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.

Last update: 2017.

Table of Contents

 About This Book

 Cryptography Terminology

 Cryptography Basic Concepts

 Introduction to AES (Advanced Encryption Standard)

 Introduction to DES Algorithm

 DES Algorithm - Illustrated with Java Programs

 DES Algorithm Java Implementation

 DES Algorithm - Java Implementation in JDK JCE

 DES Encryption Operation Modes

 DES in Stream Cipher Modes

 PHP Implementation of DES - mcrypt

 Blowfish - 8-Byte Block Cipher

 Secret Key Generation and Management

 Cipher - Secret Key Encryption and Decryption

 Introduction of RSA Algorithm

 RSA Implementation using java.math.BigInteger Class

 Introduction of DSA (Digital Signature Algorithm)

 Java Default Implementation of DSA

 Private key and Public Key Pair Generation

 PKCS#8/X.509 Private/Public Encoding Standards

 Cipher - Public Key Encryption and Decryption

 MD5 Mesasge Digest Algorithm

SHA1 Mesasge Digest Algorithm

 What Is SHA1 Message Digest Algorithm?

SHA1 Message Digest Algorithm Overview

 Using SHA1 Message Digest in Java

 Using SHA1 Message Digest in PHP

 Using SHA1 Message Digest in Perl

 OpenSSL Introduction and Installation

 OpenSSL Generating and Managing RSA Keys

 OpenSSL Managing Certificates

 OpenSSL Generating and Signing CSR

 OpenSSL Validating Certificate Path

 "keytool" and "keystore" from JDK

 "OpenSSL" Signing CSR Generated by "keytool"

 Migrating Keys from "keystore" to "OpenSSL" Key Files

 Certificate X.509 Standard and DER/PEM Formats

 Migrating Keys from "OpenSSL" Key Files to "keystore"

 Using Certificates in IE (Internet Explorer)

 Using Certificates in Firefox

 Using Certificates in Google Chrome

 Outdated Tutorials

 References

 PDF Printing Version