PlayReady Key Seed
Services implementing PlayReady must maintain a Key Management System (KMS) to store and manage content keys. Specifically, the values of {KID, Content Key} are stored for each content asset that is managed by the service. These values are stored at encryption time, and retrieved at license issuance time.
PlayReady provides a convenient way to avoid a complex KMS. The Content Key Seed algorithm allows derivation of different content keys for a collection of content assets, from a varying KID and a fixed Key Seed:
Ck(KID) = f(KID, KeySeed)
The following is the PlayReady standard algorithm:
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;
}
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https://aka.ms/ContentUserFeedback.
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