2 @unnumbered Packet format
5 @url{https://tools.ietf.org/html/rfc4506, XDR}-encoded structures.
9 * Encrypted packet: Encrypted.
15 Plain packet contains either the whole file, or file request (freq), or
16 transition packet or exec message. It is called "plain", because it
17 contains plaintext, but plain packets would never be stored on your hard
22 +--------------------------------------+--...---+
23 | MAGIC | TYPE | NICE | PATHLEN | PATH | PAYLOAD|
24 +--------------------------------------+--...---+
27 @multitable @columnfractions 0.2 0.3 0.5
28 @headitem @tab XDR type @tab Value
29 @item Magic number @tab
30 8-byte, fixed length opaque data @tab
31 @verb{|N N C P P 0x00 0x00 0x03|}
32 @item Payload type @tab
34 0 (file), 1 (freq), 2 (exec), 3 (transition), 4 (exec-fat)
37 1-255, preferred packet @ref{Niceness, niceness} level
38 @item Path length @tab
40 actual length of @emph{path} field's payload
42 255 byte, fixed length opaque data @tab
44 @item UTF-8 encoded destination path for file transfer
45 @item UTF-8 encoded source path for file request
46 @item UTF-8 encoded, zero byte separated, exec's arguments
47 @item Node's id the transition packet must be relayed on
51 Path has fixed size because of hiding its actual length -- it is
52 valuable metadata. Payload is appended to the header -- it is not stored
53 as XDR field, because XDR has no ability to pass more than 4 GiB of
54 opaque data. Moreover most XDR libraries store fields in the memory in
57 Depending on the packet's type, payload could store:
61 @item Destination path for freq
62 @item @url{https://facebook.github.io/zstd/, Zstandard} compressed exec body
63 @item Whole encrypted packet we need to relay on
64 @item Uncompressed exec body
67 Also depending on packet's type, niceness level means:
70 @item Preferable niceness level for files sent by freq
71 @item @env{NNCP_NICE} variable's value passed during @ref{CfgExec} invocation.
75 @section Encrypted packet
77 Encrypted packets are the only files found in spools, in exchangeable
78 storages and that are synchronized between TCP daemons.
80 Each encrypted packet has the following header:
83 +------------ HEADER --------------------+ +------------- ENCRYPTED -------------+
85 +--------------------------------------------+------+---------+----------...---+------+
86 | MAGIC | NICE | SENDER | RCPT | EPUB | SIGN | SIZE | BLOCK 0 | BLOCK 1 ... | JUNK |
87 +-------------------------------------/------\------+---------+----------...---+------+
89 +-------------------------------------+
90 | MAGIC | NICE | SENDER | RCPT | EPUB |
91 +-------------------------------------+
94 @multitable @columnfractions 0.2 0.3 0.5
95 @headitem @tab XDR type @tab Value
96 @item Magic number @tab
97 8-byte, fixed length opaque data @tab
98 @verb{|N N C P E 0x00 0x00 0x04|}
100 unsigned integer @tab
101 1-255, packet @ref{Niceness, niceness} level
103 32-byte, fixed length opaque data @tab
106 32-byte, fixed length opaque data @tab
108 @item Exchange public key @tab
109 32-byte, fixed length opaque data @tab
110 Ephemeral curve25519 public key
112 64-byte, fixed length opaque data @tab
113 ed25519 signature for that packet's header
116 Signature is calculated over all previous fields.
118 All following encryption is done in AEAD mode using
119 @url{https://cr.yp.to/chacha.html, ChaCha20}-@url{https://en.wikipedia.org/wiki/Poly1305, Poly1305}
120 algorithms. Data is splitted on 128 KiB blocks. Each block is encrypted with
121 increasing nonce counter.
123 Authenticated and encrypted size come after the header:
125 @multitable @columnfractions 0.2 0.3 0.5
126 @headitem @tab XDR type @tab Value
128 unsigned hyper integer @tab
132 Then comes the actual payload.
134 Each node has static @strong{exchange} and @strong{signature} keypairs.
135 When node A want to send encrypted packet to node B, it:
138 @item generates ephemeral @url{http://cr.yp.to/ecdh.html, curve25519} keypair
139 @item prepares structure for signing
140 @item signs that structure using private
141 @url{http://ed25519.cr.yp.to/, ed25519} signature key
142 @item takes remote node's exchange public key and performs
143 Diffie-Hellman computation on this remote static public key and
144 private ephemeral one
145 @item derive the keys:
147 @item initialize @url{https://blake2.net/, BLAKE2Xb} XOF with
148 derived ephemeral key and 96-byte output length
149 @item feed @verb{|N N C P E 0x00 0x00 0x04|} magic number to XOF
150 @item read 32-bytes of "size" AEAD encryption key
151 @item read 32-bytes of payload AEAD encryption key
152 @item optionally read 32-bytes pad generation key
154 @item encrypts size, appends its authenticated ciphertext to the header
155 @item encrypts payload, appends its authenticated ciphertext
156 @item possibly appends any kind of "junk" noise data to hide real
157 payload's size from the adversary (generated using XOF with
158 unlimited output length)