# Decryption by applying same transformations in

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•Decryption by applying same transformations in reverse on cipher text. •This method called symmetric key encryption as encryption and decryption performed using same key. •Normally the encryption/decryption algorithm is publicised. Only key is secret. Systems Analysis And Design Systems Analysis And Design © © V. Rajaraman 30 of 92 13.3.9
Symmetric Encryption • Problem is to ensure secrecy of key when it is sent to partner. • If the key is to be sent to many partners need for separate key for each partner.Directory of who was sent which key is to be kept and used for each transaction.Directory should be secure. • If large number of partners are there key distribution very difficult. • Advantage of symmetric key is easy and fast to transform plain text to cipher text. Systems Analysis And Design Systems Analysis And Design © © V. Rajaraman 31 of 92 13.3.10
Digital Encryption Standard DES - Proposed by IBM in 1975 Standardised by US Govt in 1977 Reasonably secure It is a combination of permutation and substitution on blocks of 64 bits. A message is broken up into 64 bit blocks and each block is separately encrypted. Systems Analysis And Design Systems Analysis And Design © © V. Rajaraman 32 of 92 13.3.11
Digital Encryption Standard #General idea used in DES M = PLAINTEXT 01101100 11011000 11011010 K = KEY 10101111 00101100 01011011 E = M + K 11000011 11110100 10000001 encryption M= E + K 01101100 11011000 11011010 decryption See simplicity of Transformation using Exclusive OR Systems Analysis And Design Systems Analysis And Design © © V. Rajaraman 33 of 92 13.3.12
Digital Encryption Standard Algorithm Before applying DES the text is split up into the 64 bit blocks. DES applied on each 64 bit block. Encryption method Step 1: Apply an initial permutation on a block.Result is B=IP(P) where P is the 64 bit block IP Initial Permutation function and B the result. Step 2: Split B into 32 bit blocks Li = leftmost 32 bits Ri = rightmost 32 bits. Step 3: Pick a 56 bit key. Permute it Step 4: Left circular shift it by 1 bit giving K 1 . Systems Analysis And Design Systems Analysis And Design © © V. Rajaraman 34 of 92 13.3.13
Digital Encryption Standard Algorithm Step 5: Perform a complex sequence of operations and obtain X 1 = F(R 1 ,K 1 ) (The complex set of operations include table look up and dropping bits). Step 6: Find R 2 = L 1 + X 1 Step 7: Set L 2 = R 1 Repeat steps 2 to 7 16 times to get B 16 = L 16 ,R 16 Step 8: Apply inverse of initial permutation on B 16 The result is the encrypted block Systems Analysis And Design Systems Analysis And Design © © V. Rajaraman 35 of 92 13.3.14
Digital Encryption Standard Algorithm • In summary the DES encryption applies the following transformation 16 times.The i th round transformation are L i+1 = R i R i+1 = L i + F(R i ,K i ) • Each round has a different key K i • For Decryption the process of encryption is reversed.The encrypted block is permuted using IP -1. On this transformations are applied starting with K 16 and going to K 1 last.The keys and F are same as those used in encryption process. Systems Analysis And Design Systems Analysis And Design © © V. Rajaraman 36 of 92 13.3.15
Digital Encryption Standard Algorithm • The encryption process uses simple binary operations. They can thus be realised in hardware as an integrated circuit chip.