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Date: Fri, 20 Dec 2013 14:04:23 -0800
From: Mark Friedenbach <mark@monetize.io>
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Subject: Re: [Bitcoin-development] BIP proposal: Authenticated prefix trees
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On 12/20/2013 11:48 AM, Gregory Maxwell wrote:
> A couple very early comments— I shared some of these with you on
> IRC but I thought I'd post them to make them more likely to not get
> lost.

I got the inputs from IRC, but thank you for posting to the list so
that others can see and review.

> Whats a VARCHAR()  A zero terminated string?  A length prefixed 
> string? How is the length encoded?  Hopefully not in a way that
> has redundancy, since things that don't survive a serialization
> round trip is a major trap.

A length-prefixed string, using the shortest representation VARINT for
the length. Same as how scripts are serialized in transactions.

> Is the 'middle' the best place for the extradata? Have you 
> contemplated the possibility that some applications might use
> midstate compression?

Yes I considered midstate compression which is why the branch hashes
come last, but "extra" was an oversight. In every application I've
considered it's either not used (and therefore a single byte), or
updated whenever the node or its children updates.

Honestly I don't expect midestate compression to offer much since in
the nodes that are updated frequently it is unlikely that there will
be enough static data at the front to fill even a 512 bit block of the
smaller hash function.

But it doesn't hurt to prepare just in case. I'll move it to the end.

> On that general subject, since the structure here pretty much
> always guarantees two compression function invocations. SHA512/256
> might actually be faster in this application.

Yes, this is a great suggestion. Moving to SHA-512/256 will let most
inner nodes fit inside a single block, so long as the "extra" field is
not too long. Also apparently SHA-512 is faster on 64-bit CPUs, which
is a nice advantage. I didn't know that.

I'm concerned about speed but I did not go with a faster hash function
because users are more likely to have hardware acceleration for the
SHA-2 family.

> Re: using sha256 instead of sha256^2, we need to think carefully
> about the implications of Merkle-Damgard generic length extension
> attacks. It would be unfortunately to introduce them here, even
> though they're currently mostly theoretical for sha256.

The serialization format encodes lengths in such a way that you cannot
extend the data structure merely by appending bits. You would have to
change the prior, already hashed bits as well. I believe this makes it
immune to length extension attacks.

> WRT hash function performance, hash functions are so ludicrously
> fast (and will be more so as processors get SHA2 instructions) that
> the performance of the raw compression function would hardly ever
> be a performance consideration unless you're using a slow
> interpreted language (... and that sounds like a personal problem
> to me). So I don't think CPU performance should be a major
> consideration in this BIP.

Well.. the UTXO tree is big. Let's assume 5,000 transactions per
block, with an average of 3 inputs/outputs per transaction. This is
close to the worst-case scenario with the current block size. That's
15,000 insert, update, or delete operations.

The number of hashes required when level-compression is used is log2
the number of items in the tree, which for bitcoin is currently about
2.5 million transactions. So that's about ~21 hashes per input/ouput,
or 315,000 hash operations. A CPU is able to do about 100,000 hashes
per second per core, that'll probably take about a second on a modern
4- or 8-core machine.

For updatable proofs, the number of hash operations is equal to the
number of bits in the key, which for the validation index is always
256. That means 3.84 million hashes, or about 10 seconds on a 4-core
machine.

The numbers for the wallet index are worse, as it scales with the
number of outputs, which is necessarily larger, and the keys are longer.

This is not an insignificant cost in the near term, although it is the
type of operation that could be easily offloaded to a GPU or FPGA.

> What I do think should be a consideration is the cost of
> validating the structure under a zero-knowledge proof. An example
> application is a blind proof for a SIN or a proof of how much coin
> you control... or even a proof that a block was a correctly
> validated one, and in these cases additional compression function
> calls are indeed pretty expensive. But they're not the only cost,
> any conditional logic in the hash tree evaluation is expensive, and
> particular, I think that any place where data from children will be
> combined with a variable offset (especially if its not word
> aligned) would potentially be rather expensive.

This is something I know less about, and I welcome constructive input.
There is *no* reason that the hash serialization needs to have fancy
space-saving features. You could even make the SIG_HASH node
serialization into fixed-size, word-aligned data structures.

But this is absolutely not my field, and I may need some hand-holding.
Do the fields need to be at fixed offsets? With fixed widths? Should I
put variable-length stuff like the level-compressed prefixes and value
data at the end (midstate be damned) to keep fixed offsets? What's
expected word alignment, 32-bit or 64-bit?

> I'm unconvinced about the prefix tree compressed applications,
> since they break compact update proofs.  If we used them in the
> Bitcoin network they could only be used for data where all
> verifying nodes had all their data under the tree. I think they add
> a lot of complexity to the BIP (esp from people reading the wrong
> section), so perhaps they should be split into another document?

I believe what you mean by "compact update proofs" is what I call
"updatable proofs", where level compression is only used in the disk
and network serialization. These are what I propose to use for the
validation and wallet indexes, if the computational costs can be
brought under control, because it allows composable proofs.

Unlike a time-ordered index, it does require that someone, somewhere
has random access to the entire UTXO set since you can't predict in
advance what your txid will be. But this is a matter of tradeoffs and
I don't believe at this time that there is a clearcut advantage of one
approach over the other. I'm pursuing a txid-indexed UTXO set because
it most closely matches the current operational model.

That said, you still want level-compression within the serialization
format itself, if for no other reason than to keep proof sizes small.

> In any case, I want to thank you for talking the time to write
> this up. You've been working on this stuff for a while and I think
> it will be lead to useful results, even if we don't end up using
> how it was originally envisioned.

Thanks,
Mark
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