This is about What Could Go Wrong with the "Elk Point 2" immutable file caps: ​http://jacaranda.org/tahoe/immutable-elkpoint-2.svg

what bad thing could happen	who could do it	what could they target	what crypto property prevents it	how expensive to brute force
shape-shifter immutable file [footnote 1]	creator of a file	their own file	the hash function's and cap format's collision resistance on the read-cap (R,T)	2(r+t)/2
unauthorized read	anyone	any file	the cipher's security and the secrecy of the read-key R	2r
forgery of immutable file	anyone	any file	the hash function's and cap format's second-pre-image resistance on (R,T)	2r+t

1. shape-shifter immutable file: creator creates more than one file matching the immutable file readcap