PRIMITIVES.md

Tink Primitives

Tink performs cryptographic tasks via so-called primitives, which provide an abstract representation of the provided functionality. Tink primitives encompass the following cryptographic operations and are supported via the corresponding interfaces:

Primitive	Interfaces
Authenticated Encryption with Associated Data (AEAD)	AEAD
Streaming AEAD	StreamingAEAD
Deterministic AEAD	DeterministicAEAD
Message Authentication Code (MAC)	MAC
Pseudo Random Function Family (PRF)	Prf, PrfSet
Hybrid encryption	HybridEncrypt, HybridDecrypt
Digital signatures	PublicKeySign, PublicKeyVerify

The interface names above correspond with the Java implementation. Other language implementations may have modified interface names due to differing naming conventions.

The tables in the section below summarize the current implementations of primitives available in the corresponding languages, and the subsequent sections describe the main properties of Tink primitives.

Supported primitives and their implementations

Primitives supported by language

Primitive	Java	C++	Objective-C	Go	Python
AEAD	yes	yes	yes	yes	yes
Streaming AEAD	yes	yes	no	yes	yes
Deterministic AEAD	yes	yes	yes	yes	yes
MAC	yes	yes	yes	yes	yes
PRF	yes	yes	no	yes	yes
Digital signatures	yes	yes	yes	yes	yes
Hybrid encryption	yes	yes	yes	yes	yes

JavaScript is currently under development.

Primitive implementations supported by language

Primitive	Implementation	Java	C++ (BoringSSL)	C++ (OpenSSL)	Objective-C	Go	Python
AEAD	AES-GCM	yes	yes	yes	yes	yes	yes
	AES-GCM-SIV	yes	yes	no	no	no	no
	AES-CTR-HMAC	yes	yes	yes	yes	yes	yes
	AES-EAX	yes	yes	yes	yes	no	yes
	KMS Envelope	yes	yes	yes	no	yes	yes
	CHACHA20-POLY1305	yes	no	no	no	yes	no
	XCHACHA-POLY1305	yes	yes	no	yes	yes	yes
Streaming AEAD	AES-GCM-HKDF-STREAMING	yes	yes	yes	no	yes	yes
	AES-CTR-HMAC-STREAMING	yes	yes	yes	no	yes	yes
Deterministic AEAD	AES-SIV	yes	yes	yes	yes	yes	yes
MAC	HMAC-SHA2	yes	yes	yes	yes	yes	yes
	AES-CMAC	yes	yes	yes	yes	yes	yes
PRF	HKDF-SHA2	yes	yes	yes	no	yes	yes
	HMAC-SHA2	yes	yes	yes	no	yes	yes
	AES-CMAC	yes	yes	yes	no	yes	yes
Digital Signatures	ECDSA over NIST curves	yes	yes	yes *	yes	yes	yes
	Ed25519	yes	yes	yes	yes	yes	yes
	RSA-SSA-PKCS1	yes	yes	yes	yes	no	yes
	RSA-SSA-PSS	yes	yes	yes	yes	no	yes
Hybrid Encryption	HPKE	yes	yes	no	no	yes	yes
	ECIES with AEAD	yes	yes	yes	yes	yes	yes
	ECIES with DeterministicAEAD	yes	yes	yes	no	yes	yes
	HKDF	yes	yes	yes	yes	yes	yes

* EC key creation from seed (DeriveKey) is unsupported.

General properties of all primitives

• stateless (hence thread-safe)
• copy-safe (for the parameters)
• at least 128-bit security (with an exception for RSA)

Authenticated Encryption with Associated Data

AEAD primitive (Authenticated Encryption with Associated Data) provides functionality of symmetric authenticated encryption. Implementations of this primitive are secure against adaptive chosen ciphertext attacks.

When encrypting a plaintext one can optionally provide associated data that should be authenticated but not encrypted. That is, the encryption with associated data ensures authenticity (ie. who the sender is) and integrity (ie. data has not been tampered with) of that data, but not its secrecy (see RFC 5116). This is often used for binding encryptions to a context. For example, in a banking database the contents of a row (e.g. bank account balance) can be encrypted using the customer's id as associated data. This would prevent swapping encrypted data between customers' records.

Minimal properties:

• plaintext and associated data can have arbitrary length (within the range 0..2³² bytes)
• CCA2 security
• at least 80-bit authentication strength
• there are no secrecy or knowledge guarantees wrt. to the value of associated data
• can encrypt at least 2³² messages with a total of 2⁵⁰ bytes so that no attack has success probability larger than 2^-32

Streaming Authenticated Encryption with Associated Data

Streaming AEAD primitive (Streaming Authenticated Encryption with Associated Data) provides authenticated encryption for streaming data, and is useful when the data to be encrypted is too large to be processed in a single step. Typical use cases include encryption of large files or encryption of live data streams.

The underlying encryption modes are selected so that partial plaintext can be obtained fast by decrypting and authenticating just a part of the ciphertext, without need of processing the entire ciphertext.

Encryption must be done in one session. There is no possibility to modify an existing ciphertext or to append to it (other than to reencrypt the entire file again).

Instances of Streaming AEAD follow the OAE2 definition proposed in the paper "Online Authenticated-Encryption and its Nonce-Reuse Misuse-Resistance" by Hoang, Reyhanitabar, Rogaway and Vizár.

Minimal properties:

• plaintext can have arbitrary length within the range 0..2³⁸ and associated data can have arbitrary length within the range 0..2³¹-1 bytes
• CCA2 security
• at least 80-bit authentication strength
• there are no secrecy or knowledge guarantees wrt. to the value of associated data
• can encrypt at least 2³² messages with a total of 2⁶⁸ bytes so that no attack with up to 2³² chosen plaintexts/chosen ciphertexts has success probability larger than 2^-32.

Deterministic Authenticated Encryption with Associated Data

Deterministic AEAD primitive (Deterministic Authenticated Encryption with Associated Data, DAEAD) provides encryption with a deterministic property: encrypting the same data always yields the same ciphertext. Such encryption is useful e.g. for key wrapping or for some schemes for searching on encrypted data (see RFC 5297, Section 1.3 for more info). However, because of deterministic property, implementations of this primitive are not semantically secure.

As for (regular) AEAD, when using Deterministic AEAD to encrypt a plaintext one can optionally provide associated data that should be authenticated but not encrypted. That is, the encryption with associated data ensures authenticity (ie. who the sender is) and integrity (ie. data has not been tampered with) of that data, but not its secrecy (see RFC 5116).

Minimal properties:

• plaintext and associated data can have arbitrary length (within the range 0..2³² bytes)
• 128-bit security level against multi-user attacks with up to 2³² keys; that means if an adversary obtains 2³² ciphertexts of the same message encrypted under 2³² keys, they need to do 2¹²⁸ computations to obtain a single key.
• at least 80-bit authentication strength
• there are no secrecy or knowledge guarantees wrt. to the value of associated data

Message Authentication Code

MAC primitive (Message Authentication Code) provides symmetric message authentication. A sender sharing a symmetric key with a recipient can compute an authentication tag for a given message, that allows for verifying that the message comes from the sender and that it has not been modified. Instances of MAC primitive are secure against existential forgery under chosen plaintext attack, and can be deterministic or randomized. This interface should be used for authentication only, and not for other purposes like generation of pseudorandom bytes.

Minimal properties:

• secure against existential forgery under CPA
• at least 128-bit security, also in multi-user scenarios (when an attacker is not targeting a specific key, but any key from a set of up to 2³² keys)
• at least 80-bit authentication strength

Pseudo Random Function Families

The PRF set primitive allows to redact data in a deterministic fashion, for example personal identifiable information or internal IDs, or to come up with a user ID from user information without revealing said information in the ID. This allows someone with access to the output of the PRF without access to the key do some types of analysis, while limiting others.

Note that while in theory PRFs can be used in other ways, for example for encryption or message authentication, the corresponding primitives should only be used for these use cases.

WARNING: Since PRFs operate deterministically on their input, using a PRF to redact will not automatically provide anonymity, but only provide pseudonymity. It is an important tool to build privacy aware systems, but has to be used carefully.

Minimal properties:

• without knowledge of the key the PRF is indistinguishable from a random function
• at least 128-bit security, also in multi-user scenarios (when an attacker is not targeting a specific key, but any key from a set of up to 2³² keys)
• at least 16 byte of output available

WARNING: While HMAC-SHA-2 and HKDF-SHA-2 behave like a cryptographically secure hash function if the key is revealed, and still provide some protection against revealing the input, AES-CMAC is only secure as long as the key is secure.

Since Tink operates on key sets, this primitive exposes a corresponding set of PRFs instead of a single PRF. The PRFs are indexed by a 32 bit key id. This can be used to rotate the key used to redact a piece of information, without losing the previous association.

Hybrid Encryption

Hybrid Encryption combines the efficiency of symmetric encryption with the convenience of public-key encryption: to encrypt a message a fresh symmetric key is generated and used to encrypt the actual plaintext data, while the recipient’s public key is used to encrypt the symmetric key only, and the final ciphertext consists of the symmetric ciphertext and the encrypted symmetric key.

WARNING Hybrid Encryption does not provide authenticity, that is the recipient of an encrypted message does not know the identity of the sender. Similar to general public-key encryption schemes the security goal of Hybrid Encryption is to provide privacy only. In other words, Hybrid Encryption is secure if and only if the recipient can accept anonymous messages or can rely on other mechanism to authenticate the sender.

The functionality of Hybrid Encryption is represented in Tink as a pair of primitives: HybridEncrypt for encryption of data, and HybridDecrypt for decryption. Implementations of these primitives are secure against adaptive chosen ciphertext attacks.

In addition to the plaintext encryption accepts an extra parameter context info, which usually is public data implicit from the context, but should be bound to the resulting ciphertext, i.e. the ciphertext allows for checking the integrity of context info but there are no guarantees wrt. to secrecy or authenticity of context info. The actual context info can be empty or null, but to ensure the correct decryption of the resulting ciphertext the same value must be provided for decryption operation.

A concrete implementation of hybrid encryption can implement the binding of context info to the ciphertext in various ways, for example:

• use context_info as "associated data"-input for the employed AEAD symmetric encryption (cf. RFC 5116).
• use context_info as "CtxInfo"-input for HKDF (if the implementation uses HKDF as key derivation function, cf. RFC 5869).

Furthermore, the DataEncapsulationMechanism can be achieved using either an Aead primitive or a Determinisitic Aead.

Minimal properties:

• plaintext and context info can have arbitrary length (within the range 0..2³² bytes)
• secure against chosen ciphertext attacks
• 128-bit security for EC based schemes, 112-bit security for RSA based schemes (i.e. allow 2048 bit keys)

Digital Signatures

Digital Signatures provide functionality of signing data and verification of signatures. It ensures the authenticity and the integrity of the signed data, but not its secrecy.

The functionality of Digital Signatures is represented in Tink as a pair of primitives: PublicKeySign for signing of data, and PublicKeyVerify for verification of signatures. Implementations of these primitives are secure against adaptive chosen-message attacks.

Minimal properties:

• data to be signed can have arbitrary length
• 128-bit security for EC based schemes
• 112-bit security for RSA based schemes (i.e. allow 2048 bit keys)