private byte[] DecryptMasterKey(KeyPair decode_key_pair, CertKey certKey1, CertKey certKey2)
{
JWE jwe1 = new JWE(decode_key_pair.key1.enc_key);
byte[] xor_key = jwe1.Decrypt(certKey1.GetKey());
JWE jwe2 = new JWE(decode_key_pair.key2.enc_key);
byte[] derived_key = jwe2.Decrypt(certKey2.GetKey());
// Now, XOR to get the master key back
byte[] master_key = new byte[xor_key.Length];
for (Int32 i = 0; i < xor_key.Length; ++i)
{
master_key[i] = (byte)(xor_key[i] ^ derived_key[i]);
}
return(master_key);
}
// Internal worker function
private PlaintextList Decrypt(SecurityDomainData data, CertKeys certKeys)
{
if (data.version == 2 && certKeys.Count() < data.SharedKeys.required)
{
throw new ArgumentException(string.Format(Resources.DecryptSecurityDomainKeyNotEnough, data.SharedKeys.required, certKeys.Count()));
}
byte[] masterKey;
if (data.version == 1)
{
// ensure that the key splitting algorithm
// is known, currently only one we know about
if (data.SplitKeys.key_algorithm != "xor_split")
{
throw new Exception($"Unknown SplitKey algorithm {data.SplitKeys.key_algorithm}.");
}
KeyPair decodeKeyPair = null;
CertKey certKey1 = null;
CertKey certKey2 = null;
foreach (KeyPair keyPair in data.SplitKeys.keys)
{
certKey1 = certKeys.Find(keyPair.key1.x5t_256);
if (certKey1 == null)
{
continue;
}
certKey2 = certKeys.Find(keyPair.key2.x5t_256);
if (certKey2 != null)
{
decodeKeyPair = keyPair;
break;
}
}
if (decodeKeyPair == null)
{
throw new Exception("Cannot find matching certs and keys for security domain");
}
masterKey = DecryptMasterKey(decodeKeyPair, certKey1, certKey2);
}
else if (data.version == 2)
{
if (data.SharedKeys.key_algorithm != "shamir_share")
{
throw new Exception($"Unknown SharedKeys algorithm {data.SharedKeys.key_algorithm}");
}
UInt32 shares_found = 0;
List <UInt16[]> share_arrays = new List <UInt16[]>();
foreach (Key key in data.SharedKeys.enc_shares)
{
CertKey cert_key = certKeys.Find(key.x5t_256);
if (cert_key != null)
{
JWE jwe = new JWE(key.enc_key);
byte[] share = jwe.Decrypt(cert_key.GetKey());
shares_found++;
share_arrays.Add(Utils.ConvertToUint16(share));
}
if (share_arrays.Count == data.SharedKeys.required)
{
break;
}
}
if (share_arrays.Count < data.SharedKeys.required)
{
throw new Exception($"Insufficient shares available. {data.SharedKeys.required} required, got {share_arrays.Count}.");
}
shared_secret secret = new shared_secret((UInt16)data.SharedKeys.required);
masterKey = secret.get_secret(share_arrays);
}
else
{
throw new Exception($"Unknown domain version {data.version}.");
}
PlaintextList plaintextList = new PlaintextList();
// Need to check KDF
foreach (Datum enc_data in data.EncData.data)
{
Plaintext p = new Plaintext();
HMACSHA512 hmac = new HMACSHA512();
byte[] enc_key = KDF.sp800_108(masterKey, enc_data.tag, "", hmac, 512);
JWE jwe_data = new JWE(enc_data.compact_jwe);
p.plaintext = jwe_data.Decrypt(enc_key);
p.tag = enc_data.tag;
plaintextList.Add(p);
}
return(plaintextList);
}