vault-based-kms-provider

Vault based KMS provider for envelope encryption of secrets in etcd3

Abstract

Kubernetes, starting with the release 1.7, adds Alpha support ( via PRs 41939 and 46460) to encrypt secrets and resources in etcd3 via a configured Provider. This release supports three providers viz. aesgcm, aescbc, secretbox. These providers store the encryption key(s) locally in a server configuration file. The provider encrypts and decrypts secrets in-process. Building upon these, a KMS provider framework with an option to support different KMS providers like google cloud KMS is being added via PRs 48574 and 49350. The new KMS provider framework uses an envelope encryption scheme.

This proposal adopts the KMS provider framework and adds a new KMS provider that uses Hashicorp Vault with a transit backend, to encrypt and decrypt the DEK stored in encrypted form in etcd3 along with encrypted secrets.

Vault is widely used for Data encryption and securely storing secrets. Externalizing encryption/decryption of kubernetes secrets to vault provides various benefits

  • Choice of industry standard encryption algorithms and strengths without having to implement specific providers for each (in K8S).
  • Reduced risk of encryption key compromise.
    • encryption key is stored and managed in Vault.
    • encryption key does not need to leave the Vault.
  • Vault provides ability to define access control suitable for a wide range of deployment scenarios and security needs.
  • Vault provides In-built auditing of vault API calls.
  • Ability for a customer already using Vault to leverage the instance to also secure keys used to encrypt secrets managed within a Kubernetes cluster
  • Separation of Kubernetes cluster management responsibilities from encryption key management and administration allowing an organization to better leverage competencies and skills within the DevOps teams.

Note, that the Vault Provider in this proposal

  1. requires Vault transit backend.
  2. supports a wide range of authentication backends supported by vault (see below for exact list).
  3. does not depend on specific storage backend or any other specific configuration.

This proposal assumes familiarity with Vault and the transit back-end.

High level design

As with existing providers, the Vault based provider will implement the interface envelope.Service. Based on value of name in the KMS provider configuration, the EnvelopeTransformer module will use an instance of the Vault provider for decryption and encryption of DEK before storing and after reading from the storage.

The KEK will be stored and managed in Vault backend. The Vault based provider configured in KMS Transformer configuration will make REST requests to encrypt and decrypt DEKs over a secure channel (must enable TLS). KMS Transformer will store the DEKs in etcd in encrypted form along with encrypted secrets. As with existing providers, encrypted DEKs will be stored with metadata used to identify the provider and KEK to be used for decryption.

The provider will support following authentication back-ends

  • Vault token based,
  • TLS cert based,
  • Vault AppRole based.

Deployers can choose an authentication mechanism best suited to their requirements. The provider will work with vault REST APIs and will not require Vault to be configured or deployed in any specific way other than requiring a Transit Backend.

Diagram illustrating interfaces and implementations

Image of interfaces

Pseudocode

Prefix Metadata

Every encrypted secret will have the following metadata prefixed.

k8s:enc:kms:<api-version>:vault:len(<KEK-key-name>:<KEK-key-version>:<DEK
encrypted with KEK>):<KEK-key-name>:<KEK-key-version>:<DEK encrypted with KEK>
  • <api-version> represents api version in the providers configuration file.
  • vault represents the KMS service kind value. It is a fixed value for Vault based provider.
  • KEK-key-name is determined from the vault service configuration in providers configuration file
  • KEK-key-version is an internal identifier used by vault to identify specific key version used to encrypt and decrypt. Vault sends kek-key-version prefixed with encrypted data in the response to an encrypt request. The kek-key-version will be stored as part of prefix and returned back to Vault during a decrypt request.

Of the above metadata,

  • EnvelopeTransformer will add k8s:enc:kms:<api-version>:vault:len(<KEK-key-name>:<KEK-key-version>:<DEK encrypted with KEK>)
  • while the vaultEnvelopeService will add <KEK-key-name>:<KEK-key-version>:<DEK encrypted with KEK>.

For each write of DEK

EnvelopeTransformer will write encrypted DEK along with encrypted secret in etcd.

Here’s the pseudocode for vaultEnvelopeService.encrypt(), invoked on each write of DEK.

KEY_NAME = <first key-name from vault provider config>
PLAIN_DEK = <value of DEK>
ENCRYPTED_DEK_WITH_KEY_VERSION  = encrypt(base64(PLAIN_DEK), KEY_NAME)

// output from vault will have an extra prefix "vault" (other than key version) which will be stripped.

STORED_DEK = KEY_NAME:<ENCRYPTED_DEK_WITH_KEY_VERSION>

For each read of DEK

EnvelopeTransformer will read encrypted DEK along with encrypted secret from etcd

Here’s the pseudocode vaultEnvelopeService.decrypt() invoked on each read of DEK.

// parse the provider kind, key name and encrypted DEK prefixed with key version
KEY_NAME = //key-name from the prefix
ENCRYPTED_DEK_WITH_KEY_VERSION = //<key version>:<encrypted DEK> from the stored value

 // add "vault" prefix to ENCRYPTED_DEK_WITH_KEY_VERSION as required by vault decrypt API

base64Encoded = decrypt(vault:ENCRYPTED_DEK_WITH_KEY_VERSION, KEY_NAME)

PLAIN_DEK = base64.Decode(base64Encoded)

Example

DEK = "the quick brown fox"
provider kind = "vault"
api version version = "v1"
Key name = "kube-secret-enc-key"
key version = v1
ciphertext returned from vault = vault:v1:aNOTZn0aUDMDbWAQL1E31tH/7zr7oslRjkSpRW0+BPdMfSJntyXZNCAwIbkTtn0=
prefixed DEK used to tag secrets = vault:kube-secret-enc-key:v1:aNOTZn0aUDMDbWAQL1E31tH/7zr7oslRjkSpRW0+BPdMfSJntyXZNCAwIbkTtn0=

Configuration

No new configuration file or startup parameter will be introduced.

The vault provider will be specified in the existing configuration file used to configure any of the encryption providers. The location of this configuration file is identified by the existing startup parameter: --experimental-encryption-provider-config .

Vault provider configuration will be identified by value “vault” for the name attribute in kms provider.

The actual configuration of the vault provider will be in a separate configuration identified by the configfile attribute in the KMS provider.

Here is a sample configuration file with the vault provider configured:

kind: EncryptionConfig
apiVersion: v1
resources:
  - resources:
    - secrets
    providers:
    - kms:
        name: vault
        cachesize: 10
        configfile: /home/myvault/vault-config.yaml

Minimal required Configuration

The Vault based Provider needs the following configuration elements, at a minimum:

  1. addr Vault service base endpoint eg. https://example.com:8200
  2. key-names list of names of the keys in Vault to be used. eg: key-name: kube-secret-enc-key.

Note : key name does not need to be changed if the key is rotated in Vault, the rotated key is identified by key version which is prefix to ciphertext.

A new key can be added in the list. Encryption will be done using the first key in the list. Decryption can happen using any of the keys in the list based on the prefix to the encrypted DEK stored in etcd

Authentication Configuration

Vault Server Authentication

For the Kubernetes cluster to authenticate the vault server, TLS must be enabled : 1. ca-cert location of x509 certificate to authenticate the vault server eg: /var/run/kubernetes/ssl/vault.crt

Client Authentication Choices

For client authentication, one of following must be used: (provider will reject the configuration if parameters for more than one authentication backends are specified )

X509 based authentication
  1. client-cert: location of x509 certificate to authenticate kubernetes API server to vault server eg. /var/run/kubernetes/ssl/valut-client-cert.pem
  2. client-key : location of x509 private key to authenticate kubernetes API server to vault server eg. /var/run/kubernetes/ssl/vault-client-key.pem

Here’s a sample vault-config.yaml configuration with client-cert:

	key-names:
	  - kube-secret-enc-key
	addr: https://example.com:8200
	ca-cert:/var/run/kubernetes/ssl/vault.crt
	client-cert:/var/run/kubernetes/ssl/vault-client-cert.pem
	client-key:/var/run/kubernetes/ssl/vault-client-key.pem
Vault token based authentication
  1. token : limited access vault token required by kubernetes API server to authenticate itself while making requests to vault eg: 8dad1053-4a4e-f359-2eab-d57968eb277f

Here’s a sample vault-config.yaml configuration using a Vault Token for authentication. the Kubernetes cluster as a client to Vault:

  key-names:
  	 -kube-secret-enc-key
  addr: https://example.com:8200
  ca-cert:/var/run/kubernetes/ssl/vault.crt
  token: 8dad1053-4a4e-f359-2eab-d57968eb277f
Vault AppRole based authentication
  1. role-id : RoleID of the AppRole
  2. secret-id : secret Id only if associated with the appRole.

Here’s a sample configuration file using a Vault AppRole for authentication.

  key-names:
    - kube-secret-enc-key
  addr: https://localhost:8200
  ca-cert: /var/run/kubernetes/ssl/vault.crt
  role-id: db02de05-fa39-4855-059b-67221c5c2f63

Key Generation and rotation

The KEK is generated in Vault and rotated using direct API call or CLI to Vault itself. The Key never leaves the vault.

Note that when a key is rotated, Vault does not allow choosing a different encryption algorithm or key size. If a key for different encryption algorithm or a different key size is desired, new key needs to be generated in Vault and the corresponding key name be added in the configuration. Subsequent encryption will be done using the first key in the list. Decryption can happen using any of the keys in the list based on the prefix to the encrypted DEK.

Backward compatibility

  1. Unencrypted secrets and secrets encrypted using other non-KMS providers will continue to be readable upon adding vault as a new KMS provider.
  2. If a Vault KMS is added as first provider, the secrets created or modified thereafter will be encrypted by vault provider.

Performance

  1. KMS provider framework uses LRU cache to minimize the requests to KMS for encryption and decryption of DEKs.
  2. Note that there will be a request to KMS for every cache miss causing a performance impact. Hence, depending on the cache size, there will be a performance impact.
  3. Response time.
    1. will depend on choice of encryption algorithm and strength.
    2. will depend on specific vault configurations like storage backend, authentication mechanism, token polices etc.