The CryptoEx library provides various coding, hash and encryption algorithms. It will run on the device and desktop and requires .NET 2.0. This documentation refers to version 1.2 of the library.


This library offers the ability to encode/decode binary to and from Base64 encoding. This is an encoding scheme that encodes binary data by treating it numerically and translating it into a base 64 representation which is printable  This means that the data is unlikely to be modified in transit through systems, such as email, which were traditionally not 8-bit clean.  Base64 uses A–Z, a–z, 0–9, + and / for the 64 printable characters.

Hash Values

Hash algorithms map binary values of an arbitrary length to small binary values of a fixed length, known as hash values. A hash value is a unique and extremely compact numerical representation of a piece of data. If you hash a paragraph of plain text and change even one letter of the paragraph, a subsequent hash will produce a different value. It is computationally improbable to find two distinct inputs that hash to the same value.

Message authentication code (MAC) hash functions are commonly used with digital signatures to sign data, while message detection code (MDC) hash functions are used for data integrity.

Two parties (Alice and Bob) might use a hash function in the following way to ensure data integrity. If Alice writes a message for Bob and creates a hash of that message, Bob can then hash the message at a later time and compare his hash to the original hash. If the hash values are identical, then the message was not altered; however, if the values are not identical, the message was altered after Alice wrote it. For this system to work, Alice must hide the original hash value from all parties except Bob.

The hashing functions provided here are MD5 (Message Digest 5) and SHA 1 (Secure Hash Algorithm 1).The hash size for the MD5CryptoServiceProvider class is 128 bits and that for the SHA1 algorithm is 160 bits.

Symmetric or Secret-Key Encryption

Secret-key encryption algorithms use a single secret key to encrypt and decrypt data. You must secure the key from access by unauthorized agents because any party that has the key can use it to decrypt data. Secret-key encryption is also referred to as symmetric encryption because the same key is used for encryption and decryption. Secret-key encryption algorithms are extremely fast (compared to public-key algorithms) and are well suited for performing cryptographic transformations on large streams of data.

Typically, secret-key algorithms, called block ciphers, are used to encrypt one block of data at a time. Block ciphers (like RC2, DES, TripleDES, and Rijndael) cryptographically transform an input block of n bytes into an output block of encrypted bytes. If you want to encrypt or decrypt a sequence of bytes, you have to do it block by block. Because n is small (n = 8 bytes for RC2, DES, and TripleDES; n = 16 [the default], n = 24, or n = 32 bytes for Rijndael), data values larger than n have to be encrypted one block at a time.

The block cipher classes provided in this library use a chaining mode called cipher block chaining (CBC), which uses a key and an initialization vector (IV) to perform cryptographic transformations on data. For a given secret key k, a simple block cipher that does not use an initialization vector will encrypt the same input block of plain text into the same output block of cipher text. If you have duplicate blocks within your plain text stream, you will have duplicate blocks within your cipher text stream. If unauthorized users know anything about the structure of a block of your plain text, they can use that information to decipher the known cipher text block and possibly recover your key. To combat this problem, information from the previous block is mixed into the process of encrypting the next block. Thus, the output of two identical plain text blocks is different. Because this technique uses the previous block to encrypt the next block, an IV is used to encrypt the first block of data. Using this system, common message headers that might be known to an unauthorized user cannot be used to reverse engineer a key.

Asymmetric or Public-Key Encryption

Public-key encryption uses a private key that must be kept secret from unauthorized users and a public key that can be made public to anyone. The public key and the private key are mathematically linked; data encrypted with the public key can be decrypted only with the private key, and data signed with the private key can be verified only with the public key. The public key can be made available to anyone; it is used for encrypting data to be sent to the keeper of the private key. Both keys are unique to the communication session. Public-key cryptographic algorithms are also known as asymmetric algorithms because one key is required to encrypt data while another is required to decrypt data.

Public-key cryptographic algorithms use a fixed buffer size whereas secret-key cryptographic algorithms use a variable-length buffer. Public-key algorithms cannot be used to chain data together into streams the way secret-key algorithms can because only small amounts of data can be encrypted. Therefore, asymmetric operations do not use the same streaming model as symmetric operations.

Two parties (Alice and Bob) might use public-key encryption as follows. First, Alice generates a public/private key pair. If Bob wants to send Alice an encrypted message, he asks her for her public key. Alice sends Bob her public key over an insecure network and Bob uses this key to encrypt a message. (If Bob received Alice's key over an insecure channel, such as a public network, Bob must verify with Alice that he has a correct copy of her public key.) Bob sends the encrypted message to Alice and she decrypts it using her private key.

During the transmission of Alice's public key, however, an unauthorized agent might intercept the key. Furthermore, the same agent might intercept the encrypted message from Bob. However, the agent cannot decrypt the message with the public key. The message can only be decrypted with Alice's private key, which has not been transmitted. Alice does not use her private key to encrypt a reply message to Bob, because anyone with the public key could decrypt the message. If Alice wants to send a message back to Bob, she asks Bob for his public key and encrypts her message using that public key. Bob then decrypts the message using his associated private key.

In a real world scenario, Alice and Bob use public key (asymmetric) encryption to transfer a secret (symmetric) key and use secret key encryption for the remainder of their session.

(c) Andrew Graham - 2008