c - Calculate the size to a Base 64 encoded message

Question

I have a binary string that I am encoding in Base 64. Now, I need to know before hand the size of the final Base 64 encoded string will be.

Is there any way to calculate that?

Something like:

BinaryStringSize is 64Kb EncodedBinaryStringSize will be 127Kb after encoding.

Oh, the code is in C.

Thanks.

Answer 1

If you do Base64 exactly right, and that includes padding the end with = characters, and you break it up with a CR LF every 72 characters, the answer can be found with:

code_size    = ((input_size * 4) / 3);
padding_size = (input_size % 3) ? (3 - (input_size % 3)) : 0;
crlfs_size   = 2 + (2 * (code_size + padding_size) / 72);
total_size   = code_size + padding_size + crlfs_size;

In C, you may also terminate with a -byte, so there'll be an extra byte there, and you may want to length-check at the end of every code as you write them, so if you're just looking for what you pass to malloc(), you might actually prefer a version that wastes a few bytes, in order to make the coding simpler:

output_size = ((input_size * 4) / 3) + (input_size / 96) + 6;