4 Ukuran Data Yang Disembunyikan 5 Teknik Pengungkapan Data

Bab 13_Steganografi dan Watermarking pada Citra Digital 203

13. 4 Ukuran Data Yang Disembunyikan

Ukuran data yang akan disembunyikan bergantung pada ukuran citra penampung. Pada citra 8-bit yang berukuran 256 × 256 pixel terdapat 65536 pixel, setiap pixel berukuran 1 byte. Setelah diubah menajdi citra 24-bit, ukuran data bitmap menjadi 65536 × 3 = 196608 byte. Karena setiap byte hanya bisa menyembunyikan satu bit di LSB-nya, maka ukuran data yang akan disembunyikan di dalam citra maksimum 1966088 = 24576 byte. Ukuran data ini harus dikurangi dengan panjang nama berkas, karena penyembunyian data rahasia tidak hanya menyembunyikan isi data tersebut, tetapi juga nama berkasnya. Semakin besar data disembunyikan di dalam citra, semakin besar pula kemungkinan data tersebut rusak akibat manipulasi pada citra penampung.

13. 5 Teknik Pengungkapan Data

Data yang disembunyikan di dalam citra dapat dibaca kembali dengan cara pengungkapan reveal atau extraction. Posisi byte yang menyimpan bit data dapat diketahui dari bilangan acak yang dibangkitkan oleh PRNG. Karena algoritma kriptografi yang digunakan menggunakan kunci pada proses enkripsi, maka kunci yang sama digunakan untuk membangkitkan bilangan acak. Bilangan acak yang dihasilkan sama dengan bilangan acak yang dipakai pada waktu penyembunyian data. Dengan demikian, bit-bit data rahasia yang bertaburan di dalam citra dapat dikumpulkan kembali. Di bawah ini ditampilkan contoh steganografi yang diambil dari program Tugas Akhir Lazarus Poli [POL98] yang diberi nama DATAhide. Untuk setiap contoh, digunakan kunci yang sama: informatika. Tampilan awal program diperlihatkan pada Gambar 13.2. Upa-menu yang ada pada menu Operasi adalah Penyembunyian data dan Pengungkapan data. 204 Pengolahan Citra Digital Gambar 13.2 Tampilan awal program DATAhide [POL98]. Contoh-contoh Hasil Steganografi 1. Citra penampung: citra 24 bit berwarna Data yang disembunyikan: citra berwarna a Penampung: citra peppers.bmp 512 × 512 pixel, 769 KB b Data yang disembunyikan: citra handshak.bmp 44 KB Bab 13_Steganografi dan Watermarking pada Citra Digital 205 c Hasil penyembunyian data peppers.bmp + handshak.bmp: d Berkas handshak-stega.bmp 44 KB hasil pengungkapan: Gambar 13.4 Penyembunyian citra handshaking ke dalam citra berwarna 24 bit peppers.bmp 2. Citra penampung: citra 24 bit berwarna Data yang disembunyikan: teks a Penampung: citra peppers.bmp 512 × 512 pixel, 769 KB b Data yang disembunyikan: hendro.doc 20 KB LETTER O F REC O MMENDA TIO N To Whom It May Concern, Herewith I highly recommend Mr. R. Hendro Wicaksono continue his postgraduate study at your university. My recommendation is based on my experience as his lecturer in several courses for the past four years. He has shown me his excellent attitude and personality. He is a hard working person and he has a lot of creative ideas. He is also a very intelligent student and he cooperates very well with his peers whenever they had to work together. During his study, he showed diligence and eagerness to achieve his goal. He sets very high standard for himself and organizes himself very well to achieve the standard. I am confident that if he can maintain his goal work, he should be able to complete the postgraduate program well within the stipulated time. I am sure that his abilities and his personal qualities along with his academic capabilities will help his to obtain his Master’s degree at your university, which will be very useful for our country. 206 Pengolahan Citra Digital Bandung, November 15, 2002 Yours Sincerely, Ir. Rinaldi Munir, M.Sc. Senior Lecturer Informatics Engineering Department, Institute Technology of Bandung ITB Jl. Ganesha No. 10, Bandung 40132 Email : rinaldiinformatika.org Phone +62-22-2508135 Ind o ne sia c Hasil penyembunyian data peppers.bmp + hendro.doc: d Hasil pengungkapan data hendro-stega.doc, 20 KB: LETTER O F REC O MMENDA TIO N To Whom It May Concern, Herewith I highly recommend Mr. R. Hendro Wicaksono continue his postgraduate study at your university. My recommendation is based on my experience as his lecturer in several courses for the past four years. He has shown me his excellent attitude and personality. He is a hard working person and he has a lot of creative ideas. He is also a very intelligent student and he cooperates very well with his peers whenever they had to work together. During his study, he showed diligence and eagerness to achieve his goal. He sets very high standard for himself and organizes himself very well to achieve the standard. I am confident that if he can maintain his goal work, he should be able to complete the postgraduate program well within the stipulated time. I am sure that his abilities and his personal qualities along with his academic capabilities will help his to obtain his Master’s degree at your university, which will be very useful for our country. Bandung, November 15, 2002 Yours Sincerely, Ir. Rinaldi Munir, M.Sc. Senior Lecturer Informatics Engineering Department, Institute Technology of Bandung ITB Jl. Ganesha No. 10, Bandung 40132 Email : rinaldiinformatika.org Phone +62-22-2508135 Ind o ne sia Gambar 13.5 Penyembunyian dokumen teks ke dalam citra berwarna 24 bit peppers.bmp Bab 13_Steganografi dan Watermarking pada Citra Digital 207 2. Citra penampung: citra 8 bit greyscale Data yang disembunyikan: audio a Penampung: citra barbara.bmp 512 × 512 pixel, 258 KB b Data yang disembunyikan: chord.wav 95 KB, yaitu berkas musik dari Windows. dimainkan dengan media player c Hasil penyembunyian data barbara.bmp + chord.wav Pada kasus ini, terjadi penurunan kualitas gambar karena pengaruh penurunan jumlah warna color quantization d Berkas chord-stega.wav 95 KB hasil pengungkapan: dimainkan dengan media player Gambar 13.6 Penyembunyian data audio ke dalam citra greyscale 8-bit 208 Pengolahan Citra Digital

13. 6 Wat ermarking