Block's technical analysis of the cold card issue: https://engineering.block.xyz/blog/predictable-rng-fallback-and-32-bit-reseed-in-coldcard-firmware Easier to understand Gemini summary regarding mk4, Q, mk5 : Core Takeaway The advisory details a critical Random Number Generator (RNG) vulnerability in COLDCARD firmware. Due to a configuration bug, COLDCARD firmware bypassed the hardware True Random Number Generator (TRNG) and defaulted to MicroPython’s software PRNG fallback (Yasmarang), which uses predictable device state (like system timers and CPU hardware IDs). What This Means for Mk4 Devices For Mk4, Q, and Mk5 models: When these devices boot, they read unpredictable random data from the secure element, but they hash it down and only apply 32 bits (4 bytes) of that entropy to reseed the RNG. The Math: Instead of having 2^{256} possible wallet combinations, your seed was selected from a maximum search space of 2^{32} (approx. 4.29 billion possibilities). The Threat: For a specialized computer or Bitcoin cracking setup, searching 2^{32} possibilities takes seconds to minutes. An attacker who knows your public key or xpub can brute-force the search space and extract your private key to steal funds. Actionable Recommendations for Mk4 Owners If you generated a wallet on an Mk4 running firmware v5.0.0 or later: 1. Assume the Seed is Compromised Do not rely on upgrading the firmware to keep your current seed safe. Firmware updates fix future RNG operations, but they cannot retroactively fix a weak seed that has already been generated. 2. Create a New Wallet on a Safe Device & Sweep Funds Immediately Obtain a secure, unaffected device (or a fresh wallet setup on software/hardware known to use proper entropy). Do NOT sweep funds into another seed created on the same affected COLDCARD firmware. Generate a fresh, safe seed and sweep (transfer) 100% of your funds to the new addresses immediately. 3. If Re-using the Mk4 in the Future Update to the latest official firmware patch provided by Coinkite once available. If creating a new wallet on the device, use physical dice rolls (BIP39 dice entropy) directly on the COLDCARD. Bypassing the automatic RNG via physical dice generation avoids relying entirely on internal hardware RNG paths.
