Encryption

Security Protocols

• A security protocol can be defined as a security

procedure for regulating data transmission

between computers. • If sensitive data is transmitted over the network in

clear text, then anyone can read the data if they

intercept it intentionally or accidentally. • To preserve the data confidentiality, we use

different encryption techniques.

Methods of Encryption At a broad level, regardless of the technique used for

encryption, the generation of ciphers from the plaintext itself can be done in two ways: Stream Ciphers Block Ciphers

Stream Cipher It involves the encryption of one plaintext bit at a time. The

decryption also happens one bit at a time Block Cipher

It involves the encryption of one block of text at a time. Decryption also takes one block of encrypted text at a time.

Stream Cipher Is one of the simplest way to encrypt data When it is employed each bit of the data is encrypted

using one bit of the key Faster than block cipher. Can encrypt the same message twice and the cipher

text will be different To make a stream cipher more difficult to crack, you

can use a crypto key that varies in length The process of continually varying the crypto key is

known as One-time pad

Block Cipher Unlike Stream Cipher which encrypts every single bit, Block

Cipher encrypts data in chunks of specific size It specifies how much data should be encrypted on each pass

and what size key should be applied to each block E.g DES specifies that DES encrypted data should be processed in

64-bit blocks using a 56-bit key You can use a number of algorithms when processing block

cipher The most basic is to simply take the data and break it into

blocks while applying the key to each A better solution is to take the earlier resultants from the

algorithm and combine them with later keys

XOR Operation An interesting property of XOR is that when used twice, it

produces the original data Example:

Two binary numbers A=101, B=110 C = A XORB C = 101 XOR110 = 011 Now, if we perform C XOR A, we will get B

011 XOR101 = 110 = B Now, if we perform C XOR B, we will get A

011 XOR110 = 101 = A

This reversibility of XOR operations has many implications in cryptographic algorithms

Data Encryption Standard (DES)

Data Encryption Standard DES is a symmetric key encryption standard Published in 1977 by the U.S. National Institute of

Standards and Technology (NIST) It is developed by IBM NIST states the goal of DES as

The goal is to completely scramble the data and key so that every bit of the ciphertext depends on every bit of data and every bit of key.

With a good algorithm, there should be no correlation between the ciphertext and either the original data or key

Data Encryption Standard It was widely adopted by the industry for use in

security products a de facto standard

It takes as an input data blocks of 64-bits length and generates 64-bit cipher blocks

56-bit key size Today, DES is not considered secure in its original

form, but in its modified form (3DES) it is still considered secure

DES Basic Operation The algorithm, which is parameterized by a 56-bit key, has

16 distinct stages It involves 16 rounds of plaintext transformations, including breaking

the plain text into two 32 bit chunks that are swapped repeatedly during rounds

Each round expands 32bit block to 48bits,which are XORd with 48bit sub-key

The sub-key has been generated by a “key schedule” An algorithm that creates the 48-bit sub-key based on the original

56bit key After XORing with sub-key, 48-bit text is divided into 6-bit chunks

(S-boxes), which then output 4-bit blocks Reducing the overall plaintext block back to its original 32 bits

Data Encryption Standard most widely used block cipher in world adopted in 1977 by NBS (now NIST)

as FIPS PUB 46 encrypts 64-bit data using 56-bit key has widespread use has seen considerable controversy over

its security

DES History IBM developed Lucifer cipher

by team led by Feistel used 64-bit data blocks with 128-bit key

then redeveloped as a commercial cipher with input from NSA and others

in 1973 NBS issued request for proposals for a national cipher standard

IBM submitted their revised Lucifer which was eventually accepted as the DES

DES Design Controversy although DES standard is public had considerable controversy over design

in choice of 56-bit key and because design criteria were classified

subsequent events and public analysis show in fact design was appropriate

DES has become widely used, especially in financial applications

Data Encryption Standard

Strength of DES –Key Size 56-bit keys have 256= 7.2 x 1016 values brute force search looks hard recent advances have shown is possible

in 1997 on Internet in a few months in 1998 on dedicated h/w (EFF) in a few days in 1999 above combined in 22hrs!

still must be able to recognize plaintext

DES Encryption Strength The strength of any encryption algorithm lies in the

fact that it would take a long time to guess the used key DES is 56 bits, that is the key to decrypt DES will be 56

bits in length 56 bit is made up of 8 bytes with 7 data bits (8*7=56) Thus we can have 128 (27) values to choose from each

character Possible number of combinations through some simple

math: (128)8 = 72 thousand plus billion combinations

DES Weaknesses DES is considered non secure for very

sensitive encryption. It is crack able in a short period of time. The short key length makes it easy to break To overcome the weaknesses of DES, Triple

DES is developed from original DES.

Triple DES

Triple-DES In Triple-DES, the DES algorithm is applied three

times using two or three different 56-bit keys This approach produces Ciphertext that is scrambled

to the equivalent of a 112-bit or 168-bit key (2)168= 3.7 * (10)50

370 trillion, trillion, trillion, trillion combinations Looking at the name, it may seem that 3DES makes

your encryption three times more difficult to break 3DES actually makes your encryption five billion,

trillion, trillion times harder to break that is 5 *(10)33

Advanced Encryption Standard (AES) In 1990s, the U.S. Government wanted to standardize a

cryptographic algorithm, which was to be used universally. Many proposals were submitted, and after a lot of debate, an

algorithm called Rijndaelwas accepted The need for coming up with a new algorithm is actually

because of the perceived weakness in DES. The 56-bit keys of DES were no longer considered safe

against attacks based on exhaustive key searches, and the 64 bit blocks were also considered as weak

AES was based on 128 bits blocks, with 128-bit key

IDEA International Data Encryption

Standard Takes input of 64-bits plain text blocks Produces output of 64-bits cipher text Key Length: 128 bits

AES Worldwide a new cryptographic protocol standard

was needed because: Key used in DES was too small (56-bit) Triple DES (3-DES) was too slow IDEA was patent protected and slow

Advanced Encryption Standard NIST chose an algorithm that supports a variety of Data

Blockand Key Sizes Two parameters (block size and key) can be chosen

independently from 128, 160, 192, 224 and 256 bit sizes

AES According to AES Designers, the main

features of AES are Symmetric and parallel structure Adapted to modern processors Suited to smart cards After seeing the review of IETF protocols( SSL,

S/MIME, SSH) will have now modifications to accommodate these algorithms

AES –The Advanced Encryption Standard Rules for AES proposals

1. The algorithm must be a symmetric block cipher.

2. The full design must be public.

3. Key lengths of 128, 192, and 256 bits supported.

4. Both software and hardware implementations required

5. The algorithm must be public or licensed on nondiscriminatory terms.