1. Calls it AES-CBC, but it's using AES-GCM https://github.com/tumblr/k8s-secret-projector/blob/master/p...
2. Encryption key generation (edited: from a password) uses a hash function rather than using a KDF. Short hashes are padded (takes first N bytes required to hit max length, appends to end) or trimmed to the appropriate length. https://github.com/tumblr/k8s-secret-projector/blob/master/p...
Do you have any examples of using a KDF that jives with the bitlength requirements of a given cipher?
Again, I'm not a cryptographer, and PBKDF2 is just an example. I would try to consult someone with a heavier background in the field. I've used PBKDF2 and Scrypt before, Argon2 is the newest, but also therefor least battle-tested.
https://godoc.org/golang.org/x/crypto/pbkdf2
Doesn't look like the Go API[1] allows you to specify an output key material length, so you'd just truncate or iterate until you get however much material you need (The HKDF Wiki implementation does this[2]).
1: https://godoc.org/golang.org/x/crypto/hkdf
2: https://en.wikipedia.org/wiki/HKDF#Example:_Python_implement...
On the other hand, it is anticipated that some applications will not be able to use HKDF "as-is" due to specific operational requirements, or will be able to use it but without the full benefits of the scheme. One significant example is the derivation of cryptographic keys from a source of low entropy, such as a user's password. The extract step in HKDF can concentrate existing entropy but cannot amplify entropy. In the case of password-based KDFs, a main goal is to slow down dictionary attacks using two ingredients: a salt value,and the intentional slowing of the key derivation computation. HKDF naturally accommodates the use of salt; however, a slowing down mechanism is not part of this specification. Applications interested in a password-based KDF should consider whether, for example, [PKCS5] meets their needs better than HKDF.
Did you consider using https://kubernetes.io/docs/tasks/administer-cluster/encrypt-... instead? It has been there for quite some time (1.10 I think) and fixes all your concerns.
(We had similar issues with storing secrets in etcd, and went with k8s encryption instead).