Internet Engineering Task Force M. Sporny
Internet-Draft Digital Bazaar
Intended status: Informational November 27, 2019
Expires: May 30, 2020
The Base58 Encoding Scheme
draft-msporny-base58-00
Abstract
This document specifies the base 58 encoding scheme, including an
introduction to the benefits of the approach, the encoding and
decoding algorithm, alternative alphabets, and security
considerations.
Status of This Memo
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Copyright (c) 2019 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. The Base58 Alphabet . . . . . . . . . . . . . . . . . . . . . 3
3. The Base58 Encoding Algorithm . . . . . . . . . . . . . . . . 4
4. The Base58 Decoding Algorithm . . . . . . . . . . . . . . . . 4
5. Test Vectors . . . . . . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
When trasmitting data, it can be useful to encode the data in a way
that survives lower fidelity transmission mechanisms. For example,
encoding data using a human alphabet, or encoding the data in a way
that a human being can visually confirm the encoded data while
reducing the opportunity to make a mistake. The Base58 encoding
scheme is similar to the Base64 encoding scheme in that it can
translate any binary data to a text string. It is different from
Base64 in that the conversion alphabet has been carefully picked to
work well in environments where a human being, such as a developer or
other human being, might need to work with the information.
Base58 is designed with a number of usability characteristics in mind
that Base64 does not consider. First, similar looking letters are
omitted such as 0 (zero), O (capital o), I (capital i) and l (lower
case L). Doing so eliminates the possibility of a human being
mistaking similar characters for a different character. Second, the
non-alphanumeric characters + (plus), = (equals), and / (slash) are
omitted to make it possible to use Base58 values in all modern file
systems and URL schemes without the need for translation. Third, by
using only alphanumeric characters, easy double-click or double tap
selection is possible in modern computer interfaces. Fourth, social
messaging systems do not line break on alphanumeric strings making it
easier to e-mail or message Base58 values for debugging or sharing
purposes. Fifth, unlike Base64, there is no padding making many
Base58 values smaller (on average) or the same size as Base64 values
for values up to 64 bytes, and less than 2% larger for values larger
than 256 bytes. Finally, Base64 has eleven encoding variations that
lead to miscommunication among developers on which variety of Base64
to use. This specification asserts that there is just one simple
encoding mechanism for Base58, making implementations and developer
interactions simpler.
While Base58 does have a number of beneficial usability features, it
is not always a good choice for an encoding format. For example,
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when encoding large amounts of data, it is 2% less efficient than
base64. Developers might avoid Base58 if a 2% increase in efficiency
over large data sets is desired.
1.1. Requirements Language
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.
2. The Base58 Alphabet
The Base58 alphabet consists of the following characters:
123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz
Each byte value from 0 to 57 maps to the alphabet above in the
following way:
+-----+---------+-----+---------+------+----------+------+----------+
| Byt | Charact | Byt | Charact | Byte | Characte | Byte | Characte |
| e | er | e | er | | r | | r |
+-----+---------+-----+---------+------+----------+------+----------+
| 0 | 1 | 1 | 2 | 2 | 3 | 3 | 4 |
| 4 | 5 | 5 | 6 | 6 | 7 | 7 | 8 |
| 8 | 9 | 9 | A | 10 | B | 11 | C |
| 12 | D | 13 | E | 14 | F | 15 | G |
| 16 | H | 17 | J | 18 | K | 19 | L |
| 20 | M | 21 | N | 22 | P | 23 | Q |
| 24 | R | 25 | S | 26 | T | 27 | U |
| 28 | V | 29 | W | 30 | X | 31 | Y |
| 32 | Z | 33 | a | 34 | b | 35 | c |
| 36 | d | 37 | e | 38 | f | 39 | g |
| 40 | h | 41 | i | 42 | j | 43 | k |
| 44 | m | 45 | n | 46 | o | 47 | p |
| 48 | q | 49 | r | 50 | s | 51 | t |
| 52 | u | 53 | v | 54 | w | 55 | x |
| 56 | y | 57 | z | | | | |
+-----+---------+-----+---------+------+----------+------+----------+
Table 1: Base58 Mapping Table
Other application-specific alphabets for Base58, such as the Ripple
alphabet and the Flickr alphabet exist. Those alphabets, while valid
in their own application spaces, are not valid encoding formats for
this specification and MUST NOT be used. Supporting more than one
Base58 encoding alphabet would harm interoperability.
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3. The Base58 Encoding Algorithm
To encode an array of bytes to a Base58 encoded value, run the
following algorithm. All mathematical operations MUST be performed
using integer arithmetic. Start by initializing a 'zero_counter' to
zero (0x0), an 'encoding_flag' to zero (0x0), a 'b58_bytes' array, a
'b58_encoding' array, and a 'carry' value to zero (0x0). For each
byte in the array of bytes and while 'carry' does not equal zero
(0x0) after the first iteration:
1. If 'encoding_flag' is not set, and if the byte is a zero (0x0),
increment the value of 'zero_counter'. If the value is not zero
(0x0), set 'encoding_flag' to true (0x1).
2. If 'encoding_flag' is set, multiply the current byte value by 256
and add it to 'carry'.
3. Set the corresponding byte value in 'b58_bytes' to the value of
'carry' modulus 58.
4. Set 'carry' to the value of 'carry' divided by 58.
Once the 'b58_bytes' array has been constructed, generate the final
'b58_encoding' using the following algorithm. Set the first
'zero_counter' bytes in 'b58_encoding' to '1'. Then, for every byte
in 'b58_array', map the byte value using the Base58 alphabet in the
previous section to its corresponding character in 'b58_encoding'.
Return 'b58_encoding' as the Base58 representation of the input array
of bytes.
4. The Base58 Decoding Algorithm
To decode a Base58 encoded array of bytes to a decoded array of
bytes, run the following algorithm. All mathematical operations MUST
be performed using integer arithmetic. Start by initializing a
'raw_bytes' array, and a 'carry' value to zero (0x0). For each input
byte in the array of input bytes:
1. Set 'carry' to the byte value associated with the input byte
character. If a mapping does not exist, return an error code.
2. While 'carry' does not equal zero and there are input bytes
remaining:
1. Multiply the input byte value by 58 and add it to 'carry'.
2. Set the output byte value to 'carry' modulus 256.
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3. Set 'carry' to the value of 'carry' divided by 256.
3. Set the corresponding byte value in 'raw_bytes' to the value of
'carry' modulus 58.
4. Set 'carry' to the value of 'carry' divided by 58.
5. Test Vectors
The following examples can be used as test vectors for the algorithms
in this specification:
The Base58 encoded value for "Hello World!" is:
2NEpo7TZRRrLZSi2U
The Base58 encoded value for "The quick brown fox jumps over the lazy
dog." is:
USm3fpXnKG5EUBx2ndxBDMPVciP5hGey2Jh4NDv6gmeo1LkMeiKrLJUUBk6Z
The Base58 encoded value for 0x0000287fb4cd is:
111233QC4
6. Acknowledgements
Thanks to Satoshi Nakamoto for inventing the Base58 encoding format
and the Bitcoin community for popularizing its usage.
7. Security Considerations
Author's Address
Manu Sporny
Digital Bazaar
Email: msporny@digitalbazaar.com
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