Re-issuing self-signed root CA without invalidating certificates signed by it

Solution 1:

Two certificates are considered the same as long as the Subject Name and the Public Key of the certificates match.

Therefore, all you need to do is to re-use the keys and ensure that the Subject Name in the new certificate is the same as the old. After that, you can change any of the other fields and/or extensions and the resulting certificate will be considered the same.

For example, create your OpenSSL configuration file:

[ req ]

prompt             = no
string_mask        = default
distinguished_name = x509_distinguished_name
x509_extensions     = x509_ext

[ x509_distinguished_name ]

# Note that the following are in 'reverse order' to what you'd expect to see.
# Adjust for your setup:

countryName = za
organizationName = My Company
organizationalUnitName = Security
commonName = My Root CA

[ x509_ext ]

basicConstraints = critical,CA:true
keyUsage = critical, keyCertSign, cRLSign
subjectKeyIdentifier = hash
crlDistributionPoints = URI:http://security.mycompany.co.za/root.crl

and save it as e.g. rootca.cnf. Ensure that the elements of the [req_distinguished_name] are identical to the ones in your original Root CA certificate (this is the identical Subject Name part).

Next, run:

openssl req -new -x509 -key rootca.key -out MyNewCA.pem -config rootca.cnf

where rootca.key is the private key used in the original certificate (this is the identical Public/Private Key part).

This creates MyNewCA.pem, which you can check with:

$ openssl x509 -noout -text -in MyNewCA.pem

Certificate:
Data:
    Version: 3 (0x2)
    Serial Number: 17564687072266118846 (0xf3c24dd49d5166be)
Signature Algorithm: sha256WithRSAEncryption
    Issuer: C=za, O=My Company, OU=Security, CN=My Root CA
    Validity
        Not Before: Jul 15 05:05:54 2017 GMT
        Not After : Aug 14 05:05:54 2017 GMT
    Subject: C=za, O=My Company, OU=Security, CN=My Root CA
    Subject Public Key Info:
        Public Key Algorithm: rsaEncryption
            Public-Key: (2048 bit)
            Modulus:
                00:c8:3d:32:8a:56:31:f6:27:1a:ce:9e:b2:1d:fb:
                ce:9f:ce:5b:42:25:aa:fe:8b:f4:34:32:ac:b3:02:
                50:71:f8:c3:43:0c:c7:2c:9f:fe:48:1b:c6:c0:e7:
                d6:44:a9:e7:d7:a0:7e:58:f4:b6:38:61:7e:d0:af:
                0f:56:21:e7:49:7a:66:13:f5:7a:fe:3d:ab:65:f8:
                01:eb:52:a7:3b:ae:a0:cf:50:57:b9:e0:09:0b:1f:
                90:14:fb:18:56:1d:57:99:a9:76:a2:63:d1:c2:d3:
                a3:f4:3a:ff:91:0d:ee:8d:44:13:58:00:09:93:da:
                e8:6a:fd:64:5f:c3:42:8e:2a:49:6e:0d:73:b7:b9:
                d4:6c:c6:ce:30:c5:82:24:a5:00:37:17:a0:1d:f1:
                c9:e9:e3:18:48:22:4f:33:96:a7:3c:a9:31:f1:3f:
                14:40:6a:74:e8:78:82:45:04:d4:4b:56:0b:cd:be:
                48:8d:03:fb:39:70:0b:91:99:70:06:bd:5e:8b:f2:
                d1:f4:6f:fc:ce:e7:f8:3c:0a:20:ea:35:b8:5f:2f:
                ee:8d:ff:d3:93:85:6b:fb:71:db:1b:e6:e9:1d:a7:
                f8:e4:ae:f4:71:fe:35:a7:89:24:af:69:a4:34:3b:
                14:66:05:02:5e:2a:1d:ac:e0:d2:48:6c:13:4e:75:
                58:93
            Exponent: 65537 (0x10001)
    X509v3 extensions:
        X509v3 Basic Constraints: critical
            CA:TRUE
        X509v3 Key Usage: critical
            Certificate Sign, CRL Sign
        X509v3 Subject Key Identifier: 
            3B:45:93:3A:2A:BC:39:29:36:13:6C:BD:B6:B4:31:C7:E7:BB:32:73
        X509v3 CRL Distribution Points: 

            Full Name:
              URI:http://security.mycompany.co.za/root.crl

Signature Algorithm: sha256WithRSAEncryption
     4d:96:d4:03:4f:e3:7c:26:be:59:f8:23:87:60:f7:4c:d3:a1:
     1c:77:a1:14:e3:e7:da:c8:2a:a3:1b:06:2a:4d:55:1c:83:26:
     73:46:0d:8a:e4:b7:d1:1e:38:cc:78:90:00:01:b3:8e:f9:3c:
     62:be:04:09:90:4e:22:87:b1:aa:bd:f9:73:bd:a7:76:ad:d5:
     ae:2d:7a:1c:1e:1a:67:c8:57:4c:f9:6d:8b:62:d6:e5:ea:e0:
     40:5c:12:28:7e:ea:f0:0c:d6:cd:f4:1d:d5:56:09:ad:43:b4:
     eb:8c:68:ce:56:a2:a8:ae:a4:d5:35:bb:58:b8:ed:82:82:b5:
     ef:cb:e2:6d:76:61:ed:ee:a5:1f:68:95:07:ed:5b:f0:65:92:
     d2:dc:1d:c6:fa:7f:e0:c9:38:c2:c6:6f:03:38:e7:3a:b0:24:
     06:e0:bc:07:dd:e7:a0:dc:74:09:e5:37:7b:66:e1:6f:47:4c:
     71:ff:02:48:7f:d4:4f:ce:cb:91:e9:ee:cd:b6:f1:0a:03:19:
     3e:19:05:7d:8f:48:e7:f1:cc:07:37:3d:91:3c:6f:54:71:3c:
     a2:6c:55:c3:03:c1:7f:eb:9e:70:f1:8f:a1:fb:62:33:8b:86:
     2c:79:bc:76:e2:01:9a:68:df:af:40:a1:b2:9c:f6:a1:e1:6e:
     2a:dd:1a:d6

Use this new certificate in place of the original.

You can change other fields and extensions, such as the validity period of the certificate, but bear in mind that you shouldn't really have any constraints (other than basicConstraints = critical,CA:true) on the Root CA certificate.

After further consideration, your issue may simply be down to the fact that your replacement Root CA certificate doesn't have the basicConstraint and possibly the keyUsage extensions. It might be worth trying adding those two extensions to your ext.conf first and testing the resulting new Root CA certificate using the -x509toreq method you posted.