HMC SHA1 hash - Java producing different hash output than C#

I think the problem is in your .NET code. Does Configuration.RecurlySection.Current.PrivateKey return a string? Is that value the key you expect?

Using the following code, .NET and Java return identical results.

.NET Code

string message = "[1312701386,transactioncreate,[account_code:ABC,amount_in_cents:5000,currency:USD]]";
string privateKey = "0123456789ABCDEF0123456789ABCDEF";

var hashedKey = SHA1.Create().ComputeHash(Encoding.UTF8.GetBytes(privateKey));
var hmac = new HMACSHA1(hashedKey);
var hash = hmac.ComputeHash(Encoding.ASCII.GetBytes(message));

Console.WriteLine("  Message: {0}", message);
Console.WriteLine("      Key: {0}\n", privateKey);
Console.WriteLine("Key bytes: {0}", BitConverter.ToString(hashedKey).Replace("-", "").ToLower());
Console.WriteLine("   Result: {0}", BitConverter.ToString(hash).Replace("-", "").ToLower());

Result:

  Message: [1312701386,transactioncreate,[account_code:ABC,amount_in_cents:5000,currency:USD]]
      Key: 0123456789ABCDEF0123456789ABCDEF

Key bytes: 4d857d2408b00c3dd17f0c4ffcf15b97f1049867
   Result: c8a9188dcf85d1378976729e50f1de5093fabb78

Java

String message = "[1312701386,transactioncreate,[account_code:ABC,amount_in_cents:5000,currency:USD]]";
String privateKey = "0123456789ABCDEF0123456789ABCDEF";

MessageDigest md = MessageDigest.getInstance("SHA-1");
byte[] keyBytes = md.digest(privateKey.getBytes("UTF-8"));

SecretKey sk = new SecretKeySpec(keyBytes, "HmacSHA1");
Mac mac = Mac.getInstance("HmacSHA1");
mac.init(sk);
byte[] result = mac.doFinal(message.getBytes("ASCII"));

System.out.println("  Message: " + message);
System.out.println("      Key: " + privateKey + "\n");
System.out.println("Key Bytes: " + toHex(keyBytes));
System.out.println("  Results: " + toHex(result));

Result:

  Message: [1312701386,transactioncreate,[account_code:ABC,amount_in_cents:5000,currency:USD]]
      Key: 0123456789ABCDEF0123456789ABCDEF

Key Bytes: 4d857d2408b00c3dd17f0c4ffcf15b97f1049867
  Results: c8a9188dcf85d1378976729e50f1de5093fabb78