Signatures use asymmetric cryptography, so there is a public key and a private key. This is important, because while the hash for a file can be calculated by anyone, a signature can only be calculated by someone who has the secret. Unlike checksums or hashes, a signature involves a secret. Connor J's answer gives examples for Windows. On Linux you can use the md5sum, sha1sum, sha256sum, etc utilities. The best example of where it makes sense to verify a hash is when retrieving the hash from the software's trusted website (using HTTPS of course), and using it to verify files downloaded from an untrusted mirror.
They may be able to create an innocent change in the original that causes it to have the same hash as a malicious file, which they could then send you. Using a hash that isn't collision resistant may be problematic if your adversary can modify the legitimate file (for example, contributing a seemingly innocent bug fix). If an attacker is able to modify files on that site or intercept and modify your connection, they can simply substitute the files for malicious versions and change the hashes to match. Retrieving the hash from the same site you're downloading the files from doesn't guarantee anything.
If you plan to use a hash to verify a file, you must obtain the hash from a separate trusted source. Using a cryptographic hash to verify integrity SHA256 is commonly used today, and is safe against both. MD5 and SHA1 are both broken in regard to collisions, but are safe against preimage attacks (due to the birthday paradox collisions are much easier to generate). Collision resistance means that it isn't feasible to create two files that have the same hash, and preimage resistance means that it isn't feasible to create a file with the same hash as a specific target file. What is a cryptographic hash?Ĭryptographic hashes provide additional properties over simple checksums (all cryptographic hashes can be used as checksums, but not all checksums are cryptographic hashes).Ĭryptographic hashes (that aren't broken or weak) provide collision and preimage resistance. Examples of checksums are CRCs, Adler-32, XOR (parity byte(s)). In general a checksum provides no guarantee that intentional modifications weren't made, and in many cases it is trivial to change the file while still having the same checksum. What is a checksum?Ī checksum simply verifies with a high degree of confidence that there was no corruption causing a copied file to differ from the original (for varying definitions of "high"). You mention checksums, PGP, and SHA in your question title, but these are all different things.
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