Generate a JSON web key set in Ruby from a .pem cert

jwkmaker.rb

require 'openssl'
require 'base64'
require 'json'

# Open certificate file
cert = OpenSSL::X509::Certificate.new(File.read('certificate.pem'))

modulus = cert.public_key.n.to_s(2)
modulus_b64 = Base64.urlsafe_encode64(modulus, padding: false)

# Extract x5c from certificate
x5c = cert.to_der
x5c_b64 = Base64.urlsafe_encode64(x5c, padding: false)

# Extract x5t from certificate
x5t = OpenSSL::Digest::SHA1.new(cert.to_der).to_s
x5t_b64 = Base64.urlsafe_encode64(x5t, padding: false)

# Extract kid from certificate
kid = OpenSSL::Digest::SHA1.new(cert.to_der).to_s
kid_b64 = Base64.urlsafe_encode64(kid, padding: false)

jwk = { keys: [{
    alg: "RS256",
    kty: 'RSA',
    use: 'sig',
    n: modulus_b64,
    e: 'AQAB',
    x5c: [x5c_b64],
    x5t: x5t_b64,
    kid: kid_b64
  }
]}

puts jwk.to_json