![]() When you save a string to a file, you might implicitly add a newline character to the end, causing the string and its corresponding file to have different hash values. You can think of a file as a string or a string as a file, but the distinction between files and strings may matter in practice. But a secure hash function makes such tampering unfeasible. A checksum could let you know if a bit was accidentally flipped in transit, but it’s easy to deliberately tamper with files without changing the checksum. If a website makes a file available for download, and publishes a hash value, you can compute the hash value yourself after downloading the file to make sure they match. Hash functions are often applied to files. This post explains how to compute hash functions from the Linux command line, from Windows, from Python, and from Mathematica. A second pre-image attack is harder than a collision attack because the attacker can only vary one file. Second pre-image resistance is like collision resistance except one file is fixed. Pre-image resistance means that starting from the hash value, it is very difficult to infer what led to that output it essentially requires a brute force attack, trying many inputs until something hashes to the given value.Ĭollision resistance means its extremely unlikely that two files would map to the same hash value, either by accident or by deliberate attack. Ideally such a function has three properties: ![]() A secure hash function maps a file to a string of bits in a way that is hard to reverse.
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