jl777
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February 17, 2016, 08:36:41 PM |
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I think the following is a relatively simple way to achieve your goals (at least pretty close to it). It requires the data provider to know how you are splitting up the data, at least at some level of granularity. Each of these blocks of data cant be changed, but you could mix and match any combination of the granular block. So if you have a terabyte total data, then say 1MB is the smallest block. you would need 1 million hashes, but with a 30,000:1 reduction in size, maybe it is acceptable. It is a tradeoff decision that doesnt affect the calculation. The larger the smallest block (lets call it a sblock) the less flexibility to distribute it, the smaller the sblock the more flexibility, but also more elements need to be stored. The data creator will need to know the size of the sblock and apply the same calculations as you do and then sign it so you have proof that is what was delivered. For each sblock worth of data (we assume the last block is zerofilled) do SHA256 hash and then treat that as a curve25519 field element. Five bits will need to be hard coded, but there are still 251 bits and odds of collision are extremely small. The following code assumes a 64bit CPU and should be close to a working sblocks calculation: /******************************************************************************
* Copyright © 2014-2016 The SuperNET Developers. *
* *
* See the AUTHORS, DEVELOPER-AGREEMENT and LICENSE files at *
* the top-level directory of this distribution for the individual copyright *
* holder information and the developer policies on copyright and licensing. *
* *
* Unless otherwise agreed in a custom licensing agreement, no part of the *
* SuperNET software, including this file may be copied, modified, propagated *
* or distributed except according to the terms contained in the LICENSE file *
* *
* Removal or modification of this copyright notice is prohibited. *
* *
******************************************************************************/
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <memory.h>
struct sha256_vstate { uint64_t length; uint32_t state[8],curlen; uint8_t buf[64]; };
union _bits256 { uint8_t bytes[32]; uint16_t ushorts[16]; uint32_t uints[8]; uint64_t ulongs[4]; uint64_t txid; };
typedef union _bits256 bits256;
union _bits320 { uint8_t bytes[40]; uint16_t ushorts[20]; uint32_t uints[10]; uint64_t ulongs[5]; uint64_t txid; };
typedef union _bits320 bits320;
// special gcc mode for 128-bit integers
typedef unsigned uint128_t __attribute__((mode(TI)));
// sha256 is ported from libtom, the curve25519 is refactored donna_curve25519.c
#define STORE32L(x, y) \
{ (y)[3] = (uint8_t)(((x)>>24)&255); (y)[2] = (uint8_t)(((x)>>16)&255); \
(y)[1] = (uint8_t)(((x)>>8)&255); (y)[0] = (uint8_t)((x)&255); }
#define LOAD32L(x, y) \
{ x = (uint32_t)(((uint64_t)((y)[3] & 255)<<24) | \
((uint32_t)((y)[2] & 255)<<16) | \
((uint32_t)((y)[1] & 255)<<8) | \
((uint32_t)((y)[0] & 255))); }
#define STORE64L(x, y) \
{ (y)[7] = (uint8_t)(((x)>>56)&255); (y)[6] = (uint8_t)(((x)>>48)&255); \
(y)[5] = (uint8_t)(((x)>>40)&255); (y)[4] = (uint8_t)(((x)>>32)&255); \
(y)[3] = (uint8_t)(((x)>>24)&255); (y)[2] = (uint8_t)(((x)>>16)&255); \
(y)[1] = (uint8_t)(((x)>>8)&255); (y)[0] = (uint8_t)((x)&255); }
#define LOAD64L(x, y) \
{ x = (((uint64_t)((y)[7] & 255))<<56)|(((uint64_t)((y)[6] & 255))<<48)| \
(((uint64_t)((y)[5] & 255))<<40)|(((uint64_t)((y)[4] & 255))<<32)| \
(((uint64_t)((y)[3] & 255))<<24)|(((uint64_t)((y)[2] & 255))<<16)| \
(((uint64_t)((y)[1] & 255))<<8)|(((uint64_t)((y)[0] & 255))); }
#define STORE32H(x, y) \
{ (y)[0] = (uint8_t)(((x)>>24)&255); (y)[1] = (uint8_t)(((x)>>16)&255); \
(y)[2] = (uint8_t)(((x)>>8)&255); (y)[3] = (uint8_t)((x)&255); }
#define LOAD32H(x, y) \
{ x = (uint32_t)(((uint64_t)((y)[0] & 255)<<24) | \
((uint32_t)((y)[1] & 255)<<16) | \
((uint32_t)((y)[2] & 255)<<8) | \
((uint32_t)((y)[3] & 255))); }
#define STORE64H(x, y) \
{ (y)[0] = (uint8_t)(((x)>>56)&255); (y)[1] = (uint8_t)(((x)>>48)&255); \
(y)[2] = (uint8_t)(((x)>>40)&255); (y)[3] = (uint8_t)(((x)>>32)&255); \
(y)[4] = (uint8_t)(((x)>>24)&255); (y)[5] = (uint8_t)(((x)>>16)&255); \
(y)[6] = (uint8_t)(((x)>>8)&255); (y)[7] = (uint8_t)((x)&255); }
#define LOAD64H(x, y) \
{ x = (((uint64_t)((y)[0] & 255))<<56)|(((uint64_t)((y)[1] & 255))<<48) | \
(((uint64_t)((y)[2] & 255))<<40)|(((uint64_t)((y)[3] & 255))<<32) | \
(((uint64_t)((y)[4] & 255))<<24)|(((uint64_t)((y)[5] & 255))<<16) | \
(((uint64_t)((y)[6] & 255))<<8)|(((uint64_t)((y)[7] & 255))); }
// Various logical functions
#define RORc(x, y) ( ((((uint32_t)(x)&0xFFFFFFFFUL)>>(uint32_t)((y)&31)) | ((uint32_t)(x)<<(uint32_t)(32-((y)&31)))) & 0xFFFFFFFFUL)
#define Ch(x,y,z) (z ^ (x & (y ^ z)))
#define Maj(x,y,z) (((x | y) & z) | (x & y))
#define S(x, n) RORc((x),(n))
#define R(x, n) (((x)&0xFFFFFFFFUL)>>(n))
#define Sigma0(x) (S(x, 2) ^ S(x, 13) ^ S(x, 22))
#define Sigma1(x) (S(x, 6) ^ S(x, 11) ^ S(x, 25))
#define Gamma0(x) (S(x, 7) ^ S(x, 18) ^ R(x, 3))
#define Gamma1(x) (S(x, 17) ^ S(x, 19) ^ R(x, 10))
#define MIN(x, y) ( ((x)<(y))?(x):(y) )
int32_t sha256_vcompress(struct sha256_vstate * md,uint8_t *buf)
{
uint32_t S[8],W[64],t0,t1,i;
for (i=0; i<8; i++) // copy state into S
S[i] = md->state[i];
for (i=0; i<16; i++) // copy the state into 512-bits into W[0..15]
LOAD32H(W[i],buf + (4*i));
for (i=16; i<64; i++) // fill W[16..63]
W[i] = Gamma1(W[i - 2]) + W[i - 7] + Gamma0(W[i - 15]) + W[i - 16];
#define RND(a,b,c,d,e,f,g,h,i,ki) \
t0 = h + Sigma1(e) + Ch(e, f, g) + ki + W[i]; \
t1 = Sigma0(a) + Maj(a, b, c); \
d += t0; \
h = t0 + t1;
RND(S[0],S[1],S[2],S[3],S[4],S[5],S[6],S[7],0,0x428a2f98);
RND(S[7],S[0],S[1],S[2],S[3],S[4],S[5],S[6],1,0x71374491);
RND(S[6],S[7],S[0],S[1],S[2],S[3],S[4],S[5],2,0xb5c0fbcf);
RND(S[5],S[6],S[7],S[0],S[1],S[2],S[3],S[4],3,0xe9b5dba5);
RND(S[4],S[5],S[6],S[7],S[0],S[1],S[2],S[3],4,0x3956c25b);
RND(S[3],S[4],S[5],S[6],S[7],S[0],S[1],S[2],5,0x59f111f1);
RND(S[2],S[3],S[4],S[5],S[6],S[7],S[0],S[1],6,0x923f82a4);
RND(S[1],S[2],S[3],S[4],S[5],S[6],S[7],S[0],7,0xab1c5ed5);
RND(S[0],S[1],S[2],S[3],S[4],S[5],S[6],S[7],8,0xd807aa98);
RND(S[7],S[0],S[1],S[2],S[3],S[4],S[5],S[6],9,0x12835b01);
RND(S[6],S[7],S[0],S[1],S[2],S[3],S[4],S[5],10,0x243185be);
RND(S[5],S[6],S[7],S[0],S[1],S[2],S[3],S[4],11,0x550c7dc3);
RND(S[4],S[5],S[6],S[7],S[0],S[1],S[2],S[3],12,0x72be5d74);
RND(S[3],S[4],S[5],S[6],S[7],S[0],S[1],S[2],13,0x80deb1fe);
RND(S[2],S[3],S[4],S[5],S[6],S[7],S[0],S[1],14,0x9bdc06a7);
RND(S[1],S[2],S[3],S[4],S[5],S[6],S[7],S[0],15,0xc19bf174);
RND(S[0],S[1],S[2],S[3],S[4],S[5],S[6],S[7],16,0xe49b69c1);
RND(S[7],S[0],S[1],S[2],S[3],S[4],S[5],S[6],17,0xefbe4786);
RND(S[6],S[7],S[0],S[1],S[2],S[3],S[4],S[5],18,0x0fc19dc6);
RND(S[5],S[6],S[7],S[0],S[1],S[2],S[3],S[4],19,0x240ca1cc);
RND(S[4],S[5],S[6],S[7],S[0],S[1],S[2],S[3],20,0x2de92c6f);
RND(S[3],S[4],S[5],S[6],S[7],S[0],S[1],S[2],21,0x4a7484aa);
RND(S[2],S[3],S[4],S[5],S[6],S[7],S[0],S[1],22,0x5cb0a9dc);
RND(S[1],S[2],S[3],S[4],S[5],S[6],S[7],S[0],23,0x76f988da);
RND(S[0],S[1],S[2],S[3],S[4],S[5],S[6],S[7],24,0x983e5152);
RND(S[7],S[0],S[1],S[2],S[3],S[4],S[5],S[6],25,0xa831c66d);
RND(S[6],S[7],S[0],S[1],S[2],S[3],S[4],S[5],26,0xb00327c8);
RND(S[5],S[6],S[7],S[0],S[1],S[2],S[3],S[4],27,0xbf597fc7);
RND(S[4],S[5],S[6],S[7],S[0],S[1],S[2],S[3],28,0xc6e00bf3);
RND(S[3],S[4],S[5],S[6],S[7],S[0],S[1],S[2],29,0xd5a79147);
RND(S[2],S[3],S[4],S[5],S[6],S[7],S[0],S[1],30,0x06ca6351);
RND(S[1],S[2],S[3],S[4],S[5],S[6],S[7],S[0],31,0x14292967);
RND(S[0],S[1],S[2],S[3],S[4],S[5],S[6],S[7],32,0x27b70a85);
RND(S[7],S[0],S[1],S[2],S[3],S[4],S[5],S[6],33,0x2e1b2138);
RND(S[6],S[7],S[0],S[1],S[2],S[3],S[4],S[5],34,0x4d2c6dfc);
RND(S[5],S[6],S[7],S[0],S[1],S[2],S[3],S[4],35,0x53380d13);
RND(S[4],S[5],S[6],S[7],S[0],S[1],S[2],S[3],36,0x650a7354);
RND(S[3],S[4],S[5],S[6],S[7],S[0],S[1],S[2],37,0x766a0abb);
RND(S[2],S[3],S[4],S[5],S[6],S[7],S[0],S[1],38,0x81c2c92e);
RND(S[1],S[2],S[3],S[4],S[5],S[6],S[7],S[0],39,0x92722c85);
RND(S[0],S[1],S[2],S[3],S[4],S[5],S[6],S[7],40,0xa2bfe8a1);
RND(S[7],S[0],S[1],S[2],S[3],S[4],S[5],S[6],41,0xa81a664b);
RND(S[6],S[7],S[0],S[1],S[2],S[3],S[4],S[5],42,0xc24b8b70);
RND(S[5],S[6],S[7],S[0],S[1],S[2],S[3],S[4],43,0xc76c51a3);
RND(S[4],S[5],S[6],S[7],S[0],S[1],S[2],S[3],44,0xd192e819);
RND(S[3],S[4],S[5],S[6],S[7],S[0],S[1],S[2],45,0xd6990624);
RND(S[2],S[3],S[4],S[5],S[6],S[7],S[0],S[1],46,0xf40e3585);
RND(S[1],S[2],S[3],S[4],S[5],S[6],S[7],S[0],47,0x106aa070);
RND(S[0],S[1],S[2],S[3],S[4],S[5],S[6],S[7],48,0x19a4c116);
RND(S[7],S[0],S[1],S[2],S[3],S[4],S[5],S[6],49,0x1e376c08);
RND(S[6],S[7],S[0],S[1],S[2],S[3],S[4],S[5],50,0x2748774c);
RND(S[5],S[6],S[7],S[0],S[1],S[2],S[3],S[4],51,0x34b0bcb5);
RND(S[4],S[5],S[6],S[7],S[0],S[1],S[2],S[3],52,0x391c0cb3);
RND(S[3],S[4],S[5],S[6],S[7],S[0],S[1],S[2],53,0x4ed8aa4a);
RND(S[2],S[3],S[4],S[5],S[6],S[7],S[0],S[1],54,0x5b9cca4f);
RND(S[1],S[2],S[3],S[4],S[5],S[6],S[7],S[0],55,0x682e6ff3);
RND(S[0],S[1],S[2],S[3],S[4],S[5],S[6],S[7],56,0x748f82ee);
RND(S[7],S[0],S[1],S[2],S[3],S[4],S[5],S[6],57,0x78a5636f);
RND(S[6],S[7],S[0],S[1],S[2],S[3],S[4],S[5],58,0x84c87814);
RND(S[5],S[6],S[7],S[0],S[1],S[2],S[3],S[4],59,0x8cc70208);
RND(S[4],S[5],S[6],S[7],S[0],S[1],S[2],S[3],60,0x90befffa);
RND(S[3],S[4],S[5],S[6],S[7],S[0],S[1],S[2],61,0xa4506ceb);
RND(S[2],S[3],S[4],S[5],S[6],S[7],S[0],S[1],62,0xbef9a3f7);
RND(S[1],S[2],S[3],S[4],S[5],S[6],S[7],S[0],63,0xc67178f2);
#undef RND
for (i=0; i<8; i++) // feedback
md->state[i] = md->state[i] + S[i];
return(0);
}
#undef RORc
#undef Ch
#undef Maj
#undef S
#undef R
#undef Sigma0
#undef Sigma1
#undef Gamma0
#undef Gamma1
void sha256_vinit(struct sha256_vstate * md)
{
md->curlen = 0;
md->length = 0;
md->state[0] = 0x6A09E667UL;
md->state[1] = 0xBB67AE85UL;
md->state[2] = 0x3C6EF372UL;
md->state[3] = 0xA54FF53AUL;
md->state[4] = 0x510E527FUL;
md->state[5] = 0x9B05688CUL;
md->state[6] = 0x1F83D9ABUL;
md->state[7] = 0x5BE0CD19UL;
}
int32_t sha256_vprocess(struct sha256_vstate *md,const uint8_t *in,uint64_t inlen)
{
uint64_t n; int32_t err;
if ( md->curlen > sizeof(md->buf) )
return(-1);
while ( inlen > 0 )
{
if ( md->curlen == 0 && inlen >= 64 )
{
if ( (err= sha256_vcompress(md,(uint8_t *)in)) != 0 )
return(err);
md->length += 64 * 8, in += 64, inlen -= 64;
}
else
{
n = MIN(inlen,64 - md->curlen);
memcpy(md->buf + md->curlen,in,(size_t)n);
md->curlen += n, in += n, inlen -= n;
if ( md->curlen == 64 )
{
if ( (err= sha256_vcompress(md,md->buf)) != 0 )
return(err);
md->length += 8*64;
md->curlen = 0;
}
}
}
return(0);
}
int32_t sha256_vdone(struct sha256_vstate *md,uint8_t *out)
{
int32_t i;
if ( md->curlen >= sizeof(md->buf) )
return(-1);
md->length += md->curlen * 8; // increase the length of the message
md->buf[md->curlen++] = (uint8_t)0x80; // append the '1' bit
// if len > 56 bytes we append zeros then compress. Then we can fall back to padding zeros and length encoding like normal.
if ( md->curlen > 56 )
{
while ( md->curlen < 64 )
md->buf[md->curlen++] = (uint8_t)0;
sha256_vcompress(md,md->buf);
md->curlen = 0;
}
while ( md->curlen < 56 ) // pad upto 56 bytes of zeroes
md->buf[md->curlen++] = (uint8_t)0;
STORE64H(md->length,md->buf+56); // store length
sha256_vcompress(md,md->buf);
for (i=0; i<8; i++) // copy output
STORE32H(md->state[i],out+(4*i));
return(0);
}
void store_limb(uint8_t *out,uint64_t in)
{
int32_t i;
for (i=0; i<8; i++,in>>=8)
out[i] = (in & 0xff);
}
uint64_t load_limb(uint8_t *in)
{
return
((uint64_t)in[0]) |
(((uint64_t)in[1]) << 8) |
(((uint64_t)in[2]) << 16) |
(((uint64_t)in[3]) << 24) |
(((uint64_t)in[4]) << 32) |
(((uint64_t)in[5]) << 40) |
(((uint64_t)in[6]) << 48) |
(((uint64_t)in[7]) << 56);
}
// Take a little-endian, 32-byte number and expand it into polynomial form
bits320 fexpand(bits256 basepoint)
{
bits320 out;
out.ulongs[0] = load_limb(basepoint.bytes) & 0x7ffffffffffffLL;
out.ulongs[1] = (load_limb(basepoint.bytes+6) >> 3) & 0x7ffffffffffffLL;
out.ulongs[2] = (load_limb(basepoint.bytes+12) >> 6) & 0x7ffffffffffffLL;
out.ulongs[3] = (load_limb(basepoint.bytes+19) >> 1) & 0x7ffffffffffffLL;
out.ulongs[4] = (load_limb(basepoint.bytes+24) >> 12) & 0x7ffffffffffffLL;
return(out);
}
void fcontract_iter(uint128_t t[5],int32_t flag)
{
int32_t i; uint64_t mask = 0x7ffffffffffffLL;
for (i=0; i<4; i++)
t[i+1] += t[i] >> 51, t[i] &= mask;
if ( flag != 0 )
t[0] += 19 * (t[4] >> 51); t[4] &= mask;
}
// Take a fully reduced polynomial form number and contract it into a little-endian, 32-byte array
bits256 fcontract(const bits320 input)
{
uint128_t t[5]; int32_t i; bits256 out;
for (i=0; i<5; i++)
t[i] = input.ulongs[i];
fcontract_iter(t,1), fcontract_iter(t,1);
// donna: now t is between 0 and 2^255-1, properly carried.
// donna: case 1: between 0 and 2^255-20. case 2: between 2^255-19 and 2^255-1.
t[0] += 19, fcontract_iter(t,1);
// now between 19 and 2^255-1 in both cases, and offset by 19.
t[0] += 0x8000000000000 - 19;
for (i=1; i<5; i++)
t[i] += 0x8000000000000 - 1;
// now between 2^255 and 2^256-20, and offset by 2^255.
fcontract_iter(t,0);
store_limb(out.bytes,t[0] | (t[1] << 51));
store_limb(out.bytes+8,(t[1] >> 13) | (t[2] << 38));
store_limb(out.bytes+16,(t[2] >> 26) | (t[3] << 25));
store_limb(out.bytes+24,(t[3] >> 39) | (t[4] << 12));
return(out);
}
// Multiply two numbers: output = in2 * in
// output must be distinct to both inputs. The inputs are reduced coefficient form, the output is not.
// Assumes that in[i] < 2**55 and likewise for in2. On return, output[i] < 2**52
bits320 fmul(const bits320 in2,const bits320 in)
{
uint128_t t[5]; uint64_t r0,r1,r2,r3,r4,s0,s1,s2,s3,s4,c; bits320 out;
r0 = in.ulongs[0], r1 = in.ulongs[1], r2 = in.ulongs[2], r3 = in.ulongs[3], r4 = in.ulongs[4];
s0 = in2.ulongs[0], s1 = in2.ulongs[1], s2 = in2.ulongs[2], s3 = in2.ulongs[3], s4 = in2.ulongs[4];
t[0] = ((uint128_t) r0) * s0;
t[1] = ((uint128_t) r0) * s1 + ((uint128_t) r1) * s0;
t[2] = ((uint128_t) r0) * s2 + ((uint128_t) r2) * s0 + ((uint128_t) r1) * s1;
t[3] = ((uint128_t) r0) * s3 + ((uint128_t) r3) * s0 + ((uint128_t) r1) * s2 + ((uint128_t) r2) * s1;
t[4] = ((uint128_t) r0) * s4 + ((uint128_t) r4) * s0 + ((uint128_t) r3) * s1 + ((uint128_t) r1) * s3 + ((uint128_t) r2) * s2;
r4 *= 19, r1 *= 19, r2 *= 19, r3 *= 19;
t[0] += ((uint128_t) r4) * s1 + ((uint128_t) r1) * s4 + ((uint128_t) r2) * s3 + ((uint128_t) r3) * s2;
t[1] += ((uint128_t) r4) * s2 + ((uint128_t) r2) * s4 + ((uint128_t) r3) * s3;
t[2] += ((uint128_t) r4) * s3 + ((uint128_t) r3) * s4;
t[3] += ((uint128_t) r4) * s4;
r0 = (uint64_t)t[0] & 0x7ffffffffffffLL; c = (uint64_t)(t[0] >> 51);
t[1] += c; r1 = (uint64_t)t[1] & 0x7ffffffffffffLL; c = (uint64_t)(t[1] >> 51);
t[2] += c; r2 = (uint64_t)t[2] & 0x7ffffffffffffLL; c = (uint64_t)(t[2] >> 51);
t[3] += c; r3 = (uint64_t)t[3] & 0x7ffffffffffffLL; c = (uint64_t)(t[3] >> 51);
t[4] += c; r4 = (uint64_t)t[4] & 0x7ffffffffffffLL; c = (uint64_t)(t[4] >> 51);
r0 += c * 19; c = r0 >> 51; r0 = r0 & 0x7ffffffffffffLL;
r1 += c; c = r1 >> 51; r1 = r1 & 0x7ffffffffffffLL;
r2 += c;
out.ulongs[0] = r0, out.ulongs[1] = r1, out.ulongs[2] = r2, out.ulongs[3] = r3, out.ulongs[4] = r4;
return(out);
}
int main(int argc,const char * argv[])
{
int32_t sblocksize,len; uint8_t *buf; bits320 prod; bits256 tmp,hash;
struct sha256_vstate md; FILE *fp,*sblocks;
if ( argc < 2 )
printf("usage: %s datafile [sblocksize]\n",argv[0]);
else if ( argc > 3 && (sblocksize= atoi(argv[2])) < 32 )
printf("usage: %s datafile [sblocksize] # min sblocksize is 32\n",argv[0]);
else if ( (fp= fopen(argv[1],"rb")) == 0 )
printf("usage: %s datafile [sblocksize] # cant find %s\n",argv[0],argv[1]);
else
{
if ( (sblocks= fopen("sblocks","wb")) == 0 )
{
printf("error creating sblocks output file\n");
exit(-1);
}
if ( sblocksize == 0 )
sblocksize = 1024*1024;
printf("calculating sblocks for %s\n",argv[1]);
memset(tmp.bytes,0,sizeof(tmp)), tmp.bytes[0] = 9;
prod = fexpand(tmp); // start with valid field element. might want to use the unit element
buf = calloc(1,sblocksize);
while ( (len= (int32_t)fread(buf,1,sblocksize,fp)) > 0 )
{
sha256_vinit(&md), sha256_vprocess(&md,buf,sblocksize), sha256_vdone(&md,hash.bytes);
fwrite(hash.bytes,1,sizeof(hash),sblocks); // save sha256 of sblock
hash.bytes[0] &= 0xf8, hash.bytes[31] &= 0x7f, hash.bytes[31] |= 0x40;
prod = fmul(prod,fexpand(hash)); // multiple field element
memset(buf,0,sblocksize);
}
tmp = fcontract(prod);
fwrite(tmp.bytes,1,sizeof(tmp),sblocks); // this is the value of all the data
free(buf);
fclose(fp);
fclose(sblocks);
return(0);
}
return(-1);
}
I didnt have a chance to verify it, but all it is doing is calculating SHA256 hash of each sblock, mapping it to a field element and multiplying it for a combined value. The field operations work at the 320 bit size so the 256 bit hash needs to be expanded and contracted. Since it is a purely field multiplication, it has no other protections, signatures and whatnot will need to be added on top of this. Note that it is unordered as the fmul is just a pure multiply, if you need it to be ordered, then just include the sequence number into the sha256 calculation. It is totally self contained portable C so save to a file, then gcc it to make the executable. Output is an sblocks file that has the corresponding hash for each sblock along with the total at the end. Hopefully it works for what you need. It is just quick proof of concept code, but if any bit is changed in any of the sblocks, the hash will change which will change the combined product. By committing to the sblock hashes, you can prove that you didnt corrupt the data you got as long as the data provider also runs the same program and signs it. Any subset that is delivered can be verified independently to match your posted set of sblock hashes. I think that satisfies your requirement? James P.S. If chance of collision (around 1 in 2^120) is too high, then a second hash can be used to create two elements per sblock. Odds of two different hash collisions with the same data is vanishingly small. |
