(weinholt crypto crc) library exports syntax for defining
procedures that calculate CRCs. There is a simple syntax that simply
requires the name of the CRC, and an advanced syntax that can define
CRCs do not really qualify as cryptography, because it is trivial to modify data so that the modified data's CRC matches the old one.
This is the simple interface that requires merely the name of the CRC algorithm. The pre-defined CRCs that can be used this way are currently:
crc-64/ecma-182.(import (weinholt crypto crc)) (define-crc crc-32)
For details on how the arguments work, and the theory behind them, see Ross N. Williams's paper A painless guide to CRC error detection algorithms, which is available at http://www.ross.net/crc/crcpaper.html. A brief description of the arguments follows.
The width is the bitwise length of the polynomial. You might be led to believe that it should sometimes be 33, but if so you've been counting the highest bit, which doesn't count.
The polynomial for CRC-16 is sometimes given as x^16 + x^15 + x^2 + 1. This translates to
#x8005). Notice that x^16 is absent. Don't use the reversed polynomial if you have one of those, instead set ref-in and ref-out properly.
After a CRC has been calculated it is sometimes xor'd with a final value, this is xor-out.
check is either
#for the CRC of the string
This is a slightly easier version of the advanced interface where you can simply specify the powers of the coefficients. CRC-16 in this syntax becomes:(import (weinholt crypto crc)) (define-crc crc-16 (16 15 2 0) #x0000 #t #t #x0000 #xBB3D) ==> (begin (define (crc-16 bv) (crc-16-finish (crc-16-update (crc-16-init) bv))) (define (crc-16-init) #x0000) (define (crc-16-finish r) (bitwise-xor r #x0000)) (define (crc-16-self-test) (if #xBB3D (if (= (crc-16 (string->utf8 "123456789")) #xBB3D) 'success 'failure) 'no-self-test)) ...)
Another example: the polynomial x^8 + x^2 + x + 1 in this syntax is
(8 2 1 0).
(define-crc crc-32) has been used, these bindings
will be available (with names that match the name of the CRC):
Calculates the final CRC of the entire bytevector and returns it as an integer.(import (weinholt crypto crc)) (define-crc crc-32) (crc-32 (string->utf8 "A fiendish scheme")) ⇒ 1384349758
Uses the state and returns a new state that includes the CRC of the given bytes.(import (weinholt crypto crc)) (define-crc crc-32) (crc-32-finish (crc-32-update (crc-32-init) (string->utf8 "A fiendish scheme"))) ⇒ 1384349758
Performs a sanity check and returns either
(1 0)– Initial version. Includes crc-32, crc-16, crc-16/ccitt, crc-32c, and crc-24.
(1 1)– Added crc-64 and the
-updateprocedures use fixnums if
(fixnum-width)is larger than the CRC's width.
(1 2)– Added crc-64/ecma-182.