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>.
(version 5.2 - ~1994).
Pre-compiled MS-DOS versions are also available.
(1997-08-08)
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