diff --git a/server/packages/kca-core/src/kca/core/keys.py b/server/packages/kca-core/src/kca/core/keys.py
index 0a2560e..3b524f3 100644
--- a/server/packages/kca-core/src/kca/core/keys.py
+++ b/server/packages/kca-core/src/kca/core/keys.py
@@ -262,6 +262,14 @@ def _int_bytes(value: int) -> bytes:
     return value.to_bytes((value.bit_length() + 7) // 8 or 1, "big")
 
 
+_JWK_EC_CURVES = {
+    "secp256r1": "P-256",
+    "secp384r1": "P-384",
+    "secp521r1": "P-521",
+    "secp256k1": "secp256k1",
+}
+
+
 def jwk_public(key: PrivateKey | PublicKey, *, kid: Optional[str] = None) -> Dict[str, Any]:
     pub = _public_key(key)
     if isinstance(pub, rsa.RSAPublicKey):
@@ -273,17 +281,15 @@ def jwk_public(key: PrivateKey | PublicKey, *, kid: Optional[str] = None) -> Dic
         }
     elif isinstance(pub, ec.EllipticCurvePublicKey):
         numbers = pub.public_numbers()
-        curve = {
-            "secp256r1": "P-256",
-            "secp384r1": "P-384",
-            "secp521r1": "P-521",
-            "secp256k1": "secp256k1",
-        }.get(pub.curve.name, pub.curve.name)
+        curve = _JWK_EC_CURVES.get(pub.curve.name)
+        if curve is None:
+            raise ValueError(f"JWK export does not support EC curve {pub.curve.name!r}")
+        coordinate_size = (pub.curve.key_size + 7) // 8
         result = {
             "kty": "EC",
             "crv": curve,
-            "x": _b64u(numbers.x.to_bytes((numbers.x.bit_length() + 7) // 8 or 1, "big")),
-            "y": _b64u(numbers.y.to_bytes((numbers.y.bit_length() + 7) // 8 or 1, "big")),
+            "x": _b64u(numbers.x.to_bytes(coordinate_size, "big")),
+            "y": _b64u(numbers.y.to_bytes(coordinate_size, "big")),
         }
     elif isinstance(pub, ed25519.Ed25519PublicKey):
         result = {
@@ -314,7 +320,8 @@ def jwk_private(key: PrivateKey, *, kid: Optional[str] = None) -> Dict[str, Any]
             }
         )
     elif isinstance(key, ec.EllipticCurvePrivateKey):
-        result["d"] = _b64u(_int_bytes(key.private_numbers().private_value))
+        private_size = (key.curve.key_size + 7) // 8
+        result["d"] = _b64u(key.private_numbers().private_value.to_bytes(private_size, "big"))
     elif isinstance(key, ed25519.Ed25519PrivateKey):
         result["d"] = _b64u(key.private_bytes(Encoding.Raw, PrivateFormat.Raw, NoEncryption()))
     else:
diff --git a/server/packages/kca-core/tests/test_core.py b/server/packages/kca-core/tests/test_core.py
index 0c72d72..eda4d6f 100644
--- a/server/packages/kca-core/tests/test_core.py
+++ b/server/packages/kca-core/tests/test_core.py
@@ -1,12 +1,14 @@
 """Core domain tests: names, extensions, keys, certificates, CRLs and exports."""
 from __future__ import annotations
 
+import base64
 import time
 import unittest
 from datetime import datetime, timedelta, timezone
 
 from cryptography import x509
 from cryptography.hazmat.primitives import serialization
+from cryptography.hazmat.primitives.asymmetric import ec
 
 from kca.core import certificates, crls, crypto, exports, imports, keys, names, requests, templates
 from kca.core.extensions import (
@@ -16,6 +18,10 @@ from kca.core.extensions import (
 )
 
 
+def _b64u_decode(value: str) -> bytes:
+    return base64.urlsafe_b64decode(value + "=" * (-len(value) % 4))
+
+
 def _name(cn: str = "Test CA") -> x509.Name:
     return names.name_from_entries(
         names.entries_from_json(
@@ -99,6 +105,52 @@ class KeyTest(unittest.TestCase):
         self.assertTrue(private)
         self.assertEqual(keys.public_der(loaded), keys.public_der(key))
 
+    def test_ec_jwk_curve_names_and_private_width(self) -> None:
+        for curve, curve_name, size in [
+            (ec.SECP256R1(), "P-256", 32),
+            (ec.SECP384R1(), "P-384", 48),
+            (ec.SECP521R1(), "P-521", 66),
+            (ec.SECP256K1(), "secp256k1", 32),
+        ]:
+            with self.subTest(curve=curve_name):
+                key = ec.derive_private_key(1, curve)
+                public = keys.jwk_public(key)
+                private = keys.jwk_private(key)
+                self.assertEqual(public["crv"], curve_name)
+                self.assertEqual(len(_b64u_decode(public["x"])), size)
+                self.assertEqual(len(_b64u_decode(public["y"])), size)
+                self.assertEqual(_b64u_decode(private["d"]), b"\0" * (size - 1) + b"\1")
+
+    def test_ec_jwk_preserves_leading_zeros(self) -> None:
+        for private_value, leading_coordinate in ((43, "y"), (379, "x")):
+            with self.subTest(private_value=private_value):
+                key = ec.derive_private_key(private_value, ec.SECP256R1())
+                numbers = key.private_numbers()
+                jwk = keys.jwk_private(key)
+                coordinate = getattr(numbers.public_numbers, leading_coordinate)
+                self.assertEqual(coordinate.to_bytes(32, "big")[0], 0)
+                for name, value in (
+                    ("x", numbers.public_numbers.x),
+                    ("y", numbers.public_numbers.y),
+                    ("d", numbers.private_value),
+                ):
+                    self.assertEqual(_b64u_decode(jwk[name]), value.to_bytes(32, "big"))
+
+    def test_ec_jwk_rejects_unregistered_curves(self) -> None:
+        for curve in (
+            ec.SECP192R1(),
+            ec.SECP224R1(),
+            ec.BrainpoolP256R1(),
+            ec.BrainpoolP384R1(),
+            ec.BrainpoolP512R1(),
+        ):
+            with self.subTest(curve=curve.name):
+                key = ec.derive_private_key(1, curve)
+                with self.assertRaisesRegex(ValueError, "does not support EC curve"):
+                    keys.jwk_public(key)
+                with self.assertRaisesRegex(ValueError, "does not support EC curve"):
+                    keys.jwk_private(key)
+
     def test_ssh_and_pvk(self) -> None:
         key = keys.generate_key("rsa", 2048)
         public = keys.ssh_public(key, "test")