[noise] Curve25519 key bitmask?

[Jonathan Rudenberg]

The Curve25519 documentation[0] says that we should do these bitwise ops while computing the secret key:

     mysecret[0] &= 248;
     mysecret[31] &= 127;
     mysecret[31] |= 64;

It’s not immediately apparent what the reason for this is and if it has any negative/positive impact. Would someone explain it to me?

Thanks,

Jonathan

[0] http://cr.yp.to/ecdh.html