Nitpick: this statement is not exactly true: "Hashes are designed to be tamper-proof".
This only applies to cryptographic hash functions, like SHA-1 and Skein. There are non-cryptogaphic hash functions which are not designed to be secure, like Dan Bernstein's DJB-family of hashes (DJBX33A and DJBX33X), that are used e.g. for hash tables with string keys.
This only applies to cryptographic hash functions, like SHA-1 and Skein. There are non-cryptogaphic hash functions which are not designed to be secure, like Dan Bernstein's DJB-family of hashes (DJBX33A and DJBX33X), that are used e.g. for hash tables with string keys.