fix(op): Add mitigation for PKCE Downgrade Attack (#741)

* fix(op): Add mitigation for PKCE downgrade attack

* chore(op): add test for PKCE verification

pkg/op/token_request_test.go

@@ -0,0 +1,75 @@
+package op_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/zitadel/oidc/v3/pkg/oidc"
+	"github.com/zitadel/oidc/v3/pkg/op"
+)
+
+func TestAuthorizeCodeChallenge(t *testing.T) {
+	tests := []struct {
+		name         string
+		codeVerifier string
+		codeChallenge    *oidc.CodeChallenge
+		want         func(t *testing.T, err error)
+	}{
+		{
+			name:         "missing both code_verifier and code_challenge",
+			codeVerifier: "",
+			codeChallenge:    nil,
+			want: func(t *testing.T, err error) {
+				assert.Nil(t, err)
+			},
+		},
+		{
+			name:         "valid code_verifier",
+			codeVerifier: "Hello World!",
+			codeChallenge: &oidc.CodeChallenge{
+				Challenge: "f4OxZX_x_FO5LcGBSKHWXfwtSx-j1ncoSt3SABJtkGk",
+				Method:    oidc.CodeChallengeMethodS256,
+			},
+			want: func(t *testing.T, err error) {
+				assert.Nil(t, err)
+			},
+		},
+		{
+			name:         "invalid code_verifier",
+			codeVerifier: "Hi World!",
+			codeChallenge: &oidc.CodeChallenge{
+				Challenge: "f4OxZX_x_FO5LcGBSKHWXfwtSx-j1ncoSt3SABJtkGk",
+				Method:    oidc.CodeChallengeMethodS256,
+			},
+			want: func(t *testing.T, err error) {
+				assert.ErrorContains(t, err, "invalid code_verifier")
+			},
+		},
+		{
+			name:         "code_verifier provided without code_challenge",
+			codeVerifier: "code_verifier",
+			codeChallenge:    nil,
+			want: func(t *testing.T, err error) {
+				assert.ErrorContains(t, err, "code_verifier unexpectedly provided")
+			},
+		},
+		{
+			name:         "empty code_verifier",
+			codeVerifier: "",
+			codeChallenge: &oidc.CodeChallenge{
+				Challenge: "f4OxZX_x_FO5LcGBSKHWXfwtSx-j1ncoSt3SABJtkGk",
+				Method:    oidc.CodeChallengeMethodS256,
+			},
+			want: func(t *testing.T, err error) {
+				assert.ErrorContains(t, err, "code_verifier required")
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := op.AuthorizeCodeChallenge(tt.codeVerifier, tt.codeChallenge)
+
+			tt.want(t, err)
+		})
+	}
+}