How to verify a wireguard public key?

[Matthias May]

On 25/12/2020 10:10, Nico Schottelius wrote:
> 
> Good morning Adam and Jason,
> 
> thanks for your qualified and fast answers! It's nice to see Dan's
> website still referenced in almost 2021 and also that it can be easily
> enough verified.
> 
> For reference and if anyone ever looks up this thread, I am using
> the following code within the Django Rest Framework [0]:
> 
> --------------------------------------------------------------------------------
>     def validate_wireguard_public_key(self, value):
>         msg = _("Supplied key is not a valid wireguard public key")
> 
>         """
>         Verify wireguard key.
>         See https://urldefense.com/v3/__https://lists.zx2c4.com/pipermail/wireguard/2020-December/006221.html__;!!I9LPvj3b!XDmxiY_v3yY5wQI9GfFvshrCIUcqg4vvKg35qvL0fFajgNHTwr3LcySSqHrNgCByOzQ$ 
>         """
> 
>         try:
>             decoded_key = base64.standard_b64decode(value)
>         except Exception as e:
>             raise serializers.ValidationError(msg)
> 
>         if not len(decoded_key) == 32:
>             raise serializers.ValidationError(msg)
> 
>         return value
> --------------------------------------------------------------------------------
> 
> Thanks again and enjoy the quite time over Christmas!
> 
> Best regards,
> 
> Nico
> 
> [0] https://urldefense.com/v3/__https://code.ungleich.ch/uncloud/uncloud/-/blob/master/uncloud_net/serializers.py*L37__;Iw!!I9LPvj3b!XDmxiY_v3yY5wQI9GfFvshrCIUcqg4vvKg35qvL0fFajgNHTwr3LcySSqHrNb-aKgNg$ 
> 
> 
> Adam Stiles <ajstiles at gmail.com> writes:
> 
>> Hi Nico,
>>
>> WireGuard uses Curve25519 keys. A Curve25519 secret key is a random 32
>> byte value with a few special bits flipped, and a public key is
>> calculated from a secret key.
>>
>> There's some good info here (https://urldefense.com/v3/__https://cr.yp.to/ecdh.html__;!!I9LPvj3b!XDmxiY_v3yY5wQI9GfFvshrCIUcqg4vvKg35qvL0fFajgNHTwr3LcySSqHrNDoxdskk$ ), including
>> this questions and answer:
>>
>> "How do I validate Curve25519 public keys?"
>>
>> "Don't. The Curve25519 function was carefully designed to allow all
>> 32-byte strings as Diffie-Hellman public keys."
>>
>> I just saw Jason's response, and so this is a bit redundant, but the
>> reference above is a good one.
>>
>> Best,
>>
>> Adam
>>
>>
>> On Thu, Dec 24, 2020 at 3:21 PM Nico Schottelius
>> <nico.schottelius at ungleich.ch> wrote:
>>>
>>>
>>> Good morning,
>>>
>>> I am currently extending uncloud [0] to support wireguard tunnels and
>>> keys. At the moment it is not entirely clear how to verify that a
>>> certain string is a valid wireguard key.
>>>
>>> I first tried checking that it is valid base64, but not all base64
>>> strings are valid wireguard keys.
>>>
>>> Then I tried using `echo $key | wg pubkey && echo ok` - which seems to
>>> check the key format, however the intended behaviour here is misused.
>>>
>>> Does anyone have a pointer on how to reliably identify wireguard public
>>> keys?
>>>
>>> Is the wireguard key always 32 bytes when decoded from base64? Tests
>>> with a number of public keys seems to indicate that.
>>>
>>> Best regards,
>>>
>>> Nico
>>>
>>>
>>> [0] https://urldefense.com/v3/__https://code.ungleich.ch/uncloud/uncloud__;!!I9LPvj3b!XDmxiY_v3yY5wQI9GfFvshrCIUcqg4vvKg35qvL0fFajgNHTwr3LcySSqHrNE6JpRjQ$ 
>>>
>>> --
>>> Modern, affordable, Swiss Virtual Machines. Visit https://urldefense.com/v3/__http://www.datacenterlight.ch__;!!I9LPvj3b!XDmxiY_v3yY5wQI9GfFvshrCIUcqg4vvKg35qvL0fFajgNHTwr3LcySSqHrNmbUvisY$ 
> 
> 
> --
> Modern, affordable, Swiss Virtual Machines. Visit https://urldefense.com/v3/__http://www.datacenterlight.ch__;!!I9LPvj3b!XDmxiY_v3yY5wQI9GfFvshrCIUcqg4vvKg35qvL0fFajgNHTwr3LcySSqHrNmbUvisY$ 
> 


Hi
On this topic, i recently implemented a check if a key is valid in cpp with the following rather crude code:

bool isValidWgKey(const string& usage, const string& key)
{
	/* Wireguard keys are BASE64 encoded */
	unsigned int _key_length = 44;
	unsigned int _key_offset = _key_length -1;
	if (key.length() != _key_length) {
		log("Wireguard " + usage + " has wrong length (" + to_string(key.length()) + " instead of 44)!");
		return false;
	}
	size_t found = key.substr(0,_key_offset).find_first_not_of("abcdefghijklmnopqrstuvwxyz"
								   "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789+/");
	if (found != std::string::npos) {
		log("Wireguard " + usage + " contains invalid character '" + key.substr(found, 1) + "'");
		return false;
	}
	if (key.substr(_key_offset,1) != "=") {
		log("Wireguard " + usage + " ends with invalid character '" +
		    key.substr(found, 1) + "' instead of '='");
		return false;
	}
	return true;
}

Maybe it's useful to someone.

BR
Matthias