12.2. Detecting Modification


Binary patches can be applied to compiled programs to alter the contents of code or data. The program needs a way of verifying its integrity at runtime.


Detecting whether portions of a binary have been modified is essentially an error-detection problem; therefore, a checksum algorithm such as CRC32, MD5, or SHA1 can be used to generate a signature for an arbitrary block of code or data. This signature can then be checked at runtime to determine whether any modification has taken place.



We have chosen the CRC32 algorithm both for its ease of implementation and for its speed. It is ideal for detecting changes to short sequences of bytes; however, because there are only 232 possible checksum values, and because it is not cryptographically secure, the likelihood of a collision is high, giving the attacker a realistic chance to replace code without changing the checksum. For this kind of application, cryptographic strength is probably overkill, as there are easier attacks than forcing a collision in the checksums (e.g., simply patch the checksumming code).

The checksum API presented here is an implementation of CRC32, which consists of macros for marking the start and end of the block to be checked, as well as a function to calculate the checksum of the block. The function crc32_calc( ) is used to compute the checksum of a buffer.

#define CRC_START_BLOCK(label) void label(void) { } #define CRC_END_BLOCK(label) void _##label(void) ...

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