# Generic Group Analyzer Unbounded "Do it unbounded"

## Introduction

Generic Group Analyzer Unbounded is a tool to automatically prove computational security statements in the Generic Group Model. It is especially designed to deal with security experiments where the attacker is allowed to interact with an Oracle an unbounded number of times and this interaction may be adaptive, i.e., new queries can depend on earlier Oracle responses.

This overcomes previous tools like the Generic Group Analyzer (Crypto 2014), paper available at http://eprint.iacr.org/2014/458, which cannot deal with adaptive adversaries; or its extension (PKC 2015), whose paper is available at http://eprint.iacr.org/2015/019 and it only deals with experiments where the number of queries is fixed.

The tool focuses on pairing-based cryptographic constructions, in particular, on schemes defined over bilinear groups.
A bilinear group is a tupple $(p, \mathbb{G}_{1}, \mathbb{G}_{2}, \mathbb{G}_{T}, g_{1}, g_{2}, e)$ where,
• $p$ is a prime number,
• $\mathbb{G}_{1}, \mathbb{G}_{2}, \mathbb{G}_{T}$ are cyclic groups of prime order $p$,
• $g_{1}$ is a generator of $\mathbb{G}_{1}$ and $g_{2}$ is a generator of $\mathbb{G}_{2}$,
• $e$, known as the pairing, is an efficiently computable map $e: \mathbb{G}_{1} \times \mathbb{G}_{2} \rightarrow \mathbb{G}_{T}$, that satisfies the bilinear property: $$\forall a,b \in \mathbb{Z}_{p}, \ e(g_{1}^{a}, g_{2}^{b}) = e(g_{1},g_{2})^{ab}$$ and it is non-degenerate, i.e, $e(g_{1},g_{2})$ is a generator of $\mathbb{G}_{T}$.

Therefore, the tool can be used to analyze the generic security of:
• Structure-Preserving Signatures (signature schemes defined over bilinear groups where the messages and signatures consist of group elements and the verification of signatures consists of evaluation pairing product equations).
• Algebraic Message-Authentication Codes.
• Assumptions.
and more cryptographic construction defined over bilinear groups whose security is defined as a computational experiment.

### Example

This is an example of a cryptographic construction that can be analyzed with the tool. It is taken from the AsiaCrypt 2015 paper, https://eprint.iacr.org/2014/635, by S. Chatterjee and A. Menezes:

#### Re-randomizable Structure-Preserving Signature scheme in Type III

• Setup ${\cal P}(1^{\lambda})$:
Return $\mathit{PP} = (p, \mathbb{G}_{1}, \mathbb{G}_{2}, \mathbb{G}_{T}, g_{1}, g_{2}, e) \leftarrow {\cal G}(1^{\lambda})$, where ${\cal G}$ is an efficient algorithm that on input $1^{\lambda}$ outputs a description of a bilinear group in Type III, with groups of order $p$ for a $\lambda$-bit prime $p$.

• Key generation ${\cal K}(\mathit{PP})$:
Choose $v,w \stackrel{\$}{\leftarrow} \mathbb{Z}_{p}^{*}$and compute$\mathit{VK} = (\mathit{PP}, V, W)$and$\mathit{SK} = (\mathit{PP}, v, w)$as $$V \leftarrow g_{1}^{v} \ \text{ and } \ W \leftarrow g_{1}^{w}$$ • Signing${\cal S}_{\mathit{SK}}(M)$: For$M \in \mathbb{G}_{2}$, choose$r \stackrel{\$}{\leftarrow} \mathbb{Z}_{p}^{*}$ and compute the signature $(T_{1},T_{2},S) \in \mathbb{G}_{1}\times \mathbb{G}_{2}^{2}$ as $$T_{1} \leftarrow g_{1}^{r}, \ \ \ \ \ T_{2} \leftarrow g_{2}^{r}, \ \ \ \text{ and } \ \ \ S \leftarrow M^{v} \cdot g_{2}^{w} \cdot g_{2}^{r^2}$$
• Verification ${\cal V}_{\mathit{VK}}(M, (T_{1},T_{2},S))$:
Accept if and only if $T_{1} \in \mathbb{G}_{1}$, $M,T_{2},S \in \mathbb{G}_{2}$ and $$e(g_{1}, S) = e(V,M) \cdot e(W,g_{2}) \cdot e(T_{1}, T_{2}) \ \ \land \ \ \ e(T_{1}, g_{2}) = e(g_{1}, T_{2})$$
A possible notion of security for a signature scheme is the Existential Unforgeability Against Chosen Message Attacks. We say that a signature scheme $({\cal P}, {\cal K}, {\cal S}, {\cal V})$ is EUF-CMA secure if for all polynomial time algorithms ${\cal A}$, $$\mathit{Pr}\left[ \begin{array}{ll} \mathit{PP} \leftarrow {\cal P}(1^{\lambda}) & \\ (\mathit{VK}, \mathit{SK}) \leftarrow {\cal K}(\mathit{PP}) & : M \not \in Q \ \land \ {\cal V}_{\mathit{VK}}(M,\Sigma) = 1 \\ (M,\Sigma) \leftarrow {\cal A}^{{\cal S}_{\mathit{SK}}(\cdot)}(\mathit{VK}) & \\ \end{array} \right] \approx \mathit{negl}(\lambda)$$ where $Q$ is the set of queries made by ${\cal A}$ to the signing oracle. Note that, since the adversary is polynomial, $|Q| \approx \mathit{poly}(\lambda)$, but this cardinality is not bounded.
Intuitively, this notion says that:

There is no polynomial time adversary that can produce a valid signature (with significant probability) on a message without the knowledge of the secret key, even if she has access to a signing Oracle that produces valid signatures for messages of her choice.

Note that the adversary must find a valid signature on a message that was not sent to the Oracle.

## Installation

1. Install Opam.
In Ubuntu,
apt-get install -y ocaml ocaml-native-compilers opam libtool libtool-bin libgmp-dev libffi-dev m4 libz-dev libssl-dev camlp4-extra
In OS X, use homebrew,
brew install opam
2. Install the right compiler and the right libraries:
opam pin add gga-unbounded GGA_UNBOUNDED_DIR -n
opam install gga-unbounded --deps-only
3. The tool uses Sage and Z3 as backend.
• For Sage, you should be able to start sage -python. (We used SageMath, Version 6.8).
• We use a Z3 wrapper written in Python. Visit the Z3 GitHub project.
4. Set the path variable:
export UBT_PATH=GGA_UNBOUNDED_DIR
5. To compile the tool use make. This will produce two executables:
• ubt.native to perform automated analysis.
• wsubt.native to communicate via web sockets with the interactive tool.
6. (Interactive Mode) The interactive mode uses MathJax. You can install it, by changing to the web directory, i.e., cd web/ and running make get-mathjax.

## Usage

### Input files

The description of the cryptographic construction and the security game is provided to the tool as an input file. As an example, we present the input file Chatterjee-Menezes.ubt expressing the Structure-Preserving Scheme described above and the EUC-CMA security experiment:
group_setting 3.

sample V,W.

input [V,W] in G1.

oracle o1(M:G2) =
sample R;
return [ R ] in G1,
[ R, M*V + R^2 + W] in G2.

win (wM:G2, wR1:G1, wR2:G2, wS:G2) =
(forall i: wM <> M_i /\ wR1 = wR2 /\ wS = V*wM + wR1*wR2 + W).


### Automated mode

You can use the automated algorithm by running, e.g.:
./ubt.native examples/Automatic/Chatterjee-Menezes.ubt
This will produce the following output,
simplify.
extract_coeffs.
simplify.
simplify.
case_distinction p9_j'1.
simplify.
simplify.
divide_by_param p9_i'1.
simplify.
Proven!
Time 3.49 seconds

which corresponds to the sequence of commands that the automatic algorithm executed to prove the security of the cryptographic scheme. Visit the Documentation section for details about these commands.

We note that there may be several ways to prove the statement, i.e., several sequences of commands representing a valid proof. The automatic algorithm follows a heuristic to apply the available rules, therefore, the produced result may not be the shortest or simplest existing proof.

### Interactive mode

We provide an interactive web interface where you can execute commands while watching the current state of the proof. This can be used to analyze given proofs or to produce customized ones.

To start the web interface, execute:
./wsubt.native examples/Interactive/Chatterjee-Menezes.ubt
and open the shown URL in your browser,
Open the following URL in your browser (websocket support required):

file:///GGA_UNBOUNDED_DIR/web/index.html

Files: examples/Interactive/Chatterjee-Menezes.ubt
This is a screenshot of the interactive mode (click on the image to enlarge it):

On the left side there is the code window, were you must define the security experiment and you can execute commands. On the right you can see the equations associated with the current state of the proof.
Visit the Documentation section for details about how use the interactive mode.