Convert an RSA public key from PKCS#8 format to PKCS#1 format

convert_key.rb

#!/usr/bin/env ruby
#
# Converts an RSA public key from PKCS#8 format to PKCS#1 format. This is useful if you don't have OpenSSL 1.1
# See https://stackoverflow.com/questions/18039401/how-can-i-transform-between-the-two-styles-of-public-key-format-one-begin-rsa

key = STDIN.read

lines = key.split("\n")
only_key = lines[1..-2].join("")
without_header = only_key[32..-1]
new_key = without_header.chars.each_slice(64).map { |chars| chars.join }.join("\n")

puts "-----BEGIN RSA PUBLIC KEY-----"
puts new_key
puts "-----END RSA PUBLIC KEY-----"