校验和

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This article is about checksums calculated using addition. The term "checksum" is sometimes used in a more general sense to refer to any kind of redundancy check. Checksums on decimal numbers are discussed under check digit.

校验和是冗余校验的一种形式, 它是通错误检测方法对经过空间（如通信）或者时间（如计算机存储）传送的数据的完整性的检查一种简单方法。 It works by adding up the basic components of a message, typically the asserted bits, and storing the resulting value. Later, anyone can perform the same operation on the data, compare the result to the authentic checksum, and (assuming that the sums match) conclude that the message was probably not corrupted.

The simplest form of checksum, which simply adds up the asserted bits in the data, cannot detect a number of types of errors. In particular, such a checksum is not changed by:

reordering of the bytes in the message
inserting or deleting zero-valued bytes
multiple errors which sum to zero

An Example of a simple checksum:

Given 4 bytes of data (can be done with any number of bytes): 25h, 62h, 3Fh, 52h
Step 1: Adding all bytes together gives 118h.  
Step 2: Drop the Carry Nibble to give you 18h.
Step 3: Get the two's complement of the 18h to get E8h. This is the checksum byte.
To Test the Checksum byte simply add it to the original group of bytes. This should give you 200h. 
Drop the carry nibble again giving 00h. Since it is 00h this means the checksum means the bytes were probably not changed.

More sophisticated types of redundancy check, including Fletcher's checksum, Adler-32, and cyclic redundancy checks (CRCs), are designed to address these weaknesses by considering not only the value of each byte but also its position. The cost of the ability to detect more types of errors is the increased complexity of computing the checksum.

These types of redundancy check are useful in detecting accidental modification such as corruption to stored data or errors in a communication channel. However, they provide no security against a malicious agent as their simple mathematical structure makes them trivial to circumvent. To provide this level of integrity, the use of a cryptographic hash function, such as SHA-256, is necessary. (Collisions have been found in the popular MD5 algorithm and finding collisions in SHA-1 seems possible, but there is no evidence as of 2006 that SHA-256 suffers similar weaknesses.)

On Unix, there is a tool called "cksum" that generates both a 32 bit CRC and a byte count for any given input file.