ASN1BitString.kt BooleanArray constructor may miscalculate numUnusedBits

[kdeus]

In the new identity/src/commonMain/kotlin/com/android/identity/asn1/ASN1BitString.kt implementation, it looks like there's a bug in the BooleanArray constructor when it's calculating numUnusedBits:

    constructor(booleanValues: BooleanArray):
            this(
                if (booleanValues.size == 0) 0 else (8 - (booleanValues.size % 8)),

If booleanValues.size % 8 == 0, then numUnusedBits is set to 8, while the standard expects it to be from 0-7. In the parsing code, if numUnusedBits is set to 8, we may end up skipping a byte.

That calculation looks like it should be: ((8 - (booleanValues.size % 8)) & 0x07)
Or, equivalently: ((8 - (booleanValues.size % 8)) % 8)

[kdeus]

And we should have a unit test covering this case...