Internet Engineering Task Force Gwerder
Internet-Draft FHNW
Intended status: Experimental February 24, 2019
Expires: August 28, 2019

MessageVortex Protocol


MessageVortex Protocol is a protocol to achieve different degrees of anonymity. It specifies messages embedded within existing transfer protocols such as SMTP or XMPP to send them via peer nodes to one or more recipients.

The protocol outperforms other protocols by decoupling transport from the final transmitter and receiver party. There is no trust put into any infrastructure except for the infrastructure of the sending and receiving party of a message. The creator of the routing block has full control over the message flow. Routing nodes gain no non-obvious knowledge about messages even when collaborating. Third-party anonymity is always achieved. Furthermore, the protocol allows achieving either sender or receiver anonymity.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on August 28, 2019.

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Table of Contents

1. Introduction

Anonymization is hard to achieve. Most of the attempts in the past rely on either trust in a dedicated infrastructure or a specialized networking protocol.

Instead of defining a transport layer, MessageVortex piggybacks on other transport protocols. A blending layer embeds Vortex messages into ordinary messages of that transport protocol. A blending layer picks the messages up, applies local operations to it and resends the new chunks to the next recipients.

A processing node learns as little as possible from the message due to the nature of the operations processed. The onionized structure of the protocol makes it impossible to follow the trace of a message without having control over the processing node itself.

MessageVortex is a protocol which allows sending and receiving messages by using a routing block instead of a destination address. The sender has full control over all parameters of the message flow.

A message is split and reassembled during transmission. Chunks of the message may carry redundant information to avoid service interruptions of the message transit. Decoy traffic and message traffic are not differentiable as the nature of the addRedundancy operation allows each generated part to be a message part or decoy. Any routing node is thus unable to differentiate between the message and decoy traffic.

Any Receiver knows after processing whether a message is destined for it (it creates a chunk with ID 1) or other nodes (processing instructions only). Due to the missing keys, no other node may do this processing.

This RFC starts with the general terminology (see Section 2). Next, a general overview of the process is given (see Section 3). Lastly, the subsequent sections describe the details of the protocol.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

1.2. Protocol Specification

Appendix A specifies all relevant parts of the protocol in ASN.1 (see [CCITT.X680.2002] and [CCITT.X208.1988]). The blocks are, if not otherwise mentioned, DER encoded.

1.3. Number Specification

All numbers within this document are, if not suffixed, decimal numbers. Numbers suffixed with a small letter 'h' followed by two hexadecimal digits are octets in hexadecimal writing. A blank in ASCII (' ') is written as 20h and a capital 'K' in ASCII as 4Bh.

2. Entities Overview

Within this document, we refer to the entities as defined below.

2.1. Node

We use the term 'node' to describe any system connected to other nodes, and supporting the MessageVortex Protocol. A 'node address' is typically an email address, an XMPP address, or any other transport protocol identity supporting the MessageVortex protocol. Any address SHOULD include a public part of an 'identity key' allowing to transmit messages safely. One or more addresses MAY belong to the same node.

2.1.1. Blocks

We use the term 'block' for an ASN.1 sequence in a transmitted message. We embed messages in the transport protocol. These messages may have any size.

2.1.2. NodeSpec

A nodeSpec block as specified in Appendix A.5 expresses in a unified format an addressable node. The nodeSpec contains a reference to the routing protocol, the routing address within this protocol and the keys required for addressing the node. This RFC specifies transport layers for XMPP and SMTP. Adding transport layers requires to write an extension to this RFC. NodeSpec for SMTP nodes

localPart         = <local part of address>
domain            = <domain part of address>
email             = localPart "@" domain
keySpec           = <BASE64 encoded AsymmetricKey [DER encoded]>
smtpAlternateSpec = localPart ".." keySpec ".." domain "@localhost"
smtpUrl           = "vortexsmtp://" smtpAlternateSpec

An alternative address representation is defined. This address allows a standard email client to address a vortex node. An alternative representation SHOULD be supported as defined below as smtpAlternateSpec (specification noted in ABNF as specified in [RFC5234]). For applications with QR code support, the smtpUrl representations SHOULD be used.

This representation does not support quoted local part SMTP addresses. NodeSpec for XMPP nodes

localPart         = <local part of address>
domain            = <domain part of address>
resourcePart      = <resource part of the address>
jid               = localPart "@" domain [ "/" resourcePart ]
keySpec           = <BASE64 encoded AsymmetricKey [DER encoded]>;
jidAlternateSpec  = localPart ".." keySpec ".." 
                    domain "@localhost" [ "/" resourcePart ]
jidUrl            = "vortexxmpp://" jidAlternateSpec

Typically a node specification is specified with the ASN.1 block NodeSpec. To allow addressing of a vortex node with an ordinary XMPP client, the alternative representation SHOULD be supported as defined below as jidAlternateSpec (specification noted in ABNF as specified in [RFC5234]).

2.2. Peer Partner

We use the term 'peer partner' as two or more message sending or receiving entities. One of these peer partners sends a message, and all other peer partners receive one or more messages. Peer partners are message specific. Every peer partner always connects directly to a node.

2.3. Encryption keys

There are several keys required for a Vortex message. For identities and ephemeral identities (see below) we use asymmetric keys. For message encryption, we use symmetric keys.

2.3.1. Identity Keys

Every participant of the network has an asymmetric key. These keys SHOULD be either EC keys with a minimum length of 384 bits or RSA keys with a minimum length of 2048 bits.

The public key needs to be known by all parties writing to or through that node.

2.3.2. Peer Key

Peer keys are symmetrical keys transmitted with a Vortex message. Peer keys are always known to the node sending a message, the node receiving the message from the sender, and to the creator of the routing block.

A peer key is included in the identity block of a Vortex message and the building instructions for a Vortex message (see RoutingCombo in Appendix A).

2.3.3. Sender Key

The sender key is a symmetrical key protecting the identity and routing block of a Vortex message. It is encrypted with the receiving peer key and prefixed to the identity block. This key decouples further identity and processing information from the previous key.

A sender key is known to precisely one peer of a Vortex message and the creator of the routing block.

2.4. Vortex Message

We use the term 'Vortex message' for a single transmission between two routing layers. A message adapted to the transport layer by the blending layer is called a 'blended Vortex message' (see Section 3).

A full vortex message contains the following items:

2.5. Message

A Message is a content to be transmitted from one sender to the recipient. The Sender uses a routing block to achieve this which is either built by him or provided by the receiver. A Message may be anonymous. There are however different degrees of anonymity:

A message is always MIME encoded as specified in [RFC2045].

2.6. Key and MAC specifications and usages

MessageVortex uses a unique encoding for keys. It is designed to be small and flexible while maintaining a specific base structure.

The following key structures exist:

MAC does not need a complete structure containing specs and value. Instead only a MacAlgorithmSpec is available. The following sections outline the constraints when specifying parameters for these structures. A node MUST NOT specify any parameter more than once.

If a crypto mode is specified requiring an IV, a node MUST provide the IV when specifying the key.

2.6.1. Asymmetric Keys

Nodes use asymmetric keys for identifying peer nodes (identities) and encrypting symmetric keys (for later de-/encryption of payload or blocks). All asymmetric keys MUST contain a key type specifying a strictly normed key. They MUST contain a public part of the key encoded as X.509 container and a private key as specified in PKCS#8 wherever possible.

RSA and EC keys must contain a keysize parameter. All asymmetric keys SHOULD contain a padding parameter. A node SHOULD assume PKCS#1 if no padding is specified.

NTRU specification MUST provide parameters "n", "p", and "q".

2.6.2. Symmetric Keys

Nodes use symmetric keys for encrypting payload and control blocks. These symmetric keys MUST contain a key type specifying a key. They MUST contain a key in an encoded form.

A node MUST provide a keysize parameter if the key (or equivalently block) size is not standardized or encoded in the name. All symmetric key specification MUST contain a mode and padding parameter. A node MAY list multiple padding or mode parameters in a ReplyCapability block to give the recipient a free choice.

2.7. Transport Address

We use the term 'transport address' for the token required to address the next immediate node on the transport layer. An email transport layer would have SMTP addresses such as '' as transport address.

2.8. Identity

2.8.1. Peer Identity

The peer identity may contain the following information of a peer partner:

2.8.2. Ephemeral Identity

Ephemeral identities are temporary identities created on a single node. These identities MUST NOT relate to any other identity on any other node. They allow bookkeeping for a node. Each ephemeral identity has a workspace assigned. Every ephemeral identity may have the following items assigned:

2.8.3. Official Identity

An official identity may have the following items assigned:

2.9. Workspace

Every official or ephemeral identity has a workspace. A workspace consists of the following elements:

2.10. Multi-Use Reply Blocks

We use the term 'multi-use reply blocks' (MURB) for a special routing block sent to a receiver of a message or request. A sender may use such a block once or several times to reply to the sender linked to the ephemeral identity. It is possible to achieve sender anonymity using these blocks.

3. Layer Overview

The protocol is designed in four layers as shown in Figure 1.

| Vortex Node                                                      |
| +--------------------------------------------------------------+ |
| |                       Accounting                             | |
| |______________________________________________________________| |
|                                                                  |
| +--------------------------------------------------------------+ |
| |                         Routing                              | |
| |______________________________________________________________| |
|                                                                  |
| +---------------------------+ +--------------------------------+ |
| |           Blending        | |             Blending           | |
| |___________________________| |________________________________| |
  +---------------------------+ +--------------+ +---------------+
  |          Transport        | | Transport in | | Transport out |
  |___________________________| |______________| |_______________|

Figure 1: Layer overview

Every participating node MUST implement the layers blending, routing, and accounting. There MUST be at least one incoming and one outgoing transport layer available to a node. All blending layers SHOULD connect to respective Transport layers for sending and receiving packets.

3.1. Transport Layer

The transport layer embeds the blended MessageVortex packets into the data stream of the existing transport layer protocol.

The transport layer infrastructure SHOULD NOT be specific to anonymous communication and should contain significant parts of non-MessageVortex traffic.

3.2. Blending Layer

The blending layer embeds MessageVortex packets into the transport layer data stream and extracts MessageVortex packets from the transport layer.

3.3. Routing Layer

The Routing Layer expands information contained in MessageVortex packets, processes them, and passes generated packets to the respective Blending Layer.

3.4. Accounting Layer

The accounting layer keeps track of all ephemeral identities authorized to use a MessageVortex node. It verifies the available quotas to an ephemeral identity.

4. Vortex Message

4.1. Overview

| |   | | |   | | C | | |   | | | R | |       | |
| |   | | |   | | P | | | H | | | O | |       | |
| | M | | | P | | R | | | E | | | U | |   P   | |
| | P | | | A | | E | | | A | | | T | |   A   | |
| | R | | | D | | F | | | D | | | I | |   Y   | |
| | E | | | D | | I | | | E | | | N | |   L   | |
| | F | | | I | | X | | | R | | | G | |   O   | |
| | I | | | N | +---+ | |___| | |___| |   A   | |
| | X | | | G |  k_h  | sk_s  | sk_s  |   D   | |
| |___| | |___|_______|_______|_______|_______| |
|  k_h  |                  sk_p                 |

Figure 2: Vortex message overview

Figure 2 shows a Vortex message. The enclosed sections denote encrypted blocks. The three to four letter abbreviations denote the key required for decryption. The abbreviation k_h stands for the asymmetric host key. sk_p is the symmetric peer key. The receiving node obtains this key by decrypting MPREFIX with its host key k_h. sk_s is the symmetric sender key. When decrypting the MPREFIX block, the node obtains this key. The sender key protects the header and the routing blocks. This key guarantees that the node assembling the message does not know about upcoming identities, operations, and requests. The peer key protects the message including structure from any third party observer.

4.2. Message Prefix Block (MPREFIX)

The PrefixBlock contains a symmetrical key as defined in Appendix A.1 and is encrypted using the host key of the receiving peer host. The symmetric key used MUST be one out of the set advertised by a CapabilitiesReplyBlock (see Section 7.2.6). A node MAY choose any parameters omitted in the CapabilitiesReplyBlock freely (unless stated otherwise in Section 7.2.6). A node SHOULD avoid sending unencrypted PrefixBlocks. A prefix block MUST contain the same forward-secret as the other prefix, the routing block, and the header block. A host MAY reply to a message with an unencrypted message block. Any reply to a message SHOULD be encrypted.

The sender MUST choose a key which may be encrypted with the host key using the padding advertised by the CapabilitiesReplyBlock.

4.3. Inner Message Block

A node MUST always encrypt (with the symmetric key of the PrefixBlock) an InnerMessageBlock. The encryption hides the inner structure of the message. The InnerMessageBlock SHOULD always accommodate four or more payload chunks.

An InnerMessageBlock always starts with a padding block. This padding guarantees that when using the same routing block multiple times, its binary structure is not repeated throughout the messages with the same routing block. The padding MUST be the first 16 bytes of the first four non-empty payload chunks (PayloadChunks). If a payload chunk is shorter than 16 bytes, the content of the padding SHOULD be filled with zero-valued bytes (00h) at very end up to the required number of bytes. An inner message block (InnerMessageBlock) SHOULD contain at least four payload chunks sized 16 bytes or bigger. If there are less than four payload chunks, then the padding MUST contain a random sequence of 16 bytes for the missing payload chunks. A node MUST NOT reuse random sequences.

An InnerMessageBlock contains so-called forwardSecrets. This random number MUST be the same in the HeaderBlock, the RoutingBlock, and the PrefixBlock. Nodes receiving Messages containing non-matching forwardSecrets MUST discard these messages, and SHOULD NOT send an error message.

4.3.1. Control Prefix Block

Control prefix block (CPREFIX) and MPREFIX block share the same structure and logic. It contains the sender key sk_s. If an MPREFIX block was unencrypted, a node MAY omit the CPREFIX block. An omitted CPREFIX block results in unencrypted control blocks (HeaderBlock and RoutingBlock).

A prefix block MUST contain the same forwardSecret as the other prefix, the routing block, and the header block.

4.3.2. Control Blocks

The control blocks contain the core information to process the payload. It contains a HeaderBlock and a RoutingBlock. Header Block

The header block (see HeaderBlock in Appendix A) contains the following information:

The list of header requests MAY one of the following:

If a header block violates these rules, then a node MUST NOT reply to any header requests. Payload and routing blocks SHOULD still be added to the workspace and processed, given the message quota is not exceeded. Routing Block

The routing block (see RoutingBlock in Appendix A) contains the following information:

4.3.3. Payload Block

Each InnerMessageBlock containing routing information SHOULD contain at least four PayloadChunks.

5. General notes

The MessageVortex protocol is a modular protocol. It allows using different encryption algorithms. For operation, a Vortex node SHOULD always support at least two completely different (i.e., relying on two different mathematical problems) types of algorithms, paddings, or modes.

5.1. Supported Symmetric Ciphers

A node MUST support the following symmetric ciphers:

A node SHOULD support any standardized, bigger key size than the smallest key.

A node MAY support Twofish ciphers (see [TWOFISH]).

5.2. Supported Asymmetric Ciphers

A node MUST support the following asymmetric ciphers:

5.3. Supported MACs

A node MUST support the following Message Authentication Codes (MAC):

A node SHOULD support the following MACs:

5.4. Supported Paddings

A node MUST support the following paddings specified in [RFC8017]:

5.5. Supported Modes

A node MUST support the following modes:

A node SHOULD NOT use the following modes:

A node SHOULD support the following modes:

6. Blending

Each node supports a fixed set of blending capabilities. They may be different for incoming and outgoing messages.

The following sections describe the blending mechanism. There are currently two blending layers specified. One layer specification for the simple mail transfer protocol (SMTP; See [RFC5321]) and one for the Extensible Messaging and Presence Protocol (XMPP; See [RFC6120]). All nodes MUST at least support "encoding=plain:0,256".

6.1. Blending in Attachments

There are two types of blending supported when using attachments.

A node MUST support PLAIN blending for reasons of interoperability. A node MAY support blending using F5.

6.1.1. PLAIN embedding into attachments

A blending layer embeds a VortexMessage in a carrier file with an offset for PLAIN blending. For replacing a file start, a node MUST use the offset 0. The routing node MUST choose the payload file for the message. A routing node SHOULD use a credible payload type (e.g., MIME type) with high entropy. It furthermore SHOULD prefix a valid header structure to avoid easy detection of the Vortex message. A routing node SHOULD use a valid footer, if any, to a payload file to improve blending.

A node SHOULD offer at least one PLAIN blending method for incoming Vortex messages. A node may offer multiple offsets for incoming Vortex messages.

A plain blending is specified as follows:

plainEncoding = "("plain:" <numberOfBytesOfOffset> 
                [ "," <numberOfBytesOfOffset> ]* ")"

6.1.2. F5 embedding into attachments

For F5, a blending layer embeds a VortexMessage into a jpeg file according to [F5]. The password for blending may be publicly known. A routing node MAY advertise multiple passwords. The use of F5 adds roughly a tenfold of transfer volume to the message. A routing block building node SHOULD only use F5 blending where appropriate.

A blending in F5 is specified as follows:

f5Encoding = "(F5:" <passwordString> [ "," <PasswordString> ]* ")"

Whereas commas or backslashes in passwords MUST be escaped with a backslash. Closing brackets are treated as normal password character unless they are the last character of the encoding specification string.

6.2. Blending into an SMTP layer

Email messages containing messages MUST be encoded with Multipurpose Internet Mail Extensions (MIME) as specified in [RFC2045]. All nodes MUST support BASE64 encoding. A node MUST test all sections of a MIME message for the presence of a VortexMessage.

A vortex message is present if a block containing the peer key at the known offset of any MIME part decodes correctly.

A node SHOULD support SMTP blending for sending and receiving. For sending SMTP as specified in [RFC5321] must be used. TLS layers MUST always be applied when obtaining messages using POP3 (as specified in [RFC1939] and [RFC2595]) or IMAP (as specified in [RFC3501]). Any SMTP connection MUST employ a TLS encryption when passing any credentials.

6.3. Blending into an XMPP layer

For interoperability, an implementation SHOULD provide XMPP blending.

Blending into XMPP traffic is done using the [XEP-0231] extension of the XMPP protocol.

PLAIN and F5 blending is acceptable for this transport layer.

7. Routing

7.1. Vortex Message Processing

7.1.1. Processing of incoming Vortex Messages

An incoming message is considered unauthenticated at first. A node should consider a VortexMessage as authenticated as soon as the ephemeral identity is known and is not temporary.

For an unauthenticated message the following rules apply:

A message is considered authenticated as soon as the identity used in the header block is known and not temporary. A node MUST NOT treat a message as authenticated if the specified maximum number of replays have been reached. For authenticated messages the following rules apply:

A routing node MUST decrement the message quota by one if a received message is authenticated and contains at least one payload block. If a message is identified as duplicate according to the reply protection, a node MUST NOT decrement the message quota.

7.1.2. Processing of Routing Blocks in Workspace

A routing workspace consists of the following items:

The accounting layer typically triggers processing. It represents either a cleanup action or a routing event. A cleanup event deletes the following information from all workspaces:

Note that maxProcessTime reflects the number of seconds since the arrival of the last octet of the message at the transport layer facility. A node SHOULD NOT take additional processing time (e.g., for anti-UBE or anti-virus) into account.

The accounting layer triggers routing events. The trigger occurs at least minProcessTime after the last octet of the message arrived at the routing layer. A node SHOULD choose the latest possible moment in such a way that the peer node receives the last octet of the assembled message before maxProcessTime is reached. The calculation of the last point in time where a message may be set SHOULD always assume that the target node is working. A sending node SHOULD choose the time within these bounds randomly. An accounting layer MAY trigger multiple routing combos in bulk to further obfuscate the identity of a single transport message.

First, the processing node escapes the payload chunk at ID 0 if needed (non-special block starting with a backslash). Next, it executes all processing instructions of a routing combo in the sequence specified. If an instruction fails, the block at the target ID of the operation remains unchanged. The routing layer proceeds with the subsequent processing instructions, ignoring the error. For a detailed description of the operations see Section 7.4. If a node succeeds in building at least one payload chunk, a VortexMessage is composed and passed to the blending layer.

7.1.3. Processing of Outgoing MessageVortex Messages

The blending layer MUST then compose a transport layer message according to the specification provided in the routing combo. It SHOULD choose any decoy message or steganographic carrier in such a way that the dead parrot syndrome as specified in [DeadParrot] is avoided.

7.2. Header Requests

Header requests are control requests for the anonymization system. Messages with requests or replies only MUST NOT affect any quota.

7.2.1. Request New Ephemeral Identity

Requesting a new ephemeral identity is done by sending a message containing a header block with the new identity and an identity creation request (HeaderRequestIdentity) to a node. The node MAY send an error block (see Section 7.3.1) if rejecting the request.

If a node accepts an identity creation request, it MUST send a reply. To accept a request without a requirement, an accepting node MUST send back a special block containing "no error". To accept a block with a requirement, an accepting node MUST send a special block containing a requirement block.

7.2.2. Request Message Quota

Any valid ephemeral identity may request to raise the current message quota to a specific value at any time. The request MUST include a reply block in the header. The request may contain other parts. If a requested value is lower than the current quota, the node SHOULD NOT refuse the quota request and SHOULD send a "No Error" status.

A node SHOULD reply to a HeaderRequestIncreaseMessageQuota request (see Appendix A) of a valid ephemeral identity. The reply MUST include a requirement, an error message or a "No Error" status message.

7.2.3. Request Increase of Message Quota

A node may request to increase the current message quota. The increase is requested by sending a HeaderRequestIncreaseMessageQuota request to the routing node. The value specified within the node is the new quota. HeaderRequestIncreaseMessageQuota requests MUST include a reply block. A node SHOULD NOT use a previously sent MURB to reply.

If the requested quota is higher than the current quota, then the node SHOULD send a "no error" reply. If the requested quota is not accepted, the node SHOULD send a requestedQuotaOutOfBand reply.

A node accepting the request MUST send a RequirementBlock or a "no error block".

7.2.4. Request Transfer Quota

Any valid ephemeral identity may request to raise the current transfer quota to a specific value at any time. The request MUST include a reply block in the header. The request may contain other parts. If a requested value is lower than the current quota, the node SHOULD NOT refuse the quota request and SHOULD send a "No Error" status.

A node SHOULD reply to a HeaderRequestIncreaseTransferQuota request (see Appendix A) of a valid ephemeral identity. The reply MUST include a requirement, an error message, or a "No Error" status message.

7.2.5. Query Quota

Any valid ephemeral identity may request the current message and transfer quota. The request MUST include a reply block in the header. The request may contain other parts.

A node MUST reply to a HeaderRequestQueryQuota request (see Appendix A). The reply MUST include the current message quota and the current message transfer quota. The reply to this request MUST NOT include a requirement.

7.2.6. Request Capabilities

Any node MAY request the capabilities of another node. The capabilities include all information necessary to create a parseable VortexMessage. Any node SHOULD reply to any encrypted HeaderRequestCapability.

7.2.7. Request Nodes

A node may ask another node for a list of routing node addresses and keys. This request may be used to bootstrap a new node and to add routing nodes increasing the anonymization of a node. The receiving node of such a request SHOULD reply with a requirement (e.g., RequirementPuzzleRequired).

A node SHOULD reply to a HeaderRequest request (see Appendix A) of a valid ephemeral identity. The reply MUST include a requirement, an error message or a "No Error" status message.

7.2.8. Request Identity Replace

This request allows a receiving node to replace an identity with the identity provided in the message. This request is required if an adversary managed to deny the usage of a node (e.g., by deleting the corresponding transport account). Any sending node may recover from such an attack by sending a validly authenticated message to another identity providing the new transport and key details.

A node SHOULD reply to a such a request of a valid known identity. The reply MUST include an error message or a "No Error" status message.

7.3. Special Blocks

Special blocks are payload messages. They reflect messages from one node to another and are not visible to the user. A special block starts with the character sequence '\special' (or 5Ch 73h 70h 65h 63h 69h 61h 6Ch) followed by a DER encoded special block (SpecialBlock). Any non-special message decoding to ID 0 in a workspace starting with this character sequence MUST escape all backslashes within the payload chunk with an additional backslash.

7.3.1. Error Block

An error block may be sent as a reply where specified as a payload. The error block is embedded in a special block and sent with any provided reply block. Error messages SHOULD contain the serial number of the offending header block and MAY contain a human-readable text providing additional messages about the error.

7.3.2. Requirement Block

If a node is receiving a requirements block, it MUST assume that the request block has been accepted, has not been processed yet, and will be processed if the contained requirement is met. A node MUST process a request as soon as the requirement is fulfilled. A node MUST resend the request as soon as the requirement is met.

A node MAY reject a request, accept a request without a requirement, accept a request upon payment (RequirementPaymentRequired), or accept a request upon solving a proof of work puzzle (RequirementPuzzleRequired). Puzzle Requirement

If a node requests a puzzle, it MUST send a RequirementPuzzleRequired block. The puzzle requirement is solved, if the node receiving the puzzle is replying with a header block containing the puzzle block and the hash of the encoded block starts with the bit sequence mentioned in the puzzle within the period specified in the field 'valid'.

To solve a puzzle posed by a node a Vortex Message needs to be sent to the requesting node. This Vortex Message MUST contain a header block which includes the puzzle block and MUST have a MAC fingerprint starting with the bit sequence as specified in the challenge. A node calculates the MAC from the unencrypted DER encoded HeaderBlock with the algorithm specified by the node. To meet this requirement, a node adds a proofOfWork field to the HeaderBlock. Payment Requirement

If a node requests a payment, it MUST send a RequirementPaymentRequired block. As soon as the requested fee is paid and confirmed, the requesting node MUST send a "No Error" status message. The usage period 'valid' describes the period in which the payment may be carried out. A node MUST accept the payment if carried out within the 'valid' period but confirmed later. A node SHOULD return all unsolicited payments to the sending address.

7.4. Routing Operations

Routing operations are contained in a routing block and processed either on arrival on a message or when a compiling new message. All Operations are reversible. No Operation is available for generating decoy traffic. For decoy traffic, either encryption of an unpadded block may be used, or the addRedundancy operation.

All payload chunk blocks inherit the validity time from the message routing combos (arrival time + max(maxProcessTime)).

When applying an operation to a source block, the resulting target block inherits the expiry of the of the source block. When having multiple different expiry times, the expiry the furthest in the future will be applied to the target block. If the operation fails, the target expiry remains unchanged.

7.4.1. Mapping Operation

A mapping operation is a straightforward operation mainly used in inOperations of a routing block to map the routing block specific blocks to a permanent workspace.

7.4.2. Split and Merge Operations

The split and merge operations allow splitting and recombining message chunks. A node MUST adhere to these constraints:

An operation MUST fail if relative values are equal to, or below zero. An operation MUST fail if a relative value is equal to or above 100. All floating point operations must be carried out according to [IEEE754] and in 64-bit precision.

7.4.3. Encrypt and Decrypt Operations

Encryption and decryption are executed according to the standards mentioned before. An encryption operation encrypts a block symmetrically and places the result in the target block. The parameters MUST contain required parameters such as IV, padding, or cipher modes. An encryption operation without a valid parameter set MUST fail.

7.4.4. Add and Remove Redundancy Operations

The addRedundancy and removeRedundancy operations are the core operations of the protocol. They may be used to split messages and distribute message content across multiple routing nodes. The operation is split into three steps.

  1. Pad the input block to a multiple of the key block size in the resulting output blocks.
  2. Apply a Vandermonde matrix with the given sizes.
  3. Encrypt each resulting block with a separate key.

The following sections describe the order of the operations in an addRedundancy operation. For a removeRedundancy operation invert the functions and order. Padding Operation

A processing node calculates the final length of all output blocks including redundancy. This is done by L=roof((<input block size in bytes>+4)/<encryption block size in bytes>)*<block size in bytes>. The block is prepended with a 32-bit uint length indicator in bytes (little-endian). This length indicator i is calculated by i=<input block size in bytes>*randominteger()*L. The rest of the input block up to length L is padded with random data. A routing block builder SHOULD specify a PRNG and a seed to be used for this padding. If GF(16) is applied, all numbers are treated as little-endian representations. Only GF(8) and GF(16) are allowed fields.

For padding removal, first, the padding i at the start is removed as a little-endian integer. Then, the length of the output block is calculated by applying <output block size in bytes>=i mod <input block size in bytes>

This padding guarantees that each resulting block matches the block size of the subsequent encryption operation and does not require any further padding. Apply Matrix

Next, the input block is organized in a data matrix D of dimensions inrows,incols where incols=(<number of data blocks>-<number of redundancy blocks>) and inrows=L/(<number of data blocks>-<number of redundancy blocks>). The input block data is distributed in this matrix first across, then down.

Next, we multiply the data matrix D by a Vandermonde matrix V. The V matrix has the number of rows equal to the incols calculated, and columns are equal to the <number of data blocks>. The content of the matrix is formed by v(i,j)=pow(i,j), whereas i reflects the row number starting at 0, and j reflects the column number starting at 0. Please note that calculations noted here have to be carried out in the GF noted in the respective operation to be successful. The operation results in matrix A. Encrypt Target Block

Each row vector of A is a new data block which is then encrypted with the corresponding encryption key noted in keys of the addRedundancyOperation. If there are not enough keys available, the keys used for encryption are reused from the beginning after the last key has been used. A routing block builder SHOULD provide enough keys so that all target blocks may be encrypted with a unique key. All encryptions SHOULD NOT use padding.

7.5. Processing of Vortex Messages

The accounting layer triggers processing according to information contained in a routing block in the workspace. All operations MUST be executed in the sequence provided in the routing block. Any failing operation must leave the result block unmodified.

All workspace blocks resulting in IDs 1 to maxPayloadBlock are then added to the message and passed to the blending layer with appropriate instructions.

8. Accounting

8.1. Accounting Operations

The accounting layer has two major kinds of operations:

Implementations MUST provide sufficient locking mechanisms to guarantee the integrity of accounting information and workspace at any time.

8.1.1. Time-Based Garbage Collection

The accounting layer SHOULD keep a list of expiry times. As soon as an entry (e.g., payload block, or identity) expires, the respective structure should be removed from the workspace. An implementation MAY choose to remove expired items periodically or when encountering them during normal operation.

8.1.2. Time-Based Routing Initiation

The accounting layer MAY keep a list of any time a routing block is activated. For improved privacy, the accounting layer should use a slotted model where, whenever possible, multiple routing blocks are handled in the same period of time, and the requests to the blending layers are mixed between the transactions.

8.1.3. Routing Based Quota Updates

A node MUST update quotas on the respective operations. It MUST decrease message quota before processing routing blocks in the workspace. A node MUST decrease the message quota after the processing of any header requests.

8.1.4. Routing Based Authorization

The transfer quota MUST be checked and decreased by the number of data bytes in the payload chunks after an outgoing message is processed and fully assembled. The message quota MUST be decreased by one on each routing block triggering the assembly of an outgoing message.

8.1.5. Ephemeral Identity Creation

Any packet may request the creation of an ephemeral identity. A node SHOULD NOT accept such a request without a costly requirement. The request includes a lifetime of the ephemeral identity. The costs for creating the ephemeral identity SHOULD raise if a longer lifetime is requested.

9. Acknowledgments

Thanks go to my family which did support me with patience and countless hours and to Mark Zeman for his feedback challenging my thoughts and peace.

10. IANA Considerations

This memo includes no request to IANA.

Additional encryption algorithms, paddings, modes, blending layers, or puzzles MUST be added by writing an extension to this or a subsequent RFC. For testing purposes, IDs above 1,000,000 should be used.

11. Security Considerations

The MessageVortex protocol may be understood more as a toolset than a fixed product. Depending on the usage of the toolset anonymity and security are affected. For a detailed analysis see [MVAnalysis].

The primary goals for security within this protocol did rely on the following focus areas:

All these factors are affected by the usage of the protocol. The following sections provide a list of factors affecting the primary goals.

The Vortex protocol does not rely on any encryption on the transport layer. Vortex messages are already encrypted. Confidentiality is not affected by the protection mechanisms of the transport layer.

If a transport layer supports encryption, a Vortex node SHOULD use it to improve the privacy of the message.

Anonymity is affected by the inner workings of the blending layer in many ways. A Vortex message cannot be read by anyone except the peer nodes and the routing block builder, but the presence of a vortex node message may be detected. This may be done either by detecting the typical high entropy of an encrypted file, broken structures of a carrier file, a meaningless content of a carrier file, or the contextless communication of the transport layer with its peer partner. A blending layer SHOULD minimize the possibility of easy detection by minimizing these effects.

A blending layer SHOULD use carrier files with high compression or encryption. Carrier files SHOULD NOT have inner structures so that the payload is comparable to valid content. To achieve undetectability by a human reviewer, a routing block builder should use F5 blending instead of PLAIN blending. This, however, increases the protocol overhead roughly by a tenfold.

The two layers 'routing' and 'accounting' have the deepest insight into a Vortex message's inner working. They know the immediate peer sender and the peer recipients of all payload chunks. As decoy traffic is generated by combining chunks and applying redundancy calculations upon them, a node can never know whether a malfunction (e.g., when doing a recovery calculation) was intended or not. Therefore a node is unable to tell a failed transaction apart from a terminated transaction. It furthermore cannot tell content apart from decoy traffic.

A routing block builder SHOULD follow the following rules in order not to compromise a Vortex message's anonymity:

An active adversary cannot use blocks from other routing block builders for his purposes. He may falsify the result by injecting wrong message chunks or by not sending a message. Such message disruptions may be detected by intentionally routing some information to the routing block builders' node. If the Vortex message does not carry the information expected the node may safely assume that one of the involved nodes is misbehaving. A block building node MAY calculate reputation for involved nodes over time. A block building node MAY build redundancy paths into a routing block to withstand such malicious nodes.

Receiver anonymity is in danger if the handling of message header and content is not done with care. An attacker might send a bugged message (e.g., with a DKIM or DMARC header) to deanonymize a recipient. Great care has to be taken when handling any other than local references when processing, verifying, or rendering a message.

12. References

12.1. Normative References

[CCITT.X208.1988] International Telephone and Telegraph Consultative Committee, "Specification of Abstract Syntax Notation One (ASN.1)", CCITT Recommendation X.208, November 1998.
[CCITT.X680.2002] International Telephone and Telegraph Consultative Committee, "Abstract Syntax Notation One (ASN.1): Specification of basic notation", November 2002.
[EAX] Bellare, M., Rogaway, P. and D. Wagner, "The EAX mode of operation", 2011.
[F5] Westfeld, A., "F5 - A Steganographic Algorithm - High Capacity Despite Better Steganalysis", October 2001.
[FIPS-AES] Federal Information Processing Standard (FIPS), "Specification for the ADVANCED ENCRYPTION STANDARD (AES)", November 2011.
[IEEE754] IEEE, "754-2008 - IEEE Standard for Floating-Point Arithmetic", August 2008.
[ISO-10118-3] International Organization for Standardization, "ISO/IEC 10118-3:2004 -- Information technology -- Security techniques -- Hash-functions -- Part 3: Dedicated hash-functions", March 2004.
[MODES] National Institute for Standards and Technology (NIST), "Recommendation for Block Cipher Modes of Operation: Methods and Techniques", December 2001.
[RFC1423] Balenson, D., "Privacy Enhancement for Internet Electronic Mail: Part III: Algorithms, Modes, and Identifiers", RFC 1423, DOI 10.17487/RFC1423, February 1993.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3610] Whiting, D., Housley, R. and N. Ferguson, "Counter with CBC-MAC (CCM)", RFC 3610, DOI 10.17487/RFC3610, September 2003.
[RFC3657] Moriai, S. and A. Kato, "Use of the Camellia Encryption Algorithm in Cryptographic Message Syntax (CMS)", RFC 3657, DOI 10.17487/RFC3657, January 2004.
[RFC3686] Housley, R., "Using Advanced Encryption Standard (AES) Counter Mode With IPsec Encapsulating Security Payload (ESP)", RFC 3686, DOI 10.17487/RFC3686, January 2004.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008.
[RFC5288] Salowey, J., Choudhury, A. and D. McGrew, "AES Galois Counter Mode (GCM) Cipher Suites for TLS", RFC 5288, DOI 10.17487/RFC5288, August 2008.
[RFC5958] Turner, S., "Asymmetric Key Packages", RFC 5958, DOI 10.17487/RFC5958, August 2010.
[RFC7253] Krovetz, T. and P. Rogaway, "The OCB Authenticated-Encryption Algorithm", RFC 7253, DOI 10.17487/RFC7253, May 2014.
[RFC8017] Moriarty, K., Kaliski, B., Jonsson, J. and A. Rusch, "PKCS #1: RSA Cryptography Specifications Version 2.2", RFC 8017, DOI 10.17487/RFC8017, November 2016.
[SEC1] Certicom Research, "SEC 1: Elliptic Curve Cryptography", May 2009.
[TWOFISH] Schneier, B., "The Twofish Encryptions Algorithm: A 128-Bit Block Cipher, 1st Edition", March 1999.
[XEP-0231] Peter, S. and P. Simerda, "XEP-0231: Bits of Binary", September 2008.

12.2. Informative References

[DeadParrot] Houmansadr, A., Burbaker, C. and V. Shmatikov, "The Parrot is Dead: Observing Unobservable Network Communications", 2013.
[KAnon] Ahn, L., Bortz, A. and N. Hopper, "k-Anonymous Message Transmission", 2003.
[MVAnalysis] Gwerder, M., "MessageVortex", 2018.
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996.
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC 2595, DOI 10.17487/RFC2595, June 1999.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, DOI 10.17487/RFC5321, October 2008.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120, March 2011.

Appendix A. The ASN.1 schema for Vortex messages

The following sections contain the ASN.1 modules specifying the MessageVortex Protocol.

A.1. The main VortexMessageBlocks

A.2. The VortexMessage Ciphers Structures

A.3. The VortexMessage Replies Structures

A.4. The VortexMessage Requirements Structures

A.5. The VortexMessage Helpers Structures

A.6. The VortexMessage Additional Structures

Author's Address

Martin Gwerder University of Applied Sciences of Northwestern Switzerland Bahnhofstrasse 5 Windisch, AG 5210 Switzerland Phone: +41 56 202 76 81 EMail: