Optimising SHA-1 for small input - c

Question

I'm hoping to optimise an implementation of SHA-1 for an 8-bit MCU (8051-based). The input data is only 8-bytes, so I wonder if something could be done to improve this macro:
#define S(x,n) ((x << n) | ((x & 0xFFFFFFFF) >> (32 - n)))
The issue I have is that when macro P calls S with S(b, 30), it takes around 60us to complete. Since there're 80 calls to P, it totals to around 4.8ms.
If I'm correct, S(x,n) expects x to be a uint32. Given the rather small input size, could the number of shifts be reduced by making x smaller, e.g., uint8?
If so, is this the only change needed? From:
#define S(x,n) ((x << n) | ((x & 0xFFFFFFFF) >> (32 - n)))
To:
#define S(x,n) ((x << n) | ((x & 0xFF) >> (8 - n)))
From:
void sha1_process( sha1_context *ctx, uint8 data[64] )
{
uint32 temp, W[16], A, B, C, D, E;
// ...
To:
void sha1_process( sha1_context *ctx, uint8 data[64] )
{
uint8 temp, W[16], A, B, C, D, E;
// ...
Here's the complete code:
#include <string.h>
#include "sha1.h"
#define GET_UINT32(n,b,i) \
{ \
(n) = ( (uint32) (b)[(i) ] << 24 ) \
| ( (uint32) (b)[(i) + 1] << 16 ) \
| ( (uint32) (b)[(i) + 2] << 8 ) \
| ( (uint32) (b)[(i) + 3] ); \
}
#define PUT_UINT32(n,b,i) \
{ \
(b)[(i) ] = (uint8) ( (n) >> 24 ); \
(b)[(i) + 1] = (uint8) ( (n) >> 16 ); \
(b)[(i) + 2] = (uint8) ( (n) >> 8 ); \
(b)[(i) + 3] = (uint8) ( (n) ); \
}
void sha1_starts( sha1_context *ctx )
{
ctx->total[0] = 0;
ctx->total[1] = 0;
ctx->state[0] = 0x67452301;
ctx->state[1] = 0xEFCDAB89;
ctx->state[2] = 0x98BADCFE;
ctx->state[3] = 0x10325476;
ctx->state[4] = 0xC3D2E1F0;
}
void sha1_process( sha1_context *ctx, uint8 data[64] )
{
uint32 temp, W[16], A, B, C, D, E;
GET_UINT32( W[0], data, 0 );
GET_UINT32( W[1], data, 4 );
GET_UINT32( W[2], data, 8 );
GET_UINT32( W[3], data, 12 );
GET_UINT32( W[4], data, 16 );
GET_UINT32( W[5], data, 20 );
GET_UINT32( W[6], data, 24 );
GET_UINT32( W[7], data, 28 );
GET_UINT32( W[8], data, 32 );
GET_UINT32( W[9], data, 36 );
GET_UINT32( W[10], data, 40 );
GET_UINT32( W[11], data, 44 );
GET_UINT32( W[12], data, 48 );
GET_UINT32( W[13], data, 52 );
GET_UINT32( W[14], data, 56 );
GET_UINT32( W[15], data, 60 );
#define S(x,n) ((x << n) | ((x & 0xFFFFFFFF) >> (32 - n)))
#define R(t) \
( \
temp = W[(t - 3) & 0x0F] ^ W[(t - 8) & 0x0F] ^ \
W[(t - 14) & 0x0F] ^ W[ t & 0x0F], \
( W[t & 0x0F] = S(temp,1) ) \
)
#define P(a,b,c,d,e,x) \
{ \
e += S(a,5) + F(b,c,d) + K + x; b = S(b,30); \
}
A = ctx->state[0];
B = ctx->state[1];
C = ctx->state[2];
D = ctx->state[3];
E = ctx->state[4];
#define F(x,y,z) (z ^ (x & (y ^ z)))
#define K 0x5A827999
P( A, B, C, D, E, W[0] );
P( E, A, B, C, D, W[1] );
P( D, E, A, B, C, W[2] );
P( C, D, E, A, B, W[3] );
P( B, C, D, E, A, W[4] );
P( A, B, C, D, E, W[5] );
P( E, A, B, C, D, W[6] );
P( D, E, A, B, C, W[7] );
P( C, D, E, A, B, W[8] );
P( B, C, D, E, A, W[9] );
P( A, B, C, D, E, W[10] );
P( E, A, B, C, D, W[11] );
P( D, E, A, B, C, W[12] );
P( C, D, E, A, B, W[13] );
P( B, C, D, E, A, W[14] );
P( A, B, C, D, E, W[15] );
P( E, A, B, C, D, R(16) );
P( D, E, A, B, C, R(17) );
P( C, D, E, A, B, R(18) );
P( B, C, D, E, A, R(19) );
#undef K
#undef F
#define F(x,y,z) (x ^ y ^ z)
#define K 0x6ED9EBA1
P( A, B, C, D, E, R(20) );
P( E, A, B, C, D, R(21) );
P( D, E, A, B, C, R(22) );
P( C, D, E, A, B, R(23) );
P( B, C, D, E, A, R(24) );
P( A, B, C, D, E, R(25) );
P( E, A, B, C, D, R(26) );
P( D, E, A, B, C, R(27) );
P( C, D, E, A, B, R(28) );
P( B, C, D, E, A, R(29) );
P( A, B, C, D, E, R(30) );
P( E, A, B, C, D, R(31) );
P( D, E, A, B, C, R(32) );
P( C, D, E, A, B, R(33) );
P( B, C, D, E, A, R(34) );
P( A, B, C, D, E, R(35) );
P( E, A, B, C, D, R(36) );
P( D, E, A, B, C, R(37) );
P( C, D, E, A, B, R(38) );
P( B, C, D, E, A, R(39) );
#undef K
#undef F
#define F(x,y,z) ((x & y) | (z & (x | y)))
#define K 0x8F1BBCDC
P( A, B, C, D, E, R(40) );
P( E, A, B, C, D, R(41) );
P( D, E, A, B, C, R(42) );
P( C, D, E, A, B, R(43) );
P( B, C, D, E, A, R(44) );
P( A, B, C, D, E, R(45) );
P( E, A, B, C, D, R(46) );
P( D, E, A, B, C, R(47) );
P( C, D, E, A, B, R(48) );
P( B, C, D, E, A, R(49) );
P( A, B, C, D, E, R(50) );
P( E, A, B, C, D, R(51) );
P( D, E, A, B, C, R(52) );
P( C, D, E, A, B, R(53) );
P( B, C, D, E, A, R(54) );
P( A, B, C, D, E, R(55) );
P( E, A, B, C, D, R(56) );
P( D, E, A, B, C, R(57) );
P( C, D, E, A, B, R(58) );
P( B, C, D, E, A, R(59) );
#undef K
#undef F
#define F(x,y,z) (x ^ y ^ z)
#define K 0xCA62C1D6
P( A, B, C, D, E, R(60) );
P( E, A, B, C, D, R(61) );
P( D, E, A, B, C, R(62) );
P( C, D, E, A, B, R(63) );
P( B, C, D, E, A, R(64) );
P( A, B, C, D, E, R(65) );
P( E, A, B, C, D, R(66) );
P( D, E, A, B, C, R(67) );
P( C, D, E, A, B, R(68) );
P( B, C, D, E, A, R(69) );
P( A, B, C, D, E, R(70) );
P( E, A, B, C, D, R(71) );
P( D, E, A, B, C, R(72) );
P( C, D, E, A, B, R(73) );
P( B, C, D, E, A, R(74) );
P( A, B, C, D, E, R(75) );
P( E, A, B, C, D, R(76) );
P( D, E, A, B, C, R(77) );
P( C, D, E, A, B, R(78) );
P( B, C, D, E, A, R(79) );
#undef K
#undef F
ctx->state[0] += A;
ctx->state[1] += B;
ctx->state[2] += C;
ctx->state[3] += D;
ctx->state[4] += E;
}
void sha1_update( sha1_context *ctx, uint8 *input, uint32 length )
{
uint32 left, fill;
if( ! length ) return;
left = ctx->total[0] & 0x3F;
fill = 64 - left;
ctx->total[0] += length;
ctx->total[0] &= 0xFFFFFFFF;
if( ctx->total[0] < length )
ctx->total[1]++;
if( left && length >= fill )
{
memcpy( (void *) (ctx->buffer + left),
(void *) input, fill );
sha1_process( ctx, ctx->buffer );
length -= fill;
input += fill;
left = 0;
}
while( length >= 64 )
{
sha1_process( ctx, input );
length -= 64;
input += 64;
}
if( length )
{
memcpy( (void *) (ctx->buffer + left),
(void *) input, length );
}
}
static uint8 sha1_padding[64] =
{
0x80, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
};
void sha1_finish( sha1_context *ctx, uint8 digest[20] )
{
uint32 last, padn;
uint32 high, low;
uint8 msglen[8];
high = ( ctx->total[0] >> 29 )
| ( ctx->total[1] << 3 );
low = ( ctx->total[0] << 3 );
PUT_UINT32( high, msglen, 0 );
PUT_UINT32( low, msglen, 4 );
last = ctx->total[0] & 0x3F;
padn = ( last < 56 ) ? ( 56 - last ) : ( 120 - last );
sha1_update( ctx, sha1_padding, padn );
sha1_update( ctx, msglen, 8 );
PUT_UINT32( ctx->state[0], digest, 0 );
PUT_UINT32( ctx->state[1], digest, 4 );
PUT_UINT32( ctx->state[2], digest, 8 );
PUT_UINT32( ctx->state[3], digest, 12 );
PUT_UINT32( ctx->state[4], digest, 16 );
}
#ifdef TEST
#include <stdlib.h>
#include <stdio.h>
/*
* those are the standard FIPS-180-1 test vectors
*/
static char *msg[] =
{
"abc",
"abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq",
NULL
};
static char *val[] =
{
"a9993e364706816aba3e25717850c26c9cd0d89d",
"84983e441c3bd26ebaae4aa1f95129e5e54670f1",
"34aa973cd4c4daa4f61eeb2bdbad27316534016f"
};
int main( int argc, char *argv[] )
{
FILE *f;
int i, j;
char output[41];
sha1_context ctx;
unsigned char buf[1000];
unsigned char sha1sum[20];
if( argc < 2 )
{
printf( "\n SHA-1 Validation Tests:\n\n" );
for( i = 0; i < 3; i++ )
{
printf( " Test %d ", i + 1 );
sha1_starts( &ctx );
if( i < 2 )
{
sha1_update( &ctx, (uint8 *) msg[i],
strlen( msg[i] ) );
}
else
{
memset( buf, 'a', 1000 );
for( j = 0; j < 1000; j++ )
{
sha1_update( &ctx, (uint8 *) buf, 1000 );
}
}
sha1_finish( &ctx, sha1sum );
for( j = 0; j < 20; j++ )
{
sprintf( output + j * 2, "%02x", sha1sum[j] );
}
if( memcmp( output, val[i], 40 ) )
{
printf( "failed!\n" );
return( 1 );
}
printf( "passed.\n" );
}
printf( "\n" );
}
else
{
if( ! ( f = fopen( argv[1], "rb" ) ) )
{
perror( "fopen" );
return( 1 );
}
sha1_starts( &ctx );
while( ( i = fread( buf, 1, sizeof( buf ), f ) ) > 0 )
{
sha1_update( &ctx, buf, i );
}
sha1_finish( &ctx, sha1sum );
for( j = 0; j < 20; j++ )
{
printf( "%02x", sha1sum[j] );
}
printf( " %s\n", argv[1] );
}
return( 0 );
}
#endif
Here's an example of the generated code for S(x,n) when called by P( E, A, B, C, D, W[1] ):
0031D0 85 18 82 MOV DPL,XSP(L)
0031D3 85 19 83 MOV DPH,XSP(H)
0031D6 78 08 MOV R0,#0x08
0031D8 12 17 85 LCALL ?L_MOV_X
0031DB 74 1E MOV A,#0x1E
0031DD 78 08 MOV R0,#0x08
0031DF 12 16 80 LCALL ?L_SHL
0031E2 85 18 82 MOV DPL,XSP(L)
0031E5 85 19 83 MOV DPH,XSP(H)
0031E8 78 10 MOV R0,#0x10
0031EA 12 17 85 LCALL ?L_MOV_X
0031ED 74 02 MOV A,#0x02
0031EF 78 10 MOV R0,#0x10
0031F1 12 16 67 LCALL ?UL_SHR
0031F4 78 08 MOV R0,#0x08
0031F6 79 10 MOV R1,#0x10
0031F8 12 17 39 LCALL ?L_IOR
0031FB 85 18 82 MOV DPL,XSP(L)
0031FE 85 19 83 MOV DPH,XSP(H)
003201 78 08 MOV R0,#0x08
003203 12 17 94 LCALL ?L_MOV_TO_X
Thanks

Answer 1

If I'm correct, S(x,n) expects x to be a uint32. Given the rather small input size, could the number of shifts be reduced by making x smaller, e.g., uint8?
No. The state of the SHA1 function consists of five 32-bit values which change every iteration, and those values are what S(x,n) is operating on. Changing those into 8-bit values would give you a completely different (and probably very broken!) hash function.
The MD5/SHA family of hash functions all rely heavily on 32-bit integer operations. Ease of implementation on 8-bit processors, like the 8051, was not a design goal for these functions, and implementations on these parts will not perform particularly well. Sorry. You'll need to either live with the slowness, use another microprocessor (or one with SHA1 hardware acceleration!), or use a different hash algorithm.

Answer 2

It sounds like your actual requirement is finding a MAC/PRF that's cheap to compute on your hardware for 8 byte inputs.
Since your data has fixed length, you can use a secure block cipher (with 128 bit blocks) as CBC-MAC. Since your data is shorter than one block, CBC-MAC simplifies to encrypting the data with the raw block cipher/ECB mode.
If your 128 bit block cipher has a similar cost-per-byte as SHA-1, this will result in an 8x speedup compared with HMAC-SHA-1 (SHA-1 has 512 bit blocks and you need to hash two blocks for HMAC). If you choose a cipher that's particularly suited for your CPU, the speedup might be even larger.
Since AES is so popular, finding implementations optimized for 8 bit CPUs shouldn't be too hard.