[taler-anastasis] branch master updated: small fixes

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dennis-neufeld pushed a commit to branch master
in repository anastasis.

The following commit(s) were added to refs/heads/master by this push:
     new a4914d4  small fixes
a4914d4 is described below

commit a4914d415b00458b1701d04d8f3dba494e4353e9
Author: Dennis Neufeld <dennis.neufeld@students.bfh.ch>
AuthorDate: Mon Jun 1 07:44:05 2020 +0000

    small fixes
---
 doc/thesis/related_work.tex | 26 ++++++++++++++------------
 1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/doc/thesis/related_work.tex b/doc/thesis/related_work.tex
index ab9ece5..72e9172 100644
--- a/doc/thesis/related_work.tex
+++ b/doc/thesis/related_work.tex
@@ -9,27 +9,28 @@ Hash functions "compress a string of arbitrary length to a 
string of fixed lengt
        \item second pre-image resistance
        \item collision resistance
 \end{itemize}
-Pre-image resistance, also called "one way property", means that for a given 
hash function H and a hash value H(x), it is computationally infeasible to find 
x \cite{SG2012}.
-The second pre-image resistance is described by following: For a given hash 
function H and a hash value H(x), it is computationally infeasible to find x 
and x' such that H(x) = H(x') \cite{SG2012}.
-The definition of collision resistance slightly differs from the second 
pre-image resistance: For a given hash function H, it is computationally 
infeasible to find a pair (x,y) such that H(x) = H(y) \cite{SG2012}.\\
-
-There are several applications for cryptographic hash functions. For example 
you can store the hash value of a pass-phrase instead of the pass-phrase itself 
in a computer to protect the pass-phrase. Another important application is 
verification of message integrity: Before and after transmission of a message 
you can calculate the hash values of it and compare them to determine if the 
message changed during transmission.
+Pre-image resistance, also called "one way property", means that for a given 
hash function H and a hash value H(x), it is computationally infeasible to find 
x \cite{SG2012}.\\
+The second pre-image resistance is described by following: For a given hash 
function H and a hash value H(x), it is computationally infeasible to find x 
and x' such that H(x) = H(x') \cite{SG2012}.\\
+The definition of collision resistance slightly differs from the second 
pre-image resistance: For a given hash function H, it is computationally 
infeasible to find a pair (x, y) such that H(x) = H(y) \cite{SG2012}.\\
+Besides these security requirements, a cryptographic hash function should also 
behave as a random function. This means that although a hash function is purely 
deterministic, the output must not be predictable.\\
 
+There are several applications for cryptographic hash functions. For example 
you can store the hash value of a pass-phrase instead of the pass-phrase itself 
in a computer to protect the pass-phrase. Another important application is 
verification of message integrity: Before and after transmission of a message 
you can calculate the hash values of it and compare them to determine if the 
message changed during transmission.\\
 In Anastasis we use SHA-512 for hashing data.
 
 \subsubsection{HMAC}
-When it comes to integrity of messages during communication of two parties 
over an insecure channel Keyed-Hash Message Authentication Codes (HMAC) are 
used as check values. An HMAC function is based on a hash function and takes 
two arguments, a key K and a message M:
+When it comes to integrity of messages during communication of two parties 
over an insecure channel Keyed-Hash Message Authentication Codes (HMAC) are 
used as check values. An HMAC function is based on a hash function and takes 
two arguments, a key K and a message M:\\
 HMAC\textsubscript{K}(M) = H(K $\oplus$ opad,H(K $\oplus$ ipad, M)) with 
"ipad" and "opad" being constants which fill up the key K to the blocksize of 
the hash function \cite{BCK1996}. The blocksize of a modern hash function like 
SHA-512 is 64 Byte.\\
 In Anastasis we use HMACs to achieve verifiability.
 
 \subsubsection{HKDF}
-A HKDF is a key derivation function (KDF) based on a HMAC. A KDF "is a basic 
and essential component of crypto-
+A HKDF is a key derivation function (KDF) based on HMAC. A KDF "is a basic and 
essential component of crypto-
 graphic systems: Its goal is to take a source of initial keying material, 
usually containing some good amount of randomness, but not distributed 
uniformly or for which an attacker has some partial knowledge, and derive from 
it one or more cryptographically strong secret keys" \cite{krawczyk2010}.\\
 Anastasis uses HKDFs to derive symmetric keys for encryption purposes.
 
 \subsubsection{Argon2}
 Hash functions like SHA-512 are very fast to compute. Therefor passwords 
stored in a hashed form are vulnerable to dictionary attacks with new hardware 
architectures like FPGAs \cite{trimberger2012} and dedicated ASIC 
\cite{madurawe2006} modules. But those architectures "experience difficulties 
when operating on large amount of memory" \cite{BDK2016}.\\
-Argon2 is a memory-hard function that won the Password Hashing Competition in 
2015. It minimizes time-memory tradeoff \cite{stamp2003} and thus maximizes the 
costs to implement an ASIC for given CPU computing time \cite{BDK2016}. Aside 
from the fact that Argon2 makes dictionary attacks much harder, we use Argon2 
for another feature too: Memory-hard schemes like Argon2 are very useful for 
key derivation from low-entropy sources \cite{BDK2016}.\\
+Argon2 is a memory-hard function that won the Password Hashing Competition in 
2015. It minimizes time-memory tradeoff \cite{stamp2003} and thus maximizes the 
costs to implement an ASIC for given CPU computing time \cite{BDK2016}. Aside 
from the fact that Argon2 makes dictionary attacks much harder, Argon2 can be 
used for another feature too: Memory-hard schemes like Argon2 are very useful 
for key derivation from low-entropy sources \cite{BDK2016}.\\
+
 Argon2 is used in Anastasis to derive an identifier for the user from some 
low-entropy material.
 
 
@@ -39,14 +40,16 @@ In a secret sharing theme the recipients of a share often 
are called \textit{pla
 
 \subsubsection{Shamir's Secret Sharing}
 The algorithm "Shamir's Secret Sharing" is one of the most well known secret 
sharing scheme. It „divide[s] data D into n pieces in such a way that D is 
easily reconstructible from any k pieces, but even complete knowledge of k - 1 
pieces reveals absolutely no information about D“ \cite{shamir_sharing}.\\
-Shamir’s simple secret sharing scheme has two key limitations. First, it 
requires a trusted dealer who initially generates the secret to be distributed, 
and second the shares are not verifiable during reconstruction. Therefore, 
malicious shareholders could submit corrupt shares to prevent the system from 
reconstructing the secret -- without these corrupt shareholders being 
detectable as malicious. Furthermore, the dealer distributing the shares could 
be corrupt and distribute some incons [...]
+Shamir’s simple secret sharing scheme has two key limitations. First, it 
requires a trusted dealer who initially generates the secret to be distributed, 
and second the shares are not verifiable during reconstruction. Therefore, 
malicious shareholders could submit corrupt shares to prevent the system from 
reconstructing the secret -- without these corrupt shareholders being 
detectable as malicious. Furthermore, the dealer distributing the shares could 
be corrupt and distribute some incons [...]
 
 \subsubsection{Verifiable Secret Sharing}
-Verifiability can be achieved by using so called commitment schemes like the 
Pederson commitment. It allows „to distribute a secret to n persons such that 
each person can verify that he has received correct information about the 
secret without talking with other persons“ \cite{pedersen_sharing_0}. In his 
paper „A Practical Scheme for Non-interactive Verifiable Secret Sharing“, Paul 
Feldman  combines the two algorithms above. His algorithm for verifiable secret 
sharing, short VSS, allows  [...]
+Verifiability can be achieved by using so called commitment schemes like the 
Pederson commitment. It allows „to distribute a secret to n persons such that 
each person can verify that he has received correct information about the 
secret without talking with other persons“ \cite{pedersen_sharing_0}. In his 
paper „A Practical Scheme for Non-interactive Verifiable Secret Sharing“ 
\cite{feldman_sharing}, Paul Feldman  combines the two schemes Shamir Secret 
Sharing and Pederson commitment. His [...]
 
 \subsubsection{Distributed Key Generation}
 Distributed key generation algorithms, short DKG, solve the problem of needing 
a trustworthy dealer by relying on a threshold of honest persons. Contrary to 
the above-mentioned schemes, in distributed key generation algorithms every 
participant is involved in key generation.\\
-The Pederson DKG is such „a secret sharing scheme without a mutually trusted 
authority“ \cite{pedersen_sharing_5.2}. Basically, this DKG works as follows: 
First, each involved party generates a pre-secret and distributes it to all 
parties using the verifiable secret sharing scheme of Feldman. Afterwards, each 
party recombines the received shares, including its own pre-secret, to a share 
of the main secret. The main secret can be reconstructed by summing up each 
recombination of the share [...]
+The Pederson DKG is such „a secret sharing scheme without a mutually trusted 
authority“ \cite{pedersen_sharing_5.2}. Basically, this DKG works as follows: \\
+First, each involved party generates a pre-secret and distributes it to all 
parties using the verifiable secret sharing scheme of Feldman.\\ 
+Afterwards, each party recombines the received shares, including its own 
pre-secret, to a share of the main secret. The main secret can be reconstructed 
by summing up each recombination of the shared pre-secrets.
 
 \subsubsection{MIDATA}
 MIDATA is a project that aims to give patients back control over their medical 
data and to enable them to share their data only with those they trust. In case 
the patient lost his device running the MIDATA application and his 
MIDATA-password, MIDATA build in a key recovery system using the Shamir Secret 
Sharing Scheme mentioned above. In their case a few "persons working at MIDATA 
have generated a public-private key pair (Recovery key) on their own computer. 
They keep the private recover [...]
@@ -56,7 +59,6 @@ In our opinion the security of MIDATA is broken in two ways:
        \item It is not clear which channel the persons working for MIDATA use 
for their decisions and activities regarding the key recovery. The channel 
could be vulnerable. For example, an attacker could illegitimately trigger a 
recovery process via e-mail if it is the chosen channel.
 \end{enumerate}
 
-
 \subsubsection{Key sharing in Anastasis}
 For Anastasis we do not need a DKG because the dealer is the user himself and 
therefore, he is fully trustworthy. But we need verifiability. In our case we 
achieve verifiability by using HMACs. Furthermore, for our purposes the 
above-mentioned algorithms are inadequate because we are dealing with a 
manageable number of sharing parties and we need a more flexible solution. 
 

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