Sis - Prvi Kolokvijum

The correct answer is Jednostavni XOR i One Time Pad. Both Jednostavni XOR and One Time Pad are encryption algorithms that use the XOR operation to encrypt and decrypt messages. XOR is a bitwise operation that takes two inputs and outputs a result based on the exclusive or (XOR) of their binary representations. One Time Pad is a type of encryption that uses a random key that is as long as the message itself, making it theoretically unbreakable if used correctly. Jednostavni XOR is a simpler version of this encryption method, where a fixed key is used instead of a random one.

The given text is encrypted using the Vigenère cipher with the key "PERA" in the English alphabet, where A is equivalent to 0. The resulting cipher text is "EIIA".

The question is asking for the size of the key space when the key length is 5 and the set of characters is Z={1,2,3,4,...,9,a,b,c,...,x,y,z,A,B,C,...,X,Y,Z,*,#,$}. The key space size is determined by the number of possible combinations of characters in the key. Since the key length is 5 and there are 64 characters in the set Z, the size of the key space is 64^5.

The given question asks to identify the algorithm that does not belong to the given group. MD5, SHA-3, and SHA-0 are all cryptographic hash functions, while AES (Advanced Encryption Standard) is a symmetric encryption algorithm. Therefore, AES is the algorithm that does not belong to the given group of hash functions.

The given correct answer is "да" (yes). This suggests that a hash function should possess the property of the avalanche effect. The avalanche effect refers to the property of a hash function where a small change in the input results in a significant change in the output. This property is desirable as it ensures that even a slight alteration in the input will produce a completely different hash value, making it difficult for attackers to predict or manipulate the output. Therefore, it is important for a hash function to have the avalanche effect to enhance its security.

The given answer "ЖЉИБ" is the result of encrypting the open text "БИЛО" using the Vigenere cipher with the key "ПЕРА". The Serbian alphabet is used, where each letter is assigned a number from 1 to 30. The Vigenere cipher is a polyalphabetic substitution cipher that uses a series of Caesar ciphers based on the letters of a keyword. In this case, the keyword "ПЕРА" is repeated to match the length of the open text. Each letter of the open text is shifted by the corresponding letter of the keyword, resulting in the encrypted text "ЖЉИБ".

The correct answer suggests that the authentication process involves the existence of "inter-node" trust. This means that there is a level of trust established between different nodes or entities involved in the authentication process. This trust allows for secure communication and verification of identities between these nodes.

The given question mentions encrypting the text "MARKO" using the Vigenere cipher with the key "MARKO" in the English alphabet where A=0. The resulting cipher text is "YAIUC". The Vigenere cipher is a polyalphabetic substitution cipher where each letter in the plaintext is shifted according to the corresponding letter in the key. In this case, the first letter "M" is shifted by 12 positions (M=M+12=Y), the second letter "A" is shifted by 0 positions (A=A+0=A), the third letter "R" is shifted by 17 positions (R=R+17=I), the fourth letter "K" is shifted by 10 positions (K=K+10=U), and the fifth letter "O" is shifted by 14 positions (O=O+14=C), resulting in the cipher text "YAIUC".

The correct answer is "Системи за детекцију упада" which translates to "Intrusion detection systems" in English. This is because the question is asking for systems that register attacks either during their occurrence or afterwards, through data analysis. Intrusion detection systems are designed to detect and respond to unauthorized access or malicious activities in a computer system or network. They monitor network traffic, analyze log files, and use various techniques to identify and report potential security breaches.

The correct answer is "samog sebe". This means that the Supreme CA is self-signed, indicating that it issues and signs its own certificates without relying on any other CA for authentication.

If MAC (Mandatory Access Control) and DAC (Discretionary Access Control) are simultaneously applied, it means that MAC is older than DAC.

This attack, which involves eavesdropping on traffic, monitoring its intensity, and gaining access to sensitive information, is an attack on confidentiality. By intercepting and accessing sensitive information, the attacker compromises the confidentiality of the data, potentially exposing it to unauthorized individuals.

The correct answer is "блок шифрата је исте дужине као блок отвореног текста" because it states that the length of the ciphertext block is the same as the length of the plaintext block. This means that each character or group of characters in the plaintext is encrypted into a corresponding character or group of characters in the ciphertext, maintaining the same length.

The correct answer is "Хеш (лозинка + случајна вредност)" because it correctly calculates the hash value by combining the password and the random value. This ensures that each user's password is securely stored and protected. By using the hash function, the password is transformed into a fixed-length string of characters, making it difficult for anyone to reverse-engineer the original password from the stored hash value. The inclusion of the random value adds an extra layer of security and unpredictability to the hash calculation.

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MAC (Message Authentication Code) се користи у процесу аутентификације. MAC функција генерише код који се прилаже поруци као доказ да је порука аутентична и да је није мењао неко други. Ово помаже у верификацији идентитета пошаљиоца поруке и осигурава да порука није промењена током преноса. Ово је битан процес у сигурној комуникацији и спречава могућност фалсификовања или измене поруке од стране нападача.

A DoS (Denial of Service) attack is an attack that aims to disrupt or deny access to a system or network, making it unavailable to legitimate users. This attack does not target the confidentiality, authentication, or integrity of the system, but rather focuses on rendering it inaccessible. Therefore, the correct answer is "Расположивост" which translates to "Availability" in English.

A cryptographic system that cannot be "broken" even with unlimited resources, manpower, and time is considered unconditionally secure. This means that no matter how much computational power or time an attacker has, they will not be able to decipher the encrypted information.

The correct answer is that it is possible to hide a BMP image within another BMP image. This means that the data from one BMP image can be embedded or concealed within the data of another BMP image, making it appear as a single image while still containing the hidden information.

The key for the Caesar cipher is determined by the order in which the letters in the encrypted message are shifted. In this case, the encrypted message is "GIDEV" and the original message is "CEZAR". By comparing the two messages, it can be observed that each letter in the original message is shifted by a certain number of positions to obtain the corresponding letter in the encrypted message. Counting the number of positions each letter is shifted, we find that the key is 4.

The given question states that a one-way hash function generates a 160-bit output. The question then asks how many possible input messages can generate a hash value of 1. The correct answer is "beskonačno mnogo poruka" which translates to "infinitely many messages." This is because a one-way hash function can have multiple input messages that produce the same output hash value, so there is no limit to the number of possible input messages that can generate a hash value of 1.

The correct answer is 2 - (M, K). The HMAC function takes two parameters: the message M and the secret key K. It computes a hash-based message authentication code using these two inputs. The code generated by HMAC can be used to verify the integrity and authenticity of the message. The answer option 2 correctly identifies the two required parameters for the HMAC function.

When creating user accounts, it is common practice to store a combination of the username and the hashed password on the disk. Storing the username and hashed password allows for authentication and verification of user credentials during the login process. Storing the hashed password instead of the plain text password adds an extra layer of security, as the hashed password cannot be easily reversed to obtain the original password. Therefore, the correct answer is "par korisničko ime - heš lozinke".

The question asks for the size of the key space, which is the number of possible keys that can be formed given the set of characters Z and the length of the key (5 in this case). The correct answer, 62*61*60*59*58, represents the number of choices for each position in the key, starting with 62 choices for the first position (since there are 62 characters in Z), then 61 choices for the second position (since one character has been used already), and so on. Multiplying these choices together gives the total number of possible keys without repeating characters.

Sequential algorithms are used where speed and real-time operation are important.

In a public-key cryptography system, the public key is used for encryption, while the private key is used for decryption. This means that anyone can use the public key to encrypt a message, but only the intended recipient, who possesses the private key, can decrypt and read the message. Therefore, the correct answer is that the system uses the public key for encryption and the private key for decryption.

The given public key (N,e) = (55,7) and private key 23 were generated using the RSA algorithm. The message M=12 is being encrypted. The result of the encryption is 23.

The given question is asking for the result of applying Caesar's cipher to the open text "JOHN" using the English alphabet. Caesar's cipher is a substitution cipher where each letter in the plaintext is shifted a certain number of positions down the alphabet. In this case, the letters in "JOHN" are shifted three positions down, resulting in "MRKQ". Therefore, the correct answer is "MRKQ".

In the Diffie-Hellman algorithm, Alice and Bob both choose a prime number (p) and a primitive root modulo p (q). They then privately select their own secret numbers (a and b) and calculate their public keys (A and B) using the formula A = q^a mod p and B = q^b mod p. Finally, they exchange their public keys and calculate the shared secret key using the formula S = B^a mod p = A^b mod p. In this case, Alice chose a=3 and Bob chose b=4. Therefore, Alice's public key is A = q^3 mod p and Bob's public key is B = q^4 mod p. When they calculate the shared secret key, they both get S = B^3 mod p = A^4 mod p = 1.

The correct answer is "само XOR." OTP (One-Time Pad) cipher is a type of encryption that uses the XOR operation exclusively. XOR (Exclusive OR) is a logical operation that takes two binary inputs and returns true if exactly one of the inputs is true. In OTP, each character of the plaintext is combined with a character from a random key using XOR. This provides a high level of security because the same key is never used twice and the ciphertext does not reveal any information about the plaintext. Therefore, OTP only uses the XOR operation for encryption.

The correct answer is S=M^d (mod N) because the process of digital signing involves raising the message M to the power of the private key d, modulo N. This calculation ensures the authenticity and integrity of the message, as it can only be verified using the corresponding public key.

The correct answer states that asymmetric algorithms are slower than symmetric algorithms. This implies that the speed of execution of asymmetric and symmetric algorithms is not the same, and that asymmetric algorithms tend to be slower.

The security of the DES algorithm relies on S-boxes. S-boxes, or substitution boxes, are used in the algorithm to perform a non-linear substitution of bits. They introduce confusion and make it difficult for attackers to analyze the relationship between the plaintext and the ciphertext. The S-boxes provide a level of complexity and resistance against various cryptographic attacks, such as differential and linear cryptanalysis. Therefore, the correct answer is S-боксовима.

Alisa želi da pošalje poruku Bobu koja je šifrovana RSA algoritmom. Kada Alisa šifruje poruku Bobovim javnim ključem, to znači da koristi javni ključ Boba za enkripciju poruke. Javni ključ Boba je dostupan javno i koristi se za šifrovanje poruka koje se šalju Bobu. Kada poruka bude poslata Bobu, on će je moći dešifrovati koristeći svoj privatni ključ koji je tajna informacija koju samo on poseduje.

The random value that is added to passwords serves to make it more difficult for attackers to guess the password using a dictionary attack. By adding a random value, the resulting password becomes more complex and less likely to be found in a dictionary of commonly used passwords. This increases the security of the password and makes it harder for attackers to gain unauthorized access to the system.

The correct answer is "ima istu dužinu i ključa i poruke" which means "it has the same length for both the key and the message." This suggests that in a bezuslovno sigurni šifarski sistem, the length of the key used for encryption is equal to the length of the message being encrypted. This ensures that the system provides unconditional security, as the key and message lengths are the same, making it difficult for an attacker to decipher the encrypted message without the correct key.

The correct answer is "Симетрични и асиметрични криптографски алгоритми и хеш функције" which translates to "Symmetric and asymmetric cryptographic algorithms and hash functions." This means that authentication protocols can utilize both symmetric and asymmetric cryptographic algorithms, as well as hash functions. These algorithms and functions are used to ensure the security and integrity of the authentication process, protecting sensitive information and verifying the identity of the communicating parties.

The correct answer is 2^128. This is because the length of the hash function is given as 128 bits, which means it can generate 2^128 different hash values.

The given matrix represents the transposition cipher where the columns are rearranged based on the key word "HUSKY". Each column is arranged in alphabetical order based on the letters of the key word. The resulting cipher text is "DMEIISFDGNTNOASESBRX".

The correct answer is 16 rounds. The DES algorithm, which stands for Data Encryption Standard, consists of 16 rounds of encryption. Each round involves various operations such as permutation, substitution, and key mixing, which collectively provide a high level of security for the encryption process. By repeating these rounds 16 times, DES ensures a strong level of encryption for the data being processed.

The correct answer is "prvo šifruje sa prvim ključem, pa dešifruje sa drugim, pa šifruje sa prvim". This is because the given encryption method follows a pattern of first encrypting with the first key, then decrypting with the second key, and finally encrypting again with the first key. The other options either do not follow this pattern or involve using the third key, which is not mentioned in the given encryption process.

The correct answer is "својим приватним кључем" because in the RSA encryption algorithm, the message is encrypted using the sender's private key, and can only be decrypted using the corresponding public key. Therefore, Bob would need to use his own private key to decrypt the message sent by Alice.

Alice wants to send a message to Bob encrypted with the RSA algorithm. In RSA encryption, the sender uses the recipient's public key to encrypt the message. Therefore, Alice would encrypt the message with Bob's public key.

The correct answer is "треба да буде велики прост број" which translates to "it should be a large prime number." In the Diffie-Hellman protocol, the number p used for exponentiation modulo p during calculations needs to be a large prime number. This is because using a prime number ensures the security and complexity of the protocol, making it difficult for attackers to compute the private key.

The given answer, 1, is the correct key for the encryption using a Caesar cipher. In a Caesar cipher, each letter in the plaintext is shifted a certain number of positions down the alphabet. In this case, the plaintext "HAL" is shifted one position down, resulting in the ciphertext "IBM". Therefore, the key for this encryption is 1.

The correct answer is 16 rounds. The DES3 algorithm, also known as Triple DES, uses a total of 16 rounds in its encryption process. Each round involves a combination of permutation, substitution, and XOR operations to provide a high level of security. By repeating these rounds multiple times, the algorithm ensures a robust encryption scheme.

The correct answer is "секвенцијални алгоритми" because this term refers to algorithms that encrypt one bit (byte) at a time. This means that the encryption process is performed sequentially, one after another. Asymmetric algorithms, on the other hand, involve the use of two different keys for encryption and decryption. Symmetric algorithms use the same key for both encryption and decryption. Block algorithms operate on fixed-size blocks of data. Therefore, "секвенцијални алгоритми" is the most appropriate term to describe algorithms that encrypt one bit at a time.

The correct answer is "блоковски". This answer is correct because it accurately describes the type of algorithms that process blocks of plaintext during encryption. The word "блоковски" is derived from the word "блок" which means block, and the suffix "-овски" which indicates belonging or relating to something. Therefore, "блоковски" can be translated as "block-related" or "block-based", indicating that these algorithms operate on blocks of plaintext.

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The given answer states that the Блоковски алгоритам (Block algorithm) affects all the bits within a block. This means that the algorithm operates on and modifies every bit within the block, rather than just a single byte of open text or a single byte within the block.

To successfully decipher a cipher encrypted with a sequential cipher system, it is necessary to possess both a random key and a pseudorandom number generator. A random key is needed to decrypt the cipher, as it contains the specific information required to reverse the encryption process. Additionally, a pseudorandom number generator is necessary to generate the sequence of numbers used in the decryption algorithm. Without both of these components, it would not be possible to accurately decipher the cipher.

For encryption and digital signing, different key pairs are used. This means that the keys used for encryption and digital signing are not the same.

During digital signature (encryption with private key), the hash of the message is encrypted.

The correct answer suggests that Bob is checking whether the message M has been altered during transmission. By comparing the hash value h(M) received from Alice with the hash value h calculated by Bob, he can determine if the message has been tampered with. If the hash values match, it indicates that the message has not been changed during transmission.

The security of a cryptosystem relies on the secrecy of the decryption key. The encryption key is used to encrypt the message, while the decryption key is used to decrypt it. If the decryption key is not kept secret and falls into the wrong hands, an attacker could easily decrypt the encrypted messages and gain access to sensitive information. Therefore, the secrecy of the decryption key is crucial for maintaining the security of the cryptosystem.

RSA is an algorithm that can be used for digital signing. It is a widely used asymmetric encryption algorithm that relies on the difficulty of factoring large prime numbers. RSA allows for secure communication and authentication by generating a public and private key pair. The private key is kept secret and used for signing messages, while the public key is shared and used for verifying the signatures. This makes RSA a suitable choice for digital signing and ensuring the integrity and authenticity of digital documents.

Hash functions are one-way functions, meaning that they cannot be reversed or decrypted. Once a message is hashed, it is nearly impossible to retrieve the original message from the hash value. Therefore, hash functions are not used for encryption because they do not provide a way to decrypt the message.

The given correct answer, "ауторизација," refers to the process of granting access rights to an authenticated user. In other words, it is the act of determining what actions a user is allowed to perform within a system or application. This process ensures that only authorized individuals can access certain resources or perform specific tasks, helping to maintain security and protect sensitive information.

The given expression can be simplified using the properties of XOR operation. The XOR operation returns true (1) if the number of true inputs is odd, and false (0) if the number of true inputs is even. In this case, since XORing a number with itself results in 0, the expression can be simplified to (0 XOR b) XOR 0, which further simplifies to b XOR 0, which is equal to b. Therefore, the correct answer is b.

The given answer states that it has not been proven whether one-way functions exist or not. The options provided in the question suggest different possibilities, but none of them have been proven to be true or false. Therefore, the correct answer is that it has not been proven whether one-way functions exist or not.

Sequential algorithms can be implemented both in hardware and software. This means that they can be executed using either dedicated hardware components or by utilizing software instructions on a general-purpose computer. The choice of implementation depends on factors such as performance requirements, cost, and available resources.

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In the ECB mode, the same block encrypted with the same key will always produce the same ciphertext. However, in the CBC mode, this may not be the case. In CBC mode, each block is XORed with the previous ciphertext block before encryption, which introduces randomness and makes the ciphertext different even if the same block and key are used. Therefore, the statement in the answer is correct.

By embedding timestamp data into the message itself, it becomes possible to detect the resending of the same message. This is because each time the message is resent, a new timestamp is generated. The other options mentioned in the question, such as authorization, detecting unauthorized data modification, and authentication, do not directly relate to the use of timestamp data in the message.

The given expression (((X xor X)xor Y)xor Y)xor X simplifies to X. This is because the xor operation with the same value cancels out, so xor-ing X with itself results in 0. Xor-ing 0 with any value Y gives Y, and xor-ing Y with itself also gives 0. Finally, xor-ing 0 with X gives X. Therefore, the answer is X.

MAC (Message Authentication Code) is a cryptographic algorithm that is used to provide integrity and authenticity to a message. It is a short encrypted message that is generated using a secret key and appended to the original message. The MAC allows the recipient to verify that the message has not been tampered with during transmission and that it originated from the expected sender. Therefore, the correct answer is "кратка шифрована порука" which translates to "a short encrypted message".

The ability of a system to provide a service to an authorized user whenever it is needed is called "расположивост" which translates to "availability" in English. This means that the system is accessible and operational, ensuring that users can access the service without any interruptions or downtime.

The given answer is the result of calculating 5^3(mod 17). In the Diffie-Hellman algorithm, Alice and Bob exchange keys by performing modular exponentiation. In this case, Alice has chosen the number 3 and Bob has chosen the number 7. The calculation 5^3(mod 17) means that Alice raises 5 to the power of 3 and then takes the remainder when divided by 17. The result is 6.

The correct answer is "псеудо случајним бинарним низом који се добија од кратког случајног кључа". This is because using a pseudo-random binary sequence generated from a short random key adds an additional layer of randomness to the encryption process. This helps to overcome the weaknesses of the OTP cipher and enhances the security of the encrypted message.

The drawback of OTP encryption is the slow process of generating a large quantity of binary sequences with random characteristics, which is only used once.

The correct answer is that a cipher system is secure if the most efficient known attack is a complete search of the key space, which needs to be sufficiently large. This means that the security of the system relies on the fact that it would take an impractical amount of time or computational power to try all possible keys and decrypt the message.

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In the protocol for public key cryptography-based authentication, it is necessary to use a different pair of keys for encryption and digital signing. This is because encryption and digital signing serve different purposes in the authentication process. Encryption is used to protect the confidentiality of the data being transmitted, while digital signing is used to verify the authenticity and integrity of the data. Using the same pair of keys for both encryption and digital signing would compromise the security of the authentication process. Therefore, it is essential to use a different pair of keys for encryption and digital signing in this protocol.

The explanation for the given correct answer is that the validity of public keys is confirmed by a trusted third party. In the context of communication, a trusted third party verifies and confirms the authenticity and integrity of the public keys being used. This third party acts as a trusted authority, ensuring that the public keys are legitimate and can be trusted for secure communication.

The correct answer is "блоковски алгоритам Фејстеловог типа" which translates to "block cipher of the Feistel type" in English. This type of algorithm divides the input data into blocks and applies a series of transformations to each block. It is named after Horst Feistel, who developed this cryptographic structure.

The given answer "радни кључ" is the correct answer because it directly translates to "working key" in English. The question states that a pseudo-random value generator generates a working key based on a short (random) key. Therefore, the working key is the result of the generator's process.

The given correct answer is "рунда" because in the context of the question, it is mentioned that the block cipher is obtained by applying a specific function multiple times on the block of plaintext and intermediate results. Each application of the function is referred to as a "рунда" in the given text.

The fixed values ipad and opad, each consisting of 64 bits, are used for calculating HMAC values.

The correct answer states that the non-repudiation service provides irrefutable evidence that the message was received from a specific person. This means that the recipient can prove that the message came from the intended sender and cannot deny receiving it.

The correct answer is 2*n. In a symmetric system with a key length of n, the security is directly proportional to the length of the key. Therefore, to achieve the same level of security in a hash function, the length should be twice the length of the key in the symmetric system. This ensures that the hash function has enough complexity and randomness to provide the same level of security.

The given question states that the Serbian alphabet is used for open tests and encryption. The letters from A to Ш are numbered from 1 to 30, while the numbers from 31 to 36 are assigned the letters A, E, H, O, P, T. The word RENTA is being encrypted. The correct answer is "35 7 32 22 31". By converting the letters of the word RENTA into their corresponding numbers (18 5 14 20 1) and using the given encryption scheme, we get the sequence "35 7 32 22 31". Therefore, the other sequences listed are all valid encryptions of the word RENTA except for "35 7 32 22 31".

After every 2 blocks, the CBC mode of operation for block ciphers automatically corrects any intentional or accidental changes to a block. This means that if there is a modification or error in a block, it will be detected and corrected within the next 2 blocks.

The correct answer is DCT метода. DCT stands for Discrete Cosine Transform, which is a commonly used method for hiding information within a video file. DCT works by converting the video frames into a frequency domain representation, where the high-frequency components can be modified to embed the hidden information. This method is widely used in video steganography to ensure that the hidden information remains undetectable to the human eye.

The correct answer is "аутентификација". This is the Serbian word for "authentication", which refers to the process of verifying the identity of a user or system. In the context of the given question, the service that provides identity verification is called "аутентификација".

The correct answer is that collision in hash functions refers to the occurrence of two different messages producing the same hash value. This means that there is a possibility of two distinct messages having identical hash values, which can lead to data integrity issues in applications that rely on hash functions for data verification and security.

The correct answer is "хибридни" (hybrid). This term refers to a cryptographic system that combines both public key and symmetric key cryptography. In this system, the public key is used to securely exchange a symmetric key, which is then used for the actual encryption and decryption of the data. This hybrid approach takes advantage of the strengths of both types of cryptography, providing a more efficient and secure solution for data encryption.

The given question is in Serbian and it asks for the property of block ciphers that makes it impossible to determine a block of plaintext even if the corresponding ciphertext is known. The correct answer is "diffusion". Diffusion refers to the property of block ciphers where a change in one bit of the plaintext or the key results in multiple changes in the ciphertext. This property ensures that small changes in the input will produce significant changes in the output, making it difficult to analyze and break the encryption.

The given question provides the public key (N,e) as (33,7) and the private key as 3, generated using the RSA algorithm. The ciphertext is C=27. The question asks for the message, which is 15.

The security of the RSA algorithm lies in the complexity of finding prime factors of large numbers. The RSA encryption scheme relies on the difficulty of factoring large numbers into their prime factors. The larger the prime factors, the more secure the encryption becomes, as it becomes exponentially more difficult to factorize the number. Therefore, the security of RSA encryption is based on the assumption that factoring large numbers into primes is a computationally difficult problem.

The correct answer is "ништа од наведеног" which means "none of the above" in English. This means that none of the options listed are the correct action to be taken with a 24-bit BMP image that has a steganographically embedded message.

The correct answer is that the task of modern encryption algorithms is to transform sequences of bits carrying information from plaintext into a new binary sequence in such a way that unauthorized parties cannot access the information contained in the plaintext through analysis of the cipher.

The given correct answer is "конфузије" because it correctly identifies the property of cryptographic algorithms where all keys are equally probable in a brute force attack. This property is known as "конфузије" in Serbian.

The correct answer is "тајни". This term refers to a communication channel that is not designed by system designers and is not under control, but can still be used for information flow.

Alice and Bob are using the Diffie-Hellman algorithm to exchange keys. They have chosen the parameters p=17 and q=11. Alice has chosen the number 7, and Bob has chosen the number 9. Bob calculates 7^11 (mod 17) and gets 14. Alice calculates 9^11 (mod 17) and gets 15. Bob then calculates 11^7 (mod 17) and gets 3. Finally, Alice calculates 11^9 (mod 17) and gets 6. Therefore, the correct answer is 11^9 (mod 17) = 6.

The correct answer is "да буду ефикасне" (to be efficient). Hash functions need to be efficient in order to process data quickly and provide fast results. Efficiency is an important factor in determining the effectiveness of a hash function.

The correct answer is "рачунски сигуран". In a cryptographic system, when the cost of breaking the cipher exceeds the value of the encrypted information, it is considered "рачунски сигуран" or computationally secure. This means that it would take an impractical amount of time, resources, or computational power to decrypt the information without the key.

The given encrypted text "OPHJAR" can be decrypted using the Affine cipher with the key (7,15). In the Affine cipher, each letter is mapped to a new letter using the formula E(x) = (ax + b) mod 26, where a and b are the key values. By applying the inverse function D(x) = a^(-1)(x - b) mod 26, where a^(-1) is the modular multiplicative inverse of a modulo 26, we can decrypt the text. In this case, the decrypted text is "лако је".

The length of the hash value of the SHA2 algorithm can vary depending on the variant used. It can be 512 bits, 256 bits, or 384 bits.

The main application of hash functions is in the process of authentication. Hash functions are commonly used to verify the integrity of data and ensure that it has not been tampered with during transmission or storage. By generating a unique hash value for a given set of data, it becomes easier to compare and authenticate the data at a later point in time. This helps in ensuring that the data has not been modified or altered in any way.

The correct answer is "имају проблем размене кључева" which translates to "have a key exchange problem". This means that symmetric algorithms require a secure way to exchange encryption keys between the sender and receiver. Unlike asymmetric algorithms, which use a pair of public and private keys, symmetric algorithms use a single shared key for both encryption and decryption. The challenge lies in securely distributing this shared key to all parties involved in the communication.