Base64 encoded data maximum size

问题:

I have a security mechanism that implements symmetric algorithm RijndaelManaged. I managed to find information what is the maximum size of encrypted data using RijndaelManaged for particular IV. According to my calculations it will be 128 bytes. However I need to convert these 128 bytes to string using Base64. Is there a way to calculate maximum number of chars that Base64 encoding will use to encode input byte array of size 128?

Thanks,Pawel

回答1:

Absolutely - Base64 takes 4 characters to represent every 3 bytes. (Padding is applied for binary data which isn't an exact multiple of 3 bytes.) So 128 bytes will always be 172 characters. (The way to work this out is that base64 represents 6 bits in each character (2⁶ = 64); therefore 3 bytes = 24 bits = 4 base-64 characters.)

回答2:

A base 64 encoded string will use 4 characters for every 3 bytes (or part thereof). So 128 bytes would be 172 base 64 characters.

回答3:

If you need to check this programatically, you can do so by checking the modulus. Here's some psudocode (no particular language) :

function base64Inflation (numBytes)
    minimumBase64Bytes = roundDown(numBytes / 3 * 4)    
    modulus = numberOfBytes % 3             // Assuming % is the modulo operator
    if modulus == 0 
        return minimumBase64Bytes           // Exact fit! No padding required.
    else
        return minimumBase64Bytes + 4       // Doesn't quite fit. We need to pad.

I've also implemented the same logic in golang:

http://play.golang.org/p/JK9XPAle5_

回答4:

In Java:

byte[] bytes = new byte[128];
int base64Length = bytes.length / 3 * 4; // Strictly integer division
if (bytes.length % 3 != 0)
{
    base64Length += 4; // Extra padding characters will be added
}
System.out.println(bytes.length + " bytes will be encoded in " + base64Length + " characters.");

So, where the input bytes.length == 128, the output will be base64Length == 172 characters.