When opening pidgin on Ubuntu Linux I received the following warning dialogue with the message “Accept certificate for gmail.com?”. Clicking on “View Certificate” showed the second dialogue.
So how do you verify this?
To start with get the actual certificate from gmail and put it in a file
> openssl s_client -connect gmail.com:443 > cert.pub
The file contents should look something like this:
CONNECTED(00000003)
depth=1 C = ZA, O = Thawte Consulting (Pty) Ltd., CN = Thawte SGC CA
verify error:num=20:unable to get local issuer certificate
verify return:0
---
Certificate chain
 0 s:/C=US/ST=California/L=Mountain View/O=Google Inc/CN=mail.google.com
  i:/C=ZA/O=Thawte Consulting (Pty) Ltd./CN=Thawte SGC CA
 1 s:/C=ZA/O=Thawte Consulting (Pty) Ltd./CN=Thawte SGC CA
  i:/C=US/O=VeriSign, Inc./OU=Class 3 Public Primary Certification Authority
---
Server certificate
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----
subject=/C=US/ST=California/L=Mountain View/O=Google Inc/CN=mail.google.com
issuer=/C=ZA/O=Thawte Consulting (Pty) Ltd./CN=Thawte SGC CA
---
No client certificate CA names sent
---
SSL handshake has read 2005 bytes and written 285 bytes
---
New, TLSv1/SSLv3, Cipher is ECDHE-RSA-RC4-SHA
Server public key is 1024 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
SSL-Session:
   Protocol : SSLv3
   Cipher   : ECDHE-RSA-RC4-SHA
   Session-ID: 45F9A9FA76661A382878C54AD89EB033C1D8CABB1840F6C154B32F406EC05D75
   Session-ID-ctx:
   Master-Key: 11FA086DFD76443E656F2C487A52B4BCF83A3F7B65C390A15FC2D876EE64E1EBF9FD1B9E8A22E5980D77CD86A11B2BE8
   Key-Arg  : None
   PSK identity: None
   PSK identity hint: None
   Start Time: 1331313945
   Timeout  : 7200 (sec)
   Verify return code: 20 (unable to get local issuer certificate)
Then calculate the fingerprint.
> openssl x509 -noout -fingerprint -in cert.pub
SHA1 Fingerprint=59:29:78:A7:2A:90:61:F7:0A:D7:C4:4C:4D:44:9D:CF:25:8C:D5:34
The above fingerprint is different than the one Pidgin was warning me about. In this case I rejected the certificate, told pidgin to re-connect and on the second attempt got a valid certificate.
