Escaping CRLF in HTTP multipart/form-data content type (iOS)?

This is an interesting question. Looking at the multipart media type RFC it appears that it is up to the composing agent to make sure that the boundary does not appear in the encapsulated data. In addition, it states the following.

Up vote 4 down vote favorite 1 share g+ share fb share tw.

I got a code piece on the web and I think it might be wrong: NSMutableURLRequest* req = NSMutableURLRequest requestWithURL: url; req setHTTPMethod: @"POST"; NSString* contentType = @"multipart/form-data, boundary=AaB03x"; req setValue:contentType forHTTPHeaderField: @"Content-type"; NSData* boundary = @"\r\n--AaB03x\r\n" dataUsingEncoding:NSUTF8StringEncoding; NSMutableData *postBody = NSMutableData data; postBody appendData: boundary; postBody appendData: @"Content-Disposition: form-data; name=\"datafile\"; filename=\"t. Jpg\"" dataUsingEncoding:NSUTF8StringEncoding; postBody appendData: @"Content-Type: image/jpeg\r\n\r\n" dataUsingEncoding:NSUTF8StringEncoding; postBody appendData: imageData; postBody appendData: boundary; req setHTTPBody:postBody; This is wrong because imageData might contain \r\n sequences, right? If so, is there a way to escape CRLFs in raw data?

Or am I missing something? Thanks in advance! Iphone http ios multipartform-data link|improve this question asked Jan 20 '11 at 18:59K J768 71% accept rate.

This is an interesting question. Looking at the multipart media type RFC it appears that it is up to the composing agent to make sure that the boundary does not appear in the encapsulated data. In addition, it states the following: NOTE: Because boundary delimiters must not appear in the body parts being encapsulated, a user agent must exercise care to choose a unique boundary parameter value.

The boundary parameter value in the example above could have been the result of an algorithm designed to produce boundary delimiters with a very low probability of already existing in the data to be encapsulated without having to prescan the data. I interpret this to mean that in order to be sure that the boundary value doesn't appear in the encapsulated data, you would have to scan the data for the boundary value. Because this is an unacceptably expensive operation in most cases, it's expected that user agents will simply choose a value that has a very low probability of occurring in the data.

Consider the probability of the boundary in your example occurring in a random string of bytes (which for the sake of argument, we will assume represents a JPEG image). The full string that would need to be matched in order to end your image data early would be "\r\n--AaB03x" - 10 bytes, or 80 bits. Starting from any bit, the chance that the next 10 bytes are that sequence is one in 2^80.

In a 1MB JPEG file, there are 2^23 bits. This means that the chance of a JPEG file containing the sequence is less than 2^23/2^80, or one in 2^57 (more than one hundred quadrillion). So, I think the answer is that to be 100% sure, you would have to check the data for the boundary sequence, and then use a different one if that boundary sequence exists in the data.

But in practice, the chances of the boundary sequence occurring are small enough that it's not worth it.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.