A hash function used to get a small integer number from a rather large information block. It results from a calculation made on network traffic information to detect errors made in the transmission or in the duplication of files. CRCs are typically calculated before and after the transmission or the duplication of files and then compared to confirm that they are, indeed, alike. The most widely used CRC calculations are done in a manner such that anticipated types of errors (such as transmission channel noise) are usually detected.

It is important to note, however, that CRCs cannot be relied upon to confirm the integrity of information (that is, that no alterations have been made in the information) because through intentional modification, some crackers can cause changes in the data that remain undetected by a CRC. However, cryptographic hash functions could be used to verify data integrity. The important operation used to calculate a CRC is binary division, with the remainder from the division operation determining the CRC. In fact, CRC types are often identified by a polynomial—the number used as the divisor, displayed in hexadecimal format. A frequently encountered CRC type is that used by Ethernet, PKZIP, WinZip, and PNG; namely, the polynomial 0x04 C11DB7 (a.k.a. CRC-32).

See Also: Crackers; Ethernet; Network.