DOKK Library

Education in [Block]Chains

Authors Jonathan A. Poritz

License CC-BY-SA-4.0

                                            Education in [Block]Chains

                                                       Jonathan A. Poritz

                                                Center for Teaching and Learning and
                                               Department of Mathematics and Physics
                                                  Colorado State University-Pueblo

                                                      10 June 2019, Domains 2019

                              This work is released under a Creative Commons Attribution-ShareAlike 4.0 International License..

                    [Note: there is no need to write down URLs or otherwise take specific notes – these slides are shared at

                   ... and even that URL will come around again at the end!]

Poritz                      Education in [Block]Chains                           Domains 2019: 10 June 2019   1 / 26
A Theme of the Indie/Open Edtech World ...

    Edtech which does not center on the
    autonomy and agency of student and
    instructor is only a tool to turn us
    all into better – more docile – factory
    robots and consumers.


                                                                 2                                                                 3

      Jean Marc Cote (if 1901) or Villemard (if 1910) [Public domain], via
Wikimedia Commons, in XXI Century. School.jpg
      Publicity photo of Charlie Chaplin for the film Modern Times (1936); [Public domain in the United State (maybe not in other jurisdictions!) since it
was published between 1924 and 1977 without a copyright notice], via Wikimedia Commons, - Modern Times.jpg
      Planking in the supermarket by TheeErin from, released under a CC-BY-SA 2.0 license.
    Poritz                       Education in [Block]Chains                          Domains 2019: 10 June 2019          2 / 26
... Which Drives Much Activity – Dangerous Activity?

   • digital literacy education,
   • instructors using tools like to shape a web that servers their pedagogy,
   • Domain of One’s Own projects,
   • etc.
But the Web can be a horrific place! So....
       Giving students and instructors autonomy and agency on the World Wild Web
       without some knowledge of security and privacy is like giving a new driver the
       car keys and a map of the big city without teaching them how to use their seat
That’s my hobby horse, and I intend to ride it:


Today’s discussion about Blockchain is a case study in this approach.

       Hobby Horse by Barney Moss via Wikimedia Commons horse (6012490195).jpg, released under a CC-BY
2.0 license.
    Poritz                  Education in [Block]Chains                       Domains 2019: 10 June 2019      3 / 26
Origins of the Blockchain Whirlwind: Bitcoin

 In October 2009, the pseudonymous
 Satoshi Nakamoto posted this paper
 to a cryptography mailing list:

 He or she shortly thereafter also
 posted, to a FLOSS sharing site,
 source code for a reference
 implementation of the proposed

        At which point, Yoda could be heard muttering, “Begun, the Bitcoin War has.”

      the full paper is here:
  Poritz               Education in [Block]Chains   Domains 2019: 10 June 2019   4 / 26
Some Bitcoin Hype

                                            Price in USD of one


       Also nonsense like

      from, here.
  Poritz      Education in [Block]Chains   Domains 2019: 10 June 2019   5 / 26
A Fly in the Ointment Here, a Few Megatons of CO2 , There



For scale, that basically amounts to the total power consumption of Austria [the country].

      from, here.
  Poritz   Education in [Block]Chains   Domains 2019: 10 June 2019   6 / 26
Diving Into the Deep End: How a Blockchain Works
Why does it consume so much power? I thought you’d never ask.
To answer, we must talk about how bitcoin works. Actually, the underlying technology is
called a blockchain, so we’ll talk for a few minutes about How a blockchain works.
Before you stick you fingers in your ears and start humming nervously, let’s remember
that almost everyone in this room has driven a car on the highway, where a lapse of
attention for a fraction of a second could have fatal consequences. And you all have
complex, sophisticated disciplinary knowledge. So bear with me for ten fucking minutes.
A blockchain is built out of three basic pieces:
  • digital signatures,
  • a cryptographic hash function, and
  • a distributed consensus protocol.

You’re probably already sweating... but chill: every academic discipline and every
commercial industry has terms of art. The above three pieces are basically
  • really distinctive personal style [for digital files],
  • a very effective blender [that grinds up digital files], and
  • an method for group decision-making [for groups that meet only on the ’net, never
    in person].
  Poritz   Education in [Block]Chains   Domains 2019: 10 June 2019   7 / 26
References for Cryptology

Crypto tends to intimidate users, because they hear that small mistakes can be fatal. But
that is also true of many things in life, and we don’t let it stop us.
The other problem with crypto is that it is fairly mathy. Which is a feature, not a bug ...
to a mathematician, but maybe not to others.8
The good news is that there are great free/open resources on the ’net. For example:
        Chapter 4 of my open textbook Yet Another Introductory Number Theory
        Textbook [YAINTT], which can be found at
        [although, being a math textbook, this is rather unabashedly mathy – even though
        that Chapter has a lot of history and terminology.]; or
        Ed Felton’s [Princeton professor of computer science and Deputy Chief Technology
        Officer under Obama] Nuts and Bolts of Encryption: A Primer for Policymakers,
        found at felten/encryption primer.pdf, which is not
        mathy at all.

      because the mathematical community has done a terrible job of sharing the joy and beauty of our subject!
  Poritz                      Education in [Block]Chains                         Domains 2019: 10 June 2019   8 / 26
Our protagonists, and an adversary

Most works of cryptology speak of two star-crossed lovers, Alice and Bob, who attempt
to keep the guttering candle of their love alight, though distance separates them and
their communications are being monitored by the evil Eve.

[Extra credit if you can name the two famous mathematicians who acted as models for
these pictures of Alice and Bob.]

  Poritz   Education in [Block]Chains   Domains 2019: 10 June 2019   9 / 26
Why is Eve so powerful?
It’s important to realize that in many – maybe most – situations, it is entirely
appropriate to assume that Eve can see all the communication between Alice and Bob
while it is in transit. All of the channels you are used to using suffer from this:
      A cell phone is basically a walkie-talkie with infrastructure [the infrastructure being
      all those cell towers all over the place]. Anyone with a radio receiver of the right
      type who is within the footprint of the same tower can hear the entire exchange.
      Satellite phones are much worse: the footprint is the size of a continent, often.
      Anything you do on the Internet is essentially public. [Ask your millennial students
      how the basic Internet Protocol works – you will be surprised/horrified at what they
Here is a diagram with some basic terminology:

  Poritz   Education in [Block]Chains   Domains 2019: 10 June 2019   10 / 26
Keys [for symmetric cryptosystems]

If we are to publish our encryption and decryption algorithms, the security must lie in
some other secret. This is an additional piece of information called the key, which is
input into those algorithms, as follows:

  Poritz   Education in [Block]Chains   Domains 2019: 10 June 2019   11 / 26
Notes on symmetric cryptosystems
The above is called symmetric (or private- or secret-key) cryptography. We shall see
an alternative in a few minutes.
    Both the encryption ek and decryption dk use the same key k, which must be shared
    in some private, pre-lapsarian moment. The keyspace K must be large, otherwise
    Eve can just try all keys and see which works [which is called a brute-force attack].
    Symmetric cryptosystems are fast you can run a video stream through one without
    noticing it on a consumer-grade PC.
    The design of symmetric cryptosystems is something of a black art. There is little
    general theory on the attack or defense side, and the algorithms tend just to be
    along the lines of scramble the bits a lot.
    Some examples:
         The Syctale – ancient Greece
         The Caesar cipher – actually used by Julius Caesar. [addition mod 26...]
         The Vigenère Cipher – thought to be unbreakable for centuries. Easy to break
         The one-time pad – completely unbreakable; hard to use in practice (but see
         Leo Marks’ Between Silk and Cyanide: A Code Maker’s War 1941-45)
         The Enigma machine – a German military coding device from WWII.
         Modern block ciphers like DES, triple-DES, AES, etc.
  Poritz   Education in [Block]Chains   Domains 2019: 10 June 2019   12 / 26
Asymmetric cryptosystems
If Alice and Bob want to be able to communicate securely without ever having met to
exchange the symmetric key, they can instead use asymmetric (or public-key)

That this is possible at all is very cool. There are a few ways we do it, now, including
RSA (named after Ron Rivest, Adi Shamir, and Leonard Adelman, who published this
idea in 1977) and elliptic curves (which are more efficient but less commonly used, since
their mathematics is significantly harder to chew that what is behind RSA).
All asymmetric crypto relies upon a mathematical function which is easy to compute in
one direction but difficult to invert. For RSA, this is essentially multiplication forward
[easy], but factoring backwards [hard]. For other asymmetric algorithms, there are other
of these one-way functions.
  Poritz   Education in [Block]Chains   Domains 2019: 10 June 2019   13 / 26
The “Man-in-the-middle attack”

A significant issue with asymmetric cryptosystems is a Public-key Infrastructure [PKI],
because of the dreaded man-in-the-middle attack:

  Poritz   Education in [Block]Chains   Domains 2019: 10 June 2019   14 / 26
Digital Signatures

Therefore, we need to be sure that the public keys we use really do belong to the people
who we think they do. We do this either by getting the key from someone in person –
but that kind of ruins the whole idea of asymmetric crypto! – or we get a key in some
way that we are sure of its provenance. One way to be sure would be to have a digital
signature on the public key, signed by someone whom we trust. Signatures work like this:

  Poritz   Education in [Block]Chains   Domains 2019: 10 June 2019   15 / 26
How to Think Critically About Security/Privacy/Cryptography

Things to think about when learning and critically assessing cryptological gizmos:
  • What exact problem was it designed to solve?
     ◦ What [exactly] are the inputs and outputs.
     ◦ What [precise] assumptions are made about who knows what, when?
     ◦ What [precise] assumptions are made about how the adversary will try to break
       the system – what is the threat [or attack] model?
     ◦ What computational power is available to the allies and to the adversaries?
  • How confident is the community in the effectiveness of the proposed technique?
      ◦ Is there a mathematical proof, based on just mathematical truths.
      ◦ Is there a mathematical proof that breaking the proposed technique would be
        equivalent to solving some well-studied problem?
      ◦ Does the security of the proposed technique rest on some engineering factors –
        e.g., is there a way known to break the technique if we could be a certain new
        kind of computer?
and, often the most important issue by far:
  • How reasonable are the assumptions in a real-world context where we want to use
    this new technique?

  Poritz   Education in [Block]Chains   Domains 2019: 10 June 2019   16 / 26
E.g., Thinking Critically About Digital Signatures

The problem: Bob has a secret key with corresponding public key that is reliably known
to the whole community as being his. He also has a message he wants to sign.
Bob will transmit to Larry the message along with an extra piece of data, the digital
signature. Larry can use the widely known public key associated to Bob, the message,
and the signature data to determine if only someone who knew Bob’s secret key could
have produced that signature on that message.
In the case of RSA-based digital signatures, there is a mathematical proof that breaking
this signature scheme would amount to figuring out how to factor large numbers quickly,
at which many mathematicians have been unsuccessful for many years.
There is a way to factor large numbers if engineers can build a quantum computer. This
used to be thought of as far in the future, if ever even possible; now we think it might be
25 years or fewer off. So it will be possible to forge RSA signatures in ≤ 25 years.9
In the real world, it is very hard to keep a secret key secret forever. It is also very hard to
associate public keys to a public identity reliably. Therefore a robust PKI is a essential
for digital signatures to work in practice, and it is hard to make one and keep it healthy.

      Actually, almost all digital signature schemes fall to quantum computers, not just RSA-based ones.
  Poritz                       Education in [Block]Chains                  Domains 2019: 10 June 2019   17 / 26
Cryptographic Hash Functions

A cryptographic hash function is an algorithm which takes arbitrarily large chunks of data
as input, and produces a hash [sometimes called a digest] of a certain, fixed size – e.g.,
the most widely used cryptographic hash function today is called SHA-256 and it always
produces hashes consisting of 256 bits, no matter how big the input.
A hash function is called pre-image resistant if it is very hard10 given any particular
possible hash value, to find an input data chunk whose digest has that value.
A hash function is called second pre-image resistant if it is very hard, given one input
data chunk, to find a second which hashes to the same digest as that chosen first chunk.

Basically, a hash function is a blender into which you pour data, which blends all its input
thoroughly and produces a small output pellet. The pellet is almost unique to that set of
ingredients, in the sense that it would take many lifetimes of the universe for you to find
another set of input ingredients which would yield the same output pellet.

The problem is described above. Known hashing algorithms are quite complex and have
little mathematical structure. Therefore, it is very hard to attack them... but, also, there
are not any proofs that they do what we hope they will. Old hash functions, for example,
are no longer used because we now know how to break them.
       meaning that, essentially, one should just have to keep trying random inputs until one gets lucky.
   Poritz                        Education in [Block]Chains                 Domains 2019: 10 June 2019   18 / 26
Hash Functions As Message Digests and For Chains

Suppose I send you a big file and we want to be sure it didn’t get corrupted in transit. I
could compute a hash of that data on my end, you could do the same on yours, and I
could read you the digest over the phone – if they agree, we’d be quite certain the files
agree as well.

You can use hash functions to make immutable chains. Suppose we all agree on some
block of starting data, called the genesis block in the blockchain community. Then we
start attaching new blocks to the list of official, on-chain blocks, by some sort of
distributed agreement process, and also putting the hash of the previous block into each
new block on the chain – this is called a hash chain. If, afterwards, we have a dispute
about what the whole chain is, we could each run down the whole chain, checking that
each hash value in the nth block is equal to the hash of the (n − 1)st block. Since it is
hard to find second-preimages for good hash functions, this is impossible to forge, and
thus it is impossible to change the old blocks on an existing chain: hash chains are

  Poritz   Education in [Block]Chains   Domains 2019: 10 June 2019   19 / 26
Hash Functions to Slow Computers Down

Suppose you want to slow someone down. You could had them a chunk of data and ask
them to find some other piece of data to add to that such that when they hashed the
big, joined data chunk, the output hash would end in at least five (or seven or ten or
whatever, depending upon how much you want to slow them down) 0s.
You couldn’t ask for them to find data which will make the hash value exactly equal
some given value, but instead this task of working to get a certain number of 0s can be
done, only with a lot of computation, by just trying over and over again.
The bitcoin protocol uses this method to slow down – and randomly assign a winner,
whoever found the second piece of data which makes the whole thing have a hash ending
in a certain number of 0s – all of the people working around the globe to verify
transactions made on the bitcoin network. The number of 0s needed to win this
competition can be changed as the total amount of computational power on planet Earth
dedicated to doing this (stupid, useless, but un-short-cut-able) work increases, so that
even with all that power, there will only be winner about once every ten minutes.
The total hashing power on planet Earth right now can do about 55 million trillion
SHA-256 hashes per second – that’s 5.5 × 1019 hashes per second. Much if this is in
special hardware in hashing farms in China....

  Poritz   Education in [Block]Chains   Domains 2019: 10 June 2019   20 / 26
Distributed Consensus
Suppose you have a block of transactions – maybe people submitting records of buying or
selling goods by transferring some artificial cryptocurrency among users – you want to
submit to a global, public ledger. It is important that everyone agrees upon what are the
valid blocks, because otherwise a bad actor could spend some cryptocurrency on one
transaction in one block, and the same cryptocurrency in another transaction with
someone else: this is the double-spend problem.
The easiest way for this to work is if there is a central party who collects all proposed
blocks and simply decides which ones will go on the chain, perhaps by
first-come-first-served, perhaps by playing favorites (or so people fear). Such a central
decision-maker is called a trusted third party [TTP] by cryptologists. Distributed
consensus is very easy, fast, and efficient with a TTP.
Without a TTP, as bitcoin tries to go, much more work is required. The bitcoin approach
is to make everyone do the “hash to get a certain number of 0s” game, and then publish
their winning extra bit of junk data. Whoever wins can prove it, with that data, and gets
to submit their preferred block to the global public ledger.
Sometimes, two people will win at the same time in different places, and the chain will
split. But if half of the network works on growing one chain and half works on the other,
pretty quickly it will be very likely that one will be quite a bit longer than the other, and
everyone will go over to the longer valid chain at that point.
  Poritz   Education in [Block]Chains   Domains 2019: 10 June 2019   21 / 26
Wrapping Up The Blockchain Bow

The term blockchain is used to describe a hash chain whose globally recognized
individual blocks are agreed upon by a distributed consensus protocol.
Generally, those who can submit blocks come from a very open group – potentially
anyone 11 – and follow some internal rules such as for maintaining a cryptocurrency ledger
or running an on-chain Turing-complete programming language [Ethereum, a company
based in the Crypto Valley, calls these on-chain programs smart contracts in an
essentially meaningless bit of excellent marketing].
Another differentiator between different blockchains is how the individuals in the group of
participants identify themselves. Generally, the choice is with some identity strictly
coupled (hopefully) to a real-world individual or entity or one where individuals act
through pseudonymous (hopefully12 IDs. Both require some kind of reliable PKI, however!
Different blockchains use different consensus protocols: bitcoin’s proof of work has a
horrible carbon cost, is slow, etc. Another protocol is based on proof of stake, which is
less energy-intensive but amounts to formalizing a strategy of the rich always get richer.

      Some people restrict this group and then talk about permissioned blockchains, but I think that is pretty much the same as freeze-dried water.
      This has turned out to be much harder that it seemed it would be, e.g., in bitcoin’s case.
  Poritz                        Education in [Block]Chains                         Domains 2019: 10 June 2019         22 / 26
Thinking Critically About Blockchains
Whatever particular version of a blockchain, the solution it provides [in the spirit of our
Thinking Critically About Security... strategy above] involves these key features:
  • A chain of linked, sequential public records.
  • This [hash] chain is immutable.
  • Individuals participate in the blockchain through either pseudonymous or
    intentionally real world-linked IDs supported by a robust PKI.
  • The progress of the blockchain is realized by a distributed consensus protocol which
    avoids putting its trust in any particular [third] party.
In short, blockchains are characterized as
   • public
   • immutable
   • relying on a robust PKI
   • eschew TTPs.
The constituent cryptographic pieces that realize these characteristics may or may not be
generally thought of as reliable, but the ways these blocks are combined is not much
subject to debate.
Whether the above characteristics are reasonable assumptions in any particular real-world
context is much more questionable....
  Poritz   Education in [Block]Chains   Domains 2019: 10 June 2019   23 / 26
Blockchains ... Of Limited Use?

The integrity of the bitcoin ledger is based on a huge amount of (useless) work, needed
because cryptocurrency fanatics don’t trust the government: it’s all about the TTP.
Now what about other use-cases?
Educational institutions should “put student credentials on the blockchain:” it’s
public, immutable, and not subject to any single party’s control.
But this is nonsense. There is a natural TTP for a credential: the issuing institution!
OK, we need a functioning PKI, but so does a blockchain-based credential system (the
credential would have to be signed, in its form “on the chain,” as well).
Walmart will “put its supply chain on the blockchain.”
But this is nonsense. There is a natural TTP for a Walmart’s supply chain: Walmart
trusts Walmart!
Marijuana producers in Colorado will put their production records on a blockchain
so that the state marijuana oversight organization will be able to check that it has all
been produced according to state law.
But this is nonsense. There is a natural TTP here as well: that state supervisory agency!
Also, just having a cryptographically verified blockchain doesn’t mean that the data
entered in its blocks corresponds correctly with the real world: this is a misreading of the
idea of “trust” here.
  Poritz   Education in [Block]Chains   Domains 2019: 10 June 2019   24 / 26
The Moral(s)

Use your critical thinking skills even with the very mathy cryptology, privacy, and
One outline of where to get started in that process is the basics of
  • What exact problem was it designed to solve?
  • How confident is the community in the effectiveness of the proposed technique?
  • How reasonable are the assumptions in a real-world context where we want to use
    this new technique?
and particularly the sub-parts of those questions as mentioned before.
You can think through those questions with a very low level of mathiness, and be highly
effective in understanding proposed cryptology/security/privacy approaches in context.
Get some comfort level with basic crypto/security/privacy, it’s your best seat belt on
the Information Superhighway. Share that knowledge and comfort (or at least
willingness to struggle for comfort) with your students, otherwise what kind of horrible
mentor are you to people about to drive those dangerous ways?

  Poritz   Education in [Block]Chains    Domains 2019: 10 June 2019   25 / 26
Questions, Comments, and Contact Info

Questions? Comments?
Also feel free to contact me at .
Get these slides at and all files for remixing13 at .
If you don’t want to write down that full URL, just remember
  or [then click Always SHARE]
  or [then click Always SHARE]
  or scan −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−→
        [then click Always SHARE]

       subject to CC-BY-SA
   Poritz   Education in [Block]Chains   Domains 2019: 10 June 2019   26 / 26