Kriptografi

Kuliah 4Computer Security

n Kriptografin Terminology n Secret-Key Encryptionn Public-Key Encryption

Eko AB-UAD 2Computer security-

Direkomendasi u/ membacan Beberapa sumber a.l :

¨ U.S. Senate hearings on Internet Security: B. Schenier http://www.senate.gov/~commerce/hearings/071601Schneier.pdf

¨ Crypto-Gram Newsletters http://www.counterpane.com/crypto-gram.html

¨ Security Focus Online, cryptanalysis tools http://online.securityfocus.com/tools/category/54

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Software Kriptografi

n Crypt pembanding : http://wombat.doc.ic.ac.uk/foldoc/foldoc.cgi?Crypt+Breakers+Workbench

n PGP, http://web.mit.edu/network/pgp.htmln Crypto dan Security

http://www.programmersheaven.com/zone16/cat731/index.htm

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Amankah komunikasi kita?

Sender

Pengintai/Penyadap

Recipient

Tidak ada jaminan jalur komunikasi aman

Confidential

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Cryptographic Protocols

§§ Messages should be transmitted to destinationMessages should be transmitted to destination§§ Only the recipient should see itOnly the recipient should see it§§ Only the recipient should get itOnly the recipient should get it§§ Proof of the sender’s identityProof of the sender’s identity§§ Message shouldn’t be corrupted in transitMessage shouldn’t be corrupted in transit§§ Message should be sent/received once onlyMessage should be sent/received once only

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Terminologi

n Cryptography adalah “crypto” yaitu rahasia dan “graphy” adalah tulisan

n tulisan yang bersifat rahasia n sbg ilmu dan seni untuk menjaga pesan

agar tetap amann Proses dalam Crypto : Enkripsi dan

Dekripsi

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Terminologi§ Plaintext (cleartext): pesan dalam format asli /

mudah dipahami§ Ciphertext (cyphertext): pesan yg sudah

dienkripsikan§ Encryption: transformasi pesan (plaintext) ke

kode yg tidak dapat(sulit) dimengerti§ Cipher: algoritma kriptografi. Fungsi matematik

yg digunakan untuk meng-enkripsi dan men-dekripsi.

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Lanjt§Decryption (decode) : mengubah ciphertext menjadi pesan asli. Proses Encryption dan Decryption butuh key yang sesuai hasil dari cipher

§Cryptanalysis (Code breaking): kegiatan untuk mengubah ciphertext menjadi pesan aslinya tanpa mengetahui kunci yang sesuai. Dengan cara coba-coba (trial and error).

§Cryptology: Ilmu yang mempelajari crypthography dan Cryptanalysis

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Enkripsi dan Dekripsi

Enkripsi DekripsiPlaintext Ciphertext Plaintext

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Kriptosistem Konvensional (Kunci simetrik )

Enkripsi DekripsiPlaintext PlaintextCiphertext

Kunci

Pengirim Penerima

Ciphertext=E(K,M)Message =D(K,C)

Kunci perlu ditransfer secara aman

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Kriptosistem Public Key

Enskripsi DekripsiPlaintext PlaintextCiphertext

Pengirim Penerima

C=E(Kpub,M)M=D(Kpriv,C)

Kunci umum (Kpub) (dari penerima)

Kunci private(Kpriv)

Kpub dapat dilihat /diketahui siapa saja

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KripanalisisTujuan kripanalisis¨Mengungkap/memecahkan pesan¨Mengungkap/memecahkan kunci¨Memecahkan / mencari celah algoritma

kriptografi¨Mengungkap unjuk kerja algoritma kriptografi¨Pembandingan suatu algoritma kriptografi dgn

algoritma krisptografi lain

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Breakable vs Practically breakablen Unconditionally secure: impossible to decrypt. No amount of ciphertext will enable a cryptanalyst to obtain the plaintext

n Computationally secure: an algorithm that is not breakable in practice based on worst case scenario

n Breakable: all algorithms (except one-time pad) are theoretically breakable

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What makes a good cryptosystem?

§ A good cryptosystem is one whose security does not depend upon the secrecy of the algorithm. § From Bruce Schneier: § “Good cryptographers rely on peer review

to separate the good algorithms from the bad.''

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Kriptosistem Konvensional (Kunci simetrik )

Enkripsi DekripsiPlaintext PlaintextCiphertext

Kunci

Pengirim Penerima

Ciphertext=E(K,M)Message =D(K,C)

Kunci perlu ditransfer secara aman

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Secret Key Cryptosystem Vulnerabilities (1)

Passive Attacker (Eavesdropper)nObtain and/or guess key and cryptosystem

use these to decrypt messagesnCapture text in transit and try a ciphertext-

only attack to obtain plaintext.

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Active AttackernBreak communication channel (denial of

service)nObtain and/or guess key and cryptosystem

and use these to send fake messages

Secret Key Cryptosystem Vulnerabilities (2)

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Kelemahan yg melekat pd Kriptografi kunci simetrik

§ Pendistribusian Kunci harus dilakukan secara aman (terlebih jika kelompok secara geografi jauh atau satu sama lain tidak kenal)§ Dibutuhkan kunci untuk setiap pasangan user§ n users membutuhkan n*(n-1)/2 kunci

§ Jika kunci (dan kriptosistem) is compromised, the adversary will be able to decrypt all traffic and produce fake messages

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Teknik Enkripsi dasar

n Substitution / substitusi : suatu karakter diganti dgn karakter lain

n Permutation / Transposition : penukaran posisi karakter dalam plaintext

n Combinations : gabungan antara kedua metode

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Caesar cipher

n C=E(K,M), e.g., C=(M+n) mod 26plaintext : A B C D E …ciphertext : A B C D E F …e.g., M=CAB

C =FDE

n Kelebihan: mudah diimplementasikann Kekurangan : mudah dipecahkan

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Security Tradeoff

COST

Security Functionality

Ease of Use