crc

I'm looking for a way to reverse a CRC32 checksum . There are solutions around, but they are either badly written , extremely technical and/or in Assembly . Assembly is (currently) beyond my ken, so I'm hoping someone can piece together an implementation in a higher level language. Ruby is ideal, but I can parse PHP, Python, C, Java, etc. Any takers? A CRC32 is only reversible if the original string is 4 bytes or less. Read the document called "Reversing CRC Theory and Practice" . This is C#: public class Crc32 { public const uint poly = 0xedb88320; public const uint startxor = 0xffffffff;

为确保消息数据的完整性，除了验证消息CRC之外，建议实现检查串行端口（UART）成帧错误的代码。 如果接收消息中的CRC与接收设备计算的CRC不匹配，则应忽略该消息。 下面的C语言代码片段显示了如何使用逐位移位和异或运算来计算Modbus消息CRC。 使用消息帧中的每个字节计算CRC，除了包含CRC本身的最后两个字节。 // Compute the MODBUS RTU CRC UInt16 ModRTU_CRC(byte[] buf, int len) { UInt16 crc = 0xFFFF; for (int pos = 0; pos < len; pos++) { crc ^= (UInt16)buf[pos]; // XOR byte into least sig. byte of crc for (int i = 8; i != 0; i--) { // Loop over each bit if ((crc & 0x0001) != 0) { // If the LSB is set crc >>= 1; // Shift right and XOR 0xA001 crc ^= 0xA001; } else // Else LSB is not set crc >>= 1; // Just shift right } } // Note, this number

I have a 10 character string key field in a database. I've used CRC32 to hash this field but I'm worry about duplicates. Could somebody show me the probability of collision in this situation? p.s. my string field is unique in the database. If the number of string fields is 1 million, what is probability of collision ? Adam Morris Duplicate of Expected collisions for perfect 32bit crc The answer referenced this article: http://arstechnica.com/civis/viewtopic.php?f=20&t=149670 Found the image below from: http://preshing.com/20110504/hash-collision-probabilities In the case you cite, at least one

When is it appropriate to use CRC for error detection versus more modern hashing functions such as MD5 or SHA1? Is the former easier to implement on embedded hardware? CRC works fine for detecting random errors in data that might occur, for example, from network interference, line noise, distortion, etc. CRC is computationally much less complex than MD5 or SHA1. Using a hash function like MD5 is probably overkill for random error detection. However, using CRC for any kind of security check would be much less secure than a more complex hashing function such as MD5. And yes, CRC is much easier

转自：https://jingyan.baidu.com/article/2c8c281dbdfa9f0009252a74.html 图片都没了，百度真差劲～～～还是博客园好！！！ ModBus通讯规约 ModBus通讯规约允许变送器与施耐德、西门子、AB、GE等多个国际著名品牌的可编程顺序控制器（PLC）、RTU、SCADA系统、DCS或与第三方具有ModBus兼容的监控系统之间进行信息交换和数据传送。 变送器只要简单地增加一套基于计算机（或工控机）的监控软件（如：组态王、Intouch、FIX、synall等）就可以构成一套电力监控系统。 通讯数据的类型及格式： 通讯信息传输过程： 当通讯命令由发送设备（主机）发送至接收设备（从机）时，符合相应地址码的从机接收通讯命令，并根据功能码及相关要求读取信息，如果CRC校验无误，则执行相应的任务，然后把执行结果（数据）返送给主机。返回的信息中包括地址码、功能码、执行后的数据以及CRC校验码。如果CRC校验出错就不返回任何信息。 地址码 地址码是每次通讯信息帧的第一字节（8位），从0到255。这个字节表明由用户设置地址的从机将接收由主机发送来的信息。每个从机都必须有唯一的地址码，并且只有符合地址码的从机才能响应回送信息。当从机回送信息时，回送数据均以各自的地址码开始。主机 发送的地址码表明将发送到的从机地址