authors==Huima
status==Experimental
title==The Bubble Babble Binary Data Encoding
number==Internet Draft
date==April 2000
Network Working Group Antti Huima
Internet Draft SSH Communications Security
draft-huima-babble-01.txt April 2000
The Bubble Babble Binary Data Encoding
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
This document describes a new encoding method for binary data that is
intended to be used in conjunction with fingerprints of
security-critical data.
1. Introduction
Hash values of certificates and public keys, known as fingerprints
or thumbprints, are commonly used for verifying that a received
security-critical datum has been received correctly. Fingerprints
are binary data and typically encoded as series of hexadecimal
digits. However, long strings hexadecimal digits are difficult for
comprehend and cumbersome to translate reliably e.g. over phone.
The Bubble Babble Encoding encodes arbitrary binary data into
pseudowords that are more natural to humans and that can be
pronounced relatively easily. The encoding consumes asymptotically
the same amount of space as an encoding of the form
HH HH HH HH ...
where `H' is a hexadecimal digit, i.e. carries 16 bits in six
characters. However, the Bubble Babble Encoding includes a
checksumming method that can sometimes detect invalid encodings.
The method does not increase the length of the encoded data.
2. Encoding
Below, _|X|_ denotes the largest integer not greater than X.
Let the data to be encoded be D[1] ... D[K] where K is the length
of the data in bytes; every D[i] is an integer from 0 to 2^8 - 1.
First define the checksum series C[1] ... C[_|K/2|_] where
C[1] = 1
C[n] = (C[n - 1] * 5 + (D[n * 2 - 3] * 7 + D[n * 2 - 2])) mod 36
The data is then transformed into _|K/2|_ `tuples'
T[1] ... T[_|K/2|_] and one `partial tuple' P so that
T[i] =
where
a = (((D[i * 2 - 3] >> 6) & 3) + C[i]) mod 6
b = (D[i * 2 - 3] >> 2) & 15
c = (((D[i * 2 - 3]) & 3) + _|C[i] / 6|_) mod 6
d = (D[i * 2 - 2] >> 4) & 15; and
e = (D[i * 2 - 3]) & 15.
The partial tuple P is
P =
where if K is even then
a = (C[i]) mod 6
b = 16
c = _|C[i] / 6|_
but if it is odd then
a = (((D[K] >> 6) & 3) + C[i]) mod 6
b = (D[K] >> 2) & 15
c = (((D[K]) & 3) + _|C[i] / 6|_) mod 6
The `vowel table' V maps integers between 0 and 5 to vowels as
0 - a
1 - e
2 - i
3 - o
4 - u
5 - y
and the `consonant table' C maps integers between 0 and 16 to
consonants as
0 - b
1 - c
2 - d
3 - f
4 - g
5 - h
6 - k
7 - l
8 - m
9 - n
10 - p
11 - r
12 - s
13 - t
14 - v
15 - z
16 - x
The encoding E(T) of a tuple T = is then the string
V[a] C[b] V[c] C[d] `-' C[e]
where there are five characters, and `-' is the literal hyphen.
The encoding E(P) of a partial tuple P = is the
three-character string
V[a] C[b] V[c].
Finally, the encoding of the whole input data D is obtained as
`x' E(T[1]) E(T[2]) ... E(T[_|K/2|_]) E(P) `x'
where `x's are literal characters.
3. Decoding
Decoding is obviously the process of encoding reversed.
To check the checksums, when a tuple or partial
tuple has been recovered from the encoded string, an
implementation should check that ((a - C[i]) mod 6) < 4 and that
((c - C[i]) mod 6) < 4. Otherwise the encoded string is not a valid
encoding of any data and should be rejected.
4. Checksum Strength
Every vowel in an encoded string carries 0.58 bits redundancy; thus
the length of the `checksum' in the encoding of an input string
containing K bytes is 0.58 * K bits.
5. Test Vectors
ASCII Input Encoding
------------------------------------------------------------------
`' (empty string) `xexax'
`1234567890' `xesef-disof-gytuf-katof-movif-baxux'
`Pineapple' `xigak-nyryk-humil-bosek-sonax'
6. Author's Address
Antti Huima
SSH Communications Security, Ltd.
[XXX]
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