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.
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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
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Pengolahan Citra Digital
13. 6 Wat ermarking