Why P-521 public key X,Y some time is 65 bytes some time is 66 bytes : <p>I use golang generate the P-521 public key.
source code look like that:</p>

<pre><code>curve:=elliptic.P521()
priv, x, y, err := elliptic.GenerateKey(curve, rand.Reader)
xBytes:=x.Bytes()
yBytes:=y.bytes()
//len(xBytes)  some time is 65 bytes ,some time is 66 bytes 
</code></pre>

<p>Why P-521 public key X,Y  not like P-256 or P-384 that have a fixed public key  length?</p>
