Provide a platform abstraction for RandBytes instead of swapping headers

The `RandBytes` functions were being placed in the `crypto` namespace,
which collides with Chromium's namespace of the same name. Within,
`RandBytes` was defined with almost-the-same API. Then in Chromium
builds, the Chromium header would be used instead (though somewhat
inconsistently).

This creates a lot of pain for Chromium development as there's a
third-party repository directly depending on headers from Chromium's
source tree, and is against the third-party policies for that reason.
There are a number of other headers that mirror Chromium and are
swapped out in the Chromium build that will cause similar pain, such as:
```
include "crypto/aead.h"
include "crypto/ec_private_key.h"
include "crypto/hkdf.h"
```

This CL provides a template for how to get rid of these header swaps and
give a platform abstraction in nearby instead.

We provide a platform abstraction in `platform/crypto.h` (really in
`platform/implementation/crypto.h`) which is implemented in
`platform/implementation/shared/crypto.cc`. However that implementation
is removed by `#ifdef` when in the Chromium build.

Then, in the Chromium repo, we will add (separately) an implementation
of the same abstraction in `//third_party/nearby/platform_impl` with GN
rules to include it in the build. It will replace the implementation
from the nearby repo.

Copybara import of the project:

--
6ca8099 by danakj <danakj@chromium.org>:

Provide a platform abstraction for RandBytes instead of swapping headers

The `RandBytes` functions were being placed in the `crypto` namespace,
which collides with Chromium's namespace of the same name. Within,
`RandBytes` was defined with almost-the-same API. Then in Chromium
builds, the Chromium header would be used instead (though somewhat
inconsistently).

This creates a lot of pain for Chromium development as there's a
third-party repository directly depending on headers from Chromium's
source tree, and is against the third-party policies for that reason.
There are a number of other headers that mirror Chromium and are
swapped out in the Chromium build that will cause similar pain, such as:
```
include "crypto/aead.h"
include "crypto/ec_private_key.h"
include "crypto/hkdf.h"
```

This CL provides a template for how to get rid of these header swaps and
give a platform abstraction in nearby instead.

We provide a platform abstraction in `platform/crypto.h` (really in
`platform/implementation/crypto.h`) which is implemented in
`platform/implementation/shared/crypto.cc`. However that implementation
is removed by `#ifdef` when in the Chromium build.

Then, in the Chromium repo, we will add (separately) an implementation
of the same abstraction in `//third_party/nearby/platform_impl` with GN
rules to include it in the build. It will replace the implementation
from the nearby repo.

--
9c2654b by danakj <danakj@chromium.org>:

Remove CryptoSpan, use absl::Span

The header swapping of Chromium crypto libraries is
problematic, but absl::Span will convert to base::span
so there's no need for the typedef even without
removing the header swapping yet.

--
df1135d by danakj <danakj@chromium.org>:

Add missing files

--
ab18a15 by danakj <danakj@chromium.org>:

Remove the random_unittest.cc from Swift build

The file moved, so the Swift package needs its path
updated.

--
2038f78 by danakj <danakj@chromium.org>:

Combine crypto unittests into crypto_test.cc

--
f7ad176 by danakj <danakj@chromium.org>:

Add stdint and stddef includes for uint8_t and size_t

--
9f04590 by danakj <danakj@chromium.org>:

Mark the shared crypto implementation compatable_with non_prod

--
18eeafd by danakj <danakj@chromium.org>:

Add IWYU pragma for crypto implementation

PiperOrigin-RevId: 632150866

internal/platform/implementation/BUILD

@@ -75,6 +75,7 @@ cc_library(
     deps = [
         "//internal/crypto_cros",
         "//internal/platform:base",
+        "//internal/platform/implementation/shared:crypto",  # Non-chromium impl
         "@com_google_absl//absl/base:core_headers",
         "@com_google_absl//absl/functional:any_invocable",
         "@com_google_absl//absl/strings",

internal/platform/implementation/crypto.h

@@ -15,12 +15,11 @@
 #ifndef PLATFORM_API_CRYPTO_H_
 #define PLATFORM_API_CRYPTO_H_
 
+#include <stddef.h>
+#include <stdint.h>
+
 #include "absl/strings/string_view.h"
-#ifdef NEARBY_CHROMIUM
-#include "crypto/random.h"
-#else
-#include "internal/crypto_cros/random.h"
-#endif
+#include "absl/types/span.h"
 #include "internal/platform/byte_array.h"
 
 namespace nearby {
@@ -36,12 +35,23 @@ class Crypto {
   static ByteArray Sha256(absl::string_view input);
 };
 
+// Fills the given buffer with |length| random bytes of cryptographically
+// secure random numbers.
+// |length| must be positive.
+//
+// TODO(crbug.com/40284755): Convert all callers in Nearby to use spans
+// and remove this RandBytes overload.
+void RandBytes(void *bytes, size_t length);
+
+// Fills |bytes| with cryptographically-secure random bits.
+void RandBytes(absl::Span<uint8_t> bytes);
+
 // Creates an object of type T initialized with random data.
 // This template should be used for simple data types: int, char, etc.
 template <typename T>
 T RandData() {
   T data;
-  ::crypto::RandBytes(&data, sizeof(data));
+  RandBytes(&data, sizeof(data));
   return data;
 }
 

internal/platform/implementation/g3/BUILD

@@ -141,6 +141,7 @@ cc_library(
         "//fastpair:__subpackages__",
         "//internal/account:__subpackages__",
         "//internal/auth:__subpackages__",
+        "//internal/crypto:__subpackages__",
         "//internal/data:__subpackages__",
         "//internal/flags:__subpackages__",
         "//internal/interop:__subpackages__",

internal/platform/implementation/shared/BUILD

@@ -13,6 +13,18 @@
 # limitations under the License.
 licenses(["notice"])
 
+cc_library(
+    name = "crypto",
+    srcs = [
+        "crypto.cc",
+    ],
+    visibility = ["//internal/platform/implementation:__subpackages__"],
+    deps = [
+        "@boringssl//:crypto",
+        "@com_google_absl//absl/types:span",
+    ],
+)
+
 cc_library(
     name = "posix_mutex",
     srcs = [

internal/platform/implementation/shared/crypto.cc

@@ -0,0 +1,31 @@
+// Copyright 2024 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
+//
+//     https://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 <stddef.h>
+#include <stdint.h>
+
+#include "absl/types/span.h"
+#include <openssl/rand.h>
+
+namespace nearby {
+
+void RandBytes(void* bytes, size_t length) {
+  RAND_bytes(reinterpret_cast<uint8_t*>(bytes), length);
+}
+
+void RandBytes(absl::Span<uint8_t> bytes) {
+  RAND_bytes(bytes.data(), bytes.size());
+}
+
+}  // namespace nearby