CODEWITHSUNDEEP
base64
Emily
Emily

Reputation: 237

Why does ToBase64String change a 16 byte string to 24 bytes

I have the following code. When I check the value of variable i it is 16 bytes but then when the output is converted to Base64 it is 24 bytes. 

   byte[] bytOut = ms.GetBuffer();
        int i = 0;
        for (i = 0; i < bytOut.Length; i++)
            if (bytOut[i] == 0)
                break;

        // convert into Base64 so that the result can be used in xml

        return System.Convert.ToBase64String(bytOut, 0, i);

Is this expected? I am trying to cut down storage and this is one of my problems.

Upvotes: 4

Views: 13235

Answers (4)

Glenn McElhoe
Glenn McElhoe

Reputation: 301

Base64 expresses the input string made of 8-bit bytes using 64 human-readable characters (64 characters = 6 bits of information).

The key to the answer of your question is that it the encoding works in 24 bit chunks, so every 24 bits or fraction thereof results in 4 characters of output.

16 bytes * 8 bits = 128 bits of information

128 bits / 24 bits per chunk = 5.333 chunks

So the final output will be 6 chunks or 24 characters.

The fractional chunks are handled with equal signs, which represent the trailing "null bits". In your case, the output will always end in '=='.

Upvotes: 5

mfanto
mfanto

Reputation: 14418

Yes, you'd expect to see some expansion. You're representing your data in a base with only 64 characters. All those unprintable ASCII characters still need a way to be encoded though. So you end up with slight expansion of the data.

Here's a link that explains how much: Base64: What is the worst possible increase in space usage?

Edit: Based on your comment above, if you need to reduce size, you should look at compressing the data before you encrypt. This will get you the max benefit from compression. Compressing encrypted binary does not work.

Upvotes: 2

Jonathan Wood
Jonathan Wood

Reputation: 67193

Base64 is a great way to represent binary data in a string using only standard, printable characters. It is not, however, a good way to represent string data because it takes more characters than the original string.

Upvotes: 0

Ehsan Zargar Ershadi
Ehsan Zargar Ershadi

Reputation: 24833

This is because a base64 string can contain only 64 characters ( and that is because it should be displayable) in other hand and byte has a variety of 256 characters so it can contain more information in it.

Upvotes: 0

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