Convert getSharedSecret output to 32 bytes and ECDH_compute_key

[vxow]

The shareKey generated by using p256.getSharedSecret does not match the sharekey generated by OpenSSL ECDH_compute_key, nor does it match the sharekey generated by using subarray(1).

[paulmillr]

I have no idea. noble multiplies point B by secret scalar A, which is the general definition of shared secret / ecdh. It must always be the same between all implementations.

Ensure the output is compressed / uncompressed depending on your needs.

noble-curves/src/abstract/weierstrass.ts

Lines 921 to 926 in a1a7dc9

	function getSharedSecret(privateA: PrivKey, publicB: Hex, isCompressed = true): Uint8Array {
	if (isProbPub(privateA)) throw new Error('first arg must be private key');
	if (!isProbPub(publicB)) throw new Error('second arg must be public key');
	const b = Point.fromHex(publicB); // check for being on-curve
	return b.multiply(normPrivateKeyToScalar(privateA)).toRawBytes(isCompressed);
	}

[vxow]

Thanks for your answer, I need your help, c++ ECDH_compute_key returns 32 bytes of data, getSharedSecret returns 33 bytes after compression, and 65 bytes without compression, I tried subarray(1), but the result still does not match. There are no examples to exchange with C++.

[TobyEalden]

I'm seeing this too, but using EVP_PKEY_derive_* functions. C++ calculates a 32 byte key but getSharedSecret calculates 33 bytes.

Did you get to the bottom of it?

[paulmillr]

if it's 32 bytes, then it's either:

a. SLICED: res.slice(1, 33)
b. HASHED: sha256(res.slice(1, 33))
c. HKDFED: hkdf_sha256(res.slice(1, 33))
d. SLICED and HASHED/HKDFED: b or d, but replace res.slice(1, 33) with decompRes.slice(1, 65), res.slice(0, 33) or decompRes.slice(0, 65)

There is no standard for ECDH. 32 bytes, 33 bytes, hashing or kdfing all make sense depending on ones needs. We provide the most flexible output, that can be converted to anything you'd like.

[TobyEalden]

res.slice(1, 33) did the trick - many thanks!

A case of a just enough knowledge to be dangerous I fear...