Convert HEX to printable string/char

Question

I'm using CNG to generate a hash. Result of BCryptFinishHash call is MD5 of a input in hex form. Example:

char *outHash = "\x02\x34\x75\01..."

I want to convert it to printable string: 02347501...

How can I do that?

Answer 1

To encode a byte array in hex and write the encoded data to a std::string, do this:

static inline char
hex_digit(unsigned int n)
{
    if (n < 10) return '0' + n;
    if (n < 16) return 'a' + (n - 10);
    abort();
}

std::string
encode_bytes(const unsigned char *bytes, size_t len)
{
    std::string rv;
    rv.reserve(len * 2);
    for (size_t i = 0; i < len; i++) {
        rv.push_back(hex_digit((bytes[i] & 0xF0) >> 4));
        rv.push_back(hex_digit((bytes[i] & 0x0F) >> 0));
    }
    return rv;
}

Note that you must know the length of the byte array. It is not safe to treat it as a NUL-terminated "C string", because binary data can contain internal zero bytes. To know the length of a hash generated by CNG, call BCryptGetProperty to get the BCRYPT_HASH_LENGTH property.

Answer 2

we can use CryptBinaryToString here with CRYPT_STRING_HEXASCII or CRYPT_STRING_HEX or CRYPT_STRING_HEXRAW or CRYPT_STRING_HEX | CRYPT_STRING_NOCRLF or CRYPT_STRING_HEXRAW | CRYPT_STRING_NOCRLF depen how you want format string. for example

void print(PUCHAR pbHash, ULONG cbHash, DWORD dwFlags = CRYPT_STRING_HEXRAW | CRYPT_STRING_NOCRLF)
{
    ULONG cch = 0;
    if (CryptBinaryToStringW(pbHash, cbHash, dwFlags, 0, &cch))
    {
        if (PWSTR sz = (PWSTR)_malloca(cch * sizeof(WCHAR)))
        {
            if (CryptBinaryToStringW(pbHash, cbHash, dwFlags, sz, &cch))
            {
                DbgPrint("%S\n", sz);
            }
            _freea(sz);
        }
    }
}

Answer 3

If you need an easy, one time solution, this is a useful tool: https://codebeautify.org/hex-string-converter

However, if you're looking to do this within your code itself, I found this from an earlier thread (AKA, this is not my work but that of @KEINE LUST from here )

int main(void)
{
unsigned char readingreg[4];
readingreg[0] = 0x4a;
readingreg[1] = 0xaa;
readingreg[2] = 0xaa;
readingreg[3] = 0xa0;
char temp[4];

sprintf(temp, "%x", readingreg[0]);
printf("This is element 0: %s\n", temp);
return 0;
}

Answer 4

You can print it like this:

for(const char *wsk=outHash; *wsk; ++wsk){
    printf("%02hhx", *wsk);
}

Edit based that cstring can have 0x00 numbers.
C

const char outHash[] = "\x02\x34\x75";
const int size = sizeof(outHash)/sizeof(char) - 1;
for(int i = 0; i < size; ++i){
    printf("%02hhx", outHash [i]);
}

C++

std::string outHash = "\x02\x34\x75";
for(int i = 0; i < outHash.size(); ++i) {
    printf("%02hhx", outHash [i]);
}

Answer 5

Loop over the characters and print the numerical value (in hex).

#include <iostream>
#include <iomanip>

int main()
{
    char*  outHash = "\x02\x34\x75\x01\x23\xff";  // Get from your Hash function.
    int    sizeOfHash = 6;                        // Use appropriate size for BCryptFinishHash()


    // Set up the characteristics of the stream.
    // setw(2):       Each printed object will use a min width of 2
    // setfill('0'):  If the object is less than 2 char then fill the space with '0'
    // hex:           Print numbers in hex.
    std::cout << std::setw(2) << std::setfill('0') << std::hex;

    // Create a view of the object.
    // Makes it simpler to loop over.
    std::string_view view(outHash, sizeOfHash);

    // Loop over the string.
    for(unsigned char val: view) {
        // Convert to `unsigned char` to make sure you don't print
        // negative numbers. Then convert from there to `int` so that
        // the `std::hex will kick in and convert to hex value.
        std::cout << static_cast<int>(val);
    }   
    std::cout << "\n";
}

Answer 6

I am working on C++ wrapper around Windows Crypto API & CNG which I am using in my projects. I plan to move all of it to github but for now it is just a work in progress, but you can find it useful for Crypto basics like HEX / Base64 encode / decode etc.

https://github.com/m4x1m1l14n/Crypto

You can use Crypto::Hex::Encode() method to achieve what you want.

#include <Crypto\Hex.hpp>
#include <Crypto\Random.hpp>

using namespace m4x1m1l14n;

char arr[] = { 0xaa, 0xbb, 0xcc, 0xdd, 0x99, 0x00 };

encoded = Crypto::Hex::Encode(arr, sizeof(arr));

/* encoded = "aabbccdd9900" */

Also you can use wrapper for MD5 which is located in Hash namespace, like this. (If you are not using large amount of data)

#include <Crypto\Hex.hpp>
#include <Crypto\Hash.hpp>

using namespace m4x1m1l14n;

encoded = Crypto::Hex::Encode(Crypto::Hash::MD5("Whatever you want to hash"));