Bubble_Babble_Encoding.txt

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
                                                          corrected 2011

		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|_ + 1] 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] = <a, b, c, d, e> 

   where

     a = (((D[i * 2 - 1] >> 6) & 3) + C[i]) mod 6
     b =   (D[i * 2 - 1] >> 2) & 15
     c = (((D[i * 2 - 1]) & 3) + _|C[i] / 6|_) mod 6
     d = (D[i * 2] >> 4) & 15; and
     e = (D[i * 2]) & 15.

   The partial tuple P is

     P = <a, b, c>

   where if K is even then

     a = (C[K/2 + 1]) mod 6
     b = 16
     c = _|C[K/2 + 1] / 6|_

   but if it is odd then

     a = (((D[K] >> 6) & 3) + C[_|K/2|_ + 1]) mod 6
     b = (D[K] >> 2) & 15
     c = (((D[K]) & 3) + _|C[_|K/2|_ + 1] / 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

   Note well that the vowel and consonant tables are indexed from 0, while
   the data and checksum series are indexed from 1.
   
   The encoding E(T) of a tuple T = <a, b, c, d, e> 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 = <a, b, c> 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 <a, b, c, d, e> or partial
   tuple <a, b, c> 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]

7. Full Copyright Statement

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

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