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 ???  Hello,

I am using C# and finding problems. Below are some calculations I have been doing with RSA.

These work for small values (a few bytes) and I am able to encrypt and decrypt succesfully.

However, when I try to scale up to, say, 96 bytes RSA keyset, then I get truncation problems. The first few bytes of my caclucated values are OK, but then the rest is wrong.

Any ideas?


HIME.hi_PutReg(pString, 10);      //p = 10
HIME.hi_PutReg(qString, 11);      //q = 11
HIME.hi_Mul(10, 11, 12);            //Modulus = 12
                                             //e = 2
HIME.hi_Decr(10,15);                //p-1 = 15
HIME.hi_Decr(11,16);                //q-1 = 16
HIME.hi_Mul(15,16,17);              //(p-1)(q-1)
HIME.hi_Modinv(2, 17, 18);
HIME.hi_Encrypt_Rsa(1, 2, 12, 4);

Any help would be most welcome. Is there a manual showing examples?

Thank you

Gwyn

Hi Gwyn,

You seem to be trying to create the RSA keys yourself ?
Unless you have a good reason not to, I highly recommend to use the HIME hi_GenerateRSAKeys or hi_GenerateRSAKeys_CRT  functions to generate the RSA public and private keys.

The HIME manual (included with the zip package you downloaded) has a page called "RSA encryption/decryption" that shows a complete example (though not in C#) of how to encrypt and decrypt data with RSA, both the 'classic' way as with using CRT (Chinese Remainder Theorem).

Kind regards
Eddy

PS. Also remember that with RSA, your plaintext length must always be smaller than the modulus length.

Eddy

Hi Gwyn,

I looked a little deeper into your code snippet. Two suggestions:
1 - Use HIME function hi_GenerateRSAKeys to generate public key, private key and modulus, because you are cutting corners generating the key in your code example. The relation between public key exponent 'e' and '(p-1)(q-1)' must be so that GCD of e and (p-1)(q-1) = 1 , where GCD is the Greatest Common Divisor.
Also: 1 < e < (p-1)(q-1) .
If these conditions are not fulfilled, you must choose new p and q (and/or e) until the conditions are fulfilled. P and q are both prime numbers, as you know.

Maybe you are doing this, but it is not shown in your code.

2 - As stated before, make sure that plaintext is smaller than the modulus.

Hope this helps.

Kind regards
Eddy

Eddy,

Thank you for your support.

My P, Q and e values are from a know good RSA key, hence I should be able to calculate all the other components from these.
The reason I am doing this is that I want to write a tool to test that RSA keys have been correctly migrated from a pre existing system to a new system - hence I have to use the existing keys.

I will do a bit more experimentation before getting back to you with an example of my truncation problem.

Best regards,

Gwyn

Gwyn,
That is correct.

Meanwhile, just thinking out loud:

• Is your plaintext pure ascii string or is it binary (perhaps containing possible zero bytes) ?
• After encryption, do you retrieve the ciphertext from HIME to restore it later in HIME, or do you just leave it in the HIME register ?
• Truncation: is you decrypted ciphertext (which should normally match your original plaintext) actually truncated (chopped off) or does it contain a series of correct characters/bytes and, following that, a series of incorrect characters ? From your first post I assume the latter, but it has me a little confused with 'truncation'  :)

Kind regards
Eddy