In addition to cryptographic operations Tink provides support for key management features like key versioning, key rotation, and storing keysets or encrypting with master keys in remote key management systems (KMS). To get a quick overview of Tink design, incl. key management features, you can also take a look at slides from a talk about Tink presented at Real World Crypto 2019.
Tink performs cryptographic tasks via so-called primitives, each of which is defined via a corresponding interface that specifies the functionality of the primitive.
A particular implementation of a primitive is identified by a cryptographic key structure that contains all key material and parameters needed to provide the functionality of the primitive. The key structure is a protocol buffer, whose globally unique name (a.k.a. type url) is referred to as key type, and is used as an identifier of the corresponding implementation of a primitive. Any particular implementation comes in a form of a KeyManager which “understands” the key type: the manager can instantiate the primitive corresponding to a given key, or can generate new keys of the supported key type.
To take advantage of key rotation and other key management features, a Tink user works usually not with single keys, but with keysets, which are just sets of keys with some additional parameters and metadata. In particular, this extra information in the keyset determines which key is primary (i.e. will be used to create new cryptographic data like ciphertexts, or signatures), which keys are enabled (i.e. can be used to process existing cryptographic data, like decrypt ciphertext or verify signatures), and which keys should not be used any more. For more details about the structure of keys, keysets and related protocol buffers see tink.proto.
The keys in a keyset can belong to different implementations/key types, but must all implement the same primitive. Any given keyset (and any given key) can be used for one primitive only. Moreover, to protect from accidental leakage or corruption, an Tink user doesn’t work directly with keysets, but rather with KeysetHandle objects, which form a wrapper around the keysets. Creation of KeysetHandle objects can be restricted to specific factories (whose visibility can be governed by a white list), to enable control over actual storage of the keys and keysets, and so avoid accidental leakage of secret key material.
Tink/Tinkey can encrypt or decrypt keysets with master keys residing in remote KMSes. Currently, the following KMSes are supported:
On iOS, Tink can also directly load or store keysets in iOS KeyChain.
You can easily add support for in-house key management systems, without having to change anything in Tink/Tinkey. For example, when Tink/Tinkey is deployed at Google, it supports encrypting keysets with master keys stored in our internal key management system.
To encrypt Tink's key material with master keys in KMSes, you first create a master key in the KMS and tell Tink/Tinkey where the master key is by providing a master key URI.
|KMS||Format of master key URIs|
Every master key URI starts with a unique prefix that identifies its KMS. The prefix for AWS KMS is
aws-kms://, Google Cloud KMS
gcp-kms://, and Android Keystore
To create master keys:
In Google Cloud KMS: https://cloud.google.com/kms/docs/quickstart.
In Android Keystore: https://developer.android.com/training/articles/keystore.html.
Tink/Tinkey needs credentials to connect to AWS KMS or Google Cloud KMS. Google Cloud credentials are service account JSON files that can be created and downloaded from Google Cloud Console. AWS credentials are properties files with the AWS access key ID is expected to be in the
accessKey property and the AWS secret key is expected to be in the
Tink/Tinkey can also load the default credentials:
Via the AEAD interface, Tink supports envelope encryption in tandem with GCP and AWS KMS. This provides one of the simplest key management options.
For more context, reference the following cloud service provider documentation:
In this mode, you first create a key encryption key (KEK) in a Key Management System (KMS) such as AWS KMS or Google Cloud KMS. To encrypt some data, you then locally generate a data encryption key (DEK), encrypt data with the DEK, ask the KMS to encrypt the DEK with the KEK, and store the encrypted DEK with the encrypted data. At a later point, you can retrieve the encrypted data and the encrypted DEK, ask the KMS to decrypt the DEK, and use the decrypted DEK to decrypt the data.