From: Alan Reiner <etotheipi@gmail•com>
To: Mark Friedenbach <mark@monetize•io>
Cc: bitcoin-development@lists•sourceforge.net
Subject: Re: [Bitcoin-development] Ultimate Blockchain Compression w/ trust-free lite node
Date: Tue, 19 Jun 2012 14:30:16 -0400 [thread overview]
Message-ID: <4FE0C538.3090001@gmail.com> (raw)
In-Reply-To: <CACh7GpEehHFEJGRTtijgM7UAa2jeEWRKrQo5dym8F_YgXAEhFA@mail.gmail.com>
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On 06/19/2012 02:18 PM, Mark Friedenbach wrote:
> On Tue, Jun 19, 2012 at 10:33 AM, Alan Reiner <etotheipi@gmail•com
> <mailto:etotheipi@gmail•com>> wrote:
>
> If we were to use a raw trie structure, then we'd have all the above
> issues solved: a trie has the same configuration no matter how
> elements
> are inserted or deleted, and accesses to elements in the tree are
> constant time -- O(1). There is no such thing as an unbalanced trie.
> But overall space-efficiency is an issue.
>
> A PATRICIA tree/trie would be ideal, in my mind, as it also has a
> completely deterministic structure, and is an order-of-magnitude more
> space-efficient. Insert, delete and query times are still O(1).
> However, it is not a trivial implementation. I have occasionally
> looked
> for implementations, but not found any that were satisfactory.
>
>
> No, a trie of any sort is dependent upon distribution of input data
> for balancing. As Peter Todd points out, a malicious actor could
> construct transaction or address hashes in such a way as to grow some
> segment of the trie in an unbalanced fashion. It's not much of an
> attack, but in principle exploitable under particular timing-sensitive
> circumstances.
>
> Self-balancing search trees (KVL, RB, 2-3-4, whatever) don't suffer
> from this problem.
>
> Mark
I was using "unbalanced" to refer to "query time" (and also
insert/delete time). If your trie nodes branch based on the next byte
of your key hash, then the max depth of your trie is 32. Period. No
one can do anything to ever make you do more than 32 hops to
find/insert/delete your data. And if you're using a raw trie, you'll
always use /exactly/ 32 hops regardless of the distribution of the
underlying data. Hence, the trie structure is deterministic
(history-independent) and cannot become unbalanced in terms of access time.
My first concern was that a malicious actor could linearize parts of the
tree and cause access requests to take much longer than log(N) time.
With the trie, that's not only impossible, you're actually accessing in
O(1) time.
However, you are right that disk space can be affected by a malicious
actor. The more branching he can induce, the more branch nodes that are
created to support branches with only one leaf.
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next prev parent reply other threads:[~2012-06-19 18:30 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-06-19 16:46 Andrew Miller
2012-06-19 17:33 ` Alan Reiner
2012-06-19 17:59 ` Gregory Maxwell
2012-06-19 18:12 ` Alan Reiner
2012-06-19 18:18 ` Mark Friedenbach
2012-06-19 18:30 ` Alan Reiner [this message]
2012-06-21 21:42 ` Mike Koss
2012-06-21 22:02 ` Gregory Maxwell
2012-06-19 18:29 Andrew Miller
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