Secure flash algorithm (SHA)

• Most widely used flash function

• Produces 160 bit hash values

• It it based on MD5

• The input is a message with length equal to $\lt 2^{64} bits$

• The output is a message digest of length 160 bit

• The processing is done of 572 bit blocks

Steps

• Step1: append padding bits Message is padded with a 1 and as many 0’s as necessary to bring the message length to 64 bits fewer/ less than an even multiple of 512.

• Step2: Append the length 54 bits are appended to the end of the padded message. These bits hold the binary format of 64 bits indicating the length of the original message.

• Step3: Initialize MD buffer

  • 160 but buffer is used to hard the intermediate and final results.

  • The buffer is represented as 5-32 bit registers A,B,C,D,E

  • They are initialized as :

• Step 4: Process the message in 572 bit word blocks

  • The number of rounds = 4

  • Each round has 20 steps

  • Four different logical functions $f_1, f_2, f_3, f_4$

  • Each round make use of additive constant $K_t$ where $0 \ltt\lt79$

  • Four different are used

  • The output of the 80^th round is added to the input to the 1^st round to produce the output

• Step 5: Output

  • After all 512 bit block is processed, the output from the I^th stage is 160 bit message digest