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<p><font size="2"><span style="font-size:10pt;"></p>
<div dir="ltr" ,="" style="text-align: left;">> The rsa_mod_exp function is only called for private key operations. You can't<br>
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> tell if it is a private encrypt or a private decrypt though but that<br>
> shouldn't matter because the operation performed at that level is the same for<br>
</span></font><font size="2"><span style="font-size:10pt;">> both.</span></font><br>
<p></p>
<p>Ah, I see. So to clarify (pls correct me if I'm wrong):</p>
<ul style="margin-bottom: 0px; margin-top: 0px;">
<li>rsa_mod_exp() is the modular exponentiation function that openSSL will attempt to use for all private key operations (if
<span>RSA_FLAG_EXT_PKEY</span> is set, or the private parameters of that method are non-null), before defaulting to bn_mod_exp().
<br>
</li><li>And bn_mod_exp() is the modular exponentiation function used by all <span>public key
</span>operations</li></ul>
<p><br>
</p>
<p>So in my case, I should set RSA_FLAG_EX_PKEY in my engine's RSA_METHOD struct, and then implement my own versions of rsa_mod_exp (for the private key encryption/decryption), and bn_mod_exp (for the public key encryption/decryption).
<br>
</p>
<p><br>
</p>
<p>Brett<br>
</p>
<br>
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<div id="x_divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" color="#000000" face="Calibri, sans-serif"><b>From:</b> openssl-dev <openssl-dev-bounces@openssl.org> on behalf of Dr. Stephen Henson <steve@openssl.org><br>
<b>Sent:</b> Sunday, August 27, 2017 6:45 PM<br>
<b>To:</b> openssl-dev@openssl.org<br>
<b>Subject:</b> Re: [openssl-dev] confusion with rsa_meth_st in a custom RSA engine</font>
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<font size="2"><span style="font-size:10pt;">
<div class="PlainText">On Sun, Aug 27, 2017, Brett R. Nicholas wrote:<br>
<br>
> <br>
> This makes sense to me, and it seems that is the desired behavior. However,<br>
> if I *only* reimplement the rsa_mod_exp() function, and leave the<br>
> encrypt/decrypt functions to the default openSSL implementations, how can my<br>
> engine know which of the four of those functions called its rsa_mod_exp<br>
> function()? To put it another way: my accelerator will need to know whether<br>
> it is meant to perform public key or private key operations, since the<br>
> inputs will be written to different memory addresses. From within<br>
> rsa_mod_exp(), the only way I can think of determining whether the function<br>
> has been called by private_encrypt/decrypt() or public_encrypt/decrypt would<br>
> be to check if the p and q fields of the RSA *rsa struct passed into the<br>
> function are NULL? But I can't tell from the source code if this will be<br>
> guaranteed just by having RSA_FLAG_EXT_PKEY set in the "flags" field of my<br>
> engine's RSA_METHOD.<br>
> <br>
<br>
The rsa_mod_exp function is only called for private key operations. You can't<br>
tell if it is a private encrypt or a private decrypt though but that<br>
shouldn't matter because the operation performed at that level is the same for<br>
both.<br>
<br>
Steve.<br>
--<br>
Dr Stephen N. Henson. OpenSSL project core developer.<br>
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