This document provides a summary of public key encryption and digital signatures. It begins by reviewing symmetric cryptography and its limitations in key distribution. It then introduces public key encryption, where each party has a public and private key pair. The document outlines the RSA algorithm and how it uses large prime number factorization problems to encrypt and decrypt messages. It also discusses how digital signatures can provide authentication, integrity, and non-repudiation for electronic messages and contracts using public key techniques like RSA.
The document summarizes the Diffie-Hellman key exchange algorithm. It was the first practical method for public key exchange proposed by Diffie and Hellman in 1976. It allows two parties to establish a shared secret key over an insecure channel. Each party generates a public/private key pair, and the secret key is derived from the exponentiation of each public key with the other party's private key. While it can securely establish a shared key, it is vulnerable to man-in-the-middle attacks without authentication of the participating identities.
This document describes the RSA algorithm for public key cryptography. RSA is based on the idea that factoring large integers into their prime factors is difficult. It involves choosing two prime numbers p and q, computing n=pq, and selecting a public key e such that gcd(e,(p-1)(q-1))=1. The private key d is computed such that ed=1 (mod φ(n)). To encrypt a message M, the sender computes C=Me (mod n) using the recipient's public key. The recipient decrypts by computing M=Cd (mod n) using their private key. The security of RSA relies on the difficulty of factoring the modulus n.
Principles of public key cryptography and its UsesMohsin Ali
This document discusses the principles of public key cryptography. It begins by defining asymmetric encryption and how it uses a public key and private key instead of a single shared key. It then discusses key concepts like digital certificates and public key infrastructure. The document also provides examples of how public key cryptography can be used, including the RSA algorithm and key distribution methods like public key directories and certificates. It explains how public key cryptography solves the key distribution problem present in symmetric encryption.
This document provides an overview of the Triple Data Encryption Standard (3DES). It first briefly describes the original Data Encryption Standard (DES) and its key components including the initial and final permutations, substitution boxes, and key schedule. It then explains that 3DES applies DES three times with three different keys to strengthen security by effectively doubling the key size to 112 bits. Simulations are included showing encryption and decryption using 3DES with equal and different keys.
cyber Security and Cryptography Elgamal Encryption Algorithm, Not-petya Case study all in one.
ElGamal encryption is a public-key cryptosystem
ElGamal Algo. uses asymmetric key encryption for communicating between two parties and encrypting the message.
This cryptosystem is based on the difficulty of finding discrete logarithm in a cyclic group
It is based on the Diffie–Hellman key exchange And It was described by Taher Elgamal in 1985.
Receiver Generates public and private keys.
Select Large Prime No. (P)
Select Decryption key/ private Key (D)
gcd(D,P)=1
Select Second part of Encryption key or public key (E1) & gcd(E1,P)=1
Third part of the encryption key or public key (E2)
E2 = E1D mod P
Public Key=(E1, E2, P) & Private key=D
In 2017 Maersk was impacted by Not-Petya ransomware attack and their network was down for a whole 9 days.
A total of 49,000 PCs and 7,000 servers were encrypted by Not-petya. Other companies that were impacted by the same attack are Merck, TNT express etc.
The tools used in Notpetya were EternalBlue and Mimikatz and hence the attack was very fast and devastating for victims.
It was The Most Devastating Cyber attack in History that’s
How a single piece of code crashed the world.
An introduction to asymmetric cryptography with an in-depth look at RSA, Diffie-Hellman, the FREAK and LOGJAM attacks on TLS/SSL, and the "Mining your P's and Q's attack".
This document provides a summary of public key encryption and digital signatures. It begins by reviewing symmetric cryptography and its limitations in key distribution. It then introduces public key encryption, where each party has a public and private key pair. The document outlines the RSA algorithm and how it uses large prime number factorization problems to encrypt and decrypt messages. It also discusses how digital signatures can provide authentication, integrity, and non-repudiation for electronic messages and contracts using public key techniques like RSA.
The document summarizes the Diffie-Hellman key exchange algorithm. It was the first practical method for public key exchange proposed by Diffie and Hellman in 1976. It allows two parties to establish a shared secret key over an insecure channel. Each party generates a public/private key pair, and the secret key is derived from the exponentiation of each public key with the other party's private key. While it can securely establish a shared key, it is vulnerable to man-in-the-middle attacks without authentication of the participating identities.
This document describes the RSA algorithm for public key cryptography. RSA is based on the idea that factoring large integers into their prime factors is difficult. It involves choosing two prime numbers p and q, computing n=pq, and selecting a public key e such that gcd(e,(p-1)(q-1))=1. The private key d is computed such that ed=1 (mod φ(n)). To encrypt a message M, the sender computes C=Me (mod n) using the recipient's public key. The recipient decrypts by computing M=Cd (mod n) using their private key. The security of RSA relies on the difficulty of factoring the modulus n.
Principles of public key cryptography and its UsesMohsin Ali
This document discusses the principles of public key cryptography. It begins by defining asymmetric encryption and how it uses a public key and private key instead of a single shared key. It then discusses key concepts like digital certificates and public key infrastructure. The document also provides examples of how public key cryptography can be used, including the RSA algorithm and key distribution methods like public key directories and certificates. It explains how public key cryptography solves the key distribution problem present in symmetric encryption.
This document provides an overview of the Triple Data Encryption Standard (3DES). It first briefly describes the original Data Encryption Standard (DES) and its key components including the initial and final permutations, substitution boxes, and key schedule. It then explains that 3DES applies DES three times with three different keys to strengthen security by effectively doubling the key size to 112 bits. Simulations are included showing encryption and decryption using 3DES with equal and different keys.
cyber Security and Cryptography Elgamal Encryption Algorithm, Not-petya Case study all in one.
ElGamal encryption is a public-key cryptosystem
ElGamal Algo. uses asymmetric key encryption for communicating between two parties and encrypting the message.
This cryptosystem is based on the difficulty of finding discrete logarithm in a cyclic group
It is based on the Diffie–Hellman key exchange And It was described by Taher Elgamal in 1985.
Receiver Generates public and private keys.
Select Large Prime No. (P)
Select Decryption key/ private Key (D)
gcd(D,P)=1
Select Second part of Encryption key or public key (E1) & gcd(E1,P)=1
Third part of the encryption key or public key (E2)
E2 = E1D mod P
Public Key=(E1, E2, P) & Private key=D
In 2017 Maersk was impacted by Not-Petya ransomware attack and their network was down for a whole 9 days.
A total of 49,000 PCs and 7,000 servers were encrypted by Not-petya. Other companies that were impacted by the same attack are Merck, TNT express etc.
The tools used in Notpetya were EternalBlue and Mimikatz and hence the attack was very fast and devastating for victims.
It was The Most Devastating Cyber attack in History that’s
How a single piece of code crashed the world.
An introduction to asymmetric cryptography with an in-depth look at RSA, Diffie-Hellman, the FREAK and LOGJAM attacks on TLS/SSL, and the "Mining your P's and Q's attack".
This document provides an overview of elliptic curve cryptography including what an elliptic curve is, the elliptic curve discrete logarithm problem (ECDLP), Diffie-Hellman key agreement and digital signatures using elliptic curves. It discusses NIST standard curves like P-256 and Curve25519 as well as choosing appropriate curves and potential issues like attacks if randomness is not properly implemented or an invalid curve is used.
PGP (Pretty Good Privacy) is an open source encryption software that provides security mechanisms like authentication, confidentiality, compression, and email compatibility. It uses strong cryptographic algorithms like IDEA, RSA, and SHA-1. PGP protects messages by signing them with the sender's private key, encrypting them with a random symmetric key, and encrypting that key with the recipient's public key. This ensures message integrity and confidentiality. Compression is applied before encryption to save space. Radix-64 encoding allows encrypted messages to be transmitted over email. PGP's features help secure email communications and stored files from unauthorized access.
Public Key Cryptography uses two keys - a public key that can encrypt messages and verify signatures, and a private key that can decrypt messages and create signatures. The RSA algorithm, the most widely used public key algorithm, is based on the mathematical difficulty of factoring large prime numbers. It works by having users generate a public/private key pair using two large prime numbers and performing modular exponentiation. The security of RSA relies on the fact that it is computationally infeasible to derive the private key from the public key and modulus.
E-MAIL, IP & WEB SECURITY
E-mail Security: Security Services for E-mail-attacks possible through E-mail – establishing keys privacy-authentication of the source-Message Integrity-Non-repudiation-Pretty Good Privacy-S/MIME. IPSecurity: Overview of IPSec – IP and IPv6-Authentication Header-Encapsulation Security Payload (ESP)-Internet Key Exchange (Phases of IKE, ISAKMP/IKE Encoding). Web Security:
This presentation contains the contents pertaining to the undergraduate course on Cryptography and Network Security (UITC203) at Sri Ramakrishna Institute of Technology. This covers the ElGamal Cryptosystem.
1) The document discusses symmetric encryption techniques including the symmetric cipher model, substitution techniques like the Caesar cipher and transposition techniques like the rail fence cipher.
2) It also covers the one-time pad cipher and its requirements for security as well as steganography techniques for hiding messages.
3) Cryptanalysis methods like brute force attacks and cryptanalytic attacks are explained for analyzing encryption algorithms.
The document discusses key management and distribution in cryptography. It covers topics such as key generation, the different types of keys including symmetric and asymmetric keys, how symmetric and asymmetric encryption works, different methods of key distribution including public key distribution and private key distribution, and an overview of public key infrastructure. The goal of key management is to support the establishment and maintenance of secure key relationships between authorized parties.
Vulnerabilities are weaknesses that attackers can exploit to gain unauthorized access to a network or its resources. Attacks are attempts to damage, access, or misuse assets without permission. Network security mechanisms detect, prevent, and recover from attacks using methods like routing control, traffic padding, encryption, access control, digital signatures, and ensuring data integrity.
RC4 is a symmetric key stream cipher algorithm invented in 1987. It operates by combining a pseudo-random keystream with plaintext using XOR operations. The keystream is generated from an initial random permutation of bytes. RC4 has been used to encrypt network traffic but weaknesses have been found, including biases in the early output bytes that allow recovery of encryption keys. While simple and fast, RC4 is no longer considered secure for many applications.
This document discusses cryptography and its history. Cryptography began as early as 2000 BC in Egypt and has evolved over three eras: the manual era involving pen and paper ciphers, the mechanical era with the invention of cipher machines, and the modern era utilizing computers. Modern cryptography combines computer science and mathematics to encrypt data for security. Key aspects include encryption, decryption, symmetric and asymmetric keys, and different cipher algorithms. The document also covers categories of cryptography, notable cryptographers, applications, and some limitations of early cryptography techniques.
This document summarizes key topics in cryptographic key management and distribution from Chapter 14 of William Stallings' book "Cryptography and Network Security". It discusses how symmetric encryption schemes require parties to share a secret key, and how public key schemes require parties to obtain valid public keys. It then covers various methods for key distribution, including using a key hierarchy with session keys and master keys, as well as alternatives like third party key distribution and the use of public key encryption to distribute secret keys. It also introduces the concept of using a key distribution center and X.509 certificates to facilitate secure key exchange through a public key infrastructure.
The document discusses the design of secure hash algorithms SHA-256 and SHA-3. SHA-256 has a block size of 512 bits and processes messages in 64 rounds. SHA-3 uses a sponge construction that absorbs data into a state and then squeezes out the output hash. Both algorithms are used to secure blockchains like Bitcoin and Ethereum.
The document summarizes the RSA encryption algorithm. It begins by explaining that RSA was developed in 1977 by Rivest, Shamir and Adleman. It then provides an example to demonstrate how RSA works step-by-step, generating keys, encrypting a message and decrypting the ciphertext. Finally, it discusses some challenges with breaking RSA encryption, including brute force attacks and mathematical attacks based on factoring the encryption keys, as well as timing attacks that aim to deduce keys based on variations in processing time.
Introduction to Public key Cryptosystems with block diagrams
Reference : Cryptography and Network Security Principles and Practice , Sixth Edition , William Stalling
A hash algorithm is a one-way function that converts a data string into a numeric string output of fixed length. It is collision resistant, meaning it is very unlikely for different data to produce the same hash value. Common hash algorithms include MD5 and SHA-1. A one-way hash function takes a variable-length input and produces a fixed-length output. It is easy to compute the hash but very difficult to reverse it or find collisions. Hash functions are used for password verification, digital signatures, and ensuring data integrity.
Public key cryptography uses asymmetric encryption with two related keys - a public key and a private key. The public key can be shared openly but the private key is kept secret. When Alice wants to send a confidential message to Bob, she encrypts it with Bob's public key. Only Bob can decrypt it using his private key. Public key infrastructure involves policies and technologies for issuing, managing, and revoking digital certificates that bind public keys to identities. Popular public key algorithms like RSA are based on the difficulty of factoring large prime numbers.
This document provides an overview of cryptography. It defines cryptography as the science of secret writing and discusses its use in applications like ATM cards and passwords. It describes the basic components of cryptography including plaintext, ciphertext, ciphers, keys, and algorithms. It differentiates between symmetric and asymmetric key cryptography. It provides examples of traditional and modern ciphers, including DES, AES, and RSA algorithms. In conclusion, it states that cryptography techniques help maintain data security, privacy, and integrity.
This document provides an overview of elliptic curve cryptography including what an elliptic curve is, the elliptic curve discrete logarithm problem (ECDLP), Diffie-Hellman key agreement and digital signatures using elliptic curves. It discusses NIST standard curves like P-256 and Curve25519 as well as choosing appropriate curves and potential issues like attacks if randomness is not properly implemented or an invalid curve is used.
PGP (Pretty Good Privacy) is an open source encryption software that provides security mechanisms like authentication, confidentiality, compression, and email compatibility. It uses strong cryptographic algorithms like IDEA, RSA, and SHA-1. PGP protects messages by signing them with the sender's private key, encrypting them with a random symmetric key, and encrypting that key with the recipient's public key. This ensures message integrity and confidentiality. Compression is applied before encryption to save space. Radix-64 encoding allows encrypted messages to be transmitted over email. PGP's features help secure email communications and stored files from unauthorized access.
Public Key Cryptography uses two keys - a public key that can encrypt messages and verify signatures, and a private key that can decrypt messages and create signatures. The RSA algorithm, the most widely used public key algorithm, is based on the mathematical difficulty of factoring large prime numbers. It works by having users generate a public/private key pair using two large prime numbers and performing modular exponentiation. The security of RSA relies on the fact that it is computationally infeasible to derive the private key from the public key and modulus.
E-MAIL, IP & WEB SECURITY
E-mail Security: Security Services for E-mail-attacks possible through E-mail – establishing keys privacy-authentication of the source-Message Integrity-Non-repudiation-Pretty Good Privacy-S/MIME. IPSecurity: Overview of IPSec – IP and IPv6-Authentication Header-Encapsulation Security Payload (ESP)-Internet Key Exchange (Phases of IKE, ISAKMP/IKE Encoding). Web Security:
This presentation contains the contents pertaining to the undergraduate course on Cryptography and Network Security (UITC203) at Sri Ramakrishna Institute of Technology. This covers the ElGamal Cryptosystem.
1) The document discusses symmetric encryption techniques including the symmetric cipher model, substitution techniques like the Caesar cipher and transposition techniques like the rail fence cipher.
2) It also covers the one-time pad cipher and its requirements for security as well as steganography techniques for hiding messages.
3) Cryptanalysis methods like brute force attacks and cryptanalytic attacks are explained for analyzing encryption algorithms.
The document discusses key management and distribution in cryptography. It covers topics such as key generation, the different types of keys including symmetric and asymmetric keys, how symmetric and asymmetric encryption works, different methods of key distribution including public key distribution and private key distribution, and an overview of public key infrastructure. The goal of key management is to support the establishment and maintenance of secure key relationships between authorized parties.
Vulnerabilities are weaknesses that attackers can exploit to gain unauthorized access to a network or its resources. Attacks are attempts to damage, access, or misuse assets without permission. Network security mechanisms detect, prevent, and recover from attacks using methods like routing control, traffic padding, encryption, access control, digital signatures, and ensuring data integrity.
RC4 is a symmetric key stream cipher algorithm invented in 1987. It operates by combining a pseudo-random keystream with plaintext using XOR operations. The keystream is generated from an initial random permutation of bytes. RC4 has been used to encrypt network traffic but weaknesses have been found, including biases in the early output bytes that allow recovery of encryption keys. While simple and fast, RC4 is no longer considered secure for many applications.
This document discusses cryptography and its history. Cryptography began as early as 2000 BC in Egypt and has evolved over three eras: the manual era involving pen and paper ciphers, the mechanical era with the invention of cipher machines, and the modern era utilizing computers. Modern cryptography combines computer science and mathematics to encrypt data for security. Key aspects include encryption, decryption, symmetric and asymmetric keys, and different cipher algorithms. The document also covers categories of cryptography, notable cryptographers, applications, and some limitations of early cryptography techniques.
This document summarizes key topics in cryptographic key management and distribution from Chapter 14 of William Stallings' book "Cryptography and Network Security". It discusses how symmetric encryption schemes require parties to share a secret key, and how public key schemes require parties to obtain valid public keys. It then covers various methods for key distribution, including using a key hierarchy with session keys and master keys, as well as alternatives like third party key distribution and the use of public key encryption to distribute secret keys. It also introduces the concept of using a key distribution center and X.509 certificates to facilitate secure key exchange through a public key infrastructure.
The document discusses the design of secure hash algorithms SHA-256 and SHA-3. SHA-256 has a block size of 512 bits and processes messages in 64 rounds. SHA-3 uses a sponge construction that absorbs data into a state and then squeezes out the output hash. Both algorithms are used to secure blockchains like Bitcoin and Ethereum.
The document summarizes the RSA encryption algorithm. It begins by explaining that RSA was developed in 1977 by Rivest, Shamir and Adleman. It then provides an example to demonstrate how RSA works step-by-step, generating keys, encrypting a message and decrypting the ciphertext. Finally, it discusses some challenges with breaking RSA encryption, including brute force attacks and mathematical attacks based on factoring the encryption keys, as well as timing attacks that aim to deduce keys based on variations in processing time.
Introduction to Public key Cryptosystems with block diagrams
Reference : Cryptography and Network Security Principles and Practice , Sixth Edition , William Stalling
A hash algorithm is a one-way function that converts a data string into a numeric string output of fixed length. It is collision resistant, meaning it is very unlikely for different data to produce the same hash value. Common hash algorithms include MD5 and SHA-1. A one-way hash function takes a variable-length input and produces a fixed-length output. It is easy to compute the hash but very difficult to reverse it or find collisions. Hash functions are used for password verification, digital signatures, and ensuring data integrity.
Public key cryptography uses asymmetric encryption with two related keys - a public key and a private key. The public key can be shared openly but the private key is kept secret. When Alice wants to send a confidential message to Bob, she encrypts it with Bob's public key. Only Bob can decrypt it using his private key. Public key infrastructure involves policies and technologies for issuing, managing, and revoking digital certificates that bind public keys to identities. Popular public key algorithms like RSA are based on the difficulty of factoring large prime numbers.
This document provides an overview of cryptography. It defines cryptography as the science of secret writing and discusses its use in applications like ATM cards and passwords. It describes the basic components of cryptography including plaintext, ciphertext, ciphers, keys, and algorithms. It differentiates between symmetric and asymmetric key cryptography. It provides examples of traditional and modern ciphers, including DES, AES, and RSA algorithms. In conclusion, it states that cryptography techniques help maintain data security, privacy, and integrity.
1. K R I P T O L O J I V E G Ü V E N L I K
P R O T O K O L L E R I
El-Gamal Asimetrik
Şifreleme Algoritması
İbrahim
Fil 927220046
2. TA R I H Ç E
Taher Elgamal ilk olarak ElGamal Cryptosystem'i, kriptolojinin
gelişmeleri üzerine bir konferans olan CRYPTO ’84'ün
bildirilerinde yayınlanan bir makalede anlattı.
3. H A K K I N D A
• ElGamal şifreleme sistemi, Diffie – Hellman anahtar
değişimine dayanan açık anahtarlı kriptografi için asimetrik bir
anahtar şifreleme algoritmasıdır.
• Bu algoritmada public ve private olmak üzere iki adet anahtar
vardır.Public metni şifrelemek için private şifreyi çözmek için
kullanılmaktadır.
4. • ElGamal'ın güvenliği, ayrık logaritma problemine
dayanmaktadır.
• Bugün ElGamal algoritması birçok kriptografik
üründe kullanılmaktadır. Açık kaynaklı yazılım GnuPG,
imzalar için standart olarak ElGamal'ı kullanır.
• Bu algoritmanın zorluğu dairesel gruplar üzerinde ayrık
algoritma işleminin uygulanmasına dayanmaktadır.
5. D A I R E S E L
G R U P L A R
Grup teorisinde, bir dairesel
grubun üyeleri tek bir
elemandan üretilebiliyorsa bu
gruba dairesel grup
denilmektedir. Bu eleman
generator olarak
isimlendirilmektedir ve bu
elemanın bütün kuvvetleri bu
grubun elemanıdır yani bu
grubun elemanları generator
değerinin kuvveti olarak
hesaplanabiliyor.
6. Örnek olarak G={g0,g1,g2,g3,g4} grubunda 5 eleman
bulunmaktadır ve dairesel olduğu için son elemandan sonra
tekrar ilk elemana dönülür yani g5=g0. Mod fonksiyonu bu
gruba örnek verilebilir.
7. AY R I K L O G A R I T M A
Ayrık logaritma işlemleri çoğunlukla soyut matematik alanında
karşımıza çıkmaktadır. Ayrık logaritma dairesel gruplar üzerinde
tanımlanmış logaritma işlemidir. Örneğin g elemanı G dairesel
grubunun bir elemanı olsun ve g^x = h olsun. Bu işlemin tersi
olan x = log(h) işlemi ayrık algoritma olarak
isimlendirilmektedir.
8. Ayrık logaritmaları anlamanın en kolay yollarından birisi de
(Zp)× ahenk sınıfı üzerindeki işlemleridir. Bir ahenk sınıfı 0 ile
p-1 arasındaki ayrık sayılar kümesi olup bu küme üzerinde
tanımlı olan herhangi bir işlem yine bu küme içinden bir sonuç
döndürmelidir. Basitçe modulo p işlem kümesi olarak
düşünülebilir ve yapılan her işlemin p tabanında modulo alındığı
düşünülebilir.
9. Daha açık bir şekilde ayrık üst (discrete exponent) denilen işlem
bir sayının verilen üst değerinin verilen modulo p için
hesaplanmasıdır. Yani sayının istenilen üstü hesaplanır ve p
değerine bölünür sonuç bölme işleminin kalan kısmıdır.
10. Ö R N E K
• 2 sayısının Z29 ‘da bir ilkel kökü (primitive root) olup olmadığını bulunuz ve
L2(15) ayrık logaritma değerini bulunuz.
21=2 mod 29
22= 4 mod 29
24=16 mod 29
28= 24 mod 29
…..
228 = 1 mod 29
11. A L G O R I T M A
• Anahtar Üretme
• Şifreleme
• Şifreyi Çözme
12. A N A H TA R Ü R E T M E
• Çok basamaklı büyük bir p asal sayısı belirlenir ve mod(p)
kümesinin elemanlarından bir g generator pozitif tam sayısı
belirlenir.
• Daha sonra şifreyi çözecek kişi 1<= x <= p-2 aralığında
kalacak şekilde kendisine bir özel anahtar(private key) seçer.
13. • Daha sonra public key'in bir parçası
olan y değeri y= gxmod(p) işlemi sonucu hesaplanır.
• (p,g,y) değerleri public key 3'lüsü olarak mesajlaşmak istenilen
kişiye iletilir.
14. Ş I F R E L E M E S Ü R E C I
M mesajını şifrelemek için, önce bir anahtar sunucusundan veya
şifrelenmemiş elektronik posta yoluyla açık anahtar
üçlüsünün (p, g, gx ) alınması gerekir. Bu mesajlaşma sürecinde
tek gizli anahtar x'dir ve karşı tarafa gx şeklinde gönderilir.
15. • M messaj metni (m1,m2,m3,...) tamsayı seti
şeklinde hazırlanır.({1,2,…,p-1} kümesinin elemanı olacak
şekilde)
• (Zp)× Kümesinden bir k üs değeri belirlenir.
• Rastgele üs k'yi iletmek için c1=gk mod(p) hesaplanır.
• Ayrıca c2=mi . gxk Mod(p) hesaplanır ve (c1,c2) ikilisi
çözülmek için gönderilir.
16. Ş I F R E Y I Ç Ö Z M E
Şifre çözme işleminde private anahtar olan x değişkeni
kullanılır.
Mi= ( c2 / c1
x )
17. Ö R N E K
g=3,p=101,k=6,m=5,x=2
1.aşama y'nin hesaplanması: y= gxmod(p)=32=9mod101
2.aşama şifreleme: C = (r,c)
r = gk mod(p)=729 mod101 = 22mod 101
c = m.yk mod(p)=5.96(mod101)=97 mod101 C=(22,97)
3.aşama şifreyi çözme: D= c.(r(p-1-x)modp) = 97.(2298 mod101)
= 5 mod101
18. K R I P TA N A L I Z
Geliştirilmiş ElGamal Algoritması (Hwang at al).
Anahtar üretimi ElGamal ile aynıdır.(p,g,gx) public key olarak
nitelendiriliyor ve x değeri private key olarak kullanılıyor.
Şifreleme sürecinde ElGamal'dan farklı olarak 2 adet üs değeri
seçilmektedir;r1,r2. 1< r1,r2 <= p-1.
19. • Mesaj gönderecek kişi M mesajını herbiri log2(p) değerinden
küçük olacak şekilde n parçaya böler ve her biri için şifreleme
yöntemini uygular. 0≤Mi< p.
• R1 = gr1mod(p), R2 = gr2mod(p),
• Şifre çözme işlemi için aşşağıdaki formül kullanılmaktadır.
20. 2d-1 = 0 mod(q) olsun.
Eğer n > d ise ve p değeri 2e.q+1 şeklinde yazılabiliyorsa
algoritmanın güvenliksiz olduğu savunulmaktadır.Burda q
değeri bir asal sayıdır.
21. Burdan şu sonuç çıkabiliyor:
2e(2d-1) = 0 mod(2eq) ---> 2e+d = 2emod(2eq) yani d değeri periyot oluyor.
(Me+a,Ce+a) mesaj ve şifeli hali olsun ve 0 ≤ a≤ n−e olsun.
Saldırgan 0 ≤ i ≤ n−e ve i ≡ a(mod (d)) koşulunu sağlayan bütün i'ler için Me+i
değerini bulabilir.
Bu formül bilindiğinden
Aşşağıdaki denklik elde edilir.
22. 2e+a=2e+i mod(2eq) olduğu için
Bu iki formül kullanılarak aşşağıdaki sonuç elde edilir.
Bu şekilde mesaj çözülmüş olur.
23. K AY N A K L A R
• http://bilgisayarkavramlari.com/2008/05/06/ayrik-logaritma-discrete-logarithm/
• Cryptanalysis of an ElGamal-Like Cryptosystem for EncipheringLarge
Messages
• http://bilgisayarkavramlari.com/2008/04/30/el-gamal-encryption-el-cemal-
sifrelemesi/
• http://bilgisayarkavramlari.com/2008/04/30/dairesel-grup-cyclic-group/
• meier_paper.pdf (tum.de)