Ledger, a French developer of hardware wallets for cryptocurrency, said Thursday that it has raised around $75 million in new venture capital funding.

Why it matters: Because the startup's popularity — a source says it's profitable with around $56 million in 2017 revenue on more than a million wallets sold — reflects the fundamental contradiction of crypto securities: They are built on a technology most notable for its computer science breakthrough in verification (i.e., solving the Byzantine Generals Problem), but their unique/unrecoverable "keys" are prone to being lost or hacked without offline storage.