Remove mentions of 32-byte slice from tweak APIs

[Kixunil]

These methods accept &Scalar, not slice and &Scalar already guarantees 32-bytes, so this failure case is impossible.

[sanket1729]

ACK e612458.

This is a strict improvement. But our code documentation can be improved, which is a separate issue.

[sanket1729]

This can only error when Scalar is zero.

[sanket1729]

Same comment as above about mutating self.

[Kixunil]

I don't think that's the case. The resulting key would be invalid if it's 0, so scalar being zero can cause error only if the key is already invalid which this library prevents. This should produce error though: priv_key.add_tweak(priv_key.negate().into())

[sanket1729]

This comment is for mul_tweak. Which happens only when the scalar is zero.

[sanket1729]

Anycase, these are unrelated to the PR itself. We can tackle this in separate issue

[Kixunil]

Ah, annoying GitHub UI...

[sanket1729]

We can be more specific and say, returns an error only if the resultant key is zero. This only happens when Scalar is the negation of Secret Key.

The upstream documentation also states:

seckey will be set to some unspecified value if this function returns 0.

We should either 1) document this or 2) Make a copy and make sure self is not mutated here.

[Kixunil]

It's already copied. :)

[sanket1729]

This comment is for tweak_add. This only happens when Scalar is the negation of Secret Key.

[sanket1729]

If we let the other parameter be SecretKey, we can safely unwrap

[Kixunil]

If other is SecretKey we can unwrap for tweak_mul but not for tweak_add. To represent it in the type system:

// This should be sealed and associated items hidden, showing unsealed for simplicity
trait IntoScalar: Into<Scalar> {
    type TweakMulError;

    fn invalid_tweak_mul() -> Self::TweakMulError;
}

impl IntoScalar for Scalar {
    type TweakMulError = Error;

    fn invalid_tweak_mul() -> Self::TweakMulError {
        Error::InvalidTweak
    }
}


impl IntoScalar for SecretKey {
    type TweakMulError = core::convert::Infallible;

    fn invalid_tweak_mul() -> Self::TweakMulError {
        unreachable!(); // _unchecked if we're brave :)
    }
}

Then accept IntoScalar in tweak_mul()

[sanket1729]

Opened #455.

[apoelstra]

ACK e612458