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''||2^''r''^||
||forgery of immutable file||anyone||any file||the hash function's and cap format's second-pre-image resistance on (''R'',''T'')||2^''r''+''t''^||

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