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The Base45 Data Encoding
Netnodpaf@netnod.seKireifredrik@kirei.seWebweavingdirkx@webweaving.org
Operations
BASE45
This document describes the base 45 encoding scheme which is
built upon the base 64, base 32 and base 16 encoding schemes.
When using QR or Aztec codes a different encoding scheme is
needed than the already established base 64, base 32 and base
16 encoding schemes that are described in RFC 4648. The difference from those and
base 45 is the key table and that the padding with '=' is not
required.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described
in RFC 2119.
Encoded data is to be interpreted as described in RFC 4648 with the exception that a
different alphabet is selected.
A 45-character subset of US-ASCII is used, the 45 characters
that can be used in a QR or Aztec code. If we look at Base 64,
it encodes 3 bytes in 4 characters. Base 45 encodes 2 bytes
over 3 characters.
The two bytes [A, B] are turned into [ C, D, E] where (A*256)
+ B = (C*45*45) + (D*45) + E. The values C, D and E are then
looked up in Table 1 to produce a three character string and
the reverse when decoding.
If the number of octets are not dividable by two, the last
remaining byte is represented by two characters.
A series of bytes is turned into groups of two. Each such 16
bit value is turned into a series of three values calculated
by doing successive calculations modulo 45. The values are
in turned looked up in what is displayed in Table 1.
Example: The string "Hello!" is the byte sequence [ 72, 101,
108, 108, 111, 33 ]. If we look at each 16 bit value, it is
[ 18633, 27756, 28449]. When looking at the values modulo
45, we get [[ 9, 9, 3], [ 13, 30, 36], [14, 2, 9]]. By
looking up these values in the table we get the encoded
string "993DU E29".
There are no considerations for IANA in this document.
When implementing encoding and decoding it is important to be
very careful so that buffer overflow does not take place, or
anything similar. This includes of course the calculations of
modulo 45 and lookup in the table of characters. Decoder also
must be robust regarding input, including proper handling of
the NUL character (ASCII 0).
Specifically it should be noted that Base 64 (for example) pad
the string so that the encoding has the correct number of
characters. This is something that Base 45 does not do,
i.e. Base 45 do not include padding. Because of this, special
care is to be taken when odd number of octets are to be
encoded which results not in N*3 characters, but (N-1)*3+2
characters in the encoded string and vice versa, when the
number of encoded characters are not divisible by 3.
The authors thank everyone that have been working with Base64
during the years that have proven the implementions are
stable.
&RFC4648;
&RFC2119;