def _generate_pkce_pair(self): """Generate code_verifier and code_challenge (S256).""" self.code_verifier = secrets.token_urlsafe(64)[:128] code_challenge = hashlib.sha256(self.code_verifier.encode()).digest() # Base64url encode without padding code_challenge = secrets.token_urlsafe(32) # simplified; use proper base64url # For correctness, implement: import base64 code_challenge = base64.urlsafe_b64encode( hashlib.sha256(self.code_verifier.encode()).digest() ).decode().rstrip("=") return code_challenge
– lexoffice, OAuth 2.0, PKCE, API security, cloud accounting, single-page application (SPA), authentication flow. 1. Introduction lexoffice is a leading cloud accounting software for small and medium-sized enterprises (SMEs) in Germany. Its API (documented at https://developers.lexoffice.io ) enables automated invoice creation, contact management, and financial reporting. All API endpoints require authenticated access, governed by the lexoffice.login process.
: Enable logging on the token exchange but redact code_verifier and refresh_token before persisting. 7. Comparison with Other Accounting APIs | Feature | lexoffice | DATEV | Xero | QuickBooks | |---------|-----------|-------|------|-------------| | OAuth2 | ✅ PKCE | ✅ PKCE | ✅ PKCE | ✅ PKCE | | Refresh token rotation | ✅ (recommended) | ❌ | ✅ | ✅ | | Sandbox environment | ✅ | ✅ | ✅ | ✅ | | Scope discovery via metadata | ✅ OIDC Discovery | ❌ | ✅ | ✅ | lexoffice.login
"access_token": "eyJhbGciOiJSUzI1NiIs...", "token_type": "Bearer", "expires_in": 3600, "refresh_token": "8xLOxBtZp8...", "scope": "invoice.read contact.read openid profile"
def __init__(self, client_id, redirect_uri, scopes=None): self.client_id = client_id self.redirect_uri = redirect_uri self.scopes = scopes or ["openid", "profile", "invoice.read"] self.code_verifier = None self.state = None Its API (documented at https://developers
Abstract Modern cloud accounting platforms like lexoffice provide RESTful APIs for integrating financial data into third-party applications. A critical component is the authentication flow, typically encapsulated as lexoffice.login . This paper analyzes the design, implementation, and security considerations of lexoffice’s login mechanism based on OAuth 2.0 with Proof Key for Code Exchange (PKCE). We present a reference implementation, discuss common integration pitfalls, and evaluate the trade-offs between security and usability. The findings provide a blueprint for developers integrating lexoffice or similar FinTech APIs.
def handle_callback(self, callback_url): """Parse callback, verify state, exchange code for tokens.""" parsed = urlparse(callback_url) query = parse_qs(parsed.query) if "state" not in query or query["state"][0] != self.state: raise ValueError("Invalid state parameter – possible CSRF attack") if "code" not in query: raise ValueError("No authorization code received") auth_code = query["code"][0] callback_url): """Parse callback
# Exchange data = "grant_type": "authorization_code", "code": auth_code, "redirect_uri": self.redirect_uri, "client_id": self.client_id, "code_verifier": self.code_verifier resp = requests.post(self.TOKEN_URL, data=data) resp.raise_for_status() tokens = resp.json() return tokens # contains access_token, refresh_token, expires_in