and on an iteration process called ``chaotic
iterations'' on which the post-treatment is based.
Proofs of chaos are given in Section~\ref{sec:pseudorandom}.
-Section~\ref{sec:efficient prng} presents an efficient
-implementation of this chaotic PRNG on a CPU, whereas Section~\ref{sec:efficient prng
+Section~\ref{sec:efficient PRNG} presents an efficient
+implementation of this chaotic PRNG on a CPU, whereas Section~\ref{sec:efficient PRNG
gpu} describes the GPU implementation.
Such generators are experimented in
Section~\ref{sec:experiments}.
\section{Efficient PRNG based on Chaotic Iterations}
-\label{sec:efficient prng}
+\label{sec:efficient PRNG}
Based on the proof presented in the previous section, it is now possible to
improve the speed of the generator formerly presented in~\cite{bgw09:ip,guyeux10}.
-\lstset{language=C,caption={C code of the sequential PRNG based on chaotic iterations},label=algo:seqCIprng}
+\lstset{language=C,caption={C code of the sequential PRNG based on chaotic iterations},label=algo:seqCIPRNG}
\begin{lstlisting}
-unsigned int CIprng() {
+unsigned int CIPRNG() {
static unsigned int x = 123123123;
unsigned long t1 = xorshift();
unsigned long t2 = xor128();
-In Listing~\ref{algo:seqCIprng} a sequential version of the proposed PRNG based on chaotic iterations
+In Listing~\ref{algo:seqCIPRNG} a sequential version of the proposed PRNG based on chaotic iterations
is presented. The xor operator is represented by \textasciicircum.
This function uses three classical 64-bits PRNGs, namely the \texttt{xorshift}, the
\texttt{xor128}, and the \texttt{xorwow}~\cite{Marsaglia2003}. In the following, we call them
stringent BigCrush battery of tests~\cite{LEcuyerS07}.
\section{Efficient PRNGs based on Chaotic Iterations on GPU}
-\label{sec:efficient prng gpu}
-
-In order to take benefits from the computing power of GPU, a program needs to have
-independent blocks of threads that can be computed simultaneously. In general,
-the larger the number of threads is, the more local memory is used, and the less
-branching instructions are used (if, while, ...), the better the performances on GPU is.
-Obviously, having these requirements in mind, it is possible to build a program similar to
-the one presented in Algorithm \ref{algo:seqCIprng}, which computes pseudorandom numbers
-on GPU.
-To do so, we must firstly recall that in
- the CUDA~\cite{Nvid10} environment, threads have a local
-identifier called \texttt{ThreadIdx}, which is relative to the block containing them.
+\label{sec:efficient PRNG gpu}
+
+In order to take benefits from the computing power of GPU, a program
+needs to have independent blocks of threads that can be computed
+simultaneously. In general, the larger the number of threads is, the
+more local memory is used, and the less branching instructions are
+used (if, while, ...), the better the performances on GPU is.
+Obviously, having these requirements in mind, it is possible to build
+a program similar to the one presented in Algorithm
+\ref{algo:seqCIPRNG}, which computes pseudorandom numbers on GPU. To
+do so, we must firstly recall that in the CUDA~\cite{Nvid10}
+environment, threads have a local identifier called
+\texttt{ThreadIdx}, which is relative to the block containing
+them. With CUDA parts of the code which are executed by the GPU are
+called {\it kernels}.
\subsection{Naive Version for GPU}
\If{threadIdx is concerned by the computation} {
retrieve data from InternalVarXorLikeArray[threadIdx] in local variables\;
\For{i=1 to n} {
- compute a new PRNG as in Listing\ref{algo:seqCIprng}\;
+ compute a new PRNG as in Listing\ref{algo:seqCIPRNG}\;
store the new PRNG in NewNb[NumThreads*threadIdx+i]\;
}
store internal variables in InternalVarXorLikeArray[threadIdx]\;
As GPU cards using CUDA have shared memory between threads of the same block, it
is possible to use this feature in order to simplify the previous algorithm,
-i.e., using less than 3 xor-like PRNGs. The solution consists in computing only
-one xor-like PRNG by thread, saving it into shared memory and using the results
+i.e., to use less than 3 xor-like PRNGs. The solution consists in computing only
+one xor-like PRNG by thread, saving it into the shared memory, and then to use the results
of some other threads in the same block of threads. In order to define which
-thread uses the result of which other one, we can use a permutation array which
-contains the indexes of all threads and for which a permutation has been
-performed. In Algorithm~\ref{algo:gpu_kernel2}, 2 permutations arrays are used.
+thread uses the result of which other one, we can use a combination array that
+contains the indexes of all threads and for which a combination has been
+performed.
+
+In Algorithm~\ref{algo:gpu_kernel2}, two combination arrays are used.
The variable \texttt{offset} is computed using the value of
-\texttt{permutation\_size}. Then we can compute \texttt{o1} and \texttt{o2}
-which represent the indexes of the other threads for which the results are used
-by the current thread. In the algorithm, we consider that a 64-bits xor-like
-PRNG is used, that is why both 32-bits parts are used.
+\texttt{combination\_size}. Then we can compute \texttt{o1} and \texttt{o2}
+representing the indexes of the other threads whose results are used
+by the current one. In this algorithm, we consider that a 64-bits xor-like
+PRNG has been chosen, and so its two 32-bits parts are used.
-This version also succeeds to the {\it BigCrush} batteries of tests.
+This version also can pass the whole {\it BigCrush} battery of tests.
\begin{algorithm}
\KwIn{InternalVarXorLikeArray: array with internal variables of 1 xor-like PRNGs
in global memory\;
NumThreads: Number of threads\;
-tab1, tab2: Arrays containing permutations of size permutation\_size\;}
+tab1, tab2: Arrays containing combinations of size combination\_size\;}
\KwOut{NewNb: array containing random numbers in global memory}
\If{threadId is concerned} {
retrieve data from InternalVarXorLikeArray[threadId] in local variables including shared memory and x\;
- offset = threadIdx\%permutation\_size\;
+ offset = threadIdx\%combination\_size\;
o1 = threadIdx-offset+tab1[offset]\;
o2 = threadIdx-offset+tab2[offset]\;
\For{i=1 to n} {
t=xor-like()\;
- t=t$\oplus$shmem[o1]$\oplus$shmem[o2]\;
+ t=t $\hat{ }$ shmem[o1] $\hat{ }$ shmem[o2]\;
shared\_mem[threadId]=t\;
- x = x $\oplus$ t\;
+ x = x $\hat{ }$ t\;
store the new PRNG in NewNb[NumThreads*threadId+i]\;
}
\subsection{Theoretical Evaluation of the Improved Version}
-A run of Algorithm~\ref{algo:gpu_kernel2} consists in three operations having
+A run of Algorithm~\ref{algo:gpu_kernel2} consists in an operation ($x=x\oplus t$) having
the form of Equation~\ref{equation Oplus}, which is equivalent to the iterative
-system of Eq.~\ref{eq:generalIC}. That is, three iterations of the general chaotic
-iterations are realized between two stored values of the PRNG.
+system of Eq.~\ref{eq:generalIC}. That is, an iteration of the general chaotic
+iterations is realized between the last stored value $x$ of the thread and a strategy $t$
+(obtained by a bitwise exclusive or between a value provided by a xor-like() call
+and two values previously obtained by two other threads).
To be certain that we are in the framework of Theorem~\ref{t:chaos des general},
we must guarantee that this dynamical system iterates on the space
$\mathcal{X} = \mathcal{P}\left(\llbracket 1, \mathsf{N} \rrbracket\right)^\mathds{N}\times\mathds{B}^\mathsf{N}$.
The left term $x$ obviously belongs into $\mathds{B}^ \mathsf{N}$.
-To prevent from any flaws of chaotic properties, we must check that each right
-term, corresponding to terms of the strategies, can possibly be equal to any
+To prevent from any flaws of chaotic properties, we must check that the right
+term (the last $t$), corresponding to the strategies, can possibly be equal to any
integer of $\llbracket 1, \mathsf{N} \rrbracket$.
-Such a result is obvious for the two first lines, as for the xor-like(), all the
-integers belonging into its interval of definition can occur at each iteration.
-It can be easily stated for the two last lines by an immediate mathematical
-induction.
+Such a result is obvious, as for the xor-like(), all the
+integers belonging into its interval of definition can occur at each iteration, and thus the
+last $t$ respects the requirement. Furthermore, it is possible to
+prove by an immediate mathematical induction that, as the initial $x$
+is uniformly distributed (it is provided by a cryptographically secure PRNG),
+the two other stored values shmem[o1] and shmem[o2] are uniformly distributed too,
+(this can be stated by an immediate mathematical
+induction), and thus the next $x$ is finally uniformly distributed.
Thus Algorithm~\ref{algo:gpu_kernel2} is a concrete realization of the general
chaotic iterations presented previously, and for this reason, it satisfies the
\label{sec:experiments}
Different experiments have been performed in order to measure the generation
-speed. We have used a computer equiped with Tesla C1060 NVidia GPU card and an
-Intel Xeon E5530 cadenced at 2.40 GHz for our experiments and we have used
-another one equipped with a less performant CPU and a GeForce GTX 280. Both
+speed. We have used a first computer equipped with a Tesla C1060 NVidia GPU card
+and an
+Intel Xeon E5530 cadenced at 2.40 GHz, and
+a second computer equipped with a smaller CPU and a GeForce GTX 280.
+All the
cards have 240 cores.
-In Figure~\ref{fig:time_xorlike_gpu} we compare the number of random numbers
-generated per second with the xor-like based PRNG. In this figure, the optimized
-version use the {\it xor64} described in~\cite{Marsaglia2003}. The naive version
-use the three xor-like PRNGs described in Listing~\ref{algo:seqCIprng}. In
-order to obtain the optimal performance we removed the storage of random numbers
-in the GPU memory. This step is time consuming and slows down the random numbers
-generation. Moreover, if one is interested by applications that consume random
-numbers directly when they are generated, their storage are completely
-useless. In this figure we can see that when the number of threads is greater
-than approximately 30,000 upto 5 millions the number of random numbers generated
-per second is almost constant. With the naive version, it is between 2.5 and
-3GSample/s. With the optimized version, it is approximately equals to
-20GSample/s. Finally we can remark that both GPU cards are quite similar. In
-practice, the Tesla C1060 has more memory than the GTX 280 and this memory
+In Figure~\ref{fig:time_xorlike_gpu} we compare the quantity of pseudorandom numbers
+generated per second with various xor-like based PRNG. In this figure, the optimized
+versions use the {\it xor64} described in~\cite{Marsaglia2003}, whereas the naive versions
+embed the three xor-like PRNGs described in Listing~\ref{algo:seqCIPRNG}. In
+order to obtain the optimal performances, the storage of pseudorandom numbers
+into the GPU memory has been removed. This step is time consuming and slows down the numbers
+generation. Moreover this storage is completely
+useless, in case of applications that consume the pseudorandom
+numbers directly after generation. We can see that when the number of threads is greater
+than approximately 30,000 and lower than 5 millions, the number of pseudorandom numbers generated
+per second is almost constant. With the naive version, this value ranges from 2.5 to
+3GSamples/s. With the optimized version, it is approximately equal to
+20GSamples/s. Finally we can remark that both GPU cards are quite similar, but in
+practice, the Tesla C1060 has more memory than the GTX 280, and this memory
should be of better quality.
+As a comparison, Listing~\ref{algo:seqCIPRNG} leads to the generation of about
+138MSample/s when using one core of the Xeon E5530.
\begin{figure}[htbp]
\begin{center}
\includegraphics[scale=.7]{curve_time_xorlike_gpu.pdf}
\end{center}
-\caption{Number of random numbers generated per second with the xorlike based PRNG}
+\caption{Quantity of pseudorandom numbers generated per second with the xorlike-based PRNG}
\label{fig:time_xorlike_gpu}
\end{figure}
-In comparison, Listing~\ref{algo:seqCIprng} allows us to generate about
-138MSample/s with only one core of the Xeon E5530.
-In Figure~\ref{fig:time_bbs_gpu} we highlight the performance of the optimized
-BBS based PRNG on GPU. Performances are less important. On the Tesla C1060 we
-obtain approximately 1.8GSample/s and on the GTX 280 about 1.6GSample/s.
+
+In Figure~\ref{fig:time_bbs_gpu} we highlight the performances of the optimized
+BBS-based PRNG on GPU. On the Tesla C1060 we
+obtain approximately 700MSample/s and on the GTX 280 about 670MSample/s, which is
+obviously slower than the xorlike-based PRNG on GPU. However, we will show in the
+next sections that
+this new PRNG has a strong level of security, which is necessary paid by a speed
+reduction.
\begin{figure}[htbp]
\begin{center}
\includegraphics[scale=.7]{curve_time_bbs_gpu.pdf}
\end{center}
-\caption{Number of random numbers generated per second with the BBS based PRNG}
+\caption{Quantity of pseudorandom numbers generated per second using the BBS-based PRNG}
\label{fig:time_bbs_gpu}
\end{figure}
-Both these experiments allows us to conclude that it is possible to
-generate a huge number of pseudorandom numbers with the xor-like version and
-about tens times less with the BBS based version. The former version has only
-chaotic properties whereas the latter also has cryptographically properties.
+All these experiments allow us to conclude that it is possible to
+generate a very large quantity of pseudorandom numbers statistically perfect with the xor-like version.
+In a certain extend, it is the case too with the secure BBS-based version, the speed deflation being
+explained by the fact that the former version has ``only''
+chaotic properties and statistical perfection, whereas the latter is also cryptographically secure,
+as it is shown in the next sections.
A cryptographic PRNG $G$ is secure if for any probabilistic polynomial time
algorithm $D$, for any positive polynomial $p$, and for all sufficiently
large $k$'s,
-$$| \mathrm{Pr}[D(G(U_k))=1]-Pr[D(U_{\ell_G(k)}=1]|< \frac{1}{p(N)},$$
+$$| \mathrm{Pr}[D(G(U_k))=1]-Pr[D(U_{\ell_G(k)})=1]|< \frac{1}{p(N)},$$
where $U_r$ is the uniform distribution over $\{0,1\}^r$ and the
probabilities are taken over $U_N$, $U_{\ell_G(N)}$ as well as over the
internal coin tosses of $D$.
then the new one is secure too.
\begin{proposition}
+\label{cryptopreuve}
If $H$ is a secure cryptographic PRNG, then $X$ is a secure cryptographic
PRNG too.
\end{proposition}
\end{proof}
+\section{Cryptographical Applications}
-
-\section{A Cryptographically Secure PRNG for GPU}
+\subsection{A Cryptographically Secure PRNG for GPU}
\label{sec:CSGPU}
-It is possible to build a cryptographically secure prng based on the previous
-algorithm (algorithm~\ref{algo:gpu_kernel2}). It simply consists in replacing
-the {\it xor-like} algorithm by another cryptographically secure prng. In
-practice, we suggest to use the BBS algorithm~\cite{BBS} which takes the form:
-$$x_{n+1}=x_n^2~ mod~ M$$ where $M$ is the product of two prime numbers. Those
-prime numbers need to be congruent to 3 modulus 4. In practice, this PRNG is
-known to be slow and not efficient for the generation of random numbers. For
-current GPU cards, the modulus operation is the most time consuming
-operation. So in order to obtain quite reasonable performances, it is required
-to use only modulus on 32 bits integer numbers. Consequently $x_n^2$ need to be
-less than $2^{32}$ and the number $M$ need to be less than $2^{16}$. So in
-pratice we can choose prime numbers around 256 that are congruent to 3 modulus
-4. With 32 bits numbers, only the 4 least significant bits of $x_n$ can be
-chosen (the maximum number of undistinguishing is less or equals to
-$log_2(log_2(x_n))$). So to generate a 32 bits number, we need to use 8 times
-the BBS algorithm, with different combinations of $M$ is required.
-
-Currently this PRNG does not succeed to pass all the tests of TestU01.
+
+It is possible to build a cryptographically secure PRNG based on the previous
+algorithm (Algorithm~\ref{algo:gpu_kernel2}). Due to Proposition~\ref{cryptopreuve},
+it simply consists in replacing
+the {\it xor-like} PRNG by a cryptographically secure one.
+We have chosen the Blum Blum Shum generator~\cite{BBS} (usually denoted by BBS) having the form:
+$$x_{n+1}=x_n^2~ mod~ M$$ where $M$ is the product of two prime numbers. These
+prime numbers need to be congruent to 3 modulus 4. BBS is
+very slow and only usable for cryptographic applications.
+
+
+The modulus operation is the most time consuming operation for current
+GPU cards. So in order to obtain quite reasonable performances, it is
+required to use only modulus on 32 bits integer numbers. Consequently
+$x_n^2$ need to be less than $2^{32}$ and the number $M$ need to be
+less than $2^{16}$. So in practice we can choose prime numbers around
+256 that are congruent to 3 modulus 4. With 32 bits numbers, only the
+4 least significant bits of $x_n$ can be chosen (the maximum number of
+indistinguishable bits is lesser than or equals to
+$log_2(log_2(x_n))$). So to generate a 32 bits number, we need to use
+8 times the BBS algorithm with different combinations of $M$. This
+approach is not sufficient to pass all the tests of TestU01 because
+the fact of having chosen small values of $M$ for the BBS leads to
+have a small period. So, in order to add randomness we proceed with
+the followings modifications.
+\begin{itemize}
+\item
+First we define 16 arrangement arrays instead of 2 (as described in
+algorithm \ref{algo:gpu_kernel2}) but only 2 are used at each call of
+the PRNG kernels. In practice, the selection of which combinations
+arrays will be used is different for all the threads and is determined
+by using the three last bits of two internal variables used by BBS.
+This approach adds more randomness. In algorithm~\ref{algo:bbs_gpu},
+character \& performs the AND bitwise. So using \&7 with a number
+gives the last 3 bits, so it provides a number between 0 and 7.
+\item
+Second, after the generation of the 8 BBS numbers for each thread we
+have a 32 bits number for which the period is possibly quite small. So
+to add randomness, we generate 4 more BBS numbers which allows us to
+shift the 32 bits numbers and add upto 6 new bits. This part is
+described in algorithm~\ref{algo:bbs_gpu}. In practice, if we call
+{\it strategy}, the number representing the strategy, the last 2 bits
+of the first new BBS number are used to make a left shift of at least
+3 bits. The last 3 bits of the second new BBS number are add to the
+strategy whatever the value of the first left shift. The third and the
+fourth new BBS numbers are used similarly to apply a new left shift
+and add 3 new bits.
+\item
+Finally, as we use 8 BBS numbers for each thread, the store of these
+numbers at the end of the kernel is performed using a rotation. So,
+internal variable for BBS number 1 is stored in place 2, internal
+variable for BBS number 2 is store ind place 3, ... and internal
+variable for BBS number 8 is stored in place 1.
+\end{itemize}
+
+
+\begin{algorithm}
+
+\KwIn{InternalVarBBSArray: array with internal variables of the 8 BBS
+in global memory\;
+NumThreads: Number of threads\;
+tab: 2D Arrays containing 16 combinations (in first dimension) of size combination\_size (in second dimension)\;}
+
+\KwOut{NewNb: array containing random numbers in global memory}
+\If{threadId is concerned} {
+ retrieve data from InternalVarBBSArray[threadId] in local variables including shared memory and x\;
+ we consider that bbs1 ... bbs8 represent the internal states of the 8 BBS numbers\;
+ offset = threadIdx\%combination\_size\;
+ o1 = threadIdx-offset+tab[bbs1\&7][offset]\;
+ o2 = threadIdx-offset+tab[8+bbs2\&7][offset]\;
+ \For{i=1 to n} {
+ t<<=4\;
+ t|=BBS1(bbs1)\&15\;
+ ...\;
+ t<<=4\;
+ t|=BBS8(bbs8)\&15\;
+ //two new shifts\;
+ t<<=BBS3(bbs3)\&3\;
+ t|=BBS1(bbs1)\&7\;
+ t<<=BBS7(bbs7)\&3\;
+ t|=BBS2(bbs2)\&7\;
+ t=t $\hat{ }$ shmem[o1] $\hat{ }$ shmem[o2]\;
+ shared\_mem[threadId]=t\;
+ x = x $\hat{ }$ t\;
+
+ store the new PRNG in NewNb[NumThreads*threadId+i]\;
+ }
+ store internal variables in InternalVarXorLikeArray[threadId] using a rotation\;
+}
+
+\caption{main kernel for the BBS based PRNG GPU}
+\label{algo:bbs_gpu}
+\end{algorithm}
+
+In algorithm~\ref{algo:bbs_gpu}, t<<=4 performs a left shift of 4 bits
+on the variable t and stores the result in t. BBS1(bbs1)\&15 selects
+the last four bits of the result of BBS1. It should be noticed that
+for the two new shifts, we use arbitrarily 4 BBSs that have previously
+been used.
+
+
+
+\subsection{A Cryptographically Secure and Chaotic Asymetric Cryptosystem}
+
+\subsubsection{Recalls of the Blum-Goldwasser Probabilistic Cryptosystem}
+
+The Blum-Goldwasser cryptosystem is a cryptographically secure asymmetric key encryption algorithm
+proposed in 1984~\cite{Blum:1985:EPP:19478.19501}. The encryption algorithm
+implements an XOR-based stream cipher using the BBS PRNG, in order to generate
+the keystream. Decryption is done by obtaining the initial seed thanks to
+the final state of the BBS generator and the secret key, thus leading to the
+ reconstruction of the keystream.
+
+The key generation consists in generating two prime numbers $(p,q)$,
+randomly and independently of each other, that are
+ congruent to 3 mod 4, and to compute the modulus $N=pq$.
+The public key is $N$, whereas the secret key is the factorization $(p,q)$.
+
+
+Suppose Bob wishes to send a string $m=(m_0, \dots, m_{L-1})$ of $L$ bits to Alice:
+\begin{enumerate}
+\item Bob picks an integer $r$ randomly in the interval $[1,N$ and computes $x_0 = r^2~mod~N$.
+\item He uses the BBS to generate the keystream of $L$ pseudorandom bits $(b_0, \dots, b_{L-1})$, as follows. For $i=0$ to $L-1$,
+\begin{itemize}
+\item $i=0$.
+\item While $i \leqslant L-1$:
+\begin{itemize}
+\item Set $b_i$ equal to the least-significant\footnote{BBS can securely output up to O(loglogN) of the least-significant bits of xi during each round.} bit of $x_i$,
+\item $i=i+1$,
+\item $x_i = (x_{i-1})^2~mod~N.$
+\end{itemize}
+\end{itemize}
+\item The ciphertext is computed by XORing the plaintext bits $m$ with the keystream: $ c = (c_0, \dots, c_{L-1}) = m \oplus b$.
+\end{enumerate}
+The ciphertext is $(c, y)$, where $y=x_{0}^{2^{L}}~mod~N.$.
+
+
+When Alice receives $(c_0, \dots, c_{L-1}), y$, she can recover $m$ as follows:
+\begin{enumerate}
+\item Using the secret key $(p,q)$, she computes $r_p = y^{((p+1)/4)^{L}}~mod~p$ and $r_q = y^{((q+1)/4)^{L}}~mod~q$.
+\item The initial seed can be obtained using the following procedure: $x_0=q(q^{-1}~{mod}~p)r_p + p(p^{-1}~{mod}~q)r_q~{mod}~N$
+\item Recompute the bit-vector $b$ by using BBS and $x_0$.
+\item Compute finally the plaintext by XORing the keystream with the ciphertext: $ m = c \oplus b$.
+\end{enumerate}
+
+
+\subsubsection{Proposal of a new Asymmetric Cryptosystem Adapted from Blum-Goldwasser}
+
\section{Conclusion}
In this paper we have presented a new class of PRNGs based on chaotic
-iterations. We have proven that these PRNGs are chaotic in the sense of Devenay.
+iterations. We have proven that these PRNGs are chaotic in the sense of Devaney.
We also propose a PRNG cryptographically secure and its implementation on GPU.
An efficient implementation on GPU based on a xor-like PRNG allows us to
generate a huge number of pseudorandom numbers per second (about
-20Gsample/s). This PRNG succeeds to pass the hardest batteries of TestU01.
+20Gsamples/s). This PRNG succeeds to pass the hardest batteries of TestU01.
In future work we plan to extend this work for parallel PRNG for clusters or
-grid computing. We also plan to improve the BBS version in order to succeed all
-the tests of TestU01.
+grid computing.
