config.go

/*
Copyright 2017 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package encryptionconfig

import (
	"context"
	"crypto/aes"
	"crypto/cipher"
	"crypto/sha256"
	"encoding/base64"
	"errors"
	"fmt"
	"io"
	"net/http"
	"os"
	"sync"
	"sync/atomic"
	"time"

	"k8s.io/apimachinery/pkg/runtime"
	"k8s.io/apimachinery/pkg/runtime/schema"
	"k8s.io/apimachinery/pkg/runtime/serializer"
	utilerrors "k8s.io/apimachinery/pkg/util/errors"
	utilruntime "k8s.io/apimachinery/pkg/util/runtime"
	"k8s.io/apimachinery/pkg/util/uuid"
	"k8s.io/apimachinery/pkg/util/wait"
	apiserverconfig "k8s.io/apiserver/pkg/apis/config"
	apiserverconfigv1 "k8s.io/apiserver/pkg/apis/config/v1"
	"k8s.io/apiserver/pkg/apis/config/validation"
	"k8s.io/apiserver/pkg/features"
	"k8s.io/apiserver/pkg/server/healthz"
	"k8s.io/apiserver/pkg/server/options/encryptionconfig/metrics"
	storagevalue "k8s.io/apiserver/pkg/storage/value"
	aestransformer "k8s.io/apiserver/pkg/storage/value/encrypt/aes"
	"k8s.io/apiserver/pkg/storage/value/encrypt/envelope"
	envelopekmsv2 "k8s.io/apiserver/pkg/storage/value/encrypt/envelope/kmsv2"
	kmstypes "k8s.io/apiserver/pkg/storage/value/encrypt/envelope/kmsv2/v2"
	envelopemetrics "k8s.io/apiserver/pkg/storage/value/encrypt/envelope/metrics"
	"k8s.io/apiserver/pkg/storage/value/encrypt/identity"
	"k8s.io/apiserver/pkg/storage/value/encrypt/secretbox"
	utilfeature "k8s.io/apiserver/pkg/util/feature"
	"k8s.io/klog/v2"
	kmsservice "k8s.io/kms/pkg/service"
)

const (
	aesCBCTransformerPrefixV1    = "k8s:enc:aescbc:v1:"
	aesGCMTransformerPrefixV1    = "k8s:enc:aesgcm:v1:"
	secretboxTransformerPrefixV1 = "k8s:enc:secretbox:v1:"
	kmsTransformerPrefixV1       = "k8s:enc:kms:v1:"
	kmsTransformerPrefixV2       = "k8s:enc:kms:v2:"

	// these constants relate to how the KMS v2 plugin status poll logic
	// and the DEK/seed generation logic behave.  In particular, the positive
	// interval and max TTL are closely related as the difference between
	// these values defines the worst case window in which the write DEK/seed
	// could expire due to the plugin going into an error state.  The
	// worst case window divided by the negative interval defines the
	// minimum amount of times the server will attempt to return to a
	// healthy state before the DEK/seed expires and writes begin to fail.
	//
	// For now, these values are kept small and hardcoded to support being
	// able to perform a "passive" storage migration while tolerating some
	// amount of plugin downtime.
	//
	// With the current approach, a user can update the key ID their plugin
	// is using and then can simply schedule a migration for 3 + N + M minutes
	// later where N is how long it takes their plugin to pick up new config
	// and M is extra buffer to allow the API server to process the config.
	// At that point, they are guaranteed to either migrate to the new key
	// or get errors during the migration.
	//
	// If the API server coasted forever on the last DEK/seed, they would need
	// to actively check if it had observed the new key ID before starting
	// a migration - otherwise it could keep using the old DEK/seed and their
	// storage migration would not do what they thought it did.
	kmsv2PluginHealthzPositiveInterval = 1 * time.Minute
	kmsv2PluginHealthzNegativeInterval = 10 * time.Second
	kmsv2PluginWriteDEKSourceMaxTTL    = 3 * time.Minute

	kmsPluginHealthzNegativeTTL = 3 * time.Second
	kmsPluginHealthzPositiveTTL = 20 * time.Second
	kmsAPIVersionV1             = "v1"
	kmsAPIVersionV2             = "v2"
	// this name is used for two different healthz endpoints:
	// - when one or more KMS v2 plugins are in use and no KMS v1 plugins are in use
	//   in this case, all v2 plugins are probed via this single endpoint
	// - when automatic reload of encryption config is enabled
	//   in this case, all KMS plugins are probed via this single endpoint
	//   the endpoint is present even if there are no KMS plugins configured (it is a no-op then)
	kmsReloadHealthCheckName = "kms-providers"
)

var codecs serializer.CodecFactory

// this atomic bool allows us to swap enablement of the KMSv2KDF feature in tests
// as the feature gate is now locked to true starting with v1.29
// Note: it cannot be set by an end user
var kdfDisabled atomic.Bool

// this function should only be called in tests to swap enablement of the KMSv2KDF feature
func SetKDFForTests(b bool) func() {
	kdfDisabled.Store(!b)
	return func() {
		kdfDisabled.Store(false)
	}
}

// this function should be used to determine enablement of the KMSv2KDF feature
// instead of getting it from DefaultFeatureGate as the feature gate is now locked
// to true starting with v1.29
func GetKDF() bool {
	return !kdfDisabled.Load()
}

func init() {
	configScheme := runtime.NewScheme()
	utilruntime.Must(apiserverconfig.AddToScheme(configScheme))
	utilruntime.Must(apiserverconfigv1.AddToScheme(configScheme))
	codecs = serializer.NewCodecFactory(configScheme)
	envelopemetrics.RegisterMetrics()
	storagevalue.RegisterMetrics()
	metrics.RegisterMetrics()
}

type kmsPluginHealthzResponse struct {
	err      error
	received time.Time
}

type kmsPluginProbe struct {
	name         string
	ttl          time.Duration
	service      envelope.Service
	lastResponse *kmsPluginHealthzResponse
	l            *sync.Mutex
}

type kmsv2PluginProbe struct {
	state        atomic.Pointer[envelopekmsv2.State]
	name         string
	ttl          time.Duration
	service      kmsservice.Service
	lastResponse *kmsPluginHealthzResponse
	l            *sync.Mutex
	apiServerID  string
	version      string
}

type kmsHealthChecker []healthz.HealthChecker

func (k kmsHealthChecker) Name() string {
	return kmsReloadHealthCheckName
}

func (k kmsHealthChecker) Check(req *http.Request) error {
	var errs []error

	for i := range k {
		checker := k[i]
		if err := checker.Check(req); err != nil {
			errs = append(errs, fmt.Errorf("%s: %w", checker.Name(), err))
		}
	}

	return utilerrors.Reduce(utilerrors.NewAggregate(errs))
}

func (h *kmsPluginProbe) toHealthzCheck(idx int) healthz.HealthChecker {
	return healthz.NamedCheck(fmt.Sprintf("kms-provider-%d", idx), func(r *http.Request) error {
		return h.check()
	})
}

func (h *kmsv2PluginProbe) toHealthzCheck(idx int) healthz.HealthChecker {
	return healthz.NamedCheck(fmt.Sprintf("kms-provider-%d", idx), func(r *http.Request) error {
		return h.check(r.Context())
	})
}

// EncryptionConfiguration represents the parsed and normalized encryption configuration for the apiserver.
type EncryptionConfiguration struct {
	// Transformers is a list of value.Transformer that will be used to encrypt and decrypt data.
	Transformers map[schema.GroupResource]storagevalue.Transformer

	// HealthChecks is a list of healthz.HealthChecker that will be used to check the health of the encryption providers.
	HealthChecks []healthz.HealthChecker

	// EncryptionFileContentHash is the hash of the encryption config file.
	EncryptionFileContentHash string

	// KMSCloseGracePeriod is the duration we will wait before closing old transformers.
	// We wait for any in-flight requests to finish by using the duration which is longer than their timeout.
	KMSCloseGracePeriod time.Duration
}

// LoadEncryptionConfig parses and validates the encryption config specified by filepath.
// It may launch multiple go routines whose lifecycle is controlled by ctx.
// In case of an error, the caller is responsible for canceling ctx to clean up any go routines that may have been launched.
// If reload is true, or KMS v2 plugins are used with no KMS v1 plugins, the returned slice of health checkers will always be of length 1.
func LoadEncryptionConfig(ctx context.Context, filepath string, reload bool, apiServerID string) (*EncryptionConfiguration, error) {
	config, contentHash, err := loadConfig(filepath, reload)
	if err != nil {
		return nil, fmt.Errorf("error while parsing file: %w", err)
	}

	transformers, kmsHealthChecks, kmsUsed, err := getTransformerOverridesAndKMSPluginHealthzCheckers(ctx, config, apiServerID)
	if err != nil {
		return nil, fmt.Errorf("error while building transformers: %w", err)
	}

	if reload || (kmsUsed.v2Used && !kmsUsed.v1Used) {
		kmsHealthChecks = []healthz.HealthChecker{kmsHealthChecker(kmsHealthChecks)}
	}

	// KMSCloseGracePeriod is the duration we will wait before closing old transformers.
	// The way we calculate is as follows:
	// 1. Sum all timeouts across all KMS plugins. (check kmsPrefixTransformer for differences between v1 and v2)
	// 2. Multiply that by 2 (to allow for some buffer)
	// The reason we sum all timeout is because kmsHealthChecker() will run all health checks serially
	return &EncryptionConfiguration{
		Transformers:              transformers,
		HealthChecks:              kmsHealthChecks,
		EncryptionFileContentHash: contentHash,
		KMSCloseGracePeriod:       2 * kmsUsed.kmsTimeoutSum,
	}, nil
}

// getTransformerOverridesAndKMSPluginHealthzCheckers creates the set of transformers and KMS healthz checks based on the given config.
// It may launch multiple go routines whose lifecycle is controlled by ctx.
// In case of an error, the caller is responsible for canceling ctx to clean up any go routines that may have been launched.
func getTransformerOverridesAndKMSPluginHealthzCheckers(ctx context.Context, config *apiserverconfig.EncryptionConfiguration, apiServerID string) (map[schema.GroupResource]storagevalue.Transformer, []healthz.HealthChecker, *kmsState, error) {
	var kmsHealthChecks []healthz.HealthChecker
	transformers, probes, kmsUsed, err := getTransformerOverridesAndKMSPluginProbes(ctx, config, apiServerID)
	if err != nil {
		return nil, nil, nil, err
	}
	for i := range probes {
		probe := probes[i]
		kmsHealthChecks = append(kmsHealthChecks, probe.toHealthzCheck(i))
	}

	return transformers, kmsHealthChecks, kmsUsed, nil
}

type healthChecker interface {
	toHealthzCheck(idx int) healthz.HealthChecker
}

// getTransformerOverridesAndKMSPluginProbes creates the set of transformers and KMS probes based on the given config.
// It may launch multiple go routines whose lifecycle is controlled by ctx.
// In case of an error, the caller is responsible for canceling ctx to clean up any go routines that may have been launched.
func getTransformerOverridesAndKMSPluginProbes(ctx context.Context, config *apiserverconfig.EncryptionConfiguration, apiServerID string) (map[schema.GroupResource]storagevalue.Transformer, []healthChecker, *kmsState, error) {
	resourceToPrefixTransformer := map[schema.GroupResource][]storagevalue.PrefixTransformer{}
	var probes []healthChecker
	var kmsUsed kmsState

	// For each entry in the configuration
	for _, resourceConfig := range config.Resources {
		resourceConfig := resourceConfig

		transformers, p, used, err := prefixTransformersAndProbes(ctx, resourceConfig, apiServerID)
		if err != nil {
			return nil, nil, nil, err
		}
		kmsUsed.accumulate(used)

		// For each resource, create a list of providers to use
		for _, resource := range resourceConfig.Resources {
			resource := resource
			gr := schema.ParseGroupResource(resource)

			// check if resource is masked by *.group rule
			anyResourceInGroup := schema.GroupResource{Group: gr.Group, Resource: "*"}
			if _, masked := resourceToPrefixTransformer[anyResourceInGroup]; masked {
				// an earlier rule already configured a transformer for *.group, masking this rule
				// return error since this is not allowed
				return nil, nil, nil, fmt.Errorf("resource %q is masked by earlier rule %q", grYAMLString(gr), grYAMLString(anyResourceInGroup))
			}

			if _, masked := resourceToPrefixTransformer[anyGroupAnyResource]; masked {
				// an earlier rule already configured a transformer for *.*, masking this rule
				// return error since this is not allowed
				return nil, nil, nil, fmt.Errorf("resource %q is masked by earlier rule %q", grYAMLString(gr), grYAMLString(anyGroupAnyResource))
			}

			resourceToPrefixTransformer[gr] = append(resourceToPrefixTransformer[gr], transformers...)
		}

		probes = append(probes, p...)
	}

	transformers := make(map[schema.GroupResource]storagevalue.Transformer, len(resourceToPrefixTransformer))
	for gr, transList := range resourceToPrefixTransformer {
		gr := gr
		transList := transList
		transformers[gr] = storagevalue.NewPrefixTransformers(fmt.Errorf("no matching prefix found"), transList...)
	}

	return transformers, probes, &kmsUsed, nil
}

// check encrypts and decrypts test data against KMS-Plugin's gRPC endpoint.
func (h *kmsPluginProbe) check() error {
	h.l.Lock()
	defer h.l.Unlock()

	if (time.Since(h.lastResponse.received)) < h.ttl {
		return h.lastResponse.err
	}

	p, err := h.service.Encrypt([]byte("ping"))
	if err != nil {
		h.lastResponse = &kmsPluginHealthzResponse{err: err, received: time.Now()}
		h.ttl = kmsPluginHealthzNegativeTTL
		return fmt.Errorf("failed to perform encrypt section of the healthz check for KMS Provider %s, error: %w", h.name, err)
	}

	if _, err := h.service.Decrypt(p); err != nil {
		h.lastResponse = &kmsPluginHealthzResponse{err: err, received: time.Now()}
		h.ttl = kmsPluginHealthzNegativeTTL
		return fmt.Errorf("failed to perform decrypt section of the healthz check for KMS Provider %s, error: %w", h.name, err)
	}

	h.lastResponse = &kmsPluginHealthzResponse{err: nil, received: time.Now()}
	h.ttl = kmsPluginHealthzPositiveTTL
	return nil
}

// check gets the healthz status of the KMSv2-Plugin using the Status() method.
func (h *kmsv2PluginProbe) check(ctx context.Context) error {
	h.l.Lock()
	defer h.l.Unlock()

	if time.Since(h.lastResponse.received) < h.ttl {
		return h.lastResponse.err
	}

	p, err := h.service.Status(ctx)
	if err != nil {
		h.lastResponse = &kmsPluginHealthzResponse{err: err, received: time.Now()}
		h.ttl = kmsPluginHealthzNegativeTTL
		return fmt.Errorf("failed to perform status section of the healthz check for KMS Provider %s, error: %w", h.name, err)
	}

	if err := h.isKMSv2ProviderHealthyAndMaybeRotateDEK(ctx, p); err != nil {
		h.lastResponse = &kmsPluginHealthzResponse{err: err, received: time.Now()}
		h.ttl = kmsPluginHealthzNegativeTTL
		return err
	}

	h.lastResponse = &kmsPluginHealthzResponse{err: nil, received: time.Now()}
	h.ttl = kmsPluginHealthzPositiveTTL
	return nil
}

// rotateDEKOnKeyIDChange tries to rotate to a new DEK/seed if the key ID returned by Status does not match the
// current state.  If a successful rotation is performed, the new DEK/seed and keyID overwrite the existing state.
// On any failure during rotation (including mismatch between status and encrypt calls), the current state is
// preserved and will remain valid to use for encryption until its expiration (the system attempts to coast).
// If the key ID returned by Status matches the current state, the expiration of the current state is extended
// and no rotation is performed.
func (h *kmsv2PluginProbe) rotateDEKOnKeyIDChange(ctx context.Context, statusKeyID, uid string) error {
	// we do not check ValidateEncryptCapability here because it is fine to re-use an old key
	// that was marked as expired during an unhealthy period.  As long as the key ID matches
	// what we expect then there is no need to rotate here.
	state, errState := h.getCurrentState()

	// allow reads indefinitely in all cases
	// allow writes indefinitely as long as there is no error
	// allow writes for only up to kmsv2PluginWriteDEKSourceMaxTTL from now when there are errors
	// we start the timer before we make the network call because kmsv2PluginWriteDEKSourceMaxTTL is meant to be the upper bound
	expirationTimestamp := envelopekmsv2.NowFunc().Add(kmsv2PluginWriteDEKSourceMaxTTL)

	// dynamically check if we want to use KDF seed to derive DEKs or just a single DEK
	// this gate can only change during tests, but the check is cheap enough to always make
	// this allows us to easily exercise both modes without restarting the API server
	// TODO integration test that this dynamically takes effect
	useSeed := GetKDF()
	stateUseSeed := state.EncryptedObject.EncryptedDEKSourceType == kmstypes.EncryptedDEKSourceType_HKDF_SHA256_XNONCE_AES_GCM_SEED

	// state is valid and status keyID is unchanged from when we generated this DEK/seed so there is no need to rotate it
	// just move the expiration of the current state forward by the reuse interval
	// useSeed can only change at runtime during tests, so we check it here to allow us to easily exercise both modes
	if errState == nil && state.EncryptedObject.KeyID == statusKeyID && stateUseSeed == useSeed {
		state.ExpirationTimestamp = expirationTimestamp
		h.state.Store(&state)
		return nil
	}

	transformer, encObject, cacheKey, errGen := envelopekmsv2.GenerateTransformer(ctx, uid, h.service, useSeed)

	if encObject == nil {
		encObject = &kmstypes.EncryptedObject{} // avoid nil panics
	}

	// happy path, should be the common case
	// TODO maybe add success metrics?
	if errGen == nil && encObject.KeyID == statusKeyID {
		h.state.Store(&envelopekmsv2.State{
			Transformer:         transformer,
			EncryptedObject:     *encObject,
			UID:                 uid,
			ExpirationTimestamp: expirationTimestamp,
			CacheKey:            cacheKey,
		})

		// it should be logically impossible for the new state to be invalid but check just in case
		_, errGen = h.getCurrentState()
		if errGen == nil {
			klogV6 := klog.V(6)
			if klogV6.Enabled() {
				klogV6.InfoS("successfully rotated DEK",
					"uid", uid,
					"useSeed", useSeed,
					"newKeyIDHash", envelopekmsv2.GetHashIfNotEmpty(encObject.KeyID),
					"oldKeyIDHash", envelopekmsv2.GetHashIfNotEmpty(state.EncryptedObject.KeyID),
					"expirationTimestamp", expirationTimestamp.Format(time.RFC3339),
				)
			}
			return nil
		}
	}

	return fmt.Errorf("failed to rotate DEK uid=%q, useSeed=%v, errState=%v, errGen=%v, statusKeyIDHash=%q, encryptKeyIDHash=%q, stateKeyIDHash=%q, expirationTimestamp=%s",
		uid, useSeed, errState, errGen, envelopekmsv2.GetHashIfNotEmpty(statusKeyID), envelopekmsv2.GetHashIfNotEmpty(encObject.KeyID), envelopekmsv2.GetHashIfNotEmpty(state.EncryptedObject.KeyID), state.ExpirationTimestamp.Format(time.RFC3339))
}

// getCurrentState returns the latest state from the last status and encrypt calls.
// If the returned error is nil, the state is considered valid indefinitely for read requests.
// For write requests, the caller must also check that state.ValidateEncryptCapability does not error.
func (h *kmsv2PluginProbe) getCurrentState() (envelopekmsv2.State, error) {
	state := *h.state.Load()

	if state.Transformer == nil {
		return envelopekmsv2.State{}, fmt.Errorf("got unexpected nil transformer")
	}

	encryptedObjectCopy := state.EncryptedObject
	if len(encryptedObjectCopy.EncryptedData) != 0 {
		return envelopekmsv2.State{}, fmt.Errorf("got unexpected non-empty EncryptedData")
	}
	encryptedObjectCopy.EncryptedData = []byte{0} // any non-empty value to pass validation
	if err := envelopekmsv2.ValidateEncryptedObject(&encryptedObjectCopy); err != nil {
		return envelopekmsv2.State{}, fmt.Errorf("got invalid EncryptedObject: %w", err)
	}

	if state.ExpirationTimestamp.IsZero() {
		return envelopekmsv2.State{}, fmt.Errorf("got unexpected zero expirationTimestamp")
	}

	if len(state.CacheKey) == 0 {
		return envelopekmsv2.State{}, fmt.Errorf("got unexpected empty cacheKey")
	}

	return state, nil
}

func (h *kmsv2PluginProbe) isKMSv2ProviderHealthyAndMaybeRotateDEK(ctx context.Context, response *kmsservice.StatusResponse) error {
	var errs []error
	if response.Healthz != "ok" {
		errs = append(errs, fmt.Errorf("got unexpected healthz status: %s", response.Healthz))
	}
	if response.Version != envelopekmsv2.KMSAPIVersionv2 && response.Version != envelopekmsv2.KMSAPIVersionv2beta1 {
		errs = append(errs, fmt.Errorf("expected KMSv2 API version %s, got %s", envelopekmsv2.KMSAPIVersionv2, response.Version))
	} else {
		// set version for the first status response
		if len(h.version) == 0 {
			h.version = response.Version
		}
		if h.version != response.Version {
			errs = append(errs, fmt.Errorf("KMSv2 API version should not change after the initial status response version %s, got %s", h.version, response.Version))
		}
	}

	if errCode, err := envelopekmsv2.ValidateKeyID(response.KeyID); err != nil {
		envelopemetrics.RecordInvalidKeyIDFromStatus(h.name, string(errCode))
		errs = append(errs, fmt.Errorf("got invalid KMSv2 KeyID hash %q: %w", envelopekmsv2.GetHashIfNotEmpty(response.KeyID), err))
	} else {
		envelopemetrics.RecordKeyIDFromStatus(h.name, response.KeyID, h.apiServerID)
		// unconditionally append as we filter out nil errors below
		errs = append(errs, h.rotateDEKOnKeyIDChange(ctx, response.KeyID, string(uuid.NewUUID())))
	}

	if err := utilerrors.Reduce(utilerrors.NewAggregate(errs)); err != nil {
		return fmt.Errorf("kmsv2 Provider %s is not healthy, error: %w", h.name, err)
	}
	return nil
}

// loadConfig parses the encryption configuration file at filepath and returns the parsed config and hash of the file.
func loadConfig(filepath string, reload bool) (*apiserverconfig.EncryptionConfiguration, string, error) {
	data, contentHash, err := loadDataAndHash(filepath)
	if err != nil {
		return nil, "", fmt.Errorf("error while loading file: %w", err)
	}

	configObj, gvk, err := codecs.UniversalDecoder().Decode(data, nil, nil)
	if err != nil {
		return nil, "", fmt.Errorf("error decoding encryption provider configuration file %q: %w", filepath, err)
	}
	config, ok := configObj.(*apiserverconfig.EncryptionConfiguration)
	if !ok {
		return nil, "", fmt.Errorf("got unexpected config type: %v", gvk)
	}

	return config, contentHash, validation.ValidateEncryptionConfiguration(config, reload).ToAggregate()
}

func loadDataAndHash(filepath string) ([]byte, string, error) {
	f, err := os.Open(filepath)
	if err != nil {
		return nil, "", fmt.Errorf("error opening encryption provider configuration file %q: %w", filepath, err)
	}
	defer f.Close()

	data, err := io.ReadAll(f)
	if err != nil {
		return nil, "", fmt.Errorf("could not read contents: %w", err)
	}
	if len(data) == 0 {
		return nil, "", fmt.Errorf("encryption provider configuration file %q is empty", filepath)
	}

	return data, computeEncryptionConfigHash(data), nil
}

// GetEncryptionConfigHash reads the encryption configuration file at filepath and returns the hash of the file.
// It does not attempt to decode or load the config, and serves as a cheap check to determine if the file has changed.
func GetEncryptionConfigHash(filepath string) (string, error) {
	_, contentHash, err := loadDataAndHash(filepath)
	return contentHash, err
}

// prefixTransformersAndProbes creates the set of transformers and KMS probes based on the given resource config.
// It may launch multiple go routines whose lifecycle is controlled by ctx.
// In case of an error, the caller is responsible for canceling ctx to clean up any go routines that may have been launched.
func prefixTransformersAndProbes(ctx context.Context, config apiserverconfig.ResourceConfiguration, apiServerID string) ([]storagevalue.PrefixTransformer, []healthChecker, *kmsState, error) {
	var transformers []storagevalue.PrefixTransformer
	var probes []healthChecker
	var kmsUsed kmsState

	for _, provider := range config.Providers {
		provider := provider
		var (
			transformer    storagevalue.PrefixTransformer
			transformerErr error
			probe          healthChecker
			used           *kmsState
		)

		switch {
		case provider.AESGCM != nil:
			transformer, transformerErr = aesPrefixTransformer(provider.AESGCM, aestransformer.NewGCMTransformer, aesGCMTransformerPrefixV1)

		case provider.AESCBC != nil:
			cbcTransformer := func(block cipher.Block) (storagevalue.Transformer, error) {
				return aestransformer.NewCBCTransformer(block), nil
			}
			transformer, transformerErr = aesPrefixTransformer(provider.AESCBC, cbcTransformer, aesCBCTransformerPrefixV1)

		case provider.Secretbox != nil:
			transformer, transformerErr = secretboxPrefixTransformer(provider.Secretbox)

		case provider.KMS != nil:
			transformer, probe, used, transformerErr = kmsPrefixTransformer(ctx, provider.KMS, apiServerID)
			if transformerErr == nil {
				probes = append(probes, probe)
				kmsUsed.accumulate(used)
			}

		case provider.Identity != nil:
			transformer = storagevalue.PrefixTransformer{
				Transformer: identity.NewEncryptCheckTransformer(),
				Prefix:      []byte{},
			}

		default:
			return nil, nil, nil, errors.New("provider does not contain any of the expected providers: KMS, AESGCM, AESCBC, Secretbox, Identity")
		}

		if transformerErr != nil {
			return nil, nil, nil, transformerErr
		}

		transformers = append(transformers, transformer)
	}

	return transformers, probes, &kmsUsed, nil
}

type blockTransformerFunc func(cipher.Block) (storagevalue.Transformer, error)

func aesPrefixTransformer(config *apiserverconfig.AESConfiguration, fn blockTransformerFunc, prefix string) (storagevalue.PrefixTransformer, error) {
	var result storagevalue.PrefixTransformer

	if len(config.Keys) == 0 {
		return result, fmt.Errorf("aes provider has no valid keys")
	}
	for _, key := range config.Keys {
		key := key
		if key.Name == "" {
			return result, fmt.Errorf("key with invalid name provided")
		}
		if key.Secret == "" {
			return result, fmt.Errorf("key %v has no provided secret", key.Name)
		}
	}

	keyTransformers := []storagevalue.PrefixTransformer{}

	for _, keyData := range config.Keys {
		keyData := keyData
		key, err := base64.StdEncoding.DecodeString(keyData.Secret)
		if err != nil {
			return result, fmt.Errorf("could not obtain secret for named key %s: %w", keyData.Name, err)
		}
		block, err := aes.NewCipher(key)
		if err != nil {
			return result, fmt.Errorf("error while creating cipher for named key %s: %w", keyData.Name, err)
		}
		transformer, err := fn(block)
		if err != nil {
			return result, fmt.Errorf("error while creating transformer for named key %s: %w", keyData.Name, err)
		}

		// Create a new PrefixTransformer for this key
		keyTransformers = append(keyTransformers,
			storagevalue.PrefixTransformer{
				Transformer: transformer,
				Prefix:      []byte(keyData.Name + ":"),
			})
	}

	// Create a prefixTransformer which can choose between these keys
	keyTransformer := storagevalue.NewPrefixTransformers(
		fmt.Errorf("no matching key was found for the provided AES transformer"), keyTransformers...)

	// Create a PrefixTransformer which shall later be put in a list with other providers
	result = storagevalue.PrefixTransformer{
		Transformer: keyTransformer,
		Prefix:      []byte(prefix),
	}
	return result, nil
}

func secretboxPrefixTransformer(config *apiserverconfig.SecretboxConfiguration) (storagevalue.PrefixTransformer, error) {
	var result storagevalue.PrefixTransformer

	if len(config.Keys) == 0 {
		return result, fmt.Errorf("secretbox provider has no valid keys")
	}
	for _, key := range config.Keys {
		key := key
		if key.Name == "" {
			return result, fmt.Errorf("key with invalid name provided")
		}
		if key.Secret == "" {
			return result, fmt.Errorf("key %v has no provided secret", key.Name)
		}
	}

	keyTransformers := []storagevalue.PrefixTransformer{}

	for _, keyData := range config.Keys {
		keyData := keyData
		key, err := base64.StdEncoding.DecodeString(keyData.Secret)
		if err != nil {
			return result, fmt.Errorf("could not obtain secret for named key %s: %s", keyData.Name, err)
		}

		if len(key) != 32 {
			return result, fmt.Errorf("expected key size 32 for secretbox provider, got %v", len(key))
		}

		keyArray := [32]byte{}
		copy(keyArray[:], key)

		// Create a new PrefixTransformer for this key
		keyTransformers = append(keyTransformers,
			storagevalue.PrefixTransformer{
				Transformer: secretbox.NewSecretboxTransformer(keyArray),
				Prefix:      []byte(keyData.Name + ":"),
			})
	}

	// Create a prefixTransformer which can choose between these keys
	keyTransformer := storagevalue.NewPrefixTransformers(
		fmt.Errorf("no matching key was found for the provided Secretbox transformer"), keyTransformers...)

	// Create a PrefixTransformer which shall later be put in a list with other providers
	result = storagevalue.PrefixTransformer{
		Transformer: keyTransformer,
		Prefix:      []byte(secretboxTransformerPrefixV1),
	}
	return result, nil
}

var (
	// The factory to create kms service. This is to make writing test easier.
	envelopeServiceFactory = envelope.NewGRPCService

	// The factory to create kmsv2 service.  Exported for integration tests.
	EnvelopeKMSv2ServiceFactory = envelopekmsv2.NewGRPCService
)

type kmsState struct {
	v1Used, v2Used bool
	kmsTimeoutSum  time.Duration
}

// accumulate computes the KMS state by:
//   - determining which KMS plugin versions are in use
//   - calculating kmsTimeoutSum which is used as transformTracker.kmsCloseGracePeriod
//     DynamicTransformers.Set waits for this period before closing old transformers after a config reload
func (s *kmsState) accumulate(other *kmsState) {
	s.v1Used = s.v1Used || other.v1Used
	s.v2Used = s.v2Used || other.v2Used
	s.kmsTimeoutSum += other.kmsTimeoutSum
}

// kmsPrefixTransformer creates a KMS transformer and probe based on the given KMS config.
// It may launch multiple go routines whose lifecycle is controlled by ctx.
// In case of an error, the caller is responsible for canceling ctx to clean up any go routines that may have been launched.
func kmsPrefixTransformer(ctx context.Context, config *apiserverconfig.KMSConfiguration, apiServerID string) (storagevalue.PrefixTransformer, healthChecker, *kmsState, error) {
	kmsName := config.Name
	switch config.APIVersion {
	case kmsAPIVersionV1:
		if !utilfeature.DefaultFeatureGate.Enabled(features.KMSv1) {
			return storagevalue.PrefixTransformer{}, nil, nil, fmt.Errorf("KMSv1 is deprecated and will only receive security updates going forward. Use KMSv2 instead.  Set --feature-gates=KMSv1=true to use the deprecated KMSv1 feature.")
		}
		klog.InfoS("KMSv1 is deprecated and will only receive security updates going forward. Use KMSv2 instead.")

		envelopeService, err := envelopeServiceFactory(ctx, config.Endpoint, config.Timeout.Duration)
		if err != nil {
			return storagevalue.PrefixTransformer{}, nil, nil, fmt.Errorf("could not configure KMSv1-Plugin's probe %q, error: %w", kmsName, err)
		}

		probe := &kmsPluginProbe{
			name:         kmsName,
			ttl:          kmsPluginHealthzNegativeTTL,
			service:      envelopeService,
			l:            &sync.Mutex{},
			lastResponse: &kmsPluginHealthzResponse{},
		}

		transformer := envelopePrefixTransformer(config, envelopeService, kmsTransformerPrefixV1)

		return transformer, probe, &kmsState{
			v1Used: true,
			// for v1 we will do encrypt and decrypt for health check. Since these are serial operations, we will double the timeout.
			kmsTimeoutSum: 2 * config.Timeout.Duration,
		}, nil

	case kmsAPIVersionV2:
		if !utilfeature.DefaultFeatureGate.Enabled(features.KMSv2) {
			return storagevalue.PrefixTransformer{}, nil, nil, fmt.Errorf("could not configure KMSv2 plugin %q, KMSv2 feature is not enabled", kmsName)
		}

		envelopeService, err := EnvelopeKMSv2ServiceFactory(ctx, config.Endpoint, config.Name, config.Timeout.Duration)
		if err != nil {
			return storagevalue.PrefixTransformer{}, nil, nil, fmt.Errorf("could not configure KMSv2-Plugin's probe %q, error: %w", kmsName, err)
		}

		probe := &kmsv2PluginProbe{
			name:         kmsName,
			ttl:          kmsPluginHealthzNegativeTTL,
			service:      envelopeService,
			l:            &sync.Mutex{},
			lastResponse: &kmsPluginHealthzResponse{},
			apiServerID:  apiServerID,
		}
		// initialize state so that Load always works
		probe.state.Store(&envelopekmsv2.State{})

		primeAndProbeKMSv2(ctx, probe, kmsName)
		transformer := storagevalue.PrefixTransformer{
			Transformer: envelopekmsv2.NewEnvelopeTransformer(envelopeService, kmsName, probe.getCurrentState, apiServerID),
			Prefix:      []byte(kmsTransformerPrefixV2 + kmsName + ":"),
		}

		return transformer, probe, &kmsState{
			v2Used:        true,
			kmsTimeoutSum: config.Timeout.Duration,
		}, nil

	default:
		return storagevalue.PrefixTransformer{}, nil, nil, fmt.Errorf("could not configure KMS plugin %q, unsupported KMS API version %q", kmsName, config.APIVersion)
	}
}

func primeAndProbeKMSv2(ctx context.Context, probe *kmsv2PluginProbe, kmsName string) {
	runProbeCheckAndLog := func(ctx context.Context, depth int) error {
		if err := probe.check(ctx); err != nil {
			klog.VDepth(1+depth, 2).ErrorS(err, "kms plugin failed health check probe", "name", kmsName)
			return err
		}
		return nil
	}

	blockAndProbeFastUntilSuccess := func(ctx context.Context) {
		_ = wait.PollUntilWithContext(
			ctx,
			kmsv2PluginHealthzNegativeInterval,
			func(ctx context.Context) (bool, error) {
				return runProbeCheckAndLog(ctx, 1) == nil, nil
			},
		)
	}

	// on the happy path where the plugin is healthy and available on server start,
	// prime keyID and DEK by running the check inline once (this also prevents unit tests from flaking)
	errPrime := runProbeCheckAndLog(ctx, 0)

	// if our initial attempt to prime failed, start trying to get to a valid state in the background ASAP
	// this prevents a slow start when the external healthz checker is configured to ignore the KMS healthz endpoint
	// since we want to support the plugin starting up async with the API server, this error is not fatal
	if errPrime != nil {
		go blockAndProbeFastUntilSuccess(ctx) // separate go routine to avoid blocking
	}

	// make sure that the plugin's key ID is reasonably up-to-date
	// also, make sure that our DEK is up-to-date to with said key ID (if it expires the server will fail all writes)
	// if this background loop ever stops running, the server will become unfunctional after kmsv2PluginWriteDEKSourceMaxTTL
	go wait.PollUntilWithContext(
		ctx,
		kmsv2PluginHealthzPositiveInterval,
		func(ctx context.Context) (bool, error) {
			if err := runProbeCheckAndLog(ctx, 0); err == nil {
				return false, nil
			}

			// TODO add integration test for quicker error poll on failure
			// if we fail, block the outer polling and start a new quicker poll inline
			// this limits the chance that our DEK expires during a transient failure
			blockAndProbeFastUntilSuccess(ctx)

			return false, nil
		})
}

func envelopePrefixTransformer(config *apiserverconfig.KMSConfiguration, envelopeService envelope.Service, prefix string) storagevalue.PrefixTransformer {
	baseTransformerFunc := func(block cipher.Block) (storagevalue.Transformer, error) {
		gcm, err := aestransformer.NewGCMTransformer(block)
		if err != nil {
			return nil, err
		}

		// v1.24: write using AES-CBC only but support reads via AES-CBC and AES-GCM (so we can move to AES-GCM)
		// v1.25: write using AES-GCM only but support reads via AES-GCM and fallback to AES-CBC for backwards compatibility
		// TODO(aramase): Post v1.25: We cannot drop CBC read support until we automate storage migration.
		// We could have a release note that hard requires users to perform storage migration.
		return unionTransformers{gcm, aestransformer.NewCBCTransformer(block)}, nil
	}

	return storagevalue.PrefixTransformer{
		Transformer: envelope.NewEnvelopeTransformer(envelopeService, int(*config.CacheSize), baseTransformerFunc),
		Prefix:      []byte(prefix + config.Name + ":"),
	}
}

type unionTransformers []storagevalue.Transformer

func (u unionTransformers) TransformFromStorage(ctx context.Context, data []byte, dataCtx storagevalue.Context) (out []byte, stale bool, err error) {
	var errs []error
	for i := range u {
		transformer := u[i]
		result, stale, err := transformer.TransformFromStorage(ctx, data, dataCtx)
		if err != nil {
			errs = append(errs, err)
			continue
		}
		// when i != 0, we have transformed the data from storage using the new transformer,
		// we want to issue a write to etcd even if the contents of the data haven't changed
		return result, stale || i != 0, nil
	}
	if err := utilerrors.Reduce(utilerrors.NewAggregate(errs)); err != nil {
		return nil, false, err
	}
	return nil, false, fmt.Errorf("unionTransformers: unable to transform from storage")
}

func (u unionTransformers) TransformToStorage(ctx context.Context, data []byte, dataCtx storagevalue.Context) (out []byte, err error) {
	return u[0].TransformToStorage(ctx, data, dataCtx)
}

// computeEncryptionConfigHash returns the expected hash for an encryption config file that has been loaded as bytes.
// We use a hash instead of the raw file contents when tracking changes to avoid holding any encryption keys in memory outside of their associated transformers.
// This hash must be used in-memory and not externalized to the process because it has no cross-release stability guarantees.
func computeEncryptionConfigHash(data []byte) string {
	return fmt.Sprintf("%x", sha256.Sum256(data))
}

var _ storagevalue.ResourceTransformers = &DynamicTransformers{}
var _ healthz.HealthChecker = &DynamicTransformers{}

// DynamicTransformers holds transformers that may be dynamically updated via a single external actor, likely a controller.
// This struct must avoid locks (even read write locks) as it is inline to all calls to storage.
type DynamicTransformers struct {
	transformTracker *atomic.Value
}

type transformTracker struct {
	transformerOverrides  map[schema.GroupResource]storagevalue.Transformer
	kmsPluginHealthzCheck healthz.HealthChecker
	closeTransformers     context.CancelFunc
	kmsCloseGracePeriod   time.Duration
}

// NewDynamicTransformers returns transformers, health checks for kms providers and an ability to close transformers.
func NewDynamicTransformers(
	transformerOverrides map[schema.GroupResource]storagevalue.Transformer,
	kmsPluginHealthzCheck healthz.HealthChecker,
	closeTransformers context.CancelFunc,
	kmsCloseGracePeriod time.Duration,
) *DynamicTransformers {
	dynamicTransformers := &DynamicTransformers{
		transformTracker: &atomic.Value{},
	}

	tracker := &transformTracker{
		transformerOverrides:  transformerOverrides,
		kmsPluginHealthzCheck: kmsPluginHealthzCheck,
		closeTransformers:     closeTransformers,
		kmsCloseGracePeriod:   kmsCloseGracePeriod,
	}
	dynamicTransformers.transformTracker.Store(tracker)

	return dynamicTransformers
}

// Check implements healthz.HealthChecker
func (d *DynamicTransformers) Check(req *http.Request) error {
	return d.transformTracker.Load().(*transformTracker).kmsPluginHealthzCheck.Check(req)
}

// Name implements healthz.HealthChecker
func (d *DynamicTransformers) Name() string {
	return kmsReloadHealthCheckName
}

// TransformerForResource returns the transformer for the given resource.
func (d *DynamicTransformers) TransformerForResource(resource schema.GroupResource) storagevalue.Transformer {
	return &resourceTransformer{
		resource:         resource,
		transformTracker: d.transformTracker,
	}
}

// Set sets the transformer overrides. This method is not go routine safe and must only be called by the same, single caller throughout the lifetime of this object.
func (d *DynamicTransformers) Set(
	transformerOverrides map[schema.GroupResource]storagevalue.Transformer,
	closeTransformers context.CancelFunc,
	kmsPluginHealthzCheck healthz.HealthChecker,
	kmsCloseGracePeriod time.Duration,
) {
	// store new values
	newTransformTracker := &transformTracker{
		transformerOverrides:  transformerOverrides,
		closeTransformers:     closeTransformers,
		kmsPluginHealthzCheck: kmsPluginHealthzCheck,
		kmsCloseGracePeriod:   kmsCloseGracePeriod,
	}

	// update new transformer overrides
	oldTransformTracker := d.transformTracker.Swap(newTransformTracker).(*transformTracker)

	// close old transformers once we wait for grpc request to finish any in-flight requests.
	// by the time we spawn this go routine, the new transformers have already been set and will be used for new requests.
	// if the server starts shutting down during sleep duration then the transformers will correctly closed early because their lifetime is tied to the api-server drain notifier.
	go func() {
		time.Sleep(oldTransformTracker.kmsCloseGracePeriod)
		oldTransformTracker.closeTransformers()
	}()
}

var _ storagevalue.Transformer = &resourceTransformer{}

type resourceTransformer struct {
	resource         schema.GroupResource
	transformTracker *atomic.Value
}

func (r *resourceTransformer) TransformFromStorage(ctx context.Context, data []byte, dataCtx storagevalue.Context) ([]byte, bool, error) {
	return r.transformer().TransformFromStorage(ctx, data, dataCtx)
}

func (r *resourceTransformer) TransformToStorage(ctx context.Context, data []byte, dataCtx storagevalue.Context) ([]byte, error) {
	return r.transformer().TransformToStorage(ctx, data, dataCtx)
}

func (r *resourceTransformer) transformer() storagevalue.Transformer {
	return transformerFromOverrides(r.transformTracker.Load().(*transformTracker).transformerOverrides, r.resource)
}

var _ storagevalue.ResourceTransformers = &StaticTransformers{}

type StaticTransformers map[schema.GroupResource]storagevalue.Transformer

func (s StaticTransformers) TransformerForResource(resource schema.GroupResource) storagevalue.Transformer {
	return transformerFromOverrides(s, resource)
}

var anyGroupAnyResource = schema.GroupResource{
	Group:    "*",
	Resource: "*",
}

func transformerFromOverrides(transformerOverrides map[schema.GroupResource]storagevalue.Transformer, resource schema.GroupResource) storagevalue.Transformer {
	if transformer := transformerOverrides[resource]; transformer != nil {
		return transformer
	}

	if transformer := transformerOverrides[schema.GroupResource{
		Group:    resource.Group,
		Resource: "*",
	}]; transformer != nil {
		return transformer
	}

	if transformer := transformerOverrides[anyGroupAnyResource]; transformer != nil {
		return transformer
	}

	return identity.NewEncryptCheckTransformer()
}

func grYAMLString(gr schema.GroupResource) string {
	if gr.Group == "" && gr.Resource == "*" {
		return "*."
	}

	return gr.String()
}