SSL certificates guide

In this article I'm going to explain how to create keys, SSL certificates and key stores. This can be required to simply migrate your website to HTTPS or to enable single sign-on authentication or in other cases. SSL certificates can be used for digital signing/verification and for encryption/decryption.

In case of digital signatures, the sender signs the message using a private key certificate, while the receiver verifies the signature of the message using the public key certificate.

In case of encryption, the sender encrypts the message using the public key certificate, while the receiver decrypts the message using the private key.

1. Generating keys.
2. Generating certificates.
3. Working with keystores.

Generating keys

The first step is generating a private/public key pair. This can be done in different ways. We'll use openssl utility as it will be used for certificates later as well. The important point is the key length - bigger length makes the key harder to crack. It's considered safe to have the length of at least 2048 bits for RSA keys nowadays.

1. Generate RSA private key of 2048 bits in PEM format

openssl genrsa -out rsaprivkey.pem 2048

2. Generate the public key in DER format

openssl rsa -in rsaprivkey.pem -pubout -outform DER -out rsapubkey.der

3. Generate the unencrypted private key in PKCS #8 and DER format

openssl pkcs8 -topk8 -inform PEM -outform DER -in rsaprivkey.pem -out rsaprivkey.der -nocrypt

Generating certificates

You have two options - generating a self signed certificate or requesting a trusted certificate from a certificate authority. While self signed certificates can be acceptable during development, any production system will require a trusted certificate. Both options are mentioned here.

1. Generate certificate signing request (CSR), in the "Common Name" set the hostname of your website

openssl req -new -key rsaprivkey.pem -out server.csr

2. Print CSR details

openssl req -in server.csr -noout -text

Example output:

Certificate Request:
    Data:
        Version: 0 (0x0)
        Subject: C=AU, ST=Some-State, O=Internet Widgits Pty Ltd
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
                Public-Key: (2048 bit)
                Modulus:
                    00:d2:02:4a:83:b1:07:4c:9f:b3:40:11:88:73:16:
                    ... more HEX data ...
                Exponent: 65537 (0x10001)
        Attributes:
            a0:00
    Signature Algorithm: sha256WithRSAEncryption
         28:f6:2e:1b:f7:4b:d9:fd:96:58:e5:ca:86:87:07:a2:a7:21:
         ... more HEX data ...

3. Generate self signed certificate

openssl x509 -req -days 365 -in server.csr -signkey rsaprivkey.pem -out server.crt

Example output:

Signature ok
subject=/C=AU/ST=Some-State/O=Internet Widgits Pty Ltd
Getting Private key

4. Request a trusted certificate (e.g. Let's Encrypt or COMODO)

There are two main encoding formats that a certificate can be stored in - a binary DER format (can also have file extensions CER or CRT) and a Base64 encoded PEM format which syntax is defined by X.509 standards (can also have file extension CRT). You can convert between the encoding formats using the following commands:

1. Convert certificate from DER to PEM format

openssl x509 -inform der -in certificate.crt -out certificate.pem

2. Convert certificate from PEM to DER format

openssl x509 -outform der -in certificate.pem -out certificate.crt

Working with keystores

There are different types of keystores. The most common types are PKCS #12 (using extensions .p12 or .pfx) and JKS (Java KeyStore, specific to Java language). We'll use keytool Java utility to work with JKS files.

1. Generate a PKCS12 keystore with our certificate (note - set non-empty password)

openssl pkcs12 -export -in server.crt -inkey rsaprivkey.pem -name server -out server.p12

Next you can either convert PKCS12 keystore type into JKS or generate an empty JKS keystore and import a certificate there. Both options are explained below.
2. Convert PKCS12 keystore to JKS keystore

keytool -importkeystore -srckeystore server.p12 -srcstoretype pkcs12 -srcalias server -destkeystore server.jks -deststoretype jks -deststorepass password -destalias server

3. Generate an empty JKS keystore (note - set non-empty password)

keytool -genkey -alias server -keystore server.jks
keytool -delete -alias server -keystore server.jks

4. Import certificate from PKCS12 to JKS keystore

keytool -importkeystore -deststorepass password -destkeystore server.jks -srckeystore server.p12 -srcstoretype PKCS12

Now when you have a JKS keystore with your private certificate, you might need to import a public trusted certificate there as well. For example, this can be needed to enable SAML SSO authentication.
5. Import a trusted DER certificate into JKS keystore

keytool -import -trustcacerts -alias trustedCert -file trustedCert.crt -keystore server.jks 

6. Check certificates in JKS keystore

keytool -list -keystore server.jks -storepass password

Example output:

Keystore type: JKS
Keystore provider: SUN

Your keystore contains 2 entries

server, 27-Jul-2017, PrivateKeyEntry,
Certificate fingerprint (SHA1): A3:11:78:7C:D7:C6:30:F9:57:DF:3C:B1:B4:C9:1F:06:83:E5:ED:39
trustedCert, 24-Jul-2017, trustedCertEntry,
Certificate fingerprint (SHA1): B8:4D:C8:5F:1C:98:A3:0B:F2:BC:04:E0:A3:22:26:66:64:F7:60:C8