Re: some RSA questions

["Mark M." <markm@voicenet.com>]

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On Sun, 27 Oct 1996, J�ri Kaljundi wrote:

> The question is easy: how should I sign and encrypt the message?
> 
> 1) calculate a digest (MD5, SHA) from the message, sign the digest (or
> should I sign the message contents + digest) with Bob's private key and
> encrypt it with Alice's public key?

This is the best way.  The message plus the signature should be encrypted
using a symmetric algorithm and encrypt the key with Alice's public key.
All RSA encryption and signing should be done according to the PKCS standard
to avoid several nasty attacks that are successful if plain RSA is used.

> 2) is there any idea to generate a DES key, encrypt the message with the
> DES key, calculate a digest on (enc. message + DES key), sign the digest
> and encrypt it with Alice's public key? The message is small, so the time
> it takes to encrypt the message is not so important compared to higher
> speed of DES.

It is generally not a good idea to sign an encrypted message.  The signature
should be calculated on the plaintext, not ciphertext.

> 3) may be it would be good to encrypt the message with Alice's public key,
> then generate a digest, sign the digest and then once more encrypt the
> whole thing with Alice's public key?

This has the same problem as #2.

> Using a DES session key helps in case someone would find out Alice's
> private key she uses to decrypt the message, but actually in this case it
> is not so important to hide the message contents (what is important are
> message integrity and sender authentication).
> 
> What are the suggestions what crypto package might I use: RSAref,
> crypto++, SSLeay or some other? 

SSLeay is pretty fast and does have code to use PKCS.  It probably doesn't make
that much of a difference.  RSAref isn't necessary; RSA isn't patented outside
the USA.  The other packages are faster and better than RSAref.

Mark
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