kopia lustrzana https://github.com/M17-Project/M17_spec
nonce-> meta field. Meta field possible values. fix phrasing in aes sections. Fix a few small bugs.
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@ -42,8 +42,8 @@ The data type specifier is used to compute the CRC, along with the payload.
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Encryption Types
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----------------
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Encryption is optional and disabled by default. The use of it is only
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allowed if local laws allow to doso.
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Encryption is optional. The use of it may be restricted within some radio
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services and countries, and should only be used if legally permissible.
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Null Encryption
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~~~~~~~~~~~~~~~
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@ -52,6 +52,54 @@ Encryption type = :math:`00_2`
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No encryption is performed, payload is sent in clear text.
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The "Encryption SubType" bits in the Stream Type field then indicate
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what data is stored in the 112 bits of the LSF META field.
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.. list-table::
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:header-rows: 1
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* - Encryption SubType bits
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- LSF META data contents
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* - :math:`00_2`
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- UTF-8 Text
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* - :math:`01_2`
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- GNSS Position Data
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* - :math:`10_2`
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- Reserved
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* - :math:`11_2`
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- Reserved
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All LSF META data must be stored in big endian byte order, as throughout
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the rest of this specification.
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GNSS Position Data stores the 112 bit META field as follows:
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.. list-table::
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:header-rows: 1
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* - Size, in bits
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- Format
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- Contents
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* - 32
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- 32-bit fixed point degrees and decimal minutes (TBD)
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- Latitude
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* - 32
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- 32-bit fixed point degrees and decimal minutes (TBD)
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- Longitude
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* - 16
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- unsigned integer
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- Altitude, in feet MSL. Stored +1500, so a stored value of 0 represents -1500 MSL. Subtract 1500 feet when parsing.
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* - 10
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- unsigned integer
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- Course in degrees true North
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* - 10
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- unsigned integer
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- Speed in miles per hour.
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* - 12
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- Reserved values
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- Transmitter/Object description field
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Scrambler
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~~~~~~~~~
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@ -110,25 +158,46 @@ Advanced Encryption Standard (AES)
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Encryption type = :math:`10_2`
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This method uses AES block cipher in counter (CTR) mode. 96-bit nonce
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value is extracted from the NONCE field, as the 96 most significant
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bits of it. The highest 16 bits of the counter are the remaining 16
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bits of the NONCE field. FN field value is then used as the
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counter. The 16 bit frame counter and 40 ms frames can provide for
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over 20 minutes of streaming without rolling over the counter [#fn_roll]_. This
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method adapts 16-bit counter to the standard 32-bit CTR for the
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encryption. FN counter always start from 0 (zero).
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This method uses AES block cipher in counter (CTR) mode, with a 96-bit
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nonce that should never be used for more than one separate stream and a 32 bit CTR.
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The 96-bit AES nonce value is extracted from the 96 most significant
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bits of the META field, and the remaining 16 bits of the META field
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form the highest 16 bits of the 32 bit counter. The FN (Frame Number)
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field value is then used to fill out the lower 16 bits of the counter,
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and always starts from 0 (zero) in a new voice stream.
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The 16 bit frame number and 40 ms frames can provide for over 20 minutes
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of streaming without rolling over the counter [#fn_roll]_.
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.. [#fn_roll] The effective capacity of the counter is 15 bits, as the
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MSB is used for transmission end signalling
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MSB is used for transmission end signalling. At 40ms per
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frame, or 25 frames per second, and 2**15 frames, we get
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2**15 frames / 25 frames per second = 1310 seconds, or 21
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minutes and some change.
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The nonce value should be generated with a hardware random number
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generator or any other method of generating non-repeating
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values. Nonce values must be used only once. It is obvious that with a
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finite number of nonce bits, the probability of nonce collision
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approaches 1. We assume that the transmission is secure for 237 frames
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using a single key. It is recommended to change keys after that
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period.
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The random part of the nonce value should be generated with a hardware
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random number generator or any other method of generating non-repeating
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values.
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To combat replay attacks, a 32-bit timestamp shall be embedded into the
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cryptographic nonce field. The field structure of the 96 bit nonce is
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shown in Table 9. Timestamp is 32 LSB portion of the number of seconds
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that elapsed since the beginning of 1970-01-01, 00:00:00 UTC, minus leap
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seconds (a.k.a. “unix time”).
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.. list-table:: 96 bit nonce field structure
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:header-rows: 1
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* - TIMESTAMP
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- RANDOM DATA
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- CTR_HIGH
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* - 32
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- 64
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- 16
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**CTR_HIGH** field initializes the highest 16 bits of the CTR, with
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the rest of the counter being equal to the FN counter.
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.. warning::
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In CTR mode, AES encryption is malleable [CTR]_ [CRYPTO]_.
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@ -140,24 +209,6 @@ period.
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In the future, use of a different mode, such as Galois/Counter Mode, could
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alleviate this issue [CRYPTO]_.
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To combat replay attacks, a 32-bit timestamp shall be embedded into
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the NONCE field. The field structure is shown in Table 9. Timestamp is 32 LSB portion of
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the number of seconds that elapsed since the beginning of 1970-01-01,
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00:00:00 UTC, minus leap seconds (a.k.a. “unix time”).
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.. list-table:: NONCE field structure
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:header-rows: 1
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* - TIMESTAMP
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- NONCE
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- CTR_HIGH
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* - 32
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- 64
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- 16
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**CTR_HIGH** field initializes the highest 16 bits of the CTR, with
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the rest of the counter being equal to the FN counter.
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.. [CTR] McGrew, David A. "Counter mode security: Analysis and recommendations." Cisco Systems, November 2, no. 4 (2002).
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.. [CRYPTO] Rogaway, Phillip. "Evaluation of some blockcipher modes of operation." Cryptography Research and Evaluation Committees (CRYPTREC) for the Government of Japan (2011).
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@ -2,22 +2,28 @@ Data Link Layer
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===============
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The Data Link layer is split into two modes:
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* Packet mode: data are sent in small bursts, on the order of 100s to 1000s of bytes at a time, after
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which the physical layer stops sending data. eg: messages, beacons, etc.
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* Stream mode: data are sent in a continuous stream for an indefinite amount of time, with no
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break in physical layer output, until the stream ends. eg: voice data, bulk data transfers, etc.
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* Packet mode
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Data are sent in small bursts, on the order of 100s to 1000s of bytes
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at a time, after which the physical layer stops sending data. e.g. messages, beacons, etc.
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* Stream mode
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Data are sent in a continuous stream for an indefinite amount of time,
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with no break in physical layer output, until the stream ends. e.g. voice data,
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bulk data transfers, etc.
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When the physical layer is idle (no RF being transmitted or received),
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the data link defaults to packet mode. ~~To switch to stream mode, a
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start stream packet (detailed later) is sent, immediately followed by
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the switch to stream mode; the Stream of data immediately follows the
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Start Stream packet without disabling the Physical layer. To switch
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out of Stream mode, the stream simply ends and returns the Physical
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layer to the idle state, and the Data Link defaults back to Packet
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mode.~~
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the data link defaults to packet mode.
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As is the convention with networking protocols, all quantities
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larger than 8 bits are encoded in bigendian.
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.. ~~To switch to stream mode, a
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.. start stream packet (detailed later) is sent, immediately followed by
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.. the switch to stream mode; the Stream of data immediately follows the
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.. Start Stream packet without disabling the Physical layer. To switch
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.. out of Stream mode, the stream simply ends and returns the Physical
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.. layer to the idle state, and the Data Link defaults back to Packet
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.. mode.~~
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As is the convention with other networking protocols, all values are
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encoded in big endian byte order.
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Stream Mode
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-----------
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@ -53,7 +59,7 @@ the **Link Setup Frame (LSF)**, and is not part of any superframes.
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* - DST
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- 48 bits
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- Destination address - Encoded callsign or a special number (eg. a group)
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- Destination address - Encoded callsign or a special number (eg. a group)
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* - SRC
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- 48 bits
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- Source address - Encoded callsign of the originator or a
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@ -61,9 +67,9 @@ the **Link Setup Frame (LSF)**, and is not part of any superframes.
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* - TYPE
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- 16 bits
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- Information about the incoming data stream
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* - NONCE
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* - META
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- 112 bits
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- Nonce for encryption
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- Metadata field, suitable for cryptographic metadata like IVs or single-use numbers, or non-crypto metadata like the sender's GNSS position.
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* - CRC
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- 16 bits
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- CRC for the link setup data
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@ -95,11 +101,12 @@ the **Link Setup Frame (LSF)**, and is not part of any superframes.
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The fields in Table 3 (except tail) form initial LSF. It contains all
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information needed to establish M17 link. Later in the transmission,
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the initial LSF is divided into 6 "chunks" and transmitted
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interleaved with data. The purpose of that is to allow late-joiners to
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receive the LICH at any point of the transmission. The process of
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collecting full LSF takes 6 frames or 6*40 ms = 240 ms. Four TAIL
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bits are needed for the convolutional coder to go back to state 0, so
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also the ending trellis position is known.
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beside the payload data. This allows late-joiners to
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reconstruct the LICH after collecting all the pieces, and start decoding
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the stream even though they missed the beginning of the transmission.
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The process of collecting full LSF takes 6 frames or 6*40 ms = 240
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ms. Four TAIL bits are needed for the convolutional coder to go back to
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state 0, so the ending trellis position is also known.
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Voice coder rate is inferred from TYPE field, bits 1 and 2.
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@ -137,7 +144,7 @@ Subsequent frames
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The most significant bit in the FN counter is used for transmission
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end signalling. When transmitting the last frame, it shall be set to 1
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(one).
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(one), and 0 (zero) in all other frames.
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The payload is used so that earlier data in the voice stream is sent first.
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For mixed voice and data payloads, the voice data is stored first, then the data.
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@ -233,8 +240,8 @@ detecting all errors up to hamming distance of 5 with payloads up to
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As M17’s native bit order is most significant bit first, neither the
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input nor the output of the CRC algorithm gets reflected.
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The input to the CRC algorithm consists of DST, SRC (each 48 bits), 16 bits of TYPE field and 128
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bits NONCE, and then depending on whether the CRC is being computed
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The input to the CRC algorithm consists of DST, SRC (each 48 bits), 16 bits of TYPE field and 112
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bits META, and then depending on whether the CRC is being computed
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or verified either 16 zero bits or the received CRC.
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The test vectors in Table 6 are calculated by feeding the given
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@ -293,6 +300,16 @@ Raw packet frames have no packet type metadata associated with them. Encapsulat
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format is discussed in :ref:`packet-superframes` in the Application Layer section. This
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provides data type information and is the preferred format for use on M17.
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When encryption type is :math:`00_2`, meaning no encryption, the
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encryption subtype bits are used to indicate the contents of the
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META field in the LSF. Since that space would otherwise go be unused,
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we can store small bits of data in that field such as free text or the
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sender's GNSS position.
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Encryption type and subtype bits, including the plaintext data formats
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when not using encryption, are described in more detail in the Application
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Layer section of this document.
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Currently the contents of the source and destination fields are arbitrary as no behavior
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is defined which depends on the content of these fields. The only requirement is that
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the content is base-40 encoded.
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@ -40,7 +40,7 @@ Later specifications may require this port.
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- Random bits, changed for each PTT or stream, but consistent from frame to frame within a stream
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* - LICH
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- sizeof(LICH)*8 bits
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- A full LICH frame (dst, src, streamtype, nonce) as defined earlier
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- A full LICH frame (dst, src, streamtype, META field) as defined earlier
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* - FN
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- 16 bits
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- Frame number (exactly as would be transmitted as an RF stream frame, including the last frame indicator at (FN & 0x8000)
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@ -101,7 +101,7 @@ Link setup frame (LSF)
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ECC stages for the link setup frame
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240 DST, SRC, TYPE, NONCE and CRC type 1 bits are convolutionally
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240 DST, SRC, TYPE, META and CRC type 1 bits are convolutionally
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coded using rate 1/2 coder with constraint K=5. 4 tail bits are used
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to flush the encoder's state register, giving a total of 244 bits
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being encoded. Resulting 488 type 2 bits are retained for type 3 bits
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@ -241,8 +241,7 @@ The puncturing schemes are defined by their partial puncturing matrices:
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.. math::
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:nowrap:
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.. only:: latex
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\setcounter{MaxMatrixCols}{32}
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\setcounter{MaxMatrixCols}{32}
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\begin{align}
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P_1 = & \begin{bmatrix}
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Plik binarny nie jest wyświetlany.
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@ -12,7 +12,7 @@
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style="font-size:6.35px;stroke-width:0.264583">TYPE</tspan></text>
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<text
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xml:space="preserve"
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