C++ BigInteger to byte array

Question

In Java, there is a BigInteger class to using large numbers, and it has a converter function: toByteInteger like this:

private static final BigInteger N = new BigInteger(HEX_N, 16);
...
byte[] digest = messageDigest.digest(N.toByteArray());

In C++, I tried Boost.Multiprecision to use large number type with mpz_int, but it has not any function which convert to mpz_int to byte array.

Is there any equivalent BigInteger type in C++? I'm working with qt framework, is there any big integer structure on qt? And is it possible to convert it to byte array?

How about just casting reference of the value inside it to `char*` or `void*`? wouldn't be your byte array? — The Quantum Physicist, Feb 23 '17 at 13:16
in the [docs](http://www.boost.org/doc/libs/1_55_0/libs/multiprecision/doc/html/boost_multiprecision/ref/number.html) I found `convert_to<>`. Did you try this? — 463035818_is_not_an_ai, Feb 23 '17 at 13:17
or v.backend().data().mpz_export(...), see: http://www.boost.org/doc/libs/1_63_0/libs/multiprecision/doc/html/boost_multiprecision/tut/ints/gmp_int.html and https://gmplib.org/manual/Integer-Import-and-Export.html#Integer-Import-and-Export — Dragos Pop, Feb 23 '17 at 13:49
@tobi303 Firstly, I convert mpz_int to std::string, than I used string's c_str() method. thank you. — gkhanacer, Feb 23 '17 at 14:15

score 0 · Answer 1 · answered Jul 05 '18 at 12:27

Don´t know any specific function, but both cases below have worked to convert cpp_int to byte array!!!

unsigned char cSHA[33], *xzh;
cpp_int ichA;
ichA = g[0];

//CONVERSION USING BINARY MATHS

for (int i = 32; i>= 1; i--)
{
    // este cast pode dar problemas em UBUNTU
    cSHA[i] = (unsigned char)(ichA & 255);
    ichA = ichA/256;
}

// OR   
//CONVERSION USING POINTER
xzh = ((unsigned char*) &ichA) + 32;

for (int i = 32; i>= 1; i--)
{
    cSHA[i] = *(xzh - i);
}

C++ BigInteger to byte array

1 Answers1