(Classic ASP) Charset Considerations when RSA Encrypting Strings

When encrypting strings, character encoding can be an issue. Encryption algorithms operate on binary data. If for example, your string includes a character such as é, the bytes used to represent it depend on the character encoding. In an "iso" encoding such as iso-8859-1, or in a "windows" encoding, such as "windows-1252", the é is represented by a single byte. In Unicode however, whether it's utf-8 or ucs-2, the é is represented by 2 bytes. When encrypting strings, the RSA component will first convert the string to the character encoding indicated by the Charset property. When decrypting strings, the RSA component converts from the character encoding indicated by the Charset property to the application's expected charset. In VB6 for example, this would be Unicode. In C++, it's ANSI. In Java, it's utf-8.

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<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body>
<%
' This example assumes the Chilkat API to have been previously unlocked.
' See Global Unlock Sample for sample code.

' For versions of Chilkat < 10.0.0, use CreateObject("Chilkat_9_5_0.Rsa")
set rsa = Server.CreateObject("Chilkat.Rsa")

' RSA public key in XML format:
publicKey = "<RSAPublicKey><Modulus>of3im3mRgd2NLXIGoK6uYLg6jj0Ug2b42rnqa5Tbwz2ieFqMJqt+++x2oqLYGurlz49nt+7/785g3XYWqoka4u9c9zul6YubIjnBM72dQy7rEkEfbUxgjcxqXyjZFx+FpaxFUecLu688XEu+9UA42VKiCgcl+E7TrqnfeeYpNXc=</Modulus><Exponent>AQAB</Exponent></RSAPublicKey>"

success = rsa.ImportPublicKey(publicKey)

' Use the utf-8 character encoding.  When encrypting, the
' plain text is first converted to utf-8 bytes and then
' RSA encrypted.
rsa.Charset = "utf-8"

' Encrypt a string and return the encrypted data base64-encoded:
rsa.EncodingMode = "base64"

plainText = "éééé"

usePrivateKey = 0
encryptedStr = rsa.EncryptStringENC(plainText,usePrivateKey)
Response.Write "<pre>" & Server.HTMLEncode( encryptedStr) & "</pre>"

%>
</body>
</html>