iOS 4.0 supports SNI, which makes it the first mobile OS to support the server_name TLS extension. Hopefully Android, WebOS, WM7, et al follow suit!

(Oh, and I’m not dead. WP 3.0 comes out shortly so expect a major CDN Tools update as well as a brand new plugin!)

As of r39934 Chromium now supports the server_name TLS extension (server name indication) in OS X (latest build). This support requires OS X 10.5.7 or later. Hopefully it’ll make its way into a dev/beta/stable release of Google Chrome itself soon.

For those who are more curious than they ought to be about how I wrote this patch… Apple added support in their Secure Transport library for the server_name TLS extension, but has not updated their documentation. As of 10.5.7 (or possibly 10.5.6) the SSLSetPeerDomainName function — which is ostensibly used for OS level certificate verification — causes OS X to send the server_name extension in the TLS client hello. However, since Chromium doesn’t use OS X’s built-in verification it wasn’t passing this data through prior to the patch.

To test you can hit up my IDN SNI site https://☣.ws/ or https://alice.sni.velox.ch/. The former will throw a certificate error if you are on a non-SNI enabled browser and the latter will have text stating that the SNI extension is missing.

Server Name Indication (SNI), an extension to TLS, allows browsers that support it to connect to SSL hosts that do not have dedicated IPs (much like standard http virtual hosting has worked for years). This extension, however, must be supported on both the server and client side. Microsoft has not yet chosen to support it (maybe IIS 8?), but the Apache project did with the 2.2.12 release. Recently, Ubuntu 9.10 Server became the first server distribution to ship with Apache and OpenSSL built with the appropriate flags, so if you’d like to follow along you can use a 9.10 VM.

In the ideal case everything is the same as a regular vhost, but you’ll first need to enable SSL. On Ubuntu this requires you to run a2enmod and type “ssl”. After that you’ll need to add

NameVirtualHost *:443

to the root conf, then make your VirtualHost much like a normal one. A very basic pair of vhosts is seen below.

<VirtualHost *:443>
	ServerAdmin webmaster@localhost
 
	DocumentRoot /my/doc/root
	ServerName mydomain.com
	SSLEngine On
	SSLCertificateFile /path/to/domain.crt 
	SSLCertificateKeyFile /path/to/domain.key
</VirtualHost>
<VirtualHost *:443>
	ServerAdmin webmaster@localhost
 
	DocumentRoot /my/doc/root
	ServerName mydomain2.com
	SSLEngine On
	SSLCertificateFile /path/to/domain2.crt 
	SSLCertificateKeyFile /path/to/domain2.key
</VirtualHost>

These vhosts should be placed in different includes ideally, but it isn’t required. If you just want to test with a self-signed certificate you can create one with

openssl req -new -nodes -keyout mykey.key -out mycert.cer -days 3650 -x509

You’ll need to specify the domain name you want in the “Common Name” section.

Once you’ve got all this done you can restart apache and test it out! If you test on a browser that doesn’t support SNI (IE on XP) you’ll get the SSL cert for the first vhost apache parses. To disable accessing it on non-SNI hosts you can add

SSLStrictSNIVHostCheck on

to the root conf. This will cause a 403 error for those browsers.

If you’d like to see an example implementation of SNI you can check out my IDN domains https://☢.ws/ and https://☣.ws/. These sites are hosted on the same IP with different SSL certificates. I have strict host checking turned on so visiting them with a non-SNI capable browser will result in a 403 error.¹

If you’re running a Microsoft CA and you want to be able to accept enrollment requests from clients supporting keygen (Firefox, Safari, Opera, et cetera) you’ve probably found that the /certsrv/ page allows enrollment, but the requests fail when you attempt to issue the certificate.  This is because the server is not parsing the subject attributes from the request.  To fix this, run the following on your server as administrator on the command line.

certutil -setreg ca\CRLFlags +CRLF_ALLOW_REQUEST_ATTRIBUTE_SUBJECT

You can also set your server to auto-issue on request for certain certificate profiles.  To do this add the CA snap-in and get properties of your CA. Under the policy module tab click properties again and click the “Follow the settings..” radio button.

Recently I needed to do some performance testing of an SSL instance on a VM. I considered using JMeter, but decided to use OpenSSL to get a rudimentary picture instead.

To obtain a basic result, we connect to the server and pull the /index.php file. You can specify whatever file you’d like to download, or none at all if you simply want to test connections.¹

openssl s_time -www /index.php -new -connect www.trustwave.com:443

Your result will look something like this:

No CIPHER specified
Collecting connection statistics for 30 seconds
ttttttttttttttttttttttttttttttttttttttttttttttttttttttttt
159 connections in 5.82s; 27.32 connections/user sec, bytes read 62328
159 connections in 31 real seconds, 392 bytes read per connection

If you’d like to get more specific with performance testing you can even use the -ciphers parameter to explicitly choose the negotiated cipher. You can obtain a list of available ciphers with “openssl ciphers”.

OpenSSL provides several tools that allow you to RSA encrypt/sign arbitrary data files. Of course, directly RSA encrypting large volumes of data is impractical because the encrypted/signed data cannot exceed the size of the key material. This is one of the reasons why SSL connections typically handshake and then pass an AES (or RC4, et cetera) key to do symmetric encryption thereafter.¹

Generate a private key. You can change the last number to the preferred modulus size. Keys greater than 4096-bit will take a long time to generate.²

openssl genrsa -out private.pem 4096

With the private key we can now encrypt the data.

openssl rsautl -encrypt -inkey private.pem -in publicfile -out privatefile

To decrypt just reverse it.

openssl rsautl -decrypt -inkey private.pem -in privatefile -out publicfile

If you would rather sign the data…

openssl rsautl -sign -inkey private.pem -in filetosign -out signed_data

To verify the signature just use -verify.³

openssl rsautl -verify -inkey private.pem -in signed_data

Continuing the howto nature of this blog (and its peculiar obsession with OpenSSL), here’s a primer on packaging an arbitrary number of certificates into a single PKCS7 container. These files are quite useful for installing multiple certificates on Windows servers. They differ from PKCS12 (PFX) files in that they can’t store private keys. If you need to generate a PKCS12 then head to that article instead.

This example assumes that you have 2 different certificate files, each in PEM (Base64) format. You can add as many -certfile elements as you want to package in the file. Additionally, concatenated certificate chains are supported. ¹

openssl crl2pkcs7 -nocrl -certfile cert1.cer -certfile cert2.cer -out outfile.p7b

If you deal with SSL/TLS long enough you will run into situations where you need to examine what certificates are being presented by a server to the client. The best way to examine the raw output is via (what else but) OpenSSL.¹

First let’s do a standard webserver connection (-showcerts dumps the PEM encoded certificates themselves for more extensive parsing if you desire. The output below snips them for readability.):

openssl s_client -showcerts -connect www.domain.com:443

CONNECTED(00000003)
--snip--
---
Certificate chain
 0 s:/C=US/ST=Texas/L=Carrollton/O=Woot Inc/CN=*.woot.com
   i:/C=US/O=SecureTrust Corporation/CN=SecureTrust CA
-----BEGIN CERTIFICATE-----
--snip--
-----END CERTIFICATE-----
 1 s:/C=US/O=SecureTrust Corporation/CN=SecureTrust CA
   i:/C=US/O=Entrust.net/OU=www.entrust.net/CPS incorp. by ref. (limits liab.)/OU=(c) 1999 Entrust.net Limited/CN=Entrust.net Secure Server Certification Authority
-----BEGIN CERTIFICATE-----
--snip--
-----END CERTIFICATE-----
---
Server certificate
subject=/C=US/ST=Texas/L=Carrollton/O=Woot Inc/CN=*.woot.com
issuer=/C=US/O=SecureTrust Corporation/CN=SecureTrust CA
---
No client certificate CA names sent
---
SSL handshake has read 2123 bytes and written 300 bytes
---
New, TLSv1/SSLv3, Cipher is RC4-MD5
Server public key is 1024 bit
--snip--

There’s a lot of data here so I have truncated several sections to increase readability. Points of interest:

1. The certificate chain consists of two certificates. At level 0 there is the server certificate with some parsed information. s: is the subject line of the certificate and i: contains information about the issuing CA.
2. This particular server (www.woot.com) has sent an intermediate certificate as well. Subject and issuer information is provided for each certificate in the presented chain. Chains can be much longer than 2 certificates in length.
3. The server certificate section is a duplicate of level 0 in the chain. If you’re only looking for the end entity certificate then you can rapidly find it by looking for this section.
4. No client certificate CAs were sent. If the server was configured to potentially accept client certs the returned data would include a list of “acceptable client CAs”.
5. Connection was made via TLSv1/SSLv3 and the chosen cipher was RC4-MD5. Incidentally, this typically means that the server you’re connecting to is IIS.

But what if you want to connect to something other than a bog standard webserver on port 443? Well, if you need to use starttls that is also available. As of OpenSSL 0.9.8 you can choose from smtp, pop3, imap, and ftp as starttls options.

openssl s_client -showcerts -starttls imap -connect mail.domain.com:139

If you need to check using a specific SSL version (perhaps to verify if that method is available) you can do that as well. -ssl2, -ssl3, -tls1, and -dtls1 are all choices here.²

openssl s_client -showcerts -ssl2 -connect www.domain.com:443

You can also present a client certificate if you are attempting to debug issues with a connection that requires one.³

openssl s_client -showcerts -cert cert.cer -key cert.key -connect www.domain.com:443

And for those who really enjoy playing with SSL handshakes, you can even specify acceptable ciphers.⁴

openssl s_client -showcerts -cipher DHE-RSA-AES256-SHA -connect www.domain.com:443

The cipher used above should work for almost any Apache server, but will fail on IIS since it doesn’t support 256-bit AES encryption.

Have you ever wondered how big the “large primes” that RSA encryption is based on really are? What exactly does a “1024-bit” key mean anyway? And if the difficulty of RSA is partially based on factoring large numbers, how do we create these large primes without determining primality via factorization?

The easiest way to demonstrate these concepts is with a simple script, so let’s take a look at a large random number generator I wrote¹ using Python. As is typical for this blog, you’ll note that the random numbers are not cryptographically secure. To make these examples simple I don’t want to introduce external dependencies, but please keep that issue in mind for any serious applications of the code presented here.

import random
import math
import sys
 
def rabinMiller(n):
     s = n-1
     t = 0
     while s&1 == 0:
         s = s/2
         t +=1
     k = 0
     while k<128:
         a = random.randrange(2,n-1)
         #a^s is computationally infeasible.  we need a more intelligent approach
         #v = (a**s)%n
         #python's core math module can do modular exponentiation
         v = pow(a,s,n) #where values are (num,exp,mod)
         if v != 1:
             i=0
             while v != (n-1):
                 if i == t-1:
                     return False
                 else:
                     i = i+1
                     v = (v**2)%n
         k+=2
     return True
 
def isPrime(n):
     #lowPrimes is all primes (sans 2, which is covered by the bitwise and operator) 
     #under 1000. taking n modulo each lowPrime allows us to remove a huge chunk 
     #of composite numbers from our potential pool without resorting to Rabin-Miller
     lowPrimes =   [3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,97
                   ,101,103,107,109,113,127,131,137,139,149,151,157,163,167,173,179
                   ,181,191,193,197,199,211,223,227,229,233,239,241,251,257,263,269
                   ,271,277,281,283,293,307,311,313,317,331,337,347,349,353,359,367
                   ,373,379,383,389,397,401,409,419,421,431,433,439,443,449,457,461
                   ,463,467,479,487,491,499,503,509,521,523,541,547,557,563,569,571
                   ,577,587,593,599,601,607,613,617,619,631,641,643,647,653,659,661
                   ,673,677,683,691,701,709,719,727,733,739,743,751,757,761,769,773
                   ,787,797,809,811,821,823,827,829,839,853,857,859,863,877,881,883
                   ,887,907,911,919,929,937,941,947,953,967,971,977,983,991,997]
     if (n >= 3):
         if (n&1 != 0):
             for p in lowPrimes:
                 if (n % p == 0):
                     return False
             return rabinMiller(n)
     return False
 
def generateLargePrime(k):
     #k is the desired bit length
     r=100*(math.log(k,2)+1) #number of attempts max
     r_ = r
     while r>0:
        #randrange is mersenne twister and is completely deterministic
        #unusable for serious crypto purposes
         n = random.randrange(2**(k-1),2**(k))
         r-=1
         if isPrime(n) == True:
             return n
     return "Failure after "+`r_` + " tries."
 
print generateLargePrime(1024)

This code is very slow, but does not represent an entirely naïve approach.²  To generate a prime we first create a random integer in the range (2^k-1,2^k), then the following rules are applied:

1. The number (n) must be >=3.  While 2 is a prime number, for our purposes we have no interest in numbers less than 3.
2. Do a bitwise and (n&1).  If the result is not 0 then we know the number is even and can throw it out.
3. Check that n%p is 0 (in other words, that n is not divisible evenly by p) for all primes <1000.  This check will eliminate a large quantity of composite numbers and prevent the expense of the next test.
4. Finally we reach the core test: Rabin-Miller.  This algorithm, if it returns true, states that there is a 75% chance that the number is prime (for the randomly chosen basis a).  To obtain a 2^-128 chance that the number is not prime, we must repeatedly run the Rabin-Miller test choosing different number bases. Since each iteration of the test increases our probability by a power of 2 we must iterate 64 times to reach this confidence level.

 
If the number passes all these tests it is returned as a valid prime number. Of course, if you don’t trust the output from this script you can always check it with openssl…

openssl prime 7337488745629403488410174275830423641502142554560856136484326749638755396267050319392266204256751706077766067020335998122952792559058552724477442839630133
8C18E5DC98684E2A15B84535635A95C4A192B73B40A780AB4CB0C58BDB9C31EF970C3AC6D804712B830FB6F1B140693A251E989F89B687EBA62781AD031D5135 is prime

 Click for an example of an 8192-bit prime created with the generateLargePrime() function. You can also check out a 1024-bit prime as well. 1024-bit keys are the minimum size recommended for end entity certificates using RSA (SSL certificates) at this time.³

Of course, a large prime is nice, but it isn’t necessarily an RSA prime. Look for another entry soon explaining the additional restrictions imposed by RSA.

On rare occasions you may find yourself with a self-signed internal CA that has expired while you are still using certificates issued from the CA. One potential solution to this problem is to self-sign a new cert with identical fields using the private key from the old certificate.¹

You can fill in almost all the fields using the interactive prompt, but to ensure maximum compatibility be sure every field matches exactly. You will also need to set the serial number of the certificate via the -set_serial parameter (openssl takes this argument in decimal form, not hex)².

openssl req -new -x509 -key previousprivatekey.pem -set_serial 0000 -out newroot.cer

You now have a new root certificate that will work with your previously issued certificates!