diff --git a/security/keymint/aidl/android/hardware/security/keymint/IKeyMintDevice.aidl b/security/keymint/aidl/android/hardware/security/keymint/IKeyMintDevice.aidl
index 9846ee9..4b63799 100644
--- a/security/keymint/aidl/android/hardware/security/keymint/IKeyMintDevice.aidl
+++ b/security/keymint/aidl/android/hardware/security/keymint/IKeyMintDevice.aidl
@@ -96,7 +96,9 @@
  *      - TRUSTED_ENVRIONMENT IKeyMintDevices must support curve 25519 for Purpose::SIGN (Ed25519,
  *        as specified in RFC 8032), Purpose::ATTEST_KEY (Ed25519) or for KeyPurpose::AGREE_KEY
  *        (X25519, as specified in RFC 7748).  However, a key must have exactly one of these
- *        purpose values; the same key cannot be used for multiple purposes.
+ *        purpose values; the same key cannot be used for multiple purposes. Signing operations
+ *        (Purpose::SIGN) have a message size limit of 16 KiB; operations on messages longer than
+ *        this limit must fail with ErrorCode::INVALID_INPUT_LENGTH.
  *        STRONGBOX IKeyMintDevices do not support curve 25519.
  *
  * o   AES
diff --git a/security/keymint/aidl/vts/functional/KeyMintAidlTestBase.cpp b/security/keymint/aidl/vts/functional/KeyMintAidlTestBase.cpp
index 8382781..49665b1 100644
--- a/security/keymint/aidl/vts/functional/KeyMintAidlTestBase.cpp
+++ b/security/keymint/aidl/vts/functional/KeyMintAidlTestBase.cpp
@@ -559,7 +559,12 @@
     std::vector<uint8_t> o_put;
     result = op_->update(vector<uint8_t>(input.begin(), input.end()), {}, {}, &o_put);
 
-    if (result.isOk()) output->append(o_put.begin(), o_put.end());
+    if (result.isOk()) {
+        output->append(o_put.begin(), o_put.end());
+    } else {
+        // Failure always terminates the operation.
+        op_ = {};
+    }
 
     return GetReturnErrorCode(result);
 }
diff --git a/security/keymint/aidl/vts/functional/KeyMintTest.cpp b/security/keymint/aidl/vts/functional/KeyMintTest.cpp
index 7f35f34..dc1d843 100644
--- a/security/keymint/aidl/vts/functional/KeyMintTest.cpp
+++ b/security/keymint/aidl/vts/functional/KeyMintTest.cpp
@@ -70,6 +70,9 @@
 
 namespace {
 
+// Maximum supported Ed25519 message size.
+const size_t MAX_ED25519_MSG_SIZE = 16 * 1024;
+
 // Whether to check that BOOT_PATCHLEVEL is populated.
 bool check_boot_pl = true;
 
@@ -3127,6 +3130,97 @@
 }
 
 /*
+ * SigningOperationsTest.EcdsaCurve25519MaxSize
+ *
+ * Verifies that EDDSA operations with curve25519 under the maximum message size succeed.
+ */
+TEST_P(SigningOperationsTest, EcdsaCurve25519MaxSize) {
+    if (!Curve25519Supported()) {
+        GTEST_SKIP() << "Test not applicable to device that is not expected to support curve 25519";
+    }
+
+    EcCurve curve = EcCurve::CURVE_25519;
+    ErrorCode error = GenerateKey(AuthorizationSetBuilder()
+                                          .Authorization(TAG_NO_AUTH_REQUIRED)
+                                          .EcdsaSigningKey(curve)
+                                          .Digest(Digest::NONE)
+                                          .SetDefaultValidity());
+    ASSERT_EQ(ErrorCode::OK, error) << "Failed to generate ECDSA key with curve " << curve;
+
+    auto params = AuthorizationSetBuilder().Digest(Digest::NONE);
+
+    for (size_t msg_size : {MAX_ED25519_MSG_SIZE - 1, MAX_ED25519_MSG_SIZE}) {
+        SCOPED_TRACE(testing::Message() << "-msg-size=" << msg_size);
+        string message(msg_size, 'a');
+
+        // Attempt to sign via Begin+Finish.
+        AuthorizationSet out_params;
+        ASSERT_EQ(ErrorCode::OK, Begin(KeyPurpose::SIGN, key_blob_, params, &out_params));
+        EXPECT_TRUE(out_params.empty());
+        string signature;
+        auto result = Finish(message, &signature);
+        EXPECT_EQ(result, ErrorCode::OK);
+        LocalVerifyMessage(message, signature, params);
+
+        // Attempt to sign via Begin+Update+Finish
+        ASSERT_EQ(ErrorCode::OK, Begin(KeyPurpose::SIGN, key_blob_, params, &out_params));
+        EXPECT_TRUE(out_params.empty());
+        string output;
+        result = Update(message, &output);
+        EXPECT_EQ(result, ErrorCode::OK);
+        EXPECT_EQ(output.size(), 0);
+        string signature2;
+        EXPECT_EQ(ErrorCode::OK, Finish({}, &signature2));
+        LocalVerifyMessage(message, signature2, params);
+    }
+
+    CheckedDeleteKey();
+}
+
+/*
+ * SigningOperationsTest.EcdsaCurve25519MaxSizeFail
+ *
+ * Verifies that EDDSA operations with curve25519 fail when message size is too large.
+ */
+TEST_P(SigningOperationsTest, EcdsaCurve25519MaxSizeFail) {
+    if (!Curve25519Supported()) {
+        GTEST_SKIP() << "Test not applicable to device that is not expected to support curve 25519";
+    }
+
+    EcCurve curve = EcCurve::CURVE_25519;
+    ErrorCode error = GenerateKey(AuthorizationSetBuilder()
+                                          .Authorization(TAG_NO_AUTH_REQUIRED)
+                                          .EcdsaSigningKey(curve)
+                                          .Digest(Digest::NONE)
+                                          .SetDefaultValidity());
+    ASSERT_EQ(ErrorCode::OK, error) << "Failed to generate ECDSA key with curve " << curve;
+
+    auto params = AuthorizationSetBuilder().Digest(Digest::NONE);
+
+    for (size_t msg_size : {MAX_ED25519_MSG_SIZE + 1, MAX_ED25519_MSG_SIZE * 2}) {
+        SCOPED_TRACE(testing::Message() << "-msg-size=" << msg_size);
+        string message(msg_size, 'a');
+
+        // Attempt to sign via Begin+Finish.
+        AuthorizationSet out_params;
+        ASSERT_EQ(ErrorCode::OK, Begin(KeyPurpose::SIGN, key_blob_, params, &out_params));
+        EXPECT_TRUE(out_params.empty());
+        string signature;
+        auto result = Finish(message, &signature);
+        EXPECT_EQ(result, ErrorCode::INVALID_INPUT_LENGTH);
+
+        // Attempt to sign via Begin+Update (but never get to Finish)
+        ASSERT_EQ(ErrorCode::OK, Begin(KeyPurpose::SIGN, key_blob_, params, &out_params));
+        EXPECT_TRUE(out_params.empty());
+        string output;
+        result = Update(message, &output);
+        EXPECT_EQ(result, ErrorCode::INVALID_INPUT_LENGTH);
+    }
+
+    CheckedDeleteKey();
+}
+
+/*
  * SigningOperationsTest.EcdsaNoDigestHugeData
  *
  * Verifies that ECDSA operations support very large messages, even without digesting.  This