Friday, June 16, 2017

Boomerang Attacks

In cryptography, a boomerang attack is a method of cryptanalysis that is based on differential cryptanalysis.

Boomerang attacks were first introduced by Wagner and allow an adversary to concatenate two high probability differential trails to attack a cipher. This is especially useful if there is a lack of long differentials with sufficient probabilities. The adversary can therefore decompose the encryption function $F$ in two subciphers $f$ and $g$ such that $F = f \circ g$. Then the adversary can search for high probability trails $\Delta \rightarrow \Delta^*$ with probability $p$ for  $f$ and  $\nabla \rightarrow \nabla^*$ with probability $q$ for $g$. The differential trails can then be combined in a chosen plaintext/adaptive chosen ciphertext attack to mount a boomerang distinguisher and then a key recovery attack based on this distinguisher to recover the secret key.

A boomerang distinguisher

The basic attack works as follows:

  1. The adversary chooses a random plaintext $X_1$ and calculates $X_2 = X_1 \oplus \Delta$.
  2. The adversary requests the ciphertexts for $X_1$ and $X_2$ from an encryption oracle which are $Y_1 = F(X_1)$ and $Y_2 = F(X_2)$
  3. Calculate ciphertexts $Y_3 = Y_1 \oplus \nabla$ and $Y_4 = Y_2 \oplus \nabla$.
  4. Request the decryptions of $Y_3$ and $Y_4$ to obtain $X_3 = F^{-1}(Y_3)$ and $X_4 = F^{-1}(Y_4)$.
  5. If the difference between $X_3$ and $X_4$ is the same as between $X_1$ and $X_2$, namely $\Delta$ we obtain a correct quartett $(X_1, X_2, X_3, X_4)$.

Calculating a correct quartet requires an attacker to consider both plaintext pairs $(X_1, X_2)$ and $(X_3, X_4)$ and results in a total probability of (pq)^2.
For an attack to succeed, for the probability of the boomerang distinguisher it must hold that $(pq) > 2^{n/2}$. For N plaintext pairs, an adversary expects about $N\cdot(pq)^2$ correct quartets in an attack, while there are only $N\cdot2^{-n}$ (where n is the blocksize) correct quartets for an ideal primitive.

Thursday, April 27, 2017

Yet Another "Ni Hao (Hello)"

Hello, everyone. I am Junwei Wang from China. It’s pleasant to be an ECRYPT-NET fellow and to receive funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie actions (MSCA).

I received my bachelor degree and master degree from Shandong University in 2012 and the University of Luxembourg in 2015, respectively. My master thesis is about the practical implementation of countermeasures resistant again high-order differential power analysis (DPA). Presently, I am doing my Ph.D. student at CryptoExperts under the supervisor of Jean-Sébastien CORON, Sihem MESNAGER, Pascal PAILLIER, and Matthieu RIVAIN.  My research interest is white-box cryptography.

The advent of content-protected applications and mobile payment systems in recent years motivates the exploration of a solution to protect sensitive information from being extracted under the white-box context in which the attacker has the power of control the whole process of the execution of software. Though some theoretical evidence reveals the fact that no generic obfuscator can be constructed to shield algorithms to be attacked, and all existed practical schemes has been broken, it is still interesting to investigate new schemes which can withstand all the existing attacks, and to propose and analyse a new attacking method which is more generic than the existing ones. I hope somebody has similar interest with me, then we can deeply explore this area together.

By the way, I am looking forward to meeting with you in Paris during EUROCRYPT.

Monday, April 17, 2017

What does privacy mean to you? A 5-day challenge

I recently came across The Privacy Paradox project, a call by WNYC Studios to take part in a five-day-long challenge on defining what does privacy mean to yourself.

Amongst its best features, the process can be started anytime, by anyone, just by registering with an e-mail account. From that moment and during the five following days, you will receive a daily e-mail explaining you the topic addressed by that day's podcast, most of them finishing with an easy demand to the listener to reflect on the matter.

With the audio's length ranging from 11 to 15 minutes, and the topics from cryptography --with Bruce Schneier starring on the first episode-- to psychology, there is something on it for everyone, and at a very accessible level. I personally found the format very nice and entertaining, and it is maybe something to learn about when we talk about privacy with people not so interested in the topic.

Finally, the people behind the project also gathered a nice list with things you can do to face the current privacy situation.

Tuesday, April 11, 2017

Robust encryption for symmetric primitives

Robustness for encryption schemes has been introduced in the context of public-key encryption in the work of Abdalla, Bellare and Neven. In a nutshell, it states that a ciphertext cannot be decrypted under two different keys. Later, their work has been extended, by taking into account the cases where the keys are adversarially generated.
The work of Farshim, Orlandi and Roşie studies the robustness in the context of symmetric primitives under the incorrect usage of keys. Roughly speaking, a  key-robust scheme does not output ciphertexts/tags that are valid with respect to distinct keys. Key-robustness is a notion that is often tacitly expected/assumed in protocol design --- as is the case with anonymous auction, oblivious transfer, or public-key encryption.

To motivate the new notion, "consider the following protocol, for constructing a ${3 \choose 2}$-OT protocol using only ${3 \choose 1}$-OTs: the sender picks $3$ random keys $k_1,k_2,k_3$ and inputs the message $x_1=(k_2,k_3),x_2=(k_1,k_3)$ and $x_3=(k_1,k_2)$ to the OT. At the same time, the sender sends encryptions of his messages under these keys, i.e., sends $c_i=E(k_i,m_i)$ for $i=1..3$. Now the receiver inputs the index of the message he does not want to learn to the ${3 \choose 1}$-OT and learns all keys except $k_i$. Intuitively the fact that the messages are sent only once (encrypted) should guarantee that the sender's choice of messages is uniquely defined. However, consider the following attack: the corrupt sender inputs $x^*_1=(k_2, k^*)$ (instead of $x_1$) such that $D(k^*,c_3)=m^*_3$ with $m^*_3\neq m_3$ and $m^*_3\neq \bot$. This means that the receiver will see two different versions of $m_3$ depending on whether the receiver asked for the pair $(2,3)$ or $(1,3)$. (This attack is an example of input-dependence and is a clear breach of security since it cannot be simulated in the ideal world)."
In terms of the results, the new work considers "both notions where the adversary has control over the keys and notions where the keys are generated honestly. The strongest notion that is formulated is called complete robustness and allows an adversary to generate the keys used in the system. The work shows that whether the adversary is in control of the keys or not makes a significant difference, by giving separations between the notions. While previous work in the public-key setting also had to deal with adversarially generated keys that were also invalid, this is not an issue in the setting, since in the symmetric world keys are often bit-strings of some pre-specified length and can be easily checked for validity. By focusing on correctly formed keys it can can shown equivalence between  complete robustness and a syntactically simpler notion, which we call full robustness." Then, it is shown that full robustness composes well: any fully robust symmetric encryption when combined with a fully robust MAC results in a fully robust AE scheme. Analogous composition results also hold for MAC-then-Encrypt and Encrypt-and-MAC.

One of the most interesting question is if "feasibility results for robustness in the public-key setting can be translated to the symmetric setting. This turns out not to be the case. The main reason for this is that in the asymmetric setting the public key can be used as a mechanism to commit to its associated secret key. In the symmetric case, on the other hand, there is no such public information. It might be tempting to think that one can just commit to the secret key and append it to the ciphertext. Unfortunately this approach cannot be proven secure due to a circular key-dependency between the encryption and the commitment components. To give a provably secure construction, with the authors constructing appropriate commitments that can be used in this setting. This requires  a right-injective PRG, that can be in turn based on one-way permutations. This result relies on the one-time security of the MAC and its collision-resistance, which once again is based on right-injective PRGs."

Monday, April 3, 2017

Three everyday git scenarios

You may remember Ralph's post on git basics from a little over a year ago. In this post, I'll share three things I've learned are possible (and practically painless) to do with git that go beyond the basic add, merge, push, and pull. Everything in this post is done from the command line.

Scenario 1

You're working on a project with others. These hardworking colleagues of yours just pushed a bunch of commits and you want to see exactly what they changed since your last commit.

What to do

Use git diff. Besides using it to show unstaged changes, you can also use git diff to show changes between any two commits. Suppose that after your last commit, your colleagues made 4 commits. To see the changes, use git diff HEAD~4 HEAD or git diff HEAD^^^^ HEAD. If you don't want to count how many commits there were, you can look up the abbreviated commit IDs (SHA-1 hashes in hex) using git log --oneline. Then, you could use something like git diff 54180a8 129ec44.

There's a lot more to git diff: you can use it to compare specific files, you can show differences at the word level rather than at the line level, etc. If you want to learn more about it, see its page on the git website. If you're working on LaTeX files, git-latexdiff is convenient: given any two commits, it will produce a PDF file showing the changes, with removed text striked through and in red, and added text underlined and in blue.

Scenario 2

You were in the zone, hacking away at multiple issues and successfully completing all of them. Well done, you! Wait—each issue should have its own commit...

What to do

Use interactive staging: use git add -p or git add --patch to add the file or files. Git will look at each file you want to add and split the changes into hunks. For each hunk, it will show you the diff and ask you what to do with it:

Stage this hunk [y,n,q,a,d,/,s,e,?]?

Here is the full list of options. They're pretty easy to remember, especially if you're a vi user who is used to navigating with HJKL.

y - stage this hunk
n - do not stage this hunk
q - do not stage this hunk or any of the remaining ones
a - stage this hunk and all later hunks in the file
d - do not stage this hunk or any of the later hunks in the file
g - select a hunk to go to
/ - search for a hunk matching the given regex
j - leave this hunk undecided, see next undecided hunk
J - leave this hunk undecided, see next hunk
k - leave this hunk undecided, see previous undecided hunk
K - leave this hunk undecided, see previous hunk
s - split the current hunk into smaller hunks
e - manually edit the current hunk
? - print help

git add -p is a powerful command that helps you keep your commits reasonably sized. It does require some care, though, since each individual commit should be consistent.

Scenario 3

You have to stop working, but you haven't finished fixing the issue you're working on.

What to do

You should do something because you don't want to have a dirty working directory.

Option 1: commit now, then use git commit --amend (introduced in Ralph's post) once you've finished what you were working on. git commit --amend is useful for a bunch of other things, like adding files you forgot to stage to a commit and fixing typos in a commit message.

Commits are local, so what should you do if you're worried about hardware failure? If you're working on a personal project, it may be acceptable to push this commit and later push the amended commit. You'll need the -f (--force) flag for the latter push. If you're working on a project with others, however, it would be bad practice to amend a commit that you've already pushed. Instead, you could create a temporary personal branch, commit your changes to this branch, and push it to the remote repository. Then, you could push the amended commit to this branch without worrying about rewriting anyone else's history and merge it with the main branch when you've finished fixing the issue.

Option 2: shelve your changes with git stash, which will sweep away both staged and unstaged changes, leaving you with a clean working directory. You can stash changes more than once. To see all stashes, use git stash list. To re-apply the most recent stash you made, use git stash pop. By default, git stash excludes new files (that haven't yet been staged) and ignored files. git stash is also useful when you want to see upstream changes made by someone else, but aren't ready to commit your work.

There's much more you can do with stashes: apply one stash to multiple branches, delete stashes you no longer need, stash parts of a file, etc. Stashes are always local; they can never be pushed to a remote repository. Atlassian has a good, detailed tutorial on git stash.

Some books you may (or may not) find useful.

Tuesday, March 21, 2017

Learn You a GPU For Great Good! (Part 1?)

Side note: I stole the title from the most famous, most awesome Haskell book I know.

If you are reading this blog you are most likely interested in cryptography. Today I want to convince you that GPUs are also, well, pretty awesome. I have personally done a few crypto-related projects using GPUs and this post is my attempt at crystallizing the knowledge and experience I built up during that time.

The purpose of this post is to provide a simple, meaningful introduction to developing GPU-accelerated programs. We will discuss setup, the two primary frameworks, basic code examples and development workflow as well as some optimization tips. In the end, I want to show that developing this type of application is not hard at all. If the post is successful I may do a follow-up with a few more detailed and tricky examples. Throughout this post I will assume you are familiar with basic C and/or C++, as the code examples will be in that language. I will not focus too much on develop complicated kernels or how to exploit multi-dimensional parallelism, I will leave that for a later post. Instead, I will focus on a few things that may help you in making the firsts steps towards GPU programming easier, as well as a few things that may help it scale a bit better.

The Why & When

GPU programming was originally designed for, and should be used for, large-scale parallel computation problems. The more parallelism you can utilize, the better GPUs will fit your problem. The most simple example is probably when you loop over a very large collection of elements, performing on each a simple operation independently.

For large-scale parallel computation problems I tend to think of three different architectural setups that you can use (they also mix). The simplest is utilizing multi-core CPUs (possibly over many machines). This has the shortest development time due to its familiarity and easy-of-use and is suitable to many applications. CPUs are of course trivially available. On the other end of the spectrum is the development of custom hardware clusters, utilizing many FPGAs or even ASICs. Development time is fairly long, even for experienced hardware designers; the upside is that this very likely gives you optimal performance.

GPUs fall somewhere in the middle. Development time is very close to that for CPUs; the primary constraint is availability. It is simply easier to get access to CPU clusters. However, these days you can also rent all the GPU power you need from Amazon EC2 instances, as was done for the recent SHA1 collision. If you solve the availability issue, you can get a lot of bang out of your buck performance-wise.

The How

First, you need to get your hands on a machine with a GPU, preferably a remote machine or otherwise a machine with more than one GPU. The reason is that if your GPU is also driving your desktop environment, programming errors may cause your computer to hang or crash. It also allows you to more easily run long-lasting kernels as well as giving you more reliable performance.

CUDA vs OpenCL

Assuming you have a GPU in your system, your next choice is between CUDA and OpenCL, two programming environments for GPU programming. If you do not plan to use an NVIDIA GPU you are stuck with OpenCL, whereas you otherwise have the choice of using CUDA.
Having used both for different projects at different times, I can say that both are perfectly usable and that the differences are mostly superficial. OpenCL is more portable and integrates easier into existing projects; CUDA has the superior tool-chain.

The examples in this post will be for CUDA, as it typically involves less boilerplate. Also, we will use the more basic CUDA C++ implementation, as it provides a better basis for understanding than special-purpose libraries. This is particularly relevant if you want to computations that are not a native part of these libraries, which is definitely true if you want to, for instance, compute CPA-like correlations in parallel.

Hello World

I am not one to break tradition and thus we start the "Hello world" of classic parallel programming, namely SAXPY. Or, more formally, given input vectors \(\textbf{x}, \textbf{y}\) of length \(n\) and a scalar \(a\), compute the output vector \(\textbf{z}\) where \(\textbf{z} = a\textbf{x} + \textbf{y}\).
First let us consider the basic C implementation of this function, where \(z = y\), i.e., we update \(y\) using the scalar \(a\) and a vector \(x\).

 1: void saxpy(int n, float a, float * __restrict__ x, float * __restrict__ y) {
 2:   for (int i = 0; i < n; ++i) {
 3:     y[i] = a*x[i] + y[i];
 4:   }
 5: }
 7: // ...
 8: int n = 1 << 20;
 9: // allocate vectors x,y of n elements each.
10: // ...
12: saxpy(n, 3.14, x, y);

Nothing too special going on here. We simply iterate over every element and perform our update with the scalar \(a=3.14\). Note the use of the __restrict__ keyword to indicate that x and y point to different objects in memory. Just giving the compiler a helping hand, which is generally a useful thing to do. Anything that makes it behave less like a random function, I say.

Conversion to CUDA is straightforward. In GPU programming you are always defining what a single parallel unit of computation is doing, this is called a kernel. When programming such a kernel, you are computing from the point of view of the thread. Before delving in too deep, let us see what the CUDA-equivalent code looks like.

 1: __global__
 2: void saxpy(int n, float a, float * __restrict__ x, float * __restrict__ y) {
 3:   int i = blockIdx.x*blockDim.x + threadIdx.x;
 4:   if (i < n) {
 5:     y[i] = a*x[i] + y[i];
 6:   }
 7: }
 9: // ...
10: const int n = 1<<20;
12: // allocate and initialize host-side buffers x,y
13: // ...
15: // allocate device-side buffers x,y
16: cudaMalloc((void **)&d_x, sizeof(float) * n);
17: cudaMalloc((void **)&d_y, sizeof(float) * n);
19: // copy host-side buffers to the device
20: cudaMemcpy(d_x, x, sizeof(float) * n, cudaMemcpyHostToDevice);
21: cudaMemcpy(d_y, y, sizeof(float) * n, cudaMemcpyHostToDevice);
23: // compute the saxpy
24: const int threads_per_block = 256;
25: const int number_of_blocks = n / threads_per_block;
26: saxpy<<<number_of_blocks, threads_per_block>>>(n, 3.14, d_x, d_y);
28: // copy the output buffer from the device to the host
29: cudaMemcpy(y, d_y, sizeof(float) * n, cudaMemcpyDeviceToHost);
31: // free the device buffers
32: cudaFree(d_x);
33: cudaFree(d_y);
35: // clean up the x, y host buffers
36: // ...

Let us consider the kernel first, denoted by the simple fact of the function definition starting with __global__. The parameters to the function are the same as before, nothing special there. Line 3 is a key first step in any kernel: we need to figure out the correct offset into our buffers x and y. To understand this, we need to understand CUDA's notion of threads and blocks (or work groups and work items in OpenCL).
The Grid
The CUDA threading model is fairly straightforward to imagine. A thread essentially computes a single instance of a kernel. These threads form groups called blocks that have somewhat-more-efficient inter-thread communication primitives. The blocks together form what is known as the grid. The grid can have up to three dimensions, i.e., the blocks can be ordered into \((x,y, z)\) coordinates. The same goes for threads inside a block, they can be addressed with \((x, y, z)\) coordinates as well.
Mostly though, I have tended to stick to 1-dimensional grids. This is simply dividing a vector of \(n\) elements into $n/m$-sized sequential blocks (even better if \(n\) is a multiple of \(m\)). 
A quick note about warps (or wavefronts in OpenCL), which is a related concept. A warp is a unit of scheduling, it determines the amount of threads that actually execute in lockstep. It is good practice to have your block size as a multiple of the warp size but other than that you should not worry overly much about warps.
In this case we find our thread by multiplying the block id with the size of block and then adding the offset of the thread within the block. The rest of the kernel is straightforward, we simply perform the same computation as in the original code but we omit the for-loop. The conditional at line 4 makes sure we do not write outside the bounds of our vector, though that should not happen if we choose our grid carefully.

The rest of the code is the standard boilerplate that you will find in most CUDA programs. A key notion is that there is a distinction between buffers allocated on the device (the GPU) and buffers allocated on the host. Note that on line 26 we schedule the kernel for execution. The first two weird-looking parameters (within angle brackets) are the number of blocks and the block size respectively.

Improving & Testing "Hello World"

To showcase a few things that I found helpful we are going to improve this simple code example. And because this is my blog post and I decide what is in it, I get to talk to you about how to test your code. GPU code tends to be a bit flaky: it breaks easily. Thus, I argue that creating simple tests for your code is essential. These do not have to be very complicated but I recommend that you use a proper framework for writing unit tests. For C++ I have had success with Catch and doctest, both single-headers that you include into your project.

Before we include these tests however, I propose that we make two more changes to the program. First of all, we are going to add better error checking. Most of the cudaFoo functions return a value indicating whether the operation was successful. Otherwise, we get something which we can use to determine the error.

1: #define check(e) { _check((e), __FILE__, __LINE__); }
3: inline cudaError_t _check(cudaError_t result, const char *file, int line) {
4:   if (result != cudaSuccess) {
5:     fprintf(stderr, "CUDA Runtime Error: %s (%s:%d)\n", cudaGetErrorString(result), file, line);
6:     assert(result == cudaSuccess);
7:   }
8:   return result;
9: }

And then simply wrap the cudaFoo functions with this check macro. Alternatively, you may want to rewrite this to use exceptions instead of asserts. Pick your poison.

Another thing I would recommend adding if you are doing CUDA in C++ is wrapping most of the allocation and de-allocation logic in a class. I generally take a more utilitarian view of classes for simple pieces of code and thus the following is not necessarily idiomatic or good C++ code.

 1: class Saxpy {
 2: public:
 3:   const int n;
 4:   float *d_x;
 5:   float *d_y;
 6:   float *x;
 7:   float *y;
 9:   Saxpy(const int n) : n(n) {
10:     x = new float[n];
11:     y = new float[n];
13:     check(cudaMalloc((void **)&d_x, sizeof(float) * n));
14:     check(cudaMalloc((void **)&d_y, sizeof(float) * n));
15:   }
17:   ~Saxpy() {
18:     check(cudaFree(d_x));
19:     check(cudaFree(d_y));
21:     delete[] x;
22:     delete[] y;
23:   }
25:   Saxpy& fill() {
26:     for (int i = 0; i < n; ++i) {
27:       x[i] = i / 12.34;
28:       y[i] = i / 56.78;
29:     }
31:     check(cudaMemcpy(d_x, x, sizeof(float) * n, cudaMemcpyHostToDevice));
32:     check(cudaMemcpy(d_y, y, sizeof(float) * n, cudaMemcpyHostToDevice));
34:     return *this;
35:   }
37:   Saxpy& run(float a) {
38:     const int threads_per_block = 256;
39:     const int number_of_blocks = n / threads_per_block;
40:     saxpy<<<number_of_blocks, threads_per_block>>>(n, a, d_x, d_y);
42:     return *this;
43:   }
45:   Saxpy& load() {
46:     check(cudaDeviceSynchronize());
47:     check(cudaMemcpy(y, d_y, sizeof(float) * n, cudaMemcpyDeviceToHost));
48:     return *this;
49:   }
50: };

Why we went through all this trouble becomes clear if we put this in a test (I am using doctest syntax as an example).

 1: TEST_CASE("testing saxpy") {
 2:   float a = 3.14;
 3:   const int n = 1024;
 4:   Saxpy s(n);
 6:   s.fill().run(a).load();
 8:   for (int i = 0; i < n; ++i) {
 9:     // we didn't keep the old y values so we recompute them here
10:     float y_i = i / 56.78;
11:     // the approx is because floating point comparison is wacky
12:     CHECK(s.y[i] == doctest::Approx(a * s.x[i] + y_i));
13:   }
14: }

That is a, for C++ standards, pretty concise test. And indeed, our tests succeed. Yay.

[doctest] test cases:    1 |    1 passed |    0 failed |    0 skipped
[doctest] assertions: 1024 | 1024 passed |    0 failed |

A Final Improvement

Because this post is already too long I will conclude with one last really nice tip that I absolutely did not steal from here. Actually, the NVIDIA developer blogs contain a lot of really good CUDA tips.
Our current kernel is perfectly capable of adapting to a situation where we give it less data than the grid can support. However, if we give it more data, things will break. This is where gride-stride loops come in. It works by looping over the data one grid at a time while maintaining coalesced memory access (which is something I will write about next time).

Here's our new kernel using these kinds of loops.

1: __global__
2: void saxpy(int n, float a, float *x, float *y) {
3:   for (int i = blockIdx.x * blockDim.x + threadIdx.x; i < n; i += blockDim.x * gridDim.x) {
4:     y[i] = a * x[i] + y[i];
5:   }
6: }


I hope this convinces you that GPU programming is actually pretty simple. The kernel here is pretty trivial, but as long as you understand that within the kernel you can basically write C/C++, you are going to do just fine.

If there is a next post I will write more about memory in GPUs, a very important topic if you want your code to actually run fast. If you want to skip ahead you should read about the different types of memory (global, local, shared, texture, etc.) and what memory coalescing entails.

Until next time.

Tuesday, March 14, 2017

What are lattices?

Do all the cool post-quantum cryptographers around you talk about lattices and all you can think about is a good salad?

Don't worry! You can be part of the hip crowd at the next cocktail party in just three easy steps!

First, I will tell you what a lattice is. It is actually really easy.
Second, we will prove a tiny fact about lattices so that those poor neurons that did all the work in the first step have some friends.
Third, nothing. There isn't even a third step. That's how easy it is!

So, let's tackle the first step. What, actually, is a lattice?
The answer could not be easier. It is simply a grid of points. Like this:

Easy, right?
The formal definition is just as simple:

Given $n$ linearly independent vectors $b_1, b_2, \ldots, b_n \in \mathbb{R}^m$, the lattice generated by them is defined as
\mathcal{L}(b_1, b_2, \ldots, b_n) = \left\{ \sum x_i b_i \middle| x_i \in \mathbb{Z} \right\}.

We can add those so called basis vectors $b_i$ to our diagram and will readily see how each point is constructed.

And now you already know what a lattice is! In the next step we will talk about some more definitions and show the proof of a theorem.

We will be discussing something related to the question of what the length of the shortest non-zero vector in a given lattice $\mathcal{L}$ is.

$\lambda_1(\mathcal{L})$ is the length of the shortest non-zero vector in $\mathcal{L}$. We use the euclidean norm for the definition of length.
And we also need the volume of a lattice which we are not going to define formally we will just take it to be the volume of that $n$ dimensional parallelogram in the picture:

Now we can already prove Blichfeld's Theorem! It makes a statement about where we can find a lattice point and roughly goes as follows:
For a lattice $\mathcal{L}\subseteq \mathbb{R}^n$ and a set $S \subseteq \mathbb{R}^n$ where the volume of $S$ is bigger than the volume of the lattice there exist two nonequal points $z_1, z_2 \in S$ such that $z_1 - z_2 \in \mathcal{L}$.
And here is the idea of the proof in a graphical way! First we simply draw an example for the set $S$ in blue:

Now we cut up our set and move all the parts into our first parallelogram!

As you can see there is quite a bit of overlap and the fact that the full volume of the set is bigger than the volume of the lattice aka the parallelogram guarantees there will be at least some overlap somewhere. But if there is overlap then we have two points, that have been moved by multiples of the base vectors and which are now at the same position!

And therefore their difference must be a multiple of base vectors as well and we have found a difference which is a lattice point! Hooray!

Now we will use a little trick to expand this result and make a statement about an actual point from the set being on the lattice not just the difference of two points!

What we will show is called Minkowski's Convex Body Theorem and it states roughly
Let $\mathcal{L}$ be a lattice. Then any centrally-symmetric convex set $S$, with volume bigger than $2^n$ times the volume of the lattice contains a nonzero lattice point.
So after this we will know such a set it will contain a lattice point and using a simple sphere as the set allows us to put a bound on $\lambda_1(\mathcal{L})$.

Let's get to it then!
First we blow up our lattice to twice its size along every dimension!

Now we add our centrally-symmetric convex set $S$. Again in blue.

And because we picked the volume of $S$ to be bigger than $2^n$ times the volume of the lattice we still get the colliding points from the last theorem EVEN in the double size lattice!

But since our set $S$ is symmetric about the origin if $z_2 \in S$ it follows that $-z_2 \in S$ and because it is convex $z_1, -z_2 \in S$ implies $\frac{z_1 - z_2}{2} \in S$. And because we've double the size of the lattice and $z_1 - z_2$ is on the bigger lattice it follows that $\frac{z_1 - z_2}{2} \in \mathcal{L}$ and we have shown that a point in our set is also in the lattice.

You can see it quite clearly in the next picture.

As already stated above if we use a simple sphere for this set we can give a bound on $\lambda_1(\mathcal{L})$ based on the volume of $\mathcal{L}$. It is known as Minkowski's First Theorem and states:
\lambda_1(\mathcal{L}) \leq \sqrt{n}(vol(\mathcal{L})^{1/n}.
Isn't that great?!

If you have now completely fallen in love with lattices but would appreciate real math instead of pictures, worry not!
You will find steps 3 - 14 on Oded Regev's course website! Enjoy!

And in case you accidentally run into him at a conference here is the picture from his website so you don't miss the opportunity to say hello.