Experiment: mmmv_symsig_t1
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Currently the mmmv_symsig_t1 lacks any code.

The mmmv_symsig_t1 wraps symmetric key encryption command line tools and implements a signing system, where parties, who have never met directly for a key exchange can probabilistically authenticate each other. Unlike the many public key encryption systems, the mmmv_symsig_t1 can be used by using only symmetric key encryption algorithms, which avoids the fundamental flaw that all public key encryption systems have. The feasibility of using one-time-pads or one-time-pad like ciphers is based on the fact that a year 2017 price for 1GiB of USB-stick based flash memory costs about 1€. With the exception of metadata related security issues, one-time-pad like ciphers tend to eliminate security flaws at the encryption algorithm side. In addition to the signing system, the mmmv_symsig_t1 allows swarms of robots with billions of robots to establish probabilistically secure channels without the need to upgrade the set of keys of older robots every time a new batch of new robots joins the swarm.

It's worth to note that if both keys of a public encryption algorithm are bundled together and the whole pair is kept secret the way  symmetric keys are kept secret, then public key encryption algorithms can be used as symmetric key encryption algorithms. That allows the "standard" tools like the GNU Privacy Guard to be used in the role of the symmetric key encryption algorithm implementation.

Definition of some Terms

In the context of the current document onionencryption stands for an encryption activity, where 

    ciphertext = encrypt_with_key_N( 
                         ) // N-2
                     )     // N-1
                 )         // N

The main thing to note about the onionencryption is that all of the keys, 1..N, are needed for decrypting the ciphertext.

The Scheme

End users(hereafter: EnU), including the Bob and the Alice, individually meet with a key exchange service provider (hereafter: KXS). The KXS gives each EnU multiple GiB worth of symmetric keys that are shared only between the KXS and the EnU. If the EnUs have not met with each other for a key exchange, then the KXS forms a central hub that decrypts the ciphertext of one EnU and encrypts the cleartext for another EnU. That is to say, if the Bob and the Alice use only a single KXS, then the KXS acts like the Eve, when the Eve conducts a man-in-the-middle attack. To probabilistically counter the man-in-the-middle attack, the Bob and the Alice use the services of multiple KXS to agree on a set of temporary encryption keys, one temporary key per one KXS. The Bob and the Alice use the set of temporary keys for onionencryption of their actual data exchange. If at least 2 KXS-es manage to keep the keys that they use for communicating with the Alice and with the Bob a secret and those same 2 KXS choose to keep the overheard temporary keys a secret, then absolutely no KXS is able to decrypt the Bob's and the Alice's onionencrypted session.

Optional Improvement Opportunities

In addition to the temporary keys that were overheard by the KXS, the Bob and the Alice may use additional temporary keys that they negotiated during previous sessions. During the session that is held through the eavesdropping KXS the Alice should generate halve of  each temporary key and the Bob should generate the other halve of each temporary key.

Worldwide Scaling

If KXSs of one town, for example, Tallinn, register themselves as users at KXSs of another town, for example, Berlin, by directly exchanging keys with the other town's KXSs, then the users of Tallinn KXSs can have a probabilistically secure onionencrypted session with the Berlin KXSs and the users of the Tallinn KXSs can register themselves as "2. class users" of the Berlin KXSs. "1. class users" of a KXS are only those users, who exchange the keys with a KXS directly, without jumping through any other onionencrypted session. If New York KXSs have been registered as 1. class users at the Berlin KXSs, then the 1. class users of Tallinn KXSs can register themselves as 2.+1.=3. class users of New York KXSs. If 7. class users of Tallinn KXSs were using the Berlin route for registering themselves at the New York KXSs, then they would become 7.+1.=8. class users of the Berlin KXSs and 8.+1.=9. class users of the New York KXSs. If London KXSs are 4. class users at the Berlin KXSs, then 3. class users of Tallinn KXSs can register themselves as 3.+1.=4. class users at the Berlin KXSs and 4.+3.=7. class users of London KXSs. 

The class of a single key that is used for a session between a KXS and its single user, EnU, equals with the class of that EnU at that KXS. The class of a user that uses an onionencrypted session for communicating with the KXS equals with the highest class of a single  key that is part of the set of keys that is used for onionencrypting the session between the EnU and the KXS. 

(Only 1. class EnUs don't use onionencryption for communicating with the KXS, where they are registered as a 1. class EnU. The same EnU can have different classes at different KXSs. One way to memorize the meaning of the class of a user at a given KXS is that the class of the user at the given KXS can be seen as the "cost" of reaching that KXS. The smaller the privacy, the higher the cost, specially if the EnU gets caught by "1984 authorities". Due to the fact that a "shortcut" can be more costly than "taking more jumps", the class is NOT a metric distance.) 

Beneficial side Effects 

The more KXSs there are, the more break-ins have to be conducted to get all the keys that the Alice and the Bob use for communicating with the KXSs. If all of the sessions between the KXSs and their users are decrypted by eavesdroppers other than the KXSs themselves, then an increase of the number of KXSs increases the number of sessions that the non-KXS-eavesdroppers have to listen in, which in turn increases the number of different geographical locations, where the non-KXS-eavesdroppers need to place their probes. If the KXSs reside at different "jurisdictions" that happen to be enemies, for example, China, Russia, EU, United States, Latin America regions, Arabic dictatorships, etc. then the lack of intelligence sharing between those "jurisdictions" partly protects the Alice-Bob onionencrypted session from absolutely everybody, except the session related metadata, but the metadata can be "salted" by using temporary keys from previous onionencrypted sessions and by choosing the KXSs  randomly for every onionencrypted session. The more KXSs there are, the greater the number of different operating systems and hardware the set of KXSs uses. The greater the variety of operating systems and hardware that the KXSs use, the more elaborate must be the software that is needed for breaking into KXSs systems. Some extra variety within the KXSs can be introduced relatively cheaply by using FPGA based embedded boards

For high security setups the KXSs core functionality might be implemented as a microcontroller board that does all the encryption and decryption and stores all of the keys at an USB flash drive that is directly connected to the microcontroller board. The microcontroller board can be connected to the internet through GPIO of some Raspberry Pi like computer, which acts as a "network card with all internet communication related software included". Since the microcontroller software is remarkably primitive, then there's practically no attack surface there and the best that any remote adversary can do is to break into the Raspberry Pi, but the Raspberry Pi acts only as a fancy network card and resides outside of the secure zone. Different KXSs can use different encryption algorithms for communicating with the Alice and the Bob. That allows the use of encryption algorithms that are usable on the microcontroller board. The microcontroller software is probably small enough to allow it to be fully formally verified. As specifications and real hardware can differ, testing is relevant even after formal verification. To reduce the number of untested states, the microcontroller board can have a dedicated microcontroller for enforcing power resets that happen not less than X times per 10 minutes, preferably at the end of each KXSs session.

Who Might want to use such a System

Probably only freelancers and privacy advocates that have the monetary means to physically meet with the KXSs will use the system. The travelling costs that come from the need to physically meet with the KXSs to get class 1 user accounts hinders worldwide adoption by free software developers. Not everybody is able to travel everywhere even, when money is not an issue(read: the Edward Snowden case, visas, etc.). Probably the first adopters of this system are robot swarm developers, Skynet developers, because the number of edges at a complete graph that has n vertices is 

   n*(n-1)/2 = (n^2-n)/2

and the mmmv_symsig_t1 allows billions of robots to establish probabilistically secure channels without the need to upgrade the set of keys of existing robots whenever a batch of new robots is added to the swarm.

Megacorporations  want to spy on their salary slaves, generally known as "employees", and the mmmv_symsig_t1, if properly implemented and used, makes spying more difficult. Banks that use checks in stead of pin calculators in 2017 certainly could not care less about securing their IT-systems and business processes. Medical institutions resort to comfort, because according to them the "public key cryptography works just fine" and many hospitals and private practices have a pretty crappy IT-support to start with, not to mention that doctors and nurses would rather do their actual work, wear bloody gloves, than do computerized "paperwork". Supermafiosi, "states", including the Estonian supermafia, won't use the mmmv_symsig_t1 either, because they are satisfied with the public key cryptography based legacy mess that they have adopted before the specification of the mmmv_symsig_t1 was even created. Small businesses that are geographically relatively close, can exchange keys directly with each other and geographically distant small businesses don't have the monetary means to meet with the KXSs that are out of their town, which limits them to higher EnU classes.

All in all, the system seems to be most relevant to robot swarm developers and IoT developers.