Cryptography and .NET

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Cryptography and .NET
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Key terms

• Symmetric Key a shared secret key between the
  sender and recipient
• Asymmetric key two keys, a public and private
  key and sometimes referred as public/ private key
  pair
• Hashing Produces a unique message digest of
  known fixed size
• Digital Signature used to authenticate sender,
  created from asymmetric and hashing algorithms

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Encryption

• Request provider for encryption algorithm and key
  length
• Create symmetric key
• Generate asymmetric key (public/ private pair)
• Key blob (securing symmetric key using asymmetric
  key
• Data encryption using symmetric key
• Persist the key blob and encrypted data for
  recipient

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Decryption

• Retrieve the persisted data
• Request provider for decryption algorithm and key
  length
• Decrypt the cipher text and obtain the original
  data

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Hashing

• Request provider for hashing algorithm and key
  length
• Create symmetric key
• Generate asymmetric key
• Key blob
• Use the hashing function and obtain the digest
• Encrypt the digest
• Persist the key blob and digest for recipient

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Verifying the Hash

• Retrieve the persisted data
• Request provider for hashing algorithm and key
  length
• Decrypt the cipher text and obtain the plain data
  and hash
• Recreate the hash from the plain data
• Compare the original and the newly created digest

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Digital Signatures

• Get the signature data
• Request provider for cryptographic algorithm and
  key length
• Create asymmetric key pair
• Key blob using public key from public/ private
  key pair
• Use hashing function and obtain the digest for
  signature data
• Encrypt the digest
• Persist the data for recepient

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Confirming the Digital Signature

• Retrieve the persisted data
• Request provider for algorithm and key length
• Decrypt the cipher text and obtain the plain data
  and hash
• Recreate the hash from the plain data
• Verify the signature with original and the newly
  created digest

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Cryptography in Microsoft .NET

• 
  Cryptography
• 
  Hierarchy

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• Microsoft .Net has classes that extend the
  cryptographic services provided by the windows
  CryptoAPI
• System.Security.Cryptography name space provides
  classes for
• Symmetric Encryption
• Asymmetric Encryption
• Hashing 
• Digital Signatures

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CryptoStream

• In .Net, CryptoStream is a channel for
  cryptographic transformations
• public CryptoStream( Stream stream,
  ICryptoTransform transform, CryptoStreamMode
  mode)
• Example 1
• byte data new byte
  1,2,3,4
• MemoryStream memData new
  MemoryStream(data)
• Rc2CryptoServiceProvider algorithm
  new Rc2CryptoServiceProvider()
• CryptoStream stream new
  CryptoStream(memData,
• algorithm.CreateEncryptor(algorithm.Key,
  algorithm.IV),
• CryptoStreamMode.Read)
• byte cipher new byte 8
• stream.Read(cipher , 0, (int) 8)
• memData.close()
• stream.close()

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• Example 2
• byte numbers new byte 1,2,3,4
• MemoryStream inmemory new
  MemoryStream()
• Rc2CryptoServiceProvider algorithm
  new Rc2CryptoServiceProvider()
• CryptoStream estream new
  CryptoStream(inmemory,
• algorithm.CreateEncryptor(algorithm.Key,
  algorithm.IV),
• CryptoStreamMode.Write)
• BinaryWriter bw new
  BinaryWriter(estreem)
• bm.Write(numbers, 0, numbers.Length)
• bm.close()

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Configuring .Net Cryptography

• Encryption with .Net
• Create cryptoStream class that wraps a data
  stream
• Based on the mode of the cryptostream, perform
  the transfomation
• Persist the data
• TripleDES algorithm
  TripleDES.create()
• Decryption with .Net
• Obtain the persisted data and perform the
  cryptographic transformations

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• Hashing with .Net
• Define the algorithm
• SHAICryptoServiceProvider sha new
  SHAICryptoServiceProvider()
• Compute hashing using hashing algorithm
• sha.ComputeHash(bytePlain, 0, filelen)
• obtain the digest
• hashsha.Hash
• Encrypt the hash
• Verifying a Hash in .Net
• Obtain persisted data and define the algorithm
  from provider
• Perform the hash and compare the old and the new
  digest.
• byte.equals(hash, bytehash)

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• Digital Signatures in .Net
• Gather the signature data
• Define the algorithm
• DSECryptoServiceProvider dsa new
  DSECryptoServiceProvider()
• Export the public key of a signature key pair
• string key ToXmlString(true)
• Call signData on the implementation algorithm to
  create the digital signatures
• byte signature dsa.signData(textstream.Ge
  tBuffer())
• Confirming Digital Signatures in .Net
• Use string key FromXmlString(true) to import
  the public key.

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Thank You !!!