diff --git a/NewCaps/WhatCouldGoWrong.md b/NewCaps/WhatCouldGoWrong.md index afa987a..0d51164 100644 --- a/NewCaps/WhatCouldGoWrong.md +++ b/NewCaps/WhatCouldGoWrong.md @@ -3,19 +3,19 @@ This is about What Could Go Wrong with the "Elk Point 2" immutable file caps: *S*|brute force on *R* is !#2| |7|unauthorized deletion|brute force KD|anyone|any one file|secrecy of *KD*|*p*/*N*.2^*d*^| |8|unauthorized deletion|figure out a working destroy key KD from Dhash|anyone|any one file|the hash function's preimage resistance on *Dhash*|*p*/*N*.2^min(*d*,*dh*)^| |9|denial of service|prevent access to servers holding sufficient shares (by controlling some of them, or by attacking them or the network)|anyone|any file|not prevented by crypto|not applicable| -|10|cause invalid share to verify|generate (*K1enc*,*Dhash*,*V*) that hash to someone else's (*T*,*U*), and copy their *S*|anyone|any one file|the hash function's second-preimage resistance on (*T*,*U*)|*p*/*N*.2^*t*+*u*^ [7]footnote| +|10|cause invalid share to verify|generate (*K1enc*,*Dhash*,*V*) that hash to someone else's (*T*,*U*), and copy their *S*|anyone|any one file|the hash function's second-preimage resistance on (*T*,*U*)|*p*/*N*.2^*t*+*u*^ [5]footnote| |11|undeletion [3]footnote|restore a deleted file's shares by controlling the relevant servers|anyone|any one file|not prevented by crypto|not applicable| -|12|undeletion [3]footnote|generate matching (*R*,*T*,*U*) for a deleted file|anyone|any one file|the hash function's and cap format's second-preimage resistance on (*R*,*T*,*U*)|*p*/*N*.2^*r*+*t*+*u*^ [7]footnote| +|12|undeletion [3]footnote|generate matching (*R*,*T*,*U*) for a deleted file|anyone|any one file|the hash function's and cap format's second-preimage resistance on (*R*,*T*,*U*)|*p*/*N*.2^*r*+*t*+*u*^ [5]footnote| |13|accidental collision|storage indices (*S1*,*T1*) and (*S2*,*T2*) collide accidentally|not applicable|any two files|approximately random distribution of hash function outputs|[4]footnote| where *k* = bitlength(*K1*), *r* = bitlength(*R*), *s* = bitlength(*S*), *t* = bitlength(*T*), *u* = bitlength(*U*), *d* = bitlength(*KD*), *dh* = bitlength(*Dhash*). @@ -33,8 +33,6 @@ where *k* = bitlength(*K1*), *r* = bitlength(*R*), *s* = bitlength(*S*), *t* = b 4. See the probability table at . The effective hash length is approximately min(*s*,*r*)+*t* bits. -5. Brute force costs assume a single-target attack that is expected to succeed with high probability. Costs will be lower for attacking multiple targets or for a lower success probability. (Should we give explicit formulae for this?) +5. On Merkle-Damgård hashes with an internal state that is the same size as the hash output (like SHA-256), there are better second-preimage attacks than brute force. See . The doubled "SHA-256d" construction used by Tahoe does not help here. This is not significant for roadblock/speedbump attacks because the internal state will be much larger than *t* bits, but it is significant for the other second-preimage attacks. 6. *roadblock*/*speedbump* attacks could be restricted to holders of a read cap by use of an extra signature, as in the Elk Point 3 design (diagram at for mutable files). - -7. On Merkle-Damgård hashes with an internal state that is the same size as the hash output (like SHA-256), there are better second-preimage attacks than brute force. See . The doubled "SHA-256d" construction used by Tahoe does not help here. This is not significant for roadblock/speedbump attacks because the internal state will be much larger than *t* bits, but it is significant for the other second-preimage attacks.