ECDHE key exchange for server authentication and a key server
A server can receive a device public key and forward the device public key to a key server. The key server can perform a first elliptic curve Diffie-Hellman (ECDH) key exchange using the device public key and a network private key to derive a secret X1. The key server can send the secret X1 to the server. The server can derive an ECC PKI key pair and send to the device the server public key. The server can conduct a second ECDH key exchange using the derived server secret key and the device public key to derive a secret X2. The server can perform an ECC point addition using the secret X1 and secret X2 to derive a secret X3. The device can derive the secret X3 using (i) the server public key, a network public key, and the device private key and (ii) a third ECDH key exchange.
1. A method for a server to authenticate a device over a wireless network, the method performed by the server, the method comprising:
a) storing, in a memory, (i) a first point on an elliptic curve, and (ii) a shared key, wherein the shared key is stored by the device;
b) generating a value from a secure hash of at least the shared key;
c) selecting (i) a first integer as a first subset of the value and (ii) a second integer as a second subset of the value;
d) receiving, via a radio connected to the wireless network and from the device, a second point on the elliptic curve from the node, the second point comprising a public key for the device;
e) deriving a shared secret comprising an elliptic curve point addition of (i) the first integer multiplied by the public key for the device and (ii) the second integer multiplied by the first point;
f) deriving a symmetric ciphering key from a key derivation function with the shared secret;
g) receiving, via a radio connected to the wireless network and from the device, a ciphertext comprising (i) a device digital signature and (ii) a device certificate;
h) decrypting the ciphertext with the symmetric ciphering key in order to read the device digital signature and the device certificate; and
i) verifying the device digital signature using a device static public key from the device certificate.
2. The method of claim 1 , wherein the elliptic curve comprises a named curve secp256r1 (p256).
3. The method of claim 1 , wherein the first integer comprises a second value N1 and the second integer comprises a third value N2.
4. The method of claim 1 , wherein the symmetric ciphering key comprises a fourth value K1.
5. The method of claim 1 , further comprising in step d), receiving a random number from the device and in step i) verifying the device digital signature using the random number.
6. The method of claim 1 , further comprising in step d), receiving an identity for the device, wherein the server selects the shared key based on the identity.
7. The method of claim 1 , further comprising in step f) deriving a message authentication code (MAC) key from the key derivation function with the shared secret and in step i) verifying the ciphertext with the MAC key.
8. The method of claim 1 , wherein the device mutually derives the shared secret.
9. The method of claim 1 , further comprising in step i), verifying the device digital signature using an elliptic curve digital signature algorithm (ECDSA).