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Multipurpose Internet Mail Extensions

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This page only lets you imagine what MIME is.  Read RFC 2045 through 2049 for first-hand information.


What is it?

MIME is the most popular method used by e-mail clients to re-encode anything in a mail-safe 7-bit format.  It has two encoding forms:


Quoted Printable

These characters can be directly encoded:

     ABCDEFGHIJKLMNOPQRSTUVWXYZ
     abcdefghijklmnopqrstuvwxyz
     0123456789
     '()+,-./:?

Any byte may be represented by an equal sign followed by the byte's hexadecimal value (e.g. an equal sign may be represented as "=3D") but a line break should remain a line break.

Encoded lines should contain no more than 76 characters.  An equal sign at the end of an encoded line indicates line continuation.

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Base64

Turn any sequence of three bytes into a four-byte sequence using the following Base64 alphabet:

     ABCDEFGHIJKLMNOPQRSTUVWXYZ
     abcdefghijklmnopqrstuvwxyz
     0123456789+/

Say you want to encode "ABC", represented in binary form as

     01000001 01000010 01000011

Turn the three bytes into four 6-bit segments

     010000 010100 001001 000011

and spell them out as QUJD using the following table:

     000000 -> A     001101 -> N     011010 -> a     100111 -> n     110100 -> 0
     000001 -> B     001110 -> O     011011 -> b     101000 -> o     110101 -> 1
     000010 -> C     001111 -> P     011100 -> c     101001 -> p     110110 -> 2
     000011 -> D     010000 -> Q     011101 -> d     101010 -> q     110111 -> 3
     000100 -> E     010001 -> R     011110 -> e     101011 -> r     111000 -> 4
     000101 -> F     010010 -> S     011111 -> f     101100 -> s     111001 -> 5
     000110 -> G     010011 -> T     100000 -> g     101101 -> t     111010 -> 6
     000111 -> H     010100 -> U     100001 -> h     101110 -> u     111011 -> 7
     001000 -> I     010101 -> V     100010 -> i     101111 -> v     111100 -> 8
     001001 -> J     010110 -> W     100011 -> j     110000 -> w     111101 -> 9
     001010 -> K     010111 -> X     100100 -> k     110001 -> x     111110 -> +
     001011 -> L     011000 -> Y     100101 -> l     110010 -> y     111111 -> /
     001100 -> M     011001 -> Z     100110 -> m     110011 -> z

If there is only 1 byte at the end, add 4 zero bits, spell out the 12 bits and signal the 4 extra bits with two equal signs (==).  If there are 2 bytes left, add 2 zero bits, spell out the 18 bits and signal the 2 extra bits with one equal sign (=).

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Header Fields

This is how you transmit an UTF-8 encoded smiley in the subject field using the Base64 alphabet:

=?utf-8?B?4pi6?=
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Source

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Gyula Zsigri [CJK]  [Home] June 11, 2002