Encrypt given data using sm4-ofb cipher algo. SM4 in OFB (Output Feedback) mode, using feedback to turn the block cipher into a stream cipher. OFB mode is used with SM4 for efficient stream encryption, though it is less commonly used than other modes like CTR. SM4-OFB is secure, but newer modes like CTR are generally preferred for modern cryptographic applications. Our encryption process prepends the IV string used during the encryption process in the encrypted data so that it can be used during decryption.