Short RSA keys combined

[Danijel Gorupec]

Hi,

 

For memory restrictions, I cannot fit RSA key generation algorithm that would generate keys longer than 32 bit. This seems too short even for my application (data only needs to remain secure for 15-20 minutes).

 

The question is: how many 32 bit RSA keys I have to be generate in order to achieve security of single 256 bit RSA key?

 

(BTW, what do you think, how long can a 32-bit RSA key hold when attacked using an average PC? Second? Minute?). Thanks.

 

 

[Cap'n Refsmmat]

How are you planning to use multiple RSA keys? Smash them together? Encrypt multiple times? I don't quite follow. Perhaps you could explain what you intend to do and what sort of encryption you're doing.

 

Also, what sort of memory restrictions prevent you from creating a key longer than 32 bits but let you generate multiple 32-bit keys?

[Danijel Gorupec]

I would like to encrypt exam results for several minutes with public key provided by teacher, so that no student can see results of any other studend while working on the exam.... This feature is not essential for my software, therefore I don't like to bloat the software (neither spend much time for development) by including variable-length-integer-math into it.

 

I can afford to store few hunderds of short public keys into the exam file as a quick solution. If this is nearly enough and makes any sense (I am affraid not - I have no experience on this matter).

 

(The idea is that teacher creates an exam and saves the exam file. While saving the exam file he will have to provide a password. From this password, public keys will be derived and stored into the exam file. Any student can then open the exam file, but after finishing the exam, the software will automatally and inevitably save the file encrypted with the public key. Nobody else, except the teacher that knows the password, will be able to open it.... It seems that Windows CryptoAPI doesn't want to derive asymmetric keys from password (?? Windows can only generate random asymmetric keys??).... To achieve this I am actually using Diffie-Hellman algorithm, not RSA, but I understand that key-sizes are about the same. I asked about RSA, because nobody talks about Diffie-Hellman much.)

 

Yes, I combine (in hopefully smart way) many short keys into single key that is then used for encryption.