base64 decoding

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base64 decoding

Dr. Rolf Jansen
I use Free Pascal for programming a small utility for two quite busy  
Mac OS X mail server (Postfix|CRM114|ANOMY|Cyrus) which prepares and  
preselects incoming mails (filtering or leraning) for the statistical  
mail filter CRM114.

Today, I managed to use the fcl base64 classes for decoding  
respective parts of the piped through e-mails. However, it took me  
some time to get it to work, and I have still some doubts if the  
base64 decoding class is stable enough for unattended operation on a  
busy mail server.

Possibly somebody has more experience with the fcl base64 classes  
than I have, and hopefully can destroy my doubts.


Base64 encoded parts of e-mails usually include line breaks (LF at  
Mac OS X Server) every 76 characters. When I write these LF into the  
TMemoryStream which is passed to TBase64DecodingStream, then the  
decoder freezes during the read beyond the first line break. If I  
pass through CRLF or only CR instead of LF then it freezes also - it  
seems that the decoder is caught into an endless loop. The obvious  
workaround is not to write the line breaks into the TMemoryStream.

Actually, I check whether the input char belongs to the base64  
character set before it is written to the TMemoryStream - also CR/LF  
is filtered out by this way.

   if ch in ['A'..'Z', 'a'..'z', '0'..'9', '+', '/', '='] then
      base64Stream.WriteByte(Char(ch));

My question here is, whether this catches every malformed base64  
code, so that the decoder does not freeze.


Another problem was, that TBase64DecodingStream.EOF() does not  
indicate properly the end of the decoded stream. Instead, I was  
forced to use the "try read() except on EStreamError" sequence for  
the eof indication.

My question here is, whether this works in any case. Again, I am a  
little bit afraid, that the decoder might be caught into an endless  
loop.

Many thanks for any helpful comments.

Best regards

Rolf

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Re: base64 decoding

Vinzent Höfler
On Wednesday 02 November 2005 22:06, Dr. Rolf Jansen wrote:

> Today, I managed to use the fcl base64 classes for decoding
> respective parts of the piped through e-mails. However, it took me
> some time to get it to work, and I have still some doubts if the
> base64 decoding class is stable enough for unattended operation on a
> busy mail server.

Sort of the same problem struck me yesterday, too, but with streams
which are perfectly valid containing only base64-chars.

// somehow the current implementation of base64 decoding stream is
// broken,  it sometimes raise StreamReadError instead of decoding
// the  values  correctly  (if  you feed $000102...7A7B7C into it,
// it  stops  just  before decoding $7C, or try feeding a constant
// stream  of  $7D  into  it,  it  will  stop after the first byte
// decoded)

Because I need this for XML-RPC server/client stuff, and thus I need the
HTTP-capabilities of it anyway, I (temporarily?) switched to Synapse's
SynaCode unit. AFAICS it doesn't have any problems with invalid base64
chars, it simply ignores them. Lukas, correct me if I'm wrong.

Currently I can't neither file a bug report nor check the current svn if
it's fixed already, because I don't have access to any of
*.freepascal.org. :-( Someone's blocking me and I haven't figured out
why and who yet.


HTH,

Vinzent.

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