Overview

• The hierarchy of algorithms in .NET Framework:

1. The abstract algorithm type classes, such as SymmetricAlgorithm, AsymmetricAlgorithm, or HashAlgorithm.
2. The abstract algorithm classes inherit from an algorithm type class; for example, Aes, RC2, or ECDiffieHellman.
3. Implementation of an algorithm class that inherits from an algorithm class; such as AesManaged, RC2CryptoServiceProvider, or ECDiffieHellmanCng.

• The CLR uses a stream-oriented design for implementing symmetric algorithms and hash algorithms.
• The core of this design is the CryptoStream class, which derives from the Stream class.
  • Because all the objects are built on a standard interface, you can chain together multiple objects (such as a hash object followed by an encryption object), and you can perform multiple operations on the data without needing any intermediate storage for it.
  • The streaming model also enables you to build objects from smaller objects.
    • For example, combined encryption and the hash algorithm can be viewed as a single stream object, although this object might be built from a set of stream objects.
• There are four ways a developer can create a cryptography object:
  1. Create an object by using the new operator.
  2. Create an object by calling the Create method on the abstract class for that algorithm.
  3. Create an object by calling the CryptoConfig.CreateFromName method.
  4. Create an object that implements a class of cryptographic algorithms by calling the Create method on the abstract class for that type of algorithm.
• When using encryption classes, it is not enough, from a security perspective, to simply force a garbage collection after you have finished using the object.
  • All cryptographic classes in the .NET Framework that hold sensitive data implement a Clear method.
  • When called, the Clear method overwrites all sensitive data within the object with zeros and then releases the object so that it can be safely garbage collected.

Hash Algorithms

✅ SHA512Managed

• SHA algorithm is a Secure Hash algorithm developed by the National Institute of Standards and Technology along with NSA.
• It is designed to compare the hash value of an original message and compare it to its message digest.
  • Can be used to find out if the message produces the same message digest.
• SHA-256, SHA-512, SHA-1, MD5, and all other hashes using the Merkle–Damgård construction are vulnerable to a length extension attack.
  • An attacker who knows the length of the MAC key and a particular value of SHA256(key|data) can compute SHA256(key|data|otherdata) for other data.
  • They can choose most of the other data, but even if they couldn't, it's a fatal flaw in a MAC scheme.
  • While SHA256(data|key) is not vulnerable to length extension, is vulnerable to "collisions" due to the same iterated construction.
  • HMAC's nesting prevents these and various other attacks.

✅ HMACSHA512

• Hash-based Message Authentication Code (HMAC) is a keyed hash algorithm that is constructed from the SHA-512.
  • The HMAC process mixes a secret key with the message data and hashes the result.
  • The hash value is mixed with the secret key again, and then hashed a second time.
  • The output hash is 512 bits in length.
• An HMAC can be used to determine whether a message sent over a non-secure channel has been tampered with, provided that the sender and receiver share a secret key.
  • The sender computes the hash value for the original data and sends both the original data and hash value as a single message.
  • The receiver recalculates the hash value on the received message and checks that the computed HMAC matches the transmitted HMAC.
• If the original and computed hash values match, the message is authenticated.
  • If they do not match, either the data or the hash value has been changed.
  • HMACs provide security against tampering because knowledge of the secret key is required to change the message and reproduce the correct hash value.
• HMACSHA512 accepts keys of any size and produces a hash sequence of length 512 bits.