Public and Private Keys in OpenSSL

Here is how to generate a private key with a size of 2048 bit:

openssl genrsa -out nicolas-rsa.pem 2048

Here is how to decompose the generated privated key:

openssl rsa -text -in nicolas-rsa.pem -noout

This displays the elements of the private key: modulus, publicExponent, privateExponent, prime1, prime2, exponent1, exponent2 and coefficient.

Here is how to get the public key:

openssl rsa -in nicolas-rsa.pem -pubout -text

Here is how to get the public key writen in a file:

openssl rsa -in nicolas-rsa.pem -pubout -out nicolas-rsa-pub.pem

Let us write a message into a file:

echo -n 'this is a message' > file.txt

This is how to encrypt this file with our public key:

openssl pkeyutl -encrypt -inkey nicolas-rsa-pub.pem -pubin -in file.txt -out encrypted-file.txt

This is how to now decrypt the encrypted file with the private key:

openssl pkeyutl -decrypt -inkey nicolas-rsa.pem -in encrypted-file.txt -out result.txt

If we want to use elliptic cryptography instead of rsa as the algorithm, let us display all of the possible types of curves:

openssl ecparam -list_curves

A prime256v1 curve is a standard option. Let us generate a private key with this option:

openssl ecparam -genkey -name prime256v1 -out ecc-key.pem

With an additional -noout option, the elliptic curve parameters are not included in the file:

openssl ecparam -genkey -name prime256v1 -noout -out ecc-key.pem

About the pem file extension: pem means "Privacy Enhanced Mail". A pem file may contain almost anything base64 encoded and wrapped with BEGIN and END lines.

About the der file extension: DER means Distinguished Encoding Rules. Data in der files (for instance X.509 certificates or private keys) are in binary format. DER files do not contain human readable plain text.

Here is how to print elements about our ecc key :

openssl ec -in ecc-key.pem -text

This is how to save the public key in a file:

openssl ec -in ecc-key.pem -pubout -out ecc-pub.pem

Creating a root certificate

A root certificate and the associated private key can be created with:

openssl req -x509 -sha256 -days 365 -key ecc-key.pem -out ecc-cert.pem

and then displayed with:

openssl x509 -in ecc-cert.pem -text -noout

Here is how to create a certificate using a configuration file called certificate.conf (and we use the private key nicolas-rsa.pem):

openssl req -x509 -days 365 -key nicolas-rsa.pem -config certificate.conf -out cert-config.pem

openssl x509 -in cert-config.pem -text -noout

The same certificate can be created without any prompting using a conf file stating prompt = no:

countryName = US
stateOrProvinceName = California
localityName = San Francisco
organizationName = Nicolas Ltd
organizationalUnitName = Information Department
commonName = nicolas-ltd.com
emailAddress = info@nicolas-ltd.com

[ v3_extensions ]
subjectKeyIdentifier = hash
# copies the subject key identifier (or if it fails then it uses the issuer and serial number)
authorityKeyIdentifier = keyid, issuer
# whether the certificate is a root or not
basicConstraints = critical, CA:true, pathlen:2
# key usage- use the certificate for signing or key encipherment
keyUsage = digitalSignature, keyEncipherment

Here is the command to create the certificate:

openssl req -x509 -days 365 -key nicolas-rsa.pem -config certificate_no_prompt.conf -out cert-config.pem

and here is how to display the certificate thus created:

openssl x509 -in cert-config.pem -text -noout

Creating a Certificate Signing Request

Here is how to generate and then display the CSR:

openssl req -key ecc-key.pem -new -out ecc-inter-cert-csr.pem -config cert-csr.conf

openssl req -in ecc-inter-cert-csr.pem -text

Given the private key ca-key.pem generated by the root authority with command:

openssl genrsa -out ca-key.pem 2048

Here is how the root authority can generate and display the root certificate:

openssl req -x509 -sha256 -days 365 -key ca-key.pem -out ca-root-cert.pem -config certificate.conf

openssl x509 -in ca-root-cert.pem -text -noout

Here is how the authority can create the signed intermediate certificate:

openssl x509 -req -CA ca-root-cert.pem -CAkey ca-key.pem -in ../ecc-inter-cert-csr.pem -out intermediate-cert.pem -days 365

In case there are extensions in the CSR to include, a -copy_extensions=copyall option has to be added to the same command. The signed intermediate certificate can be displayed with:

openssl x509 -in intermediate-cert.pem  -text -noout

Verifying the chain of trust

Here is how to verify the signature of the intermediate certificate using the public key of the root certificate:

openssl verify -CAfile ca-root-cert.pem intermediate-cert.pem

The v3 extensions of the root certificate need to include the keyCertSign value for the keyUsage extension, e.g.

keyUsage = keyCertSign, digitalSignature, keyEncipherment