From: Ruben Somsen <rsomsen@gmail•com>
To: Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
Cc: Greg Maxwell <gmaxwell@gmail•com>
Subject: Re: [bitcoindev] Re: SwiftSync - smarter synchronization with hints
Date: Sat, 3 May 2025 16:55:31 +0200 [thread overview]
Message-ID: <CAPv7TjZ_Z_zvBVnH2fH9EeSbOaVuvrVvHsuQD1bOmVq72uc2JA@mail.gmail.com> (raw)
In-Reply-To: <CAAS2fgQ1CvyxoRckZT2_6dNgKxDaKWvuK46FYMeaHc8Np_gDPg@mail.gmail.com>
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>I think you cannot escape the assumption that A is unique (well A and its
spend) thanks to TXID uniqueness
A counter-example would be a chain with two transactions with the exact
same input A and output B. SwiftSync with XOR would basically treat these
two transactions as non-existent (the two A's and B's cancel each other
out), while a regular full node (or non-XOR SwiftSync) would reject the
chain.
On Sat, May 3, 2025 at 3:57 PM Greg Maxwell <gmaxwell@gmail•com> wrote:
> Hm. Fair point, but I think you cannot escape the assumption that A is
> unique (well A and its spend) thanks to TXID uniqueness without weakening
> the security argument, since A*n eventually collides. It's essentially the
> same argument you make for characteristic 2, it just takes more
> repetitions. Without knowing the salt you'd need 2^256 repetitions to be
> certain, but e.g. see the prior suggestions on truncating the hash to 32
> bits or whatever, a practical number of A repeats would then self cancel.
>
> To be clear I'm not arguing that it should be xor here, but pointing out
> there is structure here you might not have expected.
>
>
>
>
>
> On Sat, May 3, 2025 at 1:42 PM Ruben Somsen <rsomsen@gmail•com> wrote:
>
>> Hey all,
>>
>>
>> @Saint Wenhao
>>
>> >if you take the sum of hashes, which should be finally zero, then by
>> grinding UTXOs, someone could make it zero
>>
>> That's what the secret salt prevents. You can't grind for a certain
>> number if you don't know what the number is you are trying to collide with.
>>
>> >maybe you can avoid hashing at all [...] And then, it is all about
>> mixing the salt
>>
>> Without a concrete solution I'm afraid that's wishful thinking. That last
>> part of the sentence is why we currently need the hash, as well as for
>> adding more data in the non-assumevalid version.
>>
>>
>> @Sanket Kanjalkar
>>
>> >What if instead of hash we encrypt with AES
>>
>> I can't really evaluate whether this might work, but I can see the line
>> of reasoning. Conceptually, I think what we need is something that
>> transforms the data into fixed length blocks for which an attacker can't
>> know the relationship between each block (i.e. via a secret). The
>> transformation needs to be the same on the input and output side.
>>
>>
>> @Greg Maxwell
>>
>> >Your reduction function could just be xor
>>
>> I had initially ruled XOR out. Reason for this is that XOR would lead one
>> to conclude that sets [A, B, C, C], [A, B], [A, B, D, D], etc. are all
>> equivalent because any two values cancel each other out, regardless of
>> whether the sets are on the input or output side. Modular add/sub doesn't
>> have this issue. If the speedup actually turns out to be significant then
>> there may be some clever way to salvage it like by counting the total
>> number of inputs and outputs and relying on the knowledge that every txid
>> must be unique, but that's a lot harder to reason about.
>>
>> >even if its with a quite expensive hash function that the IBD
>> performance will be heavily bottlenecked in network and parallelism related
>> issues and be far from the lowest hanging fruit for a while, considering
>> that this has eliminated the big sequential part and a number of annoying
>> to optimize components entirely
>>
>> Very true, and as you said, we can easily drop-in replace the hash
>> function at any future point we like, without adverse consequences.
>>
>>
>> Cheers,
>> Ruben
>>
>> On Sat, May 3, 2025 at 2:07 PM Greg Maxwell <gmaxwell@gmail•com> wrote:
>>
>>> On Saturday, May 3, 2025 at 11:55:28 AM UTC Sanket Kanjalkar wrote:
>>>
>>> > hash(UTXO_A||salt) + hash(UTXO_B||salt) - hash(UTXO_C||salt) -
>>> hash(UTXO_D||salt) == 0 (proving (A==C && B==D) || (A==D && B==C))
>>>
>>> What if instead of hash we encrypt with AES and modular add/subs? I
>>> cannot prove it; but I also don't see a clear way this is broken.
>>>
>>> 1. Sample random symmetric key `k`
>>> 2. Instead of above; AES_k(UTXO_A) + AES_k(UTXO_B) - AES_k(UTXO_C) -
>>> AES(UTXO_D) == 0 => (proving (A==C && B==D) || (A==D && B==C))?
>>>
>>>
>>> AES in CTR mode is, I'm not sure about other modes? Obviously CTR mode
>>> would be unsuitable! (I mean sure modular add/sub and xor are different
>>> operations but they are quite close). I think that in many modes the
>>> collision resistance would have to at least be restricted by the birthday
>>> bound with the small block size. I think CMC might be needed to avoid that
>>> sort of issue.
>>>
>>>
>>>
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>>> .
>>>
>>
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next prev parent reply other threads:[~2025-05-03 14:56 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-04-09 10:10 [bitcoindev] " Ruben Somsen
2025-05-02 6:47 ` [bitcoindev] " Greg Maxwell
2025-05-02 10:59 ` Ruben Somsen
2025-05-02 13:38 ` Saint Wenhao
2025-05-02 16:07 ` Greg Maxwell
2025-05-02 19:15 ` Saint Wenhao
2025-05-02 20:23 ` Sanket Kanjalkar
2025-05-03 12:02 ` Greg Maxwell
2025-05-03 13:42 ` Ruben Somsen
2025-05-03 13:57 ` Greg Maxwell
2025-05-03 14:36 ` Greg Maxwell
2025-05-03 14:55 ` Ruben Somsen [this message]
2025-05-03 15:54 ` Greg Maxwell
2025-05-03 16:24 ` Ruben Somsen
2025-05-03 13:53 ` Weikeng Chen
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