I need to warn you right from the start. If you think that the ‘immutability’ of a blockchain is its most useful and precious feature, think again. What you may own today, in either Bitcoin, Ether or XRP cryptocurrency (or any ‘ERC20 token’, for that matter) may become useless, once quantum computing emerges as a threat to today’s public key cryptography (PKI), which is based either on RSA or Elliptic Curve Cryptography (ECC). And all that may happen, because of blockchain’s immutability, that we all so happily admire today. Let me explain how and why.

Blockchains rely on PKI, mainly ECC flavor, which is the integral part of their DNA. That’s how you (or wallet application you use) digitally sign(s) the blockchain transactions, when paying someone (via value transfer), and that’s how that ‘payee’ proves that they are indeed the entity who should really receive and own the ‘crypto value’.

Blockchains are intentionally made to be practically immutable, i.e. nobody (in theory, at least) can modify the blockchain’s ‘distributed ledger’ of all committed blocks. That has been one of the most lauded features of almost every blockchain. Basically, immutability is one of the preconditions for being able to dtect and prevent ‘double spending’. Unless everyone trusts the ledger’s content to be immutable, prevention of double spending would be at least highly questionable or virtually impossible.

In most popular blockchain implementations, like Bitcoin or Ethereum, immutability is achieved via ‘proof of work’ mechanism, which is the integral and mandatory component of their consensus reaching algorithms. Proof of work is a computationally intense, very energy inefficient and extra slow brute force method. But you see, that’s exactly why it was chosen in the first place. If it is extra difficult to become the fastest ‘miner’, which wins the ‘proof of work’ race, for a single block to be accepted (via consensus) into the ledger, can you imagine how expensive it would be for a potential fraudster to modify the content of the existing blockchain’s block(s) and recalculate hashes of all impacted blocks in the same ledger? Other ‘proof of ...’ methods exist, like ‘proof of stake’ and ‘proof of authority’, which may be more energy efficient, but all of them basically, one way or another, aim to make blockchain’s content virtually impossible to be modified, once blocks are committed and accepted into the ledger.

That’s a very good thing indeed, for dealing with ‘double spending’, but it can become very problematic, if we need to replace the cryptographic algorithm, that was originally used to digitally sign all of the existing blockchain transactions, which are packed into the existing blocks in the current ledger. And that’s exactly what would have to happen, when mainstream quantum computing becomes feasible. Why, you may be asking? Simply because with a quantum computer, fraudster could reverse engineer the ECC private key from the corresponding ECC public key, exponentially faster than with classic computer. Using quantum computer, with today's key sizes, the reverse engineering becomes possible in matter of hours, instead of billions of years when using classic computers. Since the blocks in the blockchain are full of ECC public keys of ‘payers’, who digitally signed the transactions paying to ‘payees’, this is going to become a significant problem, in the next 5 to 10 years.

Researchers are actively working on inventing and proving quantum-safe PKI, which could be resistant to quantum attacks. There is even talk about possible emergence of quantum computing based blockchains. That is all great and exciting, but the main question still remains: what is going to happen with all of the ‘unspent transaction outputs’ of Bitcoins, Ethers, XRPs, ERC20 tokens, etc. that are already immutably captured inside public mainstream blockchain blocks - when quantum computing becomes mainstream? Could those crypto assets be ‘stolen away’ by thieves from their righteous owners, if fraudsters have access to quantum computers and could easily reverse engineer the ECC private keys from the ECC public keys freely and readily available inside each of the blockchain blocks? Unfortunately the answer to the last question is YES, EASILY.

Think hard about this, before you rush to buy and become ‘owner’ of any of those crypto assets.