Reading Base64 ...

This is the first article in an Internet Protocol/encoding Series...As developers we will at one time or another need to read base64 encoded data....heres my take...

Base64 is used to encode binary files in a text format suitable to be encapsulated in text documents. One interesting thing about base 64 is that it is one of the only data representation
schemes in modern use that doesn’t encrypt or compress the data. It just represents data
in a different form.

The solution to this problem was to create a data representation protocol that would allow
binary data to be encapsulated within another document. The defacto standard for doing
this is called base 64. In fact, some applications use base 64 as an encryption technique,
albeit a weak one. Basic authentication over the Internet encrypts the username-password
using base 64.

Here is a simple base64 decoder algorithm that has been very useful over the years...

base64decode PROCEDURE(STRING b64)

b64chars STRING('ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/')

oString ANY

CODE

oString = '' !Dont use CLEAR it will cause a GPF on an ANY Variable

! Base64 strings especially those from emails can contain carriage returns and tabs these must be removed before decoding....

b64 = StringReplace(b64,CHR(9),'')
b64 = StringReplace(b64,CHR(10),'')
b64 = StringReplace(b64,CHR(13),'')

! Base64 strings are groups of four characters and need to be stepped through by 4....

LOOP i# = 1 TO LEN(CLIP(b64)) BY 4

!Create a 24 bit Number by bit shifting the four chars values by  6 bit segments...

BigNumber# = BSHIFT(INSTRING(b64[i#],b64chars,1,1)-1,18) + BSHIFT(INSTRING(b64[i#+1],b64chars,1,1)-1,12) + BSHIFT(INSTRING(b64[i#+2],b64chars,1,1)-1,6) + (INSTRING(b64[i#+3],b64chars,1,1)-1)

!Extract the original 3 ascii chars by bit shifting in the other direction by 8 bit segments...

Char1# = BAND(BSHIFT(BigNumber#,-16),11111111b)
Char2# = BAND(BSHIFT(BigNumber#,-8),11111111b)
Char3# = BAND(BigNumber#,11111111b)

!handle trailing padding characters thus...and translate the character values...
IF b64[i#+3] = '=' AND b64[i#+2] ~= '=' THEN
oString = oString & CHR(Char1#)&CHR(Char2#)
ELSIF b64[i#+3] = '=' AND b64[i#+2] = '=' THEN
oString = oString & CHR(Char1#)
ELSE
oString = oString & CHR(Char1#)&CHR(Char2#)&CHR(Char3#)
END
!repeat operation for each group of four base64 chars

END

RETURN CLIP(oString)

The result is the unencoded text or binary data....for more info on base64 like a encoding checkout RFC 3548...