kernel-crypto-api interface allows access to the Linux kernel crypto API, which itself provides a set of cryptographic ciphers and other data transformation mechanisms.
The kernel crypto API has been designed to be used by any process such that using it requires no special privileges. As this provides a kernel surface, and has a CVE history, this interface needs to be manually connected.
The test code can be found in the snapd repository: https://github.com/snapcore/snapd/blob/master/interfaces/builtin/kernel_crypto_api_test.go
The source code for the interface is in the snapd repository: https://github.com/snapcore/snapd/blob/master/interfaces/builtin/kernel_crypto_api.go