OAuth authorization code exposure in Dex

@@ -1,6 +1,7 @@
 package server
 
 import (
+	"crypto/hmac"
 	"crypto/sha256"
 	"crypto/subtle"
 	"encoding/base64"
@@ -499,7 +500,15 @@ func (s *Server) finalizeLogin(identity connector.Identity, authReq storage.Auth
 	s.logger.Infof("login successful: connector %q, username=%q, preferred_username=%q, email=%q, groups=%q",
 		authReq.ConnectorID, claims.Username, claims.PreferredUsername, email, claims.Groups)
 
-	returnURL := path.Join(s.issuerURL.Path, "/approval") + "?req=" + authReq.ID
+	// TODO: if s.skipApproval or !authReq.ForceApprovalPrompt, we can skip the redirect to /approval and go ahead and send code
+
+	// an HMAC is used here to ensure that the request ID is unpredictable, ensuring that an attacker who intercepted the original
+	// flow would be unable to poll for the result at the /approval endpoint
+	h := hmac.New(sha256.New, authReq.HMACKey)
+	h.Write([]byte(authReq.ID))
+	mac := h.Sum(nil)
+
+	returnURL := path.Join(s.issuerURL.Path, "/approval") + "?req=" + authReq.ID + "&hmac=" + base64.RawURLEncoding.EncodeToString(mac)
 	_, ok := conn.(connector.RefreshConnector)
 	if !ok {
 		return returnURL, nil
@@ -544,6 +553,17 @@ func (s *Server) finalizeLogin(identity connector.Identity, authReq storage.Auth
 }
 
 func (s *Server) handleApproval(w http.ResponseWriter, r *http.Request) {
+	macEncoded := r.FormValue("hmac")
+	if macEncoded == "" {
+		s.renderError(r, w, http.StatusUnauthorized, "Unauthorized request")
+		return
+	}
+	mac, err := base64.RawURLEncoding.DecodeString(macEncoded)
+	if err != nil {
+		s.renderError(r, w, http.StatusUnauthorized, "Unauthorized request")
+		return
+	}
+
 	authReq, err := s.storage.GetAuthRequest(r.FormValue("req"))
 	if err != nil {
 		s.logger.Errorf("Failed to get auth request: %v", err)
@@ -556,6 +576,16 @@ func (s *Server) handleApproval(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// build expected hmac with secret key
+	h := hmac.New(sha256.New, authReq.HMACKey)
+	h.Write([]byte(authReq.ID))
+	expectedMAC := h.Sum(nil)
+	// constant time comparison
+	if !hmac.Equal(mac, expectedMAC) {
+		s.renderError(r, w, http.StatusUnauthorized, "Unauthorized request")
+		return
+	}
+
 	switch r.Method {
 	case http.MethodGet:
 		if s.skipApproval {

@@ -2,6 +2,7 @@ package server
 
 import (
 	"context"
+	"crypto"
 	"crypto/ecdsa"
 	"crypto/elliptic"
 	"crypto/rsa"
@@ -576,6 +577,7 @@ func (s *Server) parseAuthorizationRequest(r *http.Request) (*storage.AuthReques
 			CodeChallenge:       codeChallenge,
 			CodeChallengeMethod: codeChallengeMethod,
 		},
+		HMACKey: storage.NewHMACKey(crypto.SHA256),
 	}, nil
 }
 

@@ -104,7 +104,8 @@ func testAuthRequestCRUD(t *testing.T, s storage.Storage) {
 			EmailVerified: true,
 			Groups:        []string{"a", "b"},
 		},
-		PKCE: codeChallenge,
+		PKCE:    codeChallenge,
+		HMACKey: []byte("hmac_key"),
 	}
 
 	identity := storage.Claims{Email: "foobar"}
@@ -137,6 +138,7 @@ func testAuthRequestCRUD(t *testing.T, s storage.Storage) {
 			EmailVerified: true,
 			Groups:        []string{"a"},
 		},
+		HMACKey: []byte("hmac_key"),
 	}
 
 	if err := s.CreateAuthRequest(a2); err != nil {
@@ -817,6 +819,7 @@ func testGC(t *testing.T, s storage.Storage) {
 			EmailVerified: true,
 			Groups:        []string{"a", "b"},
 		},
+		HMACKey: []byte("hmac_key"),
 	}
 
 	if err := s.CreateAuthRequest(a); err != nil {

@@ -31,6 +31,7 @@ func (d *Database) CreateAuthRequest(authRequest storage.AuthRequest) error {
 		SetExpiry(authRequest.Expiry.UTC()).
 		SetConnectorID(authRequest.ConnectorID).
 		SetConnectorData(authRequest.ConnectorData).
+		SetHmacKey(authRequest.HMACKey).
 		Save(context.TODO())
 	if err != nil {
 		return convertDBError("create auth request: %w", err)
@@ -94,6 +95,7 @@ func (d *Database) UpdateAuthRequest(id string, updater func(old storage.AuthReq
 		SetExpiry(newAuthRequest.Expiry.UTC()).
 		SetConnectorID(newAuthRequest.ConnectorID).
 		SetConnectorData(newAuthRequest.ConnectorData).
+		SetHmacKey(newAuthRequest.HMACKey).
 		Save(context.TODO())
 	if err != nil {
 		return rollback(tx, "update auth request uploading: %w", err)

@@ -45,6 +45,7 @@ func toStorageAuthRequest(a *db.AuthRequest) storage.AuthRequest {
 			CodeChallenge:       a.CodeChallenge,
 			CodeChallengeMethod: a.CodeChallengeMethod,
 		},
+		HMACKey: a.HmacKey,
 	}
 }
 

@@ -45,6 +45,8 @@ const (
 	FieldCodeChallenge = "code_challenge"
 	// FieldCodeChallengeMethod holds the string denoting the code_challenge_method field in the database.
 	FieldCodeChallengeMethod = "code_challenge_method"
+	// FieldHmacKey holds the string denoting the hmac_key field in the database.
+	FieldHmacKey = "hmac_key"
 	// Table holds the table name of the authrequest in the database.
 	Table = "auth_requests"
 )
@@ -71,6 +73,7 @@ var Columns = []string{
 	FieldExpiry,
 	FieldCodeChallenge,
 	FieldCodeChallengeMethod,
+	FieldHmacKey,
 }
 
 // ValidColumn reports if the column name is valid (part of the table columns).

@@ -158,6 +158,12 @@ func (arc *AuthRequestCreate) SetNillableCodeChallengeMethod(s *string) *AuthReq
 	return arc
 }
 
+// SetHmacKey sets the "hmac_key" field.
+func (arc *AuthRequestCreate) SetHmacKey(b []byte) *AuthRequestCreate {
+	arc.mutation.SetHmacKey(b)
+	return arc
+}
+
 // SetID sets the "id" field.
 func (arc *AuthRequestCreate) SetID(s string) *AuthRequestCreate {
 	arc.mutation.SetID(s)
@@ -302,6 +308,9 @@ func (arc *AuthRequestCreate) check() error {
 	if _, ok := arc.mutation.CodeChallengeMethod(); !ok {
 		return &ValidationError{Name: "code_challenge_method", err: errors.New(`db: missing required field "AuthRequest.code_challenge_method"`)}
 	}
+	if _, ok := arc.mutation.HmacKey(); !ok {
+		return &ValidationError{Name: "hmac_key", err: errors.New(`db: missing required field "AuthRequest.hmac_key"`)}
+	}
 	if v, ok := arc.mutation.ID(); ok {
 		if err := authrequest.IDValidator(v); err != nil {
 			return &ValidationError{Name: "id", err: fmt.Errorf(`db: validator failed for field "AuthRequest.id": %w`, err)}
@@ -495,6 +504,14 @@ func (arc *AuthRequestCreate) createSpec() (*AuthRequest, *sqlgraph.CreateSpec)
 		})
 		_node.CodeChallengeMethod = value
 	}
+	if value, ok := arc.mutation.HmacKey(); ok {
+		_spec.Fields = append(_spec.Fields, &sqlgraph.FieldSpec{
+			Type:   field.TypeBytes,
+			Value:  value,
+			Column: authrequest.FieldHmacKey,
+		})
+		_node.HmacKey = value
+	}
 	return _node, _spec
 }
 

@@ -55,14 +55,16 @@ type AuthRequest struct {
 	CodeChallenge string `json:"code_challenge,omitempty"`
 	// CodeChallengeMethod holds the value of the "code_challenge_method" field.
 	CodeChallengeMethod string `json:"code_challenge_method,omitempty"`
+	// HmacKey holds the value of the "hmac_key" field.
+	HmacKey []byte `json:"hmac_key,omitempty"`
 }
 
 // scanValues returns the types for scanning values from sql.Rows.
 func (*AuthRequest) scanValues(columns []string) ([]interface{}, error) {
 	values := make([]interface{}, len(columns))
 	for i := range columns {
 		switch columns[i] {
-		case authrequest.FieldScopes, authrequest.FieldResponseTypes, authrequest.FieldClaimsGroups, authrequest.FieldConnectorData:
+		case authrequest.FieldScopes, authrequest.FieldResponseTypes, authrequest.FieldClaimsGroups, authrequest.FieldConnectorData, authrequest.FieldHmacKey:
 			values[i] = new([]byte)
 		case authrequest.FieldForceApprovalPrompt, authrequest.FieldLoggedIn, authrequest.FieldClaimsEmailVerified:
 			values[i] = new(sql.NullBool)
@@ -211,6 +213,12 @@ func (ar *AuthRequest) assignValues(columns []string, values []interface{}) erro
 			} else if value.Valid {
 				ar.CodeChallengeMethod = value.String
 			}
+		case authrequest.FieldHmacKey:
+			if value, ok := values[i].(*[]byte); !ok {
+				return fmt.Errorf("unexpected type %T for field hmac_key", values[i])
+			} else if value != nil {
+				ar.HmacKey = *value
+			}
 		}
 	}
 	return nil
@@ -297,6 +305,8 @@ func (ar *AuthRequest) String() string {
 	builder.WriteString(", ")
 	builder.WriteString("code_challenge_method=")
 	builder.WriteString(ar.CodeChallengeMethod)
+	builder.WriteString(", hmac_key=")
+	builder.WriteString(fmt.Sprintf("%v", ar.HmacKey))
 	builder.WriteByte(')')
 	return builder.String()
 }

@@ -190,6 +190,12 @@ func (aru *AuthRequestUpdate) SetNillableCodeChallengeMethod(s *string) *AuthReq
 	return aru
 }
 
+// SetHmacKey sets the "hmac_key" field.
+func (aru *AuthRequestUpdate) SetHmacKey(b []byte) *AuthRequestUpdate {
+	aru.mutation.SetHmacKey(b)
+	return aru
+}
+
 // Mutation returns the AuthRequestMutation object of the builder.
 func (aru *AuthRequestUpdate) Mutation() *AuthRequestMutation {
 	return aru.mutation
@@ -424,6 +430,13 @@ func (aru *AuthRequestUpdate) sqlSave(ctx context.Context) (n int, err error) {
 			Column: authrequest.FieldCodeChallengeMethod,
 		})
 	}
+	if value, ok := aru.mutation.HmacKey(); ok {
+		_spec.Fields.Set = append(_spec.Fields.Set, &sqlgraph.FieldSpec{
+			Type:   field.TypeBytes,
+			Value:  value,
+			Column: authrequest.FieldHmacKey,
+		})
+	}
 	if n, err = sqlgraph.UpdateNodes(ctx, aru.driver, _spec); err != nil {
 		if _, ok := err.(*sqlgraph.NotFoundError); ok {
 			err = &NotFoundError{authrequest.Label}
@@ -605,6 +618,12 @@ func (aruo *AuthRequestUpdateOne) SetNillableCodeChallengeMethod(s *string) *Aut
 	return aruo
 }
 
+// SetHmacKey sets the "hmac_key" field.
+func (aruo *AuthRequestUpdateOne) SetHmacKey(b []byte) *AuthRequestUpdateOne {
+	aruo.mutation.SetHmacKey(b)
+	return aruo
+}
+
 // Mutation returns the AuthRequestMutation object of the builder.
 func (aruo *AuthRequestUpdateOne) Mutation() *AuthRequestMutation {
 	return aruo.mutation
@@ -869,6 +888,13 @@ func (aruo *AuthRequestUpdateOne) sqlSave(ctx context.Context) (_node *AuthReque
 			Column: authrequest.FieldCodeChallengeMethod,
 		})
 	}
+	if value, ok := aruo.mutation.HmacKey(); ok {
+		_spec.Fields.Set = append(_spec.Fields.Set, &sqlgraph.FieldSpec{
+			Type:   field.TypeBytes,
+			Value:  value,
+			Column: authrequest.FieldHmacKey,
+		})
+	}
 	_node = &AuthRequest{config: aruo.config}
 	_spec.Assign = _node.assignValues
 	_spec.ScanValues = _node.scanValues

@@ -192,6 +192,13 @@ func CodeChallengeMethod(v string) predicate.AuthRequest {
 	})
 }
 
+// HmacKey applies equality check predicate on the "hmac_key" field. It's identical to HmacKeyEQ.
+func HmacKey(v []byte) predicate.AuthRequest {
+	return predicate.AuthRequest(func(s *sql.Selector) {
+		s.Where(sql.EQ(s.C(FieldHmacKey), v))
+	})
+}
+
 // ClientIDEQ applies the EQ predicate on the "client_id" field.
 func ClientIDEQ(v string) predicate.AuthRequest {
 	return predicate.AuthRequest(func(s *sql.Selector) {
@@ -1507,6 +1514,82 @@ func CodeChallengeMethodContainsFold(v string) predicate.AuthRequest {
 	})
 }
 
+// HmacKeyEQ applies the EQ predicate on the "hmac_key" field.
+func HmacKeyEQ(v []byte) predicate.AuthRequest {
+	return predicate.AuthRequest(func(s *sql.Selector) {
+		s.Where(sql.EQ(s.C(FieldHmacKey), v))
+	})
+}
+
+// HmacKeyNEQ applies the NEQ predicate on the "hmac_key" field.
+func HmacKeyNEQ(v []byte) predicate.AuthRequest {
+	return predicate.AuthRequest(func(s *sql.Selector) {
+		s.Where(sql.NEQ(s.C(FieldHmacKey), v))
+	})
+}
+
+// HmacKeyIn applies the In predicate on the "hmac_key" field.
+func HmacKeyIn(vs ...[]byte) predicate.AuthRequest {
+	v := make([]interface{}, len(vs))
+	for i := range v {
+		v[i] = vs[i]
+	}
+	return predicate.AuthRequest(func(s *sql.Selector) {
+		// if not arguments were provided, append the FALSE constants,
+		// since we can't apply "IN ()". This will make this predicate falsy.
+		if len(v) == 0 {
+			s.Where(sql.False())
+			return
+		}
+		s.Where(sql.In(s.C(FieldHmacKey), v...))
+	})
+}
+
+// HmacKeyNotIn applies the NotIn predicate on the "hmac_key" field.
+func HmacKeyNotIn(vs ...[]byte) predicate.AuthRequest {
+	v := make([]interface{}, len(vs))
+	for i := range v {
+		v[i] = vs[i]
+	}
+	return predicate.AuthRequest(func(s *sql.Selector) {
+		// if not arguments were provided, append the FALSE constants,
+		// since we can't apply "IN ()". This will make this predicate falsy.
+		if len(v) == 0 {
+			s.Where(sql.False())
+			return
+		}
+		s.Where(sql.NotIn(s.C(FieldHmacKey), v...))
+	})
+}
+
+// HmacKeyGT applies the GT predicate on the "hmac_key" field.
+func HmacKeyGT(v []byte) predicate.AuthRequest {
+	return predicate.AuthRequest(func(s *sql.Selector) {
+		s.Where(sql.GT(s.C(FieldHmacKey), v))
+	})
+}
+
+// HmacKeyGTE applies the GTE predicate on the "hmac_key" field.
+func HmacKeyGTE(v []byte) predicate.AuthRequest {
+	return predicate.AuthRequest(func(s *sql.Selector) {
+		s.Where(sql.GTE(s.C(FieldHmacKey), v))
+	})
+}
+
+// HmacKeyLT applies the LT predicate on the "hmac_key" field.
+func HmacKeyLT(v []byte) predicate.AuthRequest {
+	return predicate.AuthRequest(func(s *sql.Selector) {
+		s.Where(sql.LT(s.C(FieldHmacKey), v))
+	})
+}
+
+// HmacKeyLTE applies the LTE predicate on the "hmac_key" field.
+func HmacKeyLTE(v []byte) predicate.AuthRequest {
+	return predicate.AuthRequest(func(s *sql.Selector) {
+		s.Where(sql.LTE(s.C(FieldHmacKey), v))
+	})
+}
+
 // And groups predicates with the AND operator between them.
 func And(predicates ...predicate.AuthRequest) predicate.AuthRequest {
 	return predicate.AuthRequest(func(s *sql.Selector) {

@@ -55,6 +55,7 @@ var (
 		{Name: "expiry", Type: field.TypeTime, SchemaType: map[string]string{"mysql": "datetime(3)", "postgres": "timestamptz", "sqlite3": "timestamp"}},
 		{Name: "code_challenge", Type: field.TypeString, Size: 2147483647, Default: "", SchemaType: map[string]string{"mysql": "varchar(384)", "postgres": "text", "sqlite3": "text"}},
 		{Name: "code_challenge_method", Type: field.TypeString, Size: 2147483647, Default: "", SchemaType: map[string]string{"mysql": "varchar(384)", "postgres": "text", "sqlite3": "text"}},
+		{Name: "hmac_key", Type: field.TypeBytes},
 	}
 	// AuthRequestsTable holds the schema information for the "auth_requests" table.
 	AuthRequestsTable = &schema.Table{

@@ -1205,6 +1205,7 @@ type AuthRequestMutation struct {
 	expiry                    *time.Time
 	code_challenge            *string
 	code_challenge_method     *string
+	hmac_key                  *[]byte
 	clearedFields             map[string]struct{}
 	done                      bool
 	oldValue                  func(context.Context) (*AuthRequest, error)
@@ -2051,6 +2052,42 @@ func (m *AuthRequestMutation) ResetCodeChallengeMethod() {
 	m.code_challenge_method = nil
 }
 
+// SetHmacKey sets the "hmac_key" field.
+func (m *AuthRequestMutation) SetHmacKey(b []byte) {
+	m.hmac_key = &b
+}
+
+// HmacKey returns the value of the "hmac_key" field in the mutation.
+func (m *AuthRequestMutation) HmacKey() (r []byte, exists bool) {
+	v := m.hmac_key
+	if v == nil {
+		return
+	}
+	return *v, true
+}
+
+// OldHmacKey returns the old "hmac_key" field's value of the AuthRequest entity.
+// If the AuthRequest object wasn't provided to the builder, the object is fetched from the database.
+// An error is returned if the mutation operation is not UpdateOne, or the database query fails.
+func (m *AuthRequestMutation) OldHmacKey(ctx context.Context) (v []byte, err error) {
+	if !m.op.Is(OpUpdateOne) {
+		return v, errors.New("OldHmacKey is only allowed on UpdateOne operations")
+	}
+	if m.id == nil || m.oldValue == nil {
+		return v, errors.New("OldHmacKey requires an ID field in the mutation")
+	}
+	oldValue, err := m.oldValue(ctx)
+	if err != nil {
+		return v, fmt.Errorf("querying old value for OldHmacKey: %w", err)
+	}
+	return oldValue.HmacKey, nil
+}
+
+// ResetHmacKey resets all changes to the "hmac_key" field.
+func (m *AuthRequestMutation) ResetHmacKey() {
+	m.hmac_key = nil
+}
+
 // Where appends a list predicates to the AuthRequestMutation builder.
 func (m *AuthRequestMutation) Where(ps ...predicate.AuthRequest) {
 	m.predicates = append(m.predicates, ps...)
@@ -2070,7 +2107,7 @@ func (m *AuthRequestMutation) Type() string {
 // order to get all numeric fields that were incremented/decremented, call
 // AddedFields().
 func (m *AuthRequestMutation) Fields() []string {
-	fields := make([]string, 0, 19)
+	fields := make([]string, 0, 20)
 	if m.client_id != nil {
 		fields = append(fields, authrequest.FieldClientID)
 	}
@@ -2128,6 +2165,9 @@ func (m *AuthRequestMutation) Fields() []string {
 	if m.code_challenge_method != nil {
 		fields = append(fields, authrequest.FieldCodeChallengeMethod)
 	}
+	if m.hmac_key != nil {
+		fields = append(fields, authrequest.FieldHmacKey)
+	}
 	return fields
 }
 
@@ -2174,6 +2214,8 @@ func (m *AuthRequestMutation) Field(name string) (ent.Value, bool) {
 		return m.CodeChallenge()
 	case authrequest.FieldCodeChallengeMethod:
 		return m.CodeChallengeMethod()
+	case authrequest.FieldHmacKey:
+		return m.HmacKey()
 	}
 	return nil, false
 }
@@ -2221,6 +2263,8 @@ func (m *AuthRequestMutation) OldField(ctx context.Context, name string) (ent.Va
 		return m.OldCodeChallenge(ctx)
 	case authrequest.FieldCodeChallengeMethod:
 		return m.OldCodeChallengeMethod(ctx)
+	case authrequest.FieldHmacKey:
+		return m.OldHmacKey(ctx)
 	}
 	return nil, fmt.Errorf("unknown AuthRequest field %s", name)
 }
@@ -2363,6 +2407,13 @@ func (m *AuthRequestMutation) SetField(name string, value ent.Value) error {
 		}
 		m.SetCodeChallengeMethod(v)
 		return nil
+	case authrequest.FieldHmacKey:
+		v, ok := value.([]byte)
+		if !ok {
+			return fmt.Errorf("unexpected type %T for field %s", value, name)
+		}
+		m.SetHmacKey(v)
+		return nil
 	}
 	return fmt.Errorf("unknown AuthRequest field %s", name)
 }
@@ -2496,6 +2547,9 @@ func (m *AuthRequestMutation) ResetField(name string) error {
 	case authrequest.FieldCodeChallengeMethod:
 		m.ResetCodeChallengeMethod()
 		return nil
+	case authrequest.FieldHmacKey:
+		m.ResetHmacKey()
+		return nil
 	}
 	return fmt.Errorf("unknown AuthRequest field %s", name)
 }

@@ -27,7 +27,8 @@ create table auth_request
     expiry                    timestamp not null,
     claims_preferred_username text default '' not null,
     code_challenge            text default '' not null,
-    code_challenge_method     text default '' not null
+    code_challenge_method     text default '' not null,
+    hmac_key                  blob
 );
 */
 
@@ -86,6 +87,7 @@ func (AuthRequest) Fields() []ent.Field {
 		field.Text("code_challenge_method").
 			SchemaType(textSchema).
 			Default(""),
+		field.Bytes("hmac_key"),
 	}
 }
 

@@ -84,6 +84,8 @@ type AuthRequest struct {
 
 	CodeChallenge       string `json:"code_challenge,omitempty"`
 	CodeChallengeMethod string `json:"code_challenge_method,omitempty"`
+
+	HMACKey []byte `json:"hmac_key"`
 }
 
 func fromStorageAuthRequest(a storage.AuthRequest) AuthRequest {
@@ -103,6 +105,7 @@ func fromStorageAuthRequest(a storage.AuthRequest) AuthRequest {
 		ConnectorData:       a.ConnectorData,
 		CodeChallenge:       a.PKCE.CodeChallenge,
 		CodeChallengeMethod: a.PKCE.CodeChallengeMethod,
+		HMACKey:             a.HMACKey,
 	}
 }
 
@@ -125,6 +128,7 @@ func toStorageAuthRequest(a AuthRequest) storage.AuthRequest {
 			CodeChallenge:       a.CodeChallenge,
 			CodeChallengeMethod: a.CodeChallengeMethod,
 		},
+		HMACKey: a.HMACKey,
 	}
 }
 

@@ -2,6 +2,7 @@ package storage
 
 import (
 	"context"
+	"crypto"
 	"fmt"
 	"time"
 )
@@ -14,7 +15,8 @@ func NewCustomHealthCheckFunc(s Storage, now func() time.Time) func(context.Cont
 			ClientID: NewID(),
 
 			// Set a short expiry so if the delete fails this will be cleaned up quickly by garbage collection.
-			Expiry: now().Add(time.Minute),
+			Expiry:  now().Add(time.Minute),
+			HMACKey: NewHMACKey(crypto.SHA256),
 		}
 
 		if err := s.CreateAuthRequest(a); err != nil {

@@ -356,6 +356,8 @@ type AuthRequest struct {
 
 	CodeChallenge       string `json:"code_challenge,omitempty"`
 	CodeChallengeMethod string `json:"code_challenge_method,omitempty"`
+
+	HMACKey []byte `json:"hmac_key"`
 }
 
 // AuthRequestList is a list of AuthRequests.
@@ -384,6 +386,7 @@ func toStorageAuthRequest(req AuthRequest) storage.AuthRequest {
 			CodeChallenge:       req.CodeChallenge,
 			CodeChallengeMethod: req.CodeChallengeMethod,
 		},
+		HMACKey: req.HMACKey,
 	}
 	return a
 }
@@ -412,6 +415,7 @@ func (cli *client) fromStorageAuthRequest(a storage.AuthRequest) AuthRequest {
 		Claims:              fromStorageClaims(a.Claims),
 		CodeChallenge:       a.PKCE.CodeChallenge,
 		CodeChallengeMethod: a.PKCE.CodeChallengeMethod,
+		HMACKey:             a.HMACKey,
 	}
 	return req
 }

@@ -131,10 +131,11 @@ func (c *conn) CreateAuthRequest(a storage.AuthRequest) error {
 			claims_email, claims_email_verified, claims_groups,
 			connector_id, connector_data,
 			expiry,
-			code_challenge, code_challenge_method
+			code_challenge, code_challenge_method,
+			hmac_key
 		)
 		values (
-			$1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13, $14, $15, $16, $17, $18, $19, $20
+			$1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13, $14, $15, $16, $17, $18, $19, $20, $21
 		);
 	`,
 		a.ID, a.ClientID, encoder(a.ResponseTypes), encoder(a.Scopes), a.RedirectURI, a.Nonce, a.State,
@@ -144,6 +145,7 @@ func (c *conn) CreateAuthRequest(a storage.AuthRequest) error {
 		a.ConnectorID, a.ConnectorData,
 		a.Expiry,
 		a.PKCE.CodeChallenge, a.PKCE.CodeChallengeMethod,
+		a.HMACKey,
 	)
 	if err != nil {
 		if c.alreadyExistsCheck(err) {
@@ -175,8 +177,9 @@ func (c *conn) UpdateAuthRequest(id string, updater func(a storage.AuthRequest)
 				claims_groups = $14,
 				connector_id = $15, connector_data = $16,
 				expiry = $17,
-				code_challenge = $18, code_challenge_method = $19
-			where id = $20;
+				code_challenge = $18, code_challenge_method = $19,
+				hmac_key = $20
+			where id = $21;
 		`,
 			a.ClientID, encoder(a.ResponseTypes), encoder(a.Scopes), a.RedirectURI, a.Nonce, a.State,
 			a.ForceApprovalPrompt, a.LoggedIn,
@@ -185,7 +188,7 @@ func (c *conn) UpdateAuthRequest(id string, updater func(a storage.AuthRequest)
 			encoder(a.Claims.Groups),
 			a.ConnectorID, a.ConnectorData,
 			a.Expiry,
-			a.PKCE.CodeChallenge, a.PKCE.CodeChallengeMethod,
+			a.PKCE.CodeChallenge, a.PKCE.CodeChallengeMethod, a.HMACKey,
 			r.ID,
 		)
 		if err != nil {
@@ -207,7 +210,7 @@ func getAuthRequest(q querier, id string) (a storage.AuthRequest, err error) {
 			claims_user_id, claims_username, claims_preferred_username,
 			claims_email, claims_email_verified, claims_groups,
 			connector_id, connector_data, expiry,
-			code_challenge, code_challenge_method
+			code_challenge, code_challenge_method, hmac_key
 		from auth_request where id = $1;
 	`, id).Scan(
 		&a.ID, &a.ClientID, decoder(&a.ResponseTypes), decoder(&a.Scopes), &a.RedirectURI, &a.Nonce, &a.State,
@@ -216,7 +219,7 @@ func getAuthRequest(q querier, id string) (a storage.AuthRequest, err error) {
 		&a.Claims.Email, &a.Claims.EmailVerified,
 		decoder(&a.Claims.Groups),
 		&a.ConnectorID, &a.ConnectorData, &a.Expiry,
-		&a.PKCE.CodeChallenge, &a.PKCE.CodeChallengeMethod,
+		&a.PKCE.CodeChallenge, &a.PKCE.CodeChallengeMethod, &a.HMACKey,
 	)
 	if err != nil {
 		if err == sql.ErrNoRows {

@@ -291,4 +291,11 @@ var migrations = []migration{
 				add column code_challenge_method text not null default '';`,
 		},
 	},
+	{
+		stmts: []string{
+			`
+			alter table auth_request
+				add column hmac_key bytea;`,
+		},
+	},
 }

@@ -1,6 +1,7 @@
 package storage
 
 import (
+	"crypto"
 	"crypto/rand"
 	"encoding/base32"
 	"errors"
@@ -47,6 +48,11 @@ func newSecureID(len int) string {
 	return string(buff[0]%26+'a') + strings.TrimRight(encoding.EncodeToString(buff[1:]), "=")
 }
 
+// NewHMACKey returns a random key which can be used in the computation of an HMAC
+func NewHMACKey(h crypto.Hash) []byte {
+	return []byte(newSecureID(h.Size()))
+}
+
 // GCResult returns the number of objects deleted by garbage collection.
 type GCResult struct {
 	AuthRequests   int64
@@ -223,6 +229,9 @@ type AuthRequest struct {
 
 	// PKCE CodeChallenge and CodeChallengeMethod
 	PKCE PKCE
+
+	// HMACKey is used when generating an AuthRequest-specific HMAC
+	HMACKey []byte
 }
 
 // AuthCode represents a code which can be exchanged for an OAuth2 token response.