Using Digital Signature to validate userid and his requestid

Hi,

I am new to this digital signature. I have to validate userid and his requestid using digital signature. Like I have to encrypt the userid and requestid and save the encrpyted code in the sql database. I have done some research and I found that we have two class libraries capicom and cryptoAPI. Can any one suggest me which class library I can use as I am working with framework 3.5 sp1. And since I am new to this I am finding it difficult to know how do I start? Any suggestions would be of great help. Thanks
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