gistfile1.cs

var plainText = sha1_left16;
var cipherText = new byte[plainText.Length];

// The first 8 bytes must be the IV, then the data to be encrypted.
// get ready to encrypt
byte[] _params = new byte[8 + plainText.Length]; // 8 bytes IV vector

Array.Copy(cipherText, 0, _params, 8, plainText.Length);

// If the length of data to be encrypted by the CBC mechanism does not fit into one of the
// above PARAMS structures, the developer must produce their own byte array with the
// following layout. The first 8 bytes must be the IV, then the data to be encrypted. To
// use this array, the pParameter in the CK_MECHANISM structure must be a pointer to
// this array and the parameterLen is the length of the IV (must be 8 bytes) plus the
// length of the provided data, which must be a multiple of 8 bytes.

CK_MECHANISM m = new CK_MECHANISM();
m.mechanism = (uint)CKM.DES3_CBC;
IntPtr unmanagedPointer = Marshal.AllocHGlobal(_params.Length);
Marshal.Copy(_params, 0, unmanagedPointer, _params.Length);
m.pParameter = unmanagedPointer;
m.ulParameterLen = (uint) _params.Length;

aSession.EncryptInit(m, sec);
byte[] encData = aSession.Encrypt(plainText);

aSession.DecryptInit(m, sec);
byte[] data2 = aSession.Decrypt(encData.Take(16).ToArray());

Console.WriteLine((data2.ToHexString()));

aSession.FindObjectsFinal();