Cryptography Tutorials - Herong's Tutorial Examples - v5.41, by Herong Yang
SHA1 Message Digest Algorithm Overview
This section describes the SHA1 algorithm - a 6-step process of padding of '1000...', appending message length, preparing 80 process functions, preparing 80 constants, preparing 5 word buffers, processing input in 512 blocks.
SHA1 algorithm is well described in RFC 3174 - US Secure Hash Algorithm 1 (SHA1), see http://www.ietf.org/rfc/rfc3174.txt. Below is a quick overview of the algorithm.
SHA1 algorithm consists of 6 tasks:
Task 1. Appending Padding Bits. The original message is "padded" (extended) so that its length (in bits) is congruent to 448, modulo 512. The padding rules are:
Task 2. Appending Length. 64 bits are appended to the end of the padded message to indicate the length of the original message in bytes. The rules of appending length are:
Task 3. Preparing Processing Functions. SHA1 requires 80 processing functions defined as:
f(t;B,C,D) = (B AND C) OR ((NOT B) AND D) ( 0 <= t <= 19) f(t;B,C,D) = B XOR C XOR D (20 <= t <= 39) f(t;B,C,D) = (B AND C) OR (B AND D) OR (C AND D) (40 <= t <= 59) f(t;B,C,D) = B XOR C XOR D (60 <= t <= 79)
Task 4. Preparing Processing Constants. SHA1 requires 80 processing constant words defined as:
K(t) = 0x5A827999 ( 0 <= t <= 19) K(t) = 0x6ED9EBA1 (20 <= t <= 39) K(t) = 0x8F1BBCDC (40 <= t <= 59) K(t) = 0xCA62C1D6 (60 <= t <= 79)
Task 5. Initializing Buffers. SHA1 algorithm requires 5 word buffers with the following initial values:
H0 = 0x67452301 H1 = 0xEFCDAB89 H2 = 0x98BADCFE H3 = 0x10325476 H4 = 0xC3D2E1F0
Task 6. Processing Message in 512-bit Blocks. This is the main task of SHA1 algorithm, which loops through the padded and appended message in blocks of 512 bits each. For each input block, a number of operations are performed. This task can be described in the following pseudo code slightly modified from the RFC 3174's method 1:
Input and predefined functions: M[1, 2, ..., N]: Blocks of the padded and appended message f(0;B,C,D), f(1,B,C,D), ..., f(79,B,C,D): Defined as above K(0), K(1), ..., K(79): Defined as above H0, H1, H2, H3, H4: Word buffers with initial values Algorithm: For loop on k = 1 to N (W(0),W(1),...,W(15)) = M[k] /* Divide M[k] into 16 words */ For t = 16 to 79 do: W(t) = (W(t-3) XOR W(t-8) XOR W(t-14) XOR W(t-16)) <<< 1 A = H0, B = H1, C = H2, D = H3, E = H4 For t = 0 to 79 do: TEMP = A<<<5 + f(t;B,C,D) + E + W(t) + K(t) E = D, D = C, C = B<<<30, B = A, A = TEMP End of for loop H0 = H0 + A, H1 = H1 + B, H2 = H2 + C, H3 = H3 + D, H4 = H4 + E End of for loop Output: H0, H1, H2, H3, H4: Word buffers with final message digest
Step 5. Output. The contents in H0, H1, H2, H3, H4 are returned in sequence the message digest.
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