A checksum is computed from a data object and used to detect changes, errors or manipulation. Its design goal is efficient change detection: a newly computed value can be compared with the value expected for that data. A different result shows that the object no longer produces the expected checksum.
A cryptographic hash maps its input to a fixed-length digest. It is designed for preimage resistance, second-preimage resistance and collision resistance. Those properties add resistance to deliberate attempts to obtain selected relationships between inputs and digests, giving the hash a different design goal from a checksum.
What a comparison establishes
For a change or integrity check, either value is used in relation to a reference. Recomputing a checksum or digest supplies the new value; the expected value supplies that reference. A mismatch detects a difference, but the calculation does not authenticate the reference value or identify who created the data. Those questions require trust outside the bare comparison.
A fixed-length digest is not inherently unique for every possible input. Preimage, second-preimage and collision resistance instead express goals of computational difficulty for the named matching attempts. This is why cryptographic resistance is meaningful without claiming that collisions cannot exist. It also explains why a checksum comparison and a cryptographic-hash comparison should not be treated as equivalent under deliberate attack.
Adding trust and choosing the algorithm
A message authentication code, or MAC, uses a secret key to provide message authenticity and integrity, which a bare digest does not provide by itself. The secret key is the additional element behind those two properties. A MAC therefore supports a keyed authenticity and integrity conclusion that cannot be drawn from an unprotected digest alone.
A protected reference, digital signature or comparable trusted mechanism can supply the context needed for a digest comparison. The protected mechanism addresses trust in the value or origin used for comparison, while the hash resistance properties address deliberate attempts to find matching inputs or digests. These are separate requirements when authenticity or origin matters.
Algorithm choice is a separate part of cryptographic protection. NIST has told users to replace SHA-1 for cryptographic protection and plans to remove the remaining SHA-1 specification by 31 December 2030. That published transition applies the design discussion to a current algorithm decision: a value may still be called a hash while the chosen hash no longer meets the stated protection purpose.
