JS, C++, and C# give different Base64 Guid encoding results

I've got a C++/UnrealEngine client that talks to both an existing C# server and an existing JavaScript server (All on Windows).

My problem is when I Base64 encode Guids in C++ I get different results from C# and JS. From what I can tell it's directly related to how the Guid strings are encoded as mentioned in this SO answer.

I can't really change the C# and JS servers as they have other dependent systems, so what would I need to do in C++ (or Unreal) to ensure I get the same output?

//The Guid as a literal string
"3F73B3E6-7351-416F-ACA3-AE639A3F587F"

//C# & JSEncoded
"5rNzP1Fzb0Gso65jmj9Yfw"

//C++ Encoded
"P3Oz5nNRQW+sowAArmOaPw"

Additionally using online converters (to sanity check myself) I get the same two different results so there's no real standardized way it seems.

//https://rcfed.com/Utilities/Base64GUID
//https://toolslick.com/conversion/data/guid
"5rNzP1Fzb0Gso65jmj9Yfw"

//https://www.fileformat.info/tool/guid-base64.htm
"P3Oz5nNRQW+sowAArmOaPw"

The code used for the conversions is below:

//In C#
Guid myGuid = new Guid("3F73B3E6-7351-416F-ACA3-AE639A3F587F");

//Encodes / & + for URL & truncates the trailing "==" padding
string strBase64Guid = Convert.ToBase64String(myGuid.ToByteArray()).Substring(0, 22).Replace("/", "_").Replace("+", "-");

//In JavaScript
//https://stackoverflow.com/questions/55356285/how-to-convert-a-string-to-base64-encoding-using-byte-array-in-javascript
var strGuid = GuidToBase64("3F73B3E6-7351-416F-ACA3-AE639A3F587F");

function GuidToBase64(guid){
    //Guid to ByteArray
    var buffer = [];
    guid.split('-').map((number, index) => {
        var bytesInChar = index < 3 ? number.match(/.{1,2}/g).reverse() : number.match(/.{1,2}/g);
        bytesInChar.map((byte) => { buffer.push(parseInt(byte, 16)); })
    });
    
    var base64String = btoa(String.fromCharCode.apply(null, new Uint8Array(buffer)));
    //Encodes / & + for URL & truncates the trailing "==" padding
    return base64String.slice(0, 22).replace("/", "_").replace("+", "-");
}

//In C++ (Unreal Engine)
FGuid& myGuid = FGuid("3F73B3E6-7351-416F-ACA3-AE639A3F587F");
FString& strGuid = myGuid.ToString(EGuidFormats::Short);


//Copyright Epic Games, Inc. (AFAIK I'm allowed to paste snippets from the sources, just not the whole thing. UE's source it's all on GitHub anyway.)

//Truncated [Function overloads, & Guid format handling]

uint32 Bytes[4] = { NETWORK_ORDER32(A), NETWORK_ORDER32(B), NETWORK_ORDER32(C), NETWORK_ORDER32(D) };
TCHAR Buffer[25];
int32 Len = FBase64::Encode(reinterpret_cast<const uint8*>(Bytes), sizeof(Bytes), Buffer);
TArrayView<TCHAR> Result(Buffer, Len);
    
//Truncated [Sanitizes '+' & '/' and cuts the '==' padding]
}

Solution

The base64 in question is the result of encoding a Guid in its binary format, not in the original string format.

A Guid in binary format consists of:

4-byte integer
2-byte integer
2-byte integer
8-byte array

It does not consist of:

4-byte integer
2-byte integer
2-byte integer
2-byte integer
6-byte array

As depicted in the string format.

Your C++ code is base64-encoding an array of four 4-byte integers (because that is how FGuid stores them - I do not know why), which is not the same Guid format that C# and Javascript are using. You need to make sure you are using the correct number of integers, as well as correct byte size and endian for them, to match what C# and Javascript are using.

^{IIRC, Javascript uses Big-Endian integers, and C# uses Big- or Little-Endian integers depending on the system (see BitConverter.IsLittleEndian).}

5rNzP1Fzb0Gso65jmj9Yfw decodes as bytes:

e6 b3 73 3f 51 73 6f 41 ac a3 ae 63 9a 3f 58 7f

Which is the following Guid values in binary format:

e6 b3 73 3f = 0x3F73B3E6
51 73 = 0x7351
6f 41 = 0x416F
ac a3 ae 63 9a 3f 58 7f

So, you need something equivalent to the following in your C++ code:

uint8 Bytes[16];
memcpy(Bytes+0,  &myGuid.A, 4);
memcpy(Bytes+4,  &myGuid.B, 4);
memcpy(Bytes+8,  &myGuid.C, 4);
memcpy(Bytes+12, &myGuid.D, 4);

uint32 *ptr1 = reinterpret_cast<uint32*>(Bytes+0);
uint16 *ptr2 = reinterpret_cast<uint16*>(Bytes+4);
uint16 *ptr3 = reinterpret_cast<uint16*>(Bytes+2);

*ptr1 = NETWORK_ORDER32(*ptr1);
*ptr2 = NETWORK_ORDER16(*ptr2);
*ptr3 = NETWORK_ORDER16(*ptr3);
// not sure if FGuid stores the remaining
// 8 bytes in the correct order or not. If
// not, swap them around here as needed...

TCHAR Buffer[25];
int32 Len = FBase64::Encode(Bytes, sizeof(Bytes), Buffer);
...