Re: Unable to decrypt without Chinese Remainder Theorem factors ?

[Mounir IDRASSI]

Hi,

The problem that you are encountering comes from the fact that the 
function d2i_RSAPrivateKey, that create the key from the PEM content, 
will allocate the p, q, dmp1, dmq1 and iqmp fields and set them to zero, 
which is normal since this is their value on the PEM file. But then, the 
function RSA_eay_private_decrypt will fail with the error you are 
getting because it only tests for NULL pointers for the CRT fields not 
doesn't check if they are zeros.

You have two options here :
    - use a patched version of OpenSSL where the implementation of 
RSA_eay_private_decrypt test for NULL pointers and zeros BIGNUMs
    - convert your keys to CRT format.

For the later, I have developed an open source tool that converts 
private keys from SFM format (modulus, private exponent, public 
exponent) to CRT format. It's locate on SourceForge : 
http://rsaconverter.sourceforge.net/. It uses OpenSSL BIGNUM 
implementation. Especially, you can look at the file librsaconverter.c 
for the function SfmToCrt where the conversion algorithm is implemented.

For the first option, I can send you a patch for RSA_eay_private_decrypt 
but it's easy to do (just add tests !BN_is_zero between lines 534 and 539).
I hope this will help.

Cheers,

--
Mounir IDRASSI
IDRIX
http://www.idrix.fr


On 2/8/2010 10:12 AM, JB Van Puyvelde wrote:
Hi,

I would like to import and use in OpenSSL RSA key pairs generated by
an other program.

This program can export public and private keys to PEM files, with
X509 and PKCS#7 specifications. But, the private key doesn't contain
Chinese Remainder Theorem factors, only the private exponent and the
modulus.

So, I'm able to encrypt a file with the public key :

C:\openssl rsautl -encrypt -inkey pub.pem -pubin -in hello.txt -out hello.enc
Loading 'screen' into random state - done

But, trying to decrypt causes the following error :

C:\openssl rsautl -decrypt -inkey priv.pem -in hello.enc -out result.txt
Loading 'screen' into random state - done
RSA operation error
2548:error:0306B067:bignum routines:BN_div:div by zero:.\crypto\bn\bn_div.c:213:

I suppose the error comes from the fact that openssl.exe tries to use
the CRT factors to decrypt the file.

Could you confirm this ?

And of course, is there any way to make works decryption with my keys ?

Currently, I'm thinking about three possible solutions :
1) find a way to generates CRT factors from modulus and private
exponent (but I don't know if it's possible)
2) could the use of the RSA C functions, instead of the openssl
binary, solve the problem, because
http://openssl.org/docs/crypto/rsa.html says "p, q, dmp1, dmq1 and
iqmp may be NULL in private keys, but the RSA operations are much
faster when these values are available." ?
3) and last, generate the keys with OpenSSL, but honestly I would
really appreciate to not have to do that and continue to generate key
pairs with my first program.

An example of key pair is attached with this e-mail. And for those who
knows, the first program is coded Java, the private key comes from the
PrivateKey class and is exported to PEM file with the PemWriter class
of BountyCastle.

Thanks,

JB
   

--
--
Mounir IDRASSI
IDRIX
http://www.idrix.fr

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