Add experimental tooling for generating signed URLs

Author: alukach

src/stac_auth_proxy/signed_urls.py

@@ -0,0 +1,75 @@
+from dataclasses import dataclass
+from datetime import datetime, timezone
+import hmac
+import hashlib
+import base64
+import urllib.parse
+
+
+@dataclass
+class SignedPayload:
+    href: str
+    exp: int
+    meta: str
+    sig: str
+
+    @classmethod
+    def build(cls, *, secret_key: str, href: str, exp: int, **meta_dict) -> str:
+        """Generate a signed payload the given parameters using HMAC-SHA256."""
+        b64_meta = base64.urlsafe_b64encode(stringify_dict(meta_dict).encode()).decode()
+
+        params = {"href": href, "exp": exp, "meta": b64_meta}
+        params_str = stringify_dict(params)
+
+        signature = hmac.new(
+            secret_key.encode(),
+            params_str.encode(),
+            hashlib.sha256,
+        ).digest()
+        b64_signature = base64.urlsafe_b64encode(signature).decode()
+
+        return cls(
+            href=href,
+            exp=exp,
+            meta=b64_meta,
+            sig=b64_signature,
+        )
+
+    @property
+    def valid_expiration(self) -> bool:
+        """Verify that the signature is not expired."""
+        return datetime.now(timezone.utc).timestamp() > self.exp
+
+    @property
+    def valid_signature(self) -> bool:
+        """Verify that the payload is signed and not expired."""
+        expected_signature = self.build(href=self.href, exp=self.exp, **self.meta)
+        return hmac.compare_digest(expected_signature.sig, self.sig)
+
+    @property
+    def meta_dict(self):
+        """Decoded meta data"""
+        return {
+            k: v[0]
+            for k, v in urllib.parse.parse_qs(
+                base64.urlsafe_b64decode(self.meta).decode()
+            ).items()
+        }
+
+    def as_qs(self):
+        """Return the query string"""
+        return urllib.parse.urlencode(
+            {
+                "href": self.href,
+                "exp": self.exp,
+                "meta": self.meta,
+                "sig": self.sig,
+            }
+        )
+
+
+def stringify_dict(params: dict) -> str:
+    """
+    Sort the parameters and return them as a string.
+    """
+    return "&".join(f"{k}={params[k]}" for k in sorted(params))