Idemix

This project is a Go implementation of an anonymous identity stack for blockchain systems.

• Protocol
• Preliminaries
• Generation of issue certificate
• Generation of client certificate
• Generation of signature
• Verification of a signature
• Generation of a pseudonymous signature
• Verification of a pseudonymous signature
• Extensions
  • Adding a pseudonym as a function of the Enrollment ID (eid)
  • Signature generation
  • Signature verification
  • Auditing NymEid

Protocol

Here we describe the cryptographic protocol that is implemented.

Preliminaries

TBD (Group etc.)

Generation of issue certificate

The input for this step are the 4 attributes that are certified, namely OU, Role, EnrollmentID and RevocationHandle (call them $a_{0}, \ldots, a_{3}$).

Given these attributes, the CA samples the issuer secret key at random

$ISK \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

And then computes

$W \leftarrow g_{2}^{ISK}$

For each attribute $a_{i} \in \{a_{0}, \ldots, a_{3}\}$ the CA picks a random element $r_{i} \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$ and generates a base for that attribute

$H_{a_{i}} \leftarrow g_{1}^{r_{i}}$

The CA randomly selects $r_{ISK}, r, \bar{r}$ and computes bases

$H_{ISK} \leftarrow g_{1}^{r_{ISK}}$

$H_{r} \leftarrow g_{1}^{r}$

$\bar{g_1} \leftarrow g_{1}^{\bar{r}}$

$\bar{g_2} \leftarrow \bar{g_1}^{ISK}$

Then the CA randomly selects $r_p$ and computes

$t_1 \leftarrow g_2^{r_p}$

$t_2 \leftarrow \bar{g_1}^{r_p}$

It also generates

$C \leftarrow H(t_1||t_2||g_2||\bar{g_1}||W||\bar{g_2})$

$s \leftarrow r_{p} %2B C \cdot ISK$

The issuer public key $PK_{I}$ is

$PK_{I} \leftarrow \{ a_{0}, \ldots, a_{3}, H_{a_{0}}, \ldots, H_{a_{3}}, H_{ISK}, H_{r}, W, \bar{g_1}, \bar{g_2}, C, s, h_{CA} \}$

where $h_{CA}$ is a hash of all fields of the public key.

and the issuer private key is $SK_{I}$ is

$SK_{I} \leftarrow \{ ISK \}$

Generation of client certificate

Given a client $c$ with attributes $a_{c0}, \ldots, a_{c3}$, the client samples the secret key

$sk_{c} \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

and random elements

$r_{sk} \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

$nonce \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

and then computes

$N \leftarrow H_{ISK}^{sk_{c}}$

$t \leftarrow H_{ISK}^{r_{sk}}$

$C \leftarrow H(t||H_{ISK}||N||nonce||h_{CA})$

$s \leftarrow r_{sk} %2B C \cdot sk_{c}$

The credential request sent to the CA is $\{ N, nonce, C, s \}$.

The CA computes

$t' \leftarrow \frac{H_{ISK}^{s}}{N^C}$

and checks whether

$C = H(t'||H_{ISK}||N||nonce||h_{CA})$

If so, the CA picks random elements

$E \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

$S \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

and computes

$$B \leftarrow g_{1} \cdot N \cdot H_{r}^S \cdot \prod_{i=0}^{4} H_{a_{i}}^{a_{ci}}$$

$e \leftarrow \frac{1}{E %2B ISK}$

$A \leftarrow B^e$

The CA returns the credential $\{ A, B, S, E \}$ to the user.

The user verifies the credential by computing

$$B' \leftarrow g_{1} \cdot H_{ISK}^{sk_{c}} \cdot H_{r}^S \cdot \prod_{i=0}^4 H_{a_{i}}^{a_{ci}}$$

If $B \neq B'$ the user aborts. Otherwise it verifies the signature by checking whether the following equality

$e(g_{2}^E \cdot W, A) = e(g_{2}, B)$

holds. If so, the user accepts private key $SK_{C} \leftarrow \{ sk_{c} \}$ and the user public key is $PK_{C} \leftarrow \{ A, B, E, S \}$.

Generation of signature

To sign message $m$ and simultaneously disclose a subset of attributes $a_{c0}, \ldots, a_{c3}$ (tracked by the bits $d_{0}, \ldots, d_{3}$ such that if the bit is one the corresponding attribute is disclosed; notationally, $\bar{d}_{i} = d_{i} %2B 1 mod 2$), the client chooses a new random element $r_{n} \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$ and generates a new pseudonym

$Nym \leftarrow N \cdot H_{r}^{r_{n}}$

And then generates the new signature as follows

$n \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

$r_1 \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

$r_2 \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

$r_3 \leftarrow \frac{1}{r_1}$

$A' \leftarrow A^{r_1}$

$\bar{A} \leftarrow B^{r1} \cdot A'^{-E}$

$B' \leftarrow \frac{B^{r1}}{H_{r}^{r_2}}$

$S' \leftarrow S-r_2 \cdot r_3$

The client then generates random elements

$r_{sk_{c}} \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

$r_{e} \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

$r_{r_2} \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

$r_{r_3} \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

$r_{S'} \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

$r_{r_{n}} \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

$r_{a_{0}} \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

$r_{a_{1}} \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

$r_{a_{2}} \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

$r_{a_{3}} \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

and then generates

$t_1 \leftarrow A'^{r_{e}} \cdot H_{r}^{r_{r_2}}$

$$t_2 \leftarrow B'^{r_{r_3}} \cdot H_{ISK}^{r_{sk_{c}}} \cdot H_{r}^{r_{S'}} \cdot \prod_{i=0}^4 H_{a_{i}}^{r_{a_{i}} \bar{d}_i}$$

$t_3 \leftarrow H_{ISK}^{r_{sk_{c}}} \cdot H_{r}^{r_{r_{n}}}$

$C \leftarrow H(H(t_1||t_2||t_3||A'||\bar{A}||B'||Nym||h_{CA}||d_0||\ldots||d_3||m)||n)$

$S_{sk_{c}} \leftarrow r_{sk_{c}} %2B sk_{c} C$

$S_{E} \leftarrow r_{e} - E C$

$S_{r_2} \leftarrow r_{r_2} %2B r_2 C$

$S_{r_3} \leftarrow r_{r_3} - r_3 C$

$S_{S'} \leftarrow r_{S'} %2B S' C$

$S_{r_{n}} \leftarrow r_{r_{n}} %2B r_{n} C$

and for each attribute $a_{i}$ that requires disclosure, it generates

$S_{a_{i}} \leftarrow r_{a_{i}} %2B a_{i} C$

The signature $\sigma$ is $\sigma \leftarrow \{ Nym, A', \bar{A}, B', C, S_{sk_{c}}, S_{E}, S_{r_2}, S_{r_3}, S_{S'}, S_{r_{n}}, \ldots S_{a_{i}} \ldots, d_{0}, \ldots, d_{3}, \ldots a_{i} \ldots, n \}$.

Verification of a signature

Upon receipt of a signature $\sigma$ is $\sigma \leftarrow \{ Nym, A', \bar{A}, B', C, S_{sk_{c}}, S_{E}, S_{r_2}, S_{r_3}, S_{S'}, S_{r_{n}}, \ldots S_{a_{i}} \ldots, d_{0}, \ldots, d_{3}, \ldots a_{i} \ldots, n \}$ over message $m$ the verifier checks whether the following equality holds

$e(W, A') = e(g_{2}, \bar{A})$

If so, it recomputes

$t'_1 \leftarrow \frac{A'^{S_{E}} \cdot H_{r}^{S_{r_2}}}{\left( \bar{A} \cdot B'^{-1} \right)^C}$

$t'_2 \leftarrow H_{r}^{S_{S'}} \cdot B'^{S_{r_3}} \cdot H_{ISK}^{S_{sk_{c}}} \cdot \prod_{i=0}^4 H_{a_{i}}^{S_{a_{i}} \bar{d}_i} \cdot \left(g_{1} \cdot \prod_{i=0}^4 H_{a_{i}}^{a_{i} d_i} \right)^C$

$t'_3 \leftarrow \frac{H_{ISK}^{S_{sk_{c}}} \cdot H_{r}^{S_{r_{n}}}}{Nym^C}$

and accepts the signature if

$C = H(H(t'_1||t'_2||t'_3||A'||\bar{A}||B'||Nym||h_{CA}||d_0||\ldots||d_3||m)||n)$

This verification also verifies the disclosed subset of attributes.

Generation of a pseudonymous signature

Differently from a standard signature, a pseudonymous signature does not prove that the pseudonym possesses a user certificate signed by a CA. It only proves that the pseudonym $Nym$ signed message $m$. The signature is generated starting from the pseudonym (as generated in the section above) together with secret key $sk_{c}$ and randomness $r_{n}$ as follows: at first it picks random elements

$n \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

$r_{sk_{c}} \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

$r_{r_{n}} \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

Then it generates

$t \leftarrow H_{ISK}^{r_{sk_{c}}} \cdot H_{r}^{r_{r_{n}}}$

$C \leftarrow H(H(t||Nym||h_{CA}||m)||n)$

$S_{sk_{c}} \leftarrow r_{sk_{c}} %2B sk_{c} C$

$S_{r_{n}} \leftarrow r_{r_{n}} %2B r_{n} C$

The signature $\sigma$ is $\sigma \leftarrow \{ Nym, C, S_{sk_{c}}, S_{r_{n}}, n \}$.

Verification of a pseudonymous signature

Upon receipt of a pseudonymous signature $\sigma \leftarrow \{ Nym, C, S_{sk_{c}}, S_{r_{n}}, n \}$ over message $m$ the verifier recomputes

$t' \leftarrow \frac{H_{ISK}^{S_{sk_{c}}} \cdot H_{r}^{S_{r_{n}}}}{Nym^C}$

and accepts the signature if

$C = H(H(t'||Nym||h_{CA}||m)||n)$

Extensions

Adding a pseudonym as a function of the Enrollment ID (eid)

The enrollment id is one of the cerified attributes ($a_{2}$ with value $a_{c2}$). This extension introduces a pseudonym which is a function of the enrollment ID, together with a proof that it was correclty generated.

Signature generation

The pseudonym is computed by sampling

$r_{eid} \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

$r_{r_{eid}} \gets_{\scriptscriptstyle\$} \mathbb{Z}_{r}$

and by generating the pseudonym

$Nym_{eid} \leftarrow H_{a_{2}}^{a_{c2}} \cdot H_{r}^{r_{eid}}$

Signature generation is similar to the scheme above; in particular, the term $r_{a_{2}}$ is the same used by the original sign algorithm. The extensions include:

• the client computes an additional value $t_4 \leftarrow H_{a_{2}}^{r_{a_{2}}} \cdot H_{r}^{r_{r_{eid}}}$;

• the client includes $(Nym_{eid}, t_4)$ in the challenge computation: $C \leftarrow H(H(t_1||t_2||t_3||t_4||A'||\bar{A}||B'||Nym||Nym_{eid}||h_{CA}||d_0||\ldots||d_3||m)||n)$ (if $d_2$ is included, it should always be set to 0 otherwise the value of the enrollment ID would be revealed);

• the client computes an additional proof $S_{r_{eid}} \leftarrow r_{r_{eid}} %2B r_{eid} C$;

• The signature includes the additional proof $S_{r_{eid}}$ and pseudonym $Nym_{eid}$.

Signature verification

Signature verification is the same as above except that

• verifier computes $t'_4 \leftarrow \frac{H_{a_{2}}^{S_{a_2}} \cdot H_{r}^{S_{r_{eid}}}}{Nym_{eid}^C}$;

• verifier checks if $C \leftarrow H(H(t'_1||t'_2||t'_3||t'_4||A'||\bar{A}||B'||Nym||Nym_{eid}||h_{CA}||d_0||\ldots||d_3||m)||n)$.

Auditing NymEid

To Audit NymEid the client reveals pair $a_{c2}, r_{eid}$ and the auditor checks if $Nym_{eid} \leftarrow H_{a_{2}}^{a_{c2}} \cdot H_{r}^{r_{eid}}$.