feat (oidc): add WithVerifier

This commit adds the WithVerifier option to the
oidc package. This option allows the user to
specify a custom verifier for the PKCE code
challenge.

oidc/pkce_verifier.go

@@ -7,6 +7,7 @@ import (
 	"crypto/sha256"
 	"encoding/base64"
 	"fmt"
+	"regexp"
 
 	"github.com/hashicorp/cap/oidc/internal/base62"
 )
@@ -51,19 +52,39 @@ type S256Verifier struct {
 }
 
 // min len of 43 chars per https://tools.ietf.org/html/rfc7636#section-4.1
-const verifierLen = 43
+const (
+	// min len of 43 chars per https://tools.ietf.org/html/rfc7636#section-4.1
+	minVerifierLen = 43
+	// max len of 128 chars per https://tools.ietf.org/html/rfc7636#section-4.1
+	maxVerifierLen = 128
+)
 
 // NewCodeVerifier creates a new CodeVerifier (*S256Verifier).
+// Supported options: WithVerifier
 //
 // See: https://tools.ietf.org/html/rfc7636#section-4.1
-func NewCodeVerifier() (*S256Verifier, error) {
+func NewCodeVerifier(opt ...Option) (*S256Verifier, error) {
 	const op = "NewCodeVerifier"
-	data, err := base62.Random(verifierLen)
-	if err != nil {
-		return nil, fmt.Errorf("%s: unable to create verifier data %w", op, err)
+	var (
+		err          error
+		verifierData string
+	)
+	opts := getPKCEOpts(opt...)
+	switch {
+	case opts.withVerifier != "":
+		verifierData = opts.withVerifier
+	default:
+		var err error
+		verifierData, err = base62.Random(minVerifierLen)
+		if err != nil {
+			return nil, fmt.Errorf("%s: unable to create verifier data %w", op, err)
+		}
+	}
+	if err := verifierIsValid(verifierData); err != nil {
+		return nil, fmt.Errorf("%s: %w", op, err)
 	}
 	v := &S256Verifier{
-		verifier: data, // no need to encode it, since bas62.Random uses a limited set of characters.
+		verifier: verifierData, // no need to encode it, since bas62.Random uses a limited set of characters.
 		method:   S256,
 	}
 	if v.challenge, err = CreateCodeChallenge(v); err != nil {
@@ -72,6 +93,25 @@ func NewCodeVerifier() (*S256Verifier, error) {
 	return v, nil
 }
 
+func verifierIsValid(v string) error {
+	const op = "verifierIsValid"
+	switch {
+	case len(v) < minVerifierLen:
+		return fmt.Errorf("%s: verifier length is less than %d", op, minVerifierLen)
+	case len(v) > maxVerifierLen:
+		return fmt.Errorf("%s: verifier length is greater than %d", op, maxVerifierLen)
+	default:
+		// check that the verifier is valid based on
+		// https://datatracker.ietf.org/doc/html/rfc7636#section-4.1
+		// Check for valid characters: A-Z, a-z, 0-9, -, _, ., ~
+		validChars := regexp.MustCompile(`^[A-Za-z0-9\-\._~]+$`)
+		if !validChars.MatchString(v) {
+			return fmt.Errorf("%s: verifier contains invalid characters", op)
+		}
+	}
+	return nil
+}
+
 func (v *S256Verifier) Verifier() string        { return v.verifier }  // Verifier implements the CodeVerifier.Verifier() interface function.
 func (v *S256Verifier) Challenge() string       { return v.challenge } // Challenge implements the CodeVerifier.Challenge() interface function.
 func (v *S256Verifier) Method() ChallengeMethod { return v.method }    // Method implements the CodeVerifier.Method() interface function.
@@ -99,3 +139,35 @@ func CreateCodeChallenge(v CodeVerifier) (string, error) {
 	sum := h.Sum(nil)
 	return base64.RawURLEncoding.EncodeToString(sum), nil
 }
+
+// pkceOptions is the set of available options.
+type pkceOptions struct {
+	withVerifier string
+}
+
+// pkceDefaults is a handy way to get the defaults at runtime and
+// during unit tests.
+func pkceDefaults() pkceOptions {
+	return pkceOptions{}
+}
+
+// getPKCEOpts gets the defaults and applies the opt overrides passed in.
+func getPKCEOpts(opt ...Option) pkceOptions {
+	opts := pkceDefaults()
+	ApplyOpts(&opts, opt...)
+	return opts
+}
+
+// WithVerifier provides an optional verifier for the code verifier.  When this
+// option is provided, NewCodeVerifier will use the provided verifier. Note the
+// verifier must use the base62 character set.
+// See: https://datatracker.ietf.org/doc/html/rfc7636#section-4.1
+//
+// Valid for: NewVerifier
+func WithVerifier(verifier string) Option {
+	return func(o interface{}) {
+		if o, ok := o.(*pkceOptions); ok {
+			o.withVerifier = verifier
+		}
+	}
+}

oidc/pkce_verifier_test.go

@@ -9,6 +9,7 @@ import (
 	"errors"
 	"testing"
 
+	"github.com/hashicorp/cap/oidc/internal/base62"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -18,7 +19,7 @@ func TestNewCodeVerifier(t *testing.T) {
 		assert, require := assert.New(t), require.New(t)
 		got, err := NewCodeVerifier()
 		require.NoError(err)
-		assert.Equal(verifierLen, len(got.verifier))
+		assert.Equal(minVerifierLen, len(got.verifier))
 		assert.Equal(S256, got.Method())
 
 		challenge, err := CreateCodeChallenge(got)
@@ -53,3 +54,35 @@ func TestCreateCodeChallenge(t *testing.T) {
 		assert.True(errors.Is(err, ErrUnsupportedChallengeMethod))
 	})
 }
+
+func Test_WithVerifier(t *testing.T) {
+	t.Parallel()
+	assert, require := assert.New(t), require.New(t)
+	v, err := base62.Random(43)
+	require.NoError(err)
+	got, err := NewCodeVerifier(WithVerifier(v))
+	require.NoError(err)
+	assert.Equal(v, got.Verifier())
+
+	// Test that the verifier is too short
+	v, err = base62.Random(42)
+	require.NoError(err)
+	_, err = NewCodeVerifier(WithVerifier(v))
+	require.Error(err)
+	assert.Contains(err.Error(), "verifier length is less than 43")
+
+	// Test that the verifier is too long
+	v, err = base62.Random(129)
+	require.NoError(err)
+	_, err = NewCodeVerifier(WithVerifier(v))
+	require.Error(err)
+	assert.Contains(err.Error(), "verifier length is greater than 128")
+
+	// Test that the verifier contains invalid characters
+	v, err = base62.Random(43)
+	require.NoError(err)
+	v = v + "!"
+	_, err = NewCodeVerifier(WithVerifier(v))
+	require.Error(err)
+	assert.Contains(err.Error(), "verifier contains invalid characters")
+}