When a cryptocoin changes algorithm (eg. scrypt to X11), how does one achieve the merging of the two blockchains?

I'm assuming that they couldn't be directly compatible based on being completely different algorithms, it would cause a loadBlock() error. They also couldn't just make two blockchains, as all of the scrypt mined coins would be unspendable on the x11 blockchain and vice versa.

I have seen instances of crypto developers having a v1 to v2 exchange (usually at some ratio like 0.8:1), however that would require the v2 blockchain to have a premine for the conversion distribution.

Hopefully I'm not too far off on my thoughts here, any clarification would be greatly appreciated.

I should specify that by merging I don't mean running both algorithms on one coin (saffron-style), I mean how to implement something along the lines of

if blockNumber <= (maxScryptBlock) do
else if blockNumber >= (maxScryptBlock) do

I can't seem to find any writings anywhere on how a developer makes the switch from one algorithm to another, or really any for cloning any non-scrypt/sha256 coin.

1 Answer 1


That seems possible, but I'm not aware of any instances of it being done before.

You have three major problems:

  • Bitcoin generally assumes that there is one single ordering of transactions. That ordering can change, but there's always only one. The danger is that your two blockchains contain transactions that are mutually exclusive: transaction A and transaction B both spend the same coins, so only one can exist in the new master chain.
  • Everyone needs to upgrade to a client that supports the new merged blockchain, and the algorithms in both chains. Some people might have been screwed over by the previous step in the process (because a transaction they relied upon was essentially reversed,) so they might be a little reluctant to do this.
  • You need to avoid introducing any subtle security bugs while doing this.
  • awesome answer (I would upvote if i had more rep lol), and thank you for the explanation. However, I dont think i properly explained my question, or I'm possibly misinterpreting your answwer. I have edited my OP for better clarification. Sep 17, 2015 at 0:40
  • You're describing a hardfork, which is much easier to do correctly the first time than merging the chains after the fact.
    – Nick ODell
    Sep 17, 2015 at 22:12

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