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On machines without a TPM, we'd still like some way for the BIOS to attest that it has not been modified. With a Librem Key, we can have the BIOS use its own ROM measurement converted to a SHA256sum and truncated so it fits within an HOTP secret. Like with a TPM, a malicious BIOS with access to the correct measurements can send pre-known good measurements to the Librem Key. This approach provides one big drawback in that we have to truncate the SHA256sum to 20 characters so that it fits within the limitations of HOTP secrets. This means the possibility of collisions is much higher but again, an attacker could also capture and spoof an existing ROM's measurements if they have prior access to it, either with this approach or with a TPM. Signed-off-by: Kyle Rankin <kyle.rankin@puri.sm>