3 ms·
Not impossible, but assuming there's not a mathematical flaw that affect both algorithms the difficulty is roughly the product of the difficulty of finding a co
by femto113 7y ago
Not impossible, but assuming there's not a mathematical flaw that affect both algorithms the difficulty is roughly the product of the difficulty of finding a collision in each. AFAIK no one has come up with a joint collision for MD5+SHA1 despite collisions in each being practical for several years.
- noja 7y agoSo should we do that, as well as keep finding new algorithms?
- femto113 7y agoConcatenating two hashes is an algorithm. Mathematically concatenating two 128 bit hashes is not any stronger than a single 256 bit hash (and is likely weaker), but if it’s all you have (or all you can afford to compute) two weak hashes is definitely much better than one.
- noja 7y agoWhy is concatenating two 128 bit hashes (each with a different algorithm) not stronger than a single-algorithm 256 bit hash?
- femto113 7y agoOne reason is that it is theoretically possible to use memory instead of computation to attack the combined hashes by pre-generating a large number of collisions under one algorithm and then simply checking those using the other one, which means you don't need to do both algorithms for every check. Can't say for sure if that works out cheaper in terms of money but if the memory is available it could definitely save a lot of time. Another reason is that for any given hash its theoretical maximum strength against any attack will be less than or equal to its bit length, but the practical strength always trends lower over time as attacks are found, and having two algorithms to attack increases the chances of finding flaws.
- jwilk 7y agoCheck out this paper: https://link.springer.com/chapter/10.1007/978-3-540-28628-8_19 https://link.springer.com/chapter/10.1007/978-3-540-28628-8_...