Copy a part of NSData byte array to another NSData type

Question

You can use NSData's -(NSData *)subdataWithRange:(NSRange)range; to do that.
From your example, here is some code :

// original data in myData
NSData *d1 = [myData subdataWithRange:NSMakeRange(0, 20)];
NSData *d2 = [myData subdataWithRange:NSMakeRange(20, 80)];

Of course, the ranges are immediate here, you will probably have to do calculations, to make it work for your actual code.

Swift 3

let subdata1 = data?.subdata(in: 0..<20)
let subdata2 = data?.subdata(in: 20..<80)

Due to this is question is in very top of Google Search I wanna write here an example for swift

 NSData *mainData = /*This is you actual Data*/

 NSData *fPart = [mainData subdataWithRange:NSMakeRange(0, 20)];
 NSData *sPart = [mainData subdataWithRange:NSMakeRange(20, 80)];

Instead 80 you can use some dynamic - like data length