OpenSSL

Symmetric encryption

We can use enc command. For more information try man openssl-enc. We are going to use PBKDF2 to increase security. For a simple explanation of this technique refer to this page. For generating password we are going to use -aes-256-cbc For more information refer to AES and Cipher block chaining (CBC):

openssl enc --pbkdf2 -aes-256-cbc -in plaintext.txt -out secret.txt

You can try openssl enc --list to see all available symmetric algorithms. For decrypting we must use -d flag:

openssl enc --pbkdf2 -aes-256-cbc -d -in secret.txt -out plain.txt

Asymmetric encryption

Generating a private key

Ware going to use RSA (Rivest–Shamir–Adleman) algorithm. According to openssl documentation (try man openssl), genrsa is superseded by genpkey. So using the latter is recommended. For more information try man openssl-genpkey

openssl genpkey -algorithm RSA -out private.pem

The output is in Privacy-Enhanced Mail (PEM) format. Basically it utilizes base64 to convert the binary format Distinguished Encoding Rules (DER) into a text one that is easily transferable in text-based protocols like HTTP. The PEM format has the following structure:

-----BEGIN label-----
data
-----END label-----

In our case label is PRIVATE KEY:

-----BEGIN PRIVATE KEY-----
data
-----END PRIVATE KEY-----

The traditional method: genrsa

If we use genrsa command to generate a private key. It's in SSLeay format. Openssl documentation (try man openssl-rsa) recommend to use PKCS#8. Note that in SSLeay the label is RSA PRIVATE KEY, instead of PRIVATE KEY:

-----BEGIN RSA PRIVATE KEY-----
data
-----END RSA PRIVATE KEY-----

We're going to generate a private key in SSLeay format:

openssl genrsa -out ssleay.pem 2048

Then we convert it into PKCS#8 using the following command:

openssl pkcs8 -in ssleay.pem -topk8 -nocrypt -out private.pem

Now our private key should have the following structure (Note that label doesn't have RSA anymore):

-----BEGIN PRIVATE KEY-----
data
-----END PRIVATE KEY-----

Generating a public key

We use rsa command to generate a public key. For more information try man openssl-rsa. 

openssl rsa -in private.pem -out public.pem -pubout

The public key is in PEM format. Now we can use the following command to encrypt and decrypt a file:

echo "hello world" > ./plain.txt
openssl rsautl -encrypt -inkey public.pem -pubin -in plain.txt -out rsa-encrypted.txt
openssl rsautl -decrypt -inkey private.pem -in rsa-encrypted.txt -out rsa-decrypted.txt
cat rsa-decrypted.txt

Self-signed Certificates

If you want to have a https web server in your local network. First you need to create the certificate and then manually add it to your web browser to avoid warnings.

Self-signed Certificate

Create a config file as below. According to OpenWrt wiki, browsers usually only look at one key part of a self-signed certificate that is CN (common name). However, starting with Chrome 58, SAN (subject alt name of DNS name) is also checked. So it is important in the following config CN matches one of DNS fields with a valid local DNS. Also IP fields should have the correct local IP address.

Single Domain Config

Let's assume we want to create a certificate for foo.lan, then IP should have the result IP address of the following ping command:

$ ping foo.lan
PING foo.lan (192.168.1.1)...
...

Using above information, we have:

$ cat /etc/ssl/myconfig.conf

[req]
distinguished_name  = req_distinguished_name
x509_extensions     = v3_req
prompt              = no
string_mask         = utf8only

[req_distinguished_name]
C                   = CA
ST                  = QC
L                   = SomeCity
O                   = OpenWrt
OU                  = Home Router
CN                  = foo.lan

[v3_req]
keyUsage            = nonRepudiation, digitalSignature, keyEncipherment
extendedKeyUsage    = serverAuth
subjectAltName      = @alt_names

[alt_names]
DNS                 = foo.lan
IP                  = 192.168.1.1

Subdomains Config using Wildcards

Let's assume 192.168.1.1 hosts our web server. We want to have a certificate for foo.lan, sub1.foo.lan and sub2.foo.lan and all other future subdomains (e.g. bar.foo.lan). Let's assume from 192.168.1.2 we want to access our web server. The following ping command should be resolved to 192.168.1.1 from 192.168.1.2:

$ ping foo.lan
PING foo.lan (192.168.1.1)...
...

$ ping sub1.foo.lan
PING foo.lan (192.168.1.1)...
...

$ ping sub2.foo.lan
PING foo.lan (192.168.1.1)...
...

If they don't, you need to add domain and its subdomain to /etc/hosts of 192.168.1.2 or use better alternatives. Now we are using the following config in 192.168.1.1:

$ cat /etc/ssl/myconfig.conf

[req]
distinguished_name  = req_distinguished_name
x509_extensions     = v3_req
prompt              = no
string_mask         = utf8only

[req_distinguished_name]
C                   = CA
ST                  = QC
L                   = Montreal
O                   = FreeDove
OU                  = FreeDove
CN                  = *.foo.lan

[v3_req]
keyUsage            = nonRepudiation, digitalSignature, keyEncipherment
extendedKeyUsage    = serverAuth
subjectAltName      = @alt_names

[alt_names]
DNS.1               = foo.lan
IP                  = 192.168.1.1
DNS.2               = *.foo.lan

man x509v3_config describes the reason for DNS.1 and DNS.2:

Due to the behaviour of the OpenSSL conf library the same field name can only occur once in a section. This means that:

        subjectAltName=@alt_section

        [alt_section]

        email=steve@here
        email=steve@there

will only recognize the last value. This can be worked around by using the form:

        [alt_section]

        email.1=steve@here
        email.2=steve@there

Generate Certificate and Private Key

You can generate a certificate and a private key by:

$ sudo openssl req -x509 -nodes -days 730 -newkey rsa:4096 -keyout /etc/ssl/private/mycert.key -out /etc/ssl/certs/mycert.crt -config /etc/ssl/myconfig.conf

You can try below commands to see other options for X.509:

$ man openssl-req
$ man openssl-x509

For more information read OpenWrt wiki.

View Certificate

You can see certificate contents by:

$ openssl x509 -pubkey -fingerprint -text -in /etc/ssl/certs/mycert.crt

The output is public key, fingerprint, signature algorithm, issuer and subject names, serial number, start and expiry dates, and so on. For more information:

$ man openssl-x509

You can see the structure of X.509 certificate in this wiki.

Add Self-Signed Certificate to Linux Client

We need to manually add mycert.crt to your favorite browser in Linux. You can use scp or other alternatives to copy mycert.crt into you client computer. Then use the following command for Chromium and Evolution:

$ certutil -d sql:$HOME/.pki/nssdb -A -t "CT,C,c" -n foo -i mycert.crt

For Firefox you need to find Firefox home directory. Usually it's in ~/.mozilla/firefox/[string].default-release ([string] is a random string). Then use the following command:

$ certutil -d ~/.mozilla/firefox/[string].default-release -A -t "CT,C,c" -n foo -i mycert.crt

For viewing items in Firefox database:

$ certutil -d ~/.mozilla/firefox/[string].default-release -L
Certificate Nickname                                         Trust Attributes
                                                             SSL,S/MIME,JAR/XPI

foo                                                          CT,C,c

To delete it:

$ certutil -d ~/.mozilla/firefox/[string].default-release -D -n foo

For more information:

Checking for Expiry Date

Note that private and public keys don't have expiry date; only certificates have. Usually certificates are in X.509. An X.509 certificate contains a public key and an identity (a hostname, or an organization, or an individual), and is either signed by a certificate authority or self-signed.

Check Certificate Expiry Date

You can open certificate file (the extension can be crt, pem, ...) with a text editor or running the following command to see its expiry date:

openssl x509 -enddate -noout -in file.crt

For printing both start and end date:

openssl x509  -startdate -enddate -noout -in file.crt

or you can use the following command:

openssl x509 -dates -noout -in file.crt

For more information run man openssl-x509 or openssl x509 -help.

CRL Text Format

If you want to see a CRL file (certificate revocation list) last update and next update (expiry date), you can run:

openssl crl -lastupdate -nextupdate -noout -in crl.key

If you want to see its text version:

openssl crl -text -in crl.pem

If you don't want to see the actual CRL, use the following command:

openssl crl -text -noout -in crl.pem

For more information run man openssl-crl or openssl crl -help.

Useful Links