RSA key smaller than 512kbits?

[Bill Toulias]

I'm trying to use RSA and MD5 on my activation scheme for an app. I hash a
value with MD5 and encrypt with an RSA private key. The client pc decrypts
with my public, and verifies.
The problem is that .NET allows no less than 512bits for an RSA key, which
makes the encryption result 512bits as well and makes activation by phone an
pain (100 letters to read!).
Is there any way i can use RSA with a smaller key-pair?

 

[Nicholas Paldino [.NET/C# MVP]]

Bill,

From what I can tell, it would appear that the minimum key size for RSA
would be 384 bits, which when converted to bytes, then run through something
like Base64 encoding (in order to get letters and numbers you could enter on
a keypad), you come up with 64, which is still a bit unwieldy.

Hope this helps.