Lua

MIME Content-Transfer-Encoding Header Field

See more MIME Examples

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

Lua

local chilkat = require("chilkat")

local success = false

-- 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.

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

local mime = chilkat.Mime.new()
mime.content_type = "image/jpeg"
mime:set_body_from_encoded("base64", sbJpgBase64:get_as_string())

print(mime:get_mime())
print("-")

-- 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.
mime.encoding = "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.
print(mime:get_mime())
print("-")

-- We CAN get the binary MIME as bytes..

mimeBytes = ERROR: Method not in Chilkat for Lua: Mime.GetMimeBytes
mime:ERROR()

-- Regardless of the Content-Transfer-Encoding, the
-- body content can always be retrieved and the body bytes
-- decoded from whatever encoding is used..

jpgBytes = ERROR: Method not in Chilkat for Lua: Mime.GetBodyBinary
mime:ERROR()

-- To get the body in base64 format, first make sure
-- the Content-Transfer-Encoding is base64, then call GetBodyEncoded.
mime.encoding = "base64"
local jpgBase64 = mime:get_body_encoded()
print(jpgBase64)
print("-")

-- Let's go back to "binary" MIME..
mime.encoding = "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.
mime:convert8_bit()
local mimeStr = mime:get_mime()
print(mimeStr)
print("-")