Sharing Attachments

An attachment component in an e-mail is in the form of the three types, e.g., pure text, e-mail, and binary stream. Here, an attachment component of binary stream type means that the format of the file attached in this component is unknown. If an attachment component a is of e-mail, the component is treated as a new secret e-mail and is processed by the e-mail sharing procedure again.

As for the pure text and the binary stream types, the important information of an attachment component includes the attached file name in the header part and the file data in the body part. By concatenating the file name string followed by a separate symbol ‘|’ with the string of the file data, the resulting string s can be formed.

For the case of the pure text attachment, the hierarchical sharing technique HIERARCHI_SHARING(.) is then applied to s to generate share data SA1, …, SAn. Then, the steganographic technique proposed in Section 2.3.2 can be applied to each share data SAi to create a meaningful text.

As for the type of binary stream, the steganographic technique proposed in Section 6.2.2 is adopted and s is directly regarded as input of the procedure of the technique to generate n stego-HTML documents as the stego-attachment components.

Experimental Results

The secret e-mail document in Figure 6.6 contains a content component of pure text which appears at the top line of the browsing box, a word file attachment, an e-mail attachment, and an attachment of a jpeg image which is also shown at the bottom of the browsing box.

Suppose the number of the participants of this secret e-mail is four. Their relationship is shown as Figure 6.7. The 3^rd participant is the dominant participant, which is marked by labeling a leading star symbol in the block of the participant in Figure 6.7, of the dominant sharing between 3^rd participant and 4^th participant.

After hierarchical sharing, four stego-e-mails are generated and two of the four shares are shown in Figure 6.8 (a) and (b). The Microsoft word file and the jpeg image attachments of the secret e-mail are replaced with HTML files. The e-mail attachment is manipulated as another e-mail attachment in each share e-mail. The recovered secret e-mail is obtained by applying independent sharing recovery to the shares kept by 1^st participant and 2^nd participant, dominant sharing recovery to the shares kept by 3^rd participant and 4^th participant, and then cooperative sharing recovery to the two recovery results from the independent sharing and dominant sharing.

The final result is in Figure 6.9. Note that because only the important information of the e-mail attachment is kept, the size of the e-mail attachment is smaller than the original e-mail attachment.

Discussions and Summary

The hierarchical secret sharing is applied to secret e-mails. Because the locations of the important information of components are not identical, a secret e-mail is first parsed and classified into components of three different types, e.g. e-mail header, content, and attachment.

After collecting the important information and applying hierarchical sharing to the string to generate several pieces of share data, different steganographic techniques for components of different types are performed. For e-mail header components, a substitute e-mail header component is generated with share data, which is regarded as the value of a created attribute, embedded in. For content components, different steganographic techniques deal with the components of different types, e.g. pure text, or HTML and, for attachment components, different steganographic techniques are applied for different types, e.g. pure text, binary stream or e-mail. If the type of a content component or an attachment component is pure text, the corresponding share data is translated into simple sentences by the technique proposed in Section 2.3.2.

Else if the type of a content component or an attachment component is HTML or binary stream, a cover HTML file is selected from an HTML database for embedding the share data into the between-word space outside of the tags in the cover HTML. As for e-mail attachment components, such a component is treated and handled as a secret e-mail. A share e-mail is eventually generated by replacing the components of the secret e-mail with the accordingly generated components.

Figure 6.6 A secret e-mail.

Figure 6.7 Hierarchical framework relationship among four participants.

(a)

Cooperative

Independent Dominant

*3^rd Participant 4^th Participant 1^st Participant 2^nd Participant

(b)

Figure 6.8 Stego-e-mails. (a) through (b) Two of four stego-e-mails.

Figure 6.9 Recovered secret e-mail.

Chapter 7

Steganographic Method for Tamper Proofing of E-mail Shares

Introduction

The fidelity and integrity of a stego-e-mail may be perturbed or destroyed by the manipulations of illicit users, the mindlessness of the unwitting owner of the stego-e-mail, or transmission interference of an unstable network channel. In order to assure fidelity and integrity of a stego-e-mail, a steganographic method for tamper proofing of e-mail shares, stego-e-mails with authentication signals embedded in, is investigated in this chapter.

In the reminder of this section, the ideas of the steganographic method are proposed in Section 7.1.1 and an overview of processes of the proposed method is illustrated in Section 7.1.2. In the following sections, the detailed processes are proposed in Section 7.2. In Section 7.3, some experimental results are shown and, finally, discussions and a summary of this chapter are described.