In the previous article, In-depth Understanding of PKI System and Digital Certificate introduced the basic components of PKI system and the main role of CA certification center, as well as the basic standard of X.509 certificate. Today, we continue to apply the theoretical knowledge we have learned to build a set of our own PKI/CA digital certificate trust system.
Digital Certificate Generation Tools
There are two common tools for generating
RSA public-private key pairs:
Note: Some cases only need public-private key pairs and do not need digital certificates, such as private
SSHservices, but for some cases that require authentication, the public key needs to be digitally signed to form a digital certificate.
ssh-keygen also uses the library provided by
openssl to generate key pairs.
For example, to generate a 2048-bit RSA key pair using
ssh-keygen, simply execute the following command.
-b specifies the number of key bits, usually 2048 or more, and
-t specifies the key type, such as
Note: If the key pair is used for
ssh, then the private key is conventionally named
id_rsaand the corresponding public key is
id_rsa.pub, which can then be appended to the remote host’s
.ssh/authorized_keytrusted list using the
The default generated key pair is in
RFC 4716/SSH2 format and can be transferred to
PEM format for use by other programs.
-m specifies the converted format and supports
openssl is a powerful secure socket layer cryptographic library toolkit, the whole package can be roughly divided into three main functional parts.
- cryptographic algorithm library
- common key and certificate encapsulation management functions
- SSL communication API interface
- rich applications for testing or other purposes
The following functions can be accomplished using the
openssl development kit.
- generate X.509 certificate, certificate signing request (CSR) and CRL (certificate recovery list)
- calculate message digest (Digest)
- use various
SSL/TLSclient and server testing
S/MIMEor encrypted messages
openssl provides many different commands, and each subcommand has many options and parameters, which are not listed in detail here.
Digital certificate generation process
openssl this package can complete the CA construction, SSL certificate from generation to issuance of the whole process, when using
openssl command, you need to remember the following points.
- there are many file extensions in the generation process (
.key, etc.), although it is said that the extensions do not have any mandatory binding effect, what is important is which command the file is generated by and what format its content is. The use of these specific file extensions is just to follow certain conventions and specifications that can be understood at a glance.
- there is some functional overlap between the commands of
openssl, so we will find that the same purpose (e.g. SSL certificate generation) can often be accomplished using seemingly different sets of commands.
- the most important thing to understand CA and SSL certificate is not to remember these
opensslcommands, but to understand the operation mechanism of CA and the function of
opensslcommands, so as to understand the whole process from the principle.
Create CA root certificate
CA root certificate is an important part of PKI system. To establish a tree-like trusted system, firstly, CA itself needs to show its identity and establish the root of the tree, and other intermediate certificates rely on the root to issue certificates and establish trust relationship with each other through the root certificate. CA certificate is composed of private key (Root Key) and public key (Root Cert). CA root certificate does not directly issue server certificate and client certificate, but is only used to issue one or more intermediate certificates, and the intermediate certificate issues certificates to users and servers as proxy of CA root certificate. At the same time, in order to ensure the reliability of CA root certificate, the private key (Root Key) corresponding to CA root certificate should be offline and safely stored.
1. Create the private key storage directory for the CA root certificate
Select a directory for storing all private and public keys.
Create a directory structure that generates
serial files, which are used as a file database to track previously issued certificates.
- certs holds the issued certificates
- newcerts holds the new certificate generated by the CA
- private for private keys
- crl for revoked certificates
- index.txt Defines the text database file for certificates issued using
openssl, which is usually empty when initialized
- serial The serial number reference file used when issuing the certificate, the serial number in this file is a hexadecimal number and must be provided and include a valid serial number when initializing
2. Create the profile for the CA root certificate
Create a configuration file for the CA root certificate for
openssl, saved in
/root/ca/openssl.cnf, where the
[ ca] section is required to tell openssl the configuration information to use for the root certificate.
Configure the base information for the root certificate in the
[ CA_default ] section.
Create a root certificate signing policy to be used as the root certificate for issuing intermediate CA certificates only.
Create a relatively loose certificate signing policy for the intermediate CA certificate, and the intermediate CA certificate can issue certificates for third-party end users and servers.
[ req] section mainly defines the default options when creating certificates and certificate requests.
[ req_distinguished_name ] section defines information descriptions and default values commonly used when generating certificate requests.
openssl, in addition to configuring the basic necessary information, there are a number of extensible options available for certificate issuance.
Define the extended options for intermediate CA certificate issuance, where
pathlen:0 is to ensure that the intermediate CA certificate cannot issue new intermediate CA certificates.
[ user_cert] section constrains the policy for issuing client certificates, which are typically used for user authentication.
[ server_cert ] section, the certificate issuance policy when issuing server certificates is defined.
[ crt_ext ] section is used to define the policy when creating a certificate revocation list CRL.
Define the online certificate usage policy
3. Create root certificate private key
The generated root certificate private key must be absolutely secure and protected with
aes-256 encryption and a strong cipher.
genrsaGenerate the private key using RSA algorithm
-aes256encrypt the private key using the AES algorithm with a 256-bit key
-outoutput file directory
-4096Specify the length of the key
4. Create root certificate
Create a root certificate (
ca.cert.pem) using the root certificate private key (
ca.key.pem). You need to specify a long validity period for the root certificate; once the root certificate expires, all its issued certificates will be unusable. When using the
req command, you must specify a configuration file with
-config, otherwise the program defaults to the
/etc/pki/tls/openssl.cnf configuration file.
5. Validate the issued root certificate
The following information needs to be confirmed.
- Confirmation of the signature algorithm used in Signature Algorithm
- Confirm the validity of the certificate in Validity
- Confirm the length of the Public-Key
- Confirm the issuer information described by Issuer
- Confirm the certificate holder information specified by Subject, but note that the root certificate is a self-signed certificate, so Subject is the same as Issuer information
6. Create intermediate CA certificate key pair
Intermediate CA certificate is used to issue client certificate and server certificate instead of root certificate, and root certificate is responsible for issuing intermediate CA certificate and maintaining the trusted certificate chain. The use of intermediate certificate is mainly for security consideration because the private key of the root certificate corresponding to the root certificate always handles physical isolation and can guarantee the absolute security of the private key, and after the leakage of the private key of the intermediate CA certificate, the certificate can be revoked by the root certificate and a new intermediate certificate key pair can be reissued.
7. Create intermediate CA certificate directory structure
As with the root certificate class, we need to create a directory for the intermediate CA certificate to store the certificate. Here we save all the contents of intermediate certificate under
/root/ca/intermediate directory and create a directory structure similar to the root certificate.
Under the intermediate CA certificate directory, create a new text file named
crlnumber to save the certificate revocation list.
Create the issuing configuration file of intermediate CA certificate and save the configuration file of intermediate CA certificate in
/root/ca/intermediate/openssl.cnf, which modifies the saving directory of certificate compared with the root certificate configuration file.
The complete configuration file reads as follows.
8. Generate intermediate CA certificate private key
Create intermediate certificate private key, encrypted with
9. Generate intermediate CA certificate signing request
intermediate.key.pem for generating intermediate CA certificate signing request and use the root certificate to sign the certificate at the same time. Note that
Common Name cannot be duplicated. If you use intermediate CA to issue server certificate and client certificate, you need to specify the configuration file
10. Use root certificate to generate intermediate CA certificate
Use the root certificate to issue the intermediate CA certificate and specify the
v3_intermediate_ca extended attribute for it to issue the certificate and confirm the use of
openssl.cnf of the root certificate to issue the intermediate certificate, and note that the expiration date of the intermediate CA certificate is shorter than that of the root certificate.
index.txtis the file database of openssl, do not modify or delete the file directly.
11. Verify the content of intermediate CA certificate
Use the root certificate to verify the certificate chain trust relationship of the intermediate certificate,
OK means the certificate chain is trusted.
12. Create certificate chain file
When an application attempts to verify the validity of an issued certificate, it attempts to verify the intermediate CA certificate and the parent root certificate. In the certificate chain file, both the root certificate and the intermediate certificate must be included, because the client does not know the root certificate of the intermediate CA certificate used to issue the certificate, and the root certificate can be imported to the trusted root certificate authority in all clients that need access, so that the certificate issued by that CA is fully trusted and only the intermediate CA certificate is specified in the certificate chain file.
Issuing server certificates and client certificates
Next, we will use the intermediate CA certificate to issue a certificate for the server, which is used to encrypt the data communication between the server and the client. If a certificate issuance request (CSR) has already been created at the server using openssl, the process of creating the private key for the server certificate can be skipped.
1. Create server certificate private key
In the creation of intermediate CA certificate and root certificate,
4096bit cipher encryption is used, and server and client certificates are generally valid for several years, and
2048bit encryption can be used. Although
4096bit cipher is more secure, it will slow down the access speed when the server performs TLS handshake and digital signature verification, so it is generally recommended to use
2048Bit cipher encryption for web servers. If you set a password when you create a private key, you will need to provide this password every time the service restarts, so it is not recommended to set password protection for server certificates, and you can use the
aes256 cryptographic algorithm for encryption.
2. Create a server certificate issuance request
Use the created server private key to generate certificate issuance request. The
csr file of certificate issuance request does not need to specify intermediate CA certificate authority, but must specify
Common Name, which is usually domain name or IP address.
3. Create a server certificate from a certificate issuance request
Note: When issuing a certificate, the
-extensionsoption is used to specify the type of certificate to be issued, usually
server_certfor server certificates and
usr_certfor client certificates. After the certificate is issued, a new record will be added to the
4. View issued certificates
Use CA certificate chain to verify the trust relationship of the issued certificate.
5. Deployment Certificates
After the certificate is issued, the certificate can be deployed on the application server.
At this point, we have completed the whole process of creating private certificate center, intermediate certificate, certificate chain and issuing digital certificate. In actual use, we will also face the problem of certificate expiration rotation, including the renewal of application certificate, intermediate certificate and root certificate. For the application certificate and intermediate certificate rotation update is not a big problem, the main thing is how to update the root certificate, because once the root certificate is updated, so the certificates based on the root certificate should be updated. When performing root certificate update, the certificate center generates a new pair of secret key pair and root certificate, and then uses the new private key to generate an intermediate certificate for the old public key signature, and uses the old private key to generate another intermediate certificate for the new public key signature. In this way, both the new root certificate and the intermediate certificate issued by the old root certificate can be used normally during the update, thus realizing a smooth upgrade transition of the old and new root certificates.