Source : Free On-Line Dictionary of Computing

btoa
     
         /B too A/ A {binary}
        to {ASCII} conversion utility.
     
        btoa is a {uuencode} or {base 64} equivalent which addresses
        some of the problems with the uuencode standard but not as
        many as the base 64 standard.  It avoids problems that some
        {hosts} have with spaces (e.g. conversion of groups of spaces
        to tabs) by not including them in its character set, but may
        still have problems on non-ASCII systems (e.g. {EBCDIC}).
     
        btoa is primarily used to transfer {binary files} between
        systems across connections which are not {eight-bit clean},
        e.g. {electronic mail}.
     
        btoa takes adjacent sets of four binary {octets} and encodes
        them as five ASCII {octets} using ASCII characters '!' through
        to 'u'.  Special characters are also used: 'x' marks the
        beginning or end of the archive; 'z' marks four consecutive
        zeros and 'y' (version 5.2) four consecutive spaces.
     
        Each group of four octets is processed as a 32-bit integer.
        Call this 'I'.  Let 'D' = 85^4.  Divide I by D.  Call this
        result 'R'.  Make I = I - (R * D) to avoid {overflow} on the
        next step.  Repeat, for values of D = 85^3, 85^2, 85 and 1.
        At each step, to convert R to the output character add decimal
        33 (output octet = R + ASCII value for '!').  Five output
        octets are produced.
     
        btoa provides some integrity checking in the form of a line
        {checksum}, and facilities for patching corrupted downloads.
     
        The {algorithm} used by btoa is more efficient than uuencode
        or base 64.  ASCII files are encoded to about 120% the size of
        their binary sources.  This compares with 135% for uuencode or
        base 64.
     
        {C source (ftp://hpux.csc.liv.ac.uk/hpux/Misc/btoa-5.2/)}.
        (version 5.2 - ~1994).
     
        Pre-compiled {MS-DOS} versions are also available.
     
        (1997-08-08)