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7/28/2019 symentric & asymentric

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Subject:-303.Web Design

And E-commerce

Guided by :- Devji Chhanga


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What is an Encryption?

Encryption is for ensuring data

confidentiality.

For ex. One person wants to send his

message to other, encryption schema isused to prevent other people to read it.

In this schema the original message is

called plaintext message and the outputmessage is called Cipher text message,

which is scrambled message.


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Here, the original message is converted

in output message using an encryptionprocess.

The encryption process is controlled by

an encryption key.

The encryption key governs how the

plaintext is converted or transformed

into cipher text.

Cricket EtkemgvEncryption

Encryption key


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There are two types of encryption methods:

1) Symmetric key encryption.

2) Asymmetric key encryption.

They are also called private key encryption andpublic key encryption, respectively.

In symmetric (private) key encryption, the

encryption key and the decryption key are thesame.

In asymmetric (public) key encryption, they are

different.


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Symmetric key encryption.

Symmetric key encryption follows two basicprinciples, namely substitution andtransposition.

1).Substitution Caesar cipher is a good example of the

substitution technique.

In this encryption method, each letter of thealphabet is substituted with another letter nthplace(s) further down the alphabet.

The number n can be regarded as the

encryption key.


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For example, if we encrypt the plaintext HelloWorld with an encryption key n=3, the cipher

text becomes Khoor Zruog.

However, substitution is not a secure techniquebecause the same input will always give the

same output.

Hence, by counting how frequently a letterappears, one can guess what the letter is likely

to be, especially if the message is long.


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A better technique is transposition.

It rearranges the positions of

the letters as controlled by a key. Columnar Transposition:

1) Arrange text horizontally.

2) Use a key to generatea permutation of the

columns

3) Read vertically.


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Public Key Encryption

Diffie and Hellman proposed an innovativeprotocol to establish a secret key between

two parties even if they have never met

before.

This is the beginning of a new type of

encryption method called public key

encryption

In public key encryption, everyone has a pairof keys called the public key and the private

key.


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If a public key encryption algorithm iseffective, it is almost impossible to deduce

the corresponding private key from the

public key. If someone wants to encrypt a message

for you, he will use your public key to

encrypt the message. To decrypt thesecret message, your private key must be

used.


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Hello World! Jgnnq yqtnfEncryption

Public key

Jgnnq yqtnf Decryption Hello World!

Private key

Plain text

Plain textCipher text

Cipher text

RSA Encryption Algorithm


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Step 1: Pick two large prime numbers p and q

Step 2: Multiply p and q together to get n (i.e.n = pq )

Step 3: Choose d, such that d and w =

( p - l)(q - 1) are relatively prime (i.e. they

have no common factor other than one).

Note also that d must be smaller than w.

Step 4: Compute e such that 1 = (d x e) mod

w (i.e., when (d x e)/w, the remainder is 1.

Step 5: The public key is

Step 6: The private key is


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Suppose that there is a message with a

message code m, then the secret code s isworked out by the following formula:

s = m( pow e) mod n (i.e., the remainder

when m ( pow e) is divided by n) Decryption is the reverse process. If the

recipient receives the secret code s, the

original message code m can be worked outby the following formula:

m = s ( pow d) mod n (i.e., the remainder

when s ( pow d) is divided by n).


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Step 1: Pick two large prime numbers: p = 19

,q = 7 (here we simply use two small prime

numbers to illustrate the basic operation). Step 2: Multiply p by q to get n, so n = pq =

133.

Step 3: w =( p - l)(q - 1) = 108, we choosed = 31 because 31 and 108 have no common

factor.

Step 4: 1 = 31e mod 108, so e = 7.

Therefore, the keys are

(public key) and (private

key). Next we perform encryption for a

message code m = 29.


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Step 5: Since s = mod n, if m = 29, s = 15.

That means, the secret code is s = 15.

Finally, let us check that the original messagecode can be recovered by means of

decryption.

Step 6: m = s ( pow d) mod n (Note that d =31 and n = 133).

Step 7: m = 15 ( pow 31) mod 133 = 29.


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