Openssl Operations

Internal

• openssl
• Public Key Security

Generate a Public/Private Key Pair

Generate an RSA Private Key

This is the procedure to generate a public/private key pair. The keys can be further used in the procedure to generate digitally signed certificates, or even to configure ssh, though ssh has its own procedure to generate key pairs, which produces equivalent keys in the same PEM format.

openssl genrsa -out <keyfile-name>.pem <key-lenght>

openssl genrsa -out ./server-private-key.pem 2048

The command generates a RSA or DSA key of specified length in PEM format.

For more general considerations on private keys, see:

Private Keys

Inspect an RSA Private Key

openssl rsa -text -in ./server-private-key.pem

The output provides, between others, the length of the key.

Extract the Matching Public RSA Key from a Private Key

The matching public key can be always extracted from the private key.

openssl rsa -pubout -in ./server-private-key.pem -out ./server-public-key.pem

Note that the private key is protected by a password, the tool will require password at stdin. The key is written in a PKCS8 PEM format.

⚠️ The public key thus generated cannot be directed used with OpenSSH, for example in an authorized_key file, it has to be converted to a compatible OpenSSH format with ssh-keygen:

Convert an OpenSSL PEM Public Key for Use with OpenSSH

Generate a DSA Private Key

DSA key generation is a two-step process: the DSA parameters are created in the first step, and the key in the second:

openssl dsaparam -genkey 2048 | openssl dsa -out test-pvtkey.pem -aes128

Generate a Private Key

openssl genpkey -algorithm RSA

Get a Certificate Signed by a CA

Create a Certificate Signing Request (CSR)

This procedure generates a Certificate Signing Request (CSR) that should be sent to the certificate authority for signature. This step is part of the procedure to generate digitally signed certificates. The CSR command (openssl req) may use an existing private key, previously generated with openssl genrsa, or it can create a new private key.

If you want a field to be empty during the interactive phase, provide a single dot (.), rather then just hit Return. If you do the latter, OpenSSL will populate the corresponding field with the default value.

The challenge password is an optional field that was intended for use during certificate revocation as a way of identifying the original entity that had requested the certificate. If entered, the password will be included verbatim in the CSR and communicated to the CA. It’s rare to find a CA that relies on this field, so it's best not to include it. Having a challenge password does not increase the security of the CSR in any way. Further, this field should not be confused with the key passphrase, which is a separate feature.

To use an existing private key:

openssl req -new -key ./test-pk.pem -out ./test-csr.pem

The new CSR will be generated in PEM format as ./test-csr.pem.

To create a new private key at the time of creation of the certificate signing request, use the following command.

Note that the command will require private key password, which must be longer than four characters. The password can be provided interactively, during the certificate request creation process, or it can be specified in a file (no newline) and then provided to the command with -passout file:./password-file.txt. Provide a long, random password, and then chmod the password file to make it readable only by you. When the certificate is used, the tools using is usually offer the option of reading the password from the file. The password can be provided in-line with the -passin option.

openssl req -new -newkey rsa:2048 -sha256 -passout file:./password-file.txt -keyout ./test-pk.pem -out ./test-csr.pem

The new CSR will be generated in PEM format as ./test-csr.pem and a new private key will be written, also in PEM format as ./test-pk.pem.

Inspect a Certificate Signing Request (CSR)

openssl req -in ./test-csr.pem -noout -text

Generate the Digitally-Signed Certificate from CSR

The CSR submitted by the user will be used to generate a digitally-signed certificate. The Certificate Authority's private key will be used to sign the certificate. This operation can be used to generate a self-signed certificate, as part of the procedure to generate digitally signed certificates.

openssl x509 -req -days 365 -in ./test-csr.pem -signkey ./ca-private-key.pem -out ./test-certificate.pem

Self-Signed Certificates

Note that there is no need to create a CSR if all that is needed is a self-signed certificate. The procedure to create a self-signed certificate, based on the private key, as part of the procedure to generate digitally signed certificates.

openssl req -new -x509 -days 365 -key ./server-private-key.pem -out ./self-signed-server-certificate.pem -subj "/C=US/L=Menlo Park/O=Nova Ordis LLC/CN=www.novaordis.com"

Note that -subj is optional, if omitted, the data will be provided at stdin.

Inspect the Certificate

Certificates can be displayed with the openssl 'x509' command:

openssl x509 -noout -text -in ./server-certificate.pem

Options

-noout

Skip displaying the certificate in base64 format.

Obtain a Server Certificate

openssl s_client -connect nexus-cicd.apps.openshift.novaordis.io:443

The response includes the server's certificate:

[...]
Certificate chain
 0 s:/CN=*.apps.openshift.novaordis.io
[...]
---
Server certificate
-----BEGIN CERTIFICATE-----
MIIDRTCCAi2gAwIBAgIBEjANBgkqhkiG9w0BAQsFADAmMSQwIgYDVQQDDBtvcGVu
[...]
65vqsz8NTtde1vJ5qW31Af0pO9YehiSRfA==
-----END CERTIFICATE-----
subject=/CN=*.apps.openshift.novaordis.io
[...]

Key Format Conversions

PKCS#12 to PEM

The following command will convert PKCS#12 keys to PEM.

openssl pkcs12 -in ./test-pvtkey.p12 -out ./test-pvtkey.pem

Generate a HMAC

http://nwsmith.blogspot.com/2012/07/using-openssl-to-generate-hmac-using.html

Generates a HMAC using an MD5 digest algorithm:

echo -n "some value to be hashed and encrypted" | openssl dgst -md5 -hmac 'some-secret-key'

Generate an MD5 Hash

Generates the MD5 hash in hexadecimal format (default)

echo -n "some value to be hashed" | openssl dgst -md5 [-hex]

When using echo, make sure not to send the automatically generated "\n" to OpenSSL.

Generate the MD5 hash in binary format, which can optionally be base64 encoded:

echo -n "some value to be hashed" | openssl dgst -md5 -binary 

echo -n "some value to be hashed" | openssl dgst -md5 -binary | openssl base64

Encrypt/Decrypt

To encrypt:

openssl aes-256-cbc -pbkdf2 -in cleartext.txt | base64 > ciphertext-base64.txt

The password will be requested at the console.

-pbkdf2 triggers using PBKDF2 (Password-Based Key Derivation Function 2) algorithm.

To decrypt:

cat ciphertext-base64.txt | base64 -D | openssl aes-256-cbc -pbkdf2 -d > cleartext.txt