![]() ![]() A really good root kit assumes that hashes are being watched and causes the system to serve up different files at different times. To be correct, I need to partially retract my statement that hashes can spot changes by a root kit they can spot changes by a naïve root kit. Obviously, this method also has a lot of use in keeping a system secure. ![]() The file attributes may match, but if the hash value is different, the file is different. We then compute the hash values again and look for differences. We run though the files we're interested in and take a hash value for each. Hash values are typically 128 or 160 bits long, so are much smaller than the typical file.įor our purposes, we can use hashes to determine when files have changed, even if they are trying to hide the fact. Therefore, it isn't possible for an attacker to produce a different file that hashes to the same value. The essential property of a hash algorithm that we're interested in is that the chances of two files hashing to the same value are impossibly small. One way around this trick is to use checksums or cryptographic hash algorithms on the files and store the results.Ĭhecksums, such as a cyclic redundancy check (CRC), are also pretty easy to fake if the attacker or attacking program knows which checksum algorithm is being used to check files, so it is recommended that you use a cryptographically strong hash algorithm instead. Most applications won't bother to do this, but sometimes viruses, Trojans, or root kits do something like this to hide. It's dead simple to set the file to any size, date, and time you want. There's one central problem with relying on file attributes to determine if the files have been changed: File attributes are easy to fake. In Hack Proofing Your Network (Second Edition), 2002 Examining Checksums and Hashes Extendable-output functions are different from hash functions, but it is possible to use them in similar ways, with the flexibility to be adapted directly to the requirements of individual applications, subject to additional security considerations.” 7 ![]() ![]() The hash functions specified in this Standard supplement the SHA-1 hash function and the SHA-2 family of hash functions that are specified in FIPS 180-4, the Secure Hash Standard. Hash functions are components for many important information security applications, including (1) the generation and verification of digital signatures, (2) key derivation, and (3) pseudorandom bit generation. The SHA-3 family consists of four cryptographic hash functions, called SHA3-224, SHA3-256, SHA3-384, and SHA3-512, and two extendable-output functions (XOFs), called SHAKE128 and SHAKE256. This Standard also specifies the KECCAK-p family of mathematical permutations, including the permutation that underlies KECCAK, in order to facilitate the development of additional permutation-based cryptographic functions. Each of the SHA-3 functions is based on an instance of the KECCAK algorithm that NIST selected as the winner of the SHA-3 Cryptographic Hash Algorithm Competition. “This Standard specifies the Secure Hash Algorithm-3 (SHA-3) family of functions on binary data. Leighton Johnson, in Security Controls Evaluation, Testing, and Assessment Handbook (Second Edition), 2020 FIPS-202-SHA-3 standard: permutation-based hash and extendable-output functions ![]()
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