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Asked: 2026-05-15T11:40:37+00:00

Surprisingly enough there’s very little information on the Web about using Bouncy Castle’s lightweight

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Surprisingly enough there’s very little information on the Web about using Bouncy Castle’s lightweight API. After looking around for a while I was able to put together a basic example:

RSAKeyPairGenerator generator = new RSAKeyPairGenerator();
generator.init(new RSAKeyGenerationParameters
    (
        new BigInteger("10001", 16),//publicExponent
        SecureRandom.getInstance("SHA1PRNG"),//prng
        1024,//strength
        80//certainty
    ));

AsymmetricCipherKeyPair keyPair = generator.generateKeyPair();

I have a basic understanding of RSA and the math that happens behind the scenes, so I understand what publicExponent and strength are. I presume publicExponent refers to a coprime of phi(pq) and from what I gather it can be small (like 3) as long as appropriate padding is used. However, I have no idea what certainty refers to (some place mentioned that it might refer to a percentage but I want to be sure). The use of SecureRandom is self-explanatory. The documentation of RSAKeyGenerationParameters is completely worthless (no surprise there). My only guess is that it has something to do with the accuracy of the generated keys, but again I want to be sure. So my question is what are appropriate values for certainty and publicExponent?

P.S.
Please don’t reply with “it depends on the context – how secure you want the information to be”. It’s pretty safe to assume highest degree of security (i.e. 4096-bit RSA key or greater) unless otherwise specified… I would also appreciate links to sources that give good example of the use of Bouncy Castle’s Lightweight API (I’m not at all interested in the JCA implementation or any examples pertaining to it).

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1 Answer

  1. Editorial Team

    You are using correct values for both.

    The publicExponent should be a Fermat Number. 0x10001 (F4) is current recommended value. 3 (F1) is known to be safe also.

    The RSA key generation requires prime numbers. However, it’s impossible to generate absolute prime numbers. Like any other crypto libraries, BC uses probable prime numbers. The certainty indicate how certain you want the number to be prime. Anything above 80 will slow down key generation considerably.

    Please note that RSA algorithm still works in the unlikely event that the prime number is not true prime because BC checks for relative primeness.

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