Filip/secret felt implementation

[FilipLaurentiu]

Pull Request type

Please add the labels corresponding to the type of changes your PR introduces:

• Feature

What is the current behavior?

Felt implement the Copy trait and it could leak the secret information into memory.
Felt is not zeroized on drop, it need to be manually zeroized.

Resolves: #NA

What is the new behavior?

New type SecretFelt is introduced for this specific purpose. This type could be used to represent sensible information that will not be leaked into memory at the end of the program

Does this introduce a breaking change?

No - Only if a project use the zeroize function on Felt and upgrade to the latest version of this library, but from what I know, nobody use it (starknet-rs doesn't use it)

[tdelabro]

No - Only if a project use the zeroize function on Felt and upgrade to the latest version of this library, but from what I know, nobody use it (starknet-rs doesn't use it)

So yes haha ;)

[tdelabro]

@FilipLaurentiu CI is failing

[tdelabro]

@FilipLaurentiu ok we have this secret felt now. But we can't do much with it. Just move it around in memory safely.

What operations can we safely implement on this type? Arithmetic (Add, Mul, and so on) seems out of reach due to the way the internal representation of Felt handles it.

    /// Field addition. Never overflows/underflows.
    impl ops::Add<Felt> for Felt {
        type Output = Felt;

        fn add(self, rhs: Felt) -> Self::Output {
            Self(self.0 + rhs.0)
        }
    }

    /// Field addition. Never overflows/underflows.
    impl ops::Add<&Felt> for Felt {
        type Output = Felt;

        fn add(self, rhs: &Felt) -> Self::Output {
            Self(self.0 + rhs.0)
        }
    }

A copy of the internal value will be created if I don't mistake.

We could add a constant time comparison for the secret felt, to add one more security measure against timing attacks.

[FilipLaurentiu]

@FilipLaurentiu ok we have this secret felt now. But we can't do much with it. Just move it around in memory safely.

What operations can we safely implement on this type? Arithmetic (Add, Mul, and so on) seems out of reach due to the way the internal representation of Felt handles it.

    /// Field addition. Never overflows/underflows.
    impl ops::Add<Felt> for Felt {
        type Output = Felt;

        fn add(self, rhs: Felt) -> Self::Output {
            Self(self.0 + rhs.0)
        }
    }

    /// Field addition. Never overflows/underflows.
    impl ops::Add<&Felt> for Felt {
        type Output = Felt;

        fn add(self, rhs: &Felt) -> Self::Output {
            Self(self.0 + rhs.0)
        }
    }

A copy of the internal value will be created if I don't mistake.

We could add a constant time comparison for the secret felt, to add one more security measure against timing attacks.

I manage to implement a constant time comparison function for the SecretFelt. I don't see a way to add arithmetic functions to it, the reason will be, as you said, the way Felt work under the hood.

One big usage for SecretFelt I see is to use as a secret key for signatures. For example sign function takes a referance to a Felt and SecretFelt could be used here

[tdelabro]

@FilipLaurentiu I think once you add Eq and fix the failing CI we will be good for merging

[FilipLaurentiu]

@FilipLaurentiu I think once you add Eq and fix the failing CI we will be good for merging

Done, ready to go 👍