Zero memory used for encryuption keys.

std::vector with custom zeroing allocator is used instead of
std::string for data that can contain encryption keys.

Bug: 64201177
Test: manually created a managed profile, changed it's credentials
Test: manually upgraded a phone with profile from O to MR1.
Change-Id: Ic31877049f69eba9f8ea64fd99acaaca5a01d3dd

13 files changed, 217 insertions, 75 deletions

diff --git a/Android.mk b/Android.mk
index 9dba6512..d0b199d6 100644
--- a/Android.mk
+++ b/Android.mk
@@ -27,6 +27,7 @@ common_src_files := \
 	MoveTask.cpp \
 	Benchmark.cpp \
 	TrimTask.cpp \
+	KeyBuffer.cpp \
 	Keymaster.cpp \
 	KeyStorage.cpp \
 	KeyUtil.cpp \
diff --git a/Ext4Crypt.cpp b/Ext4Crypt.cpp
index f9d4cf8b..dc2e42ae 100644
--- a/Ext4Crypt.cpp
+++ b/Ext4Crypt.cpp
@@ -57,6 +57,7 @@
 using android::base::StringPrintf;
 using android::base::WriteStringToFile;
 using android::vold::kEmptyAuthentication;
+using android::vold::KeyBuffer;
 
 // NOTE: keep in sync with StorageManager
 static constexpr int FLAG_STORAGE_DE = 1 << 0;
@@ -80,7 +81,7 @@ std::set<userid_t> s_ephemeral_users;
 std::map<userid_t, std::string> s_de_key_raw_refs;
 std::map<userid_t, std::string> s_ce_key_raw_refs;
 // TODO abolish this map, per b/26948053
-std::map<userid_t, std::string> s_ce_keys;
+std::map<userid_t, KeyBuffer> s_ce_keys;
 
 }
 
@@ -170,7 +171,7 @@ static void fixate_user_ce_key(const std::string& directory_path, const std::str
 
 static bool read_and_fixate_user_ce_key(userid_t user_id,
                                         const android::vold::KeyAuthentication& auth,
-                                        std::string *ce_key) {
+                                        KeyBuffer *ce_key) {
     auto const directory_path = get_ce_key_directory_path(user_id);
     auto const paths = get_ce_key_paths(directory_path);
     for (auto const ce_key_path: paths) {
@@ -188,11 +189,11 @@ static bool read_and_fixate_user_ce_key(userid_t user_id,
 static bool read_and_install_user_ce_key(userid_t user_id,
                                          const android::vold::KeyAuthentication& auth) {
     if (s_ce_key_raw_refs.count(user_id) != 0) return true;
-    std::string ce_key;
+    KeyBuffer ce_key;
     if (!read_and_fixate_user_ce_key(user_id, auth, &ce_key)) return false;
     std::string ce_raw_ref;
     if (!android::vold::installKey(ce_key, &ce_raw_ref)) return false;
-    s_ce_keys[user_id] = ce_key;
+    s_ce_keys[user_id] = std::move(ce_key);
     s_ce_key_raw_refs[user_id] = ce_raw_ref;
     LOG(DEBUG) << "Installed ce key for user " << user_id;
     return true;
@@ -219,7 +220,7 @@ static bool destroy_dir(const std::string& dir) {
 // NB this assumes that there is only one thread listening for crypt commands, because
 // it creates keys in a fixed location.
 static bool create_and_install_user_keys(userid_t user_id, bool create_ephemeral) {
-    std::string de_key, ce_key;
+    KeyBuffer de_key, ce_key;
     if (!android::vold::randomKey(&de_key)) return false;
     if (!android::vold::randomKey(&ce_key)) return false;
     if (create_ephemeral) {
@@ -306,7 +307,7 @@ static bool load_all_de_keys() {
         userid_t user_id = atoi(entry->d_name);
         if (s_de_key_raw_refs.count(user_id) == 0) {
             auto key_path = de_dir + "/" + entry->d_name;
-            std::string key;
+            KeyBuffer key;
             if (!android::vold::retrieveKey(key_path, kEmptyAuthentication, &key)) return false;
             std::string raw_ref;
             if (!android::vold::installKey(key, &raw_ref)) return false;
@@ -411,7 +412,7 @@ static void drop_caches() {
 }
 
 static bool evict_ce_key(userid_t user_id) {
-   s_ce_keys.erase(user_id);
+    s_ce_keys.erase(user_id);
     bool success = true;
     std::string raw_ref;
     // If we haven't loaded the CE key, no need to evict it.
@@ -509,7 +510,7 @@ bool e4crypt_add_user_key_auth(userid_t user_id, int serial, const char* token_h
         LOG(ERROR) << "Key not loaded into memory, can't change for user " << user_id;
         return false;
     }
-    auto ce_key = it->second;
+    const auto &ce_key = it->second;
     auto const directory_path = get_ce_key_directory_path(user_id);
     auto const paths = get_ce_key_paths(directory_path);
     std::string ce_key_path;
diff --git a/KeyBuffer.cpp b/KeyBuffer.cpp
new file mode 100644
index 00000000..e7aede53
--- /dev/null
+++ b/KeyBuffer.cpp
@@ -0,0 +1,37 @@
+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ *
+ * 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 "KeyBuffer.h"
+
+#include <algorithm>
+#include <cstring>
+
+namespace android {
+namespace vold {
+
+KeyBuffer operator+(KeyBuffer&& lhs, const KeyBuffer& rhs) {
+    std::copy(rhs.begin(), rhs.end(), std::back_inserter(lhs));
+    return std::move(lhs);
+}
+
+KeyBuffer operator+(KeyBuffer&& lhs, const char* rhs) {
+    std::copy(rhs, rhs + strlen(rhs), std::back_inserter(lhs));
+    return std::move(lhs);
+}
+
+}  // namespace vold
+}  // namespace android
+
diff --git a/KeyBuffer.h b/KeyBuffer.h
new file mode 100644
index 00000000..2087187d
--- /dev/null
+++ b/KeyBuffer.h
@@ -0,0 +1,63 @@
+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ *
+ * 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.
+ */
+
+#ifndef ANDROID_VOLD_KEYBUFFER_H
+#define ANDROID_VOLD_KEYBUFFER_H
+
+#include <cstring>
+#include <memory>
+#include <vector>
+
+namespace android {
+namespace vold {
+
+/**
+ * Variant of memset() that should never be optimized away. Borrowed from keymaster code.
+ */
+#ifdef __clang__
+#define OPTNONE __attribute__((optnone))
+#else  // not __clang__
+#define OPTNONE __attribute__((optimize("O0")))
+#endif  // not __clang__
+inline OPTNONE void* memset_s(void* s, int c, size_t n) {
+    if (!s)
+        return s;
+    return memset(s, c, n);
+}
+#undef OPTNONE
+
+// Allocator that delegates useful work to standard one but zeroes data before deallocating.
+class ZeroingAllocator : public std::allocator<char> {
+    public:
+    void deallocate(pointer p, size_type n)
+    {
+        memset_s(p, 0, n);
+        std::allocator<char>::deallocate(p, n);
+    }
+};
+
+// Char vector that zeroes memory when deallocating.
+using KeyBuffer = std::vector<char, ZeroingAllocator>;
+
+// Convenience methods to concatenate key buffers.
+KeyBuffer operator+(KeyBuffer&& lhs, const KeyBuffer& rhs);
+KeyBuffer operator+(KeyBuffer&& lhs, const char* rhs);
+
+}  // namespace vold
+}  // namespace android
+
+#endif
+
diff --git a/KeyStorage.cpp b/KeyStorage.cpp
index b4f85f49..9d615550 100644
--- a/KeyStorage.cpp
+++ b/KeyStorage.cpp
@@ -195,7 +195,7 @@ static KeymasterOperation begin(Keymaster& keymaster, const std::string& dir,
 
 static bool encryptWithKeymasterKey(Keymaster& keymaster, const std::string& dir,
                                     const AuthorizationSet &keyParams,
-                                    const std::string& message, std::string* ciphertext) {
+                                    const KeyBuffer& message, std::string* ciphertext) {
     AuthorizationSet opParams;
     AuthorizationSet outParams;
     auto opHandle = begin(keymaster, dir, KeyPurpose::ENCRYPT, keyParams, opParams, &outParams);
@@ -220,7 +220,7 @@ static bool encryptWithKeymasterKey(Keymaster& keymaster, const std::string& dir
 
 static bool decryptWithKeymasterKey(Keymaster& keymaster, const std::string& dir,
                                     const AuthorizationSet &keyParams,
-                                    const std::string& ciphertext, std::string* message) {
+                                    const std::string& ciphertext, KeyBuffer* message) {
     auto nonce = ciphertext.substr(0, GCM_NONCE_BYTES);
     auto bodyAndMac = ciphertext.substr(GCM_NONCE_BYTES);
     auto opParams = AuthorizationSetBuilder()
@@ -305,7 +305,7 @@ static void logOpensslError() {
 }
 
 static bool encryptWithoutKeymaster(const std::string& preKey,
-                                    const std::string& plaintext, std::string* ciphertext) {
+                                    const KeyBuffer& plaintext, std::string* ciphertext) {
     auto key = hashWithPrefix(kHashPrefix_keygen, preKey);
     key.resize(AES_KEY_BYTES);
     if (!readRandomBytesOrLog(GCM_NONCE_BYTES, ciphertext)) return false;
@@ -351,7 +351,7 @@ static bool encryptWithoutKeymaster(const std::string& preKey,
 }
 
 static bool decryptWithoutKeymaster(const std::string& preKey,
-                                    const std::string& ciphertext, std::string* plaintext) {
+                                    const std::string& ciphertext, KeyBuffer* plaintext) {
     if (ciphertext.size() < GCM_NONCE_BYTES + GCM_MAC_BYTES) {
         LOG(ERROR) << "GCM ciphertext too small: " << ciphertext.size();
         return false;
@@ -370,7 +370,7 @@ static bool decryptWithoutKeymaster(const std::string& preKey,
         logOpensslError();
         return false;
     }
-    plaintext->resize(ciphertext.size() - GCM_NONCE_BYTES - GCM_MAC_BYTES);
+    *plaintext = KeyBuffer(ciphertext.size() - GCM_NONCE_BYTES - GCM_MAC_BYTES);
     int outlen;
     if (1 != EVP_DecryptUpdate(ctx.get(),
         reinterpret_cast<uint8_t*>(&(*plaintext)[0]), &outlen,
@@ -404,7 +404,7 @@ bool pathExists(const std::string& path) {
     return access(path.c_str(), F_OK) == 0;
 }
 
-bool storeKey(const std::string& dir, const KeyAuthentication& auth, const std::string& key) {
+bool storeKey(const std::string& dir, const KeyAuthentication& auth, const KeyBuffer& key) {
     if (TEMP_FAILURE_RETRY(mkdir(dir.c_str(), 0700)) == -1) {
         PLOG(ERROR) << "key mkdir " << dir;
         return false;
@@ -442,7 +442,7 @@ bool storeKey(const std::string& dir, const KeyAuthentication& auth, const std::
 }
 
 bool storeKeyAtomically(const std::string& key_path, const std::string& tmp_path,
-                        const KeyAuthentication& auth, const std::string& key) {
+                        const KeyAuthentication& auth, const KeyBuffer& key) {
     if (pathExists(key_path)) {
         LOG(ERROR) << "Already exists, cannot create key at: " << key_path;
         return false;
@@ -460,7 +460,7 @@ bool storeKeyAtomically(const std::string& key_path, const std::string& tmp_path
     return true;
 }
 
-bool retrieveKey(const std::string& dir, const KeyAuthentication& auth, std::string* key) {
+bool retrieveKey(const std::string& dir, const KeyAuthentication& auth, KeyBuffer* key) {
     std::string version;
     if (!readFileToString(dir + "/" + kFn_version, &version)) return false;
     if (version != kCurrentVersion) {
diff --git a/KeyStorage.h b/KeyStorage.h
index 63345f44..655cd179 100644
--- a/KeyStorage.h
+++ b/KeyStorage.h
@@ -17,6 +17,8 @@
 #ifndef ANDROID_VOLD_KEYSTORAGE_H
 #define ANDROID_VOLD_KEYSTORAGE_H
 
+#include "KeyBuffer.h"
+
 #include <string>
 
 namespace android {
@@ -46,17 +48,17 @@ bool pathExists(const std::string& path);
 // in such a way that it can only be retrieved via Keymaster and
 // can be securely deleted.
 // It's safe to move/rename the directory after creation.
-bool storeKey(const std::string& dir, const KeyAuthentication& auth, const std::string& key);
+bool storeKey(const std::string& dir, const KeyAuthentication& auth, const KeyBuffer& key);
 
 // Create a directory at the named path, and store "key" in it as storeKey
 // This version creates the key in "tmp_path" then atomically renames "tmp_path"
 // to "key_path" thereby ensuring that the key is either stored entirely or
 // not at all.
 bool storeKeyAtomically(const std::string& key_path, const std::string& tmp_path,
-                        const KeyAuthentication& auth, const std::string& key);
+                        const KeyAuthentication& auth, const KeyBuffer& key);
 
 // Retrieve the key from the named directory.
-bool retrieveKey(const std::string& dir, const KeyAuthentication& auth, std::string* key);
+bool retrieveKey(const std::string& dir, const KeyAuthentication& auth, KeyBuffer* key);
 
 // Securely destroy the key stored in the named directory and delete the directory.
 bool destroyKey(const std::string& dir);
diff --git a/KeyUtil.cpp b/KeyUtil.cpp
index a75dfbbb..7bbbf012 100644
--- a/KeyUtil.cpp
+++ b/KeyUtil.cpp
@@ -32,8 +32,23 @@
 namespace android {
 namespace vold {
 
-bool randomKey(std::string* key) {
-    if (ReadRandomBytes(EXT4_AES_256_XTS_KEY_SIZE, *key) != 0) {
+// ext4enc:TODO get this const from somewhere good
+const int EXT4_KEY_DESCRIPTOR_SIZE = 8;
+
+// ext4enc:TODO Include structure from somewhere sensible
+// MUST be in sync with ext4_crypto.c in kernel
+constexpr int EXT4_ENCRYPTION_MODE_AES_256_XTS = 1;
+constexpr int EXT4_AES_256_XTS_KEY_SIZE = 64;
+constexpr int EXT4_MAX_KEY_SIZE = 64;
+struct ext4_encryption_key {
+    uint32_t mode;
+    char raw[EXT4_MAX_KEY_SIZE];
+    uint32_t size;
+};
+
+bool randomKey(KeyBuffer* key) {
+    *key = KeyBuffer(EXT4_AES_256_XTS_KEY_SIZE);
+    if (ReadRandomBytes(key->size(), key->data()) != 0) {
         // TODO status_t plays badly with PLOG, fix it.
         LOG(ERROR) << "Random read failed";
         return false;
@@ -60,7 +75,7 @@ static std::string generateKeyRef(const char* key, int length) {
     return std::string((char*)key_ref2, EXT4_KEY_DESCRIPTOR_SIZE);
 }
 
-static bool fillKey(const std::string& key, ext4_encryption_key* ext4_key) {
+static bool fillKey(const KeyBuffer& key, ext4_encryption_key* ext4_key) {
     if (key.size() != EXT4_AES_256_XTS_KEY_SIZE) {
         LOG(ERROR) << "Wrong size key " << key.size();
         return false;
@@ -101,8 +116,11 @@ static bool e4cryptKeyring(key_serial_t* device_keyring) {
 
 // Install password into global keyring
 // Return raw key reference for use in policy
-bool installKey(const std::string& key, std::string* raw_ref) {
-    ext4_encryption_key ext4_key;
+bool installKey(const KeyBuffer& key, std::string* raw_ref) {
+    // Place ext4_encryption_key into automatically zeroing buffer.
+    KeyBuffer ext4KeyBuffer(sizeof(ext4_encryption_key));
+    ext4_encryption_key &ext4_key = *reinterpret_cast<ext4_encryption_key*>(ext4KeyBuffer.data());
+
     if (!fillKey(key, &ext4_key)) return false;
     *raw_ref = generateKeyRef(ext4_key.raw, ext4_key.size);
     key_serial_t device_keyring;
@@ -145,7 +163,7 @@ bool evictKey(const std::string& raw_ref) {
 
 bool retrieveAndInstallKey(bool create_if_absent, const std::string& key_path,
                            const std::string& tmp_path, std::string* key_ref) {
-    std::string key;
+    KeyBuffer key;
     if (pathExists(key_path)) {
         LOG(DEBUG) << "Key exists, using: " << key_path;
         if (!retrieveKey(key_path, kEmptyAuthentication, &key)) return false;
@@ -168,7 +186,7 @@ bool retrieveAndInstallKey(bool create_if_absent, const std::string& key_path,
 }
 
 bool retrieveKey(bool create_if_absent, const std::string& key_path,
-                 const std::string& tmp_path, std::string* key) {
+                 const std::string& tmp_path, KeyBuffer* key) {
     if (pathExists(key_path)) {
         LOG(DEBUG) << "Key exists, using: " << key_path;
         if (!retrieveKey(key_path, kEmptyAuthentication, key)) return false;
diff --git a/KeyUtil.h b/KeyUtil.h
index d4c97b93..412b0ae2 100644
--- a/KeyUtil.h
+++ b/KeyUtil.h
@@ -17,32 +17,21 @@
 #ifndef ANDROID_VOLD_KEYUTIL_H
 #define ANDROID_VOLD_KEYUTIL_H
 
+#include "KeyBuffer.h"
+
 #include <string>
+#include <memory>
 
 namespace android {
 namespace vold {
 
-// ext4enc:TODO get this const from somewhere good
-const int EXT4_KEY_DESCRIPTOR_SIZE = 8;
-
-// ext4enc:TODO Include structure from somewhere sensible
-// MUST be in sync with ext4_crypto.c in kernel
-constexpr int EXT4_ENCRYPTION_MODE_AES_256_XTS = 1;
-constexpr int EXT4_AES_256_XTS_KEY_SIZE = 64;
-constexpr int EXT4_MAX_KEY_SIZE = 64;
-struct ext4_encryption_key {
-    uint32_t mode;
-    char raw[EXT4_MAX_KEY_SIZE];
-    uint32_t size;
-};
-
-bool randomKey(std::string* key);
-bool installKey(const std::string& key, std::string* raw_ref);
+bool randomKey(KeyBuffer* key);
+bool installKey(const KeyBuffer& key, std::string* raw_ref);
 bool evictKey(const std::string& raw_ref);
 bool retrieveAndInstallKey(bool create_if_absent, const std::string& key_path,
                            const std::string& tmp_path, std::string* key_ref);
 bool retrieveKey(bool create_if_absent, const std::string& key_path,
-                 const std::string& tmp_path, std::string* key);
+                 const std::string& tmp_path, KeyBuffer* key);
 
 }  // namespace vold
 }  // namespace android
diff --git a/Keymaster.cpp b/Keymaster.cpp
index ffa3a7a7..1bbeb61f 100644
--- a/Keymaster.cpp
+++ b/Keymaster.cpp
@@ -31,25 +31,23 @@ KeymasterOperation::~KeymasterOperation() {
     if (mDevice.get()) mDevice->abort(mOpHandle);
 }
 
-bool KeymasterOperation::updateCompletely(const std::string& input, std::string* output) {
-    if (output)
-        output->clear();
-    auto it = input.begin();
-    uint32_t inputConsumed;
+bool KeymasterOperation::updateCompletely(const char* input, size_t inputLen,
+        const std::function<void(const char*, size_t)> consumer) {
+    uint32_t inputConsumed = 0;
 
     ErrorCode km_error;
-    auto hidlCB = [&] (ErrorCode ret, uint32_t _inputConsumed,
+    auto hidlCB = [&] (ErrorCode ret, uint32_t inputConsumedDelta,
             const hidl_vec<KeyParameter>& /*ignored*/, const hidl_vec<uint8_t>& _output) {
         km_error = ret;
         if (km_error != ErrorCode::OK) return;
-        inputConsumed = _inputConsumed;
-        if (output)
-            output->append(reinterpret_cast<const char*>(&_output[0]), _output.size());
+        inputConsumed += inputConsumedDelta;
+        consumer(reinterpret_cast<const char*>(&_output[0]), _output.size());
     };
 
-    while (it != input.end()) {
-        size_t toRead = static_cast<size_t>(input.end() - it);
-        auto inputBlob = blob2hidlVec(reinterpret_cast<const uint8_t*>(&*it), toRead);
+    while (inputConsumed != inputLen) {
+        size_t toRead = static_cast<size_t>(inputLen - inputConsumed);
+        auto inputBlob =
+                blob2hidlVec(reinterpret_cast<const uint8_t*>(&input[inputConsumed]), toRead);
         auto error = mDevice->update(mOpHandle, hidl_vec<KeyParameter>(), inputBlob, hidlCB);
         if (!error.isOk()) {
             LOG(ERROR) << "update failed: " << error.description();
@@ -61,12 +59,11 @@ bool KeymasterOperation::updateCompletely(const std::string& input, std::string*
             mDevice = nullptr;
             return false;
         }
-        if (inputConsumed > toRead) {
+        if (inputConsumed > inputLen) {
             LOG(ERROR) << "update reported too much input consumed";
             mDevice = nullptr;
             return false;
         }
-        it += inputConsumed;
     }
     return true;
 }
diff --git a/Keymaster.h b/Keymaster.h
index 4bc0df7b..dc6f1bc2 100644
--- a/Keymaster.h
+++ b/Keymaster.h
@@ -19,6 +19,8 @@
 
 #ifdef __cplusplus
 
+#include "KeyBuffer.h"
+
 #include <memory>
 #include <string>
 #include <utility>
@@ -51,7 +53,14 @@ class KeymasterOperation {
     ErrorCode errorCode() { return mError; }
     // Call "update" repeatedly until all of the input is consumed, and
     // concatenate the output. Return true on success.
-    bool updateCompletely(const std::string& input, std::string* output);
+    template <class TI, class TO>
+    bool updateCompletely(TI& input, TO* output) {
+        if (output) output->clear();
+        return updateCompletely(input.data(), input.size(), [&](const char* b, size_t n) {
+            if (output) std::copy(b, b+n, std::back_inserter(*output));
+        });
+    }
+
     // Finish and write the output to this string, unless pointer is null.
     bool finish(std::string* output);
     // Move constructor
@@ -80,6 +89,10 @@ class KeymasterOperation {
     KeymasterOperation(ErrorCode error)
         : mDevice{nullptr}, mOpHandle{0},
           mError {error} {}
+
+    bool updateCompletely(const char* input, size_t inputLen,
+                          const std::function<void(const char*, size_t)> consumer);
+
     sp<IKeymasterDevice> mDevice;
     uint64_t mOpHandle;
     ErrorCode mError;
diff --git a/MetadataCrypt.cpp b/MetadataCrypt.cpp
index 743e08c7..83118135 100644
--- a/MetadataCrypt.cpp
+++ b/MetadataCrypt.cpp
@@ -14,11 +14,13 @@
  * limitations under the License.
  */
 
+#include "KeyBuffer.h"
 #include "MetadataCrypt.h"
 
 #include <string>
 #include <thread>
 #include <vector>
+#include <algorithm>
 
 #include <fcntl.h>
 #include <sys/ioctl.h>
@@ -45,6 +47,8 @@ extern struct fstab *fstab;
 #define TABLE_LOAD_RETRIES 10
 #define DEFAULT_KEY_TARGET_TYPE "default-key"
 
+using android::vold::KeyBuffer;
+
 static const std::string kDmNameUserdata = "userdata";
 
 static bool mount_via_fs_mgr(const char* mount_point, const char* blk_device) {
@@ -68,7 +72,7 @@ static bool mount_via_fs_mgr(const char* mount_point, const char* blk_device) {
     return true;
 }
 
-static bool read_key(bool create_if_absent, std::string* key) {
+static bool read_key(bool create_if_absent, KeyBuffer* key) {
     auto data_rec = fs_mgr_get_crypt_entry(fstab);
     if (!data_rec) {
         LOG(ERROR) << "Failed to get data_rec";
@@ -93,14 +97,14 @@ static bool read_key(bool create_if_absent, std::string* key) {
     return true;
 }
 
-static std::string default_key_params(const std::string& real_blkdev, const std::string& key) {
-    std::string hex_key;
+static KeyBuffer default_key_params(const std::string& real_blkdev, const KeyBuffer& key) {
+    KeyBuffer hex_key;
     if (android::vold::StrToHex(key, hex_key) != android::OK) {
         LOG(ERROR) << "Failed to turn key to hex";
-        return "";
+        return KeyBuffer();
     }
-    auto res = std::string() + "AES-256-XTS " + hex_key + " " + real_blkdev + " 0";
-    LOG(DEBUG) << "crypt_params: " << res;
+    auto res = KeyBuffer() + "AES-256-XTS " + hex_key + " " + real_blkdev.c_str() + " 0";
+    LOG(DEBUG) << "crypt_params: " << std::string(res.data(), res.size());
     return res;
 }
 
@@ -142,7 +146,7 @@ static struct dm_ioctl* dm_ioctl_init(char *buffer, size_t buffer_size,
 }
 
 static bool create_crypto_blk_dev(const std::string& dm_name, uint64_t nr_sec,
-                                  const std::string& target_type, const std::string& crypt_params,
+                                  const std::string& target_type, const KeyBuffer& crypt_params,
                                   std::string* crypto_blkdev) {
     android::base::unique_fd dm_fd(TEMP_FAILURE_RETRY(open(
         "/dev/device-mapper", O_RDWR | O_CLOEXEC, 0)));
@@ -167,9 +171,9 @@ static bool create_crypto_blk_dev(const std::string& dm_name, uint64_t nr_sec,
         (io->dev & 0xff) | ((io->dev >> 12) & 0xfff00));
 
     io = dm_ioctl_init(buffer, sizeof(buffer), dm_name);
-    unsigned long paramix = io->data_start + sizeof(struct dm_target_spec);
-    unsigned long nullix = paramix + crypt_params.size();
-    unsigned long endix = (nullix + 1 + 7) & 8; // Add room for \0 and align to 8 byte boundary
+    size_t paramix = io->data_start + sizeof(struct dm_target_spec);
+    size_t nullix = paramix + crypt_params.size();
+    size_t endix = (nullix + 1 + 7) & 8; // Add room for \0 and align to 8 byte boundary
 
     if (endix > sizeof(buffer)) {
         LOG(ERROR) << "crypt_params too big for DM_CRYPT_BUF_SIZE";
@@ -182,7 +186,8 @@ static bool create_crypto_blk_dev(const std::string& dm_name, uint64_t nr_sec,
     tgt->sector_start = 0;
     tgt->length = nr_sec;
     target_type.copy(tgt->target_type, sizeof(tgt->target_type));
-    crypt_params.copy(buffer + paramix, sizeof(buffer) - paramix);
+    memcpy(buffer + paramix, crypt_params.data(),
+            std::min(crypt_params.size(), sizeof(buffer) - paramix));
     buffer[nullix] = '\0';
     tgt->next = endix;
 
@@ -246,7 +251,7 @@ static void async_kick_off() {
 
 bool e4crypt_mount_metadata_encrypted() {
     LOG(DEBUG) << "e4crypt_mount_default_encrypted";
-    std::string key;
+    KeyBuffer key;
     if (!read_key(false, &key)) return false;
     auto data_rec = fs_mgr_get_crypt_entry(fstab);
     if (!data_rec) {
@@ -275,7 +280,7 @@ bool e4crypt_enable_crypto() {
         return false;
     }
 
-    std::string key_ref;
+    KeyBuffer key_ref;
     if (!read_key(true, &key_ref)) return false;
 
     auto data_rec = fs_mgr_get_crypt_entry(fstab);
diff --git a/Utils.cpp b/Utils.cpp
index 395a8901..b6c7bf88 100644
--- a/Utils.cpp
+++ b/Utils.cpp
@@ -351,18 +351,20 @@ pid_t ForkExecvpAsync(const std::vector<std::string>& args) {
 }
 
 status_t ReadRandomBytes(size_t bytes, std::string& out) {
-    out.clear();
+    out.resize(bytes);
+    return ReadRandomBytes(bytes, &out[0]);
+}
 
+status_t ReadRandomBytes(size_t bytes, char* buf) {
     int fd = TEMP_FAILURE_RETRY(open("/dev/urandom", O_RDONLY | O_CLOEXEC | O_NOFOLLOW));
     if (fd == -1) {
         return -errno;
     }
 
-    char buf[BUFSIZ];
     size_t n;
-    while ((n = TEMP_FAILURE_RETRY(read(fd, &buf[0], std::min(sizeof(buf), bytes)))) > 0) {
-        out.append(buf, n);
+    while ((n = TEMP_FAILURE_RETRY(read(fd, &buf[0], bytes))) > 0) {
         bytes -= n;
+        buf += n;
     }
     close(fd);
 
@@ -434,6 +436,15 @@ status_t StrToHex(const std::string& str, std::string& hex) {
     return OK;
 }
 
+status_t StrToHex(const KeyBuffer& str, KeyBuffer& hex) {
+    hex.clear();
+    for (size_t i = 0; i < str.size(); i++) {
+        hex.push_back(kLookup[(str.data()[i] & 0xF0) >> 4]);
+        hex.push_back(kLookup[str.data()[i] & 0x0F]);
+    }
+    return OK;
+}
+
 status_t NormalizeHex(const std::string& in, std::string& out) {
     std::string tmp;
     if (HexToStr(in, tmp)) {
diff --git a/Utils.h b/Utils.h
index 7272fe18..82ac440b 100644
--- a/Utils.h
+++ b/Utils.h
@@ -17,6 +17,8 @@
 #ifndef ANDROID_VOLD_UTILS_H
 #define ANDROID_VOLD_UTILS_H
 
+#include "KeyBuffer.h"
+
 #include <utils/Errors.h>
 #include <cutils/multiuser.h>
 #include <selinux/selinux.h>
@@ -78,12 +80,15 @@ status_t ForkExecvp(const std::vector<std::string>& args,
 pid_t ForkExecvpAsync(const std::vector<std::string>& args);
 
 status_t ReadRandomBytes(size_t bytes, std::string& out);
+status_t ReadRandomBytes(size_t bytes, char* buffer);
 status_t GenerateRandomUuid(std::string& out);
 
 /* Converts hex string to raw bytes, ignoring [ :-] */
 status_t HexToStr(const std::string& hex, std::string& str);
 /* Converts raw bytes to hex string */
 status_t StrToHex(const std::string& str, std::string& hex);
+/* Converts raw key bytes to hex string */
+status_t StrToHex(const KeyBuffer& str, KeyBuffer& hex);
 /* Normalize given hex string into consistent format */
 status_t NormalizeHex(const std::string& in, std::string& out);