Keys and seeds in depth

A 24-word phrase under the BIP-39 standard encodes 256 bits of randomness plus an 8-bit check, drawn from a list of 2,048 words. The number of possible 256-bit keys is 2 to the power of 256, which is a number 78 digits long.

Every computer on earth running until the sun burns out would not get through a meaningful fraction of them. A 12-word phrase has 128 bits, and the same holds.

That holds only if the randomness was real. In July 2026 thieves worked out seeds made on Coldcard hardware wallets, whose firmware from March 2021 on had drawn on a weak software source in place of the device’s hardware random number generator.

Coinkite, the maker, gives a preliminary estimate of the real search space: about 40 bits on its Mk2 and Mk3, and about 72 on its Mk4, Mk5 and Q, which should have had 128. The thieves needed no copy of any phrase and no device, and took about 1,789 BTC by Galaxy’s count in late August. Milk Sad, in July 2023, was the same kind of theft, from phrases made by a command-line tool.