printf$("\{")$;

for$($i $=0$; i $<8$; i$++)$

printf$("0$x$\%09$"PRIx$64",",$a$[$i$])$;

printf$("$

$")$;

for$($i $=8$; i $<16$; i$++)$

printf$("0$x$\%09$"PRIx$64",",$a$[$i$])$;

printf$("$

$")$;

for$($i $=16$; i $<24$; i$++)$

printf$("0$x$\%09$"PRIx$64",",$a$[$i$])$;

printf$("$

$")$;

for$($i $=24$; i $<31$; i$++)$

printf$("0$x$\%09$"PRIx$64",",$a$[$i$])$;

printf$("0$x$\%09$"PRIx$64"\}",$a$[$i$])$;

$\}$

$//$ r $=$ a $+$ b

void add$256($bigint$256$ r$,$ const bigint$256$ a$,$ const bigint$256$ b$)\{$

int i;

uint$8\_$t carry $=0$;

$//$ Write $1$b here

$\}$

$//$ r $=$ a $-$ b

void sub$256($bigint$256$ r$,$ const bigint$256$ a$,$ const bigint$256$ b$)\{$

int i;

uint$8\_$t borrow $=0$;

$//$ Write $2$b here

$\}$

$//$ r $=$ a $*$ b using schoolbook method

void schoolbook$\_$mul$256($bigint$512$ r$,$ const bigint$256$ a$,$ const bigint$256$ b$)\{$

int i$,$ j;

$//$ Initialize r with $0$s

memset$($r$,0,256)$;

$//$ Write $3$b here

$\}$

$//$ r $=$ a $*$ b using comba method

void comba$\_$mul$256($bigint$512$ r$,$ const bigint$256$ a$,$ const bigint$256$ b$)\{$

int i$,$ j;

$//$ Write $3$c here

$\}$

$//$ r $=$ a $*$ b using Karatsuba method

void karatsuba$\_$mul$256($bigint$512$ r$,$ const bigint$256$ a$,$ const bigint$256$ b$)\{$

int i$,$ j;

$//$initialize arrays

uint$64\_$t ma$[8]$;

uint$64\_$t mb$[8]$;

uint$64\_$t z$0[16]=\{0\}$;

uint$64\_$t z$1[16]=\{0\}$;

uint$64\_$t z$2[16]=\{0\}$;

$//$ Write $3$d here

$//$ Compute ma$,$ mb

$//$ Compute z$0,$ z$1,$ z$2$

$//$ Perform subtraction z$1=$ z$1-$ z$0-$ z$2$

$//$ Implement final addition and put the result in r

$\}$