PlayReady 密钥种子

实现 PlayReady 的服务必须维护密钥管理系统 (KMS) 来存储和管理内容密钥。 具体而言，{KID，内容密钥}的值存储于由服务管理的每个内容资产。 这些值存储在加密时，并在许可证颁发时检索。

PlayReady 提供了一种方便的方式来避免复杂的KMS。 内容密钥种子算法允许从不同的 KID 和固定密钥种子为内容资产集合派生不同的内容密钥：

Ck(KID) = f(KID, KeySeed)

以下是 PlayReady 标准算法：

byte[] GeneratePlayReadyContentKey(byte[] keySeed, Guid keyId)
{
    const int DRM_AES_KEYSIZE_128 = 16;
    byte[] contentKey = new byte[DRM_AES_KEYSIZE_128];
    //
    //  Truncate the key seed to 30 bytes, key seed must be at least 30 bytes long.
    //
    byte[] truncatedKeySeed = new byte[30];
    Array.Copy(keySeed, truncatedKeySeed, truncatedKeySeed.Length);
    //
    //  Get the keyId as a byte array
    //
    byte[] keyIdAsBytes = keyId.ToByteArray();
    //
    //  Create sha_A_Output buffer.  It is the SHA of the truncatedKeySeed and the keyIdAsBytes
    //
    SHA256Managed sha_A = new SHA256Managed();
    sha_A.TransformBlock(truncatedKeySeed, 0, truncatedKeySeed.Length, truncatedKeySeed, 0);
    sha_A.TransformFinalBlock(keyIdAsBytes, 0, keyIdAsBytes.Length);
    byte[] sha_A_Output = sha_A.Hash;
    //
    //  Create sha_B_Output buffer.  It is the SHA of the truncatedKeySeed, the keyIdAsBytes, and
    //  the truncatedKeySeed again.
    //
    SHA256Managed sha_B = new SHA256Managed();
    sha_B.TransformBlock(truncatedKeySeed, 0, truncatedKeySeed.Length, truncatedKeySeed, 0);
    sha_B.TransformBlock(keyIdAsBytes, 0, keyIdAsBytes.Length, keyIdAsBytes, 0);
    sha_B.TransformFinalBlock(truncatedKeySeed, 0, truncatedKeySeed.Length);
    byte[] sha_B_Output = sha_B.Hash;
    //
    //  Create sha_C_Output buffer.  It is the SHA of the truncatedKeySeed, the keyIdAsBytes,
    //  the truncatedKeySeed again, and the keyIdAsBytes again.
    //
    SHA256Managed sha_C = new SHA256Managed();
    sha_C.TransformBlock(truncatedKeySeed, 0, truncatedKeySeed.Length, truncatedKeySeed, 0);
    sha_C.TransformBlock(keyIdAsBytes, 0, keyIdAsBytes.Length, keyIdAsBytes, 0);
    sha_C.TransformBlock(truncatedKeySeed, 0, truncatedKeySeed.Length, truncatedKeySeed, 0);
    sha_C.TransformFinalBlock(keyIdAsBytes, 0, keyIdAsBytes.Length);
    byte[] sha_C_Output = sha_C.Hash;
    for (int i = 0; i < DRM_AES_KEYSIZE_128; i++)
    {
        contentKey[i] = Convert.ToByte(sha_A_Output[i] ^ sha_A_Output[i + DRM_AES_KEYSIZE_128]
                                       ^ sha_B_Output[i] ^ sha_B_Output[i + DRM_AES_KEYSIZE_128]
                                       ^ sha_C_Output[i] ^ sha_C_Output[i + DRM_AES_KEYSIZE_128]);
    }

    return contentKey;
}