> 3.4 Attack 2: Breaking Authenticity > With the first attack, the FPGA can be used to decrypt arbitrary blocks. Hence, it can also be seen as a decryption ora

> 3.4 Attack 2: Breaking Authenticity > With the first attack, the FPGA can be used to decrypt arbitrary blocks. Hence, it can also be seen as a decryption oracle. Thus,we can also use this oracle to encrypt a bitstream, as shown by Rizzo and Duong in [41], and generate a valid HMAC tag This requires the first stage of the attack to succeed. If it fails and the FPGA cannot be used as a decryption oracle, there's no way to generate a valid encrypted bitstream with the technique outlined in the paper.