The OPC UA Secure Conversation (UASC) mechanism described 6.7 is designed for use with asymmetric cryptography algorithms, such as RSA, that allow Public Keys to be used for encryption and for digital signatures. ECC is an asymmetric cryptography algorithm that only supports digital signatures. To accommodate algorithms like ECC, the UASC handshake changes to allow negotiation of inputs used for key derivation in 6.7.6 without making the keys available to eavesdroppers. This negotiation uses a Diffie-Hellman algorithm defined in IETF RFC 8422 and is shown in Figure 15.
Figure 15 – ECC Key Negotiation
Certificates for ECC have a public-private key pair that are used to create and verify a digital signature. To negotiate the keys used for the SecureChannel the Client generates a new key pair (JC, KC) and passes the Public Key (JC) in the request. After verifying the signature on the request, the Server generates a new key pair (JS, KS) and returns the Public Key (JS) in the response. The new key pairs are used each time a SecureChannel is negotiated and they are called EphemeralKeys.
ECC public-private key pairs are always based on a specific elliptic curve function which is used for the ECC calculations. Many curves exist, however, ECC cryptography libraries support a finite set of "named curves" to allow for better interoperability. Each OPC UA SecurityPolicy defined in OPC 10000-7 specifies exactly one named curve which is used for the EphemeralKeys.
Each ECC Certificate is also based on a named curve. Each SecurityPolicy specifies a list of named curves which are permitted for use in the Certificate. This list always includes the named curve used for the EphemeralKey, however, it may allow other named curves. OPC UA applications that support ECC SecurityPolicies shall support multiple Certificates.
ECC Public Keys and digital signatures are the output of an ECC operation. The encoding of these outputs depends on the ECC curve and are described by the SecurityPolicy in OPC 10000-7.
Clause 6.7.4 specifies the contents of the OpenSecureChannel request and response messages. When using an ECC SecurityPolicy the ClientNonce is the Public Key for the Client’s EphemeralKey encoded using the Public Key encoding for the curve. Similarly, the ServerNonce is the Public Key for the Server’s EphemeralKey.
The encoding of the EphemeralKeys depends on the ECC curve used. For NIST and Brainpool curves the EphemeralKey is the x and y coordinate encoded as zero padded big-endian OctetString. For Edwards curves the EphemeralKey format is defined byIETF RFC 7748.
The EphemeralKeys are used to calculate a shared secret by using the Private Key of an EphemeralKey and the Public Key of the peer’s EphemeralKey. The exact algorithm to calculate the shared secret depends on the ECC curve and is defined by the SecurityPolicy. This shared secret is then used to derive key data using the following algorithm from IETF RFC 5869. Note that the algorithm is repeated here for clarity, however, the RFC is the normative source.
Step 1: Calculate Salts
ServerSalt = L | UTF8(opcua-server) | ServerNonce | ClientNonce
ClientSalt = L | UTF8(opcua-client) | ClientNonce | ServerNonce
Where
- L is the length of derived key material, in bytes, encoded as a 16-bit little endian integer;
- UTF8(label) is the UTF8 encoding of the string literal ‘label’;
- ServerNonce is the Server EphemeralKey from the OpenSecureChannel response;
- ClientNonce is the Client EphemeralKey from the OpenSecureChannel request;
- | concatenates sequences of bytes;
- Salt is a sequence of bytes.
Step 2: Extract
PRK = HMAC-Hash(Salt, IKM)
Where
- HMAC uses a Hash function specified by the KeyDerivationAlgorithm;
- IKM is the shared secret calculated from the EphemeralKeys;
- Salt is calculated in Step 1;
- PRK is a pseudorandom output with length equal to the Hash size.
The encoding of the x-coordinate in IKM depends on the ECC curve used. For NIST and Brainpool curves the x-coordinate is encoded as a zero padded big-endian OctetString. For Edwards curves the coordinate format is defined by RFC 7748.
If SecureChannelEnhancements = TRUE and a SecureChannel is renewed (see 6.7.4), the IKM used to derive the current set of keys is XORed with the new IKM negotiated during renewal to create a value that is used to derive the new keys.
Step 3: Expand
N = ceil(L/HashLen)
T = T(1) | T(2) | T(3) | ... | T(N)
OKM = first L octets of T
where:
T(0) = empty string (zero length)
T(1) = HMAC-Hash(PRK, T(0) | Info | 0x01)
T(2) = HMAC-Hash(PRK, T(1) | Info | 0x02)
T(3) = HMAC-Hash(PRK, T(2) | Info | 0x03)
...
-
Where
- HMAC uses a Hash function specified by the KeyDerivationAlgorithm;
- PRK is the output from Step 1;
- Info is the Salt used in Step 1;
- L is the length of keying material;
- 0x01 is the number 1 encoded as a byte.
- OKM is the output with length equal to L bytes.
The client keys are extracted from the keying material created with IKM=shared secret, Salt=ClientSalt and Info=ClientSalt as shown in Table 67.
The SymmetricEncryptionAlgorithm for the SecurityPolicy sets the DerivedSignatureKeyLength, the EncryptionKeyLength and InitializationVectorLength. All constants referenced in the table are converted to bytes for the calculation of offsets and lengths.
Table 67 – Deriving Client Keys from Keying Material
|
Name |
Offset |
Length |
|
ClientSigningKey |
0 |
DerivedSignatureKeyLength |
|
ClientEncryptingKey |
DerivedSignatureKeyLength |
EncryptionKeyLength |
|
ClientInitializationVector |
DerivedSignatureKeyLength + EncryptionKeyLength |
InitializationVectorLength |
The server keys are extracted from the keying material created with IKM=shared secret, Salt= ServerSalt and Info=ServerSalt as shown in Table 68.
Table 68 – Deriving Server Keys from Keying Material
|
Name |
Offset |
Length |
|
ServerSigningKey |
0 |
DerivedSignatureKeyLength |
|
ServerEncryptingKey |
DerivedSignatureKeyLength |
EncryptionKeyLength |
|
ServerInitializationVector |
DerivedSignatureKeyLength + EncryptionKeyLength |
InitializationVectorLength |
The SymmetricEncryptionAlgorithm for the SecurityPolicy sets the DerivedSignatureKeyLength, the EncryptionKeyLength and InitializationVectorLength..
When using AuthenticatedEncryption, the DerivedSignatureKeyLength is set to zero in the calculation of L in Step 1. The process of signing and encrypting data with AuthenticatedEncryption is discussed in 6.7.1.
When not using AuthenticatedEncryption with Sign only, the EncryptionKeyLength and InitializationVectorLength are set to 0 in the calculation of L in Step 1. The SymmetricEncryptionAlgorithm is not used and the padding has length zero. The process of signing and encrypting data when not using AuthenticatedEncryption is discussed in 6.7.1.
When using AuthenticatedEncryption, a unique InitializationVector is required for each MessageChunk. This value is constructed from the ClientInitializationVector or ServerInitializationVector where the first 8 bytes are XORed with the values in Table 69 encoded as described in 5.2.2.2.
When not using AuthenticatedEncryption, the data changes for each Message so the InitializationVector can be calculated once with the keys and reused until the keys are changed.
Table 69 – Creating a Mask for the Initialization Vector
|
Name |
Bytes |
Length |
|
TokenId |
4 |
The TokenId specified in the SecurityHeader of MessageChunk being processed. It is encoded as a UInt32 as described in 5.2.2.2. |
|
LastSequenceNumber |
4 |
The SequenceNumber specified in the SequenceHeader of last MessageChunk sent in the same direction on the SecureChannel. The value is 0 to indicate there is no LastSequenceNumber for the first MessageChunk which is always the OpenSecureChannel Message. It is encoded as a UInt32 as described in 5.2.2.2. |
The ClientInitializationVector is used when the Client encrypts the MessageChunk and the ServerInitializationVector is used when the Server encrypts the MessageChunk.
The LastSequenceNumber is the SequenceNumber from the previously sent Message which normally requires the previous Message to be decrypted. If the receiver processes incoming Messages in parallel it can calculate the expected SequenceNumber based on the order in which the encrypted Messages are received.
Once the keys are derived ECC SecureChannels behave the same as RSA SecureChannels.