Date sent: Sun, 25 Mar 2001 14:04:58 +0100
From: Dr S N Henson <[EMAIL PROTECTED]>
Organization: S N Henson
To: [EMAIL PROTECTED]
Subject: Re: RSA Private Encrypt
Send reply to: [EMAIL PROTECTED]
Dr. Henson
After I read your last note, and slept on it overnight, it all made
sense and I got everything to work this morning with the Smart Card
doing the rsa_mod_exp().
One minor suggestion, it would appear to me that the app_data field
in the RSA_METHOD structure perhaps should be a void * instead
of a char *. If I understand everything correctly, I should be able to
place a pointer to a private structure in this field and be able to pass
application dependent data relating to the key such as Smart Card
type, key id, etc.
I must say that the more I work with OpenSSL, the more I realize
how brilliant and sophisticated the whole design is. And the support
from the mailing list is first class.
Ken
"Kenneth R. Robinette" wrote:
>
>
> I was hoping that this was the case. Now if I set the
> RSA_FLAG_EXT_PKEY flag, how do I specify the function that will
> be called by OpenSSL to do the private encrypt? Is this available to
> a client program? I tried following the logic but quite frankly got lost
> at the rsa_eay_private_encrypt function. Is there any
> documentation on what the "private" function is passed and how the
> results should be returned?
>
There's some documentation in the relevant rsa manual pages.
What you do effectively is to create an RSA_METHOD structure, copy any
relevant default methods and then replace whichever ones you want. Then
create an RSA structure and set its method to the custom method just
created and of course set RSA_FLAG_EXT_PKEY.
Well that's what you do in non ENGINE builds. In the ENGINE stuff the
method would be in an ENGINE structure and you'd set the RSA structures
ENGINE... or something like that.
rsa_mod_exp() is a low level function that does the actual mathematical
private key operation:
int (*rsa_mod_exp)(BIGNUM *r0,const BIGNUM *I,RSA *rsa);
it expects an RSA private key operation to be performed on I and the
result placed in r0.
the ex_data part of the 'rsa' structure can be used to include
additional information such as key handles etc.
rsa_mod_exp() is most suitable where the hardware (or whatever) only
handles the raw private key operation. This would be the case in some
crypto accelarators or smart cards that don't do their own padding. In
the smart card case the BIGNUM structures might be converted to and from
buffers before passing to the card API.
It is also possible to override at a higher level using rsa_priv_enc,
rsa_priv_dec functions. This is more suited when the hardware (etc)
implements its own version of the RSA algorithm complete with padding
and pad checking etc, for example PKCS#11 or CryptoAPI.
Steve.
--
Dr Stephen N. Henson. http://www.drh-consultancy.demon.co.uk/
Personal Email: [EMAIL PROTECTED]
Senior crypto engineer, Celo Communications: http://www.celocom.com/
Core developer of the OpenSSL project: http://www.openssl.org/
Business Email: [EMAIL PROTECTED] PGP key: via homepage.
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