(Ruby) About RSA Public/Private Keys

This example provides some additional information for understanding public/private key pairs. In demonstrates how a private key is a superset of the public key. A public key contains the modulus and exponent. The matching private key also contains the modulus and exponent, but also contains the additional private key parts.

Chilkat Ruby Downloads install from rubygems.org gem install chilkat or download... Ruby Library for Windows, MacOS, Linux, Alpine Linux

require 'chilkat'

cert = Chilkat::CkCert.new()

# Load a digital certificate.
success = cert.LoadFromFile("digitalCert.cer")
if (success != true)
    print cert.lastErrorText() + "\n";
    exit
end

# A .cer file does not contain the private key.  It should contain
# the public key...
# pubKey is a CkPublicKey
pubKey = cert.ExportPublicKey()

# Let's have a look at it (in XML format).
print "Public Key from Certificate:" + "\n";
print pubKey.getXml() + "\n";

# An RSA public key consists of a modulus and exponent.

# An RSA private key includes both the modulus and exponent,
# as well as other "big" numbers: P, Q, D, etc.

# Let's load an RSA private key from a DER-encoded file:
privKey = Chilkat::CkPrivateKey.new()

success = privKey.LoadAnyFormatFile("PrivateKey.key","")
if (success != true)
    print privKey.lastErrorText() + "\n";

    exit
end

# If this private key is the matching half to the public key from
# the certificate, then the modulus and exponent should
# be identical.  (Thus, a "private key" really contains both the public part as well as the private parts...).
print "Private Key from DER:" + "\n";
print privKey.getXml() + "\n";