Daily Passcode Generation with RSA

[Guest]

Is there anyway to use the RSACryptoServiceProvider with a key size smaller
than 384?

I'm using VB.NET in the 1.1 Framework.

I'm trying to generate a daily passcode string by hashing the current date,
signing the hash with an assymetric private key, and converting the signature
to a Base64String. A remote system, that only has the public key, could then
hash the current date and verify it against the signature.

I'm trying to get the daily passcode to be small enough to be read over a
telephone (20-35 characters). If I generate the passcode with DSA the
Base64String is always 56 characters regardless of key size. If I use RSA,
the length of the passcode string decreases with key size. At a key size of
384 (the minimum defined size), RSA generates a 64 character long passcode.

Can the lower limit of the RSACryptoServiceProvider key size be reduced via
any overrides or reflection?

Thanks for any help.

 

[Peter Huang [MSFT]]

Hi

I think the 384 is the least limitation, the RSA based on big prime
factorization so the LegalKeySizes property give us the support keysize.

All I think you may try to take a look at the new Cryptography which will
require less keysize, you may take a look.
Elliptic Curve Cryptography
http://www.certicom.com/index.php?action=company,press_archive&view=296

Best regards,

Peter Huang
Microsoft Online Partner Support

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