Starting with version 7.3.11, rsyslog provides native support for log file encryption. In order to keep things highly flexible, both an encryption provider interface as well as an provider have been designed and implemented.
The provider interface enables to use different encryption method and tools. All that is need to be done is to write the actual provider to that interface. The encryption provider (called a "crypto provider") then implements everything necessary for the actual encryption. Note that decryption is not part of the rsyslog core. However, a new user tool - rscryutil - is provided by the project to handle this task. This tool is currently being considered to be part of the crypto provider. Consequently, there is no specific interface for it. The reasoning behind that decision is that there is very little provider-independet plumbing in this tool, so abstracting things looks a bit like over-engineering (we may change that view in the future based on how things evolve).
The initial crypto provider is based on libgcrypt, which looks like an excellent choice for (almost?) all use cases. Note that we support symmetric cryptography, only, inside the crypto provider. This is simply due to the fact that public/private key cryptography is too slow to be used for mass encryption (and this is what we do with log files!). Keep in your mind that even TLS uses symmetric cryptography for the actual session data (for the same reason), and uses public/private key cryptography only for the exchange of the symmetric key. In any case, folks using that functionality in high-secure environments are advised to check the exact security requirements. Periodic re-keying of the encrypted log files may be necessary.
No on to "how does it work?": The encryption functionality, at the action level, is enabled by specifying a crypto provider, for example as follows:
This tells the action to load the crypto provider. What then happens, is up to the crypto provider. For obvious reasons, just loading it is not sufficient, you need at least to provide a crypto key. How this is done depends on the crypto provider. It is assumed that all crypto providers user the "cry." config parameter name space. With the gcry provider, a full action may look like this:
This is the bare minimal set of parameters for gcry - it'll use defaults for the algorithm and use the key directly as specified from the configuration file.
Specifying the key directly in the configuration is usually a bad idea: in most setups, many or even all users can read the config file, so the key is far from being secret. We even thought of not permitting this option, as it is so insecure. We left it in, however, as it is a great help in initial testing.
For production use, there are two other modes: one is to read the key from a side-file (which needs to be sufficiently secured!) or obtain it via a special program that can obtain the key via any method it likes. The latter is meant to be used for higher security use cases. We assume that the (user-written) program can do all those "interesting things" that you can do to manage crypto keys in a secure manner. Access to the keyprogram, of course, needs to be properly secured in that case.
I hope this gives you a glimpse at how rsyslog log file encryption works. For details on the crypto provider parameters, see the official rsyslog grcy crypto provider documentation.