Replacing self-signed expired certificates using OpenSSL tool

I just realized that one of the certificates I use was expired and OpenVPN didn't want to connect to a server for that reason. So, it was time to generate new certificate/key pair. That's easy, using CA.pl script, part of openssl-perl package. Just do the following sequence of steps:

./CA.pl -newreq-nodes./CA.pl -sign

But the second command didn't work and I was getting the following error messsage:

Sign the certificate? [y/n]:y
failed to update database
TXT_DB error number 2
Signed certificate is in newcert.pem

Quick search revealed that I'm trying to issue new certificate that has the same name as the existing one, even though the existing one was expired. There are multiple solutions to this, as documented in the comment of the blog post I found. But the real solution is to revoke expired certificate, and then to sign a new one (note that you don't have to generate another CSR):

ca -config openssl.cnf -revoke oldcert.crt -keyfile cakey.pem -cert cacert.pem

Note that the revocation doesn't mean you have to have OCSP or CSR. Now, old certificate is revoked and you can sign a new one.

RSA parameters in PEM file...

If you've ever read anything about how RSA works you most probably know that RSA is based on arithmetic modulo some large integer. First, let N be a product of two, very large, primes p and q. N is n-bit number, those days minimally 1024 bits, and thus p and q are half that size, so that after multiplication we get required number of bits. Next, there are two numbers e and d that satisfy relation $eq = 1\ mod\ \phi(N)$, where $\phi(N)=(p-1)(q-1)$. $(N, e)$ is encryption key - or what's frequently referred to as a public key, while $(N,d)$ is decryption key, or a private key. The question now is: Given a PEM file with private key, how to find out those parameters for a specific public/private keys? In this post I'll show three ways to do it. The first two are using tools that are a standard part of OpenSSL/GnuTLS libraries. The third one is using Python.

OpenSSL/GnuTLS

Those two, especially OpenSSL, are very popular and complex cryptograhpic libraries. Both of them come with a very capable command line tools. In the case of OpenSSL the there is a single binary, openssl, that performs its function depending on the second argument. In this post, I'll use arguments rsa and genrsa that manipulate and generate, respectively, RSA keys. GnuTLS, on the other hand, has a several tools, of which we'll use certtool.

Now, first thing is to create RSA public/private key. You can do this using the following commands:

openssl genrsa -out key.pem 512

or, if you use GnuTLS, then:

certtool --generate-privkey --bits 512 --outfile key.pem

In both cases the generated RSA modulus (N) will have 512 bits, and the keys will be written into output file key.pem. You should note few things here:

1. The output file contains both private and public keys!
2. We didn't encrypt output file, which is recommended to do.
3. 512 bits these days is a way insecure. Use minimally 1024 bits.

Ok, now we can request information about this RSA keys. Again, you can do that using openssl in the following way:

openssl rsa -in key.pem -noout -text

or, using GnuTLS:

certtool -k --infile key.pem

In both cases you'll receive the following output (GnuTLS is a bit more verbose):

Private-Key: (512 bit)
modulus:
    00:ba:b6:78:3b:1c:15:f1:d9:e3:48:16:5e:e7:8e:
    fd:a0:9d:2f:ee:1b:b8:9b:3d:d3:ea:f4:ad:fb:1b:
    6e:ef:b2:b5:cd:ee:38:e9:f8:6d:64:c9:ea:95:ae:
    87:13:5a:23:8b:2f:0b:e8:bb:c6:f8:c6:c4:ee:64:
    3c:d4:97:bd:a3
publicExponent: 65537 (0x10001)
privateExponent:
    39:6b:07:ca:55:b6:c1:eb:59:a3:bf:8d:6b:f4:63:
    36:d3:5f:fb:ff:76:63:f7:3d:86:51:bc:77:2e:56:
    8d:4b:87:73:e0:53:bd:17:e8:4a:e8:df:f5:86:14:
    65:60:f2:4f:03:02:3b:e9:23:c6:d3:ce:b3:1d:e9:
    13:1a:0f:b1
prime1:
    00:d6:b1:f9:53:8c:56:96:79:c0:bd:68:6c:b9:07:
    e7:9c:70:de:f5:61:ed:bb:51:12:1d:24:37:0f:cc:
    bf:8a:95
prime2:
    00:de:a2:52:be:a1:a4:eb:d7:48:24:95:c5:2c:05:
    bd:5f:7f:74:d5:12:bd:7c:5f:f1:8e:45:a2:50:26:
    ec:d1:57
exponent1:
    66:61:30:58:1b:10:1f:69:a7:f3:aa:9c:4e:0f:ea:
    ee:bb:14:57:47:7f:aa:57:9a:9f:b2:e9:5e:eb:70:
    5b:91
exponent2:
    22:2d:8f:40:5e:b6:5f:d2:5b:eb:e9:e6:2c:1c:f1:
    76:90:ad:91:ec:5f:94:91:72:16:e2:4f:c9:b8:40:
    10:df
coefficient:
    22:9c:f3:1f:85:68:a3:36:ab:07:87:ed:a4:c0:e5:
    ef:13:a8:28:02:55:35:c1:76:96:86:97:58:08:90:
    6e:70

In this output we see parameters N (modulus), e (publicExponent), d (privateExponent), p (prime1), and q (prime2). Also, what's written in there are values d mod (p-1) (exponent1),  d mod (q-1) (exponent2) and q^-1 mod p (coefficient).

The interesting thing to note is the value of publicExponent, it is 65537. Namely, the size of public exponent isn't so important and by having such a low value it is relatively easy to encrypt messages (rising to this exponent requires 17 multiplications). This is very frequent value for encryption key, i.e. public exponent. privateExponent, on the other hand, has to be large and random, so that it isn't easy to guess.

Python

The recommended library to use to manipulate RSA keys is m2crypto which is a wrapper to OpenSSL library. There are many other libraries, of course, and you can find an older comparison here. Unfortunately, I was unable to find downloadable version on the Internet.

Anyway, to be able to manipulate RSA keys you have to import RSA module from M2Crypto, i.e.:

from M2Crypto import RSA

Then, to load RSA private/public key from a file use the following line:

rsa=RSA.load_key('key.pem')

You can also generate new RSA private key as follows:

rsa=RSA.gen_key(512, 65537)

In this case you are generating private key based on public key 65537 (we saw that this is very frequently used public key) and we require 512 bit modulus. You can find out modulus length using Python's len() function, i.e.:

>>> len(rsa)
512

To obtain N, you can inquire attribute n of the object holding RSA key, i.e.:

>>> rsa.n

To obtain pair (e,d), or public key, you can use the following method:

>>> RSA.pub()

Alternatively, you can find out only e in the following way:

>>> rsa.e

As far as I could see, there is no way to find other parameters besides N, e and bit length.

Installing certificate for Alfresco...

This post is continuation of the post about installing Alfresco using native Tomcat6 installation (on CentOS6). If you followed steps given in that post, you have running Alfresco installation but Tomcat uses self-signed certificate.

To install your own certificate first obtain it (you can use your own, self managed, CA or you can buy commercial one), then install it on your Tomcat instance. You'll find a lot of information about this in SSL Howto on Tomcat's Web pages, but that page assumes that everything you do, you are doing using keytool.

Here is a quick Howto with an assumption that you have files newcert.pem (containing certificate), newkey.pem (containing private key) and cacert.pem (your CA certificate). By default, tomcat's keystore is in its home (/usr/share/tomcat6) and it is named .keystore. Keystore file is password protected and default password for it is changeit. Note that the period isn't part of password! I suggest that you copy this file to root's home under the name keystore (note no leading dot!) or whatever else you wish so that you can restore old copy in case something goes wrong with the following steps.

The installation is two step process. First, you create keystore containing you certificate, private key and CA's certificate. In second step, you import that information to Tomcat's keystore.

First step is to pack certificate for Alfresco, its private key and CA's certificate into PKCS12 store using openssl tool as follows:

$ openssl pkcs12 -export \
        -in newcert.pem -inkey newkey.pem \
        -out mycert.p12 -name tomcat \
        -CAfile cacert.pem -caname root -chain
Enter Export Password:
Verifying - Enter Export Password:

This command assumes that all necessary files (newcert.pem, newkey.pem and cacert.pem) are in you current directory. Output of the command is also stored into current directory. Note that you are asked for password that will protect all the data. Enter something or later you'll see the following warning:

*****************  WARNING WARNING WARNING  *****************
* The integrity of the information stored in the srckeystore*
* has NOT been verified!  In order to verify its integrity, *
* you must provide the srckeystore password.                *
*****************  WARNING WARNING WARNING  *****************

And then you'll receive the following error:

keytool error: java.security.UnrecoverableKeyException: Get Key failed: / by zero

Second step is to import this pkcs12 file to tomcat's keystore using keytool as follows:

$ keytool -importkeystore -srckeystore mycert.p12 \
        -srcstoretype pkcs12 -destkeystore /usr/share/tomcat6/.keystore
Enter destination keystore password:
Enter source keystore password:
Existing entry alias tomcat exists, overwrite? [no]:  yes
Entry for alias tomcat successfully imported.
Import command completed:  1 entries successfully imported, 0 entries failed or cancelled

Again, input file is in the current directory and you are importing directly into tomcat's keystore. Note that the existing certificate with the alias tomcat will be removed and you are asked to confirm that! The default alias Tomcat searches when it start is called tomcat.

Third step is to change private key's password that has to be the same as for the keystore. Do that using the following command:

keytool -keypasswd -alias tomcat -new <keypassword> -keystore /usr/share/tomcat6/.keystore

You'll be asked for the keystore's password and the password for the key will be set to keypassword.

And that's it. Restart tomcat and check if it is using new certificate.