Secret Key Cryptography

Secret Key Cryptography

1. Single key is used by sender and receiver. This is then used for Encrypting and decrypting the message. The biggest problem is to distribute SECRET key

2. Operation

   • Stream Cipher
     • Encrypts SINGLE bit/byte of data at a time
   • Block Cipher 64bits
     • A block of fixed number of bits is processed at a time
       • When no. of bits in plaintext are not in multiple of block size, then different modes of operations needs to be used
     • Electronic code book mode -- ECB
       • every plaintext block is processed independently with same key.
     • Cipher Block Chaining mode -- CBC
       • XOR between 1st plaintext block and initial vector, further encrypted with key to produce 1st ciphertext block
         • For next, 1st ciphertext XOR with 2nd plaintext as so on ...
     • Feedback mode
       • Cipher feedback mode and Output feedback mode
     • Counter mode
       • Most imp, works as Stream Cipher.
         • Counter value is incremented for each round. Counter is encrypted using Key. Then is passes to XOR with plaintext and ciphertext

3. DES

   • Data Encryption Standard
   • Symmetric Block Cipher, Feistel Cipher. Plaintext - 64bits, Key - 64bits(56bits - actual key and 8bits - parity)
   • WORKING -- Its divided into 4 parts
     • 1. Subkey generation
       • 2. Initial Permutation
         • 3. Rounds -- 16
           • 4.Final Permutation
           • 64bit plaintext is divided into 2 halves -- left & right. Certain operations performed on it are 1.Exansion Permutation 2.XOR function (Right expanded & Subkey) 3.S-box Substitution 4. P-box Substitution 5.XOR operation (left & right halves)

4. AES

   • Advance Encryption Standard
     • Symmetric Key Algorithm having block cipher
       • Block size - 128 bits Number of rounds depends on the key size. 128 bits key --- 10 192 bits key --- 12 256 bits key --- 14
     • WORKING -- 4 sub processes
       • Substitution Bytes
         • Shift Rows
           • Mixed Columns
             • AddRoundKey