[...]
Now consider section 2.7 (definition of 7bit data), second sentence:
"No octets with decimal values greater than 127 are allowed and neither are
NULs (octets with decimal value 0). "
By this definition, 7bit data must not include NUL data, that is an octet with decimal value 0.
Now, if we look at the Base 64 vocabulary, we can see that a value of 52 is
encoded as 0, which is in opposition to the sentence above.
There is no problem once you understand that the meaning of "0" is contextual.
The first "0" is binary zero.
The second "0" is digit zero, a character representing the digit "0".
The concept of Base64 is very simple: map a bit stream on a character stream
1) The character set is equal to the regular expresion [A-Za-z0-9+/]
This is A to Z, a to z, 0 to 9, + or /
Total number of characters: 26 + 26 + 10 + 2 = 64
2) Split the binary stream up in 6 bit chunks and map each chunk on a member of character set 1
Total number of characters: 2 ** 6 = 64
As this is a 1 to 1 mapping we can reverse the process and decode to get the original bit stream.
The other stuff about identity, 7 or 8 bits is how the stream of characters enter a mail server or any other server for that part.
That's it :)
-- John Zoetebier Web site: http://www.transparent.co.nz
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