Ciphertext different each time I encode it with the same public key

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Digby - 09 Oct 2003 07:47 GMT

Hi,

I've started playing around creating public and private keys and using them
to encrypt byte[]s.

I've noticed that the same public key encrypts the same plain text as
different cipertext (or bytes) each time I run the code, although when I
decrypt with the private key, I do get the original plain text back.

Is this expected funtionality? I'm using an RSA cipher on Java1.4.1 with the
BouncyCastle provider installed.

TIA

Digby

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Michael Amling - 09 Oct 2003 14:08 GMT

> I've started playing around creating public and private keys and using them
> to encrypt byte[]s.
[quoted text clipped - 5 lines]
> Is this expected funtionality? I'm using an RSA cipher on Java1.4.1 with the
> BouncyCastle provider installed.

Yes, the naive use of c=m**e mod n is subject to certain attacks, so
randomized "padding" of the message m is needed to preclude those
attacks. I haven't looked at the padding scheme that Bouncy Castle uses,
but it could be OAEP, which is a good padding scheme. The output is
c=OAEP(m)**e mod n. OAEP xors the message with hashed output of a random
number. Bouncy Castle comes with source, right? So you could search the
source to find the padding algorithm.
You'll also find that repeated digital signatures of the same
document are all different.

--Mike Amling

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Digby - 09 Oct 2003 20:22 GMT

Thanks. Think I've got a bit to learn!

> > I've started playing around creating public and private keys and using them
> > to encrypt byte[]s.
[quoted text clipped - 17 lines]
>
> --Mike Amling

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Ciphertext different each time I encode it with the same public key

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