Base64 Encode and Decode

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Base64 Encode and Decode

• Student Ying Hong
• Course Database Security
• Instructor Dr. Yang

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Contents

• Introduction
• Algorithm
• Example
• Issues for Considerations
• Other Methods
• References

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Introduction

• The Base64 Content-Transfer-Encoding is a method
  to encode an arbitrary sequence of octets in a
  form that is unintelligible to unauthorized
  readers.
• Its used for MIME (Multipurpose Internet Mail
  Extensions) email and various other
  Internet-related applications.

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Algorithm

• Read inputs from left to right.
• Combine 3 8-bit input data to a 24-bit input
  group.
• Split each 24-bit input group to 4 6-bit units.
• If last group is not 24-bit, add zero bits to
  form 4 6-bit units.
• Translate each 6-bit input data into a single
  digit.
• The digit is used as an index into the base64
  alphabet table.

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Algorithm cont

• Represent each digit by its corresponding
  alphabet in the base64 alphabet table.
• Set the last several output characters (at most
  two) that do not represent actual input data to
  the character . That is, in the last group
  last one or two 6-bit units may be 0 you
  should either interpret it/them to A if they
  come from the plaintext, or set it/them to if
  they are added by ourselves.

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The Base64 Alphabet Table
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Example

• To encode www.cl.uh.edu
• Represent www.cl.uh.edu as its ASCII code
  number (in hexadecimal for easier to be
  represented as binary number later)
• 77 77 77 2E 63 6C 2E 75 68 2E 65 64 75.
• Represent it as binary number and combine 3 8-bit
  input data to a 24-bit group
• 01110111 01110111 01110111, 00101110 01100011
  01101100,

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Example cont

• Form 4 6-bit inputs in each 24-bit group
• 011101 110111 011101 110111, 001011 100110
  001101 101100,
• Attention
• in the last group is only 01110101(7510), so we
  need to add 0s to form 4 6-bit units, that is,
  011101 010000 000000 000000
• NOTE last 2 6-bit units dont represent actual
  input data
• Translate each 6-bit input into a single digit
  29 55 29 55, 11 38 13 44, 11 39 21 40, 11 38 21
  36, 29 16 0 0.

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Example cont

• Each digit is used as index into the base64
  alphabet table.
• Represent each digit as corresponding character
  in the base64 table
• 29 55 29 55, 11 38 13 44, 11 39 21 40, 11 38 21
  36, 29 16 0 0
• d 3 d 3 , L m N s , L n V o ,
  L m V T , d Q
• Attention
• Last two 0s are interpreted as , because
  they do NOT represent actual input data, they are
  added by ourselves.
• The final result of encoding www.cl.uh.edu is
  d3d3LmNsLnVoLmVTdQ.

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Issues for Considerations

• The encoded data are consistently about 33
  larger than the unencoded data.
• The output stream (encoded bytes) must be
  represented in lines of no more than 76
  characters each.

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Issues for Considerations cont

• All other characters (including line breaks) not
  found in the table must be ignored during
  decoding, since they probably indicate a
  transmission error, for which a warning message
  or a rejection message may be given under some
  circumstances.

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Issues for Considerations cont

• Careful decoding must be made when last group of
  input data is less than 24 bits, that is adding
  0s bits to form multiple of 6-bit unit and set
  output character that doesnt represent actual
  input data to the character .
• Three cases may occur in the last group of
  output
• inputs are 24 bits, outputs are 4 characters
  with no
• inputs are 16 bits, outputs are 3 characters
  with one
• inputs are 8 bits, outputs are 2 characters with
  two

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Issues for Considerations cont

• Care must be taken when directly encoding text
  material. That is, some special characters like
  line breaks should be converted into CRLF
  sequences prior to base64 encoding (that means
  you should be aware of both printable characters
  and those control code).

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Other Methods?? uuencode

• Uuencode is a set of algorithms that convert
  files to 7-bit ASCII characters, which can be
  transmitted over the Internet.
• Its especially popular for sending email
  attachments.
• Originally, uuencode stood for Unix-to-Unix
  encode, but it has become a universal protocol to
  transfer files between different platforms such
  as Unix, Windows, and Macintosh.

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Other Methods ?? Base85

• The Base85 encoding is used to encode IPv6
  address, which is treated as a 128-bit integer.
• The Base85 digits in ascending order are 0..9,
  A..Z, a..z, !, , , , , (, ), , ,-, , lt, ,
  gt, ?, _at_, , _, , , , , .

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Other Methods ?? Base85 cont

• Algorithm
• Convert address to decimal number.
• 10800008800200C417A (Hex.)
• 21932261930451111902915077091070067066 (Dec.)
• Divide this number by 85 and take the remainder,
  then continue to divide the quotient by 85 and
  take the remainder, and so on.
• The remainders are 51, 34, 65, 57, 58, 0, 75,
  53, 37,4, 19, 61, 31, 63, 12, 66, 46, 70, 68, 4.
• Reverse the order of the remainders.
• 4, 68, 70, 46, 66, 12, 63, 31, 61, 19, 4, 37,
  53, 75, 0, 58, 57, 65, 34, 51.

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Other Methods ?? Base85 cont

• Algorithm (cont)
• Use the numbers (reversed remainders) as index
  into the Base85 alphabet table to get the
  corresponding characters.
• 4, 68, 70, 46, 66, 12, 63, 31, 61, 19,
  (remainders)
• 4, ), , k, , C, , V, z, J,
  (encoded in Base85)
• 4, 37, 53, 75, 0, 58, 57, 65, 34, 51.
  (remainders)
• 4, b, r, gt, 0, w, v, , Y, p.
  (encoded in Base85)
• Final result is 4)kCVzJ4brgt0wvYp

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Other Methods ?? Base85 cont

• Benefits
• Reduce IPv6 address length when encoding it in
  textual format, that is from 39 characters to 20
  characters.
• Encode IPv6 address to a fixed-length
  representation.
• Its very hard to be decoded by any casual
  observer, so its more secure.
• Other issues
• So far many current processors dont have 128-bit
  arithmetic. Future processors should address this
  defect before any significant IPv6 deployment has
  been accomplished.

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References

• http//www.python.org/doc/current/lib/module-base6
  4.html (Base64)
• RFC 1521 (Base64)
• http//www.webopedia.com/TERM/U/Uuencode.html
  (uuencode)
• RFC 1924 (Base85)