Verifier

Q. How does CD-R Verifier work?

A. By computing a “signature” value for the original disc and comparing it to a signature value of a copy.  The signature can be obtained from a ISO image file on disc, although the success of this depends on the writing program used to write the image to the CD.

Q. What is this “signature” and how is it computed?

A. The signature is computed by reading every byte from the CD and using the industry-standard MD5 hash algorithm to develop a 128-bit polynomial value.  The result of this is that if a single bit changes, the signature value will be different.

Q. Why can’t you just compare the files like other software?

A. Comparing the files on a disc is one measure of the correctness of it.  There are also directory structures that may or may not be the same and other information (dates and times, volume name, etc.) which are not compared when just the files are checked.  Comparing the files does not work for audio discs, either.

Q. I can’t get a “match” on two audio discs that I know are the same.

A. Some CD drives do a better job at retrieving audio disc information than others.  While the music may sound the same and even have the same length, there may be one sector that is different at the beginning or end of the disc - this is enough to make the signatures not match.