In cryptography, a ring signature is a type of digital signature that can be performed by any member of a set of users that each have keys. Therefore, a message signed with a ring signature is endorsed by someone in a particular set of people. One of the security properties of a ring signature is that it should be computationally infeasible to determine which of the set's members' keys was used to produce the signature. Ring signatures are similar to group signatures but differ in two key ways: first, there is no way to revoke the anonymity of an individual signature; and second, any set of users can be used as a signing set without additional setup. Ring signatures were invented by Ron Rivest, Adi Shamir, and Yael Tauman Kalai, and introduced at ASIACRYPT in 2001. The name, ring signature, comes from the ring-like structure of the signature algorithm.

Definition

Suppose that a set of entities each have public/private key pairs, (P1, S1), (P2, S2), ..., (Pn, Sn). Party i can compute a ring signature σ on a message m, on input (m, Si, P1, ..., Pn). Anyone can check the validity of a ring signature given σ, m, and the public keys involved, P1, ..., Pn. If a ring signature is properly computed, it should pass the check. On the other hand, it should be hard for anyone to create a valid ring signature on any message for any set without knowing any of the private keys for that set.

Applications and modifications

In the original paper, Rivest, Shamir, and Tauman described ring signatures as a way to leak a secret. For instance, a ring signature could be used to provide an anonymous signature from "a high-ranking White House official", without revealing which official signed the message. Ring signatures are right for this application because the anonymity of a ring signature cannot be revoked, and because the group for a ring signature can be improvised. Another application, also described in the original paper, is for deniable signatures. Here the sender and the recipient of a message form a group for the ring signature, then the signature is valid to the recipient, but anyone else will be unsure whether the recipient or the sender was the actual signer. Thus, such a signature is convincing, but cannot be transferred beyond its intended recipient. There were various works, introducing new features and based on different assumptions:

Efficiency

Most of the proposed algorithms have asymptotic output size O(n); i.e., the size of the resulting signature increases linearly with the size of input (number of public keys). That means that such schemes are impracticable for real use cases with sufficiently large n (for example, an e-voting with millions of participants). But for some application with relatively small median input size such estimate may be acceptable. CryptoNote implements O(n) ring signature scheme by Fujisaki and Suzuki in p2p payments to achieve sender's untraceability. More efficient algorithms have appeared recently. There are schemes with the sublinear size of the signature, as well as with constant size.

Implementation

Original scheme

The original paper describes an RSA based ring signature scheme, as well as one based on Rabin signatures. They define a keyed "combining function" which takes a key k, an initialization value v, and a list of arbitrary values. y_i is defined as g_i(x_i), where g_i is a trap-door function (i.e. an RSA public key in the case of RSA based ring signatures). The function is called the ring equation, and is defined below. The equation is based on a symmetric encryption function E_k: It outputs a single value z which is forced to be equal to v. The equation can be solved as long as at least one y_i, and by extension x_i, can be freely chosen. Under the assumptions of RSA, this implies knowledge of at least one of the inverses of the trap door functions g^{-1}_i (i.e. a private key), since.

Signature generation

Generating a ring signature involves six steps. The plaintext is signified by m, the ring's public keys by.

Signature verification

Signature verification involves three steps.

Python implementation

Here is a Python implementation of the original paper using RSA. Requires 3rd-party module PyCryptodome. To sign and verify 2 messages in a ring of 4 users:

Cryptocurrencies

Monero and several other cryptocurrencies use this technology.