📅 Original date posted:2012-06-19
📝 Original message:On 06/19/2012 02:18 PM, Mark Friedenbach wrote:
> On Tue, Jun 19, 2012 at 10:33 AM, Alan Reiner <etotheipi at gmail.com
> <mailto:etotheipi at 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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Alan Reiner [ARCHIVE] /
npub1sm…mnq7h
2023-06-07 10:18:24
in reply to nevent1q…rz9m
Author Public Key
npub1sm6zhjmk5scuz294jmpkw99wwwjzetjgwp4fu4gn6utqgdz87hkqamnq7hPublished at
2023-06-07 10:18:24Event JSON
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"content": "📅 Original date posted:2012-06-19\n📝 Original message:On 06/19/2012 02:18 PM, Mark Friedenbach wrote:\n\u003e On Tue, Jun 19, 2012 at 10:33 AM, Alan Reiner \u003cetotheipi at gmail.com \n\u003e \u003cmailto:etotheipi at gmail.com\u003e\u003e wrote:\n\u003e\n\u003e If we were to use a raw trie structure, then we'd have all the above\n\u003e issues solved: a trie has the same configuration no matter how\n\u003e elements\n\u003e are inserted or deleted, and accesses to elements in the tree are\n\u003e constant time -- O(1). There is no such thing as an unbalanced trie.\n\u003e But overall space-efficiency is an issue.\n\u003e\n\u003e A PATRICIA tree/trie would be ideal, in my mind, as it also has a\n\u003e completely deterministic structure, and is an order-of-magnitude more\n\u003e space-efficient. Insert, delete and query times are still O(1).\n\u003e However, it is not a trivial implementation. I have occasionally\n\u003e looked\n\u003e for implementations, but not found any that were satisfactory.\n\u003e\n\u003e\n\u003e No, a trie of any sort is dependent upon distribution of input data \n\u003e for balancing. As Peter Todd points out, a malicious actor could \n\u003e construct transaction or address hashes in such a way as to grow some \n\u003e segment of the trie in an unbalanced fashion. It's not much of an \n\u003e attack, but in principle exploitable under particular timing-sensitive \n\u003e circumstances.\n\u003e\n\u003e Self-balancing search trees (KVL, RB, 2-3-4, whatever) don't suffer \n\u003e from this problem.\n\u003e\n\u003e Mark\n\nI was using \"unbalanced\" to refer to \"query time\" (and also \ninsert/delete time). If your trie nodes branch based on the next byte \nof your key hash, then the max depth of your trie is 32. Period. No \none can do anything to ever make you do more than 32 hops to \nfind/insert/delete your data. And if you're using a raw trie, you'll \nalways use /exactly/ 32 hops regardless of the distribution of the \nunderlying data. Hence, the trie structure is deterministic \n(history-independent) and cannot become unbalanced in terms of access time.\n\nMy first concern was that a malicious actor could linearize parts of the \ntree and cause access requests to take much longer than log(N) time. \nWith the trie, that's not only impossible, you're actually accessing in \nO(1) time.\n\nHowever, you are right that disk space can be affected by a malicious \nactor. The more branching he can induce, the more branch nodes that are \ncreated to support branches with only one leaf.\n\n\n-------------- next part --------------\nAn HTML attachment was scrubbed...\nURL: \u003chttp://lists.linuxfoundation.org/pipermail/bitcoin-dev/attachments/20120619/2548286d/attachment.html\u003e",
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