code: func AESEncryptWithOFB(plainStr []byte, key []byte, iv []byte) ([]byte, error) {
  	cb, err := aes.NewCipher(key)
  	if err != nil {
  		return nil, errors.Trace(err)
  	}
  	mode := cipher.NewOFB(cb, iv)
  	crypted := make([]byte, len(plainStr))
  	mode.XORKeyStream(crypted, plainStr)
  	return crypted, nil
  }
docs: // AESEncryptWithOFB encrypts data using AES with OFB mode.