[Cryptography] What is "chameleon hash"?

[Ben Laurie]

On 7 July 2017 at 20:44, Dmitry Belyavsky <beldmit at gmail.com> wrote:

> Hello,
>
> There is a very useful bitcoin-related article "The Blockchain
> Immutability Myth" by Gedeon Greenspan
>
> http://www.multichain.com/blog/2017/05/blockchain-immutability-myth/
>
>
> "With this in mind, let’s talk about the second case in which the doctrine
> of blockchain immutability has been used to ridicule ideas. Here, we’re
> referring to Accenture’s idea of using a chameleon hash  (
> http://www.coindesk.com/absolute-immutability-will-
> slow-permissioned-blockchain-progress/) to enable a block buried deep in
> a chain to be easily replaced. The primary motivation, as described by
> David Treat (https://www.accenture.com/t00010101T000000__w__/es-es/_
> acnmedia/PDF-33/Accenture-Editing-Uneditable-Blockchain.pdf) , is to
> allow an old problematic transaction to be quickly and efficiently removed.
> Under the scheme, if a block substitution does occur, a “scar” is left
> behind which all participants can see. (It should be noted that any later
> transactions that depend on the deleted one would need to be removed as
> well.)"
>
> Does anybody know if there is an implementation or at least description of
> "chameleon hash" and protocols based on them?
>

For Certificate Transparency and Trillian this has been discussed.

You don't need to do anything clever: you replace the offending entry with
an entry that says "I'm a deleted entry, and here's the reason, and here's
the hash of what used to be here". You then add an attestation to the
deletion to the end of the log and you're done.
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