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Cryptography in .Net. CS 795. Goals. Confidentiality---no one else can intercept a message as it passes from A to B---Encryption is the answer Integrity---message is not tampered as it passes from A to B --- Hashing is the answer

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  • Confidentiality---no one else can intercept a message as it passes from A to B---Encryption is the answer
  • Integrity---message is not tampered as it passes from A to B --- Hashing is the answer
  • Authentication---B wants to be sure to be sure it is A who has sent the message---digital signature is the answer
  • Cryptography is key management
hashing algorithms
Hashing Algorithms
  • Create a message digest or hash code for a given message
  • Hashing algorithms break a message into fixed blocks (512 or 1024 bits)
  • Given a seed value and the 1st block, it produces a hash code. This hash code and the next block are fed again, that produce a new hash code. This continues until the last data block. The final hash code is the message digest.
programming hashing algorithms
Programming Hashing Algorithms
  • Managed (e.g., SHA1Managed) and unmanaged (e.g., SHA1CryptoServiceProvider)
  • System.Security.Cryptography.HashAlgorithm class: Methods: Create, ComputeHash, Initialize, Clear, TransformBlock,

SHA1Managed hash_alg = new SHA1Managed();


hashAlgorithm hash_alg = HashAlgorithm.Create(“SHA1”);

byte[ ] hash_code = hash_alg.ComputeHash(message_data);

To validate hash code, generate a new hash code and compare byte-by-byte.

keyed hashing algorithms mac
Keyed Hashing Algorithms (MAC)
  • These mix a secret key with the message data blocks when creating a hash code.
  • HMAC is one standard to combine secret key and message data (e.g., HMAC-SHA-1)---here the key is used as the 1st data block
  • HMAC-SHA-1 and MAC-Triple-DES

KeyedHashAlgorithm hash_alg = KeyedHahAlgorithm.Create(“HMACSHA1”);

Hash_alg.Key = key_bytes;

byte [ ] hash_code = hasg_alg.ComputeHash(message_data);

symmetric encryption
Symmetric Encryption
  • Both parties agree on a secret key
  • Sender encrypts the message using secret key and sends the encrypted data
  • Receiver decrypts the received data using the secret key
  • To create encrypted data: (i) Data is treated as a number of fixed-size blocks
  • (ii) The fixed-size blocks are converted to encrypted blocks
programming symmetric encryption
Programming Symmetric Encryption
  • System.Security.SymmetricAlgorithm
  • Managed (DES, TripleDES, RC2, Rijndeal) and unmanaged (DESCryptoServiceProvider, TripleDESCryptoServiceProvider…)
  • Methods: Create, CreateEncryptor, CreateDecryptor, GenerateIV, ValidKeySize
  • Padding mode: PKCS7 (value of the padding byte is the numbe rof padded bytes); Zeros (0’s are padded)
  • Cipher Modes: ECB, CBC, CFB, CTS, OFB
  • KeySizes structure: MinSize, SkipSize, MaxSize of the range of key sizes
  • IV: Initialization vector; .Net Framework has some default value for it; but it can be changed
  • Secret key: Same as in the case of IV
configuring the symmetric encryption algorithms
Configuring the Symmetric Encryption Algorithms

SymmetricAlgorithm algx = SymmetricAlgorithm.Create(“Rijndael”);

//This assigns default values to the parameters; but they may be changed as follows

algx.BlockSize = 192;

algx.KeySize = 128;

KeySizes[ ] x_size_ranges = algx.LegalKeySizes;


algx.Padding = PaddingMode.Zeros;

algx.Mode = CipherMode.ECB;

byte[ ] x_secretkey = algx.Key; // Get the secret key value assigned

algx.Key = new byte[ ] {0x64, …..};

algx.IV = new bytes[ ] {…};

symmetric encryption cont
Symmetric Encryption (Cont.)
  • Encrypting and Decrypting is done by ICryptoTransform interface

SymmetricAlgorithm algx = SymmetricAlgorithm.Create(“Rijndael”);

ICryptoTransform encryptorx = algx.CreateEncryptor();

ICryptoTransform decryptorx = algx.CreateDecryptor();

See pages 356-358 have an example

asymmetric encryption
Asymmetric Encryption
  • Public-key encryption
  • A has a public-secret (or private) key pair
  • B has a public-secret (or private) key pair
  • A encrypts a message using B’s public key and sends it to B
  • B uses corresponding secret key to decrypt it
  • Main limitation: Very slow relative to symmetric encryption
creating asymmetric keys
Creating Asymmetric Keys
  • RSA (Rivest, Shamir, Adleman, 1977)
  • Algorithm:
  • Choose two large random #s, p and q, of equal length and multiply them together to create n, the RSA key modulus: n=p*q; If p=23, q=31, n=713
  • Randomly choose e, the public exponent so that e and (p-1)(q-1) are relatively prime (i.e., share no common factors except 1). In the above, (p-1)(q-1)=660; choose e=19
  • Find d such that d*e = 1 mod (p-1)(q-1)

19d= 1 mod 660; So 19d=661, 1321, 1981, 2641, .. Here, d=2641/19=139

4. Public key consists of e and n. Private key is d. Discard p and q, but do not reveal their values.

Why is RSA algorithm secure? Because it is hard to find the factors of a large number.

Here, we are given n. So we have to find factors p and q so that n=p*q

encryption with asymmetric keys
Encryption (with asymmetric keys)
  • Break the plaintext into small blocks of data
  • Encrypt each plaintext block using the public key and the encryption function
  • Concatenate the encrypted blocks
  • Length of block = trunc[(length of n in the public key -1)/8]
  • RSA Algorithm
  • Example: Encryption: If m= 25, (n=713,e=19) as the public key, c=(me)mod n

= (2519)mod 713 = 156

Decryption: c=156, use private key (n=713, d=139), compute m = cd (mod n) = 156139 mod 713 =

how secure is asymmetric encryption
How secure is Asymmetric Encryption?

Given only the public key e and n, how many computations does it take to discover the private key d?

Once we know factors p and q, it is relatively easy to calculate d, and decrypt cipher text. So the secret is in the values d,p, and q.

programming asymmetric encryption
Programming Asymmetric Encryption
  • System.Security.Cryptography.AsymmetricAlgorithm
  • System.Security.Cryptography.RSA
  • System.Security.Cryptography.RSACryptoServiceProvider
digital signatures
Digital Signatures
  • Purpose: For receiver to verify the sender (or author)
  • Use asymmetric keys
  • Sender signs the message; receiver verifies it

A generates a digital signature on a message using its private key;

B receives the message and the signature;

B uses A’s public key to verify the and that the content has not been changed

Due to the slow performance of the asymmetric algorithms, A first creates a cryptographic hash code of the message and then applies the signature algorithm on the hash code.

Joint signatures on a document

rsa algorithm for ds
RSA Algorithm for DS

Digital signing

Sender A does the following:-

  • Creates a message digest (hash code) of the information to be sent.
  • Represents this digest as an integer m between 0 and n-1.
  • Uses her private key (n, d) to compute the signature s = m^d mod n.
  • Sends this signature s to the recipient, B.

Signature verification

Recipient B does the following:-

  • Uses sender A's public key (n, e) to compute integer v = s^e mod n.
  • Extracts the message digest from this integer.
  • Independently computes the message digest (hash code) of the information that has been signed.
  • If both message digests are identical, the signature is valid.
ds and encryption decryption
DS and Encryption/Decryption
  • Decryption and signing are identical as far as the mathematics is concerned as both use the private key.
  • Similarly, encryption and verification both use the same mathematical operation with the public key.
  • That is, mathematically, m = (m^e mod n)^d mod n = (m^d mod n)^e mod n, m < n
  • However, note these important differences in implementation:-
    • The signature is derived from a message digest of the original information. The recipient will need to follow exactly the same process to derive the message digest, using an identical set of data.
    • The recommended methods for deriving the representative integers are different for encryption and signing (encryption involves random padding, but signing uses the same padding each time).
ds algorithms in net
DS Algorithms in .Net
  • RSA algorithm (used for encryption and digital signatures)
  • DSA or Digital Signature Algorithm (only digital signature, not encryption)
  • Hashing algorithms to be used prior to digital signature generation:
  • MD5, SHA-1, SHA-256 (minimum key length 256 bit ), SHA-384, SHA-512
xml signatures
XML Signatures
  • .Net supports XML signatures specification or XMLDSIG for XML documents


<title> Programming .Net Security </title>

<author>Adam Freeman </author>



  • Create a URL reference for the document (page 414, O’Reilly)
  • Create a new instance of the SignedXML class and the URL reference;
  • Create a new asymmetric signing algorithm instance and assign it to the reference object created along with all other parameters (signing key, etc.)
  • Call ComputeSignature on the reference object.
  • Use GetXml().OuterXml to get the signature.

Follow similar procedure for verification of the signature.

Performance of web services security
  • Performance Comparison: Security Design Choices