Python Serial Zmodem

File Verification Using CRCRecently I have found myself thinking a lot about file verification. By file verification, I mean the process of determining whether a file on my computer has been modified unexpectedly. Whether it happened through hardware failure, program error, or malicious tampering, I like to know when a file has had its contents altered. Likewise, I would like a convenient way to check the integrity of a file to verify that it hasnt been changed. This article will show you one way to verify the files contents checking its 3. CRC value. The problem of file integrity has been on my mind because of several nearly simultaneous incidents. First of all, I recently ran dozens of relatively untested programs through my home systems while I was judging the Dr. Dobbs Data Compression Contest. At least two of these programs caused inadvertent damage to the file systems on my computer, one under UNIX and one under MS DOS. The Sony Bravia series of HDTVs are a great piece of kit theyre nice displays that usually have enough inputs for the craziest home theatre setups. This document begin as a collection of facts about ASCII and related technologies, notably hardware serial terminals and RS232 and modems. This is lore that was at. CKermit 8. 0 Update Notes Contents CKermit Kermit Home Second Supplement to Using CKermit, 2nd Edition For CKermit 8. As of CKermit version 8. In both cases, I was able to spot a lot of the damage, but after I restored the data that looked bad, I was left feeling unsure about the rest of my system. Had other files been damaged in more subtle ways I suddenly felt as though I couldnt trust my system. An even more alarming incident occurred a couple of weeks later. A programmer who supplies us with a product for resale called us up and casually mentioned that his office had been infested with the notorious Stoned virus. Lynda Real World Xml Download here. Had we by any chance noticed anything funny in our systems We see funny things on our systems on an hourly basis, so suddenly we were once again in the position of not trusting any of the files on our computers. Fortunately this turned out to be a false alarm. Finally, as part of a recent product release at Greenleaf Software, we decided to implement a program that would allow our customers to download short patch files from our BBS to apply to the source code they purchased from us. I created a small program that could read in the patch file and make modifications to an existing source file, resulting in a corrected output file. However, to keep the program simple, we had to have a way to be sure that the input file we were patching was the file we expected it to be, and hadnt been modified in any way. Our patch program would be capable of really fouling up the file if a programmer had just changed a few lines here and there before trying to patch it. The solution to all of these problems consists of two parts. The first part of the solution is the use of the CRC 3. The second is a general purpose program called CRCMAN that can develop a catalog of CRC values for all the files in a directory tree, and can later check the files in the same directory tree against the catalog. Bsa Troop 101 Burlingame'>Bsa Troop 101 Burlingame. CRC 3. 2CRC 3. 2 is an acronym for the 3. Cyclical Redundancy Check algorithm. CRC 3. 2 generally refers to a specific 3. CRC formula sanctioned by the CCITT, an international standards body primarily concerned with telecommunications. CRC 3. 2 is used in communications protocols such as HDLC and ZMODEM to verify the integrity of blocks of data being transferred through various media. CRC calculations are done using a technique with the formidable name of polynomial division. A block of data, regardless of how long, is treated as if each bit in the block is the coefficient in a long polynomial. For example, a single hexadecimal byte, F0. H, would be considered to the polynomial 1X7 16 15 14 03 02 01 00. Since the terms with coefficients of 0 drop out, the polynomial can be expressed as 17 16 15 14. In the case where we are calculating the CRC of an entire file, the exponents will be very large, but this is not a problem. The actual value of the exponents do not come into play during the calculation of the CRC, so they can grow indefinitely without affecting the algorithm. The calculation of the CRC is done by dividing a second polynomial, known as the generator polynomial into the message polynomial, producing a quotient and a remainder. The generator polynomial used by the CRC 3. X3. 2X2. 6X2. 3X2. ZOC_Terminal__SSH__Telnet__Serial_Modem_-68348.gif' alt='Python Serial Zmodem' title='Python Serial Zmodem' />X1. X1. 2X1. 1X1. X8X7X5X4X2X1. After dividing this generator polynomial int our message polynomial, we end up with a quotient and a remainder. We simply discard the quotient, and use the remainder as our 3. CRC. Polynomial division to create a CRC was originally done using hardware shift registers and boolean glue logic. Fortunately for us, cookbook algorithms now exist to implement the CRC on desktop computers in a relatively fast and efficient manner. In this program, I use a table lookup version of the algorithm that exchanges a small increase in storage space for fast calculation. The nuts and bolts of how the CRC calculations work have been discussed many places before, so I wont go any further into the details in this article. Some excellent resources are listed in the references at the end of this book. If you are interested in exploring this topic further, they would make a good place to start. The Qualities of the CRC 3. CRCMAN uses the CRC 3. Screenshot_1_SecureCRT.jpg' alt='Python Serial Zmodem' title='Python Serial Zmodem' />SecureCRT SecureCRT client for Windows, Mac, and Linux provides rocksolid terminal emulation for computing professionals, raising productivity with advanced. SecureCRT SecureCRT combines rocksolid terminal emulation with the strong encryption, data integrity, and authentication options of the Secure Shell protocol. Paulo 49074 So 46318 do 40723 Brasil 38043 da 37922 Da 35214 US 33367 Folha 2900 Local 19724 Reportagem 1790 Jos 15364. We then treat this 3. This fingerprint differs somewhat from the human fingerprint. It often said that no two people have identical fingerprints. This cant be the case for our CRC fingerprint. Since there are more than 4,2. CRC values. However, the CRC 3. These include the following Every bit in the file contributes to the CRC. This means that changing any bit in the file should change the CRC. Relatively small changes in the file should always result in changes in the CRC. We want to be sure that it would take an extremely unlikely combination of errors to produce an identical CRC. The histogram of output CRC values for input files should tend to be flat. For a given input file, we want the probability of a given CRC being produced to be nearly equal across the entire range of possible CRCs from 0 to FFFFFFFF. These are the goals that the CCITT had in mind when selecting the CRC 3. In practice, the chances of inadvertently damaging or modifying a file without modifying the CRC is vanishingly small, so for all practical purposes a program like CRCMAN can be considered to be infallible. Another characteristic that we would like to see in the CRC 3. This characteristic isnt really necessary if we are just using the CRC to guard against accidental file corruption, but it becomes much more important if we want to detect virus infestations of our files. A typical virus might operate by modifying the MS DOS command interpreter, COMMAND. COM. My version of COMMAND. COM happens to have a CRC 3. H. In the event that a virus modifies this file, it will undoubtedly have a new CRC 3. The challenge to the virus programmer would then be to add new bytes to the end of the file so that the original CRC was restored. Unfortunately, there are techniques for doing just this that are far better than brute force solutions. So when it comes to file security against outside attack, you are better off using a digest algorithm designed for this purpose, such as one from the Secure Hash Algorithm family SHA. These calculations will be more computationally expensive, but will offer better protection against malicious agents. Implementation CRCMAN is a command line MS DOS program that can perform one of two tasks It has two operating modes, one for building a list of CRC values, and another for checking files against that list. The command line for CRCMAN has one of two forms.