Messages in this thread | | | Date | 7 Dec 2014 05:02:52 -0500 | From | "George Spelvin" <> | Subject | Re: Where exactly will arch_fast_hash be used |
| |
> For a start why don't you print out the hashes of 1-255 and then > find out how easy it is to deduce the last bit of the hash result.
They're available in lib/crc32table.h, as crc32ctable_le[0].
As a CRC is a linear function, every bit is the XOR of some selected bits of the input, i.e. the parity of the input and some bit-specific mask sequence.
Furthermore, CRCs are cyclic, so the mask sequences for adjacent bits are shifts of each other.
The lsbit of the CRC32c of x is the parity of x & 0x1f.
This is because the LFSR sequence generated by the polynomial starts 0001111110010001110010101111011000111000011011110010110000100101...
The first bit corresponds to the msbit of the last byte.
How does this implicate the low bits specifically?
| |