Base64 decode, buffer size

Question

This is probably a really basic question, but even so...
I have this code to convert base64 into a blob. However, I can't figure out why is it necessary for v_buffer to be an even number? (I have seen reference online that it must be evenly divisible by 4 - but no explanation, Also, another thread on here saying it must be divisible by 48 - again, no explanation).
declare
  v_image_input CLOB;
  v_image_output BLOB;
  v_buffer NUMBER := 256;
  v_start_index NUMBER := 1;
  v_raw_temp raw(256);
begin
  select substr(clobary, instr(clobary ,',')+1, length(clobary)) into v_image_input
  from clob_data;
  dbms_lob.createtemporary(v_image_output, true);
  for i in 1..ceil(dbms_lob.getlength(v_image_input)/v_buffer) loop
    v_raw_temp := utl_raw.cast_to_raw(substr(v_image_input, v_start_index, v_buffer));
    v_raw_temp := utl_encode.base64_decode(v_raw_temp);
    dbms_lob.writeappend(v_image_output, utl_raw.length(v_raw_temp),v_raw_temp);
    v_start_index := v_start_index + v_buffer;
  end loop;
  insert into blob_data (binary) values (v_image_output); commit;
end;

Answer 1

Basically, as you can lookup on http://en.wikipedia.org/wiki/Base64 every 3 bytes of input stream will be encoded into 4 characters by base64 algorithm. If input stream is not divisible by 3, it will be accordingly padded, so the produced base64 representation will be always a multiple of 4 bytes. You can easily construct a small test to verify it
SQL> with t as (
  2  select replace(sys_connect_by_path(chr(ascii('A') + rownum - 1),'/'),'/') input
  3  from dual
  4  connect by level <=6
  5  )
  6  select
  7  input
  8  ,utl_raw.cast_to_raw(input) raw_input
  9  ,dump(input) dump_input
10  ,utl_encode.base64_encode(utl_raw.cast_to_raw(input)) base64
11  ,dump(utl_encode.base64_encode(utl_raw.cast_to_raw(input))) dump_base64
12  from t
13  /
INPUT      RAW_INPUT            DUMP_INPUT                     BASE64               DUMP_BASE64
A          41                   Typ=1 Len=1: 65                51513D3D             Typ=23 Len=4: 81,81,61,61
AB         4142                 Typ=1 Len=2: 65,66             5155493D             Typ=23 Len=4: 81,85,73,61
ABC        414243               Typ=1 Len=3: 65,66,67          51554A44             Typ=23 Len=4: 81,85,74,68
ABCD       41424344             Typ=1 Len=4: 65,66,67,68       51554A4452413D3D     Typ=23 Len=8: 81,85,74,68,82,65,61,61
ABCDE      4142434445           Typ=1 Len=5: 65,66,67,68,69    51554A445245553D     Typ=23 Len=8: 81,85,74,68,82,69,85,61
ABCDEF     414243444546         Typ=1 Len=6: 65,66,67,68,69,70 51554A4452455647     Typ=23 Len=8: 81,85,74,68,82,69,86,71Best regards
Maxim