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Base64 and related binary to 'printable' ASCII encoding.
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Base64...

  Encodes a binary string into a printable form using a set of 64 characters
  such that 3 binary charcaters become 4 printable characters

    A-Z a-z 0-9 + /

  Base64 on the other hand can output up to 2 extra '=' pading characters on
  the final line (unused padding characters).

  Problems however can develop if the base64 encoding is being used as
  a filename, as the '/' is not permitted, and '+' represents space in URLs
  and not a 'plus'.


"Modified Base64 for URL"

  This replaces '+' and '/' with the charcaters '-' and '_' respectivally.  It
  also removes (deletes) any '=' padding on the end.

    A-Z a-z 0-9 - _

  Many other varients for the two special charcaters also exists. for example
  encfs replaces these characters with '-' and ',' (which is which is not
  known)


EncFS filename encoding

  The base64 character set is used for the binary encrypted filenames,
  but with the characters './' replaced with ',-' respectiveally.

  The filename has a checksum added, and it is then encrypted.
  The result is then base64 encoded.

  Source file from EncFS (binary to base64 indexes, then to ascii)
    http://code.google.com/p/encfs/source/browse/trunk/encfs/base64.cpp

UUencoding...

  Was originally designed in the 1970's for sending binary data in mail and
  usenet news articals. It is actually exactly the same as base64 encroding, but
  using a different set of 64 characters, that are consecutive ASCII character
  codes, where base64 charaectrs are not.  (see perl implementation below).

  That is it uses the charcaters.

    space to _    (with an sometimes optional ` characater as a pad)

  Lines are broken into lines with newline characters, and prefixed with a count
  of the number of characters on the line, typically 'M' (45) characters.
  The final line will be a single '`' on it own (zero characters on line) and
  the whole encoding wrapped by 'begin' and 'end' constructs.

  Note that while uuencode uses space and other punctunation characters,
  base64 does not, which solves a problem that has always plagued uuencoding
  when emailing binary files.

XXencoding...

  An obsolete attempt to fix the UUencode method so space is not needed.
  Characters set was

    + - 0-9 A-Z a-z   with + also used as the pad

Password base64

  (incomplete)

  http://lists.fedoraproject.org/pipermail/389-users/2009-January/008805.html

  LDAP and Unix shadow files use different form of base64, basically
  just another shuffle of the characters used.

  "A-Za-z0-9+/" --> "./0-9A-Za-z"


Base32

  Case insensitive encoding.  5 binary characters as 8 printable characters

     A-Z 0-7   and a pad of = (needing either 6,4,3,1 such pads)


Base16

Case insensitive encoding for binary data
Essentually it is just simply 'hex' encoding (using capitals)

  You can find words which are also hexadecimal number in the dictionary.
  For example...
    egrep '^[a-f]{4}$' /usr/share/dict/words

    deaf decaf feed beef decade faded defaced facade

  You generally want a sting of 32 such characters

ModHex

   A variation on Base16, designed for USB keyboard emulation.
   See "yubikey" information.

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Programs available to handle.

Base64 Encoding...

  mmencode file               # metamail package
  uuencode -m -               # sharutils package (base64 option)
  gmime-uuencode --base64     # Gmime package
  perl -ne 'use MIME::Base64; print encode_base64($_);' file

  b64encode                   # personal perl script (no wrapper)
  openssl enc -base64         # no wrapper - multiple input/output methods


Base64 Decoding

  mmencode -u mime_file
  uudecode                    # wrap with begin-base64
  gmime-uudecode --base64
  perl -ne 'use MIME::Base64; print decode_base64($_);' mime_file

  b64decode                   # personal perl script (no wrapper)
  openssl enc -base64 -d      # no wrapper - multiple input/output methods


  See also this file from EncFS (ascii to base65 indexes then to binary)
    http://code.google.com/p/encfs/source/browse/trunk/encfs/base64.cpp


Most of these packages add a start/end wrapper during encoding
and also needs the same wrapper around the data for decoding.
EG:

  begin-base64 600 -
  cGFzc3dkCg==
  ====

For Example remove and re-add the wrapper

   > echo passwd | uuencode -m - | sed -n '1d;/^====$/!p' > secret

   > cat secret
   cGFzc3dkCg==

   > ( echo 'begin-base64 600 -'; cat secret; echo '===='; ) | uudecode
   passwd

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Perl and Base64

NOTE: as base64 is the same a uuencoding except that length character at the
start of the lines has been removed and the characters have been replaced
with alphanumerics instead of puncuation and space characters.

   EG: and perl4 decoder...
     perl -ne ' tr#A-Za-z0-9+/`##cd;
                tr#A-Za-z0-9+/`# -_=#;
                $len = pack("c", 32 + 0.75*length);
                print unpack("u", $len.$_ );'          mime_file

The final length of the base64 encoded sequence uses = signs at the end to
zero out unwanted characters.  This makes it difficult for a simple
base64 encoder, but it is close to..

     perl -e 'while(read(STDIN,$_,45)) {
                $_=substr(pack("u",$_),1);
                tr# -_`#A-Za-z0-9+/A#;
                print $_; }'      file
With any 'A' characters on the end converted to '=' characters instead.

The perl package "MIME::Base64" provides better and easier methods for
conversion between base64 and Binary.  For example...

  #!/usr/bin/perl
  use MIME::Base64;
  my $passwd = 'cGFzc3dkCg==';
  my $decoded = &decode_base64($passwd);
  print "$decoded\n";

Would return

  passwd

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