diff --git a/cmd/pinniped/cmd/login_oidc.go b/cmd/pinniped/cmd/login_oidc.go
index a3517c31..0ad9998d 100644
--- a/cmd/pinniped/cmd/login_oidc.go
+++ b/cmd/pinniped/cmd/login_oidc.go
@@ -4,101 +4,75 @@
 package cmd
 
 import (
-	"fmt"
+	"encoding/json"
 
-	"github.com/coreos/go-oidc"
-	"github.com/pkg/browser"
 	"github.com/spf13/cobra"
-	"golang.org/x/oauth2"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	clientauthenticationv1beta1 "k8s.io/client-go/pkg/apis/clientauthentication/v1beta1"
 
-	"go.pinniped.dev/internal/oidc/pkce"
-	"go.pinniped.dev/internal/oidc/state"
+	"go.pinniped.dev/internal/oidcclient/login"
 )
 
 //nolint: gochecknoinits
 func init() {
-	loginCmd.AddCommand((&oidcLoginParams{
-		generateState: state.Generate,
-		generatePKCE:  pkce.Generate,
-		openURL:       browser.OpenURL,
-	}).cmd())
+	loginCmd.AddCommand(oidcLoginCommand(login.Run))
 }
 
-type oidcLoginParams struct {
-	// These parameters capture CLI flags.
-	issuer      string
-	clientID    string
-	listenPort  uint16
-	scopes      []string
-	skipBrowser bool
-
-	// These parameters capture dependencies that we want to mock during testing.
-	generateState func() (state.State, error)
-	generatePKCE  func() (pkce.Code, error)
-	openURL       func(string) error
-}
-
-func (o *oidcLoginParams) cmd() *cobra.Command {
-	cmd := cobra.Command{
-		Args:         cobra.NoArgs,
-		Use:          "oidc --issuer ISSUER --client-id CLIENT_ID",
-		Short:        "Login using an OpenID Connect provider",
-		RunE:         o.runE,
-		SilenceUsage: true,
-	}
-	cmd.Flags().StringVar(&o.issuer, "issuer", "", "OpenID Connect issuer URL.")
-	cmd.Flags().StringVar(&o.clientID, "client-id", "", "OpenID Connect client ID.")
-	cmd.Flags().Uint16Var(&o.listenPort, "listen-port", 48095, "TCP port for localhost listener (authorization code flow only).")
-	cmd.Flags().StringSliceVar(&o.scopes, "scopes", []string{"offline_access", "openid", "email", "profile"}, "OIDC scopes to request during login.")
-	cmd.Flags().BoolVar(&o.skipBrowser, "skip-browser", false, "Skip opening the browser (just print the URL).")
+func oidcLoginCommand(loginFunc func(issuer string, clientID string, opts ...login.Option) (*login.Token, error)) *cobra.Command {
+	var (
+		cmd = cobra.Command{
+			Args:         cobra.NoArgs,
+			Use:          "oidc --issuer ISSUER --client-id CLIENT_ID",
+			Short:        "Login using an OpenID Connect provider",
+			SilenceUsage: true,
+		}
+		issuer      string
+		clientID    string
+		listenPort  uint16
+		scopes      []string
+		skipBrowser bool
+	)
+	cmd.Flags().StringVar(&issuer, "issuer", "", "OpenID Connect issuer URL.")
+	cmd.Flags().StringVar(&clientID, "client-id", "", "OpenID Connect client ID.")
+	cmd.Flags().Uint16Var(&listenPort, "listen-port", 0, "TCP port for localhost listener (authorization code flow only).")
+	cmd.Flags().StringSliceVar(&scopes, "scopes", []string{"offline_access", "openid", "email", "profile"}, "OIDC scopes to request during login.")
+	cmd.Flags().BoolVar(&skipBrowser, "skip-browser", false, "Skip opening the browser (just print the URL).")
 	mustMarkRequired(&cmd, "issuer", "client-id")
+
+	cmd.RunE = func(cmd *cobra.Command, args []string) error {
+		opts := []login.Option{
+			login.WithContext(cmd.Context()),
+			login.WithScopes(scopes),
+		}
+
+		if listenPort != 0 {
+			opts = append(opts, login.WithListenPort(listenPort))
+		}
+
+		// --skip-browser replaces the default "browser open" function with one that prints to stderr.
+		if skipBrowser {
+			opts = append(opts, login.WithBrowserOpen(func(url string) error {
+				cmd.PrintErr("Please log in: ", url, "\n")
+				return nil
+			}))
+		}
+
+		tok, err := loginFunc(issuer, clientID, opts...)
+		if err != nil {
+			return err
+		}
+
+		// Convert the token out to Kubernetes ExecCredential JSON format for output.
+		return json.NewEncoder(cmd.OutOrStdout()).Encode(&clientauthenticationv1beta1.ExecCredential{
+			TypeMeta: metav1.TypeMeta{
+				Kind:       "ExecCredential",
+				APIVersion: "client.authentication.k8s.io/v1beta1",
+			},
+			Status: &clientauthenticationv1beta1.ExecCredentialStatus{
+				ExpirationTimestamp: &metav1.Time{Time: tok.IDTokenExpiry},
+				Token:               tok.IDToken,
+			},
+		})
+	}
 	return &cmd
 }
-
-func (o *oidcLoginParams) runE(cmd *cobra.Command, args []string) error {
-	metadata, err := oidc.NewProvider(cmd.Context(), o.issuer)
-	if err != nil {
-		return fmt.Errorf("could not perform OIDC discovery for %q: %w", o.issuer, err)
-	}
-
-	cfg := oauth2.Config{
-		ClientID:    o.clientID,
-		Endpoint:    metadata.Endpoint(),
-		RedirectURL: fmt.Sprintf("http://localhost:%d/callback", o.listenPort),
-		Scopes:      o.scopes,
-	}
-
-	authCodeOptions := []oauth2.AuthCodeOption{oauth2.AccessTypeOffline}
-
-	stateParam, err := o.generateState()
-	if err != nil {
-		return fmt.Errorf("could not generate OIDC state parameter: %w", err)
-	}
-
-	// We can always use PKCE (RFC 7636) because the server should always ignore the parameters if it doesn't
-	// understand them. Per https://tools.ietf.org/html/rfc7636#section-5:
-	//   As the OAuth 2.0 [RFC6749] server responses are unchanged by this specification, client implementations of
-	//   this specification do not need to know if the server has implemented this specification or not and SHOULD
-	//   send the additional parameters as defined in Section 4 to all servers.
-	pkceCode, err := o.generatePKCE()
-	if err != nil {
-		return fmt.Errorf("could not generate OIDC PKCE parameter: %w", err)
-	}
-	authCodeOptions = append(authCodeOptions, pkceCode.Challenge(), pkceCode.Method())
-
-	// If --skip-browser was passed, override the default browser open function with a Printf() call.
-	openURL := o.openURL
-	if o.skipBrowser {
-		openURL = func(s string) error {
-			cmd.PrintErr("Please log in: ", s, "\n")
-			return nil
-		}
-	}
-
-	authorizeURL := cfg.AuthCodeURL(stateParam.String(), authCodeOptions...)
-	if err := openURL(authorizeURL); err != nil {
-		return fmt.Errorf("could not open browser (run again with --skip-browser?): %w", err)
-	}
-
-	return nil
-}
diff --git a/cmd/pinniped/cmd/login_oidc_test.go b/cmd/pinniped/cmd/login_oidc_test.go
index 6d562587..b2bd9348 100644
--- a/cmd/pinniped/cmd/login_oidc_test.go
+++ b/cmd/pinniped/cmd/login_oidc_test.go
@@ -5,30 +5,29 @@ package cmd
 
 import (
 	"bytes"
-	"fmt"
-	"net/http"
-	"net/http/httptest"
-	"net/url"
-	"regexp"
-	"strings"
 	"testing"
+	"time"
 
-	"github.com/spf13/cobra"
 	"github.com/stretchr/testify/require"
 
 	"go.pinniped.dev/internal/here"
-	"go.pinniped.dev/internal/oidc/pkce"
-	"go.pinniped.dev/internal/oidc/state"
+	"go.pinniped.dev/internal/oidcclient/login"
 )
 
 func TestLoginOIDCCommand(t *testing.T) {
 	t.Parallel()
+
+	time1 := time.Date(3020, 10, 12, 13, 14, 15, 16, time.UTC)
+
 	tests := []struct {
-		name       string
-		args       []string
-		wantError  bool
-		wantStdout string
-		wantStderr string
+		name             string
+		args             []string
+		wantError        bool
+		wantStdout       string
+		wantStderr       string
+		wantIssuer       string
+		wantClientID     string
+		wantOptionsCount int
 	}{
 		{
 			name: "help flag passed",
@@ -43,7 +42,7 @@ func TestLoginOIDCCommand(t *testing.T) {
 					  --client-id string     OpenID Connect client ID.
 				  -h, --help                 help for oidc
 					  --issuer string        OpenID Connect issuer URL.
-					  --listen-port uint16   TCP port for localhost listener (authorization code flow only). (default 48095)
+					  --listen-port uint16   TCP port for localhost listener (authorization code flow only).
 					  --scopes strings       OIDC scopes to request during login. (default [offline_access,openid,email,profile])
 					  --skip-browser         Skip opening the browser (just print the URL).
 			`),
@@ -56,12 +55,46 @@ func TestLoginOIDCCommand(t *testing.T) {
 				Error: required flag(s) "client-id", "issuer" not set
 			`),
 		},
+		{
+			name: "success with minimal options",
+			args: []string{
+				"--client-id", "test-client-id",
+				"--issuer", "test-issuer",
+			},
+			wantIssuer:       "test-issuer",
+			wantClientID:     "test-client-id",
+			wantOptionsCount: 2,
+			wantStdout:       `{"kind":"ExecCredential","apiVersion":"client.authentication.k8s.io/v1beta1","spec":{},"status":{"expirationTimestamp":"3020-10-12T13:14:15Z","token":"test-id-token"}}` + "\n",
+		},
+		{
+			name: "success with all options",
+			args: []string{
+				"--client-id", "test-client-id",
+				"--issuer", "test-issuer",
+				"--skip-browser",
+				"--listen-port", "1234",
+			},
+			wantIssuer:       "test-issuer",
+			wantClientID:     "test-client-id",
+			wantOptionsCount: 4,
+			wantStdout:       `{"kind":"ExecCredential","apiVersion":"client.authentication.k8s.io/v1beta1","spec":{},"status":{"expirationTimestamp":"3020-10-12T13:14:15Z","token":"test-id-token"}}` + "\n",
+		},
 	}
 	for _, tt := range tests {
 		tt := tt
 		t.Run(tt.name, func(t *testing.T) {
 			t.Parallel()
-			cmd := (&oidcLoginParams{}).cmd()
+			var (
+				gotIssuer   string
+				gotClientID string
+				gotOptions  []login.Option
+			)
+			cmd := oidcLoginCommand(func(issuer string, clientID string, opts ...login.Option) (*login.Token, error) {
+				gotIssuer = issuer
+				gotClientID = clientID
+				gotOptions = opts
+				return &login.Token{IDToken: "test-id-token", IDTokenExpiry: time1}, nil
+			})
 			require.NotNil(t, cmd)
 
 			var stdout, stderr bytes.Buffer
@@ -76,148 +109,9 @@ func TestLoginOIDCCommand(t *testing.T) {
 			}
 			require.Equal(t, tt.wantStdout, stdout.String(), "unexpected stdout")
 			require.Equal(t, tt.wantStderr, stderr.String(), "unexpected stderr")
-		})
-	}
-}
-
-func TestOIDCLoginRunE(t *testing.T) {
-	t.Parallel()
-
-	// Start a server that returns 500 errors.
-	brokenServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
-	}))
-	t.Cleanup(brokenServer.Close)
-
-	// Start a server that returns successfully.
-	var validResponse string
-	validServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		_, _ = w.Write([]byte(validResponse))
-	}))
-	t.Cleanup(validServer.Close)
-	validResponse = strings.ReplaceAll(here.Docf(`
-		{
-		  "issuer": "${ISSUER}",
-		  "authorization_endpoint": "${ISSUER}/auth",
-		  "token_endpoint": "${ISSUER}/token",
-		  "jwks_uri": "${ISSUER}/keys",
-		  "userinfo_endpoint": "${ISSUER}/userinfo",
-		  "grant_types_supported": ["authorization_code","refresh_token"],
-		  "response_types_supported": ["code"],
-		  "id_token_signing_alg_values_supported": ["RS256"],
-		  "scopes_supported": ["openid","email","groups","profile","offline_access"],
-		  "token_endpoint_auth_methods_supported": ["client_secret_basic"],
-		  "claims_supported": ["aud","email","email_verified","exp","iat","iss","locale","name","sub"]
-		}
-	`), "${ISSUER}", validServer.URL)
-	validServerURL, err := url.Parse(validServer.URL)
-	require.NoError(t, err)
-
-	tests := []struct {
-		name              string
-		params            oidcLoginParams
-		wantError         string
-		wantStdout        string
-		wantStderr        string
-		wantStderrAuthURL func(*testing.T, *url.URL)
-	}{
-		{
-			name: "broken discovery",
-			params: oidcLoginParams{
-				issuer: brokenServer.URL,
-			},
-			wantError: fmt.Sprintf("could not perform OIDC discovery for %q: 500 Internal Server Error: Internal Server Error\n", brokenServer.URL),
-		},
-		{
-			name: "broken state generation",
-			params: oidcLoginParams{
-				issuer:        validServer.URL,
-				generateState: func() (state.State, error) { return "", fmt.Errorf("some error generating a state value") },
-			},
-			wantError: "could not generate OIDC state parameter: some error generating a state value",
-		},
-		{
-			name: "broken PKCE generation",
-			params: oidcLoginParams{
-				issuer:        validServer.URL,
-				generateState: func() (state.State, error) { return "test-state", nil },
-				generatePKCE:  func() (pkce.Code, error) { return "", fmt.Errorf("some error generating a PKCE code") },
-			},
-			wantError: "could not generate OIDC PKCE parameter: some error generating a PKCE code",
-		},
-		{
-			name: "broken browser open",
-			params: oidcLoginParams{
-				issuer:        validServer.URL,
-				generateState: func() (state.State, error) { return "test-state", nil },
-				generatePKCE:  func() (pkce.Code, error) { return "test-pkce", nil },
-				openURL:       func(_ string) error { return fmt.Errorf("some browser open error") },
-			},
-			wantError: "could not open browser (run again with --skip-browser?): some browser open error",
-		},
-		{
-			name: "success",
-			params: oidcLoginParams{
-				issuer:        validServer.URL,
-				clientID:      "test-client-id",
-				generateState: func() (state.State, error) { return "test-state", nil },
-				generatePKCE:  func() (pkce.Code, error) { return "test-pkce", nil },
-				listenPort:    12345,
-				skipBrowser:   true,
-			},
-			wantStderrAuthURL: func(t *testing.T, actual *url.URL) {
-				require.Equal(t, validServerURL.Host, actual.Host)
-				require.Equal(t, "/auth", actual.Path)
-				require.Equal(t, "", actual.Fragment)
-
-				require.Equal(t, url.Values{
-					"access_type":           []string{"offline"},
-					"client_id":             []string{"test-client-id"},
-					"redirect_uri":          []string{"http://localhost:12345/callback"},
-					"response_type":         []string{"code"},
-					"state":                 []string{"test-state"},
-					"code_challenge_method": []string{"S256"},
-					// This is the PKCE challenge which is calculated as base64(sha256("test-pkce")). For example:
-					// $ echo -n test-pkce | shasum -a 256 | cut -d" " -f1 | xxd -r -p | base64 | cut -d"=" -f1
-					// VVaezYqum7reIhoavCHD1n2d+piN3r/mywoYj7fCR7g
-					"code_challenge": []string{"VVaezYqum7reIhoavCHD1n2d-piN3r_mywoYj7fCR7g"},
-				}, actual.Query())
-			},
-			wantStderr: "Please log in: <URL>\n",
-		},
-	}
-	for _, tt := range tests {
-		tt := tt
-		t.Run(tt.name, func(t *testing.T) {
-			var stdout, stderr bytes.Buffer
-			cmd := cobra.Command{RunE: tt.params.runE, SilenceUsage: true, SilenceErrors: true}
-			cmd.SetOut(&stdout)
-			cmd.SetErr(&stderr)
-			err := cmd.Execute()
-			if tt.wantError != "" {
-				require.EqualError(t, err, tt.wantError)
-			} else {
-				require.NoError(t, err)
-			}
-
-			if tt.wantStderrAuthURL != nil {
-				var urls []string
-				redacted := regexp.MustCompile(`http://\S+`).ReplaceAllStringFunc(stderr.String(), func(url string) string {
-					urls = append(urls, url)
-					return "<URL>"
-				})
-				require.Lenf(t, urls, 1, "expected to find authorization URL in stderr:\n%s", stderr.String())
-				authURL, err := url.Parse(urls[0])
-				require.NoError(t, err, "invalid authorization URL")
-				tt.wantStderrAuthURL(t, authURL)
-
-				// Replace the stderr buffer with the redacted version.
-				stderr.Reset()
-				stderr.WriteString(redacted)
-			}
-
-			require.Equal(t, tt.wantStdout, stdout.String(), "unexpected stdout")
-			require.Equal(t, tt.wantStderr, stderr.String(), "unexpected stderr")
+			require.Equal(t, tt.wantIssuer, gotIssuer, "unexpected issuer")
+			require.Equal(t, tt.wantClientID, gotClientID, "unexpected client ID")
+			require.Len(t, gotOptions, tt.wantOptionsCount)
 		})
 	}
 }
diff --git a/go.mod b/go.mod
index a3f63b6b..f2c36c58 100644
--- a/go.mod
+++ b/go.mod
@@ -13,7 +13,6 @@ require (
 	github.com/golangci/golangci-lint v1.31.0
 	github.com/google/go-cmp v0.5.2
 	github.com/pkg/browser v0.0.0-20180916011732-0a3d74bf9ce4
-	github.com/pkg/errors v0.9.1
 	github.com/sclevine/spec v1.4.0
 	github.com/spf13/cobra v1.0.0
 	github.com/spf13/pflag v1.0.5
@@ -22,6 +21,7 @@ require (
 	go.pinniped.dev/generated/1.19/client v0.0.0-00010101000000-000000000000
 	golang.org/x/crypto v0.0.0-20200622213623-75b288015ac9
 	golang.org/x/oauth2 v0.0.0-20191202225959-858c2ad4c8b6
+	gopkg.in/square/go-jose.v2 v2.2.2
 	k8s.io/api v0.19.2
 	k8s.io/apimachinery v0.19.2
 	k8s.io/apiserver v0.19.2
diff --git a/internal/httputil/httperr/httperr.go b/internal/httputil/httperr/httperr.go
new file mode 100644
index 00000000..1fb21cb7
--- /dev/null
+++ b/internal/httputil/httperr/httperr.go
@@ -0,0 +1,67 @@
+// Copyright 2020 the Pinniped contributors. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+// Package httperr contains some helpers for nicer error handling in http.Handler implementations.
+package httperr
+
+import (
+	"fmt"
+	"net/http"
+)
+
+// Responder represents an error that can emit a useful HTTP error response to an http.ResponseWriter.
+type Responder interface {
+	error
+	Respond(http.ResponseWriter)
+}
+
+// New returns a Responder that emits the given HTTP status code and message.
+func New(code int, msg string) error {
+	return httpErr{code: code, msg: msg}
+}
+
+// Newf returns a Responder that emits the given HTTP status code and fmt.Sprintf formatted message.
+func Newf(code int, format string, args ...interface{}) error {
+	return httpErr{code: code, msg: fmt.Sprintf(format, args...)}
+}
+
+// Wrap returns a Responder that emits the given HTTP status code and message, and also wraps an internal error.
+func Wrap(code int, msg string, cause error) error {
+	return httpErr{code: code, msg: msg, cause: cause}
+}
+
+type httpErr struct {
+	code  int
+	msg   string
+	cause error
+}
+
+func (e httpErr) Error() string {
+	if e.cause != nil {
+		return fmt.Sprintf("%s: %v", e.msg, e.cause)
+	}
+	return e.msg
+}
+
+func (e httpErr) Respond(w http.ResponseWriter) {
+	// http.Error is important here because it prevents content sniffing by forcing text/plain.
+	http.Error(w, http.StatusText(e.code)+": "+e.msg, e.code)
+}
+
+func (e httpErr) Unwrap() error {
+	return e.cause
+}
+
+// HandlerFunc is like http.HandlerFunc, but with a function signature that allows easier error handling.
+type HandlerFunc func(http.ResponseWriter, *http.Request) error
+
+func (f HandlerFunc) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	switch err := f(w, r).(type) {
+	case nil:
+		return
+	case Responder:
+		err.Respond(w)
+	default:
+		http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
+	}
+}
diff --git a/internal/httputil/httperr/httperr_test.go b/internal/httputil/httperr/httperr_test.go
new file mode 100644
index 00000000..29d7e7f9
--- /dev/null
+++ b/internal/httputil/httperr/httperr_test.go
@@ -0,0 +1,52 @@
+// Copyright 2020 the Pinniped contributors. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package httperr
+
+import (
+	"errors"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestHTTPErrs(t *testing.T) {
+	t.Run("new", func(t *testing.T) {
+		err := New(http.StatusBadRequest, "bad request error")
+		require.EqualError(t, err, "bad request error")
+	})
+
+	t.Run("newf", func(t *testing.T) {
+		err := Newf(http.StatusMethodNotAllowed, "expected method %s", "POST")
+		require.EqualError(t, err, "expected method POST")
+	})
+
+	t.Run("wrap", func(t *testing.T) {
+		wrappedErr := fmt.Errorf("some internal error")
+		err := Wrap(http.StatusInternalServerError, "unexpected error", wrappedErr)
+		require.EqualError(t, err, "unexpected error: some internal error")
+		require.True(t, errors.Is(err, wrappedErr), "expected error to be wrapped")
+	})
+
+	t.Run("respond", func(t *testing.T) {
+		err := Wrap(http.StatusForbidden, "boring public bits", fmt.Errorf("some secret internal bits"))
+		require.Implements(t, (*Responder)(nil), err)
+		rec := httptest.NewRecorder()
+		err.(Responder).Respond(rec)
+		require.Equal(t, http.StatusForbidden, rec.Code)
+		require.Equal(t, "Forbidden: boring public bits\n", rec.Body.String())
+		require.Equal(t, http.Header{
+			"Content-Type":           []string{"text/plain; charset=utf-8"},
+			"X-Content-Type-Options": []string{"nosniff"},
+		}, rec.Header())
+	})
+}
+
+func TestHandlerFunc(t *testing.T) {
+	t.Run("success", func(t *testing.T) {
+
+	})
+}
diff --git a/internal/httputil/securityheader/securityheader.go b/internal/httputil/securityheader/securityheader.go
new file mode 100644
index 00000000..47fd8d26
--- /dev/null
+++ b/internal/httputil/securityheader/securityheader.go
@@ -0,0 +1,20 @@
+// Copyright 2020 the Pinniped contributors. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+// Package securityheader implements an HTTP middleware for setting security-related response headers.
+package securityheader
+
+import "net/http"
+
+// Wrap the provided http.Handler so it sets appropriate security-related response headers.
+func Wrap(wrapped http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		h := w.Header()
+		h.Set("Content-Security-Policy", "default-src 'none'; frame-ancestors 'none'")
+		h.Set("X-Frame-Options", "DENY")
+		h.Set("X-XSS-Protection", "1; mode=block")
+		h.Set("X-Content-Type-Options", "nosniff")
+		h.Set("Referrer-Policy", "no-referrer")
+		wrapped.ServeHTTP(w, r)
+	})
+}
diff --git a/internal/httputil/securityheader/securityheader_test.go b/internal/httputil/securityheader/securityheader_test.go
new file mode 100644
index 00000000..e5527cb6
--- /dev/null
+++ b/internal/httputil/securityheader/securityheader_test.go
@@ -0,0 +1,30 @@
+// Copyright 2020 the Pinniped contributors. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package securityheader
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestWrap(t *testing.T) {
+	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, _ = w.Write([]byte("hello world"))
+	})
+	rec := httptest.NewRecorder()
+	Wrap(handler).ServeHTTP(rec, httptest.NewRequest(http.MethodGet, "/", nil))
+	require.Equal(t, http.StatusOK, rec.Code)
+	require.Equal(t, "hello world", rec.Body.String())
+	require.EqualValues(t, http.Header{
+		"Content-Security-Policy": []string{"default-src 'none'; frame-ancestors 'none'"},
+		"Content-Type":            []string{"text/plain; charset=utf-8"},
+		"Referrer-Policy":         []string{"no-referrer"},
+		"X-Content-Type-Options":  []string{"nosniff"},
+		"X-Frame-Options":         []string{"DENY"},
+		"X-Xss-Protection":        []string{"1; mode=block"},
+	}, rec.Header())
+}
diff --git a/internal/mocks/mockkeyset/generate.go b/internal/mocks/mockkeyset/generate.go
new file mode 100644
index 00000000..a04777fc
--- /dev/null
+++ b/internal/mocks/mockkeyset/generate.go
@@ -0,0 +1,6 @@
+// Copyright 2020 the Pinniped contributors. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package mockkeyset
+
+//go:generate go run -v github.com/golang/mock/mockgen  -destination=mockkeyset.go -package=mockkeyset -copyright_file=../../../hack/header.txt github.com/coreos/go-oidc KeySet
diff --git a/internal/mocks/mockkeyset/mockkeyset.go b/internal/mocks/mockkeyset/mockkeyset.go
new file mode 100644
index 00000000..ff1050ff
--- /dev/null
+++ b/internal/mocks/mockkeyset/mockkeyset.go
@@ -0,0 +1,53 @@
+// Copyright 2020 the Pinniped contributors. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+//
+
+// Code generated by MockGen. DO NOT EDIT.
+// Source: github.com/coreos/go-oidc (interfaces: KeySet)
+
+// Package mockkeyset is a generated GoMock package.
+package mockkeyset
+
+import (
+	context "context"
+	gomock "github.com/golang/mock/gomock"
+	reflect "reflect"
+)
+
+// MockKeySet is a mock of KeySet interface
+type MockKeySet struct {
+	ctrl     *gomock.Controller
+	recorder *MockKeySetMockRecorder
+}
+
+// MockKeySetMockRecorder is the mock recorder for MockKeySet
+type MockKeySetMockRecorder struct {
+	mock *MockKeySet
+}
+
+// NewMockKeySet creates a new mock instance
+func NewMockKeySet(ctrl *gomock.Controller) *MockKeySet {
+	mock := &MockKeySet{ctrl: ctrl}
+	mock.recorder = &MockKeySetMockRecorder{mock}
+	return mock
+}
+
+// EXPECT returns an object that allows the caller to indicate expected use
+func (m *MockKeySet) EXPECT() *MockKeySetMockRecorder {
+	return m.recorder
+}
+
+// VerifySignature mocks base method
+func (m *MockKeySet) VerifySignature(arg0 context.Context, arg1 string) ([]byte, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "VerifySignature", arg0, arg1)
+	ret0, _ := ret[0].([]byte)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// VerifySignature indicates an expected call of VerifySignature
+func (mr *MockKeySetMockRecorder) VerifySignature(arg0, arg1 interface{}) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "VerifySignature", reflect.TypeOf((*MockKeySet)(nil).VerifySignature), arg0, arg1)
+}
diff --git a/internal/oidc/state/state.go b/internal/oidc/state/state.go
deleted file mode 100644
index 7d70e51b..00000000
--- a/internal/oidc/state/state.go
+++ /dev/null
@@ -1,37 +0,0 @@
-// Copyright 2020 the Pinniped contributors. All Rights Reserved.
-// SPDX-License-Identifier: Apache-2.0
-
-package state
-
-import (
-	"crypto/rand"
-	"crypto/subtle"
-	"encoding/hex"
-	"io"
-
-	"github.com/pkg/errors"
-)
-
-// Generate generates a new random state parameter of an appropriate size.
-func Generate() (State, error) { return generate(rand.Reader) }
-
-func generate(rand io.Reader) (State, error) {
-	var buf [16]byte
-	if _, err := io.ReadFull(rand, buf[:]); err != nil {
-		return "", errors.WithMessage(err, "could not generate random state")
-	}
-	return State(hex.EncodeToString(buf[:])), nil
-}
-
-// State implements some utilities for working with OAuth2 state parameters.
-type State string
-
-// String returns the string encoding of this state value.
-func (s *State) String() string {
-	return string(*s)
-}
-
-// Validate the returned state (from a callback parameter). Returns true iff the state is valid.
-func (s *State) Valid(returnedState string) bool {
-	return subtle.ConstantTimeCompare([]byte(returnedState), []byte(*s)) == 1
-}
diff --git a/internal/oidcclient/login/login.go b/internal/oidcclient/login/login.go
new file mode 100644
index 00000000..9f7df3da
--- /dev/null
+++ b/internal/oidcclient/login/login.go
@@ -0,0 +1,271 @@
+// Copyright 2020 the Pinniped contributors. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+// Package login implements a CLI OIDC login flow.
+package login
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"net/http"
+	"net/url"
+	"time"
+
+	"github.com/coreos/go-oidc"
+	"github.com/pkg/browser"
+	"golang.org/x/oauth2"
+
+	"go.pinniped.dev/internal/httputil/httperr"
+	"go.pinniped.dev/internal/httputil/securityheader"
+	"go.pinniped.dev/internal/oidcclient/nonce"
+	"go.pinniped.dev/internal/oidcclient/pkce"
+	"go.pinniped.dev/internal/oidcclient/state"
+)
+
+type handlerState struct {
+	// Basic parameters.
+	ctx      context.Context
+	issuer   string
+	clientID string
+	scopes   []string
+
+	// Parameters of the localhost listener.
+	listenAddr   string
+	callbackPath string
+
+	// Generated parameters of a login flow.
+	idTokenVerifier *oidc.IDTokenVerifier
+	oauth2Config    *oauth2.Config
+	state           state.State
+	nonce           nonce.Nonce
+	pkce            pkce.Code
+
+	// External calls for things.
+	generateState func() (state.State, error)
+	generatePKCE  func() (pkce.Code, error)
+	generateNonce func() (nonce.Nonce, error)
+	openURL       func(string) error
+
+	callbacks chan callbackResult
+}
+
+type callbackResult struct {
+	token *Token
+	err   error
+}
+
+type Token struct {
+	*oauth2.Token
+	IDToken       string    `json:"id_token"`
+	IDTokenExpiry time.Time `json:"id_token_expiry"`
+}
+
+// Option is an optional configuration for Run().
+type Option func(*handlerState) error
+
+// WithContext specifies a specific context.Context under which to perform the login. If this option is not specified,
+// login happens under context.Background().
+func WithContext(ctx context.Context) Option {
+	return func(h *handlerState) error {
+		h.ctx = ctx
+		return nil
+	}
+}
+
+// WithListenPort specifies a TCP listen port on localhost, which will be used for the redirect_uri and to handle the
+// authorization code callback. By default, a random high port will be chosen which requires the authorization server
+// to support wildcard port numbers as described by https://tools.ietf.org/html/rfc8252:
+//    The authorization server MUST allow any port to be specified at the
+//    time of the request for loopback IP redirect URIs, to accommodate
+//    clients that obtain an available ephemeral port from the operating
+//    system at the time of the request.
+func WithListenPort(port uint16) Option {
+	return func(h *handlerState) error {
+		h.listenAddr = fmt.Sprintf("localhost:%d", port)
+		return nil
+	}
+}
+
+// WithScopes sets the OAuth2 scopes to request during login. If not specified, it defaults to
+// "offline_access openid email profile".
+func WithScopes(scopes []string) Option {
+	return func(h *handlerState) error {
+		h.scopes = scopes
+		return nil
+	}
+}
+
+// WithBrowserOpen overrides the default "open browser" functionality with a custom callback. If not specified,
+// an implementation using https://github.com/pkg/browser will be used by default.
+func WithBrowserOpen(openURL func(url string) error) Option {
+	return func(h *handlerState) error {
+		h.openURL = openURL
+		return nil
+	}
+}
+
+// Run an OAuth2/OIDC authorization code login using a localhost listener.
+func Run(issuer string, clientID string, opts ...Option) (*Token, error) {
+	h := handlerState{
+		issuer:       issuer,
+		clientID:     clientID,
+		listenAddr:   "localhost:0",
+		scopes:       []string{"offline_access", "openid", "email", "profile"},
+		callbackPath: "/callback",
+		ctx:          context.Background(),
+		callbacks:    make(chan callbackResult),
+
+		// Default implementations of external dependencies (to be mocked in tests).
+		generateState: state.Generate,
+		generateNonce: nonce.Generate,
+		generatePKCE:  pkce.Generate,
+		openURL:       browser.OpenURL,
+	}
+	for _, opt := range opts {
+		if err := opt(&h); err != nil {
+			return nil, err
+		}
+	}
+
+	// Always set a long, but non-infinite timeout for this operation.
+	ctx, cancel := context.WithTimeout(h.ctx, 10*time.Minute)
+	defer cancel()
+	h.ctx = ctx
+
+	// Initialize login parameters.
+	var err error
+	h.state, err = h.generateState()
+	if err != nil {
+		return nil, err
+	}
+	h.nonce, err = h.generateNonce()
+	if err != nil {
+		return nil, err
+	}
+	h.pkce, err = h.generatePKCE()
+	if err != nil {
+		return nil, err
+	}
+
+	// Perform OIDC discovery.
+	provider, err := oidc.NewProvider(h.ctx, h.issuer)
+	if err != nil {
+		return nil, fmt.Errorf("could not perform OIDC discovery for %q: %w", h.issuer, err)
+	}
+	h.idTokenVerifier = provider.Verifier(&oidc.Config{ClientID: h.clientID})
+
+	// Open a TCP listener.
+	listener, err := net.Listen("tcp", h.listenAddr)
+	if err != nil {
+		return nil, fmt.Errorf("could not open callback listener: %w", err)
+	}
+
+	// Build an OAuth2 configuration based on the OIDC discovery data and our callback endpoint.
+	h.oauth2Config = &oauth2.Config{
+		ClientID: h.clientID,
+		Endpoint: provider.Endpoint(),
+		RedirectURL: (&url.URL{
+			Scheme: "http",
+			Host:   listener.Addr().String(),
+			Path:   h.callbackPath,
+		}).String(),
+		Scopes: h.scopes,
+	}
+
+	// Start a callback server in a background goroutine.
+	mux := http.NewServeMux()
+	mux.Handle(h.callbackPath, httperr.HandlerFunc(h.handleAuthCodeCallback))
+	srv := http.Server{
+		Handler:     securityheader.Wrap(mux),
+		BaseContext: func(_ net.Listener) context.Context { return h.ctx },
+	}
+	go func() { _ = srv.Serve(listener) }()
+	defer func() {
+		// Gracefully shut down the server, allowing up to 5 seconds for
+		// clients to receive any in-flight responses.
+		shutdownCtx, cancel := context.WithTimeout(h.ctx, 1*time.Second)
+		_ = srv.Shutdown(shutdownCtx)
+		cancel()
+	}()
+
+	// Open the authorize URL in the users browser.
+	authorizeURL := h.oauth2Config.AuthCodeURL(
+		h.state.String(),
+		oauth2.AccessTypeOffline,
+		h.nonce.Param(),
+		h.pkce.Challenge(),
+		h.pkce.Method(),
+	)
+	if err := h.openURL(authorizeURL); err != nil {
+		return nil, fmt.Errorf("could not open browser: %w", err)
+	}
+
+	// Wait for either the callback or a timeout.
+	select {
+	case <-h.ctx.Done():
+		return nil, fmt.Errorf("timed out waiting for token callback: %w", h.ctx.Err())
+	case callback := <-h.callbacks:
+		if callback.err != nil {
+			return nil, fmt.Errorf("error handling callback: %w", callback.err)
+		}
+		return callback.token, nil
+	}
+}
+
+func (h *handlerState) handleAuthCodeCallback(w http.ResponseWriter, r *http.Request) (err error) {
+	// If we return an error, also report it back over the channel to the main CLI thread.
+	defer func() {
+		if err != nil {
+			h.callbacks <- callbackResult{err: err}
+		}
+	}()
+
+	// Return HTTP 405 for anything that's not a GET.
+	if r.Method != http.MethodGet {
+		return httperr.Newf(http.StatusMethodNotAllowed, "wanted GET")
+	}
+
+	// Validate OAuth2 state and fail if it's incorrect (to block CSRF).
+	params := r.URL.Query()
+	if err := h.state.Validate(params.Get("state")); err != nil {
+		return httperr.New(http.StatusForbidden, "missing or invalid state parameter")
+	}
+
+	// Check for error response parameters.
+	if errorParam := params.Get("error"); errorParam != "" {
+		return httperr.Newf(http.StatusBadRequest, "login failed with code %q", errorParam)
+	}
+
+	// Exchange the authorization code for access, ID, and refresh tokens.
+	oauth2Tok, err := h.oauth2Config.Exchange(r.Context(), params.Get("code"), h.pkce.Verifier())
+	if err != nil {
+		return httperr.Wrap(http.StatusBadRequest, "could not complete code exchange", err)
+	}
+
+	// Perform required validations on the returned ID token.
+	idTok, hasIDTok := oauth2Tok.Extra("id_token").(string)
+	if !hasIDTok {
+		return httperr.New(http.StatusBadRequest, "received response missing ID token")
+	}
+	validated, err := h.idTokenVerifier.Verify(r.Context(), idTok)
+	if err != nil {
+		return httperr.Wrap(http.StatusBadRequest, "received invalid ID token", err)
+	}
+	if validated.AccessTokenHash != "" {
+		if err := validated.VerifyAccessToken(oauth2Tok.AccessToken); err != nil {
+			return httperr.Wrap(http.StatusBadRequest, "received invalid ID token", err)
+		}
+	}
+	if err := h.nonce.Validate(validated); err != nil {
+		return httperr.Wrap(http.StatusBadRequest, "received ID token with invalid nonce", err)
+	}
+
+	h.callbacks <- callbackResult{token: &Token{
+		Token:         oauth2Tok,
+		IDToken:       idTok,
+		IDTokenExpiry: validated.Expiry,
+	}}
+	_, _ = w.Write([]byte("you have been logged in and may now close this tab"))
+	return nil
+}
diff --git a/internal/oidcclient/login/login_test.go b/internal/oidcclient/login/login_test.go
new file mode 100644
index 00000000..4050ebf2
--- /dev/null
+++ b/internal/oidcclient/login/login_test.go
@@ -0,0 +1,420 @@
+// Copyright 2020 the Pinniped contributors. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package login
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"testing"
+	"time"
+
+	"github.com/coreos/go-oidc"
+	"github.com/golang/mock/gomock"
+	"github.com/stretchr/testify/require"
+	"golang.org/x/oauth2"
+	"gopkg.in/square/go-jose.v2"
+
+	"go.pinniped.dev/internal/httputil/httperr"
+	"go.pinniped.dev/internal/mocks/mockkeyset"
+	"go.pinniped.dev/internal/oidcclient/nonce"
+	"go.pinniped.dev/internal/oidcclient/pkce"
+	"go.pinniped.dev/internal/oidcclient/state"
+)
+
+func TestRun(t *testing.T) {
+	time1 := time.Date(3020, 10, 12, 13, 14, 15, 16, time.UTC)
+	testToken := Token{
+		Token: &oauth2.Token{
+			AccessToken:  "test-access-token",
+			RefreshToken: "test-refresh-token",
+			Expiry:       time1.Add(1 * time.Minute),
+		},
+		IDToken:       "test-id-token",
+		IDTokenExpiry: time1.Add(2 * time.Minute),
+	}
+	_ = testToken
+
+	// Start a test server that returns 500 errors
+	errorServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Error(w, "some discovery error", http.StatusInternalServerError)
+	}))
+	t.Cleanup(errorServer.Close)
+
+	// Start a test server that returns a real keyset
+	providerMux := http.NewServeMux()
+	successServer := httptest.NewServer(providerMux)
+	t.Cleanup(successServer.Close)
+	providerMux.HandleFunc("/.well-known/openid-configuration", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("content-type", "application/json")
+		type providerJSON struct {
+			Issuer   string `json:"issuer"`
+			AuthURL  string `json:"authorization_endpoint"`
+			TokenURL string `json:"token_endpoint"`
+			JWKSURL  string `json:"jwks_uri"`
+		}
+		_ = json.NewEncoder(w).Encode(&providerJSON{
+			Issuer:   successServer.URL,
+			AuthURL:  successServer.URL + "/authorize",
+			TokenURL: successServer.URL + "/token",
+			JWKSURL:  successServer.URL + "/keys",
+		})
+	})
+
+	tests := []struct {
+		name      string
+		opt       func(t *testing.T) Option
+		issuer    string
+		clientID  string
+		wantErr   string
+		wantToken *Token
+	}{
+		{
+			name: "option error",
+			opt: func(t *testing.T) Option {
+				return func(h *handlerState) error {
+					return fmt.Errorf("some option error")
+				}
+			},
+			wantErr: "some option error",
+		},
+		{
+			name: "error generating state",
+			opt: func(t *testing.T) Option {
+				return func(h *handlerState) error {
+					h.generateState = func() (state.State, error) { return "", fmt.Errorf("some error generating state") }
+					return nil
+				}
+			},
+			wantErr: "some error generating state",
+		},
+		{
+			name: "error generating nonce",
+			opt: func(t *testing.T) Option {
+				return func(h *handlerState) error {
+					h.generateNonce = func() (nonce.Nonce, error) { return "", fmt.Errorf("some error generating nonce") }
+					return nil
+				}
+			},
+			wantErr: "some error generating nonce",
+		},
+		{
+			name: "error generating PKCE",
+			opt: func(t *testing.T) Option {
+				return func(h *handlerState) error {
+					h.generatePKCE = func() (pkce.Code, error) { return "", fmt.Errorf("some error generating PKCE") }
+					return nil
+				}
+			},
+			wantErr: "some error generating PKCE",
+		},
+		{
+			name: "discovery failure",
+			opt: func(t *testing.T) Option {
+				return func(h *handlerState) error { return nil }
+			},
+			issuer:  errorServer.URL,
+			wantErr: fmt.Sprintf("could not perform OIDC discovery for %q: 500 Internal Server Error: some discovery error\n", errorServer.URL),
+		},
+		{
+			name: "listen failure",
+			opt: func(t *testing.T) Option {
+				return func(h *handlerState) error {
+					h.listenAddr = "invalid-listen-address"
+					return nil
+				}
+			},
+			issuer:  successServer.URL,
+			wantErr: "could not open callback listener: listen tcp: address invalid-listen-address: missing port in address",
+		},
+		{
+			name: "browser open failure",
+			opt: func(t *testing.T) Option {
+				return WithBrowserOpen(func(url string) error {
+					return fmt.Errorf("some browser open error")
+				})
+			},
+			issuer:  successServer.URL,
+			wantErr: "could not open browser: some browser open error",
+		},
+		{
+			name: "timeout waiting for callback",
+			opt: func(t *testing.T) Option {
+				return func(h *handlerState) error {
+					ctx, cancel := context.WithCancel(h.ctx)
+					h.ctx = ctx
+
+					h.openURL = func(_ string) error {
+						cancel()
+						return nil
+					}
+					return nil
+				}
+			},
+			issuer:  successServer.URL,
+			wantErr: "timed out waiting for token callback: context canceled",
+		},
+		{
+			name: "callback returns error",
+			opt: func(t *testing.T) Option {
+				return func(h *handlerState) error {
+					h.openURL = func(_ string) error {
+						go func() {
+							h.callbacks <- callbackResult{err: fmt.Errorf("some callback error")}
+						}()
+						return nil
+					}
+					return nil
+				}
+			},
+			issuer:  successServer.URL,
+			wantErr: "error handling callback: some callback error",
+		},
+		{
+			name:     "callback returns success",
+			clientID: "test-client-id",
+			opt: func(t *testing.T) Option {
+				return func(h *handlerState) error {
+					h.generateState = func() (state.State, error) { return "test-state", nil }
+					h.generatePKCE = func() (pkce.Code, error) { return "test-pkce", nil }
+					h.generateNonce = func() (nonce.Nonce, error) { return "test-nonce", nil }
+
+					h.openURL = func(actualURL string) error {
+						parsedActualURL, err := url.Parse(actualURL)
+						require.NoError(t, err)
+						actualParams := parsedActualURL.Query()
+
+						require.Contains(t, actualParams.Get("redirect_uri"), "http://127.0.0.1:")
+						actualParams.Del("redirect_uri")
+
+						require.Equal(t, url.Values{
+							// This is the PKCE challenge which is calculated as base64(sha256("test-pkce")). For example:
+							// $ echo -n test-pkce | shasum -a 256 | cut -d" " -f1 | xxd -r -p | base64 | cut -d"=" -f1
+							// VVaezYqum7reIhoavCHD1n2d+piN3r/mywoYj7fCR7g
+							"code_challenge":        []string{"VVaezYqum7reIhoavCHD1n2d-piN3r_mywoYj7fCR7g"},
+							"code_challenge_method": []string{"S256"},
+							"response_type":         []string{"code"},
+							"scope":                 []string{"test-scope"},
+							"nonce":                 []string{"test-nonce"},
+							"state":                 []string{"test-state"},
+							"access_type":           []string{"offline"},
+							"client_id":             []string{"test-client-id"},
+						}, actualParams)
+
+						parsedActualURL.RawQuery = ""
+						require.Equal(t, successServer.URL+"/authorize", parsedActualURL.String())
+
+						go func() {
+							h.callbacks <- callbackResult{token: &testToken}
+						}()
+						return nil
+					}
+					return nil
+				}
+			},
+			issuer:    successServer.URL,
+			wantToken: &testToken,
+		},
+	}
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			tok, err := Run(tt.issuer, tt.clientID,
+				WithContext(context.Background()),
+				WithListenPort(0),
+				WithScopes([]string{"test-scope"}),
+				tt.opt(t),
+			)
+			if tt.wantErr != "" {
+				require.EqualError(t, err, tt.wantErr)
+				require.Nil(t, tok)
+				return
+			}
+			require.Equal(t, tt.wantToken, tok)
+		})
+	}
+}
+
+func TestHandleAuthCodeCallback(t *testing.T) {
+	tests := []struct {
+		name           string
+		method         string
+		query          string
+		returnIDTok    string
+		wantErr        string
+		wantHTTPStatus int
+	}{
+		{
+			name:           "wrong method",
+			method:         "POST",
+			query:          "",
+			wantErr:        "wanted GET",
+			wantHTTPStatus: http.StatusMethodNotAllowed,
+		},
+		{
+			name:           "invalid state",
+			query:          "state=invalid",
+			wantErr:        "missing or invalid state parameter",
+			wantHTTPStatus: http.StatusForbidden,
+		},
+		{
+			name:           "error code from provider",
+			query:          "state=test-state&error=some_error",
+			wantErr:        `login failed with code "some_error"`,
+			wantHTTPStatus: http.StatusBadRequest,
+		},
+		{
+			name:           "invalid code",
+			query:          "state=test-state&code=invalid",
+			wantErr:        "could not complete code exchange: oauth2: cannot fetch token: 403 Forbidden\nResponse: invalid authorization code\n",
+			wantHTTPStatus: http.StatusBadRequest,
+		},
+		{
+			name:           "missing ID token",
+			query:          "state=test-state&code=valid",
+			returnIDTok:    "",
+			wantErr:        "received response missing ID token",
+			wantHTTPStatus: http.StatusBadRequest,
+		},
+		{
+			name:           "invalid ID token",
+			query:          "state=test-state&code=valid",
+			returnIDTok:    "invalid-jwt",
+			wantErr:        "received invalid ID token: oidc: malformed jwt: square/go-jose: compact JWS format must have three parts",
+			wantHTTPStatus: http.StatusBadRequest,
+		},
+		{
+			name:  "invalid access token hash",
+			query: "state=test-state&code=valid",
+
+			// Test JWT generated with https://smallstep.com/docs/cli/crypto/jwt/:
+			// step crypto keypair key.pub key.priv --kty RSA --no-password --insecure --force && echo '{"at_hash": "invalid-at-hash"}' | step crypto jwt sign --key key.priv --aud test-client-id --sub test-user --subtle --kid="test-kid" --jti="test-jti"
+			returnIDTok: "eyJhbGciOiJSUzI1NiIsImtpZCI6InRlc3Qta2lkIiwidHlwIjoiSldUIn0.eyJhdF9oYXNoIjoiaW52YWxpZC1hdC1oYXNoIiwiYXVkIjoidGVzdC1jbGllbnQtaWQiLCJpYXQiOjE2MDIyODM3OTEsImp0aSI6InRlc3QtanRpIiwibmJmIjoxNjAyMjgzNzkxLCJzdWIiOiJ0ZXN0LXVzZXIifQ.jryXr4jiwcf79wBLaHpjdclEYHoUFGhvTu95QyA6Hnk9NQ0x1vsWYurtj7a8uKydNPryC_HNZi9QTAE_tRIJjycseog3695-5y4B4EZlqL-a94rdOtffuF2O_lnPbKvoja9EKNrp0kLBCftFRHhLAEwuP0N9E5padZwPpIGK0yE_JqljnYgCySvzsQu7tasR38yaULny13h3mtp2WRHPG5DrLyuBuF8Z01hSgRi5hGcVpgzTwBgV5-eMaSUCUo-ZDkqUsLQI6dVlaikCSKYZRb53HeexH0tB_R9PJJHY7mIr-rS76kkQEx9pLuVnheIH9Oc6zbdYWg-zWMijopA8Pg",
+
+			wantErr:        "received invalid ID token: access token hash does not match value in ID token",
+			wantHTTPStatus: http.StatusBadRequest,
+		},
+		{
+			name:  "invalid nonce",
+			query: "state=test-state&code=valid",
+
+			// Test JWT generated with https://smallstep.com/docs/cli/crypto/jwt/:
+			// step crypto keypair key.pub key.priv --kty RSA --no-password --insecure --force && echo '{"nonce": "invalid-nonce"}' | step crypto jwt sign --key key.priv --aud test-client-id --sub test-user --subtle --kid="test-kid" --jti="test-jti"
+			returnIDTok: "eyJhbGciOiJSUzI1NiIsImtpZCI6InRlc3Qta2lkIiwidHlwIjoiSldUIn0.eyJhdWQiOiJ0ZXN0LWNsaWVudC1pZCIsImlhdCI6MTYwMjI4Mzc0MSwianRpIjoidGVzdC1qdGkiLCJuYmYiOjE2MDIyODM3NDEsIm5vbmNlIjoiaW52YWxpZC1ub25jZSIsInN1YiI6InRlc3QtdXNlciJ9.PRpq-7j5djaIAkraL-8t8ad9Xm4hM8RW67gyD1VIe0BecWeBFxsTuh3SZVKM9zmcwTgjudsyn8kQOwipDa49IN4PV8FcJA_uUJZi2wiqGJUSTG2K5I89doV_7e0RM1ZYIDDW1G2heKJNW7MbKkX7iEPr7u4MyEzswcPcupbyDA-CQFeL95vgwawoqa6yO94ympTbozqiNfj6Xyw_nHtThQnstjWsJZ9s2mUgppZezZv4HZYTQ7c3e_bzwhWgCzh2CSDJn9_Ra_n_4GcVkpHbsHTP35dFsnf0vactPx6CAu6A1-Apk-BruCktpZ3B4Ercf1UnUOHdGqzQKJtqvB03xQ",
+
+			wantHTTPStatus: http.StatusBadRequest,
+			wantErr:        `received ID token with invalid nonce: invalid nonce (expected "test-nonce", got "invalid-nonce")`,
+		},
+		{
+			name:  "valid",
+			query: "state=test-state&code=valid",
+
+			// Test JWT generated with https://smallstep.com/docs/cli/crypto/jwt/:
+			// step crypto keypair key.pub key.priv --kty RSA --no-password --insecure --force && echo '{"nonce": "test-nonce"}' | step crypto jwt sign --key key.priv --aud test-client-id --sub test-user --subtle --kid="test-kid" --jti="test-jti"
+			returnIDTok: "eyJhbGciOiJSUzI1NiIsImtpZCI6InRlc3Qta2lkIiwidHlwIjoiSldUIn0.eyJhdWQiOiJ0ZXN0LWNsaWVudC1pZCIsImlhdCI6MTYwMjUzMTU2NywianRpIjoidGVzdC1qdGkiLCJuYmYiOjE2MDI1MzE1NjcsIm5vbmNlIjoidGVzdC1ub25jZSIsInN1YiI6InRlc3QtdXNlciJ9.LbOA31iwJZBM4ayY5Oud-HArLXbmtAIhZv_LazDqbzA2Iw87RxoBemfiPUJeAesdnO1LKSjBwbltZwtjvbLWHp1R5tqrSMr_hl2OyZv1cpEX-9QaTcQILJ5qR00riRLz34ZCQFyF-FfQpP1r4dNqFrxHuiBwKuPE7zogc83ZYJgAQM5Fao9rIRY9JStL_3pURa9JnnSHFlkLvFYv3TKEUyvnW4pWvYZcsGI7mys43vuSjpG7ZSrW3vCxovuIpXYqAhamZL_XexWUsXvi3ej9HNlhnhOFhN4fuPSc0PWDWaN0CLWmoo8gvOdQWo5A4GD4bNGBzjYOd-pYqsDfseRt1Q",
+		},
+	}
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			tokenServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				require.Equal(t, http.MethodPost, r.Method)
+				require.NoError(t, r.ParseForm())
+				require.Equal(t, "test-client-id", r.Form.Get("client_id"))
+				require.Equal(t, "test-pkce", r.Form.Get("code_verifier"))
+				require.Equal(t, "authorization_code", r.Form.Get("grant_type"))
+				require.NotEmpty(t, r.Form.Get("code"))
+				if r.Form.Get("code") != "valid" {
+					http.Error(w, "invalid authorization code", http.StatusForbidden)
+					return
+				}
+				var response struct {
+					oauth2.Token
+					IDToken string `json:"id_token,omitempty"`
+				}
+				response.AccessToken = "test-access-token"
+				response.Expiry = time.Now().Add(time.Hour)
+				response.IDToken = tt.returnIDTok
+				w.Header().Set("content-type", "application/json")
+				require.NoError(t, json.NewEncoder(w).Encode(&response))
+			}))
+			t.Cleanup(tokenServer.Close)
+
+			h := &handlerState{
+				callbacks: make(chan callbackResult, 1),
+				state:     state.State("test-state"),
+				pkce:      pkce.Code("test-pkce"),
+				nonce:     nonce.Nonce("test-nonce"),
+				oauth2Config: &oauth2.Config{
+					ClientID:    "test-client-id",
+					RedirectURL: "http://localhost:12345/callback",
+					Endpoint: oauth2.Endpoint{
+						TokenURL:  tokenServer.URL,
+						AuthStyle: oauth2.AuthStyleInParams,
+					},
+				},
+				idTokenVerifier: mockVerifier(),
+			}
+
+			ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+			defer cancel()
+
+			resp := httptest.NewRecorder()
+			req, err := http.NewRequestWithContext(ctx, "GET", "/test-callback", nil)
+			require.NoError(t, err)
+			req.URL.RawQuery = tt.query
+			if tt.method != "" {
+				req.Method = tt.method
+			}
+
+			err = h.handleAuthCodeCallback(resp, req)
+			if tt.wantErr != "" {
+				require.EqualError(t, err, tt.wantErr)
+				if tt.wantHTTPStatus != 0 {
+					rec := httptest.NewRecorder()
+					err.(httperr.Responder).Respond(rec)
+					require.Equal(t, tt.wantHTTPStatus, rec.Code)
+				}
+			} else {
+				require.NoError(t, err)
+			}
+
+			select {
+			case <-time.After(1 * time.Second):
+				require.Fail(t, "timed out waiting to receive from callbacks channel")
+			case result := <-h.callbacks:
+				if tt.wantErr != "" {
+					require.EqualError(t, result.err, tt.wantErr)
+					return
+				}
+				require.NoError(t, result.err)
+				require.NotNil(t, result.token)
+				require.Equal(t, result.token.IDToken, tt.returnIDTok)
+			}
+		})
+	}
+}
+
+// mockVerifier returns an *oidc.IDTokenVerifier that validates any correctly serialized JWT without doing much else.
+func mockVerifier() *oidc.IDTokenVerifier {
+	mockKeySet := mockkeyset.NewMockKeySet(gomock.NewController(nil))
+	mockKeySet.EXPECT().VerifySignature(gomock.Any(), gomock.Any()).
+		AnyTimes().
+		DoAndReturn(func(ctx context.Context, jwt string) ([]byte, error) {
+			jws, err := jose.ParseSigned(jwt)
+			if err != nil {
+				return nil, err
+			}
+			return jws.UnsafePayloadWithoutVerification(), nil
+		})
+
+	return oidc.NewVerifier("", mockKeySet, &oidc.Config{
+		SkipIssuerCheck:   true,
+		SkipExpiryCheck:   true,
+		SkipClientIDCheck: true,
+	})
+}
diff --git a/internal/oidcclient/nonce/nonce.go b/internal/oidcclient/nonce/nonce.go
new file mode 100644
index 00000000..fc10112e
--- /dev/null
+++ b/internal/oidcclient/nonce/nonce.go
@@ -0,0 +1,58 @@
+// Copyright 2020 the Pinniped contributors. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+// Package nonce implements
+package nonce
+
+import (
+	"crypto/rand"
+	"crypto/subtle"
+	"encoding/hex"
+	"fmt"
+	"io"
+
+	"github.com/coreos/go-oidc"
+	"golang.org/x/oauth2"
+)
+
+// Generate generates a new random OIDC nonce parameter of an appropriate size.
+func Generate() (Nonce, error) { return generate(rand.Reader) }
+
+func generate(rand io.Reader) (Nonce, error) {
+	var buf [16]byte
+	if _, err := io.ReadFull(rand, buf[:]); err != nil {
+		return "", fmt.Errorf("could not generate random nonce: %w", err)
+	}
+	return Nonce(hex.EncodeToString(buf[:])), nil
+}
+
+// Nonce implements some utilities for working with OIDC nonce parameters.
+type Nonce string
+
+// String returns the string encoding of this state value.
+func (n *Nonce) String() string {
+	return string(*n)
+}
+
+// Param returns the OAuth2 auth code parameter for sending the nonce during the authorization request.
+func (n *Nonce) Param() oauth2.AuthCodeOption {
+	return oidc.Nonce(string(*n))
+}
+
+// Validate the returned ID token). Returns true iff the nonce matches or the returned JWT does not have a nonce.
+func (n *Nonce) Validate(token *oidc.IDToken) error {
+	if subtle.ConstantTimeCompare([]byte(token.Nonce), []byte(*n)) != 1 {
+		return InvalidNonceError{Expected: *n, Got: Nonce(token.Nonce)}
+	}
+	return nil
+}
+
+// InvalidNonceError is returned by Validate when the observed nonce is invalid.
+type InvalidNonceError struct {
+	Expected Nonce
+	Got      Nonce
+}
+
+func (e InvalidNonceError) Error() string {
+	return fmt.Sprintf("invalid nonce (expected %q, got %q)", e.Expected.String(), e.Got.String())
+}
diff --git a/internal/oidcclient/nonce/nonce_test.go b/internal/oidcclient/nonce/nonce_test.go
new file mode 100644
index 00000000..9738a245
--- /dev/null
+++ b/internal/oidcclient/nonce/nonce_test.go
@@ -0,0 +1,40 @@
+// Copyright 2020 the Pinniped contributors. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package nonce
+
+import (
+	"bytes"
+	"errors"
+	"net/url"
+	"testing"
+
+	"github.com/coreos/go-oidc"
+	"github.com/stretchr/testify/require"
+	"golang.org/x/oauth2"
+)
+
+func TestNonce(t *testing.T) {
+	n, err := Generate()
+	require.NoError(t, err)
+	require.Len(t, n, 32)
+	require.Len(t, n.String(), 32)
+
+	cfg := oauth2.Config{}
+	authCodeURL, err := url.Parse(cfg.AuthCodeURL("", n.Param()))
+	require.NoError(t, err)
+	require.Equal(t, n.String(), authCodeURL.Query().Get("nonce"))
+
+	require.Error(t, n.Validate(&oidc.IDToken{}))
+	require.NoError(t, n.Validate(&oidc.IDToken{Nonce: string(n)}))
+
+	err = n.Validate(&oidc.IDToken{Nonce: string(n) + "x"})
+	require.Error(t, err)
+	require.True(t, errors.As(err, &InvalidNonceError{}))
+	require.Contains(t, err.Error(), string(n)+"x")
+
+	var empty bytes.Buffer
+	n, err = generate(&empty)
+	require.EqualError(t, err, "could not generate random nonce: EOF")
+	require.Empty(t, n)
+}
diff --git a/internal/oidc/pkce/pkce.go b/internal/oidcclient/pkce/pkce.go
similarity index 76%
rename from internal/oidc/pkce/pkce.go
rename to internal/oidcclient/pkce/pkce.go
index 309b3d4d..d28f55df 100644
--- a/internal/oidc/pkce/pkce.go
+++ b/internal/oidcclient/pkce/pkce.go
@@ -8,9 +8,9 @@ import (
 	"crypto/sha256"
 	"encoding/base64"
 	"encoding/hex"
+	"fmt"
 	"io"
 
-	"github.com/pkg/errors"
 	"golang.org/x/oauth2"
 )
 
@@ -18,9 +18,14 @@ import (
 func Generate() (Code, error) { return generate(rand.Reader) }
 
 func generate(rand io.Reader) (Code, error) {
+	// From https://tools.ietf.org/html/rfc7636#section-4.1:
+	//   code_verifier = high-entropy cryptographic random STRING using the
+	//   unreserved characters [A-Z] / [a-z] / [0-9] / "-" / "." / "_" / "~"
+	//   from Section 2.3 of [RFC3986], with a minimum length of 43 characters
+	//   and a maximum length of 128 characters.
 	var buf [32]byte
 	if _, err := io.ReadFull(rand, buf[:]); err != nil {
-		return "", errors.WithMessage(err, "could not generate PKCE code")
+		return "", fmt.Errorf("could not generate PKCE code: %w", err)
 	}
 	return Code(hex.EncodeToString(buf[:])), nil
 }
diff --git a/internal/oidc/pkce/pkce_test.go b/internal/oidcclient/pkce/pkce_test.go
similarity index 100%
rename from internal/oidc/pkce/pkce_test.go
rename to internal/oidcclient/pkce/pkce_test.go
diff --git a/internal/oidcclient/state/state.go b/internal/oidcclient/state/state.go
new file mode 100644
index 00000000..ff71e726
--- /dev/null
+++ b/internal/oidcclient/state/state.go
@@ -0,0 +1,56 @@
+// Copyright 2020 the Pinniped contributors. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package state
+
+import (
+	"crypto/rand"
+	"crypto/subtle"
+	"encoding/hex"
+	"fmt"
+	"io"
+)
+
+// Generate generates a new random state parameter of an appropriate size.
+func Generate() (State, error) { return generate(rand.Reader) }
+
+func generate(rand io.Reader) (State, error) {
+	// From https://tools.ietf.org/html/rfc6749#section-10.12:
+	//   The binding value used for CSRF
+	//   protection MUST contain a non-guessable value (as described in
+	//   Section 10.10), and the user-agent's authenticated state (e.g.,
+	//   session cookie, HTML5 local storage) MUST be kept in a location
+	//   accessible only to the client and the user-agent (i.e., protected by
+	//   same-origin policy).
+	var buf [16]byte
+	if _, err := io.ReadFull(rand, buf[:]); err != nil {
+		return "", fmt.Errorf("could not generate random state: %w", err)
+	}
+	return State(hex.EncodeToString(buf[:])), nil
+}
+
+// State implements some utilities for working with OAuth2 state parameters.
+type State string
+
+// String returns the string encoding of this state value.
+func (s *State) String() string {
+	return string(*s)
+}
+
+// Validate the returned state (from a callback parameter).
+func (s *State) Validate(returnedState string) error {
+	if subtle.ConstantTimeCompare([]byte(returnedState), []byte(*s)) != 1 {
+		return InvalidStateError{Expected: *s, Got: State(returnedState)}
+	}
+	return nil
+}
+
+// InvalidStateError is returned by Validate when the returned state is invalid.
+type InvalidStateError struct {
+	Expected State
+	Got      State
+}
+
+func (e InvalidStateError) Error() string {
+	return fmt.Sprintf("invalid state (expected %q, got %q)", e.Expected.String(), e.Got.String())
+}
diff --git a/internal/oidc/state/state_test.go b/internal/oidcclient/state/state_test.go
similarity index 68%
rename from internal/oidc/state/state_test.go
rename to internal/oidcclient/state/state_test.go
index ff181839..a12a3b16 100644
--- a/internal/oidc/state/state_test.go
+++ b/internal/oidcclient/state/state_test.go
@@ -5,6 +5,7 @@ package state
 
 import (
 	"bytes"
+	"errors"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -15,8 +16,11 @@ func TestState(t *testing.T) {
 	require.NoError(t, err)
 	require.Len(t, s, 32)
 	require.Len(t, s.String(), 32)
-	require.True(t, s.Valid(string(s)))
-	require.False(t, s.Valid(string(s)+"x"))
+	require.NoError(t, s.Validate(string(s)))
+	err = s.Validate(string(s) + "x")
+	require.Error(t, err)
+	require.True(t, errors.As(err, &InvalidStateError{}))
+	require.Contains(t, err.Error(), string(s)+"x")
 
 	var empty bytes.Buffer
 	s, err = generate(&empty)