In a previous post I mentioned how Google & Microsoft are starting to deprecate the use of SHA-1 Certificate Signatures in their browsers.

Google’s announcement can be found here at http://googleonlinesecurity.blogspot.co.uk/2014/09/gradually-sunsetting-sha-1.html

Following on from this post I wondered how to change the default settings of OpenSSL to make sure that all future certificates don’t use SHA-1.

OpenSSL on Ubuntu 12.04 / 14.04

First edit the OpenSSL config file

$ sudo vim /etc/ssl openssl.cnf

Look for the following section

[ CA_default ]   

default_days    = 1000          # how long to certify for
default_crl_days= 30            # how long before next CRL
default_md  = default        # use public key default MD
preserve    = no            # keep passed DN ordering

Make the following change and save the file.

default_md  = sha256        # use SHA-256 for Signatures

From this point onwards all Certificates generated will be signed with SHA-256.

Check the Certificate Signature

The following command will output information about the Certificate

$ openssl x509 -in cert.crt -text -noout

The output should look something like this.

Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number: 11485830970703032316 (0x9f65de69ceef2ffc)
    Signature Algorithm: sha256WithRSAEncryption
        Issuer: C=US, ST=MD, L=Baltimore, CN=Test CA/emailAddress=test@example.com
        Validity
            Not Before: Jan 24 14:24:11 2014 GMT
            Not After : Feb 23 14:24:11 2014 GMT
        Subject: C=US, ST=MD, L=Baltimore, CN=Test CA/emailAddress=test@example.com
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (4096 bit)
                Modulus:
                    00:b1:7f:29:be:78:02:b8:56:54:2d:2c:ec:ff:6d:
                    ...
                    39:f9:1e:52:cb:8e:bf:8b:9e:a6:93:e1:22:09:8b:
                    59:05:9f
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Subject Key Identifier: 
                4A:9A:F3:10:9E:D7:CF:54:79:DE:46:75:7A:B0:D0:C1:0F:CF:C1:8A
            X509v3 Authority Key Identifier: 
                keyid:4A:9A:F3:10:9E:D7:CF:54:79:DE:46:75:7A:B0:D0:C1:0F:CF:C1:8A

            X509v3 Basic Constraints: critical
                CA:TRUE
            X509v3 Key Usage: 
                Certificate Sign, CRL Sign
    Signature Algorithm: sha256WithRSAEncryption
         4a:6f:1f:ac:fd:fb:1e:a4:6d:08:eb:f5:af:f6:1e:48:a5:c7:
         ...
         cd:c6:ac:30:f9:15:83:41:c1:d1:20:fa:85:e7:4f:35:8f:b5:
         38:ff:fd:55:68:2c:3e:37

 Hopefully in the not too distant future this will become the default on new installs. It may also be changed with a security update.