What are MD5 and SHA1?
MD5 and SHA1 can be thought of as more advanced alternatives to CRC32. Both MD5 and SHA1 are more secure than CRC32 and it is harder to create fake files of a given MD5 or SHA1. For a full description of how MD5 and SHA1 work, take a look at the Crypto FAQ by RSA Security. At this moment in time, standard archives such as ZIP files do not contain MD5 or SHA1 details in the header record. In order to validate that files within a ZIP have the correct MD5 or SHA1, they must be uncompressed in full and the checksum calculated before it can be checked. Validating CRCs is much faster because they are stored within the ZIP header and can be retrieved without uncompressing the ZIP.
