Ruby OpenSSL::SSL::SSLError on CentOS 6.2 - ruby

Question

I'm trying to run the following code (taken from codeacademy) in CentOS 6.2:
require 'rubygems'
require 'oauth'
# Change the following values to those provided on dev.twitter.com
# The consumer key identifies the application making the request.
# The access token identifies the user making the request.
consumer_key = OAuth::Consumer.new(
"MY_KEY",
"MY_SECRET")
access_token = OAuth::Token.new(
"STRING1",
"STRING2")
# All requests will be sent to this server.
baseurl = "https://api.twitter.com"
# The verify credentials endpoint returns a 200 status if
# the request is signed correctly.
address = URI("#{baseurl}/1.1/account/verify_credentials.json")
# Set up Net::HTTP to use SSL, which is required by Twitter.
http = Net::HTTP.new address.host, address.port
http.use_ssl = true
http.verify_mode = OpenSSL::SSL::VERIFY_PEER
# Build the request and authorize it with OAuth.
request = Net::HTTP::Get.new address.request_uri
request.oauth! http, consumer_key, access_token
# Issue the request and return the response.
http.start
response = http.request request
puts "The response status was #{response.code}"
and get the following error message:
/usr/lib/ruby/1.8/net/http.rb:586:in `connect': SSL_connect returned=1
errno=0 state=SSLv3 read server certificate B: certificate verify
failed (OpenSSL::SSL::SSLError)
The keys have been omitted (tehy are, after all, secret), but i'm using correct ones.
The neccessary gems are installed.
What might the problem be?

Answer 1

http = Net::HTTP.new address.host, address.port
http.use_ssl = true
http.verify_mode = OpenSSL::SSL::VERIFY_PEER
...
You also need:
http.ca_file = File.join(File.dirname(__FILE__), "ca-cert.pem")
Since its Tweeter:
$ openssl s_client -connect api.twitter.com:443
CONNECTED(00000003)
depth=1 C = US, O = "VeriSign, Inc.", OU = VeriSign Trust Network, OU = Terms of use at https://www.verisign.com/rpa (c)10, CN = VeriSign Class 3 Secure Server CA - G3
verify error:num=20:unable to get local issuer certificate
verify return:0
---
Certificate chain
0 s:/C=US/ST=California/L=San Francisco/O=Twitter, Inc./OU=Twitter Security/CN=api.twitter.com
i:/C=US/O=VeriSign, Inc./OU=VeriSign Trust Network/OU=Terms of use at https://www.verisign.com/rpa (c)10/CN=VeriSign Class 3 Secure Server CA - G3
1 s:/C=US/O=VeriSign, Inc./OU=VeriSign Trust Network/OU=Terms of use at https://www.verisign.com/rpa (c)10/CN=VeriSign Class 3 Secure Server CA - G3
i:/C=US/O=VeriSign, Inc./OU=VeriSign Trust Network/OU=(c) 2006 VeriSign, Inc. - For authorized use only/CN=VeriSign Class 3 Public Primary Certification Authority - G5
...
You need the top issuer, (i: at level 1), which is VeriSign Class 3 Public Primary Certification Authority - G5. You can get that from Public Root CA - VeriSign. The filename is PCA-3G5.pem.
After you download the root, you can then run s_client again and the server certificate will verify:
$ openssl s_client -connect api.twitter.com:443 -CAfile PCA-3G5.pem