hello Martinas,
well you can encrypt the data in the infopath form before saving or submiting it
below is the code to encrypt a field in InfoPath 2007.
Rijndael rKey = new RijndaelManaged();
XPathNavigator xnFormName = null;
XPathNavigator totalNode = this.MainDataSource.CreateNavigator();
xnFormName = totalNode.SelectSingleNode("/my:myFields/my:field1", this.NamespaceManager);
string totalNodeOriginal = xnFormName.ToString();
byte[] totalInfo =
System.Text.Encoding.Unicode.GetBytes(xnFormName.ToString());
MemoryStream ms = new MemoryStream();
CryptoStream csBase64 = new CryptoStream(ms,
new ToBase64Transform(), CryptoStreamMode.Write);
CryptoStream csRijndael = new CryptoStream(csBase64,
rKey.CreateEncryptor(), CryptoStreamMode.Write);
csRijndael.Write(totalInfo, 0, (int)totalInfo.Length);
csRijndael.FlushFinalBlock();
string base64enc = System.Text.Encoding.ASCII.GetString
(ms.GetBuffer(), 0, (int)ms.Length);
xnFormName.SetValue(base64enc);
hope this solves your problem