@node Packet
@unnumbered Packet format

All packets are
@url{https://tools.ietf.org/html/rfc4506, XDR}-encoded structures.

@menu
* Plain packet: Plain.
* Encrypted packet: Encrypted.
@end menu

@node Plain
@section Plain packet

Plain packet contains either the whole file, or file request (freq), or
transition packet or exec message. It is called "plain", because it
contains plaintext, but plain packets would never be stored on your hard
drive.

@verbatim
            HEADER
+--------------------------------------+--...---+
| MAGIC | TYPE | NICE | PATHLEN | PATH | PAYLOAD|
+--------------------------------------+--...---+
@end verbatim

@multitable @columnfractions 0.2 0.3 0.5
@headitem @tab XDR type @tab Value
@item Magic number @tab
    8-byte, fixed length opaque data @tab
    @verb{|N N C P P 0x00 0x00 0x02|}
@item Payload type @tab
    unsigned integer @tab
    0 (file), 1 (freq), 2 (exec), 3 (transition)
@item Niceness @tab
    unsigned integer @tab
    1-255, preferred packet @ref{Niceness, niceness} level
@item Path length @tab
    unsigned integer @tab
    actual length of @emph{path} field's payload
@item Path @tab
    255 byte, fixed length opaque data @tab
    @itemize
    @item UTF-8 encoded destination path for file transfer
    @item UTF-8 encoded source path for file request
    @item UTF-8 encoded, zero byte separated, exec's arguments
    @item Node's id the transition packet must be relayed on
    @end itemize
@end multitable

Path has fixed size because of hiding its actual length -- it is
valuable metadata. Payload is appended to the header -- it is not stored
as XDR field, because most XDR libraries will store all that data in the
memory.

Depending on the packet's type, payload could store:

@itemize
@item File contents
@item Destination path for freq
@item @url{http://zlib.net/, zlib} compressed exec body
@item Whole encrypted packet we need to relay on
@end itemize

Also depending on packet's type, niceness level means:

@itemize
@item Preferable niceness level for files sent by freq
@item @env{NNCP_NICE} variable's value passed during @ref{CfgExec} invocation.
@end itemize

@node Encrypted
@section Encrypted packet

Encrypted packets are the only files found in spools, in exchangeable
storages and that are synchronized between TCP daemons.

Each encrypted packet has the following header:

@verbatim
  +------------ HEADER --------------------+   +------------- ENCRYPTED -------------+
 /                                          \ /                                       \
+--------------------------------------------+------+---------+----------...---+------+
| MAGIC | NICE | SENDER | RCPT | EPUB | SIGN | SIZE | BLOCK 0 | BLOCK 1  ...   | JUNK |
+-------------------------------------/------\------+---------+----------...---+------+
                                     /        \
                      +-------------------------------------+
                      | MAGIC | NICE | SENDER | RCPT | EPUB |
                      +-------------------------------------+
@end verbatim

@multitable @columnfractions 0.2 0.3 0.5
@headitem @tab XDR type @tab Value
@item Magic number @tab
    8-byte, fixed length opaque data @tab
    @verb{|N N C P E 0x00 0x00 0x04|}
@item Niceness @tab
    unsigned integer @tab
    1-255, packet @ref{Niceness, niceness} level
@item Sender @tab
    32-byte, fixed length opaque data @tab
    Sender node's id
@item Recipient @tab
    32-byte, fixed length opaque data @tab
    Recipient node's id
@item Exchange public key @tab
    32-byte, fixed length opaque data @tab
    Ephemeral curve25519 public key
@item Signature @tab
    64-byte, fixed length opaque data @tab
    ed25519 signature for that packet's header
@end multitable

Signature is calculated over all previous fields.

All following encryption is done in AEAD mode using
@url{https://cr.yp.to/chacha.html, ChaCha20}-@url{https://en.wikipedia.org/wiki/Poly1305, Poly1305}
algorithms. Data is splitted on 128 KiB blocks. Each block is encrypted with
increasing nonce counter.

Authenticated and encrypted size come after the header:

@multitable @columnfractions 0.2 0.3 0.5
@headitem @tab XDR type @tab Value
@item Size @tab
    unsigned hyper integer @tab
    Payload size.
@end multitable

Then comes the actual payload.

Each node has static @strong{exchange} and @strong{signature} keypairs.
When node A want to send encrypted packet to node B, it:

@enumerate
@item generates ephemeral @url{http://cr.yp.to/ecdh.html, curve25519} keypair
@item prepares structure for signing
@item signs that structure using private
    @url{http://ed25519.cr.yp.to/, ed25519} signature key
@item takes remote node's exchange public key and performs
    Diffie-Hellman computation on this remote static public key and
    private ephemeral one
@item derive the keys:
    @enumerate
    @item initialize @url{https://blake2.net/, BLAKE2Xb} XOF with
    derived ephemeral key and 96-byte output length
    @item feed @verb{|N N C P E 0x00 0x00 0x04|} magic number to XOF
    @item read 32-bytes of "size" AEAD encryption key
    @item read 32-bytes of payload AEAD encryption key
    @item optionally read 32-bytes pad generation key
    @end enumerate
@item encrypts size, appends its authenticated ciphertext to the header
@item encrypts payload, appends its authenticated ciphertext
@item possibly appends any kind of "junk" noise data to hide real
    payload's size from the adversary (generated using XOF with
    unlimited output length)
@end enumerate