CODEWITHSUNDEEP
c++copensslcryptographybignum
The Quantum Physicist
The Quantum Physicist

Reputation: 26326

OpenSSL: BIGNUM's bin2bn doesn't allocate a new BIGNUM when output ptr is null

According to OpenSSL 1.1.0's manual, it says:

BN_bin2bn() converts the positive integer in big-endian form of length len at s into a BIGNUM and places it in ret. If ret is NULL, a new BIGNUM is created.

But then in the following minimal example:

#include <iostream>
#include <boost/algorithm/hex.hpp>
#include <openssl/bn.h>
#include <vector>

int main()
{
    std::string in = "200ec31326d7a933222e3b43a7d6c920a1d2e8a74d1e6f4980ca78b2d9c1aaba6c2ad71f0f1d0cbb40695f27be048982589bccf30066a8735db4a6b0928925077e";
    std::vector<unsigned char> out;
    // convert hex to binary bytes
    boost::algorithm::unhex(in.begin(), in.end(), std::back_inserter(out));
    BIGNUM *eccsig_r = nullptr;
    // convert bytes to a BIGNUM object
    BN_bin2bn(&out[1],  32, eccsig_r);
    std::cout<<eccsig_r<<std::endl; // prints 0!
    return 0;
}

The pointer address of eccsig_r remains 0 (or nullptr). If I understand what the manual says, it should be that eccsig_r is never nullptr again after calling bin2bn().

Why is eccsig_r still nullptr? I can't understand this. Please advise. I'm on Debian 9.

PS: For full disclosure, that hex you see up there is a simple ECC signature that I serialized. I don't believe that has any effect on this. Please correct me if I'm wrong.

Upvotes: 1

Views: 1047

Answers (1)

The Quantum Physicist
The Quantum Physicist

Reputation: 26326

OK. I think I figured it out. If ret is nullptr, then the return value will create a new BIGNUM in the return of bin2bn(). It's a weird way of doing things, in my opinion. I don't understand the purpose, but this works:

#include <iostream>
#include <boost/algorithm/hex.hpp>
#include <openssl/bn.h>
#include <vector>

int main()
{
    std::string in = "200ec31326d7a933222e3b43a7d6c920a1d2e8a74d1e6f4980ca78b2d9c1aaba6c2ad71f0f1d0cbb40695f27be048982589bccf30066a8735db4a6b0928925077e";
    std::vector<unsigned char> out;
    // convert hex to binary bytes
    boost::algorithm::unhex(in.begin(), in.end(), std::back_inserter(out));
    BIGNUM *eccsig_r = nullptr;
    // convert bytes to a BIGNUM object
    eccsig_r = BN_bin2bn(&out[1],  32, nullptr);
    std::cout<<eccsig_r<<std::endl; // doesn't print zero anymore!
    return 0;
}

Please correct me if I'm wrong.

Upvotes: 1

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