CN101673334B - Information embedding and extracting method with a type of PDF text as shelter - Google Patents

Abstract

The invention provides an information embedding method with a type of PDF text as a shelter. The method comprises the following steps: generating a Flag Str 1 and a Flag Str 2 according to embedded information, and generating an alternate numeric string according to a steganographic key; adding '0.' in front of the Flag Str 2, and transforming the Flag Strs into real numbers between (0, 1); constructing two logistic chaotic sequence password generators Chao 1 and Chao 2; traversing and replacing numbers in all the TJ operation objects of a PDF file; and converting the PDF file containing the complete embedded information into a steganographic file. The invention further provides an information extraction method with the type of PDF text as the shelter.

Description

With the information embedding method of PDF text as blindage

The application is to be dividing an application of 200410009983.1 " with the information steganography method of type of PDF text as blindage " in the application number that on Dec 9th, 2004 submitted.

Technical field

The present invention relates to the computer network information technical field of communication safety and comprising, particularly a kind of with information embedding and the extracting method of a class P DF text as blindage.

Background technology

Information encryption is the important way that guarantees communication security, but only uses in actual applications encryption normally inadequate.Although the modern password art has developed into the subject of a comparative maturity, carry out again hidden writing after in many application that need confidential corespondence, still tending to information is encrypted, not attract much attention.Has randomness owing to process the ciphertext that generates through encryption method, thereby be easy to cause supervisor's attention, and can launch on this basis to the decoding of ciphertext or to sender and recipient's attack, if and transmission is can by the information that anyone reads, so just can reduce the possibility that people throw doubt upon to it and attempt to seek secret information on the surface.Therefore, the research of communication security not only comprises cryptological research, also comprise the research of Information Hiding Techniques, and steganography is the important branch of Information Hiding Techniques.Different from the purpose of cryptology protection information content; steganography is the position for the fact of hidden information existence or information existence; being about to important information is hidden among the out of Memory; so that people are not aware of its existence; perhaps know its existence, but the person can't determine its position without permission.

Pdf document is the abbreviation of Portable Document Format, means the Portable file layout, is delivered in 1992 by Adobe company.It has become cross-platform general format now, can be unblockedly in almost arbitrarily platform demonstration and reading, become in the world general electronic information transmission and in Web publishing printing standard format files commonly used, be one of the most popular transfer mode such as online e-magazine.The PDF file with the two-dimension vector graphics formation method established in the PostScript page-description language as basic imaging model.

As format file, the PDF page elements not only comprises the body text pel, also comprise graph primitive and image pel etc., can be with reference to existing document (as: Proceedings of the First-Sixth Information Hiding Workshop to the information steganography method in the graph image pel, Spinger-Verlag, 1996-2004 etc.).The present invention only studies with the body text of the PDF document information steganography method as blindage, consider the diversity of PDF document, the present invention transforms the PDF document (each character in the text is accurately located separately) of generation as example take Justified English WORD document.With sound, the data such as video are different, contain the redundant information that can be used to carry out confidential corespondence seldom in the text data, and as blindage and with sound, video is not quite similar as the information steganography method of blindage with text.Existing in format file the method for hidden write information few (as: Brassil J.et al.Electronic Marking and Identification Techniques to Discourage Document Copying " .Proceeding of INFORCOM ' 94; 1994; 1278-1287 and Low S.H.et al.Document Identification for Copyright Protection Using Centroid Detection.IEEE Transaction on Communications; Vol.46; No.3; 1998,372-383 etc.), typical way is to be stored in information in line space or the column pitch and method that to adopt the hidden unit of writing such as font style characteristic coding be 1Bit.The quantity of information that existing information steganography method take format file as blindage has following deficiency: A. hiddenly to write is very little; B. the quantity of information that can hiddenly write is difficult to estimate; C. easily broken through by steganalysis etc.These deficiencies have restricted the practicality of steganography method.

Summary of the invention

The objective of the invention is in order to overcome the at present deficiency of the information steganography method take format file as blindage, the present invention is directed to the PDF document of one of the most popular current network delivery Doctype, but the hidden information of writing large estimated information amount in its body text, support blind extraction, accomplish simple and practical, and the method for invention itself is observed the Kerckhoffs principle: the security of communication does not rely on the confidentiality to institute's using method itself, and only depends on stego-key.

Overall technological scheme of the present invention is: exist Secret channel in the numeral owing to the body text TJ operand that transforms the PDF document (each character the text is accurately located separately) that generates from Justified English WORD document, utilize the page-describing mechanism of display operation symbol TJ, can be on the basis that guarantees the transparency, in the TJ operand, have strategy ground selected part integer " numeral " n (1≤n≤16) come hidden write can blind extraction information.The telescopiny of steganography method and leaching process are by following conceptual design: the strategy that exchanges security with certain redundancy for is adopted in choosing of integer " numeral " n, selected part (the redundance parameter is adjustable) integer " numeral " comes hidden write information, the security of Algorithms of Selecting depends on the iterative initial value (being generated by stego-key) of pseudorandom number generator, in addition, also utilized one-way hash function SHA algorithm and MOD (16) computing to guarantee the data integrity of the information that embeds.

The invention has the advantages that:

The steganographic capacity of the inventive method is estimable.The present invention is owing to adopted scope to come hidden write information in integer (scope adds 1 in the integer number of the 0-15) numeral of 1-16, so a numeral can represent the hidden write information of 4Bit, this more above-mentioned method (the hidden unit of writing is 1Bit) that is stored in information in line space or the column pitch and adopts font style characteristic to encode will hiddenly be write larger quantity of information.Fig. 1 for choose the PDF reference manual (Adobe Systems Incorporated.Portable Document Format Reference Manual.Version 1.3.http: //www.Adobe.com.March, 1999.) the PDF blindage file PDFCoverFile.pdf that generates of the content of Chapterl.1～2.3.1, totally 6 pages, comprise 24026 characters; Fig. 2 is the embedding information (treated Lena picture) of 4668 bytes; The hidden hidden written document PDFStegoFile.pdf that write Fig. 2 of Fig. 3 for processing through the inventive method; When the scope of choosing 90% (the redundance parameter is 10%) when the integer " numeral " of 1-16 comes hidden write information (scope subtracts 1 at the integer " numeral " of 1-16), significantly, hidden written document PDFStegoFile.pdf has kept the transparency that shows, and can hiddenly write rate (hidden write information byte number/English body text number of characters) and be: (4668/24026) * 100% ≈ 19.43%.The present invention is to the PDF blindage of different typesetting formats (as: single-row, biserial etc.) and to containing each class text (mathematical formulae etc.), figure, and the PDF blindage of the page elements such as image all is suitable for fully.In addition, the Kerckhoffs principle is observed in the security of the inventive method; Possess the assurance mechanism that embeds the information data integrality and support the characteristics such as blind extraction, these characteristics have improved the practicality of the inventive method.

1.PDF the page-describing mechanism of document body text

PDF is encapsulated the formation text object to the operation to text, and each text object includes character code, text status parameter and the coordinate operational character of oneself.PDF regards text character as iconic model, the corresponding specific figure (glyph) of each character. character graphics is organized into groups in font (font) mode, a font has comprised one group of specific character code and figure, and has defined the corresponding relation between character code and the figure.When program is carried out, each font generates is with a denominative dictionary (font dictionary), to each character that need to draw, program at first searches out the dictionary name (fontname) that contains this character, from dictionary, obtain character code, obtain character graphics according to character code again.

The body text object Text Object of PDF document: begin with the BT operational character, finish with the ET operational character, wherein comprised and shown text character or character string, mobile text status, text status and other some parameter informations are set.Comprise following operational character in the PDF document: text status operational character Tf, text positional operand Tm, text positional operand Td/TD and text display operational character TJ/Tj etc.

The 1st parameter of text status operational character Tf:Tf is font information.

Text positional operand Tm: the operand of text matrix T m be [Sx 00 Sy Tx Ty] wherein, Tx and Ty are the length that former coordinate system is changed at x axle and y direction of principal axis.New coordinate system x axle and axial 1 unit of y are equivalent to Sx unit and Sy unit of former coordinate system.Font size and line feed information: font size information can be embodied by the 1st operand of Tm.Be worth greatlyr, illustrate that font size is larger, on the contrary less.The existing position of value element body of the 6th operand of Tm.Because the difference of font, with the 6th operand of the Tm of delegation minute differences may be arranged, but it is poor much smaller than 1.

Two operands of text positional operand Td/TD:Td/TD are Tx, Ty.The Td operational character is to move to next line, and the side-play amount of putting apart from the current line first place is (Tx, Ty).The TD operational character also will arrange leading text status parameter except doing the work identical with Td.Behind the Td/TD location, the text matrix T m of current line makes corresponding modification, detailed content is asked for an interview: Adobe Systems Incorporated.Portable Document Format Reference Manual.Version 1.3.http: //www.Adobe.com.March, 1999.

Text status parameter and operational character relevant with character pitch in the PDF page-describing are: character pitch parameter Tc, word space parameter Tw and display operation symbol TJ, wherein Tw is mainly for the English design, and it only is applied to space symbol (space is the word separator in English).When operational character TJ carries out, if run into character, then show this character by parameter Tc (or Tw); If run into numeral, establishing this digital numerical value is n, and then n will " deduct the n/1000 of current character unit from current the coordinate.So-called character unit refers to the standard length of certain number word, and for example, for the character of 12 dot matrix, character unit is 12 length.More detailed content is asked for an interview: Adobe Systems Incorporated.Portable Document Format Reference Manual.Version 1.3.http: //www.Adobe.com.March, 1999.Page-describing mechanism and experimental result by TJ show: when changing digital numerical value n, if n value is hour, as: less than 16, the transparency that maintenance PDF document is shown in reader.The application example of display operation symbol TJ is seen Fig. 8.

For making the PDF document possess completely device independence.Each character in the text does not adopt parameter Tc and Tw in the text of pinpoint PDF English document separately, only accurately locates the coordinate of each character in TJ display operation symbol with intercharacter integer.Fig. 9 accurately locates the central a certain TJ operand of PDF document (blindage file) separately for each character that transforms the text that generates from a Justified English WORD document, is made as CoverTJObj; Figure 10 is the TJ operand of corresponding CoverTJObj in the hidden written document after the embedding information, is made as StegoTJObj.Fig. 9, the content among Figure 10 in each "＜＞" is a character code.

2. the existence of Secret channel

English WORD document for the justify align type-setting mode that generally adopts (does not adopt parameter Tc and Tw in the body text, only in TJ display operation symbol, accurately locate the coordinate of each character with intercharacter numeral), the horizontal coordinate location of every this character of style of writing is normally by " numeral " the coefficient result in the parameter among operational character Tf, Tm, the TD/Td and the TJ operational character.Because Justified composing constraint, in every delegation of document, character pitch often itself just has uncertainty, the uncertainty of character pitch has caused the uncertainty of integer " numeral " in the TJ operational character, and integer " numeral " sequence that has in probabilistic TJ operational character provides a Secret channel that can supply hidden write information to us.

3. exchange the strategy of security for certain redundancy

The inventive method in the TJ operand of PDF blindage file, have strategy ground selected part integer " numeral " n (1≤n≤16) come hidden write can blind extraction information.The strategy that exchanges security with certain redundancy for is adopted in choosing of integer " numeral " n, selected part (the redundance parameter is adjustable) scope is come hidden write information (scope adds 1 at the integer " numeral " of 0-15) at the integer " numeral " of 1-16, the security of choosing method depends on the iterative initial value of pseudorandom number generator, and this iterative initial value is generated by stego-key.Adopt above strategy so that the Kerckhoffs principle is observed in the security of the inventive method.Figure 11 has represented " numeral " in the TJ operand, as the relation of scope between the integer " numeral " and the scope of actual hidden write information is at integer of 1-16 " numeral " of 1-16 of Secret channel.

4. the assurance of hidden write information integrality is machine-processed

By Fig. 6 and Fig. 7, and the explanation of accompanying drawing specific implementation step, as can be known: the present invention has utilized one-way hash function SHA method and MOD (16) computing to generate FlagStr1 string and FlagStr2 string." scope is in the integer storehouse of 0-15 " that embeds is by the FlagStr1 string, and " scope is at the integer of 0-15 " string and FlagStr2 string that embedding information changes into form, and the order of embedding is the composition order in " scope is in the integer storehouse of 0-15 " also according to this, such as Figure 12.So not only make the inventive method support blind extraction, also guaranteed the data integrity of the information that embeds, can effectively verify the correctness of the embedding information of extracting through extracting method, thereby improved the practicality of the inventive method.

The order that information embeds is: embedding information is through 20 " scope is at the integer of 0-15 " string of SHA and MOD (16) computing generation; Read embedding information, every 4Bit changes into one " scope is at the integer of 0-15 " string; Stego-key is through 20 " scope is at the integer of 0-15 " string of SHA and MOD (16) computing generation.

5. steganographic capacity is estimated

If cm represents the character sum of PDF text; That supposes to comprise in the text sk% relaxes and tightens character to (as: AV, wa etc.); The absolute value that is located at the integer that se% is arranged in the TJ operand is more than or equal to 1 and less than or equal to 16. in addition, we establish the redundance parameter be pr%. so, we have following steganographic capacity estimator:

Capacity=((cm-cm * sk%) * se%) * (1-pr%), establish the byte number that ed represents embedding data, when

The time, then embedding data can hiddenly be write in the PDF text.

Here, cm, sk% has different values with se% to different PDF texts, and pr% is decided according to concrete condition by us.

A large amount of experiment and the analysis showed that: when the redundance parameter is 10%, can hiddenly writes rate (hidden write information byte number/PDF document text number of characters) and be about: 20%.In addition, the present invention is to the PDF blindage of different typesetting formats (as: single-row, biserial etc.) and to containing each class text (mathematical formulae etc.), figure, and the PDF blindage of the page elements such as image all is suitable for fully.The Kerckhoffs principle is observed in the security of the inventive method; Steganographic capacity is estimable; Possess the assurance mechanism that embeds the information data integrality and support the characteristics such as blind extraction, these characteristics have improved the practicality of the inventive method.This programme can be used for making up the Information Hiding Techniques application of net environment.

Description of drawings

The invention will be further described below in conjunction with accompanying drawing.

Fig. 1 be choose the PDF reference manual the PDF blindage file map that generates of content.

Fig. 2 is embedding information (Lena picture) figure of 4668 bytes.

The hidden hidden written document PDFStegoFile.pdf figure that write Fig. 2 of Fig. 3 for processing through the inventive method.

Fig. 4 is the telescopiny of the steganography method take the PDF document as blindage;

Fig. 5 is the leaching process of steganography method.

Fig. 6 is to the process flow diagram of the specific implementation of telescopiny.

Fig. 7 is to the process flow diagram of the specific implementation of leaching process.

The application example figure of Fig. 8 display operation symbol TJ.

Fig. 9 transforms the text figure that generates from a Justified English WORD document.

Figure 10 is the TJ operand figure of corresponding CoverTJObj in the hidden written document after the embedding information.

" numeral " figure in Figure 11 TJ operand.

The order that Figure 12 embeds is the composition sequence chart in " scope is in the integer storehouse of 0-15 " also according to this.

Embodiment

The present invention will be described below in conjunction with the drawings and specific embodiments.

Blindage file among Fig. 4 is: transform the PDF document (do not adopt parameter Tc and Tw the body text, only accurately locate the coordinate of each character with intercharacter numeral in TJ display operation symbol) that generates from Justified English WORD document; In addition, the inventive method is symmetrical steganography method, so stego-key and extraction key need pass through other safe lane transmission.

Fig. 5 is the leaching process of steganography method.Detailed process describes in detail in Fig. 7.

Below in conjunction with Fig. 6 the specific implementation step of telescopiny is elaborated.

S6-1. generate " scope is in the integer storehouse of 0-15 " that comprises following content;

A. the information that embeds is made as identification string FlagStr1 through 20 " scope is at the integers of 0-15 " of SHA and MOD (16) computing generation;

B. read embedding information, every 4Bit changes into one " scope is at the integer of 0-15 ";

C. stego-key is made as identification string FlagStr2 through 20 " scope is at the integers of 0-15 " of SHA and MOD (16) computing generation;

S6-2. add before identification string FlagStr2 " 0. ", the real number between generation (0 1) is as the iterative initial value ChaoKey of following Chaotic stream cipher maker;

S6-3. construct two Logistic Chaotic stream cipher maker Chao1 and Chao2, all take ChaoKey as iterative initial value, establish iterative value and be respectively Chao1Num, Chao2Num;

S6-4. travel through the numeral in all TJ operands of blindage file, if the absolute value of the numeral that runs into is between 1 to 16, if Chao2Num is greater than " redundance parameter " (as: 10%), then order is got a numeral in " scope is in the integer storehouse of 0-15 " that S6-1 generates, add " 1 " and obtain scope at the integer of 1-16, the scope in this TJ operand of replacing is at the integer of 1-16; Otherwise, if Chao2Num less than or equal to " redundance parameter " (as: 10%), then adds " 1 " to the scope that is changed into by Chao1Num and obtains scope and replace numeral in this TJ operand at the integer of 1-16 at the integer of 0-15; If the absolute value of the numeral that runs in the blindage file TJ operand then keeps this numeral constant greater than 16;

S6-5. by Chao1 and Chao2 grey iterative generation Chao1Num, Chao2Num, if " scope is in the integer storehouse of 0-15 " takes, the scope that is then changed into by Chao1Num adds separately " 1 " and obtain the numeral of absolute value 1 to 16 that scope runs in the integer replacement TJ of 1-16 operand at the integer of 0-15, turn S6-6; Otherwise, if " scope is in the integer storehouse of 0-15 " do not take, then turn S6-4;

S6-6. finally the blindage file that embeds information is fully changed into hidden written document.

Below in conjunction with Fig. 7 the specific implementation step of leaching process is elaborated.

S7-1. generate 20 " scope is at the integer of 0-15 " extracting key through SHA and MOD (16) computing, add again separately " 1 " and obtain 20 scopes at the integer of 1-16, be made as identification string FlagStr;

S7-2. add before identification string FlagStr " 0. ", the real number between generation (01) is as the iterative initial value ChaoKey of following Chaotic stream cipher maker;

S7-3. construct Logistic Chaotic stream cipher maker Chao2, take ChaoKey as iterative initial value, establishing iterative value is Chao2Num;

S7-4. travel through the numeral in all TJ operands of hidden written document, if Chao2Num, then takes out the numeral of absolute value between 1 to 16 that runs in the hidden written document TJ of the PDF operand in turn greater than " redundance parameter " (as: 10%);

If S7-5. obtain identification string FlagStr fully, then to acquired " scope is at the integer of 1-16 " long string, carry out S7-5.1-S5.2; Otherwise, if traveled through all numerals in the TJ operand of hidden written document, then report an error: may be to extract password bad etc., otherwise, turn S7-4.

S7-5.1 as the data integrity identifying code, is made as CheckStr to 20 " scope is at the integers of 1-16 " of beginning;

S7-5.2 to the 21st " scope is at the integer of 1-16 " reciprocal, is reduced into the embedding information that wish is extracted to the 21st " scope is at the integer of 1-16 ", and is write as file, is made as EmbeddedFile;

S7-6. file EmbeddedFile is generated 20 scopes at the integer of 1-16 through SHA and MOD (16) computing, if identical with CheckStr, the information that then embeds is successfully extracted; Otherwise, report an error: may be that hidden file has suffered the reasons such as malicious modification, make the embedding information of extraction unavailable.

It should be noted last that above embodiment is only unrestricted in order to technical scheme of the present invention to be described.Although with reference to embodiment the present invention is had been described in detail, those of ordinary skill in the art is to be understood that, technical scheme of the present invention is made amendment or is equal to replacement, do not break away from the spirit and scope of technical solution of the present invention, it all should be encompassed in the middle of the claim scope of the present invention.

Claims (2)

1. one kind with the information embedding method of PDF text as blindage, comprising:

Step 1), embedding information is done one-way hash function SHA operation and MOD (16) computing, obtain the integer of 20 scopes between " 0-15 ", resulting result queue is identification string FlagStr1; Every 4Bit of the information of embedding is changed into one " scope is at the integer of 0-15 "; Stego-key is done one-way hash function SHA operation and MOD (16) computing, obtain the integer of 20 scopes between " 0-15 ", resulting result queue is identification string FlagStr2; Obtain " scope is in the integer storehouse of 0-15 " by described FlagStr1, described one " scope is at the integer of 0-15 ", described FlagStr2;

Step 2), before described identification string FlagStr2, add " 0. ", this identification string is converted into real number between (0,1);

Step 3), two Logistic Chaotic stream cipher maker Chao1 of structure and Chao2, the iterative initial value ChaoKey of described Chao1, Chao2 is set to step 2) the resulting real number that is transformed by identification string FlagStr2, the iterative value of described Chao1, Chao2 is labeled as respectively Chao1Num, Chao2Num;

Numeral in all TJ operands of step 4), traversal pdf document, if meet the numeral absolute value between 1 to 16, and the value of described Chao2Num is greater than user predefined " redundance parameter ", then order is chosen a numeral of not chosen from " scope is in the integer storehouse of 0-15 " that described step 1) generates, and this numeral is added the current numeral of meeting in " 1 " rear TJ of replacement operand; If meet the numeral absolute value between 1 to 16, but the value of described Chao2Num is less than or equal to described " redundance parameter ", the scope that then described Chao1Num changed into adds the current numeral of meeting in " 1 " rear TJ of replacement operand with this integer behind the integer between 0 to 15; If meet the numeral absolute value greater than 16 or less than 1, then keep this numeral constant;

Step 5), by Chao1 and Chao2 grey iterative generation Chao1Num, Chao2Num, if " scope 015 integer storehouse " takes, then when the absolute value of the numeral in the chance TJ operand is between 1 to 16, described Chao1Num is changed into scope behind the integer between 0 to 15, this integer is added the current numeral of meeting in " 1 " rear TJ of replacement operand, then carry out next step; If the numeral that described " scope 015 integer storehouse " is not selected in addition then re-executes step 4);

Step 6), the pdf document that will comprise complete embedding information are converted to hidden written document;

Wherein, described TJ refers to the text display operational character TJ that comprises in the PDF document.

2. according to claim 1ly it is characterized in that described redundance parameter big or small adjustable with the information embedding method of PDF text as blindage.

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