What is this Java code doing (need to convert to C#)

What is the meaning of these 2 lines of code in Java? In .Net, I know i need to use cryptography but not sure how to convert them since I dont know the meaning....

This is the example I'm following

https://firstdata.zendesk.com/entries/452518-generating-the-payment-request-form-programmatically

// Use Java Cryptography functions to generate the x_fp_hash value
// generate secret key for HMAC-SHA1 using the transaction
 keySecretKey key = new SecretKeySpec(transactionKey.getBytes(), "HmacSHA1");


// Get instance of Mac object implementing HMAC-SHA1, and
// Initialize it with the above secret key
Mac mac = Mac.getInstance("HmacSHA1");mac.init(key);

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