OPC UA supports both RSA and ECC for security. This annex provides a brief overview of each and also describes some benefits and limitations of each. Both RSA and ECC utilize the mathematical concept of one-way functions.

RSA provides security using the mathematical concept that it is much more difficult to factor a number than to multiply two large numbers together. RSA uses two keys – a public key and a private key. RSA can be used for digital signatures and for encryption of data. The digital signature is generated using the private key and the public key can be used to validate that it was generated using the private key. For encryption, the data is encrypted using the public key and it can only be decrypted using the private key. There are a number of different algorithms that provide for signatures and encryption using these concepts.

Asymmetric encryption (using the two keys) can be very computationally intensive, can result in large keys and is rather slow, but it is well understood and publicly available. Typically, it is only used to exchange a SymmetricKey which is then used for all communication (signing and encryption).

ECC is based on the algebraic structure of elliptical curves or finite fields (see Figure B.1 for illustration). It depends on being able to compute a point (multiplication) and the difficulty in computing the multiplicand given the original point and the resulting product. The size of the elliptical curve determines how difficult the problem is. For ECC the parameters that define the curve must be known by all. There are a number of well-known curves that can be used.

Figure B.1 – Elliptical Curve Example

Compared to RSA, ECC requires smaller integers and thus much less computational power and has a smaller key to transmit. ECC is only used for signing not encryption, but it does allow for the generation of a shared secret over an un-secured channel (ECDH). Once the shared secrete is generated then Symmetric encryption can be used. ECC is not as well-known as RSA and there are many different curves, some of which are restricted by patents. The patents can apply to the algorithm that are used and / or the technique used. For interoperability it is important that a selected curve is supported by multiple libraries/platforms.

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