Why does C# Convert.ToBase64String() give me 88 as a length when I'm passing in 64 bytes?

Question

I'm trying to understand the following:

If I am declaring 64 bytes as the array length (buffer). When I convert to a base 64 string, it says the length is 88. Shouldn't the length only be 64, since I am passing in 64 bytes? I could be totally misunderstanding how this actual works. If so, could you please explain.

//Generate a cryptographic random number
RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider();

// Create byte array
byte[] buffer = new byte[64];

// Get random bytes
rng.GetBytes(buffer);

// This line gives me 88 as a result. 
// Shouldn't it give me 64 as declared above?
throw new Exception(Convert.ToBase64String(buffer).Length.ToString());

// Return a Base64 string representation of the random number
return Convert.ToBase64String(buffer);

Answer 1

No, base-64 encoding uses a whole byte to represent six bits of the data being encoded. The lost two bits is the price of using only alphanumeric, plus and slash as your symbols (basically, excluding the numbers representing not visible or special characters in plain ASCII/UTF-8 encoding). The result that you are getting is (64*4/3) rounded up to the nearest 4-byte boundary.

Answer 2

Base64 encoding converts 3 octets into 4 encoded characters; therefore

(64/3)*4 ≈ (22*4) = 88 bytes.

Read here.