(Tcl) MIME Content-Transfer-Encoding Header Field

Explains the Content-Transfer-Encoding header field and how it affects how data is stored in the MIME.

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load ./chilkat.dll

# The Content-Transfer-Encoding header field is typically set to one of these values:
# 
#   base64
#   quoted-printable
#   binary
#   8bit
#   7bit

# If the encoding is base64 or quoted-printable, then the bytes of the content are encoded as such.
# The values "8bit", "7bit", and "binary" all imply that NO encoding has been performed.

# Here are some guidelines and hints:
# 
# 1) "8bit", "7bit", and "binary" encodings (i.e. NO encoding) results in the smallest MIME.

# 2) A "binary" encoding implies non-text data.  Binary MIME CANNOT be retrieved as a string.  It can only be retrieved as a byte array.

# 3) "8bit" and "7bit" encodings imply text.  
#    7bit implies that the text is us-ascii (all byte values less than or equal to 0x7F).
#    8bit implies that the text contains non-us-ascii chars.

# 4) Base64 is the best choice for encoding non-text data, such as PDF's, images, etc.

# 5) quoted-printable is the best choice for encoding text data where most chars are going to be us-ascii. This would typically 
#    include all Western European languages.  For Asian, Arabic, Hebrew, etc. where most chars in the text are non-us-ascii,
#    the most efficient encoding would be base64.  

# Let's demonstrate with this small JPG image.

set sbJpgBase64 [new_CkStringBuilder]

CkStringBuilder_Append $sbJpgBase64 "/9j/4AAQSkZJRgABAQEASABIAAD//gAmRmlsZSB3cml0dGVuIGJ5IEFkb2JlIFBob3Rvc2hvcD8g\r\n"
CkStringBuilder_Append $sbJpgBase64 "NC4w/9sAQwAQCwwODAoQDg0OEhEQExgoGhgWFhgxIyUdKDozPTw5Mzg3QEhcTkBEV0U3OFBtUVdf\r\n"
CkStringBuilder_Append $sbJpgBase64 "YmdoZz5NcXlwZHhcZWdj/9sAQwEREhIYFRgvGhovY0I4QmNjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj\r\n"
CkStringBuilder_Append $sbJpgBase64 "Y2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj/8IAEQgAFAAUAwERAAIRAQMRAf/EABcAAAMBAAAA\r\n"
CkStringBuilder_Append $sbJpgBase64 "AAAAAAAAAAAAAAIDBAX/xAAYAQADAQEAAAAAAAAAAAAAAAABAgMEAP/aAAwDAQACEAMQAAAB2kZY\r\n"
CkStringBuilder_Append $sbJpgBase64 "NNEijWKddfTmLgALWH//xAAbEAACAgMBAAAAAAAAAAAAAAABAgMRAAQSE//aAAgBAQABBQL0XqN+\r\n"
CkStringBuilder_Append $sbJpgBase64 "pM2aqJGMiqFFCyg7z//EABwRAAICAgMAAAAAAAAAAAAAAAERAAIQIQMSUf/aAAgBAwEBPwHqU5aq\r\n"
CkStringBuilder_Append $sbJpgBase64 "Axx+y1tMQl4elj//xAAcEQEAAQUBAQAAAAAAAAAAAAABEQACEBIhA1H/2gAIAQIBAT8B3Bhqy7Zc\r\n"
CkStringBuilder_Append $sbJpgBase64 "enyiwmGgDhiOzj//xAAdEAABAwUBAAAAAAAAAAAAAAABAAIREBIhIkFR/9oACAEBAAY/ArZyn+Cg\r\n"
CkStringBuilder_Append $sbJpgBase64 "xtxWuJaoCnqDuin/xAAcEAABBAMBAAAAAAAAAAAAAAABABEhYRAxQVH/2gAIAQEAAT8hkEwPUUR9\r\n"
CkStringBuilder_Append $sbJpgBase64 "DYfE4nxtRpIkBTsayuALIiuY/9oADAMBAAIAAwAAABDWPTsf/8QAGhEAAwADAQAAAAAAAAAAAAAA\r\n"
CkStringBuilder_Append $sbJpgBase64 "AAEREDFBIf/aAAgBAwEBPxC0DVPcWm+Ce4OesrkE6bjH/8QAGBEBAQEBAQAAAAAAAAAAAAAAAREA\r\n"
CkStringBuilder_Append $sbJpgBase64 "QRD/2gAIAQIBAT8QahMiOc8YgSrnTY3ELclHXn//xAAcEAEBAAIDAQEAAAAAAAAAAAABEQAhMUFx\r\n"
CkStringBuilder_Append $sbJpgBase64 "EFH/2gAIAQEAAT8Qn3igmSZSj+c4N4zapMy9IjFV98wncN2iuLFsCEbDGxQkI6RO/n//2Q==\r\n"

set mime [new_CkMime]

CkMime_put_ContentType $mime "image/jpeg"
CkMime_SetBodyFromEncoded $mime "base64" [CkStringBuilder_getAsString $sbJpgBase64]

puts [CkMime_getMime $mime]
puts "-"

# The result:
# 
# 	Content-Type: image/jpeg
# 	Content-Transfer-Encoding: base64
# 
# 	/9j/4AAQSkZJRgABAQEASABIAAD//gAmRmlsZSB3cml0dGVuIGJ5IEFkb2JlIFBob3Rvc2hvcD8g
# 	NC4w/9sAQwAQCwwODAoQDg0OEhEQExgoGhgWFhgxIyUdKDozPTw5Mzg3QEhcTkBEV0U3OFBtUVdf
# 	YmdoZz5NcXlwZHhcZWdj/9sAQwEREhIYFRgvGhovY0I4QmNjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj
# 	Y2NjY2NjY2NjY2NjY2NjY2NjY2NjY2NjY2Nj/8IAEQgAFAAUAwERAAIRAQMRAf/EABcAAAMBAAAA
# 	AAAAAAAAAAAAAAIDBAX/xAAYAQADAQEAAAAAAAAAAAAAAAABAgMEAP/aAAwDAQACEAMQAAAB2kZY
# 	NNEijWKddfTmLgALWH//xAAbEAACAgMBAAAAAAAAAAAAAAABAgMRAAQSE//aAAgBAQABBQL0XqN+
# 	pM2aqJGMiqFFCyg7z//EABwRAAICAgMAAAAAAAAAAAAAAAERAAIQIQMSUf/aAAgBAwEBPwHqU5aq
# 	Axx+y1tMQl4elj//xAAcEQEAAQUBAQAAAAAAAAAAAAABEQACEBIhA1H/2gAIAQIBAT8B3Bhqy7Zc
# 	enyiwmGgDhiOzj//xAAdEAABAwUBAAAAAAAAAAAAAAABAAIREBIhIkFR/9oACAEBAAY/ArZyn+Cg
# 	xtxWuJaoCnqDuin/xAAcEAABBAMBAAAAAAAAAAAAAAABABEhYRAxQVH/2gAIAQEAAT8hkEwPUUR9
# 	DYfE4nxtRpIkBTsayuALIiuY/9oADAMBAAIAAwAAABDWPTsf/8QAGhEAAwADAQAAAAAAAAAAAAAA
# 	AAEREDFBIf/aAAgBAwEBPxC0DVPcWm+Ce4OesrkE6bjH/8QAGBEBAQEBAQAAAAAAAAAAAAAAAREA
# 	QRD/2gAIAQIBAT8QahMiOc8YgSrnTY3ELclHXn//xAAcEAEBAAIDAQEAAAAAAAAAAAABEQAhMUFx
# 	EFH/2gAIAQEAAT8Qn3igmSZSj+c4N4zapMy9IjFV98wncN2iuLFsCEbDGxQkI6RO/n//2Q==
# 

# Set the Content-Transfer-Encoding to "binary" by setting the Encoding property.
CkMime_put_Encoding $mime "binary"

# Try to get the MIME string.  This CANNOT be done.  The bytes of the JPG image do not
# represent chars, and to return a string means that bytes must be interpreted according
# to some character encoding (such as utf-8).  Non-text binary bytes can only be 
# contained in a string IF encoded in some way. Encodings such as Base64, quoted-printable,
# URL, etc. exist to make it possible to represent binary data in string format.   
puts [CkMime_getMime $mime]
puts "-"

# We CAN get the binary MIME as bytes..
set mimeBytes [new_CkByteData]

set success [CkMime_GetMimeBytes $mime $mimeBytes]

# Regardless of the Content-Transfer-Encoding, the 
# body content can always be retrieved and the body bytes
# decoded from whatever encoding is used..
set jpgBytes [new_CkByteData]

set success [CkMime_GetBodyBinary $mime $jpgBytes]

# To get the body in base64 format, first make sure
# the Content-Transfer-Encoding is base64, then call GetBodyEncoded.
CkMime_put_Encoding $mime "base64"
set jpgBase64 [CkMime_getBodyEncoded $mime]
puts "$jpgBase64"
puts "-"

# Let's go back to "binary" MIME..
CkMime_put_Encoding $mime "binary"

# Let's say we have MIME, and it was loaded directly from a file, or from
# a byte array.  (It was not loaded from the contents of a string variable.)
# We don't know whether the MIME contains binary or 8bit encodings, and thus
# we dont' know if the MIME is safe to get as a string.
# The Convert8Bit method can be called to recursively traverse the MIME and set
# all 8bit or binary encodings to "base64".  This makes the MIME safe for storing in
# a string.
CkMime_Convert8Bit $mime
set mimeStr [CkMime_getMime $mime]
puts "$mimeStr"
puts "-"

delete_CkStringBuilder $sbJpgBase64
delete_CkMime $mime
delete_CkByteData $mimeBytes
delete_CkByteData $jpgBytes