References:
OpenSSL
http://git.openssl.org/gitweb/?
p=openssl.git;a=blob;f=crypto/ecdsa/ecs_ossl.c;h=adab1f74b41daf6e719ca1fdae1ba81
7085c7802;hb=HEAD#l309
Nettle:
http://git.lysator.liu.se/nettle/nettle/blobs/master/ecc-ecdsa-sign.c#line86
http://git.lysator.liu.se/nettle/nettle/blobs/master/ecc-hash.c
NSS:
https://hg.mozilla.org/projects/nss/file/49360b638350/lib/freebl/ec.c#l746