So why SHA-1 over MD5 which probably is move available and order of magnitude faster ? what problems MD5 has outside of its security that SHA-1 does not ?
This actually ends up not being true. Try
openssl speed md5
openssl speed sha1
First of all, it is not a single algorithm, it is a family of algorithms
For a CRC of size N, you have to also choose a N-bit
polynomial, N-bit starting value, and N-bit output XOR
value. There is no single standard, there are tens or
hundreds of popular options
http://www.evanjones.ca/crc32c.html
Most programming languages have some CRC32 implementation
- but do not document what parameters they use, or use
different notation for the same parameters (forward and
reverse), or do not let you change the parameters, or do
not let you change the CRC size, or all of these. There is
no easy way to get a "standard CRC64 or CRC128" compatible
across platforms without putting it together from github
snippets and example code yourself.
- C: https://software.intel.com/en-us/node/503522
- Java: http://download.java.net/java/jdk9/docs/api/java/util/zip/CR...
- JavaScript: https://www.npmjs.com/package/fast-crc32c https://www.npmjs.com/package/crc32c
- Go: https://github.com/jacobsa/gcloud/blob/master/gcs/gcsutil/cr...
- Python: https://github.com/ludios/pycrc32c
- Ruby: http://www.rubydoc.info/gems/digest-crc/0.4.1/Digest/CRC32c
And again, I'm not actually recommending git switch to CRC (as a fan of security, I would prefer they use an actual collision resistant hash function). But CRC better meets Linus's stated requirements:
- CRC will be faster (much faster as in ~4X, your numbers seem off to me) in almost all cases
- CRC will *not* fail to detect a bitflip, whereas there's a certain probability a random oracle-based construction will
CRC-X is complicated to use and terrible choice.
First of all, it is not a single algorithm, it is a family of algorithms. For a CRC of size N, you have to also choose a N-bit polynomial, N-bit starting value, and N-bit output XOR value. There is no single standard, there are tens or hundreds of popular options [1]. The optimal polynomial depends on both the CRC size, and the length of the data you are feeding into it [2]. If you choose poor parameters, you will get terrible error detection characteristics (like allowing extra 0 bits at the start of data with no change to the checksum). If you choose good parameters, certain classes of common errors like zeroing out a block of more than N bits will still have much worse characteristics than 1 / 2^N random chance of collision.
Second, implementation in standard libraries is patchy. Most programming languages have some CRC32 implementation - but do not document what parameters they use, or use different notation for the same parameters (forward and reverse), or do not let you change the parameters, or do not let you change the CRC size, or all of these. There is no easy way to get a "standard CRC64 or CRC128" compatible across platforms without putting it together from github snippets and example code yourself.
Third, CRC is fast when implemented in hardware, but not that much faster than SHA-1 or SHA-512 in software. It's only 1.5-2.5x faster [3], and when you're doing one checksum per uploaded file or something, it really does not matter. It's going to be even slower when you don't have a suitable-length CRC available in optimised native code, and have to write it yourself in simple C or pure Python.
The obvious and simple solution is to pick a known popular cryptographic hash like (SHA-X) that is available in the standard library of every programming language under the exact same API and no parameters to configure, truncate its output to the digest size you want, and call it a day. No need to worry about error detection performance on specific error cases like long bursts of zeros, and you get some defense against malicious tampering as a free bonus.
[1] https://en.wikipedia.org/wiki/Cyclic_redundancy_check#Standa...
[2] http://repository.cmu.edu/cgi/viewcontent.cgi?article=1672&c...
[3] https://www.cryptopp.com/benchmarks.html