Network Working Group P. Faltstrom
Internet-Draft Netnod
Intended status: Standards Track F. Ljunggren
Expires: October 5, 2021 Kirei
D. van Gulik
Webweaving
April 03, 2021
The Base45 Data Encoding
draft-faltstrom-base45-03
Abstract
This document describes the base 45 encoding scheme which is built
upon the base 64, base 32 and base 16 encoding schemes.
Status of This Memo
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This Internet-Draft will expire on October 5, 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . 2
3. Interpretation of Encoded Data . . . . . . . . . . . . . . . 2
4. The Base 45 Encoding . . . . . . . . . . . . . . . . . . . . 2
4.1. When to use Base45 . . . . . . . . . . . . . . . . . . . 3
4.2. The alphabet used in Base45 . . . . . . . . . . . . . . . 3
4.3. Encoding example . . . . . . . . . . . . . . . . . . . . 3
4.4. Decoding example . . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 4
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
8. Normative References . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
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 [RFC4648]. The difference
from those and base 45 is the key table and that the padding with '='
is not required.
2. Conventions Used in This Document
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 [RFC2119].
3. Interpretation of Encoded Data
Encoded data is to be interpreted as described in RFC 4648 [RFC4648]
with the exception that a different alphabet is selected.
4. The Base 45 Encoding
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 in 3 characters.
The two bytes [A, B] are turned into [C, D, E] where (A*256) + B = C
+ (D*45) + (E*45*45). The values C, D and E are then looked up in
Table 1 to produce a three character string and the reverse when
decoding.
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If the number of octets are not dividable by two, the last remaining
byte is represented by two characters. [A] is turned into [C, D]
where A = C + (D*45).
4.1. When to use Base45
If binary data is to be stored in a QR-Code one possible way is to
use the Alphanumeric encoding that uses 11 bits for 2 characters as
defined in section 7.3.4 in ISO/IEC 18004:2015 [ISO18004]. The ECI
mode indicator for this encoding is 0010.
If the data is to use some other transport a transport encoding
suitable for that transport should be used. It is not recommended to
for example first encode data in Base45 and then encode the Base45
blob in for example Base64 if the data is to be sent via email.
Instead the Base45 encoding should be removed, and the data itself
should be encoded in Base64.
4.2. The alphabet used in Base45
The alphanumeric code is defined to use 45 characters as specified in
this alphabet.
Table 1: The Base 45 Alphabet
Value Encoding Value Encoding Value Encoding Value Encoding
00 0 12 C 24 O 36 Space
01 1 13 D 25 P 37 $
02 2 14 E 26 Q 38 %
03 3 15 F 27 R 39 *
04 4 16 G 28 S 40 +
05 5 17 H 29 T 41 -
06 6 18 I 30 U 42 .
07 7 19 J 31 V 43 /
08 8 20 K 32 W 44 :
09 9 21 L 33 X
10 A 22 M 34 Y
11 B 23 N 35 Z
4.3. Encoding example
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.
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It should be noted that although the examples are all text, Base45 is
an encoding for binary data where each octet can have any value
0-255.
Encoding example 1: The string "AB" is the byte sequence [65 66].
The 16 bit value is 65 * 256 + 66 = 16706. 16706 equals 11 + 45 * 11
+ 45 * 45 * 8 so the sequence in base 45 is [11 11 8]. By looking up
these values in the table we get the encoded string "BB8".
Encoding example 2: The string "Hello!!" is the byte sequence [72 101
108 108 111 33 33]. If we look at each 16 bit value, it is [18533
27756 28449 33]. Note the 33 for the last byte. When looking at the
values modulo 45, we get [[38 6 9] [36 31 13] [9 2 14] [33 0]] where
the last byte is represented by two. By looking up these values in
the table we get the encoded string "%69 VD92EX0".
Encoding example 3: The string "base-45" is the byte sequence [98 97
115 101 45 52 53]. If we look at each 16 bit value, it is [25185
29541 11572 53]. Note the 53 for the last byte. When looking at the
values modulo 45, we get [[30 19 12] [21 26 14] [7 32 5] [8 1]] where
the last byte is represented by two. By looking up these values in
the table we get the encoded string "UJCLQE7W581".
4.4. Decoding example
The series of characters are lookup up in Table 1, and the indices
three and three are interpreted as the numbers
Decoding example 1: The string "QED8WEX0" represents when lookup in
Table 1 the values [26 14 13 8 32 14 33 0]. We look at the numbers
in three number sequences (except last) and get [[26 14 13] [8 32 14]
[33 0]]. In base 45 we get [26981 29798 33] where the bytes are
[[105 101] [116 102] [33]]. If we look at the ascii values we get
the string "ietf!".
5. IANA Considerations
There are no considerations for IANA in this document.
6. Security Considerations
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 any byte value 0-255,
including the NUL character (ASCII 0).
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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.
Further that a base45 encoded piece of data includes non-URL-safe
characters so if base45 encoded data have to be URL safe, one have to
use %-encoding.
7. Acknowledgements
The authors thank Alan Barrett, Tomas Harreveld, Anders Lowinger,
Jakob Schlyter, Peter Teufl and Gaby Whitehead for the feedback.
Also everyone that have been working with Base64 during the years
that have proven the implementions are stable.
8. Normative References
[ISO18004]
ISO/IEC JTC 1/SC 31, "ISO/IEC 18004:2015 Information
technology - Automatic identification and data capture
techniques - QR Code bar code symbology specification",
ISO/IEC
18004:2015 https://www.iso.org/standard/62021.html,
February 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.
Authors' Addresses
Patrik Faltstrom
Netnod
Email: paf@netnod.se
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Fredrik Ljunggren
Kirei
Email: fredrik@kirei.se
Dirk-Willem van Gulik
Webweaving
Email: dirkx@webweaving.org
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