Analysis of CRC Methods and Potential Data Integrity Exploits

Cyclic redundancy checks (CRCs) are commonly used to detect errors from noise in networks. They also have been used to verify the integrity of files in a system to prevent tampering and suggested as a possible algorithm for manipulation detection codes. It has been known that a CRC will not detect all errors but with random noise it is unlikely. But a malevolent adversary can efficiently modify a file without changing its length to maintain the same CRC. In this paper, we present an efficient algorithm for modifying an individual byte of a file and other additional bytes to maintain the same CRC. A previous attack on CRC padded a file with additional bytes to maintain the same CRC. In this work, the original file length is maintained and it demonstrates that a CRC, although good for error detection, may not be the best choice for a hash function without additional modifications.