SFSP (Secure Frame Separation Protocol) v1.0
Invented by Giovanni Blu Mitolo Originally published: 24/11/2017, latest revision: 31/10/2018 Related work: https://github.com/gioblu/PJON/ Compliant implementations: PJON v10.0 and following Released into the public domain
SFSP has been specified to obtain reliable frame separation using byte stuffing and xor based flag obfuscation. It is similar to SLIP and its variations, although uses a more cautious approach, appending at the end of the frame an
END flag to exclude a set of vulnerabilities. Its overhead is 1.33 times higher if compared to SLIP's but it is more reliable thanks to the presence of the
START flag. SFSP uses a xor based flag obfuscation method to avoid the occurrence of flags within a frame. Its procedure is lightweight and more efficient if compared with the flag transposition method used by SLIP that requires more memory and operations to be executed. The suggested sampling and transmission strategy is byte-by-byte. SFSP is designed to support fast on the fly encoding and decoding implementations with no need of additional buffers.
Frame transmission starts with a
START flag (decimal 149), followed by data bytes. When
END (decimal 234) or
ESC flag (decimal 76) occurs in data, it is prepended with an
ESC flag and its value is xored with the
ESC flag to avoid the presence of
ESC flag in data.
END is appended at the end of the frame.
START: 149 - 10010101 - 0x95 - END: 234 - 11101010 - 0xea - ê ESC: 187 - 10111011 - 0xBB - » ______________________________ | DATA 1-65535 bytes | _______ |______ _____ _______ ______| _____ | START | | BYTE || ESC || BYTE || BYTE | | END | |-------| |------||-----||-------||------| |-----| | 149 | | 23 || 76 ||149^ESC|| 52 | | 234 | |_______| |______||_____||_______||______| |_____| | | | 2 START is xored with ESC | 1 START or 149 is escaped
Receiver identifies the start of a frame when a
START flag is received. If
ESC flag occurs during reception, it is ignored and the next byte xored with
ESC flag to get back its original value; any value out
ESC flag causes the receiver to discard the frame and be ready to receive the next one nominally. The end of the frame is detected when an unescaped
END flag is received.
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