1 71 adler 32 checksum many checksum algorithms

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: ￿ 7.1 Adler-32 Checksum Many checksum algorithms compute the hash of a message by adding together bits of the message. Usually the bits are collected into 8-bit bytes or 32-bit words and the addition is performed 8 or 32 bits at a time. A simple checksum isn’t very robust when there is more than one error: it’s all too easy for the second error to mask the first error, e.g., an earlier 1 that was corrupted to 0 can be offset in the sum by a later 0 corrupted to a 1. So most checksums use a formula where a bit’s effect on the sum is dependent on its position in the message as well as its value. The Adler-32 checksum is an elegant and reasonably effective check-bit computation that’s particularly easy to implement in software. It works on 8-bit bytes of the message, assembled from eight consecutive message bits. Processing each byte (D1 , D2 , . . . , Dn ), compute two sums A and B : A = (1 + sum of the bytes) mod 65521 = (1 + D1 + D2 + . . . + Dn ) mod 65521 B = (sum of the A values after adding each byte) mod 65521 = ((1 + D1 ) + (1 + D1 + D2 ) + . . . + (1 + D1 + D2 + . . . + Dn )) mod 65521 After the modulo operation the A and B values can be represented as 16-bit quantities. The Adler-32 checksum is the 32-bit quantity (B ￿ 16) + A. The Adler-32 checksum requires messages that are several hundred bytes long before it reaches its full effectiveness, i.e., enough bytes so that A exceeds 65521.2 Methods like the Adler-32 checksum are used to check whether large files being transferred have errors; Adler-32 itself is used in the popular zlib compression utility and (in rolling window form) in the rsync file synchroniza...
View Full Document

This document was uploaded on 02/26/2014 for the course CS 6.02 at MIT.

Ask a homework question - tutors are online