Coldcard firmware forces user entropy for new seed generation

Coinkite has released firmware updates for Coldcard Mk4, Mk5 and the Coldcard Q that require newly created wallet seeds to include user-supplied entropy combined with the device’s hardware randomness. The company warned that existing seed phrases remain vulnerable and must be replaced before funds are moved.

The updates are version 5.6.1 for Mk4 and Mk5 and 1.5.1Q for the Coldcard Q, announced in a company blog post on Thursday. Under the new process, users must enter at least one of the approved entropy sources during seed creation: 65 keypresses with unpredictable timing, 50 six-sided die rolls or 128 coin flips. The device mixes that input with randomness from secure elements and the hardware random-number generator to produce the mnemonic seed.

Coinkite noted that a July 31 update had already fixed the seed-generation failure for newly created wallets. The Thursday release follows a three-week security review and adds multiple protections around USB handling, transaction signing and the hardware RNG.

Firmware changes add a transaction re-verification step immediately before signing to address attacks that could originate from a compromised computer USB port. The software introduces additional checks on the hardware RNG and a boot-time test to confirm the wallet is using its intended secure hardware path. The update narrows the USB data window by allowing downloads only for the device’s most recent output and requires an encrypted USB session. It also blocks certain Bitcoin signature hash modes that permit transaction outputs to remain modifiable by default.

Other firms have moved to detect addresses exposed by weak seed generation. Blockchain security company Coinspect released Unlukey, a free tool that reproduces known weak-seed patterns and checks public addresses against the affected dataset. Security analysis has traced the weakness to a firmware bug introduced in March 2021 that reduced effective seed entropy for some devices from 128 bits to about 40 bits, making keys brute-forceable without physical access. Confirmed losses tied to the vulnerability reached 1,778 BTC, about $112 million, according to an Aug. 14 report by Galaxy Research.

Coinkite urged users to update devices, generate new seeds and migrate funds only after securely storing replacement seed phrases. Users are advised to verify firmware versions, recreate seeds on the updated device and follow best practices for transaction verification and USB hygiene when signing.