Recently, I began to work on OpenSSL program. When I
try to sign/verify a file with RSA algarism, my
program shows some strange fault.
The OpenSSl I worked on is version 0.97d.
I read the file into a buffer and call EVP_SignUpdate
function repeatedly as below:
const int SIG_STEP = 5; // means the buffer size
which I read the file content into
…….
// Do the signature
EVP_SignInit (&md_ctx, EVP_sha1());
while (!endflag){
read_count = fread( buffer, sizeof(char), SIG_STEP,
appfile );
if( read_count != SIG_STEP ) {
if(ferror(appfile) == 0) {
printf( "fread error\n" );
return 1;
} else if(feof(appfile) != 0) {
printf("reach the end\n");
EVP_SignUpdate (&md_ctx, buffer, strlen(buffer));
endflag = true;
}
}else
EVP_SignUpdate (&md_ctx, buffer, strlen(buffer));
endflag = true;
}
sig_len = sizeof(sig_buf);
err = EVP_SignFinal (&md_ctx, (unsigned char
*)sig_buf, &sig_len, pkey);
……
and then I verify it as below:
……
while (!endflag) {
if(ftell(sigedfile) < (endpos - 64 - (SIG_STEP -
1))) //SIG_STEP-1 means that the step length buffer
will read
read_count = fread( buffer, sizeof(char), SIG_STEP,
sigedfile );
else {
read_count = fread( buffer, sizeof(char), (endpos -
64 - ftell(sigedfile) + 1), sigedfile);
endflag = true;
}
if( read_count != SIG_STEP ) {
if(ferror(sigedfile) == 0) {
printf( "file read error\n" );
return 1;
} else if(feof(sigedfile) != 0) {
printf("reach the end\n");
EVP_VerifyUpdate (&md_ctx, buffer,
strlen((char*)buffer));
fwrite(buffer, sizeof(char), read_count, appfile);
endflag = true;
}
} else {
EVP_VerifyUpdate (&md_ctx, buffer,
strlen((char*)buffer));
fwrite(buffer, sizeof(char), read_count, appfile);
}
}
err = EVP_VerifyFinal (&md_ctx, (unsigned char
*)signature, sig_len, pkey);
……..
When I change the SIG_STEP to different value, and
run the program again, I get the signature with
different length.
My problem is when the signature length less than 64,
the verification will fail. But it really works well
when the signature is 64 byte long.
Is there anyone have some idea about this? Please let
me know, thanks for your help
Regards,
sleepy
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