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diff --git a/fcp/secagg/shared/aes_key_test.cc b/fcp/secagg/shared/aes_key_test.cc
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+++ b/fcp/secagg/shared/aes_key_test.cc
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+// Copyright 2021 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "fcp/secagg/shared/aes_key.h"
+
+#include <string>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/strings/str_cat.h"
+#include "fcp/secagg/shared/math.h"
+#include "fcp/secagg/shared/shamir_secret_sharing.h"
+
+namespace fcp {
+namespace secagg {
+namespace {
+
+using ::testing::Eq;
+
+// For testing purposes, make an AesKey out of a string.
+AesKey AesKeyFromString(const std::string& key) {
+  return AesKey(reinterpret_cast<const uint8_t*>(key.c_str()),
+                static_cast<int>(key.size()));
+}
+
+// Suppose the randomly chosen key can be expressed in bit_length <= 128 bits.
+// Java did not express the key in 128 bits, but rather will have used
+// (bit_length + 1) bits. The extra bit is the highest-order bit, and is a sign
+// bit, guaranteed to be 0. The next highest-order bit is guaranteed to be 1.
+std::string JavaStyleKey(int bit_length) {
+  EXPECT_TRUE(0 < bit_length && bit_length <= 128);
+  std::string key = "16 byte test key";
+  int byte_with_sign_bit = (127 - bit_length) / 8;
+  int pos_of_sign_bit = (127 - bit_length) % 8;
+  if (bit_length == 128) {
+    pos_of_sign_bit = 7;
+    key = absl::StrCat("\0", key);
+  } else {
+    key.erase(0, byte_with_sign_bit);
+  }
+  // Make sure the high-order bit is the sign bit 0, and the next highest-order
+  // bit is 1.
+  key[0] = static_cast<char>(127 >> pos_of_sign_bit);
+  if (pos_of_sign_bit == 7) {
+    key[1] = static_cast<char>(255);
+  }
+  return key;
+}
+
+TEST(AesKeyTest, CreateFromSharesHandles32BKeys) {
+  AesKey original_key = AesKeyFromString("32 byte AES key for testing only");
+  ShamirSecretSharing shamir;
+  auto shares = shamir.Share(5, 7, original_key);
+  auto key_or_error = AesKey::CreateFromShares(shares, 5);
+  EXPECT_THAT(key_or_error.ok(), Eq(true));
+  EXPECT_THAT(key_or_error.value(), Eq(original_key));
+}
+
+TEST(AesKeyTest, CreateFromSharesHandlesShortKeys) {
+  ShamirSecretSharing shamir;
+  for (int i = 1; i <= 128; ++i) {
+    std::string original_key_string = JavaStyleKey(i);
+    AesKey original_key = AesKeyFromString(original_key_string);
+    std::string key_string_for_sharing;
+    if (original_key_string.size() < 16) {
+      key_string_for_sharing =
+          absl::StrCat(std::string(16 - original_key_string.size(), '\0'),
+                       original_key_string);
+    } else if (original_key_string.size() == 17) {
+      key_string_for_sharing = original_key_string.substr(1);
+    } else {
+      key_string_for_sharing = original_key_string;
+    }
+    auto shares = shamir.Share(5, 7, AesKeyFromString(key_string_for_sharing));
+    auto key_or_error = AesKey::CreateFromShares(shares, 5);
+    EXPECT_THAT(key_or_error.ok(), Eq(true));
+    EXPECT_THAT(key_or_error.value(), Eq(original_key))
+        << i << " bit key fails";
+  }
+}
+
+}  // namespace
+}  // namespace secagg
+}  // namespace fcp