Task at hand: to decipher a string using a given raw DES key and an initialization
vector (IV) value.
The following information is provided by another party (all values are Base64-encoded):
DES Key: mInO4JRGtQ4=
IV: X6PfOtMlNmk=
Encrypted string: dBUMGWhRA7dAbOuOvwBna1fwEHKAXW4wVozBoqc7R8o=
Cipher mode: CBC
The following code snippet performs the decryption:
<!--METADATA TYPE="TypeLib" UUID="{B72DF063-28A4-11D3-BF19-009027438003}"-->
<%
Set CM = Server.CreateObject("Persits.CryptoManager")
Set context = CM.OpenContext("", True)
' DES encryption key (Base64-encoded)
Set KeyBlob = CM.CreateBlob
KeyBlob.Base64 = "mInO4JRGtQ4="
' Import key into CryptoKey object. Reverse byte order
Set Key = context.ImportRawKey(KeyBlob, calgDES, True)
' Specify initialization vector (IV, Base64-encoded)
Set IVblob = CM.CreateBlob
IVblob.Base64 = "X6PfOtMlNmk="
Key.SetIV IVblob
' Text to decipher (Base64-encoded)
Set TextBlob = CM.CreateBlob
TextBlob.Base64 = "dBUMGWhRA7dAbOuOvwBna1fwEHKAXW4wVozBoqc7R8o="
' Decrypt
Response.Write Key.DecryptText(TextBlob)
%>
This code snippet should produce the output FiveLittleMonkiesJumpingOnABed.
Two points to note:
1. True is passed to ImportRawKey as the third parameter to reverse the byte order
of the key. Most non-CryptoAPI implementations
use the opposite byte order for symmetric keys. Whether you should
use or omit the third parameter can be determined by trial and error.
2. A cipher mode is not explicitly specified because,
according to the other party, the string is encrypted using the CBC mode,
and AspEncrypt uses CBC by default.
If the string were encoded using, say, the ECB mode, our code
would have to include the line
Key.Mode = ccmECB
