Scarica Appunti Network Security e più Appunti in PDF di Sicurezza delle reti solo su Docsity!
From: f. of Network security: 1.why computer and network security is important? (a) Protects the company assets Security for computer and network is very important because it protects the company assets not the hardware and software but it also includes the information and data which are stored on the pc of the company. (b) To gain competitive advantage: Network security is important for places like e commerce .. online purchasing and banking system because people trust the secured system especially when it belongs to financial transactions. So the more banking system are secured the more people will fill save in doing purchasing and transaction through online systems. (c) Comply with regulatory requirements / fiduciary responsibilities Companies applies such policies and procedures as responsibility to keep the company safe as we said policies of each company are security requirements which they protect the company assets and protect company from fraud & (liability) (d) Keep your job You should secure your position inside the company (organization assets) Security should be part of every network inside the organization because without security for network it cause termination for the network and the organization Addition to that security is not an easy issue because it costs money in the following 1.hire 2.training 1 1
3.Buying software and hardware 4.securing the organization’s network Notes from lecture (9/10/2012) sec key 2/ Block cipher vs Stream cipher: Note: Block cipher operates plain text as block of (n) bit to produce cipher text Block cipher divide message in block with fix length Block cipher is slower than stream cipher Block cipher are used in all other cases A block cipher we must encode the block If the message is short than the block size then we use padding: waste of memory/transmission Stream cipher: Are good in all application when message is short and speed essential Reviews-groups: a set G with binary operation(+ addition) is called commutative group if (check the example sec key 2.1 page 36/91) (Advanced encryption standard )AES Specifications: Advanced encryption standard has four techniques 1.substitution 2.shift rows 3.Xor round key 4.Mix columns (check the example sec key 2.1 page 63/91) 2 2
(check the example sec key 2.1 page 17/91) Transposition Ciphers (check the example sec key 2.1 page 16/91) Monoalphabetic Cipher Security (lecture 1) Short notes from last (one time pad) Ex: Sometimes in one time pad it uses same key for decrypting 2 different messages which is not good that one key decrypted 2 messages. So it is better that every message has it is own key.(key is only used once) For ex: M1 K = C M2 K = C C1 C2= listen to cipher text 4 4
http://en.wikipedia.org/wiki/Encrypted_key_exchange Encrypted Key Exchange (also known as EKE) is a family of password-authenticated key agreement methods described by Steven M. Bellovin and Michael Merritt.[1] Although several of the forms of EKE in this paper were later found to be flawed[clarification needed], the surviving, refined, and enhanced forms of EKE effectively make this the first method to amplify a shared password into a shared key, where the shared key may subsequently be used to provide a zero-knowledge password proof or other functions. In the most general form of EKE, at least one party encrypts an ephemeral (one-time) public key using a password, and sends it to a second party, who decrypts it and uses it to negotiate a shared key with the first party. A second paper describes Augmented-EKE,[2] and introduced the concept of augmented password-authenticated key agreement for client/server scenarios. Augmented methods have the added goal of ensuring that password verification data stolen from a server cannot be used by an attacker to masquerade as the client, unless the attacker first determines the password (e.g. by performing a brute force attack on the stolen data). A version of EKE based on Diffie-Hellman, known as DH-EKE, has survived attack and has led to improved variations, such as the PAK family of methods in IEEE P1363.2. With the US patent on EKE expiring in late 2011, an EAP authentication method using EKE was published as an IETF RFC.[3] The EAP method uses the Diffie-Hellman variant of EKE. 5 5
Stream ciphers can be viewed as approximating the action of a proven unbreakable cipher, the one-time pad (OTP), sometimes known as the Vernam cipher. A one-time pad uses a keystream of completely random digits. The keystream is combined with the plaintext digits one at a time to form the ciphertext. This system was proved to be secure by Claude E. Shannon in 1949. However, the keystream must be (at least) the same length as the plaintext and be generated completely at random. This makes the system very cumbersome to implement in practice, and as a result the one-time pad has not been widely used, except for the most critical applications. A stream cipher makes use of a much smaller and more convenient key such as 128 bits. Based on this key, it generates a pseudorandom keystream which can be combined with the plaintext digits in a similar fashion to the one-time pad. However, this comes at a cost. The keystream is now pseudorandom and so is not truly random. The proof of security associated with the one- time pad no longer holds. It is quite possible for a stream cipher to be completely insecure. Types of stream ciphers A stream cipher generates successive elements of the keystream based on an internal state. This state is updated in essentially two ways: if the state changes independently of the plaintext or ciphertext messages, the cipher is classified as a synchronous stream cipher. By contrast, self- synchronising stream ciphers update their state based on previous ciphertext digits. Synchronous stream ciphers In a synchronous stream cipher a stream of pseudo-random digits is generated independently of the plaintext and ciphertext messages, and then combined with the plaintext (to encrypt) or the ciphertext (to decrypt). In the most common form, binary digits are used (bits), and the keystream is combined with the plaintext using the exclusive or operation (XOR). This is termed a binary additive stream cipher. In a synchronous stream cipher, the sender and receiver must be exactly in step for decryption to be successful. If digits are added or removed from the message during transmission, synchronisation is lost. To restore synchronisation, various offsets can be tried systematically to obtain the correct decryption. Another approach is to tag the ciphertext with markers at regular points in the output. If, however, a digit is corrupted in transmission, rather than added or lost, only a single digit in the plaintext is affected and the error does not propagate to other parts of the message. This 7 7
property is useful when the transmission error rate is high; however, it makes it less likely the error would be detected without further mechanisms. Moreover, because of this property, synchronous stream ciphers are very susceptible to active attacks: if an attacker can change a digit in the ciphertext, he might be able to make predictable changes to the corresponding plaintext bit; for example, flipping a bit in the ciphertext causes the same bit to be flipped in the plaintext. Self-synchronizing stream ciphers Another approach uses several of the previous N ciphertext digits to compute the keystream. Such schemes are known as self-synchronizing stream ciphers , asynchronous stream ciphers or ciphertext autokey (CTAK). The idea of self-synchronization was patented in 1946, and has the advantage that the receiver will automatically synchronise with the keystream generator after receiving N ciphertext digits, making it easier to recover if digits are dropped or added to the message stream. Single-digit errors are limited in their effect, affecting only up to N plaintext digits. An example of a self-synchronising stream cipher is a block cipher in cipher feedback (CFB) mode. http://en.wikipedia.org/wiki/Keystream Jump to: navigation, search 8 8
http://en.wikipedia.org/wiki/Pseudorandom A pseudorandom process is a process that appears to be random but is not. Pseudorandom sequences typically exhibit statistical randomness while being generated by an entirely deterministic causal process. Such a process is easier to produce than a genuinely random one, and has the benefit that it can be used again and again to produce exactly the same numbers - useful for testing and fixing software. To generate truly random numbers requires precise, accurate, and repeatable system measurements of absolutely non-deterministic processes. Linux uses, for example, various system timings (like user keystrokes, I/O, or least-significant digit voltage measurements) to produce a pool of random numbers. It attempts to constantly replenish the pool, depending on the level of importance, and so will issue a random number. This system is an example, and similar to those of dedicated hardware random number generators. http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation In cryptography, modes of operation is the procedure of enabling the repeated and secure use of a block cipher under a single key.[1][2] A block cipher by itself allows encryption only of a single data block of the cipher's block length. When targeting a variable-length message, the data must first be partitioned into separate cipher blocks. Typically, the last block must also be extended to match the cipher's block length using a suitable padding scheme. A mode of operation describes the process of encrypting each of these blocks, and generally uses randomization based on an additional input value, often called an initialization vector, to allow doing so safely.[1] Modes of operation have primarily been defined for encryption and authentication.[1][3] Historically, encryption modes have been studied extensively in regard to their error propagation properties under various scenarios of data modification. Later development regarded integrity protection as an entirely separate cryptographic goal from encryption. Some modern modes of operation combine encryption and authentication in an efficient way, and are known as authenticated encryption modes.[2] While modes of operation are commonly associated with symmetric encryption,[2] they may also be applied to public-key encryption primitives such as RSA in principle (though in practice public-key encryption of longer messages is generally realized using hybrid encryption).[1] http://en.wikipedia.org/wiki/Stream_cipher_attack 10 10
Stream ciphers, where plaintext bits are combined with a cipher bit stream by an exclusive-or operation (xor), can be very secure if used properly. However they are vulnerable to attack if certain precautions are not followed: keys must never be used twice valid encryption should never be relied on to indicate authenticity http://en.wikipedia.org/wiki/Block_cipher In cryptography, a block cipher is a deterministic algorithm operating on fixed-length groups of bits, called blocks, with an unvarying transformation that is specified by a symmetric key. Block ciphers are important elementary components in the design of many cryptographic protocols, and are widely used to implement encryption of bulk data. The modern design of block ciphers is based on the concept of an iterated product cipher. Product ciphers were suggested and analyzed by Claude Shannon in his seminal 1949 publication Communication Theory of Secrecy Systems as a means to effectively improve security by combining simple operations such as substitutions and permutations.[1] Iterated product ciphers carry out encryption in multiple rounds, each which uses a different subkey derived from the original key. A widespread implementation of such ciphers is called a Feistel network, named after Horst Feistel, and notably implemented in the DES cipher.[2] Many other realizations of block ciphers, such as the AES, are classified as substitution-permutation networks.[3] The publication of the DES cipher by the U.S. National Bureau of Standards (now National Institute of Standards and Technology, NIST) in 1977 was fundamental in the public understanding of modern block cipher design. In the same way, it influenced the academic development of cryptanalytic attacks. Both differential and linear cryptanalysis arose out of studies on the DES design. Today, there is a palette of attack techniques that a block cipher must be secure against, in addition to being robust against brute force attacks. Even a secure block cipher is suitable only for the encryption of a single block under a fixed key. A multitude of modes of operations have been designed to allow their repeated use in a secure way, commonly to achieve the security goals of encryption and authentication. However, block ciphers may also be used as building blocks in other cryptographic protocols, such as universal hash functions and pseudo-random number generators. 11 11
http://en.wikipedia.org/wiki/Cryptographic_key In cryptography, a key is a piece of information (a parameter) that determines the functional output of a cryptographic algorithm or cipher. Without a key, the algorithm would produce no useful result. In encryption, a key specifies the particular transformation of plaintext into ciphertext, or vice versa during decryption. Keys are also used in other cryptographic algorithms, such as digital signature schemes and message authentication codes. http://en.wikipedia.org/wiki/Kerckhoffs%27_principle In cryptography, Kerckhoffs's principle (also called Kerckhoffs's Desiderata , Kerckhoffs's assumption , axiom , or law ) was stated by Auguste Kerckhoffs in the 19th century: A cryptosystem should be secure even if everything about the system, except the key, is public knowledge. Kerckhoffs's principle was reformulated (perhaps independently) by Claude Shannon as "The enemy knows the system." In that form, it is called Shannon's maxim. In contrast to "security through obscurity," it is widely embraced by cryptographers. Contents Explanation of the principle Stated simply, the security of a cryptosystem should depend solely on the secrecy of the key and the private randomizer.[4]^ Another way of putting it is that a method of secretly coding and transmitting information should be secure even if everyone knows how it works. Of course, despite the attacker's familiarity with the system in question, the attacker lacks knowledge as to which of all possible instances is being presently observed. Advantage of secret keys Certificate Revocation List (CRL) http://searchsecurity.techtarget.com/definition/Certificate-Revocation-List Certificate Revocation List (CRL) is one of two common methods when using a public key infrastructure for maintaining access to servers in a network. The other, newer method, which has superseded CRL in some cases, is Online Certificate Status Protocol (OCSP). 13 13
The CRL is exactly what its name implies: a list of subscribers paired with digital certificate status. The list enumerates revoked certificates along with the reason(s) for revocation. The dates of certificate issue, and the entities that issued them, are also included. In addition, each list contains a proposed date for the next release. When a potential user attempts to access a server, the server allows or denies access based on the CRL entry for that particular user. The main limitation of CRL is the fact that updates must be frequently downloaded to keep the listn, search http://en.wikipedia.org/wiki/Revocation_list In the operation of some cryptosystems, usually public key infrastructures (PKIs), a certificate revocation list (CRL) is a list of certificates (or more specifically, a list of serial numbers for certificates) that have been revoked, and therefore should not be relied upon. current. OCSP overcomes this limitation by checking certificate status in real time. There are two different states of revocation defined in RFC 3280: Revoked: A certificate is irreversibly revoked if, for example, it is discovered that the certificate authority (CA) had improperly issued a certificate, or if a private-key is thought to have been compromised. Certificates may also be revoked for failure of the identified entity to adhere to policy requirements such as publication of false documents, mis-representation of software behavior, or violation of any other policy specified by the CA operator or its customer. The most common reason for revocation is the user no longer being in sole possession of the private key (e.g., the token containing the private key has been lost or stolen). Hold: This reversible status can be used to note the temporary invalidity of the certificate (e.g., if the user is unsure if the private key has been lost). If, in this example, the private key was found and nobody had access to it, the status could be reinstated, and the certificate is valid again, thus removing the certificate from future CRLs. Revocation vs. expiration Certificate expiration dates are not a substitute for a CRL. While all expired certificates are considered invalid, not all unexpired certificates are necessarily valid. CRLs or other certificate validation techniques are a necessary part of any properly operated PKI, as mistakes in certificate vetting and key management are expected to occur in real world operations. 14 14
encryption algorithm, turning it into an unreadable ciphertext (ibid.). This is usually done with the use of an encryption key, which specifies how the message is to be encoded. Any adversary that can see the ciphertext, should not be able to determine anything about the original message. An authorized party, however, is able to decode the ciphertext using a decryption algorithm, that usually requires a secret decryption key, that adversaries do not have access to. For technical reasons, an encryption scheme usually needs a key-generation algorithm, to randomly produce keys. There are two basic types of encryption schemes [1]:375-376: Symmetric-key and public-key encryption. In private-key schemes, the encryption and decryption keys are the same. Thus communicating parties must agree on a secret key before they wish to communicate. By contrast, in public-key schemes, the encryption key is public: that is, anyone (friend or foe) has access to the encryption key, and can encrypt messages. However only the receiving party has access to the decryption key and thus is the only one capable of reading the encrypted messages. Public- key encryption is a relatively recent invention: historically, all encryption schemes have been private-key schemes[1]:478. Encryption has long been used by militaries and governments to facilitate secret communication. It is now commonly used in protecting information within many kinds of civilian systems. For example, the Computer Security Institute reported that in 2007, 71% of companies surveyed utilized encryption for some of their data in transit, and 53% utilized encryption for some of their data in storage.[2]^ Encryption can be used to protect data "at rest", such as files on computers and storage devices (e.g. USB flash drives). In recent years there have been numerous reports of confidential data such as customers' personal records being exposed through loss or theft of laptops or backup drives. Encrypting such files at rest helps protect them should physical security measures fail. Digital rights management systems which prevent unauthorized use or reproduction of copyrighted material and protect software against reverse engineering (see also copy protection) are another somewhat different example of using encryption on data at rest. [ citation needed ] Encryption is also used to protect data in transit, for example data being transferred via networks (e.g. the Internet, e-commerce), mobile telephones, wireless microphones, wireless intercom systems, Bluetooth devices and bank automatic teller machines. There have been numerous reports of data in transit being intercepted in recent years.[3]^ Encrypting data in transit also helps to secure it as it is often difficult to physically secure all access to networks.[ citation needed ] Encryption, by itself, can protect the confidentiality of messages, but other techniques are still needed to protect the integrity and authenticity of a message; for example, verification of a message authentication code (MAC) or a digital signature. Standards and cryptographic software 16 16
and hardware to perform encryption are widely available, but successfully using encryption to ensure security may be a challenging problem. A single slip-up in system design or execution can allow successful attacks. Sometimes an adversary can obtain unencrypted information without directly undoing the encryption. See, e.g., traffic analysis, TEMPEST, or Trojan horse.[ citation needed ] One of the earliest public key encryption applications was called Pretty Good Privacy (PGP). It was written in 1991 by Phil Zimmermann and was purchased by Symantec in 2010.[4] Digital signature and encryption must be applied at message creation time (i.e. on the same device it has been composed) to avoid tampering. Otherwise any node between the sender and the encryption agent could potentially tamper it. It should be noted that encrypting at the time of creation only adds security if the encryption device itself has not been tampered with. http://en.wikipedia.org/wiki/Stream_cipher In cryptography, a stream cipher is a symmetric key cipher where plaintext digits are combined with a pseudorandom cipher digit stream (keystream). In a stream cipher each plaintext digit is encrypted one at a time with the corresponding digit of the keystream, to give a digit of the cyphertext stream. An alternative name is a state cipher, as the encryption of each digit is dependent on the current state. In practice, a digit is typically a bit and the combining operation an exclusive-or (xor). The pseudorandom keystream is typically generated serially from a random seed value using digital shift registers. The seed value serves as the cryptographic key for decrypting the ciphertext stream. Stream ciphers represent a different approach to symmetric encryption from block ciphers. Block ciphers operate on large blocks of digits with a fixed, unvarying transformation. This distinction is not always clear-cut: in some modes of operation, a block cipher primitive is used in such a way that it acts effectively as a stream cipher. Stream ciphers typically execute at a higher speed than block ciphers and have lower hardware complexity. However, stream ciphers can be susceptible to serious security problems if used incorrectly (see stream cipher attacks); in particular, the same starting state (seed) must never be used twice Types of stream ciphers A stream cipher generates successive elements of the keystream based on an internal state. This state is updated in essentially two ways: if the state changes independently of the plaintext or ciphertext messages, the cipher is classified as a synchronous stream cipher. By contrast, self- synchronising stream ciphers update their state based on previous ciphertext digits. 17 17
http://stackoverflow.com/questions/5635235/block-ciphers-and-stream-ciphers A stream cipher is an encryption system which works over a given sequence of input bits. Most stream ciphers work by generating from the key a long sequence of random-looking bits, which are then combined (by bitwise XOR) with the data to encrypt. This is a (crude) emulation of one- time pad. A block cipher is a generic cryptographic element which works over "blocks" which are sequences of bits with a fixed length (e.g. 128 bits for AES). The block cipher is a permutation of the blocks; the key selects which permutation we are talking about. A block cipher alone cannot process an arbitrary long message; the block cipher and the data must be used within an elaborate construction called a mode of operation (also often called a "chaining mode"). There is a chaining mode for block ciphers called "CTR" as "counter mode": in this mode, the block cipher is used to encrypt successive values of a counter (the counter having the size of a block). The resulting encrypted blocks are then concatenated, resulting in an arbitrarily long sequence of bits which depend only on the key. It suffices then to XOR that sequence with the data to encrypt. In other words, CTR mode turns a block cipher into a stream cipher. Another popular chaining mode is CBC, which does not fit the model of a stream cipher. With stream ciphers, what must be avoided at all costs is reusing the same key-dependent sequence of bits for two distinct messages; this would yield the infamous "two-times pad" which can be broken quite easily (by exploiting redundancies in the two encrypted messages). With a block cipher in CTR mode, this translates to reusing the same counter values. This is why CTR mode requires a random Initial Value (IV) which is the counter value you begin encryption with. By choosing a new random IV, with sufficiently large blocks, you avoid with very high probability any overlap in the sequences of counter values that you use. The concept of IV is not specific to block ciphers; some stream ciphers also use an IV (e.g. the one in the eSTREAM portfolio). When a stream cipher has an IV, reusing the key is no problem -- provided that you use proper IV (i.e. IV generated with a cryptographically strong RNG in the complete space of possible IV, with uniform probability). However, some other stream ciphers do not have an IV, in particular the widely used RC4. Reusing the same key would mean reusing the exact same sequence of generated bits, and that's bad. 19 19
Note that some chaining modes other than CTR also need an IV, which should be unique for each message encrypted with a given key. Block ciphers do not alleviate the need for that. http://en.wikipedia.org/wiki/Public-key_cryptography Public-key cryptography refers to a cryptographic system requiring two separate keys, one of which is secret and one of which is public. Although different, the two parts of the key pair are mathematically linked. One key locks or encrypts the plaintext, and the other unlocks or decrypts the ciphertext. Neither key can perform both functions by itself. The public key may be published without compromising security, while the private key must not be revealed to anyone not authorized to read the messages. Public-key cryptography uses asymmetric key algorithms (such as RSA), and can also be referred to by the more generic term "asymmetric key cryptography." The algorithms used for public key cryptography are based on mathematical relationships (the most notable ones being the integer factorization and discrete logarithm problems) that presumably have no efficient solution. Although it is computationally easy for the intended recipient to generate the public and private keys, to decrypt the message using the private key, and easy for the sender to encrypt the message using the public key, it is extremely difficult (or effectively impossible) for anyone to derive the private key, based only on their knowledge of the public key. This is why, unlike symmetric key algorithms, a public key algorithm does not require a secure initial exchange of one (or more) secret keys between the sender and receiver. The use of these algorithms also allows the authenticity of a message to be checked by creating a digital signature of the message using the private key, which can then be verified by using the public key. In practice, only a hash of the message is typically encrypted for signature verification purposes. Public-key cryptography is a fundamental, important, and widely used technology. It is an approach used by many cryptographic algorithms and cryptosystems. It underpins such Internet standards as Transport Layer Security (TLS), PGP, and GPG. There are three primary kinds of public key systems: public key distribution systems, digital signature systems, and public key cryptosystems, which can perform both public key distribution and digital signature services. Diffie–Hellman key exchange is the most widely used public key distribution system, while the Digital Signature Algorithm is the most widely used digital signature system. How it works 20 20
