Fix test message format for cast receiver

Merge of https://widevine-internal-review.git.corp.google.com/c/cdm/+/206431

Message to be signed by CAST funciton is supposed to be in a certain
format: "constant prefix + SHA1(message)".

Some of our current CAST tests uses random message which break this
specification. This fixes the input message.

Test: Cast tests with run_fake_l1_tests
Bug: 359893908
Change-Id: I6b318d749971d837f13daa7b147313e8e0b1d3d0

libwvdrmengine/oemcrypto/test/oemcrypto_cast_test.cpp

@@ -5,8 +5,6 @@
 
 #include "oemcrypto_cast_test.h"
 
-#include "oemcrypto_usage_table_test.h"
-
 using ::testing::Range;
 
 namespace wvoec {
@@ -260,18 +258,8 @@ class OEMCryptoCastReceiverTest : public OEMCryptoLoadsCertificateAlternates {
     ASSERT_NO_FATAL_FAILURE(s.open());
     ASSERT_NO_FATAL_FAILURE(s.LoadWrappedRsaDrmKey(wrapped_drm_key_));
 
-    // The application will compute the SHA-1 Hash of the message, so this
-    // test must do that also.
-    uint8_t hash[SHA_DIGEST_LENGTH];
-    if (!SHA1(message.data(), message.size(), hash)) {
-      dump_boringssl_error();
-      FAIL() << "boringssl error creating SHA1 hash.";
-    }
-
-    // The application will prepend the digest info to the hash.
-    // SHA-1 digest info prefix = 0x30 0x21 0x30 ...
-    vector<uint8_t> digest = wvutil::a2b_hex("3021300906052b0e03021a05000414");
-    digest.insert(digest.end(), hash, hash + SHA_DIGEST_LENGTH);
+    vector<uint8_t> digest;
+    ASSERT_NO_FATAL_FAILURE(PrepareCastDigestedMessage(message, digest));
 
     // OEMCrypto will apply the padding, and encrypt to generate the
     // signature.