STET (Split-Trust Encryption Tool)

The Split-Trust Encryption Tool (STET) allows users to encrypt and decrypt data while splitting trust between multiple Key Management Systems and providing environment attestations to aid in key release policy decisions.

Building STET

STET can be built manually using either Go's native toolchain (e.g. go get and go build commands), or via Bazel.

Using Go's Native Toolchain

Install make and curl via apt-get install build-essential curl.
Install Go 1.22+.
Install protoc version 3.
Install Go plugins for the protocol compiler.
- Specifically, run go install google.golang.org/protobuf/cmd/protoc-gen-go@v1.26 and go install google.golang.org/grpc/cmd/protoc-gen-go-grpc@v1.1, then add $(go env GOPATH)/bin to your PATH.
From the root of this source repository, run make all to vendor .proto files and compile pb.go sources.
Build STET via go build github.com/GoogleCloudPlatform/stet/cmd/stet.
Copy the resulting binary to an executable location, granting it suid permissions (root permissions are required to generate attestations in a Confidential VM):
```
$ sudo mv stet /usr/local/bin
$ sudo chown root /usr/local/bin/stet
$ sudo chmod u+sx,a+rs /usr/local/bin/stet
```

Using Bazel

Install Bazel
From the root of this source repository, run bazel build cmd/stet:main
- This will download all necessary dependencies
- The resulting binary will be found at bazel-bin/cmd/stet/main_/main
Copy the resulting binary to an executable location, granting it suid permissions (root permissions are required to generate attestations in a Confidential VM):
```
$ sudo mv bazel-bin/cmd/stet/main_/main /usr/local/bin
$ sudo chown root /usr/local/bin/stet
$ sudo chmod u+sx,a+rx /usr/local/bin/stet
```

Prebuilt Binaries

Alternatively, prebuilt binaries are automatically compiled and bundled with releases of STET, which can be found on the Releases page. As with the above instructions, the binary should be copied to an executable location and granted suidpositions as well:

$ tar -zxf stet_1.0.0_linux_x86_64.tar.gz
$ sudo mv stet /usr/local/bin
$ sudo chown root /usr/local/bin/stet
$ sudo chmod u+sx,a+rx /usr/local/bin/stet

Using STET

Create configuration

STET configurations are written in YAML.

A key_config provides information about the encryption and decryption process. It includes the fields listed below.

KEK Information

One or more kek_infos fields must be defined, indicating all the KEKs to use in encryption and decryption for that configuration. Each kek_info contains the Google Cloud KMS URI of a KEK.

The number of kek_infos in a configuration must match the number of shares the DEK will be split into, as indicated by the Key Splitting Algorithm (see below).

DEK Algorithm

The DEK Algorithm indicates the algorithm used to generate the DEK.

Currently, only AES256-GCM is supported.

Key Splitting Algorithm

This indicates the number of shares the split the DEK into - if at all. It requires exactly one of the following fields to be defined:

no_split: true indicates the DEK will not be split into any shares.
- The key_config should contain a single kek_infos indicating the KEK to encrypt/decrypt the entire DEK with
shamir indicates the DEK will be split into multiple shares using Shamir's Secret Sharing. Each share will be encrypted/decrypted with a separate KEK. This configuration requires two fields to be defined:
- shares defines the total number of shares to split the DEK into.
- threshold defines the minimum number of shares needed to reconstitute the DEK during decryption.
- The key_config should contain a number of kek_infos equal to the values of shares

Examples

key_config using a single KMS (no split):

key_config:
  kek_infos:
  - kek_uri: "gcp-kms://projects/my-project/locations/us-east1/keyRings/my-key-ring/cryptoKeys/my-key"
  dek_algorithm: AES256_GCM
  no_split: true

key_config using Shamir's Secret Sharing:

key_config:
  kek_infos:
  - kek_uri: "gcp-kms://projects/my-project/locations/us-east1/keyRings/my-key-ring/cryptoKeys/my-key-1"
  - kek_uri: "gcp-kms://projects/my-project/locations/us-east1/keyRings/my-key-ring/cryptoKeys/my-key-2"
  dek_algorithm: AES256_GCM
  shamir:
    shares: 2
    threshold: 2

Creating a Configuration File

A configuration file for STET consists of two configurations: one for encryption and one for decryption.

An encrypt_config contains a single key_config with information about the key and scheme to encrypt with.

A decrypt_config contains all the key_configs known to the client, one of which should match the key_config the ciphertext was encrypted with. During decryption, the STET will search through the provided key_configs to find the matching one.

Note that the encryption and decryption configurations could be identical if the key_config used in encryption is the only known to the client. However, they must still be defined separately in the configuration file.

Example of a STET configuration file:

encrypt_config:
  key_config:
    kek_infos:
    - kek_uri: "gcp-kms://projects/my-project/locations/us-east1/keyRings/my-key-ring/cryptoKeys/my-key-1"
    - kek_uri: "gcp-kms://projects/my-project/locations/us-east1/keyRings/my-key-ring/cryptoKeys/my-key-2"
    dek_algorithm: AES256_GCM
    shamir:
      shares: 2
      threshold: 2

decrypt_config:
  key_configs:
  - kek_infos:  # Note the extra hyphen here, because key_configs is a repeated field.
    - kek_uri: "gcp-kms://projects/my-project/locations/us-east1/keyRings/my-key-ring/cryptoKeys/my-key0"
    dek_algorithm: AES256_GCM
    no_split: true
  key_configs:
  - kek_infos:  # Note the extra hyphen here, because key_configs is a repeated field.
    - kek_uri: "gcp-kms://projects/my-project/locations/us-east1/keyRings/my-key-ring/cryptoKeys/my-key-1"
    - kek_uri: "gcp-kms://projects/my-project/locations/us-east1/keyRings/my-key-ring/cryptoKeys/my-key-2"
    dek_algorithm: AES256_GCM
    shamir:
      shares: 2
      threshold: 2

Generating asymmetric keypairs for offline encryption

Another method of wrapping shares is to generate an asymmetric keypair locally. In your STET configuration file, you can specify the fingerprint of the keypair rather than the URI of a KMS-stored key and add another stanza describing the path to the public/private keys on disk.

Generate a private, 2048-bit RSA key:

$ openssl genrsa -out my_key.pem 2048

Create a public key file from that private key:

$ openssl rsa -in my_key.pem -pubout -out my_key.pub

Compute the fingerprint of the keypair:

$ openssl rsa -in stet.pem -pubout -outform DER | openssl sha256 -binary | openssl base64

Here is an example config file that encrypts data using a single RSA key:

encrypt_config:
  key_config:
    kek_infos:
    - rsa_fingerprint: "0FkfGnh4KEUBJGLoLUEcIIFTdlcx61ec1M/H2Gdh7tY="
    dek_algorithm: AES256_GCM
    no_split: true

decrypt_config:
  key_configs:
  - kek_infos:
    - rsa_fingerprint: "0FkfGnh4KEUBJGLoLUEcIIFTdlcx61ec1M/H2Gdh7tY="
    dek_algorithm: AES256_GCM
    no_split: true

asymmetric_keys:
  public_key_files:
  - "/home/me/stet.pub"
  private_key_files:
  - "/home/me/stet.pem"

Execute STET

Example invocations:

$ stet encrypt --config-file=~/stet.cfg ./plaintext.txt ./ciphertext.dat
$ stet decrypt --config-file=~/stet.cfg ./ciphertext.dat ./plaintext2.txt

Using stdin and stdout

In place of providing input and output files, stdin and stdout can be used for input and output respectively by using - in place of the corresponding file in the command.

When stdout is used as the output destination, logging from STET is written to stderr to avoid interference with the outputted plaintext or ciphertext.

Using stdin as input:

$ some-process | stet encrypt - ./ciphertext.dat
$ some-process | stet decrypt - ./plaintext.txt

Using stdout as output:

$ stet encrypt ./plaintext.txt - | some-process
$ stet decrypt ./ciphertext.dat - | some-process

Using both stdin as input and stdout as output:

$ some-process | stet encrypt - - | some-other-process
$ some-process | stet decrypt - - | some-other-process

Validating blob IDs and key URIs

When encrypting and decrypting data using STET, it is important to validate the blob ID and key URIs used. STET will output the blob ID stored in the metadata of the encrypted file, as well as the key URI of the key actually used (e.g. the specific external key URI when using an external key management system):

Blob ID of decrypted data: fa0fc0a8-b7b3-4ae2-bcc8-0aa862d1ff61
Used these key URIs: [https://my-ekm.io/keys/12345]

Checking the key URI used during encryption prevents accidental or malicious misconfiguration resulting in the incorrect (with a potentially less restrictive decryption policy) encryption key.

If the blob ID was set to a known value (e.g. the location of the file when uploaded to a bucket), ensuring a match at decryption time confirms that the data was not swapped out with another piece of data unknowingly.

`gsutil` integration

STET is also integrated with gsutil 5.0 and higher. Instead of invoking STET manually, if gsutil can find STET in your $PATH and the --stet flag is passed, file uploads will be encrypted via STET (and the blob ID is set to the final upload location), and file downloads will be decrypted via STET before being written to disk.

$ gsutil cp --stet secrets.txt "gs://my-bucket/my-secrets"
$ gsutil cp --stet "gs://my-bucket/my-secrets" plaintext.txt

The stet_binary_path and stet_config_path variables can be set in your boto configuration file to override the default path and config file lookups.

License

STET is not an officially supported Google product.

STET is released under the Apache 2.0 license.

Copyright 2021 Google LLC

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.

GoogleCloudPlatform/stet