How to create byte[] with length 16 using FromBase64String [duplicate]

Answer 1

A base64 string for 16 bytes will always be 24 characters and have == at the end, as padding.

(At least when it's decodable using the .NET method. The padding is not always inlcuded in all uses of base64 strings, but the .NET implementation requires it.)

Answer 2

You can use PadRight() to pad the string to the end of it with a char that you will later remove once decrypted.

Answer 3

In Base64 encoding '=' is a special symbol that is added to end of the Base64 string to indicate that there is no data for these chars in original value.

Each char is equal to 6 original bits of data, so to produce 8 bit values the string length has to be dividable by 4 without remainder. (6 bits * 4 = 8 bits * 3). When the resulting BASE64 string is shorter than 4n then '=' are added at the end to make it valid.

Update

Last char before '==' encodes only 2 bits of information, so by replacing it with all possible Base64 chars will give you only 4 different keys out of 64 possible combinations. In other words, by generating strings in format "bbbbbbbbbbbbbbbbbbbbbb==" (where 'b' is valid Base64 character) you'll get 15 duplicate keys per each unique key.