Document every exported symbol and how callers use the library.

Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

@@ -4,66 +4,38 @@ Short-lived RS256 user tokens from a Nextcloud ExApp to Go Microservices.
 
 Import: `gitea.neitzel.de/konrad/go-usertoken` (package `usertoken`).
 
+```bash
+go get gitea.neitzel.de/konrad/go-usertoken
+```
+
 The ExApp mints a token after AppAPI has named the user. Each Microservice checks that token. One process trusts either a static public key or an OIDC issuer. The same bearer is forwarded when a Microservice calls another.
 
-Procedure and claim rules: Knowledge `platforms/nextcloud/exapps/authentication.md`. Domain words: [CONTEXT.md](./CONTEXT.md).
+- [Guide](docs/guide.md) — how to mint and how to verify
+- [API](docs/api.md) — every exported symbol
+- [Domain language](CONTEXT.md)
 
-This module does not speak AppAPI and does not see `APP_SECRET`.
+Procedure and claim rules: Knowledge `platforms/nextcloud/exapps/authentication.md`.
 
-## ExApp
+## Included
 
-```go
+- [ExApp](docs/guide.md#exapp) — mint a token, including key generation
-signer, err := usertoken.NewSignerFromEnv()
+- [Microservice](docs/guide.md#microservice) — verify a bearer and forward it
-raw, err := signer.Mint(time.Now(), usertoken.MintInput{
-    Subject: userID,
-    Groups:  &groupIDs, // nil omits groups
-})
-req.Header.Set("Authorization", "Bearer "+raw)
-```
 
-| Variable | Role |
+## Excluded
-| --- | --- |
-| `USER_TOKEN_PRIVATE_KEY_FILE` | RSA private key PEM |
-| `USER_TOKEN_ISSUER` | `iss` string |
-| `USER_TOKEN_AUDIENCE` | `aud` string |
-| `USER_TOKEN_TTL` | optional, default `5m` |
 
-## Microservice
+- AppAPI and `APP_SECRET`
+- Choosing the Nextcloud user (the ExApp already has that id)
 
-```go
+## Testing
-auth, err := usertoken.FromEnv(ctx)
-handler = auth.Middleware()(handler)
 
-caller, _ := usertoken.CallerFromContext(r.Context())
+Unit tests cover minting, static verification, and the HTTP and gRPC interceptors. OIDC tests do not need a live issuer unless a test starts one.
-raw, _ := usertoken.BearerFromContext(r.Context())
-out.Header.Set("Authorization", "Bearer "+raw)
-```
 
-`/health` is not authenticated. Any other path without a valid bearer is 401.
+`go test ./...` fails when [`docs/api.md`](docs/api.md) does not match the exported API. Regenerate it with:
-
-Set one of these. Setting both, or neither, makes `FromEnv` fail.
-
-Static public key (ExApp-minted tokens):
-
-| Variable | Role |
-| --- | --- |
-| `USER_TOKEN_PUBLIC_KEY_FILE` | RSA public key PEM |
-| `USER_TOKEN_ISSUER` | expected `iss` |
-| `USER_TOKEN_AUDIENCE` | expected `aud` |
-| `USER_TOKEN_SKEW` | optional, default `1m` |
-
-OIDC issuer (Keycloak or another identity server):
-
-| Variable | Role |
-| --- | --- |
-| `OIDC_ISSUER` | issuer URL that serves discovery |
-| `OIDC_AUDIENCE` | expected `aud`; empty skips the check |
-| `OIDC_GROUPS_CLAIM` | group array claim, default `groups` (`identity_groups` for current Keycloak tokens) |
-| `USER_TOKEN_SKEW` | optional, default `1m` |
 
 ```bash
-openssl genrsa -out user-token.key 2048
+UPDATE_API_DOCS=1 go test -run TestAPIDoc -count=1
-openssl rsa -in user-token.key -pubout -out user-token.pub
 ```
 
-The private key stays on the ExApp. Static mode copies `user-token.pub` to each Microservice.
+## Related
+
+- Knowledge `platforms/nextcloud/exapps/authentication.md` — procedure and claims

apidoc_test.go

@@ -0,0 +1,278 @@
+package usertoken_test
+
+import (
+	"bytes"
+	"fmt"
+	"go/ast"
+	"go/doc"
+	"go/doc/comment"
+	"go/parser"
+	"go/printer"
+	"go/token"
+	"os"
+	"strings"
+	"testing"
+)
+
+const apiModulePath = "gitea.neitzel.de/konrad/go-usertoken"
+
+func TestAPIDoc(t *testing.T) {
+	got, err := renderAPIDoc(".", apiModulePath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	const path = "docs/api.md"
+	if os.Getenv("UPDATE_API_DOCS") == "1" {
+		if err := os.MkdirAll("docs", 0o755); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(path, []byte(got), 0o644); err != nil {
+			t.Fatal(err)
+		}
+		return
+	}
+	want, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read %s: %v", path, err)
+	}
+	if string(want) != got {
+		t.Fatalf("docs/api.md is stale; regenerate with UPDATE_API_DOCS=1 go test -run TestAPIDoc -count=1\n%s", firstDiff(string(want), got))
+	}
+}
+
+func renderAPIDoc(dir, modulePath string) (string, error) {
+	fset := token.NewFileSet()
+	pkgs, err := parser.ParseDir(fset, dir, func(fi os.FileInfo) bool {
+		return !strings.HasSuffix(fi.Name(), "_test.go")
+	}, parser.ParseComments)
+	if err != nil {
+		return "", err
+	}
+	if len(pkgs) != 1 {
+		return "", fmt.Errorf("expected 1 package in %s, found %d", dir, len(pkgs))
+	}
+	var astPkg *ast.Package
+	for _, p := range pkgs {
+		astPkg = p
+	}
+	pkg := doc.New(astPkg, modulePath, 0)
+	lookup := symLookup(pkg)
+
+	var b strings.Builder
+	b.WriteString("# API\n\n")
+	b.WriteString("Generated from the package comment and every exported declaration. Do not edit.\n")
+	b.WriteString("Regenerate with `UPDATE_API_DOCS=1 go test -run TestAPIDoc -count=1`.\n\n")
+	fmt.Fprintf(&b, "## Package %s\n\n", pkg.Name)
+	writeDoc(&b, pkg.Doc, lookup)
+	writeValues(&b, fset, "Constants", pkg.Consts, "### ", lookup)
+	writeValues(&b, fset, "Variables", pkg.Vars, "### ", lookup)
+	writeFuncs(&b, fset, "Functions", pkg.Funcs, "### ", lookup)
+	if len(pkg.Types) > 0 {
+		b.WriteString("## Types\n\n")
+		for _, typ := range pkg.Types {
+			writeType(&b, fset, typ, lookup)
+		}
+	}
+	return b.String(), nil
+}
+
+func writeType(b *strings.Builder, fset *token.FileSet, typ *doc.Type, lookup func(string, string) bool) {
+	fmt.Fprintf(b, "### %s\n\n", typ.Name)
+	sig, err := formatGenDecl(fset, typ.Decl)
+	if err != nil {
+		fmt.Fprintf(b, "_(signature unavailable: %v)_\n\n", err)
+	} else {
+		writeCode(b, sig)
+	}
+	writeDoc(b, typ.Doc, lookup)
+	writeValues(b, fset, "", typ.Consts, "#### ", lookup)
+	writeValues(b, fset, "", typ.Vars, "#### ", lookup)
+	writeFuncs(b, fset, "", typ.Funcs, "#### ", lookup)
+	if !isInterface(typ) {
+		writeFuncs(b, fset, "", typ.Methods, "#### ", lookup)
+	}
+}
+
+func writeValues(b *strings.Builder, fset *token.FileSet, title string, vals []*doc.Value, heading string, lookup func(string, string) bool) {
+	if len(vals) == 0 {
+		return
+	}
+	if title != "" {
+		fmt.Fprintf(b, "## %s\n\n", title)
+	}
+	for _, v := range vals {
+		for _, name := range v.Names {
+			fmt.Fprintf(b, "%s%s\n\n", heading, name)
+		}
+		sig, err := formatGenDecl(fset, v.Decl)
+		if err != nil {
+			fmt.Fprintf(b, "_(signature unavailable: %v)_\n\n", err)
+		} else {
+			writeCode(b, sig)
+		}
+		writeDoc(b, v.Doc, lookup)
+	}
+}
+
+func writeFuncs(b *strings.Builder, fset *token.FileSet, title string, fns []*doc.Func, heading string, lookup func(string, string) bool) {
+	if len(fns) == 0 {
+		return
+	}
+	if title != "" {
+		fmt.Fprintf(b, "## %s\n\n", title)
+	}
+	for _, fn := range fns {
+		name := fn.Name
+		if fn.Recv != "" {
+			name = strings.TrimPrefix(fn.Recv, "*") + "." + fn.Name
+		}
+		fmt.Fprintf(b, "%s%s\n\n", heading, name)
+		sig, err := formatFunc(fset, fn.Decl)
+		if err != nil {
+			fmt.Fprintf(b, "_(signature unavailable: %v)_\n\n", err)
+		} else {
+			writeCode(b, sig)
+		}
+		writeDoc(b, fn.Doc, lookup)
+	}
+}
+
+func writeCode(b *strings.Builder, sig string) {
+	b.WriteString("```go\n")
+	b.WriteString(sig)
+	b.WriteString("\n```\n\n")
+}
+
+func writeDoc(b *strings.Builder, raw string, lookup func(string, string) bool) {
+	text := renderComment(raw, lookup)
+	if text == "" {
+		return
+	}
+	b.WriteString(text)
+	if !strings.HasSuffix(text, "\n") {
+		b.WriteByte('\n')
+	}
+	b.WriteByte('\n')
+}
+
+func symLookup(pkg *doc.Package) func(string, string) bool {
+	syms := map[string]bool{}
+	methods := map[string]bool{}
+	addNames := func(vals []*doc.Value) {
+		for _, v := range vals {
+			for _, name := range v.Names {
+				syms[name] = true
+			}
+		}
+	}
+	addFuncs := func(fns []*doc.Func) {
+		for _, fn := range fns {
+			syms[fn.Name] = true
+			if fn.Recv != "" {
+				recv := strings.TrimPrefix(fn.Recv, "*")
+				methods[recv+"."+fn.Name] = true
+			}
+		}
+	}
+	addNames(pkg.Consts)
+	addNames(pkg.Vars)
+	addFuncs(pkg.Funcs)
+	for _, typ := range pkg.Types {
+		syms[typ.Name] = true
+		addNames(typ.Consts)
+		addNames(typ.Vars)
+		addFuncs(typ.Funcs)
+		addFuncs(typ.Methods)
+	}
+	return func(recv, name string) bool {
+		if recv == "" {
+			return syms[name]
+		}
+		return methods[recv+"."+name]
+	}
+}
+
+func renderComment(raw string, lookup func(string, string) bool) string {
+	raw = strings.TrimSpace(raw)
+	if raw == "" {
+		return ""
+	}
+	var parser comment.Parser
+	parser.LookupSym = lookup
+	var printer comment.Printer
+	printer.HeadingLevel = 4
+	printer.DocLinkURL = func(link *comment.DocLink) string {
+		if link.ImportPath != "" {
+			return link.DefaultURL("https://pkg.go.dev")
+		}
+		name := link.Name
+		if link.Recv != "" {
+			name = link.Recv + "." + link.Name
+		}
+		return "#" + githubAnchor(name)
+	}
+	return strings.TrimSpace(string(printer.Markdown(parser.Parse(raw))))
+}
+
+// githubAnchor matches GitHub heading slugs: lowercase, drop punctuation, keep
+// letters, digits, hyphens, and underscores. "Client.Chat" becomes "clientchat".
+func githubAnchor(name string) string {
+	var b strings.Builder
+	for _, r := range strings.ToLower(name) {
+		if (r >= 'a' && r <= 'z') || (r >= '0' && r <= '9') || r == '-' || r == '_' {
+			b.WriteRune(r)
+		}
+	}
+	return b.String()
+}
+
+func formatGenDecl(fset *token.FileSet, decl *ast.GenDecl) (string, error) {
+	if decl == nil {
+		return "", fmt.Errorf("nil decl")
+	}
+	copyDecl := *decl
+	copyDecl.Doc = nil
+	return formatNode(fset, &copyDecl)
+}
+
+func formatFunc(fset *token.FileSet, decl *ast.FuncDecl) (string, error) {
+	if decl == nil {
+		return "", fmt.Errorf("nil decl")
+	}
+	copyDecl := *decl
+	copyDecl.Doc = nil
+	copyDecl.Body = nil
+	return formatNode(fset, &copyDecl)
+}
+
+func formatNode(fset *token.FileSet, node ast.Node) (string, error) {
+	var buf bytes.Buffer
+	if err := printer.Fprint(&buf, fset, node); err != nil {
+		return "", err
+	}
+	return strings.TrimSpace(buf.String()), nil
+}
+
+func isInterface(typ *doc.Type) bool {
+	if typ.Decl == nil || len(typ.Decl.Specs) != 1 {
+		return false
+	}
+	spec, ok := typ.Decl.Specs[0].(*ast.TypeSpec)
+	if !ok {
+		return false
+	}
+	_, ok = spec.Type.(*ast.InterfaceType)
+	return ok
+}
+
+func firstDiff(want, got string) string {
+	w := strings.Split(want, "\n")
+	g := strings.Split(got, "\n")
+	n := min(len(g), len(w))
+	for i := 0; i < n; i++ {
+		if w[i] != g[i] {
+			return fmt.Sprintf("line %d:\n got: %s\nwant: %s", i+1, g[i], w[i])
+		}
+	}
+	return fmt.Sprintf("length got %d lines, want %d lines", len(g), len(w))
+}

auth.go

@@ -29,8 +29,14 @@ type Auth struct {
 	v tokenVerifier
 }
 
-// FromEnv reads the process environment. See the authentication procedure for names.
+// FromEnv reads the process environment and builds an Auth.
-// Both USER_TOKEN_PUBLIC_KEY_FILE and OIDC_ISSUER set, or neither set, is an error.
+// Set USER_TOKEN_PUBLIC_KEY_FILE or OIDC_ISSUER, not both and not neither.
+// Static mode also reads USER_TOKEN_ISSUER and USER_TOKEN_AUDIENCE.
+// OIDC mode reads OIDC_AUDIENCE and optional OIDC_GROUPS_CLAIM.
+// Optional USER_TOKEN_SKEW is a time.Duration; empty uses 1 minute.
+// It returns an error when the mode is ambiguous, the key file is unreadable,
+// USER_TOKEN_SKEW cannot be parsed, or OIDC discovery fails.
+// ctx is used for OIDC discovery.
 func FromEnv(ctx context.Context) (*Auth, error) {
 	pubFile := strings.TrimSpace(os.Getenv("USER_TOKEN_PUBLIC_KEY_FILE"))
 	oidcIss := strings.TrimSpace(os.Getenv("OIDC_ISSUER"))
@@ -89,7 +95,9 @@ func New(ctx context.Context, cfg Config) (*Auth, error) {
 	return &Auth{v: v}, nil
 }
 
-// Verify checks a raw JWT (no "Bearer " prefix).
+// Verify checks a raw JWT. raw has no "Bearer " prefix.
+// A failed check wraps ErrUnauthorized.
+// Startup and configuration failures do not.
 func (a *Auth) Verify(ctx context.Context, raw string) (Caller, error) {
 	return a.v.verify(ctx, raw)
 }

docs/api.md

@@ -0,0 +1,179 @@
+# API
+
+Generated from the package comment and every exported declaration. Do not edit.
+Regenerate with `UPDATE_API_DOCS=1 go test -run TestAPIDoc -count=1`.
+
+## Package usertoken
+
+Package usertoken mints and checks the short-lived RS256 user token an ExApp sends to Microservices.
+
+An ExApp calls [NewSignerFromEnv](#newsignerfromenv) and [Signer.Mint](#signermint) after AppAPI has named the user. A Microservice calls [FromEnv](#fromenv) and [Auth.Middleware](#authmiddleware) or [Auth.UnaryServerInterceptor](#authunaryserverinterceptor). One process trusts either a static public key or an OIDC issuer, not both.
+
+The procedure, claims, and environment variables are in knowledge/platforms/nextcloud/exapps/authentication.md.
+
+## Variables
+
+### ErrUnauthorized
+
+```go
+var ErrUnauthorized = errors.New("unauthorized")
+```
+
+ErrUnauthorized is wrapped by every failed token check. Startup and configuration failures do not wrap it.
+
+## Functions
+
+### BearerFromContext
+
+```go
+func BearerFromContext(ctx context.Context) (string, bool)
+```
+
+BearerFromContext returns the raw JWT [Auth.Middleware](#authmiddleware) stored, without the "Bearer " prefix. Outbound calls send "Bearer " plus this string.
+
+## Types
+
+### Auth
+
+```go
+type Auth struct {
+	// contains filtered or unexported fields
+}
+```
+
+Auth verifies bearers for one Microservice process.
+
+#### FromEnv
+
+```go
+func FromEnv(ctx context.Context) (*Auth, error)
+```
+
+FromEnv reads the process environment and builds an Auth. Set USER\_TOKEN\_PUBLIC\_KEY\_FILE or OIDC\_ISSUER, not both and not neither. Static mode also reads USER\_TOKEN\_ISSUER and USER\_TOKEN\_AUDIENCE. OIDC mode reads OIDC\_AUDIENCE and optional OIDC\_GROUPS\_CLAIM. Optional USER\_TOKEN\_SKEW is a time.Duration; empty uses 1 minute. It returns an error when the mode is ambiguous, the key file is unreadable, USER\_TOKEN\_SKEW cannot be parsed, or OIDC discovery fails. ctx is used for OIDC discovery.
+
+#### New
+
+```go
+func New(ctx context.Context, cfg Config) (*Auth, error)
+```
+
+New builds an Auth from explicit configuration. ctx is used for OIDC discovery.
+
+#### Auth.Middleware
+
+```go
+func (a *Auth) Middleware() func(http.Handler) http.Handler
+```
+
+Middleware requires a bearer on every path except /health. Success stores the Caller and the raw JWT on the request context.
+
+#### Auth.UnaryServerInterceptor
+
+```go
+func (a *Auth) UnaryServerInterceptor() grpc.UnaryServerInterceptor
+```
+
+UnaryServerInterceptor requires a bearer on every RPC. Success stores the Caller and the raw JWT on the handler context, same as [Auth.Middleware](#authmiddleware). Native clients send metadata key "authorization". The HTTP gateway's forwarded header is "grpcgateway-authorization".
+
+#### Auth.Verify
+
+```go
+func (a *Auth) Verify(ctx context.Context, raw string) (Caller, error)
+```
+
+Verify checks a raw JWT. raw has no "Bearer " prefix. A failed check wraps ErrUnauthorized. Startup and configuration failures do not.
+
+### Caller
+
+```go
+type Caller struct {
+	Subject		string
+	Username	string
+	Groups		[]string
+}
+```
+
+Caller is the user a verified token names. Groups is nil when the token omitted the claim, and non-nil (possibly empty) when the claim was present.
+
+#### CallerFromContext
+
+```go
+func CallerFromContext(ctx context.Context) (Caller, bool)
+```
+
+CallerFromContext returns the user [Auth.Middleware](#authmiddleware) stored.
+
+### Config
+
+```go
+type Config struct {
+	PublicKey	*rsa.PublicKey
+	Issuer		string
+	Audience	string
+	OIDCIssuer	string
+	OIDCAudience	string
+	GroupsClaim	string
+	Skew		time.Duration
+}
+```
+
+Config selects one verify mode. Set PublicKey for static RS256, or OIDCIssuer for discovery. Not both. Skew of zero uses 1 minute. GroupsClaim empty uses "groups" (OIDC only). Audience empty is allowed only for OIDC, where it skips the audience check.
+
+### MintInput
+
+```go
+type MintInput struct {
+	Subject	string
+	Groups	*[]string
+}
+```
+
+MintInput is one user token. Groups nil omits the claim. A non-nil pointer includes it, including an empty list.
+
+### SignConfig
+
+```go
+type SignConfig struct {
+	PrivateKey	*rsa.PrivateKey
+	Issuer		string
+	Audience	string
+	TTL		time.Duration
+}
+```
+
+SignConfig is the ExApp mint configuration. TTL of zero uses 5 minutes.
+
+### Signer
+
+```go
+type Signer struct {
+	// contains filtered or unexported fields
+}
+```
+
+Signer mints RS256 user tokens. Only the ExApp should hold one.
+
+#### NewSigner
+
+```go
+func NewSigner(cfg SignConfig) (*Signer, error)
+```
+
+NewSigner checks the key, issuer, and audience. It returns an error when PrivateKey is nil or issuer or audience is empty. A TTL of zero or less uses 5 minutes.
+
+#### NewSignerFromEnv
+
+```go
+func NewSignerFromEnv() (*Signer, error)
+```
+
+NewSignerFromEnv reads USER\_TOKEN\_PRIVATE\_KEY\_FILE, USER\_TOKEN\_ISSUER, USER\_TOKEN\_AUDIENCE, and optional USER\_TOKEN\_TTL. It returns an error when the private-key path is empty, the PEM is not an RSA key, USER\_TOKEN\_TTL is set and is not a time.Duration, or issuer or audience is empty. An empty USER\_TOKEN\_TTL uses 5 minutes.
+
+#### Signer.Mint
+
+```go
+func (s *Signer) Mint(now time.Time, in MintInput) (string, error)
+```
+
+Mint signs one RS256 token at now. Subject becomes both sub and preferred\_username. It returns an error when Subject is empty or the key cannot sign.
+

docs/guide.md

@@ -0,0 +1,133 @@
+# go-usertoken guide
+
+How to use the library. Symbol signatures and doc comments are in [API](api.md). Words for the domain are in [CONTEXT.md](../CONTEXT.md). Claim layout and the full environment list are in Knowledge `platforms/nextcloud/exapps/authentication.md`.
+
+## ExApp
+
+### Purpose
+
+The ExApp mints a short-lived RS256 token for the user AppAPI already named. Only the ExApp holds the private key. Microservices hold the matching public key, or they trust an OIDC issuer instead.
+
+### When to use it
+
+Mint a token when the ExApp calls a Microservice on behalf of the requesting user. Do not mint inside a Microservice.
+
+### Call sequence
+
+1. Generate an RSA key once and keep the private key on the ExApp. Copy `user-token.pub` to each Microservice that uses static mode.
+
+```bash
+openssl genrsa -out user-token.key 2048
+openssl rsa -in user-token.key -pubout -out user-token.pub
+```
+
+2. Set the ExApp environment:
+
+| Variable | Role |
+| --- | --- |
+| `USER_TOKEN_PRIVATE_KEY_FILE` | RSA private key PEM (PKCS#1 or PKCS#8) |
+| `USER_TOKEN_ISSUER` | `iss` string |
+| `USER_TOKEN_AUDIENCE` | `aud` string |
+| `USER_TOKEN_TTL` | optional, default `5m` |
+
+3. Call `NewSignerFromEnv` at startup, or `NewSigner` with a `SignConfig`.
+4. After AppAPI has named the user, call `Signer.Mint` with the current time and a `MintInput`.
+5. Send `Authorization: Bearer` plus the returned string.
+
+`MintInput.Subject` becomes both `sub` and `preferred_username`. `Groups` nil omits the groups claim. A non-nil pointer includes the claim, including an empty list.
+
+A `SignConfig.TTL` of zero uses 5 minutes, same as an empty `USER_TOKEN_TTL`.
+
+### Errors
+
+`NewSignerFromEnv` returns an error when the private-key path is empty, the PEM is not an RSA key, `USER_TOKEN_TTL` is set and is not a `time.Duration`, or issuer or audience is empty.
+
+`NewSigner` returns an error when the private key is nil or issuer or audience is empty.
+
+`Mint` returns an error when the subject is empty or the key cannot sign. Configuration failures are not `ErrUnauthorized`. That sentinel is for failed checks on the Microservice side.
+
+### Example
+
+```go
+signer, err := usertoken.NewSignerFromEnv()
+if err != nil {
+    return err
+}
+raw, err := signer.Mint(time.Now(), usertoken.MintInput{
+    Subject: userID,
+    Groups:  &groupIDs, // nil omits groups
+})
+if err != nil {
+    return err
+}
+req.Header.Set("Authorization", "Bearer "+raw)
+```
+
+## Microservice
+
+### Purpose
+
+A Microservice checks the bearer and stores the caller on the request context. One process uses either a static public key or an OIDC issuer.
+
+### When to use it
+
+Wrap HTTP handlers, or install the gRPC interceptor, at process start. Forward the same raw JWT when this service calls another.
+
+### Call sequence
+
+1. Set exactly one mode.
+
+Static public key (tokens this ExApp minted):
+
+| Variable | Role |
+| --- | --- |
+| `USER_TOKEN_PUBLIC_KEY_FILE` | RSA public key PEM |
+| `USER_TOKEN_ISSUER` | expected `iss` |
+| `USER_TOKEN_AUDIENCE` | expected `aud` |
+| `USER_TOKEN_SKEW` | optional, default `1m` |
+
+OIDC issuer:
+
+| Variable | Role |
+| --- | --- |
+| `OIDC_ISSUER` | issuer URL that serves discovery |
+| `OIDC_AUDIENCE` | expected `aud`; empty skips the audience check |
+| `OIDC_GROUPS_CLAIM` | group array claim, default `groups` |
+| `USER_TOKEN_SKEW` | optional, default `1m` |
+
+2. Call `FromEnv` with a context. The context is used for OIDC discovery. Or call `New` with a `Config` that sets either `PublicKey` or `OIDCIssuer`.
+3. Wrap handlers with `Auth.Middleware`. For gRPC, use `Auth.UnaryServerInterceptor`. Native clients send metadata `authorization`. The HTTP gateway's forwarded header is `grpcgateway-authorization`.
+4. Inside a handler, read `CallerFromContext` and `BearerFromContext`.
+5. On an outbound call, set `Authorization` to `Bearer ` plus the raw string from `BearerFromContext`.
+
+`/health` is not authenticated. Any other HTTP path without a valid bearer is 401. A gRPC call without a valid bearer is `Unauthenticated`.
+
+`Caller.Groups` is nil when the token omitted the claim, and non-nil (possibly empty) when the claim was present.
+
+### Errors
+
+`FromEnv` and `New` return an error when both modes are set, when neither mode is set, when the public key file is unreadable, when `USER_TOKEN_SKEW` cannot be parsed, or when OIDC discovery fails. Those are configuration errors. They do not wrap `ErrUnauthorized`.
+
+`Verify` returns an error that wraps `ErrUnauthorized` when the token is missing, expired, signed by the wrong key, or has the wrong issuer or audience. `Middleware` and `UnaryServerInterceptor` turn that into 401 or `Unauthenticated` and do not call the handler.
+
+An empty `OIDCAudience` skips the audience check. Static mode always requires an audience.
+
+### Example
+
+```go
+auth, err := usertoken.FromEnv(ctx)
+if err != nil {
+    return err
+}
+handler = auth.Middleware()(handler)
+
+// inside a handler
+caller, ok := usertoken.CallerFromContext(r.Context())
+if !ok {
+    http.Error(w, "unauthorized", http.StatusUnauthorized)
+    return
+}
+raw, _ := usertoken.BearerFromContext(r.Context())
+out.Header.Set("Authorization", "Bearer "+raw)
+_ = caller
+```

signer.go

@@ -38,6 +38,9 @@ type Signer struct {
 
 // NewSignerFromEnv reads USER_TOKEN_PRIVATE_KEY_FILE, USER_TOKEN_ISSUER,
 // USER_TOKEN_AUDIENCE, and optional USER_TOKEN_TTL.
+// It returns an error when the private-key path is empty, the PEM is not an
+// RSA key, USER_TOKEN_TTL is set and is not a time.Duration, or issuer or
+// audience is empty. An empty USER_TOKEN_TTL uses 5 minutes.
 func NewSignerFromEnv() (*Signer, error) {
 	path := strings.TrimSpace(os.Getenv("USER_TOKEN_PRIVATE_KEY_FILE"))
 	if path == "" {
@@ -63,6 +66,8 @@ func NewSignerFromEnv() (*Signer, error) {
 }
 
 // NewSigner checks the key, issuer, and audience.
+// It returns an error when PrivateKey is nil or issuer or audience is empty.
+// A TTL of zero or less uses 5 minutes.
 func NewSigner(cfg SignConfig) (*Signer, error) {
 	if cfg.PrivateKey == nil {
 		return nil, fmt.Errorf("private key is nil")
@@ -85,7 +90,9 @@ type tokenClaims struct {
 	Groups            *[]string `json:"groups,omitempty"`
 }
 
-// Mint signs one token at now. Subject becomes sub and preferred_username.
+// Mint signs one RS256 token at now.
+// Subject becomes both sub and preferred_username.
+// It returns an error when Subject is empty or the key cannot sign.
 func (s *Signer) Mint(now time.Time, in MintInput) (string, error) {
 	subject := strings.TrimSpace(in.Subject)
 	if subject == "" {