with the increasing popularity of technologies such as digital television , internet streaming video and video conferencing , video compression has became an essential component of broadcast and entertainment media . among various kinds of approaches , block - based motion estimation and compressionare most widely accepted ones .the _ block - matching algorithm_(bma ) for _ motion estimation_(me ) has been adopted in many international standards for digital video compression , such as h.264 and mpeg 4 . in the framework of video coding, the statistical redundancies can be categorized by either temporal or spatial . for the purpose of reducing the temporal redundancies among frames ,motion estimation was applied .block - based matching algorithms consider each frame in the video sequence formed by many nonoverlapping small regions , called the marcoblocks(mb ) which are often square - shaped and with fixed - size( or ) .let represents the mb and the number of blocks , and ; let be the entire frame and the partition into mbs should satisfy and .given a mb in the anchor frame , the motion estimation problem is to determine a corresponding matching mb in the target frame such that the matching error between these two blocks is minimized .then , a motion vector is computed by subtracting the coordinates of the mb in the anchor frame from that of the matching mb in the target frame . instead of sending the entire frame pixel - by - pixel , a set of motion vectorsis transmitted through the channel which greatly reduces the amount of transmission . in the decoder side, a motion compensated procedure is applied to reconstruct frames using the received motion vectors and the anchor frame . referred to many researches ,the motion estimation and encoding part consumes nearly percent of the total amount of computation in the whole video compression procedure thus making it an active research topic in the last two decades .there are many proposals of bmas in literature .the most basic one is the _ exhaustive search_(es ) , also known as full search which simply compares the given mb in the anchor frame with all candidate mbs in the target frame exhaustively within a predefined search region .previous research showed that es can obtain high matching accuracy but requires a very large amount of computation thus infeasible to implement in real - time video applications . to speed up the search , various fast algorithms for block matching which reduce the number of search candidateshave been developed .well known examples are _2-d logarithmic search_(logs) , _ three step search_(tss) , _ four step search_(4ss) , _ diamond search_(ds) which is accepted in the mpeg-4 verification model and widely implemented in vlsi , and the recent proposed _ adaptive rood pattern search_(arps) which is almost two or three times faster than ds and even achieves higher _ peak signal - to - noise ratio_(psnr ) than that using ds .  from the optimization point of view, block - based methods can be described by the following minimization , :   - i_{k-1}[\mathbf{n}+\mathbf{d}_{m}])\ ] ]  where is the target frame ; is the anchor frame ; is the matching error ; are the motion vectors and is the search area to which belongs , defined as .sign of is positive when motion of the block is towards positive direction from frame to frame . and negative when motion of the blcok is in negative direction from frame to frame . is an size mb with the top - left corner coordinate at .the goal is to find the best displacement motion vector for each mb , in the sense of the criterion ._ particle swarm optimization_(pso ) was originally proposed by kennedy and eberhart in 1995 .it is widely accepted and focused by researchers due to its profound intelligence background and simple algorithm structure .currently , pso has been implemented in a wide range of research areas such as functional optimization , pattern recognition , neural network training , fuzzy system control etc . and obtained significant success . like _genetic algorithm_(ga ) , pso is also an evolutionary algorithm based on swarm intelligence .but , on the other side , unlike ga , pso has no evolution operators such as crossover and mutation . in pso ,the potential solutions , called particles , fly through the solution space by following the current optimum particles . the original intent was to graphically simulate the graceful but unpredictable choreography of a bird flock . through competitions and cooperations ,particles follow the optimum points in the solution space to optimize the problem .many proposals indicate that pso is relatively more capable for global exploration and converges more quickly than many other heuristic algorithms .the rest of the paper is organized as follows .section ii introduces the pso algorithm and we propose the pso - zmp block - matching algorithm for motion estimation in section iii .simulation results and analysis on five video sequences are given in section iv .section v concludes the paper .particle swarm algorithm is a kind of evolutionary algorithm based on swarm intelligence .each potential solution is considered as one particle , and these particles are distributed stochastically in the high - dimensional solution space in the initialization period of the algorithm . through following the optimum discovered by itself and the entire group , each particle periodically updates its own velocity and position .      where , is the number of particles and is the dimensionality ; ,  ] is the position vector of particle which is a potential solution in the solution space .the quality of the solution is measured by a fitness function ; is the inertia weight which decreases linearly during a run ; are both positive constants , called the acceleration factors which are generally set to 2.0 ; and are two independent random number distributed uniformly over the range  ] as its motion vector(mv ) .   based on the center - biased characteristics in video sequences , that is ,certain mbs are highly correlated in local regions of the frame , the encoder creates a prediction of a region of the current frame based on previously encoded and transmitted frames .if the frame is processed in raster order , the current - encoded mb should have four patterns of _ region of support_(ros ) that consists of the neighboring blocks whose mvs will be used to compute the predicted mv for prediction in fig .[ pre.pattern ] due to the limited computational cost . experiment mentioned in shows there is little psnr difference using these four ros patterns in the predictive coding routine , and ros type d consumes least amount of computation because of its simplest structure .thus , ros pattern d is adopted in this paper .   due to the spatial correlation characteristics between mbs in one frame , during the initiation period of the pso matching routine , we distribute the particles in four specific patterns(fig . [ pattern.all ] ) with a view to reduce the computational cost but to achieve higher psnr .since frames are processed in raster order , the mb in the top - left corner in the frame , can not be predictive coded because there is no reference mb for prediction in the current - encoded frame .thus , for this condition , we simply skip the predictive coding and begin pso searching routine directly with the initial positions of particles in the pattern type b in fig .[ pattern.all ] .  for those mbs located at the leftmost column of frames ,their reference mbs used in predictive coding are in the other side of the frame , thus may not be highly correlated and inefficient in prediction .so , we also solely perform the pso searching routine in this case , with the pattern type d in fig .[ pattern.all ] . and , for the last leftmostmb been processed in the frame , that is , the mb in the bottom - left corner , we use the pattern type c in fig .[ pattern.all ] instead .otherwise , pattern type a in fig .[ pattern.all ] is adopted .we put four particles in a rood shape with size zero(size refers to the distance between any vertex point and the center - point ) in the adjacent mbs and four particles in a rood shape with size one , and then rotate it by angle . with two rood shape in difference size, we try to balance the global exploration and local refined search in order for broader searching space as well as higher matching accuracy .moreover , we distribute particles equally in all directions(8 particles in 8 directions ) with a view to , in stochastic condition , find the matching mb in each direction with equal possibility .notably , if the position of a particle in the during initialization and a pso run is out of the boundary of the image frame , we simply put the particle in the position nearest to its intended position .generally , there are two widely adopted stopping criteria .fixed - iteration _ , that is , given a certain iteration time , saying , the search stops after times of iteration. the other is _ specified - threshold_. during a pso run , the most - fitted value found by the entire group , called the _ best so far _ " value will be updated by the particles . for minimization problems , we specify a very small threshold , and if the change of during times of iteration is smaller than the threshold , we consider the group best value very near to the global optimum , thus the matching procedure stops . due to the center - biased characteristics of real - world motion fields ,we adopt the fixed - iteration method in this paper for reducing the computational cost .we incorporate the zmp , the predictive coding and the pso matching routines together and propose a block - matching algorithm for motion estimation based on pso and zmp .the algorithm can be summarized in the pseudocode below : +we ve tested our pso - zmp algorithm on five test video sequences : akiyo , container , mother & daughter , news and silent within 100 image frames(except 90 frames in akiyo due to the limitation of the sequence length ) .pso matching is the core routine in our algorithm . in this paper , to balance between computational cost and compensated video quality , we adopt the standard pso with inertia weight which is widely considered as the defacto pso standard .we use the fixed - iteration stopping criterion with max 5 iterations .the max velocity is set to 5 .the inertia weight decreases linearly from 0.9 to 0.4 during a pso run and two acceleration factors , are set to 2.0 , as commonly did .* we divide a whole image frame into mbs in the simulation . *we select a zmp threshold for each test video sequence correspondingly based on data obtained in experiments .the parameters are shown in table [ zmp.table ] .* we do not restrict the range of candidate matching mbs rigidly by a search window . instead , through the fixed - iteration and the setting of max velocity , particles search for the matching mb in an area more flexible and adaptable .  .zmpthreshold for five test video sequences [ cols="^,^,^",options="header " , ]in this paper , we have proposed a fast block - based motion estimation algorithm based on _ particle swarm optimization_(pso ) with novel particle initiation patterns . applied successfully in many functional and combinatorial optimization problems, pso is proved to have a relevant stronger ability in global exploration .in addition , a _ zero - motion prejudgment_(zmp ) routine is incorporated into the pso bma to further reduce the computational cost of the algorithm .simulation results show that the pso - zmp bma proposed requires less amount of computation and achieves psnr in a acceptable degree of drop . while close and acceptable psnr performance compared towidely accepted arps and ds bma .moreover pso just consumes a few lines of codes due to its simplicity which makes the pso - zmp algorithm attractive for hardware implementation .  in the future, variants of pso might be applied to strengthen the global searching ability and the accelerate the convergence speed . and , to speed up the search and avoid being trapped in local minima , a multiresolution procedure may be used .the authors would like to thank qian wu for helping us make the nice search pattern figures and yuxuan wang for the invaluable discussion and proofreading .s.zhu and k.k.ma . a new diamond search algorithm for fast block - matching motion estimation . in _ proc .information , communications and signal processing(icics ) _ , volume 1 , pages 292296 , sept.9 - 12 1997 .

Abstract: in this paper , we propose a fast 2-d block - based motion estimation algorithm called _ particle swarm optimization - zero - motion prejudgment_(pso - zmp ) which consists of three sequential routines : 1)zero - motion prejudgment . the routine aims at finding static macroblocks(mb ) which do not need to perform remaining search thus reduces the computational cost ; 2)predictive image coding and 3)pso matching routine . simulation results obtained show that the proposed pso - zmp algorithm achieves over 10 times of computation less than diamond search(ds ) and 5 times less than the recent proposed adaptive rood pattern searching(arps ) . meanwhile the psnr performances using pso - zmp are very close to that using ds and arps in some less - motioned sequences . while in some sequences containing dense and complex motion contents , the psnr performances of pso - zmp are several db lower than that using ds and arps but in an acceptable degree .


the design of a secure and efficient user authentication scheme is one of the major concerns for most enterprises and organizations .a significant amount of money , time and effort are invested every year to carry out research in this direction . according to cybersecurity ventures ,the u.s government has invested more than $ million over the past four years in multi - factor authentication ( mfa ) techniques , aiming to improve a simple password - based authentication scheme  .additionally , many academic studies in the past studied extensively the security and usability of password - based authentication techniques .  despite the fact that several methods of authentication , such as hardware tokens , biometrics , mouse and keyboard keystroke analytics , have been developed in the past few years, a simple password - based scheme is still the primary mean of authentication for many online services .this is mainly due to the fact that password - based authentication is a cheap , efficient and secure ( at least in theory ) method of authenticating users . as shown in  , designing other than simple password - based authentication scheme might be a very complex task mostly due to the fact that not only the best security engineering practices , e.g. , usability and privacy have to be applied , but also the human factor has to be taken into consideration .the security of password - based systems relies on the user to choose a strong enough password .if this password is not complex enough , then brute - force or dictionary attacks could potentially breach the security of a system .brute - force attacks assume that the distribution of human - chosen passwords is uniform which is not a practical assumption , as human tend to select passwords based on patterns or structures arising from their natural language .relatively recent research has revealed that this curve ( of user - selected passwords ) is skewed and more sound mathematical metrics for the security against guessing attacks using large dictionaries are presented in .in addition to the above - mentioned human factor , more sophisticated attacks using , e.g. , malware could be performed .these types of attacks predominantly exploit various phishing campaigns convincing either directly or by other means like social engineering approaches the potential victim to unintentionally install malicious software on the target computing device . upon infection , the victim s machineis completely controlled by the attacker who can easily obtain user s passwords in a singe step .researchers have realized this problem , therefore other identification methods in an attempt to mitigate single - step disclosure of shared - secret by introducing time - varying challenges have been proposed  .the partial password scheme is an example of such method where authentication takes place in the form of challenge - response pairs , with the challenge requesting a set of characters chosen randomly from a pre - shared password .it is considered as a very cheap and effective method against several attacks that could otherwise compromise a shared - secret in a single step .it is claimed to be more secure than the simple password implementation due to the fact that the size of the responses space grows in a combinatorial way , depending on the implementation .for example , for a password of length and a partial - password implementation requesting characters out of , the number of possible responses is if no repetitions are allowed and if repetitions are allowed .partial password method is widely deployed in the banking sector especially in uk as a part of ( at least ) 2-factor authentication method  for authenticating users in internet banking .it decreases the probability of success of malware - based attacks since the fraudster can not really provide to their command - and - control ( cc ) server the full password in a single step . even though, the fraudsters can sometimes bypass this mechanism by exploiting the weakest link , the human , using html injections to modify the page presented to the user and request the full password , this scenario is out of the scope of this paper .  in general ,all type of attacks applied to the simple password implementations , apply also to partial - password implementation schemes .the only difference is that the attacker requires more data to launch a successful attack , i.e. , intersepting more times the authentication handshake in order to either reconstruct the full password or get enough data to respond correctly to a new challenge with an overwhelming probability .thus , we have three main type of attacks applied also to partial - password implementations , as follows :  1 .* brute force * : an attacker uses a computer program or a script that produces all possible password combinations using a fixed alphabet .then , the attacker tries each password , one by one , until authentication is successful .* dictionary attack * : an attacker uses a program or script to try to authenticate by cycling through combinations of common words or using dictionaries based on information related to passwords obtained from compromised servers .key logger * : an attacker uses a program to track all of a user s keystrokes ._ outline of contributions : _ our motivation is to investigate some open questions  , such as how security of the partial password scheme varies if challenges are generated using a different method , e.g. , allowing the same positions to be requested in the same challenge and how information about user s responses only could be used to speed - up dictionary attacks .the later scenario is close to the scenario of a hardware - keylogger or to a scenario where the malware has limited capabilities in terms of intercepting also the challenge presented to the end - user .considering the fact that half of online users access their banking account at least twice a week  , there is sufficient information exposed that could be used to launch succesfful attacks .in addition , we discuss possible server - side partial password implementations as ( unofficially ) indicated by several information security experts in different online forums . unfortunately , there is no formal academic literature in this direction and we aim to motivate other researchers to work in this direction , as partial password implementations are deployed by several major banks in their internet banking .this paper is organised as follows . in section 2related studies are discussed .section 3 presents the partial password implementations .section 4 discusses the security of partial password implementations under different attack scenarios and settings .finally , section 5 concludes the paper and gives future research directions in the field .in this section we present related studies that fall into partial password mode of authentication .for example , we describe hardware keyloggers attack scenarios in which an attacker has data related to the responses but nothing related to the associated challenges .  _hardware keyloggers : _ a paper by goring _ et al . _ studies the case of a hardware keylogger attack , where the attacker can obtain responses but not challenges .however , their method is limited to a very particular case where whenever authentication fails , the server presents again the same challenge to the user .this potentially allows the attacker to construct challenge - response pairs by just repeating the authentication process . in this paper, we further investigate this attack model and we study how we can use data obtained in a keylogger setting combined with large - dictionaries of user - selected passwords in order to speed up dictionary attacks ._ partial password schemes : _ another paper by aspinall _ et al . _ studies the security of a particular partial password implementation , where the positions requested in the challenges are chosen uniformly at random without replacement .furthermore , they study how the security of the system is related to the number of challenge - response pairs that the attacker has obtained ( defined in  as _ recording _ attacks ) . in order to speed - up their attacks they applied frequency analysis of letters of user - selected passwords , as appearing in the rockyou dataset  . in this paper, we study a more generic scheme in which the challenges are chosen uniformly at random and repetitions of positions is allowed .this is claimed to be a more complex scenario and left as future work in  and this is the major contribution of this paper .a partial password is a challenge on a subset of characters from a full password . the overall protocol consist of two phases which could be described as follows  :  * a. registration phase : * the user selects a password of a desired length and usually on a restricted alphabet.  * b. login phase : * the authentication phase is based on the following challenge - response protocol .  1 .* challenge * : the server selects a subset of integers from the set and presents the challenge to the user .+ [ cols="<,^ , > , > , > , > , > , > , > " , ]  2 .* response * : the response will be of the form . the user passes this step only if for all .  if the user s response is not correct , then either the same or a fresh challenge is presented , while on a subsequent login trial a fresh challenge is generated in case of a previous successful authentication .the scenario where the same challenge is presented to the user was studied in  .in addition , aspinall and just studied the security of the scheme when the integers are chosen uniformly at random but without replacement , while the scenario of repetitions allowed is left as open question as it is considered more complex .one of the major contributions of this paper is that we study also this scenario .   in classical password implementationsonly the hash of the password is enough to be stored on the server . finding a message for a given hash value ( or two different messages with the same hash value ) for secure cryptographic hash functionsis considered computationally hard , thus even an adversary with unrestricted access to the hash values can not deduce the password from the hashes , if a secure cryptographic hash functions is employed , such as sha-256 .however , in partial password schemes , a new level of complexity in both storage and validation of the shared - secret on the server side is introduced .it is not enough anymore to store the hash of the full password and hence standard password hashing schemes do not apply . instead one has to either store the password in a plaintext , or the hashes of different combinations of each password  . for the latter solution , it is not trivial to store the hashes of all the combinations of variable length passwords .possibly , this is the reason why most banks are restricting both the length and the alphabet of the user passwords and only request for up to 4 ( maximum ) different characters in their partial - password implementation schemes  .  to the best of our knowledge, there is no formal academic literature discussing the problem of server - side implementation of partial password authentication mechanisms .we would like to motivate academic research in this direction as those schemes are widely deployed by major banks around the globe . based on our research findings by searching several online security related forums we have indicated that possible implementations deployed in industry might be as follows  :  1 .the password is stored in plaintext  .this imposes a significant risk from a security point of view as an administrator is likely to have a direct access to the password in plaintext form . furthermore , if the database is compromised then an adversary has access to all plaintext passwords. this solution might be also not complied with policies requiring hashed or encrypted password storage .the hashes of all possible combinations of letters are stored per password per user  . in a general case , where there are not many constraints applied on a password , this solution might lead to significant database issues in terms of required storage space .however survey conducted in  showed that many banking online systems , that are based on the partial password mode of authentication , impose more or less rigorous restrictions on the length of the password and the size of a character set . in extreme cases passwordcould be restricted to a size of only four characters allowing a character set of size 10 ( pin case ) . by applying such restrictions ,database storage issues become less demanding and thus this extensive hashing method is more applicable in practice . under this setting , for a password of length and a partial password scheme that requests positions we need to store possible hashes , which is translated to bits of information per user , if a -bit hash function is employed , e.g. , =256 for sha-256 .another practical implementation that one can think of is the following :  1 .the password could be stored on the server in an encrypted form with a use of some symmetric - key scheme , like aes . in this case , to mitigate any practical key management issues , keys could be managed by a tamper - resistant hardware , i.e. , hardware security module ( hsm ) or a separate authentication server with employed appropriate access control systems in order to avoid unauthorized users to access the cryptographic key .this would provide a black - box interface for encryption and substring verification such that when the password characters are passed to the application they are fed into the hsm or the authentication server along with the encrypted password .the hsm could then decrypt the password and confirm ( or reject ) the validity of the provided characters . however , the drawback of this method is that during authentication , the full password is decrypted and under certain circumstances leakage of this fully decrypted password could occur .considering the survey conducted in  , there are surprisingly many tight constraints imposed on passwords used in partial password schemes , i.e. , the size of acceptable alphabet and length of the password are relatively small , as well as the number of requested characters in the challenges . in the case of internet banking authentication , most banks request a password within a given range and restricted to a given alphabet , usually the alphanumeric of size 36 or numeric of size 10 characters ( pin ) .in this section we focus on questions like how many challenge - response pairs are sufficient to reconstruct the shared - secret and how many are needed in order to guess correctly the next challenge in a partial password protocol with sufficiently high probability .we essentially study the following three attack scenarios :  * * recording attacks : * a malware or a keylogger installed on the user s device is recording several ( challenge , response ) pairs which are sent to the fraudster s server .the main goal of the fraudster is to reconstruct the password . ** next - challenge attacks : * same setting as in recording attacks but in this scenario the attacker would like to know the success rate of providing the correct response given some pairs . ** attacks with unknown challenges : * the attacker runs a dictionary attack and for some reason has only a set of responses , without necessarily knowing the corresponding positions . the idea is to examine if such limited information could benefit a lot a dictionary attack .since human - selected password distribution is known to be skewed this could be seen as another confirmation of this empirical result .  in order to tackle the scenario where the positions in the challenge could repeat in the same challenge, we resemble the definition of a _ multiset _ ( cf . definition [ def1 ] ) .[ def1 ] a * multiset * is a 2-tuple where is some set and a function from to the set .the number of multisets of cardinality , with elements taken from a finite set of cardinality , is called the _multiset coefficient_. this number is denoted by and is given by .suppose that the user has agreed on a password of length with , , where the pre - defined alphabet .we have evaluated the security of partial password implementation in two different scenarios .  1 .* scenario a ( without replacement ) : * the challenge is of the form with , for all and for all .* scenario b ( with replacement ) : * the challenge is of the form with , for all .consider the case where malware , installed on the user computing device , is capturing the responses of the user before the http post being encrypted with ssl and sends these responses to the command - and - control server .then , the threat scenario is that after sufficient data the attacker would be able either to reconstruct the full password or have a sufficiently high probability to response correctly to fresh challenges .the security analysis of both scenarios is based on theorem [ theorem1 ] .[ theorem1 ]  let the number of different positions of the password that the malware posses after capturing challenge - response pairs .the probability , that the malware knows exactly out of the total positions is given by equation [ eq1 ] and for scenario a and b respectively ,      if at step , the malware obtained distinct indices and the aim is exactly by having another pair , this implies we need to select exactly from the unseen ones and select the rest depending on the scenario . for scenario a , we choose out of the already known indices , while for b we choose from indices , allowing repetitions .figure 1 presents how probability varies against the number of challenge - response pairs .as we observe , in case of and , an attacker can reconstruct the password with probability higher that after recording pairs in scenario a and for scenario b. for and , pairs are needed in scenario a while for scenario b for a success probability .another question of significant interest is the probability to respond correctly to a new challenge given pairs .denote these probabilities as and for scenario a and b respectively .then , we have the following :    after runs , if the attacker knows positions , the expected number of pairs learned is given by and respectively ,    in figure 2 we observe that an attacker has probability higher than to correctly reply to the next challenge , by having 8 pairs in scenario a or equivalently 9 pairs in scenario b , for and .thus , security of both schemes is similar for average passwords regarding guessing the next challenge .   in this section, we study the scenario where an attacker has obtained some information regarding user s responses , but has no knowledge to which challenge they correspond .this is similar to the hardware keyloggers scenario as mentioned by goring _et al _ in .we call this scenario as the `` attacks with unknown challenges '' scenario .we have experimentally demonstrated that in case of a dictionary attack if information available from keyloggers is used , then we have a significant reduction in the dictionary size , ending up with a reduced number of candidates .this confirms even more the claim that the probability distribution of human - selected passwords is skewed .this is due to the fact that even with having a set of characters randomly selected from a word , we can limit down tremendously the number of possible candidates in a dictionary attack . in our experiments , we used as a dictionary the well - studied rockyou dataset and results are presented in table 1 .  denoting by the set of available characters corresponding to a target password ( i.e. , for  ), we have performed the following three experiments :  1 .* experiment a * : and two characters positions of the password are known .* experiment b * : and the length of the password are known 3 .  *experiment c * : , the length of the password and two characters positions are known .the algorithm we employed to filter down possible password candidates is described in algorithm  .note that is a parameter which is used in order to search for passwords which are close to the length of password up to a desired margin . in our casewe study the scenario , i.e targeting passwords of known length .table  presents some of the results of our experiments .initialize an empty list compute , the set of distinct character appearing in the word compute * experiment a : * * if * and known : * experiment b : * * if * and : * experiment c : * * if * and and :   l|c|r||r|r|r|r| password & & & experiment a & experiment b & experiment c + password & & 1.0 & 2456 & 36 & 12 + baseball & & 1.0&1435&39 & 1 + dragon & & 1.0 & 3378 & 29 & 3 + admin & & 1.0 & 3695 & 17 & 7 + querty & &1.0 & 381&4 & 1 +  from table 1 we can observe that by knowing the set of distinct characters we can speed up the dictionary attack tremendously .this is expected to happen since humans tend to select words from their natural language and thus the distribution of possible -grams follow a certain distribution . in our future workwe plan to study how the number of possible candidates varies with , i.e. , the attacker posses a fraction of the password s characters. this would be complex to implement and study .partial passwords is a mode of authentication which is widely deployed by the industry and especially in uk banking sector  .it was proposed as a countermeasure against attacks that could reveal a shared secret in a single step  .it is a challenge - response protocol , where the challenge is of the form , `` _ _ what are the characters of your password at positions 1,5 and 9 ? _ _ '' .  in this paper , we extend the work of aspinall _ et al . _ , and study some of the open questions stated in the same paper .we investigate and compare the security of several partial password implementations in which the elements in the challenges are generated uniformly at random but without replacement against the one where the replacements are allowed .the latter cases seems to be more secure , especially for attackers aiming to fully reconstruct the password .they also benefit from simpler implementation since we do nt need to check if the next positions in the challenge were already asked .finally , we study the scenario where the attacker has access to responses but not challenges and whether this information is valuable to dictionary - type attacks .we have experimentally demonstrated that such information can tremendously reduce the number of potential password candidates from a given dictionary and this confirms again the claim that the probability distribution of human - chosen secrets is skewed ._ further work : _ there are several areas that we would like to investigate in more details .we would like to extend the _ hardware keylogger attack _ to other scenarios like having a percentage , , of characters from the password , how this affects the number of possible candidates from the dictionary .in addition , we plan to explore the usability of partial passwords which is still not studied despite the wide practical adoption of such mechanisms  .m. , aspinall , d. : `` on the security and usability of dual credential authentication in uk online banking . '' in : 7th international conference for internet technology and secured transactions ( icitst 2012 ) , ieee ( december 2012 )

Abstract: a _ partial password _ is a mode of password - based authentication that is widely used , especially in the financial sector . it is based on a challenge - response protocol , where at each login attempt , a challenge requesting characters from randomly selected positions of a pre - shared secret is presented to the user . this mode could be seen as a `` cheap way '' of preventing for example a malware or a keylogger installed on a user s device to learn the full password in a single step . despite of the widespread adoption of this mechanism , especially by many uk banks , there is limited material in the open literature . questions like how the security of the scheme varies with the sampling method employed to form the challenges or what are the existing server - side implementations are left unaddressed . in this paper , we study questions like how the security of this mechanism varies in relation to the number of challenge - response pairs available to an attacker under different ways of generating challenges . in addition , we discuss possible server - side implementations as ( unofficially ) listed in different online forums by information security experts . to the best of our knowledge there is no formal academic literature in this direction and one of the aims of this paper is to motivate other researchers to study this topic .   authentication , passwords , partial passwords , server - side implementation , recording attacks , dictionary attacks , keyloggers


high - dose - rate ( hdr ) brachytherapy is a form of radiation therapy in which the tumor is temporarily exposed to a highly radioactive source which dwells at different positions in or around the planning target volume ( ptv ) . for prostate tumors ,the dwell positions are offered by a temporary transperineal implant of catheters which run through the prostate .a remote afterloader , which is connected to all the catheters , then sends a radioactive source through the catheters one by one , stopping at several dwell positions inside the ptv .we assume that the catheter locations and the dwell positions are known .the goal of the treatment planner is to irradiate the ptv while sparing the surrounding organs at risk ( oars ) by optimizing the dwell time at each dwell position .traditionally , treatment planning was a forward process , where dwell times were adjusted until the dose distribution was satisfactory . in the last fifteen years, inverse planning has been developed as a computerized technique where the dwell times are optimized according to the treatment planner s preferences on the dose distribution .  in the remainder of this introduction ,we explain how dose distributions are evaluated , we provide a literature review on the ongoing progress of inverse planning , and we show how this work contributes to these developments .  _ dvh criteria . _ the dose distribution within the ptv or an oar is described in a dose - volume histogram ( dvh ) .such a histogram displays the percentage of the structure receiving at least a certain dose .an example histogram is displayed in figure [ fig : dvhexample ] .points on this histogram are denoted by or , e.g. , is the minimum dose received in the hottest 10% of the structure under consideration and represents the percentage of the volume receiving 100% of the prescribed dose . in a slight deviation from the above notation , is the maximal dose received in the structure .these statistics are currently the most important quantitative evaluation criteria .besides these , the treatment planner inspects the isodose lines in order to avoid undesired hot spots and cold spots .table [ tab : dvhcriteria ] displays clinically used dvh criteria from a local hospital . in order to achieve tumor control , of the ptvneeds to be at least , i.e. , at least of the ptv s volume need to receive at least of the prescribed dose . the limitation of complications is expressed in dvh constraints on the rectum and urethra . for the rectum imposes that of the rectum s tissue may not receive more than . a hard upper bound for tissue in the rectumis set by .for the urethra , is the upper bound for of the tissue .the remaining are allowed to reach radiation levels of at most ..dvh protocol for a prescribed dose of gy for hdr prostate brachytherapy based on . [ cols="^,^,^",options="header " , ]   [ tab : datasetdwellpos ]   dopsa has been tested on three patients .since it is a random local search procedure , the result can be subject to random effects .therefore , the heuristic has been run 250 times for each patient in order to provide a detailed analysis of the average treatment plan quality and the variation in quality .if a low variation can be ensured for this optimization algorithm , only a single optimization run per patient will be necessary in a clinical setting to obtain treatment plans with consistent quality .figures [ fig : sapt2]-[fig : sapt4 ] show the results for all three patient data sets . the average and standard deviation in ptv coverage over 250 replicationsare displayed over the running time of 180 seconds .the positioning of catheters determines the maximally achievable level of in the ptv , which explains the difference in objective values between patients .the high dependence on catheter positioning is well observable for patient 2 for whom only solutions with can be attained : inspecting the corresponding ultrasound scans reveals badly positioned catheters .  in all three cases, the ptv coverage increases rapidly within the first 30 seconds and further improvements on the treatment plan quality are achieved over the remaining running time .every treatment plan determined by dopsa is feasible , i.e. , it always satisfies the dvh criteria imposed on the oars and dwell time constraints ( see constraints - ) .the absolute standard deviations in ptv coverage after a running time of 3 minutes for each of the 250 replications are 0.09% , 0.33% , and 0.23% , respectively . therefore , dopsa delivers treatment plans with consistent quality across replications .   for the comparison with existing dvh - based optimizers, we have run an milp solver with the solver settings from and we have implemented ipip . the original ipip has been extended by the dtmr to ensure a fair comparison .these two methods have been run on the same three patient data sets and on the same model .it is important to recognize that ipip can only provide a single solution per data set , whereas milp and dopsa continue improving the solution .this is observable in figures [ fig : dvhpt2]-[fig : dvhpt4 ] , where the line for ipip remains flat after a single increase . in theory ,milp could stop when it proves that the solution is optimal , but this did not happen for any of the patients .figures [ fig : dvhpt2 ] to [ fig : dvhpt4 ] show the average ptv coverage obtained by dopsa and the coverage obtained by the existing methods milp and ipip .the running time for patient 3 has been extended to 5 minutes since milp requires more than 3 minutes to find non - zero feasible points .ipip attains adequate ptv coverage within less than 6 seconds for each patient .all those solutions are initially superior to treatment plans found by dopsa . over the running time of 3 minutes, however , dopsa consistently outperforms ipip for all patient data sets . in each of the 250 replications for all three patients , the ptv coverage after3 minutes is higher for dopsa than for ipip .the average ptv coverage attained by dopsa exceeds the results by ipip after only 18 , 6 , and 12 seconds , respectively .milp requires substantially more time to determine good treatment plans . for 2 out of 3 patients, milp requires more than 1 minute to determine a treatment plan with non - zero ptv coverage .however , after 3 minutes for patients 1 - 2 and 5 minutes for patient 3 it has found the best plan among the three algorithms .  unfortunately , the optimal objective values are not known .there is still a large gap between the best solution found by any of the algorithms and the upper bound provided by the milp solver , and this gap does not decrease substantially after running the solver for a week .dopsa differs greatly from the existing dose - volume based methods while it finds similar objective values .this could be an indication that much better solutions do not exist .the significance of the strong performance of dopsa compared to the milp by becomes apparent when closely investigating the structure of the milp - .below , we show that the milp is equivalent to the well - known -norm minimization , and that an milp solver implicitly uses a method that is known to be effective in approximating the -norm .therefore , we conclude that dopsa displays a remarkable performance by outperforming milp in terms of solution speed .the objective is to maximize the number of calculation points that receive the prescribed dose of 8.5 gy .let us first define the vector , indexed by in as follows : since is zero if calculation point receives at least the prescribed dose , minimizing the number of non - zero elements in yields a treatment plan with maximal .the number of non - zero elements is ( recall that the -norm counts the number of non - zero elements in a vector ) . and can be rewritten similarly .thus , the lagrangean of the milp - is equivalent to an -norm minimization . attaining sparse vectors by minimizing the -normhas been shown to be np - hard .an milp solver first solves the following lp relaxation : ,t\geq 0}\left\{\sum\limits_{i\in i_{ptv}}x_{i}:(\dot{d}t)_{i}\geq 8.5x_{i},(x , t)\in \mathcal{x}\right\ } , \label{eq : lprelaxation}\ ] ] where is a polyhedron .substituting yields the following equivalent program : ,t\geq 0}\left\{\sum\limits_{i\in i_{ptv}}|y_{i}|:y_{i}\geq \tfrac{8.5-(\dot{d}t)_{i}}{8.5},(1-y , t)\in \mathcal{x}\right\}.\ ] ] so , the lp relaxation is equivalent to -norm minimization .literature , predominately in the field of image processing , suggests that an optimal solution for the -norm minimization ( linear relaxation of the milp ) provides a good approximation of an optimal solution for the -norm minimization ( milp ) : shows that , in most cases , the -approximation is also the sparsest solution for underdetermined systems of equations . discusses necessary and sufficient conditions for the equivalence of -norm and -norm solutions in general .these conditions do not hold in our case .nevertheless , provide numerical evidence for the power of these -approximations .so , when the milp - is solved with an milp solver , the solver implicitly uses an approximation that is known to be accurate .this explains the results by , where an milp solver quickly determines good treatment plans .however , our results in section [ sec : comparison ] show that dopsa still outperforms an milp solver in finding good solutions in a short time - span , which advocates the power of dopsa .compared to milp - based formulations , dopsa has another advantage .when more dvh - constraints are added , the time for constraint verifications increases linearly , or sublinearly when solutions are removed in the process .contrarily , both the number of variables and the number of constraints of the milp formulation grow linearly in the number of dvh criteria , which slows down an milp solver at least quadratically .for example , when the lp relaxation is solved with an interior point method , the time complexity of solving the kkt system increases at least quadratically in the number of variables . a dual solver can not alleviate this slowdown since the number of constraints increases as well .the existing dose - volume based optimization methods milp and ipip have not been compared before .our results show that each method has its own advantages .ipip is faster while milp determines a better solution .dopsa , our new algorithm , is a viable alternative to both .its main advantage is that it does not require a costly solver . the time it takes to determine a good solution is less than milp but slightly more than ipip . since ipip returns a single solution quickly but does not keep improving the solution , dopsa eventually determines better treatment plans .this leads to two observations : first , advances in computing power will reduce the absolute time difference between both methods , making ipip lose its edge .second , it is possible to combine the strength of two methods and use the ipip solution as a starting point for dopsa .however , the additional costs for an lp solver required by ipip and the added complexity is not justified by the slight improvement in solution time .we have disclosed all information necessary to implement dopsa .we hope it receives more testing by the community , and will eventually become available to treatment planners .the authors would like to thank elekta brachytherapy for supplying anonymous patient data , t. siauw ( ucsf , san francisco ca , usa ) and anonymous referees for their constructive feedback .d. baltas , z. katsilieri , v. kefala , s. papaioannou , a. karabis , p. mavroidis , and n. zamboglou .influence of modulation restriction in inverse optimization with hipo of prostate implants on plan quality : analysis using dosimetric and radiobiological indices . in _world congress on medical physics and biomedical engineering , september 7 - 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Abstract: high - dose - rate brachytherapy is a tumor treatment method where a highly radioactive source is brought in close proximity to the tumor . in this paper we develop a simulated annealing ( sa ) algorithm to optimize the dwell times at preselected dwell positions to maximize tumor coverage under dose - volume constraints on the organs at risk . compared to existing algorithms , our algorithm has advantages in terms of speed and objective value and does not require an expensive general purpose solver . its success mainly depends on exploiting the efficiency of matrix multiplication and a careful selection of the neighboring states . in this paper we outline its details and make an in - depth comparison with existing methods using real patient data .  at ( current page.south ) ;


the method of surrogate data was proposed by theiler as a general procedure to test whether data are consistent with some class of models .it has been used to test a proposed nonlinear dynamics in geophysics , certain properties of neural spike dynamics , but the most common application is testing for linear models . in order to test the hypothesis that the data are consistent with being generated by a linear system , the ft algorithm is applied .based on an example and the theory of linear stochastic systems we will show that this algorithm does not produce the correct distribution of time series and therefore might not generally yield the correct distribution of the test statistic .the surrogate data method is similar to a monte carlo implementation of a standard hypothesis test in which one specifies a procedure for generating an ensemble of new time series with a distribution that matches a model to be tested .one generates several sample time series , the surrogate data .next , a selected feature function is evaluated for the original time series and for each of the surrogates .if the distribution of the feature values is well approximated by a gaussian it can be characterized by its mean and standard deviation .if , the feature value calculated for the measured time series , is more than a few standard deviations away from , the null hypothesis that the original data were generated by the tested model is rejected , and the significance is reported as the distance . if the distribution of the feature is not well approximated by a gaussian distribution , nonparametric rank statistics should be used .the surrogate data method differs from a simple monte carlo implementation of a hypothesis test in that it tests not against a single model , but a class of models , i.e. linear systems driven by gaussian noise .the idea is to select a single model from the class on the basis of the measured data , and then do a monte carlo hypothesis test for the selected model and the original data .the statistical properties of a time series generated by a linear process are specified by the autocovariance function ( acf ) or equivalently by its fourier transform , the power spectrum .theiler use the periodogram , , the squared modulus of the fourier transform of the data , to specify the linear model to be tested .they generate surrogates ( ft algorithm ) by drawing complex numbers with amplitude and random phases and fourier transforming back into the time domain .there are two problems in this procedure : first , the periodogram is not a consistent estimator of the power spectrum , and second , the distribution of surrogates generated by randomizing the phases but not the amplitudes does not correspond to any linear stochastic system at all .the consequences of these two points on the estimated distribution of the feature , in case of gaussianity and , are the primary focus of this paper .  in the next sectionwe briefly summarize the main ideas of surrogate data testing for linearity based on the periodogram and demonstrate by a simple example that this procedure might misspecificy the distribution of the test statistic . to explain this result we refer to some results of the theory of linear systems in section [ review ] and discuss the consequences for surrogate data testing for linearity in section [ consequ ] .the key to a hypothesis test is the distribution of a test statistic when the null hypothesis is true , i.e. . given a measured sample , one calculates and decides if it is reasonable to suppose that was drawn from , in general by an a priori choosen significance level .the surrogate data method was proposed by theiler primarily to test the null hypothesis that the data come from a linear gaussian process . once again , the key to designing a hypothesis test is the identification of the distribution of a test statistic . for a class as large and complex as all linear processes, it seems impossible .the idea of theiler is to use the observation itself to limit the size of the whole class to a single member .surrogate data testing needs two ingredients : a procedure to generate data from the class of models to be tested and a feature or statistic to perform the testing .if a single model is given , the first step is simple .one generates a large number of time series and approximates the distribution . for instance , see the paper by witt in which a nonlinear difference equation proposed to describe the dynamics of the reversals of the earth s magnetic field was rejected using a feature that describes the transition probabilities .theiler proposed their ft algorithm to determine if a given time series could have been produced by a linear system driven by gaussian noise .the basic idea is that the statistical properties of a linear process are captured completely by its spectral properties .therefore , they first calculate the periodogram , the squared modulus of the fourier transform , for the positive frequencies . in order to generate the surrogate data they draw complex numbers with amplitude and random phases , symmetrize the negative frequencies with to obtain a real time series by fourier transforming back into the time domain .we now give a simple example that demonstrates that the ft algorithm , might not reproduce the parent distribution of a feature for a linear process .we used a simple gaussian white noise process with mean zero and unit variance and evaluated the simple feature :  [ ableit ] f(*x * ) = _ t=1^n-1|x(t+1 ) - x(t)|  for new realizations of the process and for ensembles generated by the ft algorithm .these ensembles were based on different realizations of the process .the results were independent from the number of data points used .we only show the results for in fig .[ surro_ex ] . to excludeany variance dependent effect , all time series are normalized to mean zero and variance 1 .the lowest line displays the cumulative distribution of the feature from new realizations of the process .the lines above show the results for realizations based on the ft algorithm .the ft algorithm fails to recover the parent distribution .the surrogate distributions are grouped too tightly around the value of the test statistic ( marked by ) for that time series on which the surrogates were based .  the kolmogorov smirnov test for gaussianity does not reject the consistency with a normal distribution at the level of confidence for the shown distributions .thus , they may be described by mean and standard deviation .the standard deviation of the true distribution is underestimated by a factor of two .the kolmogorov smirnov test for consistency of two distributions applied to each surrogate distributions and the distribution of new realizations of the process rejected the null hypothesis in every case at the level of confidence .the choosen test statistic is indeed able to detect nonlinearities since its expectation value for a gaussian white noise process with unit variance is and for gaussified data of the logistic map , , which shows also a flat spectrum , it is .the misallocation of and the underestimation of the true standard deviation by that appear in fig . [ surro_ex ] can be explained in terms of the theory of linear processes .   in order to model the fluctuations in the periodicity of sunspot datayule introduced linear stochastic systems :  x(t ) = a_1 x(t-1 ) + a_2 x(t-2 ) + ( t ) , ( t)~n(0,^2 ) .these are now called autoregressive ( ar ) processes can be generalized to autoregressive moving average ( arma ) processes by including not only past values of the observations but also past values of the noise .the general arma[p , q ] process reads :  x(t ) = _i=1^p a_i x(t - i ) + _j=1^q b_j ( t - j ) + ( t )  state space models are a further generalization .they consist of a vector valued ar[1 ] process and a linear observation function with observation noise :    where is the covariance matrix of the dynamical noise , and the variance of the observation noise. state space models model observation noise explicitly . for ar and arma models, observation noise causes the estimated parameters to be biased towards zero .once the type of model has been chosen , one must select the model order .generally , one fits a sequence of higher and higher order models and tests the prediction errors for consistency with white noise , e.g. by a kolmogorov test on the periodogram .if a particular class and order of model is selected , when the parameters of the model are estimated using data from a random process , the estimated parameters are random variables . by restricting the number of parameters to be fit , one insures that as the number of observed data increases , the variance of the estimates goes to zero . in the next subsectionwe emphasize that periodograms do not have this property .we now examine the spectrum of linear processes and the relationship between the spectrum and the periodogram .the autocovariance function is given by    and the spectrum is defined by :    the spectrum of a state space model is given by :  [ specar ] s ( ) = * c * ( * 1 * -*a*e^-i ) ^-1 * q * ( * 1 * -*a*e^i ) ^-1 t * c*^t + * r * .thus the spectrum of this process is a smooth function of .spectra of ar or arma processes are special cases of eq .( [ specar ] ) .we now turn to the distribution of the periodogram . for details see , we only summarize the results .asymptotically , the fourier transform of a linear stochastic process x ( ) = _ t x(t ) ( t ) + _ t x(t ) ( t ) is a complex gaussian random variable [ fourverteil ] x ( ) = ( 0,s ( ) ) + i ( 0,s ( ) ) where is independent of .furthermore , fourier components are asymptotically uncorrelated for different frequencies . in terms of modulus and phase instead of real and imaginary part, this means that not only is the phase a random variable but so is the modulus .  the distribution of the periodogram ( ) = ( _ t x(t)(t ) ) ^2 + ( _ t x(t ) ( t))^2 follows from eq .( [ fourverteil ] ) to be a distritubion with two degrees of freedom : ( ) ~s ( ) ^2_2 again independent of .since the mean and variance of are and respectively , the standard deviation of the periodogram is equal to its mean , thus , the periodogram fluctuates wildly regardless of the number of data points used to calculate it .it is an unbiased but not a consistent estimator of the spectrum since its variance does not decrease with .this is true not only for linear stochastic processes but also for nonlinear stochastic and even for chaotic processes if only the autocovariance function decays fast enough .the characteristics of the spectrum and periodogram described in the previous section lead to the following two points concerning the ft algorithm :  1 .the ft algorithm does not use its estimate of the spectrum correctly . both phases and amplitudesshould be randomized .2 .  a better estimator of the spectrum than the periodogram is required .below we elaborate on these points .( [ fourverteil ] ) means that at each frequency the fourier transform of a time series from a linear stochastic system is a two dimensional gaussian random variable .the ft algorithm , by fixing the modulus and only randomizing the phases , restricts the new realization to a set of measure zero in the set of all possible realizations of the linear stochastic system under consideration .it interprets the periodogram of a stochastic process as the spectrum of linear deterministic oscillators with fixed amplitudes .thus , instead of testing for linear stochastic systems , the ft algorithm by construction tests for the time discrete version of the following deterministic process :    note , that this process depends on the length of the time series .if the true spectrum of the process were known the following algorithm would yield the correct distribution of surrogates .the algorithm is based on eq .( [ fourverteil ] ) that connects the spectrum with the variance of the complex random variable .  * for each frequency , draw two gaussian distributed random numbers , multiply them by and take the result as the real and imaginary parts of the fourier transform of the desired data . * for reasons of symmetry the last frequency is always real for even number of data points .thus , in this case , only one gaussian distributed random number has to be drawn .* to obtain a real valued time series , chose the fourier components for the negative frequencies as the complex conjugates of the components at positive frequencies , i.e. , . * do an inverse fourier transformation of to get the surrogate time series .[ surro_wahr ] shows the results of a simulation study analogously to fig .[ surro_ex ] .again , the bottom line displays the cumulative distribution of the feature from eq .( [ ableit ] ) for new realizations of the white noise process .the upper lines show the distributions based on the proposed algorithm using the true flat spectrum of white noise .the critical issue for the procedure described in the previous section is spectral estimation .there are two general approaches commonly used for this purpose : fit an ar or arma model to the data and calculate the spectrum from the resulting parameters or smooth the periodogram . for more than the brief discussion given here ,see .this method can not be recommended for two reasons . on the one hand ,as discussed in section [ review ] , in presence of observation noise these models do not give correct results even if the underlying dynamic is linear .on the other hand , if the process is nonlinear they are , in general , not consistent estimators of the spectrum .of course , the parameters of linear models determine the spectrum .but not every spectrum , e.g. of a chaotic process , is well described by a linear model , since the prediction errors will not be consistent with white noise .increasing the order of the model does not help , because the `` spectrum '' calculated from the parameters will begin to interpolate the periodogram .for the same reason , surrogate data generated by realizations from a fitted arma model test , in general , for some linear model , but not for that specified by the spectrum .   estimating the spectrum by smoothing the periodogram rests on three results of the theory of linear processes : the expectation value of the periodogram is the spectrum ,the spectrum is a continuous function , and the random variables and are asymptotically uncorrelated .the idea is to smooth the periodogram to reduce the variance ( or equivalently split the time series into segments and average the periodograms ) . denoting the number of data points by , the number of frequency bins included in the smoothing by , one can show that in the limit of and , with constrained to increase slowly enough to insure , this procedure leads to a consistent estimator for the spectrum .  for finite data sequences , the smoothing will introduce a bias andthere is a trade off between bias and variance of the estimator which is discussed at length in .   the bias and the variance of the spectral estimate lead to a misspecification of the surrogate data distribution that vanishes asymptotically .[ right_ex ] is analogous to fig .[ surro_ex ] .it shows the results of a simulation study using a moving average kernel of width 2000 frequency bins to estimate the spectrum and the algorithm of section [ randi_all ] to generate the surrogate data .the kolmogorov - smirnov - test for consistency of two distributions does not reject the null hypothesis in 7 of the 9 cases shown at the level of confidence , demonstrating the convergence .the appeal of the general idea of surrogate data testing is clarified by a comparison with the tests for linearity proposed in the stochastic systems literature .these tests rely on asymptotic results for linear processes , mainly concerning third order properties .there are two classes of these tests .the first class is based on fitting an ar model to the data and investigating third order properties of the prediction errors .examples of these parametric tests are given by .the other class of tests is based on the asympotitic property that the three point correlations : c(_1 , _ 2 ) = _ t x(t ) x(t+_1 ) x(t+_2 ) are zero for any linear process . for examples of these nonparametric tests see . in order to perform the test , is fourier transformed and the normalized bispectrum is estimated . for lineargaussian processes it should be zero and the consistency with zero can be tested .both classes of tests test data of a given process by a property that is fulfilled by all possible linear processes .the sampling properties of a test statistic of a linear process depend on the the details of the process and the statistic .in particular , the variance will often be larger for processes with sharper peaks in the spectrum or equivalently longer coherence times .for example , the variance of the sample mean for an iid gaussian process is , but if the process has correlations , the variance of the sample mean will decrease more slowly as a function of . by construction , the surrogate data method produces the process specific sampling properties .the power of any test , i.e. the ability to detect a violation of the null hypothesis , depends on the statistic .the bispectral tests considering the three point correlations are sensitive to quadratic nonlinearities but can be fooled completely by processes showing only cubic nonlinearities like the duffing - oscillator .the general scheme of surrogate data testing can use any statistic , and the power of the test will depend on the statistic chosen .thus one can detect a larger class of violations of the null hypothesis .we showed by an example that the ft algorithm proposed by theiler et al . might specify the distribution of the test statistic incorrectly , independent of the number of data points used .based on the theory of linear stochastic systems , we proposed an alternative that involves spectral estimation .therefore , testing for linearity by the idea of surrogate data is , in general , also a procedure that is only valid asymptotically . for a finite 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Abstract: the method of surrogate data is a tool to test whether data were generated by some class of model . tests based on the periodogram have been proposed to decide if linear systems driven by gaussian noise could have generated a sample time series . we show that this procedure based on the periodogram , in general , misspecifies the test statistic . based on the theory of linear systems we suggest an alternative procedure to obtain the correct distribution of the test statistic and discuss problems of this approach .


a meteoroid enters the earth s atmosphere at hypersonic speed and it collides with air molecules . the high kinetic energy involved in the processdetermine the transformation of a solid body into a plasma , which can scatter radio waves and can emit light ( meteor ) .  during sixties and seventies several worksinvestigated the formation and evolution of the meteor , with particular attention to diffusion , in order to study mesospheric winds .a complete review of standard meteor science can be found in ceplecha et al .( ) .however , there are still some aspects not well understood about the physical properties of a meteor , specifically whether it is an ionized gas or a plasma . during past years, these two appellatives were often used as synonymous in meteor physics , even though they indicate two different states of the matter . in some studies , such as those about diffusion ,specific plasma properties are taken into account ( e.g. ambipolar diffusion ) ; however in other studies , such as about radiowave scattering , the meteor is simply considered a long narrow column of ionized gas .this can appear as a futile debate , but it hides important concepts .specifically , a plasma has collective properties ( e.g. langmuir frequency ) that an ionized gas has not .a first attempt to study the meteor as a plasma was carried out by herlofson ( ) .he investigated the proper oscillations in the meteor and their interaction with radio waves .but , at our knowledge , none continued his studies . only in 1999 the question of collective oscillations in meteoric plasmawas reprised ( foschini ) .perhaps , this gap may be explained by taking into account that , according to purposes of meteor astronomy , it was sufficient to use the approximation of the long narrow cylinder .however , the meteoric plasma is something more complex than a reflecting rod and it is necessary to study it .there are several types of oscillations and instabilities , which can interact with radio waves .the scattering is not the only process : for example , fluctuations from equilibrium may lead to transformation of waves ( longitudinal to transverse and vice versa ) .the question is : are such processes present in a meteoric plasma ?we think that the study of plasma collective oscillations may give new useful tools to understand the physics of meteors. some basic concepts about meteoric plasma were settled in a previous paper ( foschini 1999 ) , thereafter called paper i. according to the theory exposed there , radio echoes can be divided into two classes and two subclasses. then we have underdense and overdense echoes , according to whether the langmuir frequency is higher or lower than the radio wave frequency .overdense echoes totally reflect electromagnetic waves , but the presence of binary collisions among ions and electrons weaken the collective oscillations of the plasma , allowing the propagation of the waves , even though with strong attenuation .therefore , we can divide the overdense echoes into two subclasses : type i , when there is total reflection ; type ii , when binary collisions allow the propagation .the division between overdense type i and ii depends on the electron ion collision frequency , which in turn depends on electron density and ion cross section . in paperi , for the sake of simplicity , we considered potassium ion , that is the chemical element with lower ionization energy .in addition , recent studies show that potassium seems to be much more important in the evolution of meteor than previously thought ( von zahn et al . ) . with this assuption, the division between overdense echoes occurs at about  m .it is worth noting that this border can be moved by considering other elements .but the calculation of particle distribution and evolution in a meteoric plasma will be object of other papers .the overdense type i echoes derives from total reflection of radio waves ( see fig .[ fig1 ] for an example ) .this allows to calculate the height of the meteor in an easy way , as shown in the paper i. here we want to present a statistical sample of several meteor showers , for which we have calculated the height .data will be discussed and compared with available data in literature .we shortly recall how to calculate the meteor height , as described in paper i. first , we have to take into account that the plasma has not a definite boundary and then , the incident electromagnetic wave penetrates a little into it before reaching the density necessary to allow total reflection .we can consider this something similar to skin effect in metals .we can consider a simple geometry , as shown in fig . 6 of paper i , and then use the definition of the attenuation in decibel units :     \label{e : decib}\ ] ]  where subscripts and stand for incident and reflected wave .> from the solution of maxwell s equations we obtain that , for overdense meteors type i , the electric field is :    where the wave vector in eq .( [ e : solu ] ) has the form :    we refer to paper i for explanation of symbols , even though they are commonly used in literature about electromagnetic fields .we substitute eqs .( [ e : solu ] ) and ( [ e : wv ] ) in eq .( [ e : decib ] ) and , taking into account that the amplitude of a totally reflected wave is equal to the amplitude of the incident wave , we can obtain an attenuation value of about , where is the path of the wave into the plasma :    where is the incidence angle and is the penetration depth .then , eq . ( [ e : decib ] ) becomes :     \label{e : decib1}\ ] ]  in the case of overdense type i ( total reflection ) the attenuation is simply a function of the angle of incidence .( [ e : decib1 ] ) refers to an idealized case .when we deal with real meteors and radio waves , we have to take into account of several factors , i.e. antenna gains , losses in radio receiver and trasmitter , atmospheric absorption , and the distance of reflecting point from trasmitter and receiver . strictly speaking , eq .( [ e : decib1 ] ) can be considered as the `` meteor cross section '' in the radar equation .we can consider common factors in radar theory , as described in kingsley & quegan ( ) . by means ofcommonly used values for forward scatter radar , we obtain that the attenuation recorded with our receiver is :     \label{e : meyere}\ ] ]  where is the received signal amplitude [ v ] . > from the amplitude of the reflected wave , we can calculate the incidence angle with the eq .( [ e : decib1 ] ) .therefore , we can calculate the meteor height by considering the geometry of fig .we can see that :    taking into account that the earth s mean radius is much larger than , we can calculate :    by substituting eq .( [ e : hei2 ] ) in eq .( [ e : hei1 ] ) , we obtain the height of the reflection for an overdense meteor type i :    where .the angle is the half angular distance between receiver and transmitter .the distance plays an important role in the derivation of the above formula . indeed , from a geometric point of view , the specular reflection in a forward scatter system occurs when the meteor trail lies along a tangent to an ellipsoidal surface , with the transmitter and the receiver stations in foci ( forsyth & vogan ) .this condition is fulfilled by different values of distances of the reflecting point from the source and from the receiver .if we do not know the path source meteor receiver , this introduces an uncertainty of about 40 km in the height of the reflecting point ( for our system ) .one way to overcome this problem is to set up a third station , but if it is not possible , as in our case , we can reduce uncertainties by making heuristic considerations .indeed , as explained by forsyth & vogan ( ) , the forward scatter system is most sensitive to meteor trails which are nearly horizontal and directed along the transmission line .a meteor perpendicular to the source receiver line gives an echo that is about five times lower than the case of parallel direction ( forsyth & vogan ) .therefore , the choice of the reflecting point located closely to the middle of the transmission path appears to be reasonable and , as we shall see , is justified by facts .the general principle of meteor observation by forward scattering of radio waves is the following : a vhf radio receiver ( 30100 mhz ) is located at a large distance ( about 6002000 km ) from a transmitter at the same frequency .direct radio communication is not possible , owing to the earth s curvature , but the meteor allows the commuication over the horizon , by reflecting the transmitted signal .  in this study , observations were carried out mainly by m. de meyere .his radio receiver is located in deurle , belgium ( longitude  e , latitude  n ) , while the trasmitter is located in sofia ( bulgary ) .it transmits radio signals at 66.50 mhz all over the day with 10 kw power . in this case , values of and are , respectively ,  km and .the receiving station consists of a crossed yagi antenna ( 4 elements ) , that is linked to a computer with a digital acquisition interface ( 150 samples per second , 8 bit resolution ) .data are recorded and stored into a file .  .observed meteor showers .in the last column , the percentage indicates the number of overdense meteors type i compared with the total number of recorded meteors . [ cols="<,^,^",options="header " , ]   observations of several meteor shower were carried out during several years ( see table [ tab1 ] ) , with a total number of meteors recorded equal to 13401 .the overdense i meteors are 131 ( about 1% of the total ) and the height distribution for analysed showers are shown in figs .[ fig3][fig6 ] ( each bin is 1 km wide ) .values in table [ tab1 ] refer to all echoes recorded during shower days . in order to evaluate also the sporadic background, we have analysed some days in february , without any shower .we have found a mean value for sporadic overdense type i meteors of about 0.2 meteor per hour , so that the contribution of background can be considered negligible .measured heights are in the range between 70 and 110 km , in good agreement with typical meteor heights ( 60110 km ) , even though the large part of meteors are in the range 95110 km .the peaks of distributions are not centered , but are located toward right .however , this seems to be an effect dues to a low number of data .indeed , the best fit for the observations , calculated with the test , is a gaussian distribution ( therefore is calculated with standard methods for this type of distribution ) . in table[ tab2 ] values of mean speed ( allen ) and height of overdense i meteor are shown .data obtained here show that the mean height is independent from the entry speed of the meteoroids ( see table [ tab2 ] ) . on the other hand, it is known that the height depends on entry speed of meteoroids : for example , greenhow & lovell ( ) wrote that the highest speed sporadic meteors , moving at 6070 km / s , ionize at a mean height of 100 km , whereas those with minimum speed ( 11.2 km / s ) reach a mean height of 85 km .the theory of radio meteor height was elaborated by kaiser ( , b ) and recently belkovich et al .( ) proposed some changes , in order to take into account the fragmentation .kaiser found that the width of the height distribution depends on the atmospheric scale height and the mass distribution of incoming meteoroids .the mean height depends strongly on meteor speed , through two coefficients named and , and depends also on the probability of ionization .kaiser s theory refers to the point of maximum ionization , but it is known that in experimental radio observations the height of reflecting point does not necessarily lie in the point of maximum ionization ( greenhow & lovell ) .  if we observe a meteor shower with a given mean speed and mass distribution , the height distribution is related to the length of ionization curve .the point of maximum ionization corresponds to the most probable height .it is worth noting that kaiser s theory refers to underdense meteors . only mckinley ( ) referred to overdense meteors and found no clear dependence on speed .moreover , he found a two peaks distribution : the main peak is located at about 95 km , and the second one at 106 km .we can try to explain the differences between our results and data available in literature .the first reason is that we try to analyse overdense meteors , while the large part of published data refer to underdense meteors .it is very interesting to note that for bright meteors , which are surely overdense , the radio height of maximum echo duration is well above the mean height of maximum light , obtained from photographic data ( millman & mckinley ) . on the other hand ,the situation is reversed for faint meteors .the reason for this difference is that overdense type i meteors reflect totally the incoming electromagnetic wave .total reflection is allowed only when plasma frequency is higher than the radio frequency and the collision frequency in the plasma is negligible ( see paper i ) .these conditions are achieved indipendently from speed of incoming of meteoroid , but it depends on the mass and chemical composition of the body .once an overdense type i meteor is created , the signal amplitude of the reflected wave remains constant until the collision frequency in the plasma or recombination and attachment processes subtract energy to the plasma frequency .we can say that collective properties of the plasma , which generate the long plateau of overdense type i meteor , `` hide '' in some way some properties of the incoming meteoroid .concerning the two peaks found by mckinley ( ) , we note that our distributions show only the secondary peak .this can be explained by taking into account that while mckinley made no distinction between overdense meteors , we have considered only overdense type i meteors .we have carried out an analysis of several overdense radio echoes , recorded during last years by a radio observer located in belgium .we have analysed a particular class of overdense meteors ( type i ) and measured height distributions are in good agreement with previous results obtained by mckinley ( ) , even though only for the secondary peak .we suppose that the first peak in mckinley s work should be due to overdense type ii meteors , while the secondary peak , recorded also by our system , appeared to be due to overdense type i meteors .we think that collective properties of the meteoric plasma ( langmuir oscillations ) hide some characteristics of the originary cosmic body , specifically there is no clear dependence on speed .further study , mainly theoretical and able to take into account collective properties of plasma , are required to assess the particle dynamics in the meteor .it should be noted that our studies were carried out with an amateur forward scatter system and we have no full control on it .moreover , heuristic considerations were introduced in order to minimize uncertainties , but results showed that they were justified by facts .the agreement with previous works , with other techniques , supports our conclusions .the future availability of a full forward scatter system would be of great help in more detailed studies .lf wishes to thank p. farinella for useful discussions . authors are also grateful to the referee , p. pecina , for useful comments .this research has made use of _ nasa s astrophysics data system abstract service_.

Abstract: it is known from theory that , by means of a plasma physics approach , it is possible to obtain a simple formula to calculate the approximate height of a meteor ( foschini , 1999 ) . this formula can be used in case of forward scatter of radio waves and has the advantage that it does not depend on the diffusion coefficient . on the other hand , it is possible to apply the formula to a particular type of meteor only ( overdense meteor type i ) , which is a small fraction of the total number observed . we have carried out a statistical analysis of several radio echoes from meteor showers recorded during last years by a radio observer located in belgium . results are compared and discussed with those obtained with other methods and available in literature .