ISC_SHA512_BLOCK_LENGTH
unsigned char key[ISC_SHA512_BLOCK_LENGTH];
isc_hmacsha512_init(hmacsha512ctx, hkey->key, ISC_SHA512_BLOCK_LENGTH);
if (r.length > ISC_SHA512_BLOCK_LENGTH) {
#define ISC_HMACSHA512_KEYLENGTH ISC_SHA512_BLOCK_LENGTH