Convert.ToBase64String(byte(15)).length = 24 og ikke 22 ?

De 24 tegn er OK.

http://rfc.sunsite.dk/rfc/rfc1864.html

The output of the MD5 algorithm is a 128 bit digest.  When viewed in
  network byte order (big-endian order), this yields a sequence of 16
  octets of binary data.  These 16 octets are then encoded according to
  the base64 algorithm in order to obtain the value that is placed in
  the Content-MD5 field.  Thus, if the application of the MD5 algorithm
  over the raw data of a MIME entity results in a digest having the
  (unlikely) value of "Check Integrity!", then that MIME entity's
  header could contain the field

        Content-MD5:  Q2hlY2sgSW50ZWdyaXR5IQ==

(tæl selv)

Og forklaringen er her:

http://rfc.sunsite.dk/rfc/rfc2045.html

  Special processing is performed if fewer than 24 bits are available
  at the end of the data being encoded.  A full encoding quantum is
  always completed at the end of a body.  When fewer than 24 input bits
  are available in an input group, zero bits are added (on the right)
  to form an integral number of 6-bit groups.  Padding at the end of
  the data is performed using the "=" character.  Since all base64
  input is an integral number of octets, only the following cases can
  arise: (1) the final quantum of encoding input is an integral
  multiple of 24 bits; here, the final unit of encoded output will be
  an integral multiple of 4 characters with no "=" padding, (2) the
  final quantum of encoding input is exactly 8 bits; here, the final
  unit of encoded output will be two characters followed by two "="
  padding characters, or (3) the final quantum of encoding input is
  exactly 16 bits; here, the final unit of encoded output will be three
  characters followed by one "=" padding character.

Det skal altid være et multipla af 3 bytes. Og da 21 ikek er nok så må
det jo blive 24 med 2 padding bytes (=).

Det kan man da kalde et brugbart svar :) Takker