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Revision as of 20:05, 7 August 2024 by William (talk | contribs) (Added signing a CSR)
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OpenSSLis the widely used encryption layer in UNIX operating systems. The most common day to day use is providing transport layer security to websites, such as this one and indicated by the use of https in the URL.

Points to be covered in this document

  • Generating a private key
  • Generating a Certificate Signing Request
  • Fitting an SSL key
  • Verification
  • Starting a Certificate Authority.

Will be fleshed out as time allows.

Creating and using S/MIME

This is how to create a self-signed S/MIME certificate, used for email encryption and decryption in an email client.

First, generate your new key for the sender: 

$ openssl genrsa -out smime.key 2048

Then create a Certificate Signing Request: 

$ openssl req -new -key smime.key -out smime.csr
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:GB
State or Province Name (full name) [Some-State]:Leeds
Locality Name (eg, city) []:Leeds
Organization Name (eg, company) [Internet Widgits Pty Ltd]:Palfreman Trading Ltd
Organizational Unit Name (eg, section) []:
Common Name (e.g. server FQDN or YOUR name) []:William Palfreman
Email Address []:william@palfreman.com

Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:
An optional company name []:

Then sign the CSR using your own Certificate Authority. 

$ openssl x509 -req -days 730 -in csr/smime.csr -CA certs/intermediate.crt -CAkey private/intermediate.key -set_serial 1 -out ../smime/smime.crt

Then the receiver needs to create a key and a certificate signing request. This is their server. 

$ openssl genrsa -out sender-smime.key 2048
Generating RSA private key, 2048 bit long modulus (2 primes)
........................................................................................+++++
........+++++

Then create the Certificate Request using the new key 

$ openssl req -new -key sender-smime.key -out sender-smime.csr
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:GB
State or Province Name (full name) [Some-State]:London
Locality Name (eg, city) []:LONDON
Organization Name (eg, company) [Internet Widgits Pty Ltd]:University of Grantchester
Organizational Unit Name (eg, section) []:Grantchester HostCo
Common Name (e.g. server FQDN or YOUR name) []:William Palfreman
Email Address []:william.palfreman@grantchester.ac.uk

Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:
An optional company name []:

The creates a certificate request (CSR) which isn't confidential but I won't list here. Take that CSR to your Certificate Authority (CA) and issue the certificate 

$ openssl x509 -req -days 365 -in csr/grant.csr -CA certs/intermediate.crt -CAkey private/intermediate.key -out grant.crt
Certificate request self-signature ok
subject=C = GB, ST = London, L = LONDON, O = University of Granchester, OU = HostCo, CN = William Palfreman, emailAddress = william.palfreman@grantchester.ac.uk
Enter pass phrase for private/intermediate.key:

Then pass the receiver sender certificate back to the sending server.

[Next, details about the openssl pipeline to sign and encrypt the smime attachment.]

Keys and CSRs for websites

Create Key

Create the new RSA key. This should be done for each new certificate and should remain on the server it is intended for. 

$ openssl genrsa -out www.mydomain.com.key 2048
$ chmod 400 www.mydomain.com.key

Create CSR

  1. Make a local copy of openssl.cnf
  2. Uncomment the line about v3_extensions
    req_extensions = v3_req
  3. Add a subjectAltName under basicConstraints and keyUsage in [ v3_req ] section.
[ v3_req ]
basicConstraints = CA:FALSE
keyUsage = nonRepudiation, digitalSignature, keyEncipherment
subjectAltName = DNS:www.mydomain.com,DNS:mydomain.com,DNS:other.mydomain.com

There is some debate whether wildcard domains can be SANs. I think they can.

Now create the CSR for the website.

$ openssl req -new -key www.mydomain.com.key -sha256 -config openssl.cnf -subj '/C=GB/ST=Yorkshire/L=Leeds/O=Company Name/OU=Company IT Dept/CN=www.mydomain.com' > www.mydomain.com.csr

The file www.mydomain.com.csr can be provided to the certificate authority for signing.

Signing Cert with own CA

This assumes you have your own certificate authority as many people do for internal use. In this example the above CSR has been sent to you to sign.

  1. Create this file in the directory where the CSR is
cat mydomain-extensions.cnf

[ v3_req ]

# Extensions to add to a certificate request

basicConstraints = CA:FALSE
keyUsage = nonRepudiation, digitalSignature, keyEncipherment
subjectAltName = DNS:www.mydomain.com,DNS:mydomain.com,DNS:other.mydomain.com

[ ca_extensions ]

subjectKeyIdentifier   = hash
authorityKeyIdentifier = keyid:always, issuer
basicConstraints       = critical, CA:true
keyUsage               = keyCertSign, cRLSign
  1. Sign the CSR to make the certificate
intermediate$ openssl x509 -req -days 1830 -in csr/www.mydomain.com.csr -CA certs/intermediate.crt -CAkey private/intermediate.key -extensions v3_req -extfile extensions.cnf -set_serial 1 -out mydomain.com/www.mydomain.com.crt
Certificate request self-signature ok
subject=C = GB, ST = Yorkshire, L = Leeds, O = Company Name, CN = www.mydomain.com
Enter pass phrase for private/intermediate.key:

Now you have created the certificate, check all the details are correct: 

$ openssl x509 -text -noout -in www.mydomain.com.crt 
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number: 1 (0x1)
        Signature Algorithm: sha256WithRSAEncryption
        Issuer: C = GB, ST = England, O = My Company, OU = My Company Certificate Authority, CN = My Company Intermediate CA
        Validity
            Not Before: Aug  7 18:57:24 2024 GMT
            Not After : Aug 11 18:57:24 2029 GMT
        Subject: C = GB, ST = Yorkshire, L = Leeds, O = My Company Name, CN = www.mydomain.com
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (4096 bit)
                Modulus:<snipped>
                Exponent: 65537 (0x10001)
        X509v3 extensions:
            X509v3 Basic Constraints: 
                CA:FALSE
            X509v3 Key Usage: 
                Digital Signature, Non Repudiation, Key Encipherment
            X509v3 Subject Alternative Name: 
                DNS:mydomain.com, DNS:www.mydomain.com
            X509v3 Subject Key Identifier: 
                <snipped>
            X509v3 Authority Key Identifier: 
                <snipped>
    Signature Algorithm: sha256WithRSAEncryption
    Signature Value:<snipped>

Moduli

You must check the moduli line up on the server. Each file, the RSA, the CSR and the certificate should have the same modulus. You can append you ca-chain.pem to the certificate file. OpenSSL will only look at the first. 

$ sudo openssl rsa -modulus -noout -in private/www.mydomain.com.key | openssl md5
(stdin)= 5e7b29b4369f6f7a7f79e1d78c5dd672
$ openssl x509 -modulus -noout -in www.mydomain.com.crt | openssl md5
(stdin)= 5e7b29b4369f6f7a7f79e1d78c5dd672
$ openssl req -modulus -noout -in www.mydomain.com.csr | openssl md5
(stdin)= 5e7b29b4369f6f7a7f79e1d78c5dd672

Then the certificate can be fitted to the webserver.


Writing random seed with writerand

Sometimes openssl lacks a random seed and fails. You can easily create a file for this. 

$ openssl rand -writerand $HOME/.rnd
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