[mnet-devel] Re: [web-calculus] YURLs

[Zooko]

[Removing Cc:'s except Trevor Perrin and mnet-devel.]

 Trevor Perrin wrote:
>
> it looks like a bad symmetric key will cause the inode to decrypt 
> improperly, so the code that parses the inode will probably complain that 
> it's gibberish.

I agree.  This is doubly true for inodes which contain blockIds -- for the 
file to finish reconstruction would require that the inode decrypt into 
something containing real blockIds that map to real blocks!

> It might be better to catch this error before parsing - if 
> the inodeId was an HMAC-SHA1 of the encrypted inode and the encryption key, 
> I think that would work.

Hm.  The problem with that is that servers store the encrypted inode block, 
indexed by the inodeId (== SHA1(encryptedinode)), and the servers are not 
allowed to know the encryption key.

Therefore, we need the mapping between encryptedinode and inodeId to be 
verifiable without knowledge of the key (so that servers can verify the 
validity of blockId->block mappings for the blocks they store).

I agree that it would be better to explicitly include some information to 
double-check the encryption key.  Then I could say "If the file reconstructs 
without an error, then either the result is the original file or an attacker 
has found a collision in SHA1.".

As it is, I can only say "If the file reconstructs without an error, then 
either the result is the original file, or an attacker has found a collision 
in SHA1, or an attacker has found an AES key which decrypts this ciphertext 
into some plaintext which has at least 64 bits of his choosing.".

I really don't know how plausible is that last threat (the funny AES key).

So here is a proposal -- the last 20 bytes of the inode are the HMAC of the 
rest of the inode with the symmetric key.  This is the simplest solution 
I can think of which preserves our requirements for mnet URIs and which allows 
us once again to say "Either the resulting file is the original file or an 
attacker has found a collision in SHA1.".

Regards,

Zooko



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