Git-SHAs are computed by prefixing "blob $DecimalMessageLength\0" to a message and then SHA1-checksumming the prefixed message.
From the properties of the SHA1 algorithm, is it possible to do this in a streaming fashion, i.e., prepend the prefix after the message body has been hashed?
C example below (link with -lcrypto with libssl-dev installed; it's probably not very useful since this one doesn't even expose the SHA1 algorithm but I was playing...):
#include <openssl/sha.h>
#include <stdio.h>
#include <stdlib.h>
int pr_dgst(unsigned char const Dgst[static SHA_DIGEST_LENGTH])
{
char const digits[]="0123456789abcdef";
char digest_pr[(SHA_DIGEST_LENGTH)*2+1];
for(size_t i=0;i<SHA_DIGEST_LENGTH;i++){
digest_pr[i*2+0]=digits[Dgst[i]/16];
digest_pr[i*2+1]=digits[Dgst[i]%16];
}
digest_pr[(SHA_DIGEST_LENGTH)*2]='\0';
return puts(digest_pr);
}
int main()
{
system("echo gitsha; printf '%s' 'abc' | git hash-object --stdin");
#define STR_STRLEN(A) A, (sizeof(A)/sizeof(*(A))-1) //paste string literal and its length
unsigned char digest[SHA_DIGEST_LENGTH];
SHA_CTX ctx;
SHA1_Init(&ctx); SHA1_Update(&ctx,STR_STRLEN("blob 3\0abc")); SHA1_Final(digest,&ctx);
pr_dgst(digest); //prints the same as the system command
//do this in a streaming fashion??
SHA1_Init(&ctx);
size_t len = 0;
SHA1_Update(&ctx,STR_STRLEN("a")); len++;
SHA1_Update(&ctx,STR_STRLEN("b")); len++;
SHA1_Update(&ctx,STR_STRLEN("c")); len++;
//"prepend" "blob 3\0" now?
SHA1_Final(digest,&ctx);
/*pr_dgst(digest);*/
}
