Random numbers that meet security needs can be generated by suitable functions that are provided by cryptography libraries. Common random functions such as using rand() provided by the “C” standard library do not generate enough entropy. As an alternative, implementers could use the random number generators provided by the Microsoft Windows Crypto library (WinCrypt library) or by OpenSSL. Even the random functions provided in cryptography libraries require a source of entropy to initialize and the required entropy is not always available on embedded devices. PCs can use several individual pieces of information (hardware ids like CPU, Mac, addresses, USB devices, screen resolution, installed software …) to generate entropy, but embedded devices are built completely identically. Often only the time and maybe a MAC address is left for entropy. These sources of entropy can be guessed or discovered. This makes the embedded devices very vulnerable.
A common mistake is to generate cryptographic keys during the first boot. Thus even the time information is predictable (creation time is stored e.g. in a certificate). Some alternate solutions a vendor might want to consider:
- Add specific entropy generator hardware when designing embedded devices.
- Do not generate certificates on embedded devices. Use an external tool or the GDS to generate the certificate and load it onto the device. A problem could still remain for the symmetric keys, as these are normally not created directly during the boot phase; rather they are created when a client connects.
- Wait long enough until enough entropy information is available. Some operating systems provide hints when they have reached this point.
- For embedded systems without a good entropy source it may help to store the cryptographic pseudo-random number generator (CPRNG) state, so that it will not produce the same random numbers after every boot. Vendor should ensure that cryptographic functions they use are initialized with suitable entropy and that the generated certificates are not created in a predictable manner.