On Public-key Steganography in the Presence of an Active
Warden
Scott Craver
Intel Corporation
Microcomputer Research Labs
2200Mission College Blvd.,Santa Clara,CA95052-8119
Department of Mathematical Sciences
Northern Illinois University
DeKalb,IL60115
Abstract.The so-called prisoners’problem,in which two individuals attempt to
communicate covertly without alerting a“warden”who controls the communications
channel,has taken a number of forms,adorned with various assumptions or require-
ments which make the problem more or less dif?cult.One assumption which makes
the problem considerably more managable is that the participants are allowed to share
some secret information(such as an encryption key)prior to imprisonment.Another
assumption,which makes the problem much more dif?cult,is that the warden be al-
lowed to modify messages sent between the prisoners as well as read them.This paper
describes techniques for pure steganography,in which no secret information needs to
be shared before imprisonment.First,a modi?cation of an existing protocol will be
shown to admit pure steganography if the warden is not allowed to modify the contents
of the channel.Then,a technique will be described that allows pure steganography be-
tween two prisoners in the presence in the presence of an active(content-modifying)
warden.
This technique is possible through the use of two distinct channels rather than one:the
subliminal channel for steganographic communication is augmented by a supralimi-
nal channel,one in which information is not hidden from the warden but cannot be
modi?ed.
1The prisoners’problem
The prisoners’problem was?rst posed by G.J.Simmons in1983,and is generally con-sidered to be the de facto model of covert communication.In this problem,two people, usually named Alice and Bob,are thrown in prison and intend to co-author an escape plan.The problem is that all communication between them is arbitrated by a warden,here named Wendy,who will place both parties in solitary con?nement at the?rst sign of any suspicious communication.Alice and Bob must trade inconspicuous-seeming transmissions which contain hidden information that,they hope,Wendy will not notice.
Using terminology agreed upon in[6],the inconspicuous data that is used to hide the real message is usually referred to as cover-data or cover-objects:a letter is often called a cover-text,for instance,while an image may be called a cover-image.The hidden or embedded message is placed therein,turning the cover-object into a stego-object.Alice’s and Bob’s goal,then,is to trade stego-objects without Wendy realizing that they are in fact stego-objects.
Fig.1.The prisoners’problem,illustrated
Further complications may hinder Alice’s and Bob’s escape.Wendy the warden may,in certain situations,be allowed to slightly modify messages as they are passed between the prisoners,to foil any hidden codes depending on the exact wording of the communication between them.In this case we call Wendy an active warden;without this ability she is considered a passive warden.One real-world example of an active warden is the censoring of telegrams by the United States government during World War II:the semantic content of telegrams could not be changed,but censors would slightly alter their exact wording, replacing words with close synonyms to foil possible secret codes[1].
It may be bene?cial at this point to describe some common variations of the prisoners’
problem.First,the warden’s power to alter the transmissions between the prisoners affects the dif?culty of the problem:
–A passive warden can do nothing but spy on the communications channel between the prisoners.
–An active warden is allowed to modify(slightly)the data being sent between the https://www.doczj.com/doc/387359843.html,d modi?cation of text which does not alter its semantic content(say,replacing words with close synonyms)is an example of an active warden being active.The active warden must not modify data so much that innocent communication would be foiled.–The case of a malicious warden is not often addressed.A malicious warden would be one who may alter the prisoners’messages with impunity,perhaps composing entire messages for one prisoner while pretending to be the other.In this environment the pris-oners can not hope to do much of anything!Fortunately,real-world situations prevent a warden from grossly altering the content of messages.Imagine the confusion if a large number of telegrams sent during World War II were altered in meaning,suppressed,or entirely fabricated by crafty censors on the lookout for spies!
As for the prisoners themselves,it should be pointed out that in the best case,they would not have to communicate prior to imprisonment,so as to(say)trade an encryption key.This best-case scenario,here called pure steganography,is very dif?cult to engineer. Current steganographic protocols generally assume that some information is shared between the prisoners prior to imprisonment.If this assumption was not allowed,little progress in information hiding could have been made to date.
The remainder of this paper is organized in the following fashion.In section2it will be shown how steganographers have managed to send information covertly in the presence of an active(and in some cases malicious)warden,provided that information such as secret and public keys can be traded beforehand.In section3,we will see how a protocol,described by Ross Anderson in[1],allows steganography in the presence of a passive warden with only one prisoner knowing the other’s public key.A modi?cation of this protocol will be shown to admit pure steganography in the presence of a passive warden.Finally,section4 will describe what are here called supraliminal channels,which allow pure steganography in the presence of an active warden.The paper will close with a discussion of the feasibility of supraliminal channels.
2Private-key steganography
Let us assume that Alice and Bob are allowed to share a secret key prior to imprisonment, or even to trade public keys.This gives them the opportunity not only to communicate covertly,but to defeat an active warden.In the former case,steganography consists merely of encrypting a message in such a way that the ciphertext appears statistically random,and embedding the bits of the text in a known subliminal channel.The embedded information, of course,must be made to have the same distribution as the channel noise in order to foil statistical tests.
In the presence of an active warden,it is not enough to embed a message in a known place.If Alice can subtly alter the bits in an image,it follows that Wendy could scramble those same bits with as little impact,erasing whatever was being sent via the subliminal channel.In this case it is possible to use what is referred to in[1]as a“selection channel.”
Essentially,the secret information shared between Alice and Bob is used to determine where the message is hidden.A cryptographically secure pseudo-random generator,seeded by a secret key,can be used to pick a subset of pixels in an image,for instance,to be used to conceal the data.If Wendy attempts to make subtle changes to the image,she may only be able to scramble a small percentage of the actual channel bits,since she doesn’t know exactly where they are.This scrambling can then be?xed using an error-correcting code.
The sharing of keys before imprisonment,however,is a requirement that we would ultimately like to see removed.It allows a great deal of freedom on the part of Alice and Bob —indeed,if they share public keys before imprisonment,they can even defeat a malicious warden by signing their secret messages to prevent impersonation—but it is not reassuring to think that if two people ever need to communicate covertly,they must know so far in advance that they can trade secret keys before a real-world“warden”starts listening in.
3Public-key steganography
3.1Boiling out the impurities
In public-key cryptography,it is not necessary for two people to share a secret key to estab-lish a secure channel.One only needs to know the the other’s public key.This suggests a possible approach to steganography in which a secret key does not have to be agreed upon by Alice and Bob prior to imprisonment.Some information must still be known a priori—one prisoner must know the other’s public key—but from a practical perspective this is a much more reasonable requirement.
A protocol which allows public-key steganography in the presence of a passive warden is described by Ross Anderson in[1].It relies on the fact that an encrypted message can be random enough to hide“in plain sight:”
1.Alice,knowing Bob’s public key,encrypts her message with it to obtain an apparently
meaningless ciphertext.
2.Alice embeds in a channel known to Bob(and,hence,to Wendy).The resulting
stego-object is sent to Bob.
3.Bob has no idea if any message is hidden in the channel or not,but we can assume that
the technique is standard enough that if he suspects a message,he will look for it.He retrieves a random-seeming string,attempts to decrypt it with his private key,and out pops Alice’s message.
One problem with this approach is that Bob has no idea if anything is being sent:he may not even know Alice,and certainly does not know if she intends to use a steganographic channel.If the two traded a private key before imprisonment,at least Bob would know that some secret transmission was pending.In this case,Bob will just have to suspect that a hidden message might be present in any cover object he receives.
This is not too serious a problem,however:it is already assumed(as it usually is in cryptography)that the information-hiding technique used by Alice is known to all,and standard enough that Wendy would suspect its use.Certainly Bob can,too.As long as hidden content is suspected and can be easily extracted by a known method if it does exist, it is not unfair to assume that it will be discovered.
A more practical,related problem is that in a large group of possible recipients every single recipient must suspect hidden content in each object.Only the intended recipient will ?nd it,of course;but in an environment such as a USENET newsgroup,where a large num-ber of people may send stego-objects,each of which may be targeted toward an unspeci?ed recipient,all the parties involved may spend the better part of each day looking inside every object.This is related to the previous problem,since it is the result of the message sender not being able to give the intended recipient any warning that a message is being sent.
3.2Pure steganography using the Anderson protocol
The assumption made above is not to be taken lightly,for it suggests that the protocol described in[1]is actually more powerful than may initially be suspected.If we can assume that Bob will attempt to decode what might or might not be a hidden message,then we can also assume(at least in the two-prisoner case)that Bob may attempt to participate in what might or might not be an ongoing steganography protocol.For example,suppose that Alice and Bob are astronomers residing on separate planets,each searching for signs of intelligent life in the universe.Bob receives from Alice’s planet what might be a message, or what might be noise.He may decide to treat the signal as if it were a message,send one back in the same“language,”and wait for a second signal to con?rm his suspicion.
In the context of steganography,the situation is complicated by the fact that the signals being traded must be meaningless to everyone but Alice and Bob.We can imagine that if the search for extra-terrestrial Alice ends up beaming a meaningful signal to the nearby warlike Wendy,Bob may give away that his planet is inhabitable and ripe for colonization. On the other hand,the situation is made less complicated by the cryptologic assumption that whatever data-hiding or encryption algorithm is used is known to all.
In any case,the possibility that Bob may participate in a protocol on suspicion of a steganographic message suggests a method by which Alice may send Bob her public key while they are both imprisoned,entirely eliminating the need for prior communication.A passive warden is still required for this to happen,but this too can be relaxed,as will be demonstrated in the next section.
Protocol1A modi?cation of the protocol described in[1]
This is essentially Anderson’s protocol with an extra step at the beginning.It is assumed that a method of information-hiding is known to all,as well as a public-key encryption algorithm.Wendy is assumed to be passive.
1.Alice generates a random public key/private key pair.If she is using RSA,for instance,
she could generate the large prime numbers used for the key by picking cryptographi-cally pseudo-random strings and testing for primality until one is found.She then selects another pseudo-random string for the public key.This is usually the way RSA keys are created.
2.Alice embeds the key in the subliminal channel,viewable by both Bob and Wendy,and
sends the resultant stego-object to Bob.Neither Bob nor Wendy can determine if the channel contains noise or a key.If Wendy can only snoop,then Bob can attempt to communicate:
3.Bob suspects that the data in question is a public key,uses it to encrypt a short note of
acknowledgement with a randomly chosen secret key for future covert transmissions, and sends this back to Alice embedded in a stego-object.
4.Alice suspects that the returned data is an encrypted key,decrypts it with her private key,
and out pops a message from Bob.Now the two can communicate by an appropriate secret-key protocol.
At no point in the protocol does Wendy have any idea that the randomness means any-thing:Alice alone can deduce content from Bob’s reply.As long as she is not allowed to insert her own information(and thus either foil the transmission or maliciously insert her own key to catch the two in the act),she can not conclude that communication has taken place.
If Wendy is capable of writing to the channel,then there is no way communication can take place:if Wendy does not utterly destroy the“in plain sight”bits,she can attempt a man-in-the-middle attack by overwriting Alice’s key with her own.A more malicious warden could entirely spoof either Alice’s or Bob’s response.In that case,however,the original protocol would not work either,since in neither case would Bob have any way of identifying the author of the original message.
4Public-key steganography in the presence of an active warden Cover-plots and Supraliminal Channels
If we assume that the Warden can only make minor modi?cations to the possible stego-objects sent between Alice and Bob,then we can assume that there is some amount of per-ceptually signi?cant information that the warden cannot change whatsoever.For instance,if Alice sends Bob a picture of a cow and Wendy can only modify1bit in every100,we can assume that Wendy will not be able to turn the cow into a pig.In a novel,there could be ex-plicit states of affairs or descriptions of characters so relevant to the plot that no information about those states can be changed without a signi?cant rewrite of a number of portions of the book.If we develop some formal encoding of object and state-of-affairs descriptions,we have the makings for a channel through which Alice can send a small amount of information to Bob out in the open,but with high integrity.What we are describing here is a supralimi-nal channel rather than a subliminal one:information is hidden in plain sight,so obviously, in fact,that it is impossible to modify without gross modi?cations to the transmitted object.
Formally,we can de?ne a cover-plot function,where is a set of pos-sible encodings of state-of-affairs descriptions,or“cover-plots”—so called because they can be considered the plot,or content,of a cover-object.A cover-plot is simply a formal description of content appropriate to the medium of the cover-object.Such a description could be used by an author or content-creator to compose a covertext containing an em-bedding of,and hence an embedding of the bit-string.The cover-plot function would have the following properties:
–The function would be known to both Bob and Alice(and hence we must assume that it is also known to Wendy).
–A member of the inverse image under of any particular bitstring is feasible to com-pute,as is itself.
–is onto.It is not necessary for to be one-to-one—indeed,if is many-to-one Alice’s job will be signi?cantly easier,since she will have more freedom in choosing a cover-object which embeds a particular bit-string.
–A cover-object can be created to contain a state-of-affairs description such that an active warden cannot signi?cantly change the encoding of(that is,she cannot change to such that)without making unreasonable,wide-sweeping changes to the entire cover-object.
–A human being(or,ideally,a computer program)can deduce the exact state-of-affairs description embedded within a cover-object.The composer of the covertext can fa-cilitate this deduction by making the embedding very clear,but this should not be so necessary that an object with an easy-to-interpret embedding is conspicuous.
–Clarifying the last property,an innocent cover-object without an explicitly embedded state-of-affairs description can yet be interpreted as having one.That is,innocent cover-objects should have clearly deducible cover-plot descriptions(which map under to random garbage)so that the presence of a clearly deducible cover-plot description is not indicative of a hidden message,unless the bit string possesses some meaning.
This can be summarized as a channel that is robust,blatant and inconspicuous.Here, a robust channel is one whose content cannot be altered without making unreasonably drastic changes to the stego-object(i.e.,requiring a malicious,instead of an active,warden).
A blatant channel is one whose content is publicly accessible,a necessity if Alice is to send Bob information when no key is shared between them prior to transmission.For a channel to be inconspicuous as well as blatant may sound like a contradiction,but it is not:an inconspicuous channel is one whose existence alone is not indicative of a hidden message.A perfectly innocent image,containing no covert information,will seem to the warden to contain random cover-plot bits.If Alice sends Bob a cover-object with an obvious message in this supraliminal channel(“Bob:here is the secret plan to break of out of jail. Love,Alice”)then Wendy can throw the two into solitary con?nement.However,if Alice sends Bob apparently random cover-plot bits,perhaps a random session key to be used later, Wendy cannot conclude that the message is anything other than innocent,since innocent messages also contain random cover-plot bits.Only when that key is later used can Wendy know that covert communication is taking place.
One cannot help but notice the connection to digital watermarking,a form of informa-tion hiding in which the hidden data is required to be robust to signi?cant modi?cation of an image.In particular,a recent approach to watermarking,advocated by Ingemar Cox,et al in [2],has resulted in robust watermarking schemes using what largely resembles a subliminal channel with supraliminal aspirations.Cox,et al emphasize the importance of hiding a wa-termark in perceptually signi?cant components of an image,such as high-magnitude DCT matrix coef?cients,so that an ownership label can survive a signi?cant amount of abuse by a forger.Further,the watermark can still be made invisible,as the technique presented in[2]demonstrates.One may wonder if a similar scheme would allow the embedding of a supraliminal channel invisibly in an existing image,so that Alice and Bob need not compose a new image for each message for the explicit purpose of embedding a particular string of cover-plot bits.
Unfortunately,conceptual differences between robust watermarking and embedding bits in a supraliminal channel prevent the application of one to the other.In the former case,some sort of secret key is often used to embed a watermark,so that only those who know the key
can detect or remove the watermark.A cover-plot,on the other hand,must be readable by all but removable by none.Also,the purpose of a supraliminal channel(and,indeed, steganography in general)is to hide a speci?c message of some meaning to the recipient;In the case of invisible watermarking,the embedded labels need not have any semantic content at all,or may be a function of the image.The scheme described in[2],for instance,embeds a vector of pseudo-random numbers picked from a normal distribution.In[3],it is discovered that in order for this scheme to become secure it may be necessary for the vector to be a function of a one-way hash of the image itself.In short,we have a difference of priorities: in watermarking,an ownership label can be meaningless,or picked to?t the cover-object, while the cover-object’s content is given beforehand and is important.In steganography, the image can be meaningless,or picked to?t the embedded message,while the hidden message’s content is given beforehand and is important.
Finally,if it was possible to embed a supraliminal channel in an existing image(for ex-ample)without signi?cantly altering it,it would be equally possible for a warden to scram-ble the channel without signi?cantly altering the image,since everybody knows where the information is and how to bury cover-plot bits inside.Hence the content of the cover-object should itself be a function of the information embedded within it.It may be hasty to declare it impossible for a supraliminal channel to be subliminal,but the two concepts seem to have irreconcilable differences.
Example1.A rudimentary supraliminal channel for audio/video clips
In this example,the cover-object may be an audio-video clip in which people are speak-ing.The noise within the clip could be used to embed a subliminal channel.The set of cover-plots is the set of all texts which can be spoken by Alice and unambiguously under-stood by Bob.The cover-plot function is computed by hashing the text of each suf?ciently long word,assigning each a numeric value.
For instance,suppose that each letter of the alphabet is assigned some agreed-upon numeric value.When an audio clip is received by Bob,he types into his computer all words of(say)at least six letters.For each word the computer calculates the product of the letters’numeric values,modulo some small prime.These values can then be arranged left-to-right and treated as the base-p representation of an integer.
Alice must be able to compose a convincing body of text which hashes to a desired array of values.She has some?exibility in that smaller words can be used with impunity to generate a context for the larger words.She can then use a dictionary(or,more likely,a thesaurus)which can be searched by cover-plot values.Notice that the larger the number of values to which a word can hash(the larger the value of in the above example),the harder Alice’s job will be,and the more conspicuous a cover-plot might be given a small amount of time to create one.On the other hand,if the number of possible hash values is made small, the ease in composition on Alice’s part is offset by a lower bandwidth.
Once this composition step is complete,Alice turns on a video camera and records herself(or other people)reciting the body of text she composed.
This channel is blatant,as long as the technique is made publicly available and the letters’numeric values are a known standard.The channel is inconspicuous,since every text
document,innocent or not,has a cover-plot.Finally,the channel is robust unless Wendy is capable of seamlessly altering both the sound of Alice’s voice and her lip movements so as to change the text of her speech.While this is possible with today’s technology,there exist real-world domains where Wendy does not possess the time to do this(say,where she has to monitor hundreds of such transmissions per day or per hour),or where the cost in time and computation is too high to justify the altering of video clips on the mere suspicion that their text may have some future steganographic signi?cance.Alternatively,she could completely re-record the message herself,hoping that Bob does not know what Alice looks or sounds like,but this would likely classify her as a malicious warden.
A formalized method for describing the plot,characters,etc.of a letter or book,or a describing the overall content of an image may also serve as good cover-plot functions;one could use details so central to a cover-object’s content that to change them would require signi?cant changes throughout the object.Perceptually signi?cant components of certain transforms of the cover-object may also do the trick,provided one can feasibly compose a cover-object when given particular transform components that it must contain.This could make Alice’s job much more dif?cult,but Bob’s would be so simple that it could be com-pletely automated.If a robust supraliminal channel can be entirely automated,requiring no (or negligible)human intervention in composing or interpreting a work,it could be quite a signi?cant contribution to the?eld of steganography.
Public-key steganography with a supraliminal channel
Of course,a supraliminal channel is not appropriate for subliminal communication between Alice and Bob,for a number of reasons.First,the bandwidth will likely be very small,since the channel is engineered to be highly robust.The number of deducible states-of-affairs may be larger for a novel than for an image or video clip,but in either case it is unlikely that a single object could hold a reasonably sized message.A robust channel of this kind is more appropriate for transmission of a session key,as it will be used below.
Secondly,it is expected that Wendy also knows how to compute,so any information passed through the channel is available to her.Unless the cover-plot bits have no apparent meaning,Alice and Bob are in serious trouble(or at least more serious than what previously got them thrown in prison).
With public-key cryptography,however,such a channel could be used to defeat an active https://www.doczj.com/doc/387359843.html,ually it is assumed that Alice and Bob share a secret key.With a supraliminal channel,however,a secret key can be exchanged covertly with no prior information,right in front of an active warden.The protocol is described below and in?gures2and3.
Protocol2Key exchange in the presence of an active warden
In this protocol,Alice and Bob have no information prior to imprisonment.It is assumed that a public-domain cover-plot algorithm(as well as this protocol)is known to all,including Wendy.Alice and Bob perform the following steps:
1.Alice generates a random public key and a private key.
2.Alice then?nds a cover-plot within the inverse image of.
Wendy
Bob Alice
1100110..possibly altered object Cover-Object (embedding)Fig.2.How a cover-plot function is used to establish a supralminal channel.
3.Alice composes a cover-object
containing an embedding of the cover-plot S,and sends this to Bob.At this point,Wendy can inspect the channel,and will ?nd a random-looking string of cover-plot bits.There is no reason to expect anything unless the data is later used in some way,say as a session key.The cover-plot bits pass unmolested through the channel to Bob.
4.Bob extracts the cover-plot bits,suspecting that they represent a public key.He cre-ates a pseudo-random secret key ,which he will later use to embed information in a
hide information in a
Wendy can see:The string E(r)Alice possesses:
Wendy can see:Bob now possesses:Her public key E
Her private key D The string E Alice’s public key E
Alice now possesses:
The session key r=D(E(r)
(Optionally) the message T Bob possesses:A session key r, used to
(optionally) A message T subliminal channel
Fig.3.Protocol using supraliminal and subliminal channels.Beneath each character is listed the in-formation he or she knows once the illustrated step has taken place.subliminal channel,and encrypts it with the purported public key to get a pseudo-random string Alice .He then ?nds a cover-plot ,and creates a cover-object containing an embedding of .5.
Bob now uses his secret key to securely embed a message in the subliminal channel of the image,using an existing secret-key-based technique which the active warden cannot defeat.If the medium used to implement the supraliminal channel is not rich enough to support a subliminal channel,Alice and Bob can always postpone this step and embed their secret messages in the subliminal channels of subsequent cover-objects.6.
Bob sends to Alice.Now,Wendy can again snoop the channel,and now has (what she might suspect is)both and .Even if she does suspect this,there is no indi-cation that the strings are related in this way,and no way she can determine the value of ,and thus no way she can determine the existence of any subliminal message.Fur-ther,both strings are random-seeming enough that there is no indication that any covert communication is occurring.She sends the message through the channel to Alice.7.
Alice decrypts what she assumes to be to get ,and,optionally,snoops the subliminal channel using the secret key to ?nd a message.8.Alice and Bob now communicate with impunity using a secure secret-key based scheme,with the random key that they now both share.Wendy has no reason to suspect that a key exchange has occurred.
Statistical tests on the cover-plot bits may yield some deviation from what would be
expected in an innocent object.However,the strings Alice and Bob share are already the product of cryptographically secure pseudo-random generators,and if the cover-plot bits of a certain medium typically fall into a non-uniform distribution,the bits can always be encoded in a manner that matches this distribution.Further,the strings transmitted via this channel are very short,and only two transmissions are needed to share a key.This may make statistical tests dif?cult.
5Are supraliminal channels“fair?”
There are a number of possible objections that may be made to the protocols presented here.First,just how fair is it to assume that the warden is not malicious?Why not imper-sonate Alice or Bob,or grossly modify their work for the purpose of foiling the possible covert communication?After all,Bob has no idea what cover-object to expect from Al-ice.He would not necessarily know if Wendy completely changed the transmitted object to something else.
The reason why Wendy can not be malicious in real-world situations is quite clear,but is sometimes forgotten when we focus entirely on the situation of two prisoners whose very names indicate that they intend to communicate covertly.A real-world warden,be she a postal censor or a system administrator,may deal with communications between a large number of people.At best she can suspect that trading of secret information is taking place,and may not be sure where or between whom.To signi?cantly alter the meaning of large number of e-mail documents or telegrams is simply unacceptible behavior.Even if the recipients of those documents have no idea what content to expect,all subsequent communication would either give away the ruse or need to be intercepted and modi?ed as well.David Kahn in[5]describes the concept of a“semagram,”a secret communication not in written form:
For example,during World War2,censors intercepted a shipment of watches.Fear-ing that the position of the hands of the watches could be spelling out a secret message,they changed the position of the hands.
If no secret message existed in those watch-hands,the recipient would likely not care about the modi?cation.On the other hand,if the recipient ultimately recieved a shipment of radial tires instead of watches,he or she would have legitimate reason to complain,as would everyone else who received a package subjected to a similar censoring methodology. Even if the recipient had no idea what content to expect,the resulting“thank you”note would be confusing indeed.Unless the United States could intercept all future transmissions from the recipient which allude to the shipment’s content,the censorship will ultimately be discovered.In any case,such wide-scale modi?cation of potentially innocent content is unreasonable.As Einstein may have put it,“Raf?niert ist der V orsteher,aber boshaft ist er nicht.”
The big question-marks,however,regard the very feasibility of the supraliminal channel: can it be done ef?ciently?Further,is it fair to allow the cover-object to be a function of the information being sent?It is often assumed that the cover-object is given,and any message
could be inserted therein.But the cover-object is assumed to be meaningless to Alice and Bob anyway,a mere front for the real message;we might as well use the opportunity to hide actual content.
Ef?ciency is the real problem,in this author’s opinion.Cover-plot functions mapping plot descriptions of stories or other high-level content would probably require a human being to perform both the composition and interpretation steps,and quite possibly the generation step as well(see?gure2.)One could develop a scheme which makes the interpretation step easy enough for a computer to manage,but in order to automate the entire process it may not help to turn a cryptographic problem into an arti?cial intelligence problem.If a supraliminal channel could be based on perceptually signi?cant information that a computer is suited to both interpret and convincingly compose,this hurdle would seem much less daunting.
6Conclusion
The prisoners’problem exists in many variations,often assuming that the prisoners share some information prior to imprisonment.Pure steganography,without any a priori secret knowledge between the communicating parties,is very dif?cult,but not necessarily impos-sible:a public-key steganography protocol has been described in[1]which requires only that one party’s public key be known to the other.Furthermore,certain assumptions allow this protocol to be modi?ed so as to allow pure steganography in the presence of a passive warden.Finally,if a supraliminal channel as described in this paper can be feasibly imple-mented,secret key exchange and hence pure steganography can take place in the presence of an active warden.
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in，on，at的时间用法和地点用法 一、in, on, at的时间用法 ①固定短语： in the morning/afternoon/evening在早晨/下午/傍晚, at noon/night在中午/夜晚, （不强调范围，强调的话用during the night） early in the morning=in the early morning在大清早， late at night在深夜 on the weekend在周末（英式用at the weekend在周末，at weekends每逢周末） on weekdays/weekends在工作日/周末， on school days/nights在上学日/上学的当天晚上， ②不加介词 this, that, last, next, every, one, yesterday, today, tomorrow, tonight，all，most等之前一般不加介词。如， this morning 今天早晨 （on）that day在那天（that day更常用些） last week上周 next year明年 the next month第二个月（以过去为起点的第二个月，next month以现在为起点的下个月） every day每天 one morning一天早晨 yesterday afternoon昨天下午 tomorrow morning明天早晨 all day/morning/night整天/整个早晨/整晚（等于the whole day/morning/night） most of the time （在）大多数时间 ③一般规则 除了前两点特殊用法之外，其他≤一天，用on，＞一天用in，在具体时刻或在某时用at（不强调时间范围） 关于on 生日、on my ninth birthday在我九岁生日那天 节日、on Teachers’Day在教师节 （注意：节日里有表人的词汇先复数再加s’所有格，如on Children’s Day, on Women’s Day, on Teachers Day有四个节日强调单数之意思，on Mother’s Day, on Father’s Day, on April Fool’s Day, on Valenti Day） 星期、on Sunday在周日，on Sunday morning在周日早晨 on the last Friday of each month 在每个月的最后一个星期五 日期、on June 2nd在六月二日 on the second (of June 2nd) 在六月的第二天即在六月二日 on the morning of June 2nd在六月二日的早晨，on a rainy morning在一个多雨的早晨 on a certain day 在某天 on the second day在第二天（以过去某天为参照） 注意：on Sunday在周日，on Sundays每逢周日（用复数表每逢之意），every Sunday每个周日，基本一个意思。 on a school day 在某个上学日，on school days每逢上学日。on the weekend在周末，on weekends每逢 周末。 关于in in June在六月 in June, 2010在2010年六月
慕课、翻转课堂、微课的介绍和特点 所谓“慕课”(MOOC)，顾名思义，“M”代表Massive(大规模)，与传统课程只有几十个或几百个学生不同，一门MOOCs课程动辄上万人，最多达16万人；第二个字母“O”代表Open(开放)，以兴趣导向，凡是想学习的，都可以进来学，不分国籍，只需一个邮箱，就可注册参与；第三个字母“O”代表Online(在线)，学习在网上完成，无需旅行，不受时空限制；第四个字母“C”代表Course，就是课程的意思。[1] MOOC是新近涌现出来的一种在线课程开发模式，它发端于过去的那种发布资源、学习管理系统以及将学习管理系统与更多的开放网络资源综合起来的旧的课程开发模式。通俗地说，慕课是大规模的网络开放课程，它是为了增强知识传播而由具有分享和协作精神的个人组织发布的、散布于互联网上的开放课程。 这一大规模在线课程掀起的风暴始于2011年秋天，被誉为“印刷术发明以来教育最大的革新”，呈现“未来教育”的曙光。2012年，被《纽约时报》称为“慕课元年”。[2]多家专门提供慕课平台的供应商纷起竞争，Coursera、edX和Udacity是其中最有影响力的“三巨头”，前两个均进入中国。 主要特点: 1)、大规模的：不是个人发布的一两门课程：“大规模网络开放课程”(MOOC)是指那些由参与者发布的课程，只有这些课程是大型的或者叫大规模的，它才是典型的的MOOC。 2)、开放课程：尊崇创用共享（CC）协议；只有当课程是开放的，它才可以成之为MOOC。 3)、网络课程：不是面对面的课程；这些课程材料散布于互联网上。人们上课地点不受局限。无论你身在何处，都可以花最少的钱享受美国大学的一流课程，只需要一台电脑和网络联接即可。
常用标点符号用法简表 标点符号栏目对每一种汉语标点符号都有详细分析，下表中未完全添加链接，请需要的同学或朋友到该栏目查询。名称符号用法说明举例句号。表示一句话完了之后的停顿。中国共产党是全中国人民的领导核心。逗号，表示一句话中间的停顿。全世界各国人民的正义斗争，都是互相支持的。顿号、表示句中并列的词或词组之间的停顿。能源是发展农业、工业、国防、科学技术和提高人民生活的重要物质基础。分号；表示一句话中并列分句之间的停顿。不批判唯心论，就不能发展唯物论；不批判形而上学，就不能发展唯物辩证法。冒号：用以提示下文。马克思主义哲学告诉我们：正确的认识来源于社会实践。问号？用在问句之后。是谁创造了人类？是我们劳动群众。感情号①！1.表示强烈的感情。2.表示感叹句末尾的停顿。战无不胜的马克思主义、列宁主义、毛泽东思想万岁！引号 ②“ ” ‘ ’ ╗╚ ┐└1.表示引用的部分。毛泽东同志在《论十大关系》一文中说：“我们要调动一切直接的和间接的力量，为把我国建设成为一个强大的社会主义国家而奋斗。”2.表示特定的称谓或需要着重指出的部分。他们当中许多人是身体好、学习好、工作好的“三好”学生。 3.表示讽刺或否定的意思。这伙政治骗子恬不知耻地自封为“理论家”。括号③（）表示文中注释的部分。这篇小说环境描写十分出色，它的描写（无论是野外，或是室内）处处与故事的发展扣得很紧。省略号④……表示文中省略的部分。这个县办工厂现在可以生产车床、电机、变压器、水泵、电线……上百种产品。破折号⑤——1.表示底下是解释、说明的部
分，有括号的作用。知识的问题是一个科学问题，来不得半点的虚伪和骄 傲，决定地需要的倒是其反面——诚实和谦逊的态度。2.表示意思的递进。 团结——批评和自我批评——团结3.表示意思的转折。很白很亮的一堆洋 钱！而且是他的——现在不见了！连接号⑥—1.表示时间、地点、数目等 的起止。抗日战争时期（1937-1945年）“北京—上海”直达快车2.表 示相关的人或事物的联系。亚洲—太平洋地区书名号⑦《》〈〉表示 书籍、文件、报刊、文章等的名称。《矛盾论》《中华人民共和国宪法》《人 民日报》《红旗》杂志《学习〈为人民服务〉》间隔号·1.表示月份和日期 之间的分界。一二·九运动2.表示某些民族人名中的音界。诺尔曼·白求 恩着重号.表示文中需要强调的部分。学习马克思列宁主义，要按照毛泽 东同志倡导的方法，理论联系实际。······
In\ on\ at (time) at 用在具体某一时刻eg at 11:00 at 4:30 在节假日的全部日子里at Christmas 习惯用法at noon at weekends\ at the weekend at night at breakfast\lunch\supper on 具体到某一天；某一天的早晨，中午或晚上on May the first on Sunday morning 对具体某一天的早晨，中午，晚上进行详细的描述on a sunny morning on a windy night 节日的当天；星期on Women?s Day on Monday In 用在年；月；季节in spring in 2012 in August 后面+一段时间表示将来时in two days 习惯用法in the morning\in the afternoon\in the evening “\”以this, that, last, next, some, every, one, any,all开始的时间副词之前的at\on\in 省略在today, tomorrow, yesterday, the day after tomorrow, tomorrow morning，yesterday afternoon，the day before yesterday 之前的介词必须省略 Practice ___ summer ____ 2012 ____ supper ___ 4:00 ___ June the first ___yesterday morning ____ New Year?s Day ___ Women?s Day ___ the morning ____ the morning of July the first ____ 2014 ___ tomorrow morning ____ midnight 1.—What are you doing ____ Sunday? And what is your wife doing ___ the weekend? 2. He?ll see you ____ Monday. And he…ll see your brother ____next Monday. 3. They often go out ___ the evenings. But they don?t go out ____ Sunday evenings. 4. Do you work ____ Fridays? Does she work _____ every Friday? 5. They usually have a long holiday ___ summer. But their son can only have a short holiday ___ Christmas. 6. Paul got married ___ 2010, He got married ___ 9 o?clock ___ 19 May 2010. His brother got married ___ May, 2011. His sister is getting married ___ this year. 1.—When will Mr Black come to Beijing? ---_______ September 5 A. on B. to C. at D. in 2. The twins were born ____ a Friday evening. A. on B. of C. at D. in 3. It?s the best time to plant ____ spring. A. on B. in C. at D.\ 4. ____ the age of twelve, Edison began selling newspaper on train. A. On B. At C. In D.By 5. She has been an English teacher ____ 2000. A. for B. since C. in D.on 6.I have studied English _____ 2003. A. since B. for C. from D.in
一、基本定义 句子，前后都有停顿，并带有一定的句调，表示相对完整的意义。句子前后或中间的停顿，在口头语言中，表现出来就是时间间隔，在书面语言中，就用标点符号来表示。一般来说，汉语中的句子分以下几种： 陈述句： 用来说明事实的句子。 祈使句： 用来要求听话人做某件事情的句子。 疑问句： 用来提出问题的句子。 感叹句： 用来抒发某种强烈感情的句子。 复句、分句： 意思上有密切联系的小句子组织在一起构成一个大句子。这样的大句子叫复句，复句中的每个小句子叫分句。 构成句子的语言单位是词语，即词和短语（词组）。词即最小的能独立运用的语言单位。短语，即由两个或两个以上的词按一定的语法规则组成的表达一定意义的语言单位，也叫词组。 标点符号是书面语言的有机组成部分，是书面语言不可缺少的辅助工具。它帮助人们确切地表达思想感情和理解书面语言。 二、用法简表 名称
句号① 问号符号用法说明。？1．用于陈述句的末尾。 2．用于语气舒缓的祈使句末尾。 1．用于疑问句的末尾。 2．用于反问句的末尾。 1．用于感叹句的末尾。 叹号！ 2．用于语气强烈的祈使句末尾。 3．用于语气强烈的反问句末尾。举例 xx是xx的首都。 请您稍等一下。 他叫什么名字？ 难道你不了解我吗？为祖国的繁荣昌盛而奋斗！停止射击！ 我哪里比得上他呀！ 1．句子内部主语与谓语之间如需停顿，用逗号。我们看得见的星星，绝大多数是恒星。 2．句子内部动词与宾语之间如需停顿，用逗号。应该看到，科学需要一个人贡献出毕生的精力。 3．句子内部状语后边如需停顿，用逗号。对于这个城市，他并不陌生。 4．复句内各分句之间的停顿，除了有时要用分号据说苏州园林有一百多处，我到过的不外，都要用逗号。过十多处。 顿号、用于句子内部并列词语之间的停顿。
介词in on at 表示时间的用法及区别 Step1 Teaching Aims 教学生掌握时间介词in,on和at的区别及用法。 Step2 Teaching Key and Difficult Points 教学生掌握时间介词in,on和at的区别及用法。 Step3 Teaching Procedures 1.用in的场合后所接的都是较长时间 (1)表示“在某世纪/某年代/特定世纪某年代/年/季节/月”这个含义时，须用介词in Eg: This machine was invented in the eighteenth century. 这台机器是在18世纪发明的。 、 She came to this city in 1980. 他于1980年来到这个城市。 It often rains here in summer. 夏天这里常常下雨。 (2)表示“从现在起一段时间以后”时，须用介词in。(in+段时间表将来) Eg: They will go to see you in a week. 他们将在一周后去看望你。
I will be back in a month. 我将在一个月后回来。 (3)泛指一般意义的上、下午、晚上用in, in the morning / evening / afternoon Eg: They sometimes play games in the afternoon. 他们有时在下午做游戏。 Don't watch TV too much in the evening. 晚上看电视不要太多。（4）A. 当morning, evening, afternoon被of短语修饰，习惯上应用on, 而不用in. Eg: on the afternoon of August 1st & B. 但若前面的修饰词是early, late时，虽有of短语修饰，习惯上应用in, 而不用on. Eg: in the early morning of September 10th 在9月10的清晨； Early in the morning of National Day, I got up to catch the first bus to the zoo. 国庆节一清早，我便起床去赶到动物园的第一班公共汽车。 2.用on的场合后所接的时间多与日期有关 （1）表示“在具体的某一天”或（在具体的某一天的）早上、中午、晚上”，或“在某一天或某一天的上午，下午，晚上”等，须用介
介词in, on, at在表示时间时的用法区别 ①in时间范围大（一天以上）如：in Tanuary, in winter, in 1999;泛指在上午，下午，晚上，如：in the morning(afternoon, evening). 习惯用法：in the daytime 在白天。 ②on指在某一天或某一天的上午，下午，晚上，如：on Monday, on Sunday afternoon, on July 1, 1999 ③at时间最短，一般表示点时间，如at six o’clock, at three thirty.习惯用法：at night, at noon, at this time of year. in, on和at在表达时间方面的区别 in 表示在某年、某季节、某月、某周、某天和某段时间 in a year在一年中 in spring 在春季 in September 在九月 in a week 在一周中 in the morning/afternoon/evening 在上午/下午/傍晚 但在中午，在夜晚则用at noon/night on 表示某一天或某一天的某段时间 on Monday 在周一 on Monday afternoon 在周一下午 on March 7th 在3月7日 on March 7th, 1998. 在1998年3月7日 on the morning of March 7th, 1998. 在1998年3月7日上午
at 表示某个具体时刻。 at eight o’clock 在8点钟 at this time of the year 在一年中的这个时候 at the moment 在那一时刻 at that time 在那时 注意：在英语中，如果时间名词前用this, last, next 等修饰时，像这样的表示，“在某时”的时间短语前，并不需要任何介词。 例如：last month, last week, this year, this week, next year, the next day, the next year 等。 1．What’s the weather like in spring/summer/autumn/winter in your country? 你们国家春天/夏天/秋天/冬天的天气怎么样？ in 在年、月、周较长时间内 in a week 在里面 in the room 用某种语言 in English 穿着 in red on 某日、某日的上下午on Sunday afternoon 在……上面 on the desk 靠吃……为生live on rice 关于 a book on Physics 〔误〕We got to the top of the mountain in daybreak. 〔正〕We got to the top of the mountain at day break. 〔析〕at用于具体时刻之前，如：sunrise, midday, noon, sunset, midnight, night。〔误〕Don't sleep at daytime 〔正〕Don't sleep in daytime. 〔析〕in 要用于较长的一段时间之内，如：in the morning / afternoon, 或in the week / month / year. 或in spring / supper /autumn / winter等等。 〔误〕We visited the old man in Sunday afternoon. 〔正〕We visited the old man on Sunday afternoon. 〔析〕in the morning, in the afternoon 如果在这两个短语中加入任何修饰词其前面的介
作品简介 本微课视频的教学内容为牛津小学英语5B Unit8 At the weekends (B)部分的单词教学。本部分的教学内容和教学重点是掌握6个昆虫类单词及insects的发音。教学目标是让学生通过微视频的学习，能够正确、流利地朗读6种昆虫类的单词以及单词insects，并能够熟练运用。教学难点是让学生读准单词发音，掌握发音规律，掌握以字母y结尾的可数名词变为复数时的变化规则。情感态度方面，需要学生具有通过微视频学习知识的兴趣，能够积极、主动的参与到教学活动中来，通过微视频学习的方式，掌握语言技能；能够根据自己对知识的掌握程度和实际需要借助视频学习的优势，反复观看，解决自己的难点问题，积极参与到视频教学中的操练项目，巩固和运用知识。 作品的创作说明： 教学设计 教学内容：牛津小学英语5B Unit8 At the weekends (B) 教学目标：让学生通过微视频学习，能够正确、流利地朗读6种昆虫类的单词以及单词insects，并能够熟练运用。 教学重点：熟练掌握6个昆虫类单词及insects的发音 教学难点：让学生读准6个昆虫类单词及insects的发音，掌握发音规律，知道以y结尾的可数名词变为复数时的变化规律。 教学过程：
Step 1借助情境图，激发兴趣，揭示课题 1.出示情境图，公园的一角 T: Here’s a picture. Is it nice? What can you see in the picture? T: Today, let’s learn some words about insects. 2.揭示课题，读2遍 Step2利用猜谜的游戏，呈现新单词，学习新单词 1.边听边猜，激发兴趣，学习新单词a grasshopper , a bee 教师讲两个昆虫的特点，让学生猜他们是什么？学生猜对后，教师呈现昆虫的图和单词，带领学生跟读，并指导单词grasshopper中a 和o的发音以及字母组合ee的发音。利用已知的单词学习新单词的发音。 2.自主阅读，猜一猜，他们是哪些昆虫 （1）学生自己读文本，猜文本中所指的是哪些昆虫。学生抓住昆虫的特点，思考。 （2）核对答案，并学习他们的发音，同时学习他们的复数形式及发音。 （3）引导学生观察发现，掌握以辅音字母+y 结尾的可数名词复数形式的变化规律。 3．整体呈现单词，齐读 整体呈现新单词，播放录音让学生整体跟读一遍，加以巩固。Step3 巩固操练 1.仔细阅读，根据歌谣内容，选择正确的图，填空。
精心整理in，on，at的时间用法和地点用法 一、in,on,at的时间用法 1、固定短语： inthemorning/afternoon/evening在早晨/下午/傍晚, 2 （on thenextmonth第二个月（以过去为起点的第二个月，nextmonth以现在为起点的下个月） everyday每天 onemorning一天早晨 yesterdayafternoon昨天下午
tomorrowmorning明天早晨 allday/morning/night整天/整个早晨/整晚（等于thewholeday/morning/night）mostofthetime（在）大多数时间 3、一般规则 除了前两点特殊用法之外，其他≤一天，用on，＞一天用in，在具体时刻或在某时用at（不强调时间范围） 关于 On 1 2) 3) （注意：节日里有表人的词汇先复数再加s’所有格，如 onChildren’sDay,onWomen’sDay,onTeachers’Day有四个节日强调单数之意思，onMother’sDay,onFather’sDay,onAprilFool’sDay,onValentine’sDay） 星期、onSunday在周日，onSundaymorning在周日早晨onthelastFridayofeachmonth在每个月的最后一个星期五
日期、onJune2nd在六月二日 onthesecond(ofJune2nd)在六月的第二天即在六月二日onthemorningofJune2nd在六月二日的早晨，onarainymorning在一个多雨的早晨 onacertainday在某天 onthesecondday在第二天（以过去某天为参照） 关于 In 1 2) InJune在六月 inJune,2010在2010年六月 in2010在2010年 inamonth/year在一个月/年里（在将来时里翻译成一个月/年之后） inspring在春天
创作编号：BG7531400019813488897SX 创作者：别如克* 微课的类型介绍 按照不同的划分方法，微课可以被分为许多种类型，其中按照教学方法来分类，微课大致可以划分为分别为五类，分别为：讲授类、讨论类、启发类、演示类、练习类等。那么今天我们就为大家介绍几种常见的微课类型，以便于大家在今后的微课制作中有更好设计思路和创意。 一、讲授类 讲授类微课适用于教师运用口头语言向学生传授知识,如描绘情境、叙述事实、解释概念、论证原理和阐明规律。这是中小学最常见、最主要的一种微课类型。 1.课堂实录微缩版 这种类型的微课中有教师、有学生、有活动、有交互……然而学生不是教研员，也不是评课专家。他不需要观看师生活动过程，学生需要的仅仅是学习内容本身。 2.画中画版 这种方式的优点在于，老师的头像出现在视频局部，仿佛老师就在自己身边一对一讲解，亲切而自然，更易于被学生接受从而愿意去学习。 二、讨论类 讨论类微课适用于在教师指导下，由全班或小组围绕某一种中心问题通过发表各自意见和看法，共同研讨，相互启发，集思广益地进行学习。 三、演示类 1.幻灯片动画式 演示类微课适用于教师在课堂教学时，把实物或直观教具展示给学生看，或者作示范性的实验，或通过现代教学手段，通过实际观察获得感性知识以说明和印证所传授知识。 PPT类微课是指教师借助PPT的视频录制功能将嵌入有课程讲解配音的PPT直接录制成流媒体视频。这类微课视频的制作门槛低，要求老师必须具备一定的动画设计制作能力。 2.手机+白纸 工具随手可得，技术难度低，操作快捷方便，画面真实亲切，并且易于分享
结语： 但在实际微课的设计与拍摄时，老师还得根据课程的性质、教学的要求、学校的实际硬件条件及自身的技术能力来选择微课的类型，只有这样，微课才能真正与课堂教学相结合，并能通过翻转课堂教学来提升课程教学的质量。 创作编号：BG7531400019813488897SX 创作者：别如克*
常用标点符号主要用法 问号 1、用在特指问句后。如：(7)你今年多大了？ 2、用在反问句后。如：(8)为什么我们不能刻苦一点呢？ ?提示：反问句若语气缓和，末尾可用句号；若语气重可用感叹号。如：(9)国家 主席可以活活被整死；堂堂大元帅受辱骂；……这哪里还有什么尊重可言！ 3、用在设问句后。如：(10)我们能让你计划实现吗？不会的。 4、用在选择问句中。如：(11)我们是革命呢，还是要现大洋？ ( 12)你到底是去，还是不去？ 5、用在表疑问的独词句后。如：(13)我？不可能吧。 ?提示：若疑问句为倒装句，问号应放在句末。如：(14)到底出了什么问题，你的 车？(若说成：“到底出了什么问题？你的车。”则错误。) ?特别提示： 句号、问号均表示句末停顿。句号用于陈述句末尾，问号用于疑问句末尾。有些句 中虽有疑问词，但全句并不是疑问句，句末只能用句号，不能用问号。 例如：(17)……最后应求出铜块的体积是多少？ (18)面对千姿百态、纷繁芜杂的期刊世界，有哪位期刊编辑不想通过期刊版面设 计为刊物分朱布白、添花增色呢？ (19)关于什么是智力？国内外争论多年也没有定论。 (17) (18) ( 19)三句都是非疑问句，(17) (18)句中问号均应改为句号，(19)句中的问号应改为逗号。 感叹号 ?特别提示： 1、在表感叹或祈使语气的主谓倒装句中，感叹号要放在句末。 如：(20)多么雄伟壮观啊，万里长城！ 2、句前有叹词，后是感叹句，叹号放在句末。 如：(21)啊，这儿多么美丽！ 下面介绍句中点号的用法。句中点号包括逗号、分号、顿号、和冒号四种。 逗号 提示：复句内各分句之间的停顿，除了有时用分号外，都要用逗号。 顿号 用于句中并列的词、词组之间较小的停顿。 如：(22)邓颖超的品德、人格、风范为中华民族树立了一座精神丰碑。 (23)从1918年起，鲁迅陆续发表了《狂人日记》、《药》、《祝福》等短篇小说。 ?特别提示：以下九种情况不用顿号。 1、不定数的两个数字间不用顿号。 如：(24)你的年龄大概是十六七岁。(不能写成“十六、七岁”) ?【注意】相邻的两个数字而非约数之间要用顿号。
i n o n a t的时间用法和 地点用法版 集团档案编码：[YTTR-YTPT28-YTNTL98-UYTYNN08]
i n，o n，a t的时间用法和地点用法 一、in,on,at的时间用法 1、固定短语： inthemorning/afternoon/evening在早晨/下午/傍晚, atnoon/night在中午/夜晚,（不强调范围，强调的话用duringthenight）earlyinthemorning=intheearlymorning在大清早， lateatnight在深夜 ontheweekend在周末（英式用attheweekend在周末，atweekends每逢周末）onweekdays/weekends在工作日/周末， onschooldays/nights在上学日/上学的当天晚上， 2、不加介词 this,that,last,next,every,one,yesterday,today,tomorrow,tonight，all，most等之前一般不加介词。如， thismorning今天早晨 （on）thatday在那天（thatday更常用些） lastweek上周 nextyear明年 thenextmonth第二个月（以过去为起点的第二个月，nextmonth以现在为起点的下个月） everyday每天 onemorning一天早晨 yesterdayafternoon昨天下午 tomorrowmorning明天早晨
allday/morning/night整天/整个早晨/整晚（等于 thewholeday/morning/night） mostofthetime（在）大多数时间 3、一般规则 除了前两点特殊用法之外，其他≤一天，用on，＞一天用in，在具体时刻或在某时用at（不强调时间范围） 关于on On指时间表示： 1）具体的时日和一个特定的时间，如某日，某节日，星期几等。Hewillcometomeetusonourarrival. OnMay4th(OnSunday,OnNewYear’sday,OnChristmasDay),therewillbeacelebra tion. 2)在某个特定的早晨，下午或晚上。 Hearrivedat10o’clocko nthenightofthe5th. Hediedontheeveofvictory. 3)准时，按时。 Iftherainshouldbeontime,Ishouldreachhomebeforedark. 生日、onmyninthbirthday在我九岁生日那天 节日、onTeachers’Day在教师节 （注意：节日里有表人的词汇先复数再加s’所有格，如 onChildren’sDay,onWomen’sDay,onTeachers’Day有四个节日强调单数之意思， onMother’sDay,onFather’sDay,onAprilFool’sDay,onValentine’sDay）星期、onSunday在周日，onSundaymorning在周日早晨
本微课视频有以下几个优点： 1、通过本节课的教学，让学生进一步体会，数学来源于生活，又用于生活，提供学生生活中熟悉的材料作背景，学生学习兴趣很高。 2、教学设施中，我非常重视开头的引入教学，激发学生学习的兴趣。注意鼓励学生大胆发言，注意从生活中的两幅图片出发，展现知识的形成过程，使学生能够利用已有的生活知识和数学知识，通过知识迁移、类比的方法归纳得出同类项的概念以及合并同类项的法则。使他们不会觉得数学概念学习的单调乏味，逐步提高学生抽象概括的能力。 3、教学的设计主要是体现以“学生为主体，教师为主导”的教学理念。整个教学过程都是学生在思考、交流，相互比赛，并解决问题，老师只是进行适当的点拨，从而真正体现了以学生为主体，教师为主导的教学理念。学生通过自学，再小组讨论，把不懂的问题都在组内消化完成。全班交流展示时，所举同类项的例子，能检验该学生是否真正掌握了同类项的概念，突出了合作交流的重要性。 4、通过议一议，汇报学习成果，培养了学生的数学语言表达能力，促使学生对合并同类项的法则理解记忆。通过比一比，看谁反应快；填一填，看谁写得最快；全班分为两组进行比赛能检验学生对同类项的概念及合并同类项的法则是否掌握，学生通过两组比赛活动，也能发扬学生合作学习的精神，进一步培养学生团结协助、严谨求实的学习精神。从而激发了学生探究数学的兴趣，更能发挥学生解决问题的主动性，使每个学生在比赛活动中都有收获。 5、通过学生谈收获，教师做补充，培养学生的数学语言表达能力和自我整理的学习习惯。因此整堂课主要突出了以下三个方面：贯穿了一个原则——以学生为主体的原则；培养了一种意识——合作交流的意识；锻炼了一种能力——数学语言表达能力。
介词in,on与at表时间的用法 at < 天(eg. noon, dawn, night, one’ clock) on = 天(Monday, 30th June, New Year’s Day, Mother’s Day) in > 天(2008, summer, April, 还有早午晚) 用in的场合后所接的都是较长时间 (1)表示“在某世纪/某年代/特定世纪某年代/年/季节/月”这个含义时,须用介词in Eg: This machine was invented in the eighteenth century. 这台机器是在18世纪发明的。 This incident happened in the 1970s. 该事件发生在20世纪70年代。 She came to this city in 1980. 他于1980年来到这个城市。 It often rains here in summer. 夏天这里常常下雨。 (2)表示“从现在起一段时间以后”时,须用介词in。(in+段时间表将来) Eg: They will go to see you in a week. 他们将在一周后去看望你。 I will be back in a month. 我将在一个月后回来。 (3)泛指一般意义的上、下午、晚上用in, in the morning / evening / afternoon Eg: They sometimes play games in the afternoon. 他们有时在下午做游戏。
Don't watch TV too much in the evening. 晚上看电视不要太多。 (4)A. 当morning, evening, afternoon被of短语修饰,习惯上应用on, 而不用in. Eg: on the afternoon of August 1st (5)B. 但若前面的修饰词是early, late时,虽有of短语修饰,习惯上应用in, 而不用on. Eg: in the early morning of September 10th 在9月10的清晨; in the late afternoon of September 12th 在9月12日的傍晚。 Early in the morning of National Day, I got up to catch the first bus to the zoo. 国庆节一清早,我便起床去赶到动物园的第一班公共汽车。 用on的场合后所接的时间多与日期有关 (1)表示“在具体的某一天”或(在具体的某一天的)早上、中午、晚上”,或“在某一天或 某一天的上午,下午,晚上”等,须用介词on。 Eg: Jack was born on May 10th, 1982. 杰克生于1982年5月10日。 They left on a rainy morning. 他们是在一个雨天的早上离开的。 He went back to America on a summer afternoon. 他于一个夏天的下午返回了美国。
公文写作中标点符号的使用规范 《中华人民共和国国家标准》（GB|T15834—1995）中有《标点符号用法》，其中对标点符号的使用作了明确的规定： 标点符号是辅助文字记录语言的符号，是书面语的有机组成部分，用来表示停顿、语气以及词语的性质和作用。 常用的标点符号有16种，分点号和标号两大类。 点号的作用在于点断，主要表示说话时的停顿和语气。点号又分为句末点号和句内点号——句末点号用在句末，有句号、问号、叹号3种，表示句末的停顿，同时表示句子的语气；句内点号用在句内，有逗号、顿号、分号、冒号4种，表示句内的各种不同性质的停顿。 标号的作用在于标明，主要标明语句的性质和作用。常用的标号有9种，即：引号、括号、破折号、省略号、着重号、连接号、间隔号、书名号和专名号。 一、常用标点符号用法简表 名称符号用法说明举例 句号。表示一句话完了之后的停 顿。 中国共产党是全中国人民的领导核心。 逗号，表示一句话中间的停顿。全世界各国人民的正义斗争，都是互相支持的。 顿号、表示句中并列的词或词组 之间的停顿。 能源是发展农业、工业、国防、科学技术和提高人 民生活的重要物质基础。 分号；表示一句话中并列分句之 间的停顿。 不批判唯心论，就不能发展唯物论；不批判形而上 学，就不能发展唯物辩证法。 冒号：用以提示下文。马克思主义哲学告诉我们：正确的认识来源于社会实践。 问号？用在问句之后。是谁创造了人类？是我们劳动群众。 感情号！1.表示强烈的感情。 2.表示感叹句末尾的停 顿。 战无不胜的马克思主义、列宁主义、毛泽东思想万 岁！ 引号“”1.表示引用的部分。毛泽东同志在《论十大关系》一文中说：“我们要
七年级语文常用标点符号用法简表 七年级常用标点符号用法简表 一、基本定义 句子，前后都有停顿，并带有一定的句调，表示相对完整的意义。句子前后或中间的停顿，在口头语言中，表现出来就是时间间隔，在书面语言中，就用标点符号来表示。 二、复习标点符号的写法，明确标点符号的书写位置。 常用的标点符号有16种，分为点号和标号。 点号 句号 。 问号 ？ 感叹号 ！ 逗号 顿号 分号 ； 冒号 标号
引号“”‘’括号[] 破折号——省略号……书名号 着重号· 间隔号· 连接号— 专名号____ 备注占两格左上角
右上角 各占 一格 各占 一格 占两格 每格 三个点 占两格 标字下 标字间 占一格 标字下点号表示语言中的停顿，一般用在句中或句末。标号主要标明语句的性质和作用。 标点符号的书写位置。 在横行书写的文稿中，句号、问号、叹号、逗号、顿号、分号和冒号都占一个字的位置，放在句末的左下角。这七种符号通常不能放在一行的开头，因为这些符号表示语气的停顿，应该紧跟在一句话的末尾。如果一行的最后的一个格正好被文字占用了，那么这个标点就必须点标在紧靠文字的右下角。 引号、括号、书名号的前一半和后一半都各占一个字的
位置，它们的前一半可以放在一行的开头，但不出现在一行的末尾，后一半不出现在一行的开头。 破折号和省略号都占两个字的位置，可以放在一行的开头，也可以放在一行的末尾，但不可以把一个符号分成两段。这两种符号的位置都写在行次中间。） 引用之语未独立，标点符号引号外；引用之语能独立，标点符号引号里。 注意事项： 冒号 表示提示性话语之后的停顿，用来提引下文。 ①同志们，朋友们：现在开会了。 ②他十分惊讶地说：“啊，原来是你！” ③北京紫禁城有四座城门：午门、神武门、东华门和西华门。 注意：“某某说”在引语前，用冒号；在引语中或引语后，则不用冒号。如： ⑴老师说：“李白是唐代的大诗人，中学课本有不少李白的诗。” ⑵“李白是唐代的大诗人，”老师说，“中学课本里有不少李白的诗。” ⑶“李白是唐代的大诗人，中学课本里有不少李白的诗。”老师说。
时间介词(at, in ,on) 的用法 1. at （1）时间的一点、时刻等。如： They came home at seven o’clock. (at night, at noon, at midnight, at ten o’clock, at daybreak, at dawn）. （2）后面接表示岁数的词。 Children in China start school at 6 years old. （3）较短暂的一段时间。可指某个节日或被认为是一年中标志大事的日子。如： He went home at Christmas （at New Year, at the Spring Festival）. 2. in （1）在某个较长的时间（如世纪、朝代、年、月、季节以及泛指的上午、下午或傍晚等） 内。如： in 2004, in March, in spring, in the morning, in the evening, etc （2）在一段时间之后,常用于将来时。 He will arrive in two hours. These products will be produced in a month. 3. on （1）具体的时日和一个特定的时间，如某日、某节日、星期几等。如： On Christmas Day（On May 4th）, there will be a celebration. （2）在某个特定的早晨、下午或晚上。如： He arrived at 10 o’clock on the night of the 5th. （3）准时，按时。如： If the train should be on time, I should reach home before dark. 表示频率的副词用法详解 一、常见的频率副词 always，usually，often，sometimes， seldom，never. 1) always表示的频率为100%，意思是"总是、一直、始终"。 I always do my cleaning on Sundays. 我总是在星期天搞卫生。 2）usually与always相比，表示的频率要低些，约为70%-80%。意思是"通常"。 Plants are usually green. 植物通常是绿色的。 Usually she goes to work by bus. 她通常乘公共汽车去上班。 3）often的频率比usually又略低些，约为60%-70%，意思是"经常"、"常常"。 Do you often write to them? 你常给他们写信吗？ Does Fred come here often? 弗雷德常来这儿吗？ 4）sometimes的频率比often又低些，约为50%＞sometimes＞30%，意思是“有时、不时”。(window.cproArray = window.cproArray || []).push({ id: "u3054369" }); 3