Information Security Presentation

Information Security Third Class-Cybersecurity Part 6 Data Integrity and Cryptographic Hash Functions 1. Hash Functions. 2. Cryptographic Hash Functions Requirements 3. Hash Function Algorithm 4. Applications of Hash Functions 5. End Data Integrity and Cryptographic Hash Functions 1. Hash Functions A hash function accepts a variable-length data block as input and produces a fixed-size hash value. A good hash function will produce outputs that are evenly distributed and random. In general, the principle of a hash function is data integrity. Changing any bit or bits in the input data changes the hash code. The kind of hash function needed for security applications is called a cryptographic hash function. Data Integrity and Cryptographic Hash Functions 2. Cryptographic Hash Functions Requirements Before entering into the discussion, defining the hash function terms would be helpful. For a hash value: h = H(x) The x is a data block whose hash value is h using the function H. Thus, x is the preimage of h. The accepted requirements for a cryptographic hash function are listed below: 1. Variable input size Hash function H can be applied to a block of data of any size. 2. Fixed output size Hash function H produces a fixed-length output. 3. Efficiency Hash function H is relatively easy to compute for any given x, making hardware and software implementations practical. Data Integrity and Cryptographic Hash Functions 4. Preimage resistant (one-way property) For any given hash value h, it is computationally infeasible to find data x such that H(x) = h. It is easy to generate a hash code given a message but impossible to generate a message given a hash code. An attacker can quickly discover the secret message if the hash function is not one-way. 5. Second preimage resistant (weak collision resistant) For any given block x, finding y ≠ x with H(y) = H(x) is computationally infeasible. This property guarantees that an alternative message with the same hash value as a given message cannot be found. 6. Collision resistant (strong collision resistant) It is computationally impossible to find any pair of data (x, y) such that H(x) = H(y). A strong hash function protects against an attack in which one party generates a message for another party to sign. The first three properties are requirements for the practical application of a hash function. A hash function that satisfies the first five properties is called a weak hash function. If the sixth property, collision resistant, is also satisfied, it is called a strong hash functin . Data Integrity and Cryptographic Hash Functions 3. Hash Function Algorithm All hash functions operate using a general principle. The input message is viewed as a sequence of n-bit blocks. The input is processed one block at a time to produce an n-bit hash function. Then, the simplest hash function is the bit-bybit exclusive-OR (XOR) of every block. This can be expressed as: Hash code= b1 ⊕ b2 ⊕…. ⊕ bm This hash function may result in an unchanged hash value, meaning it is less efficient. To improve this, perform a onebit circular shift on the hash value after each block is processed. This hash function has the effect of randomizing the input more completely and overcoming any regularity that appears in the input. The most widely used hash function is the Secure Hash Algorithm (SHA). The three SHA algorithms are structured differently and are called as SHA-1, SHA-2 and SHA-3. Data Integrity and Cryptographic Hash Functions 4. Applications of Hash Functions The cryptographic hash is used in various security applications and internet protocols. It is helpful to look at the range of applications it employs. 4.1 Message Authentication Message authentication is a service used to verify the integrity of a message. It assures that the data received are exactly as sent (i.e., contain no modification, insertion, deletion, or replay), and in many cases, the sender's identity is valid. • The basic steps to use a hash function to authenticate a message are as follows: 1. The sender computes a hash value of the message and transmits both the hash value and the message. 2. The receiver performs the same hash calculation on the message and compares this value with the incoming hash value. If there is a mismatch, the receiver knows that the message or the hash value has been changed. Data Integrity and Cryptographic Hash Functions There are a variety of methods to provide message authentication using the hash function: 1. The message plus hash code is encrypted using symmetric encryption (see Figure 1). Because only A and B share the secret key, authentication, and confidentiality are provided because encryption is applied to the entire message plus the hash code. Figure 1. 2. Only the hash code is encrypted using symmetric encryption (see Figure 2). This reduces the computation time for applications that do not require confidentiality. 3. A hash function can be used for message authentication without encryption. Figure 2 The sender and receiver share a secret value, S. The A computes the hash value of M and S and then appends the result to the M (see Figure 3). Because B also has S, it can be used to recomputed the hash value to verify. Because the secret value S itself is not sent, an attacker cannot modify the message and cannot generate a false message. Figure 3. Data Integrity and Cryptographic Hash Functions 4.2. Digital Signatures Another important application, which is similar to the message authentication application, is the digital signature. The simplified method of digital signature uses a hash code and public-key cryptography. • The message's hash code is encrypted using the sender’s private key. The encrypted hash code is appended to the data (digital signature), providing authentication because the attacker needs to know the user’s private key to alter the message. • The receiver decrypts the incoming hash code using the sender’s public key, computes the message hash code, and compares. Anyone who knows the sender’s public key can verify the integrity of the message associated with the digital signature. • If confidentiality is needed, the message and the encrypted hash code can be encrypted using a symmetric secret key. This is a common technique. Data Integrity and Cryptographic Hash Functions 5. Message Authentication Code Message authentication is commonly achieved using a Message Authentication Code (MAC), also known as a Keyed Hash Function. A MAC function involves using a secret key to generate a small, fixed-size block of data, known as a cryptographic checksum or MAC, that is appended to the message. This technique assumes sender A and receiver B share a common secret key K. When A has a message to send to B, it calculates the MAC as: MAC = C (K, M). Where: M = input message. C = MAC function. K = shared secret key. MAC = Message Authentication Code. Data Integrity and Cryptographic Hash Functions • The sender sends the message and MAC to the receiver. • The receiver performs the same calculation on the received message using the same secret key to generate a new MAC. • The received MAC is compared to the calculated MAC. Message authentication code. A MAC function is similar to encryption. One difference is that the MAC algorithm need not be reversible, as it must be for decryption. Because of the mathematical properties of the authentication function, a MAC is less vulnerable to being broken than encryption. The End Data Integrity and Cryptographic Hash Functions