Message Authentication and Hash function 2 · SHA-512 | Preprocessing 2. Parsing the Padded Message...

Message Authentication and Hash function 2

Concept and Example

1 IT352 | Network Security

|Najwa AlGhamdi

SHA : Secure Hash Algorithm

• Four secure hash algorithms, SHA-11, SHA-256, SHA-384, and SHA-512.

• All four of the algorithms are iterative, one-way hash functions that can process a message to produce a message digest.

• This property is useful in the generation and verification of 1. Digital signatures

2. Message authentication codes

3. random numbers (bits).

IT352 | Network Security |Najwa AlGhamdi

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SHA 512

• The most widely used hash function • Input max size is 2128 • Output size is 512 bit digest. • The input is processed in1024 bit blocks. • SHA algorithm described into two steps 1. Preprocessing

– Padding the message, M. – Parsing the padded message into message blocks – Setting the initial hash value, H(0)

2. Hash computation – It generates a message schedule from the

padded message. – uses that schedule, along with functions,

constants, and word operations to iteratively generate a series of hash values

• The final hash value generated by the hash computation is used to determine the message digest.

IT352 | Network Security

|Najwa AlGhamdi 3

SHA-512 | Preprocessing

1. Padding the Message • The message, M, shall be padded

before hash computation begins. • The purpose of this padding is to

ensure that the padded message 1024 bits.

• If the length of the message M, in bits, is l bits the – Append the bit “1” to the end of the

message – followed by k zero bits, where k is the

smallest non-negative solution to the equation

• l +1+ k = 896(mod1024)


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1. Padding the Message

Example:

• For example, the (8-bit ASCII) message “abc” has length

• l=8×3 = 24, so the message is padded with a one bit, then 896 - (24 + 1) = 871 zero bits

• and then the message length to become the 1024-bit padded message.

• The length of the padded message should now be a multiple of 1024 bits.


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2. Parsing the Padded Message

• After a message has been padded, it must be parsed into N m-bit blocks before the hash computation can begin.

• the padded message is parsed into N 1024-bit blocks, M(1), M(2),…,M(N).

• 1024 bits of the input block may be expressed as sixteen 16 words of size 64 bits (W0—W15).

3. Setting the Initial Hash Value (H(0))

• Before hash computation begins for each of the secure hash algorithms, the initial hash value, H(0), must be set.


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3. Setting the Initial Hash Value (H(0))

• the initial hash value, H(0), shall consist of the following eight 64-bit words, in hex

• These words were obtained by taking the first sixty-four bits of the fractional parts of the square roots of the first eight prime numbers.


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SHA-512 | Hash Computing

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2. Initialize these values

1. Prepare the message

schedule, {Wt}

3. Compute intermediate values of a,b,c,d,e,f,g,h using constant K



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4. Compute the ith intermediate hash value H(i):


• After all N 1024 bit block have been processed , the output from the Nth stage is the 512 bit message digest.


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HMAC

• There has been increased interest in developing MAC derived from hash code such as SHA.

– Hash function execution are faster than encryption function.

– Library code for hash function is wieldy used.

• However , Hash function like SHA cannot be used as MAC . Why?


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HMAC

• HMAC : Incorporating a secrete key in a hash function.

• HMAC has advantages over SHA

– More secure, because the embedded hash function has the strength of cryptography.


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HMAC Algorithm

• Variables 1. H= embedded hash

function (SHA) 2. M = Message. 3. Yi = ith block of M 4. L= number of blocks

in M 5. b= number of bits in

block 6. n= length of hash

code. 7. K= secret key. 8. K+= K padded with 0s

so that the result is b bit in the length .

9. Ipad = 00110110 repeated b/8 times

10. Opad = 01011100 repeated b/8 times


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HMAC Algorithm

1. Append zeros to the left end of K to create a b-bit string K+.

2. XOR K+ With ipad to produce the b-bit block Si

3. Append M to Si.

4. Apply H to the stream generated in the step 3

5. XOR K+ with opad to produce the b-bit block So.

6. Append the hash result from step 4 to So.

7. Apply H to the stream generated in step6 and output the result.

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|Najwa AlGhamdi 15

HMAC Algorithm

So HMAC can be expressed as

HMAC (K,M) =

H[ (K+ xor opad) ||

H [(K+ xor ipad) ||M] ]


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Message Authentication and Hash function 2 · SHA-512 | Preprocessing 2. Parsing the Padded Message...

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