Thursday, January 21, 2016

Comparing SSL/TLS Facilities for z/VSE

Comparing SSL/TLS Facilities for z/VSE

We have recently received a number of questions about the SSL/TLS functionality available for IPv6/VSE. These questions are generally about the differences between IPv6/VSE SSL/TLS functionality and the SSL/TLS functionality available in the SSL for VSE feature of TCP/IP for VSE.

The question of SSL/TLS support is far more complex that just having an SSL/TLS client that is able to connect to z/VSE.

Most of the information presented here is also in the IBM publication z/VSE 6.1 TCP/IP Support SC34-2706-00 which provides information about SSL/TLS features available for both IPv6/VSE and TCP/IP for VSE. However, I will try to layout the information in a way that will allow you to easily compare the types of available SSL/TLS support.

For reference, when we talk about SSL/TLS sockets we are referring to a secure and trusted connection. The word secure means the connection is encrypted. The word trusted means the connection has been authenticated.

Authentication itself means the certificate presented to the SSL/TLS client by the server has been checked against the Certificate Authority's certificate to verify correctness, expiration, etc. Optionally, the SSL/TLS client can present a certificate to the SSL/TLS server allowing the server to authenticate the client too. This means that the SSL/TLS client and the SSL/TLS server are known to each other and have verified they are authorized to establish a connection.

The article will also refer to "Control Program Assist for Cryptographic Function" (CPACF). This is a feature of System z machines. The feature is disabled by default. After ordering and installing a System z machine, you much contact IBM to have this feature enabled. The feature provides hardware micro/millicode assist instructions to dramatically improve the performance of encryption and message digest/hash functions. CPACF features discussed in this article require using a z10 or newer machine.


To begin, lets look at the support itself. IPv6/VSE uses the IBM z/VSE port of the Open Source package OpenSSL. The OpenSSL package used by IBM has US government FIPS 142-2 certification and IBM itself updates the OpenSSL port with security patches and makes these updates available to customers in a timely fashion. US government certification of OpenSSL is a time consuming and expensive process where every part of the OpenSSL code is tested for correctness.

The SSL for VSE feature of TCP/IP for VSE was developed by Connectivity Systems, Inc. (CSI) and is exclusive to TCP/IP for VSE. I could find no information about the certification status of SSL for VSE.

A certified SSL/TLS solution is important because the certification process tests to make sure the SSL/TLS connections are made correctly and that no information is leaked in the process. Certification is far more that just being able to establish a connection or having someone tell you "Hey! I got connected!".

SSL/TLS Connection Types

IPv6/VSE supports SSLv3, TLSv1 and TLSv1.2 using RSA and Diffie-Hellmann key exchange.
SSL for VSE supports SSLv3 and TLSv1 using RSA key exchange.

While SSLv3 is supported, it should not be used. SSLv3 is completely broken and is little, if any, better than using plain text socket connections. The IBM OpenSSL port plans to discontinue SSLv3 support in the near future.

While TLSv1 is secure, IPv6/VSE supports TLSv1.2 connections. TLSv1.2 is the current TLS standard and is more secure than TLSv1.

While both SSL/TLS solutions support RSA keys, IPv6/VSE supports RSA keys sizes up to 4096 bits in length in both software and hardware.

While the SSL for VSE feature of TCP/IP for VSE Version 2.1 supports these RSA key sizes too, RSA keys sizes of 2048 and 4096 bits require Crypto Express2 (or later) adapter hardware. The SSL for VSE feature of TCP/IP for VSE Version 1.4/1.5 does not support RSA key sizes greater than 1024 bits.

Since RSA keys sizes less than 2048 bits are now deprecated and insecure, the SSL for VSE feature of TCP/IP for VSE Version 2.1 and a Cyrpto Express2 (or later) adapter hardware is required for secure sockets using SSL for VSE.

IPv6/VSE also supports using the more secure Diffie-Hellmann (DH) key exchange. DH key exchange is different method of managing session keys and is more secure than RSA key exchange. SSL for VSE does not support this type of key exchange. 

Why is DH Important?

The following diagram shows an SSL session key exchange using RSA.

The weak point is that the secret session is part of the SSL session data. The key is sent from the client to the server, encrypted via the public server RSA key. If the private server RSA key is compromised, stolen, or broken, the session key is no longer secure and it is possible to decrypt and read the complete session data. 

Another method of exchanging the session key, is by using Diffie-Hellman. Using Diffie-Hellman, the session key is never sent over the network and is therefore never part of the network session data. The following example shows how the session key is negotiated using DH. Therefore, the common term is not “exchanging a session key”, but rather “agreeing on a common session key” through the DH key agreement process.

The important point is that the encryption key is created independently on both sides. Therefore, it is not possible to reveal the session key from a given recorded network session later. For simplification, the example does not show how the two communication partners are authenticated. In practice, DH is mostly used together with certificate authentication by using RSA.

DH key exchange is the most secure method currently available and is fully supported by IPv6/VSE. Again, SSL for VSE does not support DH key exchange.

Encryption Methods

Supported by SSL for VSE ...

Of these encryption methods 01, 02, 08 and 09 should not be used because they are insecure.

Supported by IPv6/VSE ...
2F AES128-SHA 
35 AES256-SHA 
3C AES128-SHA256 
3D AES256-SHA256 
67 DHE-RSA-AES128-SHA256

Note: The z/VSE 6.1 TCP/IP Support manual states the IPv6/VSE supports method 09 and C011 but these algorithms are insecure and no longer supported.

IPv6/VSE supports far more encryption methods using both software and hardware.

Message Digest/Hash Algorithms

Each message (packet) transferred using SSL/TLS includes a message digest or hash value. This value is used to verify the data has not been altered. A Message Digest is also used to secure certificates and ensure the certificate has not been altered. Therefore the algorithm used to calculate this value is very important.

The message digest MD5 is completely broken/insecure and should no longer be used.
The SHA1 family of message digests can be used for secure sockets but should not be used to authenticate certificates. The difference is how long the hash value is used. For secure sockets the hash value is used only for the duration of an SSL/TLS transfer. For certificates the value is used for a long time, perhaps years. This gives hackers far to much time to hack/break the digest key.
The SHA2 family of message digests are current and secure.
The SHA3 family of message digests are also secure and the industry is moving to SHA3.

IPv6/VSE supports using SHA1 and SHA2. It is planned to add SHA3 in the near future. These values are supported using either software or hardware (CPACF).
SSL for VSE supports using SHA1 and with the introduction of TCP/IP for VSE 2.1 under z/VSE 6.1 SHA2 was added. However, the CPACF hardware on a z10 (or newer machine) must be available and enabled for SHA2 to be used.

The industry is now using SHA2 and moving to SHA3 to secure certificates and communications.

Sample Connection

Google Chrome was used to create this blog entry. I used the information available from Chrome to display the type of SSL/TLS connection.

So, Chrome created a TLSv1.2 secure socket using ECDHE_ECDSA. This is an Elliptic Curve cryptographic session created using DH key exchange. After the key exchange process completed a 128-bit AES key is being used to encrypt each message. IPv6/VSE fully supports this type of TLS session while SSL for VSE does not.

Viewing the certificate information shows that the certificate's fingerprint or message digest hash uses both SHA-1 and SHA-2 (SHA-256). This allows both older (less secure) and newer (more secure) clients and servers to authenticate the certificate.


Maybe the real question is simple.

Can I get a secure and trusted socket using IPv6/VSE? Yes, easily. Many options exist to provide secure communications using both software and hardware.

Can I get a secure and trusted socket using the SSL for VSE feature of TCP/IP for VSE?

Maybe. Only if you ...
  1. Are using TCP/IP for VSE 2.1 and
  2. Running on a z10 (or newer) machine and
  3. Have CPACF  available and enabled and
  4. Have a Crypto Express2 (or later) adapter

Basically, if you need truly secure and trusted socket support I strongly recommend you move to IPv6/VSE.


IBM publication z/VSE 6.1 TCP/IP Support SC34-2706-00

The diagrams in the "Why is DH Important?" section and the text around them was lifted directly from the IBM manual with minor edits to the text by me. Actual credit here is likely to be to Joerg Schmidbauer at the IBM z/VSE lab in Germany. Joerg is the person who did the actual port of OpenSSL to z/VSE. Very impressive stuff if you ask me.

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