Cryptographic Algorithms – Guidance for Developers

8/14/2019 Cryptographic Algorithms Guidance for Developers

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privatebyte[] GetRandomNumber(int keyLength)

{

//create an instance of the default random number generator

RandomNumberGenerator rng = RandomNumberGenerator.Create();

//obtain the required length of the byte array - 128 bits

//divided by 8 is 16

int btLength = keyLength / 8;

//create a byte array for the random number

byte[] bt = newbyte[btLength];

//fill the byte array with the random number

rng.GetBytes(bt);

return bt;

}


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//alg would actually be configurable in practice

string alg = "MD5";


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//use System.Text.Encoding to convert to a byte array

byte[] inputText = System.Text.Encoding.Default.GetBytes(plainText);

//instantiate a base class, this allows either an SHA1Managed or

//MD5CryptoServiceProvider to be configured

HashAlgorithm ha = HashAlgorithm.Create(alg);

byte[] hashInput = null;

//create the hash

using (ha)

{

hashInput = ha.ComputeHash(inputText);

}

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string plainText = "Here is the message";

string hashKey = "Here is the key";

//alg would be configurable in practice

string alg = "HMACSHA1";

//create the base class, so that different a different

//algorithm class can be configured

KeyedHashAlgorithm kha = KeyedHashAlgorithm.Create(alg);

//convert the key and plaintext to byte arrays

byte[] hashKeyBytes = System.Text.Encoding.Default.GetBytes(hashKey);

byte[] plainTextBytes = System.Text.Encoding.Default.GetBytes(plainText);

//set the key

kha.Key = hashKeyBytes;

byte[] hashResultBytes = null;

//compute the hash

using(kha)

{

hashResultBytes = kha.ComputeHash(plainTextBytes);

}


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Messagedata block 2

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privatebyte[] EncryptSymmetric(

byte[] key,

string plainText,

byte[] IV,

int blockSize,

string alg,

CipherMode mode)

{

//calculate keysize based on length of array

int keySize = key.Length * 8;


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//convert the text to a byte array

byte[] btPlainText =

System.Text.Encoding.Default.GetBytes(plainText);

//use a base class so that the algorithm used is configurable

SymmetricAlgorithm sa = SymmetricAlgorithm.Create(alg);

MemoryStream ms = null;

using(sa)

{

//set the blocksize and keysize for the symmetric

//algorithm class

sa.BlockSize = blockSize;

sa.KeySize = keySize;

//set the initialisation vector

sa.IV = IV;

//set the padding to PKCS#7

sa.Padding = PaddingMode.PKCS7;

//no parallel computation, no requirement to encrypt

//as data is assembled, so use CBC mode

//It may be good practice to configure this

sa.Mode = mode;

//set the secret key for the encryption

sa.Key = key;

//the ICryptoTransform interface exposes details to the

//algorithm for handling data in blocks

ICryptoTransform ict = sa.CreateEncryptor();

//the encrypted data is written to a memory stream

ms = new MemoryStream();

//CryptoStream transforms blocks of data

//using the ICryptoTransform object

CryptoStream cs = new CryptoStream(

ms,

ict,

CryptoStreamMode.Write);

//perform the encryption

using (cs)

{

cs.Write(btPlainText, 0, btPlainText.Length);

}

}

return ms.GetBuffer();

}

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Message text

Asymmetricencryption of

text using partyAs privatekey

Ajsadvdvcl

Party A sends a message

Bbjbdc&&$

Message text

Bbjbdc&&$

Public Key Cert

Message, encryptedhash and Party Aspublickey are

enveloped together

Message sentto Party B

Public Key Cert

Message text

Bbjbdc&&$

Public Key Cert

Public keycertificate

extracted andVERIFIED -this is VERYIMPORTANT

Encrypted hashextracted

Ajsadvdvcl

Bbjbdc&&$

Encrypted hashdecrypted

using publickey fromcertificate

Message text

Message textextracted

Hash createdfrom message

text

Ajsadvdvcl

Do the twohashes match?

If yes, then thesignature has

been verified

Party B has verified the digitalsignature

Hash createdfrom message

text

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foreach (Certificate cert in certStore.Certificates)

{

//obtain the name of the certificate

string name =

cert.GetInfo(CAPICOM_CERT_INFO_TYPE.CAPICOM_CERT_INFO_SUBJECT_SIMPL

E_NAME);

//is this the certificate we wish to use?if (name == certName)

{

x509cert = cert;

}

}

//create a signer

Signer messageSigner = new SignerClass();

//set the certificate to use

messageSigner.Certificate = x509cert;

//create a signed data class

SignedData sd = new SignedDataClass();

//text to sign

sd.Content = "This is the piece of text that requires signing";

//sign the message

string signedMessage = sd.Sign(messageSigner,

false,

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ICATE);

//to check the signatures, we have to iterate through all of the

signatures.

//Messages can be cosigned

foreach (Signer sr in sd.Signers)

{

Signer s = sr;

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s.Certificate.GetInfo(

CAPICOM_CERT_INFO_TYPE.CAPICOM_CERT_INFO_SUBJECT_SIMPLE_NAME)

+ ":" + s.Certificate.IsValid().Result);

//this is useful - CAPICOM will also check CRL status

//If you set the CheckFlag parameter

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CAPICOM_CHECK_FLAG.CAPICOM_CHECK_TRUSTED_ROOT |

CAPICOM_CHECK_FLAG.CAPICOM_CHECK_TIME_VALIDITY |

CAPICOM_CHECK_FLAG.CAPICOM_CHECK_SIGNATURE_VALIDITY |

CAPICOM_CHECK_FLAG.CAPICOM_CHECK_ONLINE_REVOCATION_STATUS ;

//call the IsValid.Result property again

MessageBox.Show(s.Certificate.GetInfo(CAPICOM_CERT_INFO_TYPE.CAPICOM_CERT_INFO_SUBJECT_SIMPLE_NAME) + ":" +

s.Certificate.IsValid().Result.ToString());

}

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Cryptographic Algorithms – Guidance for Developers