Add ValidateSignature helper function

[thomaspanf]

We need this to validate inputs from: https://stake.rocketpool.net/manage/signalling-address

V1 reference: https://github.com/rocket-pool/smartnode/blob/master/rocketpool-cli/pdao/commands.go#L102

[jshufro]

hate to be a party pooper

[jshufro]

yikes, uh, this doesn't actually validate a signature so it shouldn't be called ValidateSignature.

It just checks that it's 0x-prefixed hex of 65 bytes.

There's no reason to have a function that does that specifically for EIP-712... if we're sanitizing user input i suggest:

const EIP712Length = 65

// SanitizeHexInput checks for and strips a 0x prefix, returning an error if one is not found.
// It returns the byte slice representing the decoded hex, or a decoding error
func SanitizeHex(inp string) ([]byte, error) {
  if !strings.HasPrefix(inp, "0x") {
    return nil, errors.New("expected hex prefix")
  }
  return hex.DecodeString(strings.TrimPrefix(inp, "0x"))
}

// SanitizeEIP712String checks that a 0x-prefixed hex string is the proper length to be
// parsed as a EIP-712 signature, and parses it, returning an error if the length is invalid
// or the hex cannot be decoded.
func SanitizeEIP712String(inp string) ([]byte, error) {
  inp, err := SanitizeHexInput(inp)
  if err != nil {
    return nil, fmt.Errorf("error sanitizing EIP-712 signature string as hex: %w", err)
  }
  if len(out) != EIP712Length {
    return nil, fmt.Errorf("error sanitizing EIP-712 signature string: invalid length %d (expected %d)", len(out), EIP712Length)
  }
  return out, nil
}

[jclapis]

Suggestion for the sake of common usage: move the r, s, v parsing logic out of the SN and put it into the ValidateSignature function (renamed to ValidateEip712Signature in my PR), perhaps with a struct for the components if Geth doesn't already have one. If the intent of this is "take a string that's supposed to be an EIP-712 signature and turn it into an EIP-712 signature" like the other Validate methods do then that'd make sense here.

[jshufro]

https://google.github.io/styleguide/go/decisions#initialisms

But yes, either sanitize the input and don't validate, or just do everything here. Both are acceptable- validation of the signature should happen in the front end and the backend.

[thomaspanf]

This piece of functionality is going to included in package eip712 within smartnode pr #599.

Closing #17 now