is 1

(meaning no common divisors)• Compute d such that d*e = 1 mod ((n) )• Public key is (e, n)• Private key is (d,n)

Bits and bytes

• Bit is 0 or 1

• Bit stands for binary digit

• Byte is 8 bits

Is a picture worth a 1000 words?

• Assume straight encoding of picture in which each pixel element is– black or white for how many bits?– one of 256 different colors (i.e., reference to a

pallette) for how many bits?– 0 to 255 levels of Red, 0 to 255 levels of

Green, 0 to 255 levels of Blue for how many bits?

Is a picture…, cont.

• How big is the picture?

• How many pixels wide and how many high?

How much space is taken up by 1000 words?

• Standard ASCII encoding is 8 bits for 1 byte for character.

• What would be reasonable estimate for a word?– word plus space?

Do computations!

Image encodings

• Good topic for presentation

• lossless means that the full original can be restored– gif is lossless

• lossy means that it cannot– jpeg is lossy

Redundancy in messages

• In some sense, the opposite of steganography and cryptography

• Make a message longer so receiver can check if message is correct.

• Send extra information.• Ultimate form: send the whole message

twice.• Receiver can detect a problem and

request new transmission.

Check bit, parity bit

• Using a 7 bit encoding,

• Decide between even or odd parity. Say odd

• Send 8 bits, when the 8th bit makes it be an odd number of bits0000010 would be expanded to 00000100

0101011 would be expanded to 01010111

check digit

• Various methods

• One: compute sum of digits of message and compute modulo 10 and make this be the extra digit sent

• Alternative (that can catch transpositions of numbers) Give weighting to numbers, compute the sum module 10 and make this be extra digit sent

ISBN-10

http://en.wikipedia.org/wiki/Check_digit

• Uses modulo 11. Weights positions 1, 2, ….10. Computes sum modulo 11

• and adds 0, 1, 2, …, X to message.

Today

• one-way encryption meaning no attempt to decode

• Typical use is passwords!

Passwords

• What does using input type="password" do?

My term

• over the shoulder security

• password still sent over the web.

• A secure connection means that it will be encrypted and then decrypted.

One-way encryption

• Typical use: take password and immediately encrypt it using one-way encryption and store the encrypted form.

• Your program makes sure that the plaintext is the only way…

• Protects against inside jobs!

• Other uses???

SHA256 function

• One of several possibilities

• aka 'hash' or 'digest'.

• http://www.webtoolkit.info/javascript-sha256.html

Where to do this?

• One choice is to do this on the client, that is, using JavaScript and send it (along with other information) to the server (the middleware program = the php program)

• Client side (HTML & JavaScript)– or other languages

• Server side (php and also MySql)– OR other languages…

HTML form handling

• onSubmit indicates program done immediately on the client

• action indicates program on the server

• So, this example does work on client and then on server

• Note: in the book chapter, I use a table to format the form.

HTML<form name="f" action="completereg.php"

onSubmit="return encode();" method="post">

User id (email address)

<input type="email" name="un" required />

Password <input type="password" name="pw" required />

Confirm password <input type="password" name="cpw" required/>

<input type="submit" value="Register"/>

</form>

Notice

• use of input type="password"

• Notice typical device of having user/customer/client/…. enter password twice.

• SOME HTML5 implementations will check that any input field that has the required attribute have a value. My code also checks.

The encode function

• checks if user name and password entered

• checks if two passwords match

• if both true, invokes the SHA256 function to produce the digest and returns true

• else returns false, and so action is not taken.

function encode() { var pw1 = document.f.pw.value; if ((document.f.un.value.length<1) ||(pw1.length<1)) { alert("Need to enter User Name and Password.

Please try again."); return false; } else if (pw1 == document.f.cpw.value) {document.f.pw.value = SHA256(pw1);

document.f.cpw.value = document.f.pw.value; return true; }

else { alert("passwords do not match. Please try again.");

return false; } }

Discussion

and possible posting

• How can this be done using cloud computing? That is, cloud computing to all the computation on server computers…– simple answer: start authentication on the

client.– Research security, authentication, passwords,

etc. on cloud computing

Homework

• Review chapter 10

• Read AI article

• Read about visualizations

• Read about duolingo

• Be prepared for next guest speakers