This document describes the US Secure Hash Algorithm 1 (SHA-1), which has been adopted as a Federal Information Processing Standard in the United States. The document provides a detailed explanation of the SHA-1 algorithm, including its purpose, definitions, operations, message padding, functions, and constants. It outlines two methods for computing the message digest, with Method 1 being the primary approach. The algorithm is based on principles similar to those used in the MD4 algorithm and is designed to produce a 160-bit message digest from a message of up to 2^64 bits. The message digest is used for data integrity and digital signature verification. The document also includes a C implementation of the SHA-1 algorithm, along with a test driver to validate its functionality. The SHA-1 algorithm is considered secure because it is computationally infeasible to find a message that corresponds to a given message digest or to find two different messages that produce the same digest. The document also includes references to related standards and RFCs, as well as the authors' addresses and copyright information.This document describes the US Secure Hash Algorithm 1 (SHA-1), which has been adopted as a Federal Information Processing Standard in the United States. The document provides a detailed explanation of the SHA-1 algorithm, including its purpose, definitions, operations, message padding, functions, and constants. It outlines two methods for computing the message digest, with Method 1 being the primary approach. The algorithm is based on principles similar to those used in the MD4 algorithm and is designed to produce a 160-bit message digest from a message of up to 2^64 bits. The message digest is used for data integrity and digital signature verification. The document also includes a C implementation of the SHA-1 algorithm, along with a test driver to validate its functionality. The SHA-1 algorithm is considered secure because it is computationally infeasible to find a message that corresponds to a given message digest or to find two different messages that produce the same digest. The document also includes references to related standards and RFCs, as well as the authors' addresses and copyright information.