I’ve built a candidate fix for the RNCryptor HMAC vulnerability. It’s on a branch (issue44) currently, but I’ll merge it to master next week if no one sees problems, and update the docs. By default it is incompatible with v1 files. If you need to be able to decrypt v1 files, you can define RNCRYPTOR_ALLOW_V1_BAD_HMAC when building.
Please let me know if anyone encounters issues.
Thank you to Allen Mann for discovering the following significant security vulnerability in RNCryptor.
RNCryptor computes the HMAC over the ciphertext only. It does not compute an HMAC over the header, particularly the IV. This allows an attacker to modify the IV and thus manipulate the first block (16-bytes) of decrypted plaintext. This is a major issue, and is exactly the kind of problem that RNCryptor is designed to avoid.
I will be modifying RNCryptor shortly to fix this. This will rev the file version to 2. I will provide a backward-compatibly option to read v1 files (default off). I strongly recommend that developers move to the v2 format.
I will try to get this out as quickly as I can, hopefully this week.
During CocoaConf-2012-Raleigh, I discussed my PinchView from Laying out text with Core Text. It’s a text view that squeezes the glyphs towards your finger when you touch it. I built it to demonstrate per-glyph layout in Core Text. While demonstrating it, I was pretty unsatisfied with how it looked when you touched it or let go. When you drag your finger on the view, the glyphs move around like water. It’s quite pretty. But when you initially touch the screen, the glyphs suddenly jump to their new locations, and then they jump back when you release the screen. Well, that’s no good. So I wanted to add animations.
But here’s the thing: what do you animate? While you do want to animate the glyph positions, you’re not doing it directly. The location of each glyph is dependent on the location of the current touch. What you want to animate is how much the touch impacts the glyph positions. A quick look over CALayer’s list of animatable properties confirmed that there’s nothing like that. But no problem, I added a custom property called
touchPointScale and animated that. (I cover animating custom properties in the Layers chapter of iOS:PTL, and I still have to pull out that chapter every time to remind myself how to do it. Ole Begemann has a good, quick writeup on Stack Overflow.)
OK, so great. But one comment I got at CocoaConf was that it should handle multitouch. So I started playing with that, but now I had a problem. I could have lots of touches, so my single
touchPointScale doesn’t…er…scale. What I want to do is take a collection of
TouchPoint objects that the layer owns, animate each of their scales independently, and have the layer do its animation thing. But how do we animate based on changes in properties of things in a layer’s collection?
The pain of maintaining this thing has gotten to be too much, though. I’m releasing it today on GitHub in its current form, which is based on the older, synchronous form of RNCryptor. I may not have updated all the license text yet; if I miss any, it is under the MIT license. Thanks to those who purchased Cryptor-Titanium during its commercial life. Anyone who is interested in continuing development, please contact me (or submit a pull request).
There was an interesting question on StackOverflow that was unfortunately closed as off topic. It was off topic, but it’s still a useful question. When phone screening potential Cocoa developers, what kinds of questions should you ask? I’ve helped several clients screen potential candidates over the years, and so I have several questions I use to help with that.
I know this exists out there somewhere already, but I couldn’t find it anywhere and I was sick of writing it over and over again…. If someone knows of previous art, please point me in the right direction. I know Fiery Robot’s and Mike Ash’s, but they solve different problems.
Have you ever noticed how hard it is to write a repeating
NSTimer that doesn’t create a retain loop? Almost always you wind up with something like this:
Seems easy enough, except it’s a retain loop. Mike Ash does a nice job of explaining it and walking you through the hoops you need to avoid it. For such a common thing, you’d think this would be easy. And it should be, so I fixed it. I just still can’t quite believe I’m the first to do so.
Anyway, for your consideration I present a very simple class called
RNTimer. Right now it just handles the most common case: a repeating timer that does not generate a retain loop and automatically invalidates when it is released. It could of course be expanded to handle more NSTimer functionality if there is interest. Let me know if you have a use case that the current implementation doesn’t address.
You may find it along with further information at GitHub.
Last night’s CocoaHeads was a blast. Alondo, you have to come give an intermediate/advanced talk on Storyboards. Thanks a lot to @scottpenrose for his lightning and thunder, and @flightblog for his inspiration (METAR gave me a lot of headaches, too). And of course, thanks especially to Josh for organizing everything. And a new place for NSCoder? You rock.
For those who didn’t scribble notes fast enough, my Core Foundation presentation is attached (including the fix for my memory leak; was it Jay who found that?) I’ll make sure to fix the code in the book for the next edition.
Building a Core Foundation.pdf
When last we looked at Bézier curve calculations, we were able to calculate five million points in about 0.6s (~8.3Mp/s or megapoints-per-second). That’s 1000 points per curve, 100 curves, at 50fps. That was 5x faster than the original
-Os optimized function. But we’re just getting warmed up. We haven’t yet gotten half of the performance available.
[If you want the answer to last time's homework, skip to the end.]
So you want to hand-calculate Bézier curves. Good for you. It comes up more often then you’d think on iOS, even though
UIBezierPath is supposed to do it all for you. The truth is, sometimes you need the numbers yourself. For instance if you want to calculate intersections, or you want to draw text along the curve (like in CurvyText from iOS:PTL chapter 18).
The most up-to-date source for this is now available at github.
Your programs need to deal gracefully with being offline. Mugunth Kumar has built an excellent toolkit that manages REST connections while offline called MKNetworkKit, and Chapter 17 of our book is devoted to the ins-and-outs of this subject.
But sometimes you just have a simple
UIWebView, and you want to cache the last version of the page. You’d think that
NSURLCache would handle this for you, but it’s much more complicated than that.
NSURLCache doesn’t cache everything you’d think it would. Sometimes this is because of Apple’s decisions in order to save space. Just as often, however, it’s because the HTTP caching rules explicitly prevent caching a particular resource.
What I wanted was a simple mechanism for the following case:
- You have a UIWebView that points to a website with embedded images
- When you’re online, you want the normal caching algorithms (nothing fancy)
- When you’re offline, you want to show the last version of the page
My test case was simple: a webview that loads cnn.com (a nice complicated webpage with lots of images). Run it once. Quit. Turn off the network. Run it again. CNN should display.