@unnumbered Packet format
All packets are
-@url{https://en.wikipedia.org/wiki/External_Data_Representation,
-XDR}-encoded structures.
+@url{https://tools.ietf.org/html/rfc4506, XDR}-encoded structures.
@menu
* Plain packet: Plain.
contains plaintext, but plain packets would never be stored on your hard
drive.
-Each packet has the following header:
-
@verbatim
HEADER
+-------------------------------+--...---+
@end verbatim
@multitable @columnfractions 0.2 0.3 0.5
-@headitem @tab XDR type @tab Value
-
+@headitem @tab XDR type @tab Value
@item Magic number @tab
- 8-byte, fixed length opaque data @tab
- @code{NNCPP0x10x00x00}
-
+ 8-byte, fixed length opaque data @tab
+ @verb{|N N C P P 0x00 0x00 0x01|}
@item Payload type @tab
- unsigned integer @tab
- 0 (file), 1 (freq), 2 (mail), 3 (transition)
-
+ unsigned integer @tab
+ 0 (file), 1 (freq), 2 (mail), 3 (transition)
@item Path length @tab
- unsigned integer @tab
- actual length of following field's payload
-
+ 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, space separated, email recipients list
- @item Node id the transition packet must be relayed on
- @end itemize
-
+ 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, space separated, email recipients list
+ @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.
+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.
-Actual payload comes after that header to the very end:
+Depending on the packet's type, payload could store:
@itemize
@item File contents
Each encrypted packet has the following header:
@verbatim
- HEADER
-+--------------------------------------------+-------...--------+
-| MAGIC | NICE | SENDER | EPUB | SIGN | SIZE | CIPHERTEXT | MAC |
-+------------------------------/------\------+-------...--------+
+ +------------ HEADER -------------+ +-------- ENCRYPTED --------+
+ / \ / \
++-------------------------------------+------------+----...-----------+------+
+| MAGIC | NICE | SENDER | EPUB | SIGN | SIZE | MAC | CIPHERTEXT | MAC | JUNK |
++------------------------------/------\------------+----...-----------+------+
/ \
- +--------------------------------------------+
- | MAGIC | NICE | RCPT | SENDER | EPUB | SIZE |
- +--------------------------------------------+
+ +-------------------------------------+
+ | MAGIC | NICE | RCPT | SENDER | EPUB |
+ +-------------------------------------+
@end verbatim
@multitable @columnfractions 0.2 0.3 0.5
-@headitem @tab XDR type @tab Value
-
+@headitem @tab XDR type @tab Value
@item Magic number @tab
- 8-byte, fixed length opaque data @tab
- @code{NNCPE0x10x00x00}
-
+ 8-byte, fixed length opaque data @tab
+ @verb{|N N C P E 0x00 0x00 0x01|}
@item Niceness @tab
- unsigned integer @tab
- 1-255, packet niceness level, its priority.
- Lower value means higher precedence
-
+ unsigned integer @tab
+ 1-255, packet @ref{Niceness, niceness} level
@item Sender @tab
- 32-byte, fixed length opaque data @tab
- Sender node's id
-
+ 32-byte, fixed length opaque data @tab
+ Sender node's id
@item Exchange public key @tab
- 32-byte, fixed length opaque data @tab
- Ephemeral curve25519 public key
-
+ 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 encrypted packet
-
-@item Size @tab
- unsigned hyper integer @tab
- Encrypted payload size
-
+ 64-byte, fixed length opaque data @tab
+ ed25519 signature for that packet's header
@end multitable
Signature is calculated over the following structure:
@item Recipient (32-byte recipient node's id)
@item Sender
@item Exchange public key
-@item Size
@end itemize
-Actual encrypted payload comes after that header. Payload is encrypted
-using @url{https://www.schneier.com/academic/twofish/, Twofish}
-algorithm with 256-bit key in
+All following encryption is done using
+@url{https://www.schneier.com/academic/twofish/, Twofish} algorithm with
+256-bit key in
@url{https://en.wikipedia.org/wiki/Counter_mode#Counter_.28CTR.29, CTR}
mode of operation with zero initialization vector (because each
-encrypted packet has ephemeral exchange key). Ciphertext's length is
-equal to plaintext. @url{https://blake2.net/, BLAKE2b-256} MAC is
-appended to the ciphertext.
+encrypted packet has ephemeral exchange key). @url{https://blake2.net/,
+BLAKE2b-256} MAC is appended to the ciphertext.
+
+After the headers comes an encrypted payload size and MAC of that size.
+
+@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
+
+Next comes the actual encrypted payload with corresponding MAC.
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 (underlying payload size must be
-already known)
-@item signs that structure using private @url{http://ed25519.cr.yp.to/,
-ed25519} signature key
+@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 derived ephemeral key used as an input to
-@url{https://en.wikipedia.org/wiki/HKDF, HKDF}-BLAKE2b-256 key
-derivation function
-@item two 256-bit keys are derived from it for using with Twofish and
-BLAKE2b-MAC functions
-@item Twofish encryption and BLAKE2b-MACing is performed over the
-plaintext. Ciphertext and MAC tag are appended to the header
+ Diffie-Hellman computation on this remote static public key and
+ private ephemeral one
+@item derived ephemeral key is used as an input to
+ @url{https://en.wikipedia.org/wiki/HKDF, HKDF}-BLAKE2b-256 KDF
+@item derives four session keys using
+ @url{https://en.wikipedia.org/wiki/HKDF, HKDF}-BLAKE2b-256 KDF:
+ @enumerate
+ @item "Size" encryption (for Twofish) key
+ @item "Size" authentication (for BLAKE2b-MAC) key
+ @item Payload encryption key
+ @item Payload authentication key
+ @end enumerate
+@item encrypts size, appends its ciphertext to the header
+@item appends MAC tag over that ciphertext
+@item encrypts and appends payload ciphertext
+@item appends MAC tag over that payload ciphertext
+@item possibly appends any kind of "junk" noise data to hide real
+ payload's size from the adversary
@end enumerate