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''||''how''||''who could do it''||''what could they target''||''what crypto property prevents it''||''how expensive to brute force''||
||1||shape-shifter immutable file [footnote 1]||collide read-cap (''R'',''T'')||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^||
||2||unauthorized read||attack the encryption of ''K'' with ''R''||anyone||any one file||the cipher's security and the secrecy of the read-key ''R''||2^''r''^||
||3||forgery of immutable file||generate a matching read-cap (''R'',''T'') for someone else's file||anyone||any one file||the hash function's and cap format's second-pre-image resistance on (''R'',''T'')||2^''r''+''t''^||
||4||roadblock or speedbump [footnote 2]||generate (''V'',''K'',''D'') which hash to a someone else's ''T'', and copy their ''S''||anyone||any one file||the hash function's and cap format's collision resistance on ''T''||2^''t''^||
||5||unauthorized read||attack the encryption of the plaintext with ''K''||anyone||any one file||the cipher's security and the secrecy of the encryption key ''K''||2^''k''^||
||6||unauthorized read||figure out the input to the hash function that generates ''S''||anyone||any one file||the hash function's pre-image resistance on ''S''||brute force attack on ''R'' is !#2||

1. ''shape-shifter immutable file'': creator creates more than one file matching the immutable file readcap
2. ''roadlblock'': attacker prevents uploader (including repairer) from being able to write a real share into the right storage index; ''speedbump'': attacker adds his bogus share into the list of shares stored under the storage index by the same method; downloader has to download, examine, and discard the bogus (''V'',''K'',''D'')'s until it finds the real one

http://allmydata.org/pipermail/tahoe-dev/2009-October/002959.html