Quantum-Ready Blockchains Compared, Part 4: Retrofitting — Ethereum, the XRP Ledger, Solana and Cardano

Disclosure: I hold CKB. This series presents information, not recommendations, and nothing in it is investment advice. Previously: Part 2 looked at QRL, quantum-safe from its first block. Part 3 looked at four chains built to swap their cryptography, and found that only Algorand and Nervos have shipped it, and that staking chains face a second migration of their consensus that none has yet completed. The four chains in this part are worth, together, more than $440 billion. They are among the busiest networks in crypto. None of them was designed to change its signature scheme, and on three of the four, an ordinary holder cannot do anything about quantum risk today. That is not negligence. These are running systems with millions of users and enormous amounts of software built on top of them, and changing their cryptography is like changing the foundations of an occupied building. What they have instead are plans, some detailed and dated, some deliberately cautious. This part looks at those plans, at the one design choice that makes some of these chains more exposed than others, and at a promising idea two of them share for rescuing holders who move too late. Bitcoin, covered in depth in our Nervos series, appears at the end as the yardstick. The short version: the retrofitters have the deepest research teams and the clearest deadlines in this series, and almost nothing on the main network yet. First, who is exposed by default? Before looking at plans, it helps to know what each plan has to fix. The key question is Part 1's fourth test: is your public key visible before you spend? Solana is the outlier. A Solana address is the Ed25519 public key itself, written in a different alphabet. Every funded Solana account, including ones that have never sent a single transaction, already shows a future quantum attacker exactly what it needs. (Classic Algorand accounts work the same way, as Part 3 noted.) Ethereum and the XRP Ledger hide the key at first, then expose it for…