Lots been written about this recently and this is from Professor Matthew Greens blog, him and his students developed the attack:
A Few Thoughts on Cryptographic Engineering: Attack of the Week: Apple iMessage
Then today I saw this on Bruce Schneiers blog which was written back in November and while it doesn’t mention iMessage, it’s obviously about that.
https://www.schneier.com/blog/archives/2016/03/cryptography_is.html
