Objective-C

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.

Objective-C

#import <CkoStringBuilder.h>
#import <CkoMime.h>
#import <NSString.h>

BOOL success = NO;

//  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.

CkoStringBuilder *sbJpgBase64 = [[CkoStringBuilder alloc] init];
[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"];

CkoMime *mime = [[CkoMime alloc] init];
mime.ContentType = @"image/jpeg";
[mime SetBodyFromEncoded: @"base64" str: [sbJpgBase64 GetAsString]];

NSLog(@"%@",[mime GetMime]);
NSLog(@"%@",@"-");

//  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.   
NSLog(@"%@",[mime GetMime]);
NSLog(@"%@",@"-");

//  We CAN get the binary MIME as bytes..
NSData mimeBytes;
mimeBytes = [mime GetMimeBytes];

//  Regardless of the Content-Transfer-Encoding, the 
//  body content can always be retrieved and the body bytes
//  decoded from whatever encoding is used..
NSData jpgBytes;
jpgBytes = [mime GetBodyBinary];

//  To get the body in base64 format, first make sure
//  the Content-Transfer-Encoding is base64, then call GetBodyEncoded.
mime.Encoding = @"base64";
NSString *jpgBase64 = [mime GetBodyEncoded];
NSLog(@"%@",jpgBase64);
NSLog(@"%@",@"-");

//  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 Convert8Bit];
NSString *mimeStr = [mime GetMime];
NSLog(@"%@",mimeStr);
NSLog(@"%@",@"-");