bridge

README

bridge

An OpenID Connect provider in front of a SAML federation. People log in at their own university, through RENATER's Fédération Éducation-Recherche or eduGAIN, and applications get an OpenID Connect token. Pure Go, CGO_ENABLED=0, one binary.

go install github.com/go-authn/bridge/cmd/authn-bridge@latest

authn-bridge keygen   --key /var/lib/authn-bridge/oidc.key --salt /var/lib/authn-bridge/salt
authn-bridge metadata --config /etc/authn-bridge > sp.xml     # register this with the federation
authn-bridge check    --config /etc/authn-bridge
authn-bridge          --config /etc/authn-bridge   # serve

Release binaries, and installing one as a hardened systemd service: docs/install.md. Before v0.20.0 the command was bridge (see Upgrading to v0.20.0).

It does what SATOSA does for RENATER, which RENATER's technical framework names as the gateway a bilateral-trust product such as Keycloak needs. The SAML half is go-authn/saml.

The configuration

issuer            = "https://login.example.org"
signing_key_file  = "/var/lib/authn-bridge/oidc.key" # authn-bridge keygen --key
subject_salt_file = "/var/lib/authn-bridge/salt"     # authn-bridge keygen --salt; NEVER change it
listen            = "127.0.0.1:8080"                 # the public listener: loopback by default, behind a TLS proxy

saml {
  key_file  = "/etc/authn-bridge/saml.key"   # IdPs encrypt assertions to this
  cert_file = "/etc/authn-bridge/saml.crt"

  metadata_url         = "https://pub.federation.renater.fr/metadata/fer/idps.xml"
  metadata_cert_file   = "/etc/authn-bridge/metadata-signature-2026.pem"
  metadata_fingerprint = "68:2F:20:58:41:9E:D0:79:EB:FE:3C:27:D6:A4:A4:09:39:6F:AE:3F:10:5F:22:EA:04:0F:04:F3:78:2D:5C:C0"

  discovery = "https://discovery.renater.fr/renater"   # or leave it out for this provider's own list
  # idps    = ["https://idp.univ-example.example/idp/shibboleth"]   # restrict; one entry skips the choice

  names             = { fr = "Passerelle OIDC de l'IRHC", en = "IRHC OIDC gateway" }
  technical_contact = "admin@example.org"
}

claims {
  username = "eppn"            # becomes preferred_username: eppn, subject_id, uid or mail
  groups   = ["entitlement"]   # becomes groups: entitlement, scoped_affiliation, affiliation, is_member_of
}

client "fileshare-web" {
  secret_file   = "/etc/authn-bridge/fileshare-web.secret"   # absent: a public client
  redirect_uris = ["https://files.example.org/callback"]
  audience      = ["fileshare"]                         # what the access token is for
}

The metadata signing certificate is pinned by its fingerprint, which is checked at every start: a key that arrives over the same channel as the document it vouches for vouches for nothing. RENATER publishes the fingerprint on its metadata page; the one above is the 2026 certificate's.

What an application gets

flow	authorization code, with PKCE S256 required for every client
ID token	RS256, aud = the client, nonce, auth_time, acr (the IdP's authentication context), at_hash, and nothing about the person: what the scopes release is at /userinfo (OIDC Core §5.4, since an access token is always issued). A client whose ID token is itself the credential, such as opkssh, sets id_token_claims = true to have them copied in
access token	RS256, typ: at+jwt (RFC 9068), aud = the client's audience, or the one resource it asked for (RFC 8707), with preferred_username, groups and idp (the entity ID of the IdP that vouched) so that a resource server can decide without asking anybody. A token carrying ssh, nfs, app_password, wireguard or wireguard_peers is addressed to this provider alone (aud = the issuer), and those endpoints refuse any other: a token a resource server received cannot be replayed here
sub	an HMAC of the institution's identifier under the salt: stable, not reversible, not an address. public (the same for every client) or pairwise per client (OIDC Core 8.1)
scopes	profile (name, given_name, family_name, preferred_username), email, eduperson (AARC-G056 names: eduperson_principal_name, eduperson_scoped_affiliation, eduperson_entitlement, entitlements, schac_home_organization, voperson_id...), groups

The identifier behind sub is chosen in RENATER's order of preference: subject-id, then pairwise-id, then eduPersonPrincipalName, then the deprecated eduPersonTargetedID, then a persistent NameID. Mail is never an identifier, and there is no email_verified: the IdP asserted an address, which is not the same as anybody having verified that the person reads it.

prompt=none becomes IsPassive, and an IdP with no session answers login_required. prompt=login and max_age=0 become ForceAuthn; any other max_age goes without it, and when the IdP's AuthnInstant is older than max_age the person is sent back to it with ForceAuthn (OIDC Core 3.1.2.1: re-authenticate only "if the elapsed time is greater"). acr_values becomes RequestedAuthnContext -- and an IdP that ignores the request (they may) is refused, so a client that asked for REFEDS MFA is never told it got a second factor when it did not.

Groups are held to the IdP that asserted them. A federation has hundreds of IdPs, and groups decide what a person may do -- at every resource server, in the SSH and X.509 certificates. With more than one IdP allowed to log in, an eduPersonEntitlement or isMemberOf value is kept only if its namespace is one of that IdP's shibmd scopes or under one: urn:mace:<domain>:..., urn:geant:<domain>:... (AARC-G002), https://<host>/.... Any other value is dropped, in groups and in entitlements alike. affiliation, which names no institution, needs saml { idps } to list exactly one IdP. A community proxy asserting groups for people of many institutions is named explicitly:

claims {
  trusted_groups = {
    "https://proxy.eduteams.org/saml2/idp/metadata.php" = ["urn:geant:eduteams.org:"]
  }
}

With exactly one IdP, that IdP is trusted for every value, as before.

What it refuses

	why
an authorization request without PKCE S256	RFC 7636 makes plain the default when no method is given, so a missing method is refused rather than read as S256. A confidential client configured pkce = "or_nonce" may send an OpenID nonce instead, as RFC 9700 2.1.1 allows; then a code_verifier for that code is refused, which is the RFC's downgrade check. A public client cannot be so configured.
a redirect URI not registered exactly	shown on the page, never sent to the URI: an error sent anywhere a request names is an open redirector. Loopback URIs may change port (RFC 8252)
a parameter twice	RFC 6749 3.1
a code used twice	refused, and the token the first use bought stops working (RFC 6749 4.1.2)
a code exchanged by another client, with another redirect URI, or the wrong verifier	a code is bound to all three
a public client that sends a secret	it is configured as something it is not
an IdP response delivered into another browser	the login is found through a cookie, not through the RelayState the response carries -- otherwise anybody could log somebody else in as themselves
an IdP that releases no persistent identifier	nobody could be recognised twice
a username with a comma, a space, a quote or a control character, for an SSH or X.509 certificate	sshd reads principals="a,b" and AuthorizedPrincipalsFile entries as lists: one such name is two
everything go-authn/saml refuses	unsigned or SHA-1 assertions, several assertions, replay, foreign scopes, unauthenticated CBC...

And at configuration time: an issuer that is not https, a salt under 32 bytes, an RSA key under 2048 bits, a code lifetime over ten minutes, a client secret somebody can guess, a redirect over cleartext http, a restricted IdP list together with RENATER's discovery service (which cannot be restricted).

Verified against things this repository did not write

• The federation and the IdP are xmlsec1: it signs the federation's metadata and signs and encrypts every response (AES-128-GCM, RSA-OAEP, Response signed -- a Shibboleth IdP v5's defaults).
• The relying party is golang.org/x/oauth2 and coreos/go-oidc: they discover the provider, build the request with PKCE, exchange the code, verify the ID token with its nonce and at_hash, and ask /userinfo.
• The access token is checked by go-authn/oidc with audience fileshare, exactly as go-fileshare checks the tokens WebDAV clients bring it; and the ID token is refused there.
• go-fileshare itself, the released binary: an X.509 certificate issued here opens a share over NFS with TLS, an SSH certificate over SFTP, and once the person is disabled here the next NFS call, the SFTP session already open and a new SFTP login are all refused within seconds -- through the CRL and the KRL go-fileshare fetches from this provider.
• go-fileshare against the EuroHPC SSH CA profile: go-fileshare v0.22.1 is configured as a site trusting EFP's CA would be. The CA key comes from GET /ssh/config into trusted_user_ca_file, and ssh_domains names the host. A certificate from an EFP-profile client logs in only where both of these hold: its ssh_domain_grants name the host, and its ssh_source_address allows the address the connection comes from. A certificate granted another host is refused and logged as such. One pinned elsewhere is refused. On Linux, the same certificate that gets in from 127.0.0.1 is refused from 127.0.0.2 (interop_efp_test.go).
• The OpenID Foundation conformance suite, the one OpenID Provider certification runs, executes its oidcc-basic-certification-test-plan (static clients, discovery) against the bridge built from every pull request, logging in through a SAML IdP on the way (conformance/). Every module passes or is listed, with why, in conformance/expected.txt -- the two deliberate refusals: a request with neither PKCE nor a nonce, and acr_values an IdP did not honour. The workflow fails on any other, and on a listed one that starts passing. The same run executes its openid-ssf-transmitter-caep-test-plan (poll delivery) against the Shared Signals transmitter, the CAEP event triggered through the admin API as an operator would; every module passes.
• The relying party is also openid-client, an OpenID Certified one, over TLS: discovery's issuer, the RFC 9207 iss parameter, the ID token, userinfo against its sub, the access token as an RFC 9068 resource server checks it (validateJwtAccessToken), refresh rotation and reuse, and the device grant (openidclient_test.go).
• The ACME CA is Pebble, Let's Encrypt's test CA, with External Account Binding required: it refuses a wrong MAC key, validates tls-alpn-01 against the listener itself, and the certificate served chains to its root.
• The refusals were sabotage-checked: PKCE made optional, redirect URIs matched by prefix, the login found through RelayState, the replay revocation, the code's binding to its client, verifier and redirect URI, the scopes, the token type, email_verified, and a public client's secret each turn a test red.

Without a browser: the device grant, and authn-bridge token

A WebDAV client, a script, a terminal has no browser to send to a university. It asks for a code instead (RFC 8628), and the person logs in on any other device:

client "rclone" {
  device           = true
  audience         = ["fileshare"]
  refresh_lifetime = "720h"     # refresh tokens, off unless set
  name             = "rclone (WebDAV)"
}

authn-bridge token --issuer https://login.example.org --client rclone

prints an access token, logging in the first time and refreshing quietly after that -- which is exactly rclone's bearer_token_command for go-fileshare over WebDAV. It is golang.org/x/oauth2's device client, not one written here.

user codes	eight letters from RFC 8628's alphabet (no vowels, no digits); ten wrong codes per host per ten minutes, where an IPv6 host is its /64, and a hundred for the /48 or IPv4 /24 around it (before v0.16.6, per address, so one /64 had no limit)
the confirmation page	names the application and says to refuse a code somebody else sent: RFC 8628 5.4's remote phishing, where the ATTACKER's device gets the token
polling	slow_down adds five seconds for good; a device code buys one set of tokens
refresh tokens	rotate (RFC 9700 4.14.2) inside a family whose end is fixed at the login; a retired one used again revokes the family and the access tokens it bought

Several resource servers: a token for one at a time

A client whose audience names several resource servers -- a go-fileshare/portal in front of several fileshare servers -- would get tokens addressed to all of them, and any one of them could replay a token it received to the others (RFC 8707 §3). Since v0.21.0 it can ask for a token for one, with RFC 8707's resource:

client "portal" {
  secret_file      = "/etc/authn-bridge/portal.secret"
  redirect_uris    = ["https://files.example.org/callback"]
  audience         = ["https://fs-paris.example.org/", "https://fs-lyon.example.org/"]
  refresh_lifetime = "8h"
}

at the token endpoint	resource=https://fs-paris.example.org/ on the authorization_code or refresh_token grant: the access token's aud is that one alone (RFC 8707 §2.2). The refresh token keeps the whole grant, so the next refresh can ask for another server
at the authorization endpoint	resource may be repeated, and bounds the grant: a later token request can narrow it to one of those, never widen it. A refresh token is bound to what was consented (RFC 9700 §4.14.2), across a restart too
what may be named	an absolute URI, without a fragment (RFC 8707 §2), that the client's audience lists. With a scope of this provider's own (ssh, nfs, …) the token is for the provider alone, so only the issuer may be named
refused	invalid_target, at the authorization endpoint as a redirect, at the token endpoint as an error. A refused refresh does not spend the refresh token. resource on the device or client-credentials grants is refused rather than ignored: a client that asked for a token for one server must not be handed one for another
without resource	exactly as before: the grant's resources if the authorization request named some, otherwise the client's whole audience

Refresh tokens still rotate for every client, confidential ones included: RFC 9700 §4.14.2 requires it only for public clients, but it costs a confidential client nothing that does not refresh twice with one token -- and the portal spends its refresh token in one place for exactly that reason.

SSH certificates, for SFTP

certificates_file = "/var/lib/authn-bridge/certificates.json"   # every certificate issued, and the revoked
ssh_ca {
  key_file = "/var/lib/authn-bridge/ssh-ca"   # authn-bridge keygen --ssh-ca
  validity = "12h"
}
client "sftp" {
  device           = true
  ssh_certificates = true
}

authn-bridge ssh-cert --issuer ... --client sftp logs in with a code and writes ~/.ssh/id_ed25519-cert.pub, where ssh and sftp find it. The certificate has exactly one principal, the preferred_username -- PROTOCOL.certkeys makes an empty list valid for ANY user, so there is never one -- no permit-* extension, the person's groups in groups@go-authn.org, and it ends with the IdP's session when that is sooner. go-fileshare trusts it with oidc { ssh_ca_file }.

EuroHPC SSH CA profile

A client can have its certificates issued in the profile of the EuroHPC Federation Platform SSH CA (GÉANT / MyAccessID): Ed25519, one hour, ONE principal that names the person across the federation, and the hosting entity's domain in an extension. Every key below is per client and optional; a client that sets none of them gets the certificate above, unchanged byte for byte apart from serial, nonce, signature and times.

ssh_ca {
  key_file = "/var/lib/authn-bridge/ssh-ca"
  validity = "12h"                       # the most any client may ask for
}
client "efp" {
  device              = true
  ssh_certificates    = true
  ssh_principal_claim = "voperson_id"    # preferred_username (default), voperson_id, eduperson_principal_name, sub
  ssh_validity        = "1h"             # never longer than ssh_ca's, nor past the IdP's session
  ssh_extensions      = ["permit-pty"]   # none by default
  # ssh_source_address = ["192.0.2.0/24"]          # the source-address critical option
  # ssh_domain_grants  = ["login.my-hpc.eu"]       # the hosting entity's hosts: see below
}

key
ssh_principal_claim	the claim the one principal comes from. voperson_id and eduperson_principal_name are released by the eduperson scope, which the token must hold (authn-bridge ssh-cert --scope eduperson); a token without it, or a person whose IdP released no such attribute, gets a 403 saying which. The value goes through the same check as a username: a comma, a quote, a space or a control character is refused, since sshd would read two names.
ssh_extensions	OpenSSH's permit-pty, permit-user-rc, permit-port-forwarding, permit-agent-forwarding, permit-X11-forwarding, and nothing else: an unknown name stops the provider from starting. Not no-touch-required: it waives the user-presence test of the person's own security key, which is not the provider's to give away.
ssh_source_address	addresses or CIDR prefixes (no host bits under the mask), in the source-address critical option. OpenSSH's sshd enforces it, and so does go-fileshare: with ssh_domains as without it, since fileshare v0.22.1.
ssh_validity	shorter than ssh_ca's validity, for this client.
ssh_domain_grants	domain patterns for GÉANT's ssh-domain-grant@core.aai.geant.org extension, checked at load by go-authn/sshcert's ValidatePattern (the specification's syntax), then lower case, at least two labels, and a wildcard only under a registrable domain (what stays fixed to its right is not a public suffix: *.ac.uk, *.gouv.fr, *.github.io are refused, *.hpc.example.ac.uk is not); encoded by go-authn/sshcert.

GET /ssh/config publishes the CA key as EFP publishes its own, {"PublicKey":"ssh-ed25519 AAAA..."}, so a site follows EFP's trust instructions with this provider's URL in place of EFP's:

curl -s https://login.example.org/ssh/config | jq -r '.PublicKey' > /etc/ssh/bridge-ssh-ca.pub
echo "TrustedUserCAKeys /etc/ssh/bridge-ssh-ca.pub" >> /etc/ssh/sshd_config

and authorizes the principal as EFP's authorization instructions say: an AuthorizedPrincipalsCommand that reads the domain grant and refuses a certificate meant for another hosting entity, and the person's principal mapped to a local account. Use go-authn/sshcert's sshcert-authorize, a drop-in for GÉANT's ssh-cert-authorize with one difference that matters: GÉANT's lets in a certificate that carries no domain grant, so on a host that trusts this CA and EFP's, or any second CA, the grant filters nothing; sshcert-authorize refuses it unless told otherwise (--allow-no-grant). It is not advertised in the discovery document: no OpenID Connect metadata names an SSH CA, and this provider does not invent one.

What remains different from EFP:

• The principal is voperson_id, the SAML subject-id the person's own institution released, lower-cased (a1b2c3@univ-example.example) -- not a MyAccessID CUID (<id>@myaccessid.org). A site maps the one or the other; a person known to both has two.
• There is a KRL. GET /ssh/krl, for RevokedKeys, lists the certificates of the people disabled here, these included; EFP documents no revocation, so a site configured only as EFP says is not told.
• The groups extension (groups@go-authn.org) is still there when the token carries groups; EFP's certificates have none.
• The domain grant is ssh-domain-grant@core.aai.geant.org, as GÉANT's specification and EFP's authorisation page spell it, encoded by go-authn/sshcert. Its patterns follow the specification, and this provider refuses one more thing: a wildcard over a public suffix (*.ac.uk, by the Public Suffix List), which would grant every institution under it.

X.509 certificates, for NFS over TLS

NFS over TLS (RFC 9289) can name the client by its certificate. This provider issues one to a person who logged in, for a key made on their machine:

certificates_file = "/var/lib/authn-bridge/certificates.json"
x509_ca {
  key_file  = "/var/lib/authn-bridge/nfs-ca/ca.key"    # authn-bridge keygen --x509-ca /var/lib/authn-bridge/nfs-ca
  cert_file = "/var/lib/authn-bridge/nfs-ca/ca.crt"    # what go-fileshare trusts
  validity  = "12h"
}
client "nfs" {
  device            = true
  x509_certificates = true
}

authn-bridge nfs-cert --issuer ... --client nfs makes a P-256 key, sends a certificate request with the "nfs" scope, and writes the certificate and key in PEM, and in the DER the kernel keyring takes, with the mount command.

• The person is a subjectAltName otherName 1.3.6.1.4.1.2238.1.1.1, a UTF8String user@domain, exactly one: what FreeBSD's rpc.tlsservd -u reads (draft-cel-nfsv4-rpc-tls-othername). FreeBSD maps it only when domain is its own NFSv4 domain and user is in its passwd; go-fileshare maps the whole name.
• The groups are URI names beside it, tag:go-authn.github.io,2026:group:<group, percent-encoded> (RFC 4151). Not an extension of the UUID arc: x509.ParseCertificate refuses a whole certificate whose extension OID has an arc larger than an int (measured), so every Go server would.
• Client authentication only, the CN the username (empty, and the SAN critical, past 64 characters), a CRL distribution point, and no longer than validity, the IdP's session or the CA's own certificate.
• GET /x509/crl: DER, signed by the CA that signs the certificates, NextUpdate an hour on and issued again every half hour, its number rising at every issue (RFC 5280 5.2.3: two CRLs whose thisUpdate differ have different numbers), its ETag the content's. An issue is never dated before the one it replaces, even if the clock steps back. Disabling or revoking a person lists their certificates there until they expire; enabling them again takes nothing off. openssl verifies the certificates and says revoked once they are listed. A certificate request held open across a disabling is answered 401, and what it had already recorded is revoked. The token is checked when the request starts and the body read after, so before v0.16.6 such a request came back with a certificate that no list named.

⛔ A certificate names the machine, not the user of it. RFC 9289: the server 'cannot utilize the remote TLS peer identity to authenticate RPC users'. Linux sets the client certificate per mount (cert_serial, privkey_serial), so everybody using that mount is the person it names: this is for a machine one person uses, and authn-bridge nfs-cert says so.

On the Linux client (measured by go-fileshare against Linux 6.17 and ktls-utils 0.9): tlshd verifies the SERVER's certificate against the system trust store only, ignoring x509.truststore; certificate files named in /etc/tlshd.conf must be root's, the key mode 600 -- either mistake shows only as gnutls: Error in the certificate (-43); and MOUNT's MNT goes in the clear, so a refused person sees "access denied" at the first access, not at mount. authn-bridge nfs-cert prints all three.

Every certificate, SSH and X.509, is recorded in certificates_file before it is handed out: one that cannot be recorded is not issued, since it could never be revoked. The file is required with ssh_ca or x509_ca.

Application passwords, for SMB and S3

NTLMv2 and SigV4 prove a secret the server must already hold, so no token will do. authn-bridge app-password --issuer ... --client files sets one, generated, shown once, into a database go-fileshare reads with a users "sql" block:

client "files" {
  device        = true
  app_passwords = true         # its tokens, with the app_password scope, may set one
}

app_passwords {
  driver   = "sqlite"          # or postgres, mysql
  dsn_file = "/etc/authn-bridge/apppw.dsn"
  store    = ["nt_hash"]       # SMB only: the default. Add "password" for S3.
  lifetime = "2160h"
}

What is stored is what the protocols need: the NT hash for SMB, the password itself only when S3 is asked for.

The table is login, password, nt_hash, expires, idp (idp, the institution that vouched, since v0.4.0). Name the columns in go-fileshare's query, never *: go-authn/directory's sqldir reads them by position and refuses more than it knows, so a * breaks at the first restart after a column is added:

-- sqlite; PostgreSQL: expires > extract(epoch from now())
select login, password, nt_hash from app_passwords where expires > strftime('%s','now')

(This is the query the tests run through sqldir.)

WireGuard keys, for a VPN such as claimward

WireGuard authenticates a peer by its public key and by nothing else: the protocol has no certificates, no expiry and no revocation. bridge keeps the answer to whose is this key, and may it still connect?:

• a person's VPN client registers the public key of its device, for a lease;
• the private key never leaves the device;
• a gateway reads the live keys back as a list signed for it alone.

The format both sides share, and the gateway's side, are in go-authn/wireguard.

certificates_file = "/var/lib/authn-bridge/certs.json"   # keys are recorded beside the certificates

wireguard {
  lifetime = "24h"              # a key is listed this long; registering it again renews it
  max_keys = 10                 # devices per person
}

client "claimward" {            # the VPN client people log in with
  device           = true
  wireguard_keys   = true       # its tokens, with the wireguard scope, may register a key
  refresh_lifetime = "720h"
}

client "claimward-gw" {         # the gateway's own client
  secret_file     = "/etc/authn-bridge/claimward-gw.secret"
  wireguard_peers = ["claimward"]   # it reads the keys registered through these clients
  ssf_receiver    = true            # optional: told at once when somebody is disabled
}

POST /wireguard/key	{"public_key": "...", "device": "laptop"} with a token of the wireguard scope. Answers the lease's end.
DELETE /wireguard/key	{"public_key": "..."}: the owner takes a key back
GET /wireguard/peers	for the gateway's own client_credentials token, scope wireguard_peers: a JWT of type wireguard-peers+jwt, signed with the access-token key (in the JWKS), addressed to that gateway, valid five minutes

What it refuses:

• A key someone else holds (409). A public key is public, so whoever reads one off a configuration must not be able to take it over.
• A key that was taken back (409). It may have been taken back because its device was lost.
• One key too many (409).
• A point of low order, a key in any but its canonical spelling, or a device name that does not print (400).
• A token for the list from a person, or a gateway's token registering a key (403).

Taking keys back. Disabling a person or an institution takes their keys back with their certificates, and the list's version goes up. go-authn/wireguard's Source refuses a list older than one it has seen, so the list from before the disabling, still signed and still in its five minutes, cannot be replayed. A gateway that is also an SSF receiver is told at once and fetches the list then; otherwise its next fetch does it. A registration held open across a disabling is answered 401, and its key taken back.

sub in the list is the person as the registering client sees them, public or pairwise. It is the same value the gateway finds in that person's tokens for that client.

One login, two tokens. A token with the wireguard scope is addressed to bridge alone, like every scope of bridge's own: it is never one a VPN server can be shown. A client therefore logs in once for openid wireguard, registers its key with that token, and refreshes asking for scope=openid. RFC 6749 §6 lets a refresh ask for less than was granted, which bridge honours since v0.17.0. The narrower token is addressed to the client's own audience (claimward, unless audience says otherwise), and the refresh token keeps everything it had.

OpenPubkey and opkssh

OpenPubkey commits the user's key in the ID token's nonce, and opkssh turns that into SSH logins with no CA at all. This provider does what that needs: the nonce comes back verbatim in the code flow and in the device flow (where the openpubkey client sends it on the device authorization request), RS256 so that GQ signatures work, and

retired_signing_key_files = ["/var/lib/authn-bridge/oidc-2026-03.key"]

because a PK Token outlives the ID token inside it and the openpubkey verifier only fetches the keys published now: a retired key stays in the JWKS, verifying and never signing, for at least the longest PK Token lifetime any verifier allows (opkssh: a day by default, up to a week). Give opkssh a client of its own -- http://localhost:3000/login-callback and its two siblings as redirect URIs -- because its ID token travels to every server it logs into. That client needs id_token_claims = true: opkssh matches people on the PK Token's claims (email, preferred_username), and since v0.18.0 an ID token carries none unless its client asks.

Judged by the openpubkey library itself: its client makes PK Tokens through the device flow, with GQ signatures, and through the loopback code flow; its verifier accepts them, and refuses one signed by a key no longer published.

TLS: from files, or from an ACME CA

Both are go-authn/servercert's, shared with go-fileshare. The public listener serves TLS itself when told to, or plain HTTP behind a reverse proxy. From files -- certbot's, a Kubernetes secret -- which are read again when they change (compared by content at most every 10 seconds; a pair that does not load yet, the certificate renewed before its key, keeps the one that did):

cert_file = "/etc/letsencrypt/live/login.example.org/fullchain.pem"
key_file  = "/etc/letsencrypt/live/login.example.org/privkey.pem"

Or from an ACME CA, for the issuer's host and no other:

acme {
  accept_terms_of_service = true             # the operator's agreement; no default
  cache_dir = "/var/lib/authn-bridge/acme"  # account key and certificates
  email     = "noc@example.org"
  # Let's Encrypt by default: tls-alpn-01 on the listener itself, which then
  # has to be the one on port 443; or http-01 with
  # http_listen = ":80"                      # which also redirects to https
}

GÉANT TCS -- what French institutions get through RENATER, issued by HARICA since 2025 -- is ACME with External Account Binding. With an Enterprise Admin ACME account the domains are validated in HARICA's portal beforehand, so no challenge is asked and nothing has to reach the provider from outside:

acme {
  accept_terms_of_service = true
  directory_url     = "<the ACME directory URL cm.harica.gr shows for the account>"
  eab_key_id        = "<key id from cm.harica.gr>"
  eab_hmac_key_file = "/etc/authn-bridge/harica-eab.key"   # the HMAC key, base64url
  email             = "noc@example.org"             # HARICA requires one
  cache_dir         = "/var/lib/authn-bridge/acme"
}

directory_ca_file trusts a private ACME CA (step-ca) for the directory's own TLS.

cache_dir holds private keys, so the provider refuses to start on one that anybody else could reach or re-point. That means a symbolic link, mode beyond 0700, or an owner other than the provider's user. Every directory above it is held to sshd's StrictModes rule: owned by root or that user, and not writable by group or others unless sticky. A missing cache_dir is created 0700. Since bridge v0.16.5 (go-authn/servercert v0.3.0) the directories above are checked too, not only the last one. A /var/lib/authn-bridge that a group can write now stops the provider at startup, naming the path.

⛔ golang.org/x/crypto/acme (v0.57.0) polls a finalized order at the URL in the finalize response's Location header, which RFC 8555 does not put there (golang/go#77704): Let's Encrypt sends one, Pebble and Buypass do not, and a CA that does not gets its certificate never fetched. servercert learns each order's URL from the new-order response, where RFC 8555 does require it, and supplies it. Whether HARICA sends the header is not known here; with this, it does not matter.

⛔ The SAML key and certificate (saml { key_file cert_file }) are not this certificate and never change with it: they are in the federation's metadata, and every IdP encrypts to them.

ssh-oidc (KIT: oidc-agent, mccli, motley-cue, pam-ssh-oidc)

ssh-oidc logs people into SSH with an access token: oidc-agent gets it, mccli sends it, motley-cue on the server checks it at this provider's /userinfo and maps the person to a local account. It needs nothing this provider lacks -- no dynamic registration, no introspection, no token exchange -- only a client for oidc-agent:

client "oidc-agent" {
  # public: oidc-gen --pub --client-id oidc-agent
  redirect_uris    = ["http://localhost:8080", "http://localhost:4242", "http://localhost:43985"]
  device           = true
  refresh_lifetime = "720h"   # oidc-agent refuses a provider that gives no refresh token
  audience         = ["ssh"]  # what motley-cue's audience says
}

and on the server, in motley_cue.conf:

[authorisation.bridge]
op_url = https://login.example.org
scopes = ["openid", "profile", "email", "eduperson"]
vo_claim = eduperson_entitlement   # or groups
audience = ssh

What to know, read in their sources (not yet run end to end):

• sub must be public (the default): motley-cue keys an account on sub, and a pairwise sub is one per client -- the same person through two clients would get two accounts.

• Tokens and the 1023 characters OpenSSH reads as a keyboard-interactive answer: an RS256 access token with a 3072-bit key is about 1150 characters before any group. Sign access tokens with a P-256 key instead -- ID tokens stay RS256 --

  access_token_key_file = "/var/lib/authn-bridge/access-token.key"   # authn-bridge keygen --access-token-key
  

  and the access token leaves out auth_time and acr, which nothing reads from it (they are in the ID token). Measured by TestES256AccessTokens for a long eppn and three AARC entitlements, against the shibmd:Scope lengths of RENATER's metadata (743 scopes):

  scope length		characters
  13	median	885
  22	95th percentile	945
  29	99th percentile	992
  44	the longest	1092: over

  Before v0.22.0 the same token was 1014 / 1074 / 1121 / 1221: over from the 95th percentile on. What still does not fit is the longest scopes and, above all, many groups: every entitlement adds its whole URN, and nothing bounds how many an IdP sends. Past 1023, mccli asks motley-cue for a one-time password instead, which motley-cue allows by default; a token pasted into plain ssh has to fit. go-authn/oidc (go-fileshare), coreos/go-oidc and flaat (motley-cue) all verify ES256.

• /userinfo is how motley-cue checks every token. Give the provider a state block (below) so that a restart does not refuse every token handed out before it.

State: what a restart must not forget

state {
  driver   = "postgres"                  # or sqlite, mysql
  dsn_file = "/etc/authn-bridge/state.dsn"
}

Refresh token families, the refresh tokens and the retired ones (so that one used again after a restart still revokes its family), and the access tokens honoured at /userinfo and for certificates, written through to one table, bridge_state, and read at start. Without it they live in memory, and a restart logs everybody out.

A login in progress is not among them: it travels in its own cookie, sealed (AES-256-GCM, under a key derived from the subject salt) so that the browser can neither read nor change it -- the way SATOSA, the SAML-OIDC proxy of the R&E federations, carries its state. Starting one costs this provider nothing it keeps, a restart between the application and the IdP's answer costs the person nothing, and an authorization request too large to carry (state, nonce, redirect URI together past what a cookie holds) is refused at the start. Once an IdP has answered for a login, its handle is remembered until the login would have expired, so it is answered once; with a state block, that and the IDs of the assertions accepted are written there too, so that a restart, or another instance, does not accept the same answer twice. A login ends against the configuration in force then: a redirect URI or an IdP removed meanwhile is not honoured.

• A refresh token is kept as its SHA-256, never as itself: whoever reads the database cannot use what is in it (Ory Hydra's fosite keeps a signature for the same reason). Access tokens are kept by their jti, not a secret.
• A revocation the database did not take would come back at the next start: a delete that fails is retried at every later write, and until it succeeds the provider is not ready (/readyz, the gRPC health).
• Tested across restarts on SQLite and PostgreSQL: tokens still work, revoked ones stay revoked, a retired refresh token used again still revokes its family; with no state block the same restart forgets, which is the control.

Shared Signals: telling the services that verify on their own

An access token, an opkssh certificate, are checked where they are used, against keys, and nothing brings them back here: a person disabled here keeps them until they expire. So this provider is an OpenID Shared Signals transmitter: on DisablePerson, RevokePerson and DisableIdP it queues a CAEP session-revoked event -- everything issued to this subject before event_timestamp is void -- for every receiver that asked, go-fileshare among them.

state { ... }            # required: a queue a restart empties loses revocations
ssf {}                   # event_retention = "192h" by default
client "fileshare-ssf" { # a receiver: client credentials, scope ssf
  secret_file  = "/etc/authn-bridge/fileshare-ssf.secret"
  ssf_receiver = true
  audience     = ["fileshare"]
}

• Poll delivery only (RFC 8936): the receiver asks; nothing here dials out. /.well-known/ssf-configuration names the endpoints, under /ssf/.
• A receiver authenticates with an access token from the client credentials grant (RFC 6749 4.4, ssf_receiver clients only, scope ssf), and sees its own streams only.
• Events are SETs (RFC 8417) signed with the ID token key, typ secevent+jwt, no exp, no sub. The subject is aliases with the account, acct:<username>, by default -- a sub may be pairwise, one per client. A receiver configured ssf_subject_format = "iss_sub" gets instead the issuer and the public sub of each identity known under that person, one event each, which is what the CAEP Interoperability Profile (2.5) has transmitters send; a person nobody knows a sub for still reaches it, by account, since a revocation that does not arrive fails open. session-revoked always carries reason_admin (CAEP Interop 3.1). An institution disabled is one event per person this provider has a trace of there, and one whose subject is the tenant, with the institution's scopes: its people this provider has forgotten are covered by their domain.
• Streams and undelivered events are in the state database, until acknowledged or event_retention.
• A stream that is paused or disabled answers a poll with an error, not an empty 200 a receiver would read as "up to date"; paused, its events wait for it (SSF 1.0 8.1.1), disabled, none are kept. An event a receiver reports in error (setErrs) is handed out again, not dropped -- a key rotated inside its refresh window, a full disk -- until 10 reports or event_retention.
• HTTP, wire types and handlers are go-ssf's. Judged by its client and its Poller, by go-jose against the published key set, and by go-fileshare itself, the release, over WebDAV: a token it verifies on its own is refused within seconds of the person -- or, for somebody forgotten here, their institution -- being disabled.

Running it: the admin API, health and metrics

Both are off unless the configuration asks for them, and neither is ever on the public listener:

admin {
  listen = "unix:///run/authn-bridge/admin.sock"      # mode 0600
  # or, over the network, mutual TLS and nothing less (loopback included):
  # listen         = "10.0.0.5:9443"
  # tls_cert_file  = "/etc/authn-bridge/admin.crt"
  # tls_key_file   = "/etc/authn-bridge/admin.key"
  # client_ca_file = "/etc/authn-bridge/operators-ca.crt"
  # reflection     = true                       # for grpcurl; off by default
}

metrics { listen = "127.0.0.1:9101" }           # /healthz /readyz /metrics

The admin API is gRPC, bridge.admin.v1.AdminService (proto/bridge/admin/v1/admin.proto), with grpc.health.v1 beside it:

Status	version, the federation's metadata (IdPs, valid until, last refresh, last error), what is held in memory
RefreshMetadata	fetch the federation's metadata now
ListIdPs, ListClients	what the institution list shows, and the configured relying parties
RevokePerson	end somebody's refresh token families, the access tokens they bought, their device grants in progress and their application password, and revoke everything the certificate store holds for them: SSH and X.509 certificates (into the next KRL and CRL) and WireGuard keys
DisablePerson, EnablePerson	refuse somebody here -- at login and at every token, whatever their institution says -- and revoke what they hold; optionally until a given time
DisableIdP, EnableIdP	the same for everybody one institution vouches for: when its IdP is compromised, say. A metadata refresh does not lift it
ListDisabled	who is disabled, why, by whom, until when

A person is disabled as who they are: usernames compare without case (eppn, uid, mail and subject-id all compare so), the entry records the stable identity of everybody known under that name, so another spelling from their IdP does not let them back in, and a person whose IdP releases no username is disabled by the sub a relying party knows them by -- public or pairwise, matched at every login, whether or not anything of theirs is still live here. Their application password and their SSF events reach every spelling of their name the provider handed out.

Disabling is kept in a file, and refused without one -- somebody disabled until the next restart would be let back in by the next deployment:

disabled_file = "/var/lib/authn-bridge/disabled.json"   # written whole, mode 0600

A file that cannot be read stops the provider from starting, for the same reason. Disabling an institution also removes its people's application passwords: those set from this version on carry the IdP that vouched for them, and older ones are found by their scope when the username is scoped (eppn, subject-id).

⛔ A removed application password stops working in go-fileshare when it next reads its directory: at its reload interval, on SIGHUP, or when asked (fileshare.admin.v1.AdminService/ReloadDirectory, since fileshare v0.10.0), which also closes that person's open SMB, WebDAV and S3 sessions. Call it after DisablePerson for the effect to be immediate. Everything that comes back to this provider is refused at once.

SSH and X.509 certificates already issued are revoked with their person or institution: they are listed in GET /ssh/krl (an OpenSSH KRL, what sshd's RevokedKeys and ssh-keygen -Q read) and GET /x509/crl until they expire, and go-fileshare refuses them at login -- failing closed when it cannot fetch a list recent enough.

Both lists are issued as go-authn/revocation's protocol says, so that they can travel through anything -- a mirror, plain HTTP, a file -- and still be checked:

• The KRL is signed: GET /ssh/krl.sig is a detached SSHSIG signature by the SSH CA key, namespace krl@go-authn.github.io, what OpenSSH recommends since it stopped verifying the KRL's own signature section. ssh-keygen -Y verify checks it. Fetch it with If-Match: <the list's ETag>; a list issued again in between answers 412.
• Both say when they stop being current: the CRL's nextUpdate, and the KRL's expires@go-authn.github.io extension, which sshd ignores. Each is an hour after issue, and a list is issued again every half hour, or at once when something is revoked.
• The ETag is the content's. It used to be the revocation counter while the body changed at every request, so a reader polling with If-None-Match kept a copy whose expiry passed: a reader that enforces the expiry then failed closed for good against a healthy provider. TestAPollingReaderStaysCurrent polls for two hours of clock.

For a plain sshd, or nginx and other TLS servers, revokd (go-authn/revocation) fetches the lists, keeps only those that verify, and writes them where they are read. When a list lapses it fails closed for sshd, and with listen it serves the verified lists to other servers as a mirror. The KRL is written by go-authn/krl and checked here with ssh-keygen -Q.

There are no users or groups to add or delete here. People exist because their institution vouches for them, and their groups are what it asserts; the closest provider to this one, Dex, has no call to change them either. Who may use what is decided by the service they reach -- a share in go-fileshare names the groups it admits -- and a local list of people and groups is go-authn/directory's.

Every call that changes something is logged with who made it: the client certificate's cn= over TLS, the peer's uid= on the socket. The listener is grpc-transports/control, shared with go-fileshare: the socket is never reachable by another user, not even between bind and chmod, and never taken from an instance still running.

⛔ What RevokePerson cannot end: an access token already handed out is a signed statement a resource server checks on its own, valid there until it expires (an hour by default). Everything that comes back to this provider -- a refresh, /userinfo, an SSH certificate or an application password asked with that token -- is refused at once; an SSH certificate already issued lasts its validity, which is why that is short.

Health and metrics are go-net-health/endpoint's, as in go-fileshare: /healthz is up while the process answers; /readyz is ready while the federation's metadata is loaded and still valid (a provider past its validUntil knows no IdP); /metrics is Prometheus text 0.0.4 with bridge_* families -- logins by outcome, tokens by grant, SSH certificates, application passwords, metadata validity and refreshes, what is held in memory. No label names a person or an institution: which universities' people use the service, and when, is not this provider's to publish.

-tags nogrpc leaves gRPC out (4.1 MB of the binary) and refuses a configuration with an admin block, rather than starting without it.

Behind a reverse proxy, and under load

trusted_proxies     = ["10.0.0.0/8"]   # whose X-Forwarded-For is believed
requests_per_minute = 120             # per address; 0 turns the limit off

Starting a login, a device grant and posting to the ACS cost memory or an RSA operation before anybody is authenticated, so each address gets so many a minute (429 past it). An address is an IPv4 address or an IPv6 /64 -- one host holds a whole /64 -- and each IPv4 /24 and IPv6 /48 shares ten times that, so a flood needs many networks rather than many addresses. A login in progress is kept by its browser, not here (see State). The device grants waiting for their person are capped at 20,000; past that, the oldest is pushed out rather than everybody being refused. Behind a proxy, name it in trusted_proxies, or every request is the proxy's: one address for the whole internet, and the wrong device codes of one person lock out everybody. Every request is bounded: 1 MiB, 30 s to read, 60 s to answer, 2 min idle.

claims { username = "uid" } or "mail" needs saml { idps } to list exactly one IdP: neither is held to an IdP's scopes, so with two, one names the other's people.

Upgrading to v0.20.0

The command is now authn-bridge. It was bridge, which is also iproute2's /sbin/bridge, installed on practically every Linux distribution: whichever came first on PATH won. The repository and the module path stay github.com/go-authn/bridge; only the command and the binary are renamed.

• go install github.com/go-authn/bridge/cmd/authn-bridge@latest. The old path, go install github.com/go-authn/bridge@..., no longer builds a command from v0.20.0 on: the program moved to internal/app, with cmd/authn-bridge calling it.
• The release assets are authn-bridge-<os>-<arch>, no longer bridge-<os>-<arch>.
• Scripts, cron jobs, systemd units and bearer_token_command lines that run bridge ... must run authn-bridge .... The subcommands, flags and configuration are unchanged.
• The start-up log line and error prefix say authn-bridge; a log alert that matched bridge v0. matches authn-bridge v0. now. Metric names (bridge_*), cookie names and the database table bridge_state are unchanged.
• The user-side cache (~/.config/go-authn-bridge) is unchanged, so authn-bridge token and ssh-cert reuse the logins bridge cached.

docs/install.md installs it as a hardened systemd service (packaging/systemd/authn-bridge.service).

Upgrading to v0.18.0

Two changes a deployment must look at.

The ID token carries nothing about the person. email, name, preferred_username and the eduperson claims are at /userinfo, as OIDC Core §5.4 says when an access token is issued; the OpenID Foundation's suite warned about them. An application that read them from the ID token calls /userinfo instead, or its client sets id_token_claims = true (an opkssh client must).

Some people get a new sub. go-authn/saml v0.3.0 holds an unscoped identifier to the IdP that signed it, and that changes two shapes of the identifier sub is derived from:

the IdP releases	sub before (an HMAC of)	from v0.18.0
an opaque string eduPersonTargetedID v	idp!v	idp!idp!sp!v
an ePTID or persistent NameID with no NameQualifier	idp!!spq!v	idp!idp!spq!v

subject-id, pairwise-id, eppn and correctly qualified identifiers are unchanged. A person in the two rows above is a new person to every relying party: their refresh grants, application passwords, WireGuard keys and disable-by-sub entries no longer match. The change cannot be undone after the fact, since two old identifiers can map to one new one. Before upgrading, find the IdPs that release only those shapes (a log line login: ... via <idp> and the attribute the IdP sends) and tell their users that their accounts at the relying parties start afresh. TestASubjectKeepsItsShape pins the new shapes, so the next change will show up as a failing test, not as renamed people.

What it is not, yet

• One process. With a state block, refresh tokens and the access tokens honoured at /userinfo survive a restart; logins in progress, codes and device grants (minutes) do not. It is written through and read at start, so two instances on one database would each miss what the other wrote since: one instance still.
• No dynamic registration, no front- or back-channel logout.

Release binaries

Each release carries authn-bridge for linux, darwin and windows on amd64 and arm64 (pure Go, CGO_ENABLED=0), a SHA256SUMS manifest, and a build provenance attestation per binary, made by this repository's release workflow at the tag. Check a download before running it:

sha256sum -c SHA256SUMS --ignore-missing
gh attestation verify authn-bridge-linux-amd64 --repo go-authn/bridge

authn-bridge --version prints the tag it was built from. docs/install.md takes a release binary to a running systemd service, upgrades it and rolls it back.

Licence

BSD-3-Clause.