Hash multiple files - c++

Question

I'm trying to hash multiple files, but there is an error.
My files name start from Cheque 083654.tif - 08365122.tif
My code:
for (int i = 4; i < 123; i++)
{
stringstream file;
file<< "C:/Users/user/Desktop/datasets/Cheque 08365" << i << ".tif";
string filename = file.str();
cout << filename << '\n';
unsigned char *sha256digest = calculateSHA256(filename);
char *sha256hash = (char *)malloc(sizeof(char) * 65);
sha256hash[65] = '\0';
for (int i = 0; i < SHA256_DIGEST_LENGTH; i++)
{
sprintf(&sha256hash[i * 2], "%02x", sha256digest[i]);
}
printf("SHA256 HASH: %s\n", sha256hash);
system("pause");
}
The error states that no suitable conversion function from string to char * exists at the filename in:
unsigned char *sha256digest = calculateSHA256(filename);
How can I solve this error?

Answer 1

If calculateSHA256 returns std::string, that assignment is illegal in more ways than one. char * is just a pointer to that storage, string returned by function is a temporal object, which stops existing after semicolon. First, you have to save that string, second, to access its data by appropriate member function. There is no way to convert string directly to a pointer.
Or don't use pointer at all. You would find it better to avoid using C idioms at all.
std::string sha256digest = calculateSHA256(filename);
// FORMATTED OUTPUT
std::stringstream hashstr;
hashstr << std::hex << std::setfill('0');
for( auto x : sha256digest ) // this would iterate through entirety of string
{
hashstr << std::setw(2) << static_cast<int>(static_cast<unsigned char>(x));
}
std::string output;
hashstr >> output;
std::cout << "SHA256 HASH: " << output;

Answer 2

You do not need a stringstream to construct the filename. Use std::to_sting().
I think most of the simplification can be done inside your own calculateSHA256() function. Let it return a std::vector or std::string instead of a char*.
Here's an example where I let it return a std::vector<std::uint8_t> instead:
#include <openssl/evp.h>
#include <openssl/sha.h>
#include <cstdint>
#include <cstdio>
#include <fstream>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <stdexcept>
#include <string>
#include <vector>
// An EVP_MD_CTX helper class
class EvpMdCtx {
public:
explicit EvpMdCtx(const EVP_MD* type, ENGINE* impl = nullptr) : EvpMdCtx() {
if(init(type, impl) == 0)
throw std::runtime_error("EVP_DigestInit_ex failed");
}
EvpMdCtx() : ctx(EVP_MD_CTX_new()) {
if(ctx == nullptr) throw std::runtime_error("EVP_MD_CTX_new failed");
}
EvpMdCtx(const EvpMdCtx&) = delete;
EvpMdCtx& operator=(const EvpMdCtx&) = delete;
~EvpMdCtx() { EVP_MD_CTX_free(ctx); }
int init(const EVP_MD* type, ENGINE* impl = nullptr) {
return EVP_DigestInit_ex(ctx, type, impl);
}
int update(const void* d, size_t cnt) { return EVP_DigestUpdate(ctx, d, cnt); }
auto finalize() {
std::vector<std::uint8_t> md_value(EVP_MAX_MD_SIZE);
unsigned md_len;
if(EVP_DigestFinal_ex(ctx, md_value.data(), &md_len) == 0)
md_value.clear();
else
md_value.resize(md_len);
return md_value;
}
private:
EVP_MD_CTX* ctx;
};
std::vector<std::uint8_t> calculateSHA256(const std::string& filename) {
std::ifstream is(filename);
if(not is) return {};
EvpMdCtx ctx(EVP_sha256());
char buf[BUFSIZ]; // a buffer to fill
while(true) {
is.read(buf, std::size(buf));
auto len = is.gcount();
if(len > 0) {
if(ctx.update(buf, static_cast<size_t>(len)) == 0) return {};
} else {
break;
}
}
// finalize
return ctx.finalize();
}
int main() {
const std::string file = "C:/Users/user/Desktop/datasets/Cheque 08365";
for(int i = 4; i <= 122; ++i) {
std::string filename = file + std::to_string(i) + ".tif";
auto res = calculateSHA256(filename);
if(res.empty()) {
std::cout << "Failed: " << filename << '\n';
} else {
std::cout << std::hex << std::setfill('0');
for(auto v : res) {
std::cout << std::setw(2) << static_cast<int>(v);
}
std::cout << ' ' << filename << '\n';
}
}
}