SIWE / EIP-4361
Also called SIWE, Sign-In with Ethereum, EIP-4361
Sign-In with Ethereum (SIWE) lets you log into a website using your Ethereum wallet instead of a username, password, or a third-party account like Google's. Click "Sign in with Ethereum," your wallet (MetaMask or similar) pops up a message to sign, not a transaction, just a signature, which costs no gas, and the website verifies that signature against your wallet's public address. If it checks out, you're logged in as that address, with nothing sent to the site except a signature over a message you can read before approving.
The signed message itself is the actual standard: EIP-4361 defines a precise plain-text format, domain, wallet address, a human-readable statement, the resource URIs the session applies to, a nonce, an issued-at timestamp, and an optional expiry, so that any wallet can render it legibly (not just a wall of hex) and any relying party can parse it the same way every other relying party does. That message format is what makes SIWE portable across different sites and wallets, rather than every site inventing its own bespoke "sign this string" convention with different security properties.
Where it comes from
EIP-4361 was standardized through the Ethereum Improvement Proposal process in 2022, championed by Spruce Systems, among its listed authors are Spruce co-founder Wayne Chang and Gregory Rocco, working with contributors from the Ethereum Name Service and the Ethereum Foundation. Solidus had no part in writing it.
Solidus today
Solidus has not implemented SIWE. Solidus's own login is built around
did:solidus (an Ed25519 challenge/response flow against a decentralized
identifier Solidus itself defines (see DID and did:solidus)) which is a
different design point: SIWE proves control of an Ethereum address;
Solidus's own flow proves control of a DID, which can (but doesn't have
to) be wallet-derived. Solidus does operate a public EVM Subnet (see that
entry), which is the part of the stack where SIWE would be the more
natural fit if it were ever added, but nothing has been built, and
nothing here should be read as a commitment that it will be.
See also
OIDC is the closest conceptual analogue for non-wallet login. DID and did:solidus are what Solidus's own authentication is built on instead. EVM Subnet is the part of Solidus's chain where a wallet-signature login would apply, if built.
Where it comes from
Someone else specified this. Solidus assembles it.
Sign-In with Ethereum is Ethereum Improvement Proposal EIP-4361, standardized in 2022. It was championed and driven primarily by Spruce Systems (Wayne Chang and Gregory Rocco among its authors), built in collaboration with the Ethereum Name Service and the Ethereum Foundation. Solidus has no involvement in it and, as of this writing, no implementation of it either, see Solidus status below.
How to check this
Solidus has not built this. The entry explains the concept.
There is no Solidus proof to offer for this one, no endpoint, no package, no demo, and that's stated plainly rather than invented. The general concept is independently checkable: siwe.xyz publishes a public reference implementation and demo maintained outside Solidus entirely.