Tink for C++ HOW-TO

The following subsections present instructions and/or C++ snippets for some common tasks in Tink.

Installing Tink

Tink is built with Bazel, so it is quite easy to use Tink in a project built with Bazel, and this is the recommended way. See for example tink-examples repo on how to import Tink using Bazel's WORKSPACE file.

Still, there are definitely projects for which using Bazel is not an option, and for such situations we offer a library that can be used with other build tools. Currently only we offer support for Linux machines, but are working on supporting other operating systems as well.

Warning: In any case, the use of Tink without Bazel is at experimental stage, so the instructions given below might not work in some environments.

Supported Platforms

Linux
mac OS X (coming soon)

Caveats

Tink depends on Abseil and Protocol Buffers, so any project that wants to use Tink should either depend on the same versions of these libraries (cf. versions in the corresponding entries in WORKSPACE file), or not depend directly on these libraries at all (i.e. have only the indirect dependence via Tink).

Installing pre-build binaries (TODO)

Installing from the source

Prerequisites

To install Tink from the source code, the following prerequisites must be installed:

git - to download the source of Tink
Bazel - to build the Tink library

Step-by-step instructions to build and use `libtink.so`

clone Tink from GitHub:

git clone https://github.com/google/tink/

build the library and header-file bundles, and install them in appropriate directories of the target project (TARGET_DIR):

bazel build cc:libtink.so
bazel build cc:tink_headers cc:tink_deps_headers
TARGET_DIR="/usr/local"
mkdir -p $TARGET_DIR/lib
mkdir -p $TARGET_DIR/include
sudo cp bazel-bin/cc/libtink.so $TARGET_DIR/lib/
sudo tar xfv bazel-genfiles/cc/tink_headers.tar -C $TARGET_DIR/include/
sudo tar xfv bazel-genfiles/cc/tink_deps_headers.tar -C $TARGET_DIR/include/

If in Step 2 you specified a system directory (for example, /usr/local) as the TARGET_DIR, then run ldconfig to configure the linker. For example:
```
sudo ldconfig
```
If you assigned a TARGET_DIR other than a system directory (for example, ~/mydir), then you must append the extraction directory (for example, ~/mydir/lib) to two environment variables:
```
export LIBRARY_PATH=$LIBRARY_PATH:$TARGET_DIR/lib
export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$TARGET_DIR/lib
```

Validate your installation

To validate the installation compile and run hello_world.cc.

Copy the source code and a test cryptographic key, create some plaintext to encrypt.

cd /tmp
wget https://raw.githubusercontent.com/google/tink/master/examples/helloworld/cc/hello_world.cc
wget https://raw.githubusercontent.com/google/tink/master/examples/helloworld/cc/aes128_gcm_test_keyset_json.txt
echo "some message to be encrypted" > plaintext.txt

Compile the source code.

 g++ -I$TARGET_DIR/include/ -L$TARGET_DIR/lib/ hello_world.cc -ltink -o hello_world

Run hello_world application to encrypt and decrypt some data.

./hello_world aes128_gcm_test_keyset_json.txt encrypt plaintext.txt "associated data" ciphertext.bin
./hello_world aes128_gcm_test_keyset_json.txt decrypt ciphertext.bin "associated data" decrypted.txt
cat decrypted.txt

(additionally export LD_RUN_PATH=$LD_RUN_PATH:$TARGET_DIR/lib/ might be needed to ensure that the resulting binary can find libtink.so upon startup)

Initializing Tink

Tink provides customizable initialization, which allows for choosing specific implementations (identified by key types) of desired primitives. This initialization happens via registration of the implementations. Registration

For example, if you want to use all implementations of all primitives in Tink 1.1.0, the initialization would look as follows:

   #include "tink/config/tink_config.h"

   // ...
   auto status = TinkConfig::Init();
   if (!status.ok()) /* ... handle failure */;
   status = Config::Register(TinkConfig::Tink_1_1_0());
   // ...

To use only implementations of the AEAD primitive:

   #include "tink/aead/aead_config.h"

   // ...
   auto status = AeadConfig::Init();
   if (!status.ok()) /* ... handle failure */;
   status = Config::Register(AeadConfig::Tink_1_1_0());
   // ...

For custom initialization the registration proceeds directly via Registry-class:

   #include "tink/registry.h"
   #include "custom_project/custom_aead_key_manager.h"

   // ...
   auto status = Registry::RegisterKeyManager(
       CustomAeadKeyManager.kKeyType, new CustomAeadKeyManager());
   if (!status.ok()) /* ... handle failure */;

Generating New Key(set)s

Each KeyManager-implementation provides NewKey(template)-method that generates new keys of the corresponding key type. However to avoid accidental leakage of sensitive key material one should be careful with mixing key(set) generation with key(set) usage in code. To support the separation between these activities Tink package provides a command-line tool called Tinkey, which can be used for common key management tasks.

Still, if there is a need to generate a KeysetHandle with fresh key material directly in C++ code, one can use KeysetHandle:

    auto new_keyset_result = KeysetHandle::GenerateNew(key_template);
    if (!new_keyset_result.ok()) return new_key_result.status();
    auto keyset = std::move(new_keyset_result.ValueOrDie());
    // use the keyset...

where key_template can be initialized with one of pre-generated templates from examples/keytemplates-folder.

Loading Existing Keysets

To load cleartext keysets, use CleartextKeysetHandle and an appropriate KeysetReader, depending on the wire format of the stored keyset, for example a BinaryKeysetReader or a JsonKeysetReader.

    #include "tink/binary_keyset_reader.h"
    #include "tink/cleartext_keyset_handle.h"

    // ...
    std::string binary_keyset = ...;
    auto reader_result = BinaryKeysetReader::New(binary_keyset);
    if (!reader_result.ok()) return reader_result.status();
    auto reader = std::move(reader_result.ValueOrDie());
    auto handle_result = CleartextKeysetHandle::Read(std::move(reader));
    if (!handle_result.ok()) return handle_result.status();
    auto keyset_handle = std::move(handle_result.ValueOrDie());

To load encrypted keysets, one can use KeysetHandle and an appropriate KeysetReader:

    #include "tink/aead.h"
    #include "tink/json_keyset_reader.h"
    #include "tink/cleartext_keyset_handle.h"
    #include "tink/integration/aws_kms_client.h"

    // ...
    std::string json_encrypted_keyset = ...;
    auto reader_result = JsonKeysetReader::New(json_encrypted_keyset);
    if (!reader_result.ok()) return reader_result.status();
    auto reader = std::move(reader_result.ValueOrDie());
    std::string master_key_uri =
        "aws-kms://arn:aws:kms:us-east-1:007084425826:key/84a65985-f868-4bfc-83c2-366618acf147";
    auto aead = std::move(AwsKmsClient::NewAead(master_key_uri).ValueOrDie());
    auto handle_result = KeysetHandle::Read(std::move(reader), *aead);
    if (!handle_result.ok()) return handle_result.status();
    auto keyset_handle = std::move(handle_result.ValueOrDie());

Obtaining and Using Primitives

Primitives represent cryptographic operations offered by Tink, hence they form the core of Tink API. A primitive is just an interface that specifies what operations are offered by the primitive. A primitive can have multiple implementations, and user chooses a desired implementation by using a key of corresponding type (see the this section for details).

The following table summarizes C++ implementations of primitives that are currently available or planned (the latter are listed in brackets).

Primitive	Implementations
AEAD	AES-GCM, AES-CTR-HMAC, (AES-EAX)
MAC	HMAC-SHA2
Digital Signatures	ECDSA over NIST curves, (Ed25519)
Hybrid Encryption	ECIES with AEAD and HKDF

Tink user accesses implementations of a primitive via a factory that corresponds to the primitive: AEAD via AeadFactory, MAC via MacFactory, etc. where each factory offers corresponding getPrimitive(...) methods.

Symmetric Key Encryption

Here is how you can obtain and use an AEAD (Authenticated Encryption with Associated Data primitive to encrypt or decrypt data:

    #include "tink/aead.h"
    #include "tink/keyset_handle.h"
    #include "tink/aead/aead_factory.h"


    // 1. Get a handle to the key material.
    KeysetHandle keyset_handle = ...;

    // 2. Get the primitive.
    auto aead_result= AeadFactory.GetPrimitive(keyset_handle);
    if (!aead_result.ok()) return aead_result.status();
    auto aead = std::move(aead_result.ValueOrDie());

    // 3. Use the primitive.
    auto ciphertext_result = aead.Encrypt(plaintext, aad);
    if (!ciphertext_result.ok()) return ciphertext.status();
    auto ciphertext = std::move(ciphertext_result.ValueOrDie());

Hybrid Encryption

To decrypt using a combination of public key encryption and symmetric key encryption:

    #include "tink/hybrid_decrypt.h"
    #include "tink/keyset_handle.h"
    #include "tink/hybrid/hybrid_decrypt_factory.h"


    // 1. Get a handle to the key material.
    KeysetHandle keyset_handle = ...;

    // 2. Get the primitive.
    auto hybrid_decrypt_result= HybridDecryptFactory.GetPrimitive(keyset_handle);
    if (!hybrid_decrypt_result.ok()) return hybrid_decrypt_result.status();
    auto hybrid_decrypt = std::move(hybrid_decrypt_result.ValueOrDie());

    // 3. Use the primitive.
    auto plaintext_result = hybrid_decrypt.Decrypt(ciphertext, context_info);
    if (!plaintext_result.ok()) return plaintext.status();
    auto plaintext = std::move(plaintext_result.ValueOrDie());