Data Encryption Standard Mirza Abu Daud Baig 315139 Comp. Sc. II

Introduction What is DES ? It is an encryption algorithm. An encryption scheme has five ingredients: Plaintext original data in unencrypted format Encryption Algorithm applied to plaintext to encrypt Secret Key input to the encryption algorithm Cipher text result of plaintext and secret key Decryption Algorithm encryption algorithm run in reverse to produce plaintext

Introduction What is DES ? It is a Symmetric system. Symmetric, Single key or private key encryption. These algorithms require that the sender and the receiver agree on a key before they can communicate securely. Asymmetric, two key or public key encryption. In these Algorithms there exists 2 keys viz. public and private keys, thereby eliminating the options of sharing any secret keys.

Encryption

Encryption Algorithm Symmetric DES AES 3DES Asymmetric RSA

FEATURES DES is a cryptographic algorithm (cipher). It is a block cipher. It encrypts data in blocks in size of 64 bits each i.e. 64 bit of plain text goes as the input to DES which produce 64 bits of cipher text. The same algorithm and key are used for encryption and decryption, with minor differences. The key length is 56 bits. Actually, the initial key consists of 64 bits before the DES process even starts, every eight bit of the key is discarded to produce a 56-bit key. DES consists of 16 rounds.

Data Encryption Standard DES has 16 rounds, meaning the main algorithm is repeated 16 times to produce the ciphertext.As the number of rounds increases, the security of the algorithm increases exponentially. To do the encryption, DES uses "keys" where are also apparently 16 hexadecimal numbers long, or apparently 64 bits long. However, every 8th key bit is ignored in the DES algorithm, so that the effective key size is 56 bits. But, in any case, 64 bits (16 hexadecimal digits) is the round number upon which DES is organized.

THE PHASES DES Algorithm consists of 2 stages. Key Scheduling Text Scheduling

Key Scheduling STEP - 1 The first step is to pass the 64-bit key through a permutation called Permuted Choice 1

Key Scheduling Step-2 The next step is to use this 56 bit key to generate 16, 48 bit subkeys, called K[1]-K[16], which are used in the 16 rounds of DES for encryption and decryption.

Key Scheduling STEP - 3 The output after rotation is submitted to PC-2.

Text Scheduling step-1 The first step is to pass the plaintext through a permutation called the Initial Permutation, or IP.

Text Scheduling step-2 These 32 bits are passed to the E- table to get 48 bits as output.

Text Scheduling step-3 These 48 bits from E- table is XORed with the key K[i] to get 48 bit output. The 48 bit is passed to the Substitution table called S-box.

Text Scheduling step-4 Each table returns 4 bits i.e.32 bits are submitted to Permutation table which gives 32 bits output.

Text Scheduling step-5 32 bits output from Permutation table is XORed with left half of Original Text. The 32 bits output from XORed operation is now the right half of entire 64 bits output. This entire cycle is repeated 16 times for the 16 phases of DES to get the 64 bits of output. At the end of the sixteenth round we left shift the 64 bits by 32 bits.

Text Scheduling step-6 A final permutation IP-1 is applied as defined by the following table giving the complete encrypted cipher text.

DECRYPTION The same algorithm can be used for encryption as well as decryption. The method described above encrypts a block of plaintext and returns a block of cipher text. In order to decrypt the cipher text and get the original plaintext again, the procedure is simply repeated but the subkeys are applied in reverse order, from K[16]-K[1].

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