Document the maximum length in EcdhPrivateKey.deriveBits for each curve

[jonasfj]

Documentation for EcdhPrivateKey.deriveBits could have bullet point list mentining the maximum length of bits that can be derived from each curve.

Example

[length] specifies the length of the derived secret in bits. The maximum length depends on the [EllipticCurve] used in the keys:
...
 * [EllipticCurve.p521] can provide up to 528 bits.

See:

webcrypto.dart/lib/src/webcrypto/webcrypto.ecdh.dart

Lines 185 to 205 in b295d8d

	/// Derive a shared secret from two ECDH key pairs using the private key from one pair
	/// and the public key from another.
	///
	/// The shared secret is identical whether using A's private key and B's public key,
	/// or B's private key and A's public key, enabling secure key exchange between the
	/// two parties.
	///
	/// [length] specifies the length of the derived secret in bits.
	/// [publicKey] is [EcdhPublicKey] from the other party's ECDH key pair.
	///
	/// Returns a [Uint8List] containing the derived shared secret.
	///
	/// {@macro EcdhPrivateKey:example}
	// Note some webcrypto implementations (chrome, not firefox) supports passing
	// null for length (in this primitive). However, you can always know the right
	// length from the curve. Note p512 can provide up to: 528 bits!!!
	//
	// See: https://tools.ietf.org/html/rfc6090#section-4
	// Notice that this is not uniformly distributed, see also:
	// https://tools.ietf.org/html/rfc6090#appendix-B
	Future<Uint8List> deriveBits(int length, EcdhPublicKey publicKey);

---

• Find maximum length for each curve.
• Add tests for each curve + maximum length in deriveBits.
• Document maximum length that can be derived for each curve.

[HamdaanAliQuatil]

Starting with the tests. Will post my progress here.

[harrshita123]

I have opened a PR adding documentation for the maximum deriveBits length per ECDH curve, along with tests covering both valid and invalid lengths.

Thanks!