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Creating and managing keys is an important part of the cryptographic process. Symmetric algorithms require the creation of a key and an initialization vector (IV). The key must be kept secret from anyone who should not decrypt your data. The IV does not have to be secret, but should be changed for each session. Asymmetric algorithms require the creation of a public key and a private key. The public key can be made public to anyone, while the private key must known only by the party who will decrypt the data encrypted with the public key. This section describes how to generate and manage keys for both symmetric and asymmetric algorithms.

Symmetric Keys

The symmetric encryption classes supplied by the .NET Framework require a key and a new initialization vector (IV) to encrypt and decrypt data. Whenever you create a new instance of one of the managed symmetric cryptographic classes using the parameterless constructor, a new key and IV are automatically created. Anyone that you allow to decrypt your data must possess the same key and IV and use the same algorithm. Generally, a new key and IV should be created for every session, and neither the key nor IV should be stored for use in a later session.

1. Feb 07, 2018  x = 0 For i in range(10): String = “var%d =%d”%(x, x) exec(String) x+=1 Now you have 11 variables.
2. Create your own class derived from dict where the init method takes a list and a single value as inputs and iterate through the list setting the keys to value, define an update method that takes a list and a new value and for each item that is not already a key set it to the new value, (assuming that is what you need).

@miigotu 'youthinks' wrong. E should be chosen so that e and λ(n) are coprime. It is not chosen at random, and since it is usually small for computation reasons, and included in the public key, it can always be known by an attacker anyway.

To communicate a symmetric key and IV to a remote party, you would usually encrypt the symmetric key by using asymmetric encryption. Sending the key across an insecure network without encrypting it is unsafe, because anyone who intercepts the key and IV can then decrypt your data. For more information about exchanging data by using encryption, see Creating a Cryptographic Scheme.

The following example shows the creation of a new instance of the TripleDESCryptoServiceProvider class that implements the TripleDES algorithm.

Python Dynamic Method

When the previous code is executed, a new key and IV are generated and placed in the Key and IV properties, respectively.

Sometimes you might need to generate multiple keys. In this situation, you can create a new instance of a class that implements a symmetric algorithm and then create a new key and IV by calling the GenerateKey and GenerateIV methods. The following code example illustrates how to create new keys and IVs after a new instance of the symmetric cryptographic class has been made.

When the previous code is executed, a key and IV are generated when the new instance of TripleDESCryptoServiceProvider is made. Another key and IV are created when the GenerateKey and GenerateIV methods are called.

Asymmetric Keys

The .NET Framework provides the RSACryptoServiceProvider and DSACryptoServiceProvider classes for asymmetric encryption. These classes create a public/private key pair when you use the parameterless constructor to create a new instance. Asymmetric keys can be either stored for use in multiple sessions or generated for one session only. While the public key can be made generally available, the private key should be closely guarded.

A public/private key pair is generated whenever a new instance of an asymmetric algorithm class is created. After a new instance of the class is created, the key information can be extracted using one of two methods:

• The ToXmlString method, which returns an XML representation of the key information.

• The ExportParameters method, which returns an RSAParameters structure that holds the key information.

Both methods accept a Boolean value that indicates whether to return only the public key information or to return both the public-key and the private-key information. An RSACryptoServiceProvider class can be initialized to the value of an RSAParameters structure by using the ImportParameters method.

Asymmetric private keys should never be stored verbatim or in plain text on the local computer. If you need to store a private key, you should use a key container. For more on how to store a private key in a key container, see How to: Store Asymmetric Keys in a Key Container.

Python Code For Dynamic Key Generation 10

The following code example creates a new instance of the RSACryptoServiceProvider class, creating a public/private key pair, and saves the public key information to an RSAParameters structure.

Python Code For Dynamic Key Generation 2

See also

This is an exercise in secure symmetric-key encryption, implemented in purePython (only built-in libraries used), expanded from Bo Zhu's (http://about.bozhu.me)AES-128 implementation at https://github.com/bozhu/AES-Python

• AES-128, AES-192 and AES-256 implementations in pure python (very slow, butworks).Results have been tested against the NIST standard (http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf)
• CBC mode for AES with PKCS#7 padding (now also PCBC, CFB, OFB and CTR thanks to @righthandabacus!)
• encrypt and decrypt functions for protecting arbitrary data with apassword

Python Dynamic Programming Example

Note: this implementation is not resistant to side channel attacks.

Although this is an exercise, the encrypt and decrypt functions shouldprovide reasonable security to encrypted messages. It ensures the data iskept secret (using AES), blocks are encrypted together (CBC), the samemessage encrypted twice will have different ciphertexts (salt), the ciphertexthasn't been tampered with (HMAC) and the key has some defense against brute-force(PBKDF2).

The algorithm is as follows:

1. 16 random bytes of salt are extracted from the system's secure random numbergenerator (usually /dev/urandom)>

2. The given master key is stretched and expanded by PKBDF2-HMAC(SHA256) usingthe salt from 1), to generate the AES key, HMAC key and IV (initializationvector for CBC).

3. The given message is encrypted with AES-128 using the AES key and IV fromstep 2), in CBC mode and PKCS#7 padding.

4. A HMAC-SHA256 is generated from the concatenation of the salt from 1) andthe ciphertext from 3).

5. The final ciphertext is HMAC + salt + ciphertext.

Security overview:

Python Dynamic Programming

• The random salt ensures the same message will map to different ciphertexts.

• The HMAC ensures the integrity of both the entire ciphertext and the PKBDF2salt; encrypt-then-mac prevents attacks like Padding Oracle.

• Bytes from keys, iv and salt are not reused in different algorithms.

• PBKDF2 key stretching allows for relatively weak passwords to be used as AESkeys and be moderately resistant to brute-force, but sacrificing performance.