Security/Server Side TLS

The goal of this document is to help operational teams with the configuration of TLS on servers. All Mozilla sites and deployment should follow the recommendations below.

The Operations Security (OpSec) team maintains this document as a reference guide to navigate the TLS landscape. It contains information on TLS protocols, known issues and vulnerabilities, configuration examples and testing tools. Changes are reviewed and merged by the OpSec team, and broadcasted to the various Operational teams.

Document Status Major Versions READY Version 2: Public release. r=ulfr r=kang Version 1.5: Julien Vehent (ulfr) added details for PFS DHE handshake, added nginx configuration details; Guillaume Destuynder (kang): added Apache recommended conf Version 1.4: ulfr: revised ciphersuite. Prefer AES before RC4. Prefer 128 before 256. Prefer DHE before non-DHE. Version 1.3: ulfr: added netscaler example conf Version 1.2: ulfr: ciphersuite update: bump DHE-AESGCM above ECDH-RC4 Version 1.1: ulfr: integrated review comments from Infra; kang: SPDY information Version 1: ulfr: creation

Recommended Ciphersuite

The general purpose ciphersuite at the time of this writing is:

ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:ECDHE-RSA-RC4-SHA:ECDHE-ECDSA-RC4-SHA:RC4-SHA:HIGH:!aNULL:!eNULL:!EXPORT:!DES:!3DES:!MD5:!PSK

If your version of OpenSSL is old, unavailable ciphers will be discarded automatically. Always use the full ciphersuite above and let OpenSSL pick the ones it supports.

The ordering of a ciphersuite is very important because it decides which algorithms are going to be selected in priority. The recommendation above prioritizes algorithms that provide perfect forward secrecy.

The listing below shows the list of algorithms returned by this ciphersuite. If you have to pick them manually for your application, make sure you keep this ordering.

Older versions of OpenSSL may not return the full list of algorithms. AES-GCM and some ECDHE are fairly recent, and not present on most versions of OpenSSL shipped with Ubuntu or RHEL. This listing below was obtained from a freshly built OpenSSL.

$ openssl version
OpenSSL 1.1.0-dev xx XXX xxxx
 
$ openssl ciphers -v 'ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:
ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:kEDH+AESGCM:
ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:
ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:
ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:
DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:ECDHE-RSA-RC4-SHA:ECDHE-ECDSA-RC4-SHA:
AES128-GCM-SHA256:AES256-GCM-SHA384:RC4-SHA:HIGH:!aNULL:!eNULL:!EXPORT:!DES:!3DES:
!MD5:!PSK' |column -t
 
ECDHE-RSA-AES128-GCM-SHA256    TLSv1.2  Kx=ECDH        Au=RSA    Enc=AESGCM(128)    Mac=AEAD
ECDHE-ECDSA-AES128-GCM-SHA256  TLSv1.2  Kx=ECDH        Au=ECDSA  Enc=AESGCM(128)    Mac=AEAD
ECDHE-RSA-AES256-GCM-SHA384    TLSv1.2  Kx=ECDH        Au=RSA    Enc=AESGCM(256)    Mac=AEAD
ECDHE-ECDSA-AES256-GCM-SHA384  TLSv1.2  Kx=ECDH        Au=ECDSA  Enc=AESGCM(256)    Mac=AEAD
DHE-DSS-AES256-GCM-SHA384      TLSv1.2  Kx=DH          Au=DSS    Enc=AESGCM(256)    Mac=AEAD
DHE-RSA-AES256-GCM-SHA384      TLSv1.2  Kx=DH          Au=RSA    Enc=AESGCM(256)    Mac=AEAD
DHE-DSS-AES128-GCM-SHA256      TLSv1.2  Kx=DH          Au=DSS    Enc=AESGCM(128)    Mac=AEAD
DHE-RSA-AES128-GCM-SHA256      TLSv1.2  Kx=DH          Au=RSA    Enc=AESGCM(128)    Mac=AEAD
ECDHE-RSA-AES128-SHA256        TLSv1.2  Kx=ECDH        Au=RSA    Enc=AES(128)       Mac=SHA256
ECDHE-ECDSA-AES128-SHA256      TLSv1.2  Kx=ECDH        Au=ECDSA  Enc=AES(128)       Mac=SHA256
ECDHE-RSA-AES128-SHA           SSLv3    Kx=ECDH        Au=RSA    Enc=AES(128)       Mac=SHA1
ECDHE-ECDSA-AES128-SHA         SSLv3    Kx=ECDH        Au=ECDSA  Enc=AES(128)       Mac=SHA1
ECDHE-RSA-AES256-SHA384        TLSv1.2  Kx=ECDH        Au=RSA    Enc=AES(256)       Mac=SHA384
ECDHE-ECDSA-AES256-SHA384      TLSv1.2  Kx=ECDH        Au=ECDSA  Enc=AES(256)       Mac=SHA384
ECDHE-RSA-AES256-SHA           SSLv3    Kx=ECDH        Au=RSA    Enc=AES(256)       Mac=SHA1
ECDHE-ECDSA-AES256-SHA         SSLv3    Kx=ECDH        Au=ECDSA  Enc=AES(256)       Mac=SHA1
DHE-RSA-AES128-SHA256          TLSv1.2  Kx=DH          Au=RSA    Enc=AES(128)       Mac=SHA256
DHE-RSA-AES128-SHA             SSLv3    Kx=DH          Au=RSA    Enc=AES(128)       Mac=SHA1
DHE-RSA-AES256-SHA256          TLSv1.2  Kx=DH          Au=RSA    Enc=AES(256)       Mac=SHA256
DHE-DSS-AES256-SHA             SSLv3    Kx=DH          Au=DSS    Enc=AES(256)       Mac=SHA1
ECDHE-RSA-RC4-SHA              SSLv3    Kx=ECDH        Au=RSA    Enc=RC4(128)       Mac=SHA1
ECDHE-ECDSA-RC4-SHA            SSLv3    Kx=ECDH        Au=ECDSA  Enc=RC4(128)       Mac=SHA1
AES128-GCM-SHA256              TLSv1.2  Kx=RSA         Au=RSA    Enc=AESGCM(128)    Mac=AEAD
AES256-GCM-SHA384              TLSv1.2  Kx=RSA         Au=RSA    Enc=AESGCM(256)    Mac=AEAD
RC4-SHA                        SSLv3    Kx=RSA         Au=RSA    Enc=RC4(128)       Mac=SHA1
SRP-DSS-AES-256-CBC-SHA        SSLv3    Kx=SRP         Au=DSS    Enc=AES(256)       Mac=SHA1
SRP-RSA-AES-256-CBC-SHA        SSLv3    Kx=SRP         Au=RSA    Enc=AES(256)       Mac=SHA1
DH-DSS-AES256-GCM-SHA384       TLSv1.2  Kx=DH/DSS      Au=DH     Enc=AESGCM(256)    Mac=AEAD
DH-RSA-AES256-GCM-SHA384       TLSv1.2  Kx=DH/RSA      Au=DH     Enc=AESGCM(256)    Mac=AEAD
DHE-DSS-AES256-SHA256          TLSv1.2  Kx=DH          Au=DSS    Enc=AES(256)       Mac=SHA256
DH-RSA-AES256-SHA256           TLSv1.2  Kx=DH/RSA      Au=DH     Enc=AES(256)       Mac=SHA256
DH-DSS-AES256-SHA256           TLSv1.2  Kx=DH/DSS      Au=DH     Enc=AES(256)       Mac=SHA256
DHE-RSA-AES256-SHA             SSLv3    Kx=DH          Au=RSA    Enc=AES(256)       Mac=SHA1
DH-RSA-AES256-SHA              SSLv3    Kx=DH/RSA      Au=DH     Enc=AES(256)       Mac=SHA1
DH-DSS-AES256-SHA              SSLv3    Kx=DH/DSS      Au=DH     Enc=AES(256)       Mac=SHA1
DHE-RSA-CAMELLIA256-SHA        SSLv3    Kx=DH          Au=RSA    Enc=Camellia(256)  Mac=SHA1
DHE-DSS-CAMELLIA256-SHA        SSLv3    Kx=DH          Au=DSS    Enc=Camellia(256)  Mac=SHA1
DH-RSA-CAMELLIA256-SHA         SSLv3    Kx=DH/RSA      Au=DH     Enc=Camellia(256)  Mac=SHA1
DH-DSS-CAMELLIA256-SHA         SSLv3    Kx=DH/DSS      Au=DH     Enc=Camellia(256)  Mac=SHA1
ECDH-RSA-AES256-GCM-SHA384     TLSv1.2  Kx=ECDH/RSA    Au=ECDH   Enc=AESGCM(256)    Mac=AEAD
ECDH-ECDSA-AES256-GCM-SHA384   TLSv1.2  Kx=ECDH/ECDSA  Au=ECDH   Enc=AESGCM(256)    Mac=AEAD
ECDH-RSA-AES256-SHA384         TLSv1.2  Kx=ECDH/RSA    Au=ECDH   Enc=AES(256)       Mac=SHA384
ECDH-ECDSA-AES256-SHA384       TLSv1.2  Kx=ECDH/ECDSA  Au=ECDH   Enc=AES(256)       Mac=SHA384
ECDH-RSA-AES256-SHA            SSLv3    Kx=ECDH/RSA    Au=ECDH   Enc=AES(256)       Mac=SHA1
ECDH-ECDSA-AES256-SHA          SSLv3    Kx=ECDH/ECDSA  Au=ECDH   Enc=AES(256)       Mac=SHA1
AES256-SHA256                  TLSv1.2  Kx=RSA         Au=RSA    Enc=AES(256)       Mac=SHA256
AES256-SHA                     SSLv3    Kx=RSA         Au=RSA    Enc=AES(256)       Mac=SHA1
CAMELLIA256-SHA                SSLv3    Kx=RSA         Au=RSA    Enc=Camellia(256)  Mac=SHA1
SRP-DSS-AES-128-CBC-SHA        SSLv3    Kx=SRP         Au=DSS    Enc=AES(128)       Mac=SHA1
SRP-RSA-AES-128-CBC-SHA        SSLv3    Kx=SRP         Au=RSA    Enc=AES(128)       Mac=SHA1
DH-DSS-AES128-GCM-SHA256       TLSv1.2  Kx=DH/DSS      Au=DH     Enc=AESGCM(128)    Mac=AEAD
DH-RSA-AES128-GCM-SHA256       TLSv1.2  Kx=DH/RSA      Au=DH     Enc=AESGCM(128)    Mac=AEAD
DHE-DSS-AES128-SHA256          TLSv1.2  Kx=DH          Au=DSS    Enc=AES(128)       Mac=SHA256
DH-RSA-AES128-SHA256           TLSv1.2  Kx=DH/RSA      Au=DH     Enc=AES(128)       Mac=SHA256
DH-DSS-AES128-SHA256           TLSv1.2  Kx=DH/DSS      Au=DH     Enc=AES(128)       Mac=SHA256
DHE-DSS-AES128-SHA             SSLv3    Kx=DH          Au=DSS    Enc=AES(128)       Mac=SHA1
DH-RSA-AES128-SHA              SSLv3    Kx=DH/RSA      Au=DH     Enc=AES(128)       Mac=SHA1
DH-DSS-AES128-SHA              SSLv3    Kx=DH/DSS      Au=DH     Enc=AES(128)       Mac=SHA1
DHE-RSA-CAMELLIA128-SHA        SSLv3    Kx=DH          Au=RSA    Enc=Camellia(128)  Mac=SHA1
DHE-DSS-CAMELLIA128-SHA        SSLv3    Kx=DH          Au=DSS    Enc=Camellia(128)  Mac=SHA1
DH-RSA-CAMELLIA128-SHA         SSLv3    Kx=DH/RSA      Au=DH     Enc=Camellia(128)  Mac=SHA1
DH-DSS-CAMELLIA128-SHA         SSLv3    Kx=DH/DSS      Au=DH     Enc=Camellia(128)  Mac=SHA1
ECDH-RSA-AES128-GCM-SHA256     TLSv1.2  Kx=ECDH/RSA    Au=ECDH   Enc=AESGCM(128)    Mac=AEAD
ECDH-ECDSA-AES128-GCM-SHA256   TLSv1.2  Kx=ECDH/ECDSA  Au=ECDH   Enc=AESGCM(128)    Mac=AEAD
ECDH-RSA-AES128-SHA256         TLSv1.2  Kx=ECDH/RSA    Au=ECDH   Enc=AES(128)       Mac=SHA256
ECDH-ECDSA-AES128-SHA256       TLSv1.2  Kx=ECDH/ECDSA  Au=ECDH   Enc=AES(128)       Mac=SHA256
ECDH-RSA-AES128-SHA            SSLv3    Kx=ECDH/RSA    Au=ECDH   Enc=AES(128)       Mac=SHA1
ECDH-ECDSA-AES128-SHA          SSLv3    Kx=ECDH/ECDSA  Au=ECDH   Enc=AES(128)       Mac=SHA1
AES128-SHA256                  TLSv1.2  Kx=RSA         Au=RSA    Enc=AES(128)       Mac=SHA256
AES128-SHA                     SSLv3    Kx=RSA         Au=RSA    Enc=AES(128)       Mac=SHA1
CAMELLIA128-SHA                SSLv3    Kx=RSA         Au=RSA    Enc=Camellia(128)  Mac=SHA1

The ciphers are described here: http://www.openssl.org/docs/apps/ciphers.html

Prioritization logic

1. ECDHE+AESGCM ciphers are selected first. These are TLS 1.2 ciphers and not widely supported at the moment. No known attack currently target these ciphers.
2. PFS ciphersuites are preferred, with ECDHE first, then DHE.
3. AES 128 is preferred to AES 256. There has been [discussions] on whether AES256 extra security was worth the cost, and the result is far from obvious. At the moment, AES128 is preferred, because it provides good security, is really fast, and seems to be more resistant to timing attacks.
4. AES is preferred to RC4. BEAST attacks on AES are mitigated in TLS 1.1 and above, and difficult to achieve in TLS 1.0. In comparison, attacks on RC4 are not mitigated and likely to become more and more dangerous.

Mandatory discards

• aNULL contains non-authenticated Diffie-Hellman key exchanges, that are subject to Man-In-The-Middle (MITM) attacks
• eNULL contains null-encryption ciphers (cleartext)
• EXPORT are legacy weak ciphers that were marked as exportable by US law
• DES and 3DES contains all legacy ciphers that used the deprecated Data Encryption Standard
• SSLv2 contains all ciphers that were defined in the old version of the SSL standard, now deprecated
• MD5 contains all the ciphers that use the broken message digest 5 as the hashing algorithm

Forward Secrecy

The concept of forward secrecy is simple: client and server negotiate a key that never hits the wire, and is destroyed at the end of the session. The RSA private from the server is used to sign a Diffie-Hellman key exchange between the client and the server. The pre-master key obtained from the Diffie-Hellman handshake is then used for encryption. Since the pre-master key is specific to a connection between a client and a server, and used only for a limited amount of time, it is called Ephemeral.

With Forward Secrecy, if an attacker gets a hold of the server's private key, it will not be able to decrypt past communications. The private key was only used to sign the DH handshake, which does not reveal the pre-master key. Diffie-Hellman ensures that the pre-master keys never leave the client and the server, and cannot be intercepted by a MITM.

Diffie-Hellman is slow. Faster implementation, such as Elliptic Curve Diffie-Hellman (ECDH) are promising but not widely supported. Therefore, forward secrecy is still considered the privilege of a few.

DHE hanshake and dhparam

When an ephemeral Diffie-Hellman cipher is used, the server and the client negotiate a pre-master key using the Diffie-Hellman algorithm. This algorithm requires that the server sends the client a prime number and a generator. Neither are confidential, and are sent in clear text. However, they must be signed, such that a MITM cannot hijack the handshake.

As an example, TLS_DHE_RSA_WITH_AES_128_CBC_SHA256 works as follow:

1. Server sends Client a [SERVER KEY EXCHANGE] message during the SSL Handshake. The message contains:
   1. Prime number p
   2. Generator g
   3. Server's Diffie-Hellman public value A = g^X mod p, where X is a private integer chosen by the server at random, and never shared with the client.
   4. signature S of the above (plus two random values) computed using the Server's private RSA key
2. Client verifies the signature S
3. Client sends server a [CLIENT KEY EXCHANGE] message. The message contains:
   1. Client's Diffie-Hellman public value B = g^Y mod p, where Y is a private integer chosen at random and never shared.
4. The Server and the Client can now calculate the pre-master secret using each other's public values:
   1. server calculates PMS = B^X mod p
   2. client calculates PMS = A^Y mod p
5. Client sends a [CHANGE CIPHER SPEC] message to the server, and both parties continue the handshake using ENCRYPTED HANDSHAKE MESSAGES

The size of the prime number p constrains the size of the pre-master key PMS, because of the modulo operation. A smaller prime almost means weaker values of A and B, which could leak the secret values X and Y. Thus, the prime p should not be smaller than the size of the RSA private key.

$ openssl dhparam 2048
Generating DH parameters, 2048 bit long safe prime, generator 2
..+..+...............+
-----BEGIN DH PARAMETERS-----
MBYCEQCHU6UNZoHMF6bPtj21Hn/bAgEC.....
......
-----END DH PARAMETERS-----

OCSP Stapling

When connecting to a server, clients should verify the validity of the server certificate using either a Certificate Revocation List (CRL), or an Online Certificate Status Protocol (OCSP) record. The problem with CRL is that the lists have grown huge and take forever to download. OCSP is much more lightweight, as only one record is retrieved at a time. But the side effect is that OCSP requests must be made to a 3rd party OCSP responder when connecting to a server, which adds latency and potential failures.

The solution is to allow the server to send the OCSP record during the TLS handshake, therefore bypassing the OCSP responder. This mechanism saves a roundtrip between the client and the OCSP responder, and is called OCSP Stapling.

The location of the OCSP responder is taken from the Authority Information Access field of the signed certificate:

Authority Information Access: 
      OCSP - URI:http://ocsp.startssl.com/sub/class1/server/ca

Recommended Server Configurations

Nginx

Nginx provides the best TLS support at the moment. It is the only daemon that provides OCSP Stapling, custom DH parameters, and the full flavor of TLS versions (from OpenSSL).

The detail of each configuration parameter, and how to build a recent Nginx with OpenSSL, is at the end of this document.

server {
    listen 443;
    ssl on;
    # certs sent to the client in SERVER HELLO are concatenated in ssl_certificate
    ssl_certificate /path/to/signed_cert_plus_intermediates;
    ssl_certificate_key /path/to/private_key;
    # Diffie-Hellman parameter for DHE ciphersuites, recommended 2048 bits
    ssl_dhparam /path/to/dhparam.pem;
    ssl_session_timeout 5m;
    ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
    ssl_ciphers '<recommended ciphersuite from top of this page>';
    ssl_prefer_server_ciphers on;
    ssl_session_cache shared:SSL:50m;
 
    # Enable this if your want HSTS (recommended, but be careful)
    # add_header Strict-Transport-Security max-age=15768000;
 
    # OCSP Stapling ---
    # fetch OCSP records from URL in ssl_certificate and cache them
    ssl_stapling on;
    ssl_stapling_verify on;
    ## verify chain of trust of OCSP response using Root CA and Intermediate certs
    ssl_trusted_certificate /path/to/root_CA_cert_plus_intermediates;
    resolver <IP DNS resolver>;
 
    ....
}

Apache

In Apache 2.4.6, the DH parameter is always set to 1024 bits and is not user configurable. Future versions of Apache will automatically select a better value for the DH parameter. The configuration below is recommended.

<VirtualHost *:443>
    ...
    SSLEngine on
    SSLCertificateFile      /path/to/signed_certificate
    SSLCertificateChainFile /path/to/intermediate_certificate
    SSLCertificateKeyFile   /path/to/private/key
    SSLCACertificateFile    /path/to/all_ca_certs
    SSLProtocol             all -SSLv2 -SSLv3
    SSLCipherSuite          <recommended ciphersuite from top of this page>
    SSLHonorCipherOrder     on
    SSLCompression          off
    SSLUseStapling          on
    SSLStaplingResponderTimeout 5
    SSLStaplingReturnResponderErrors off
    SSLStaplingCache        shmcb:/var/run/ocsp(128000)
 
    # Enable this if your want HSTS (recommended, but be careful)
    # Header add Strict-Transport-Security "max-age=15768000"
 
    ...
</VirtualHost>

Haproxy

SSL support in Haproxy is still Beta and shouldn't be used to terminate production SSL traffic. Haproxy lacks support for OCSP Stapling. All other features are available, including custom dhparams.

frontend ft_test
  mode    http
  bind    0.0.0.0:443 ssl crt /path/to/<cert+privkey+intermediate+dhparam> ciphers <recommended_ciphersuite>
  # Enable this if your want HSTS (recommended, but be careful)
  # rspadd  Strict-Transport-Security:\ max-age=15768000

Stud

Stud is a lightweight SSL termination proxy. It's basically a wrapper for OpenSSL. Stud is not being heavily developed, and features such as OCSP stapling are missing. But it is very lightweight and efficient, and with a recent openssl, supports all the TLS 1.2 ciphers.

# SSL x509 certificate file. REQUIRED.
# List multiple certs to use SNI. Certs are used in the order they
# are listed; the last cert listed will be used if none of the others match
#
# type: string
pem-file = "<concatenate cert + privkey + dhparam>"
 
# SSL protocol.
#
tls = on
ssl = on
 
# List of allowed SSL ciphers.
#
# Run openssl ciphers for list of available ciphers.
# type: string
ciphers = "<recommended ciphersuite from top of this page>"
 
# Enforce server cipher list order
#
# type: boolean
prefer-server-ciphers = on

Amazon Web Services Elastic Load Balancer (AWS ELB)

ELBs support TLS 1.2, but lack support for ciphers ordering, custom DH parameters and OCSP Stapling. The default configuration of ELBs doesn't enable the correct ciphers or versions of TLS. This can be done by hand in the Web Console, but is tedious. Gene Wood, from Identity Ops, wrote a script that configures the proper TLS policy on ELB: https://github.com/mozilla/identity-ops/blob/master/aws-tools/apply_security_assurance_elb_ciphersuite_policy.py

Because of the lack of server side ordering, it is preferable to terminate TLS connection on something than ELBs. ELBs can be used at layer 4 to load balance TCP connections, and terminate SSL on Nginx, Apache or any suitable TLS stack. When using ELBs as L4 load balancer, the following limitations apply:

• Client IP will be hidden to the backend servers. The application behind the ELB will only see the IP of the ELB. Headers such as X-Forwarded-For cannot be used to store the client IP, because the ELB does not decrypt the SSL.
• Only layer 4 type heartbeats can be used (connection establishment on target port).
• Session stickiness will only be possible by source IP: one source IP will always reach the same application server. Session stickiness via cookie cannot be used, because the ELB does not decrypt the SSL.

ELBs support HAproxy's proxy protocol, that removes the need for X-Forwarded-For and operates with a header placed right before the TCP packet. While still in beta, a solution composed of L4 ELBs that send TCP traffic to HAproxy for SSL termination would solve the limitations above.

Zeus (Riverbed Stingray)

Zeus lacks support for TLS 1.2, Elliptic Curves, AES-GCM and OCSP Stapling.

The recommended prioritization is:

1. DHE-RSA-AES128-SHA
2. DHE-RSA-AES256-SHA
3. AES128-SHA
4. AES256-SHA
5. RC4-SHA
6. DES-CBC3-SHA
7. EDH-RSA-DES-CBC3-SHA

While the recommended DH prime size is 2048, problems with client libraries, such as Java 6, make this impossible to deploy for now. Therefore, a DH prime of 1024 bits should be used until all clients are compatible with larger primes.

Zeus uses RSA BSAFE crypto library.

# ./zeus.zxtm -vv | grep ^Crypto
Crypto library           : RSA CryptoC6.4

The following ciphersuites are supported by Zeus.

ssl!ssl3_ciphers
    This is a list (space, comma or colon separated) of SSL ciphers that will be used with performing SSL decryption or SSL encryption. The order of the supplied list determines the priority of the ciphers for SSL decryption.
    The default order is:
        SSL_RSA_WITH_RC4_128_SHA
        SSL_RSA_WITH_RC4_128_MD5
        SSL_RSA_WITH_AES_256_CBC_SHA
        SSL_DHE_RSA_WITH_AES_256_CBC_SHA
        SSL_RSA_WITH_3DES_EDE_CBC_SHA
        SSL_DHE_RSA_WITH_3DES_EDE_CBC_SHA
        SSL_RSA_WITH_AES_128_CBC_SHA
        SSL_DHE_RSA_WITH_AES_128_CBC_SHA
    In addition, the following ciphers are supported but disabled by default:
        SSL_RSA_EXPORT_WITH_RC4_56_SHA
        SSL_RSA_EXPORT_WITH_RC4_56_MD5
        SSL_RSA_WITH_DES_CBC_SHA
        SSL_DHE_RSA_WITH_DES_CBC_SHA
        SSL_RSA_EXPORT_WITH_DES_CBC_SHA
        SSL_RSA_EXPORT_WITH_RC4_40_MD5
        SSL_RSA_EXPORT_WITH_DES40_CBC_SHA
        SSL_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA
        SSL_RSA_WITH_NULL_SHA
        SSL_RSA_WITH_NULL_MD5

Citrix Netscaler

The configuration sample below shows how a default ciphersuite object can be created and attached to a vserver.

add ssl cipher MozillaDefault
bind ssl cipher MozillaDefault -cipherName TLS1-DHE-DSS-AES-128-CBC-SHA
bind ssl cipher MozillaDefault -cipherName TLS1-DHE-RSA-AES-128-CBC-SHA
bind ssl cipher MozillaDefault -cipherName TLS1-DHE-DSS-AES-256-CBC-SHA
bind ssl cipher MozillaDefault -cipherName TLS1-DHE-RSA-AES-256-CBC-SHA
bind ssl cipher MozillaDefault -cipherName TLS1-AES-256-CBC-SHA
bind ssl cipher MozillaDefault -cipherName TLS1-AES-128-CBC-SHA
bind ssl cipher MozillaDefault -cipherName SSL3-RC4-SHA

add ssl certKey <domain> -cert <cert> -key <key>
add ssl certKey <intermediateCertName> -cert <intermediateCertName>
link ssl certKey <domain> <intermediateCertName>
set ssl vserver <domain>:https -eRSA ENABLED
 
bind ssl vserver <domain>:https -cipherName MozillaDefault

The configuration can be viewed with the following commands: show ssl cipher MozillaDefault

> show ssl vserver marketplace.firefox.com:https
 
    Advanced SSL configuration for VServer marketplace.firefox.com:https:
    DH: DISABLED
    Ephemeral RSA: ENABLED        Refresh Count: 0
    Session Reuse: ENABLED        Timeout: 120 seconds
    Cipher Redirect: DISABLED
    SSLv2 Redirect: DISABLED
    ClearText Port: 0
    Client Auth: DISABLED
    SSL Redirect: DISABLED
    Non FIPS Ciphers: DISABLED
    SNI: DISABLED
    SSLv2: DISABLED    SSLv3: ENABLED    TLSv1: ENABLED
    Push Encryption Trigger: Always
    Send Close-Notify: YES

CipherScan

See https://github.com/jvehent/cipherscan

Cipherscan is a small Bash script that connects to a target and list the preferred Ciphers. It's an easy way to test a web server for available ciphers, but not as comprehensive as SSLLabs.

The example below shows the expected output of CipherScan with the recommended ciphersuite, on a properly configured Nginx.

$ ./CiphersScan.sh jve.linuxwall.info:443
prio  ciphersuite                  protocol  pfs_keysize
1     ECDHE-RSA-AES128-GCM-SHA256  TLSv1.2   ECDH,P-256,256bits
2     ECDHE-RSA-AES256-GCM-SHA384  TLSv1.2   ECDH,P-256,256bits
3     DHE-RSA-AES256-GCM-SHA384    TLSv1.2   DH,4096bits
4     DHE-RSA-AES128-GCM-SHA256    TLSv1.2   DH,4096bits
5     ECDHE-RSA-AES128-SHA256      TLSv1.2   ECDH,P-256,256bits
6     ECDHE-RSA-AES128-SHA         TLSv1.2   ECDH,P-256,256bits
7     ECDHE-RSA-AES256-SHA384      TLSv1.2   ECDH,P-256,256bits
8     ECDHE-RSA-AES256-SHA         TLSv1.2   ECDH,P-256,256bits
9     DHE-RSA-AES128-SHA256        TLSv1.2   DH,4096bits
10    DHE-RSA-AES128-SHA           TLSv1.2   DH,4096bits
11    DHE-RSA-AES256-SHA256        TLSv1.2   DH,4096bits
12    AES128-GCM-SHA256            TLSv1.2
13    AES256-GCM-SHA384            TLSv1.2
14    ECDHE-RSA-RC4-SHA            TLSv1.2   ECDH,P-256,256bits
15    RC4-SHA                      TLSv1.2
16    DHE-RSA-AES256-SHA           TLSv1.2   DH,4096bits
17    DHE-RSA-CAMELLIA256-SHA      TLSv1.2   DH,4096bits
18    AES256-SHA256                TLSv1.2
19    AES256-SHA                   TLSv1.2
20    CAMELLIA256-SHA              TLSv1.2
21    DHE-RSA-CAMELLIA128-SHA      TLSv1.2   DH,4096bits
22    AES128-SHA256                TLSv1.2
23    AES128-SHA                   TLSv1.2
24    CAMELLIA128-SHA              TLSv1.2

SSL Labs (Qualys)

Available here: https://www.ssllabs.com/ssltest/

Qualys SSL Labs provides a very nice and comprehensive SSL testing suite.

GlobalSign has a modified interface of SSL Labs with a few more bells and whistles: https://sslcheck.globalsign.com/

Appendices

Supported ciphers on various systems

On a variety of ~900 systems (RHEL5 & 6, CentOS 5 & 6 and Ubuntu), the following versions of OpenSSL were found:

37	OpenSSL 0.9.8e-fips-rhel5 01 Jul 2008
35	OpenSSL 0.9.8k 25 Mar 2009
777	OpenSSL 1.0.0-fips 29 Mar 2010
18	OpenSSL 1.0.1 14 Mar 2012

The recommended ciphersuite was tested on each system. The list below shows the ciphersuites supported by all tested systems. However old your setup may be, it is safe to assume that the following ciphers are going to be available, in the following order:

Cipher	Has Forward Secrecy	Issues
RC4-SHA	No	RC4 Warning
DHE-RSA-AES128-SHA	Yes	vulnerable to BEAST
DHE-RSA-AES256-SHA	Yes	vulnerable to BEAST
AES256-SHA	No	vulnerable to BEAST
DHE-DSS-AES128-SHA	Yes	vulnerable to BEAST
DHE-DSS-AES256-SHA	Yes	vulnerable to BEAST
AES128-SHA	No	vulnerable to BEAST

Attacks on TLS

BEAST CVE-2011-3389

Beast is a vulnerability in the Initialization Vector (IV) of the CBC mode of AES, Camellia and a few other ciphers that use CBC mode. The attack allows a MITM attacker to recover plaintext values by encrypted the same message multiple times.

BEAST is mitigated in TLS1.1 and above.

more: https://blog.torproject.org/blog/tor-and-beast-ssl-attack

LUCKY13

Lucky13 is another attack on CBC mode that listen for padding checks to decrypt ciphertext.

more: https://www.imperialviolet.org/2013/02/04/luckythirteen.html

RC4 weaknesses

It has been proven that RC4 biases in the first 256 bytes of a cipherstream can be used to recover encrypted text. If the same data is encrypted a very large amount of time, then an attacker can apply statistical analysis to the results and recover the encrypted text. While hard to perform, this attack shows that it is time to push RC4 down the ciphersuite.

more: http://security.stackexchange.com/questions/32497/tls-rc4-or-not-rc4

CRIME CVE-2012-4929

The root cause of the problem is information leakage that occurs when data is compressed prior to encryption. If someone can repeatedly inject and mix arbitrary content with some sensitive and relatively predictable data, and observe the resulting encrypted stream, then he will be able to extract the unknown data from it.

more: https://community.qualys.com/blogs/securitylabs/2012/09/14/crime-information-leakage-attack-against-ssltls

BREACH

This is a more complex attack than CRIME, which does not require TLS-level compression (it still needs HTTP-level compression).

In order to be successful, it requires to:

1. Be served from a server that uses HTTP-level compression
2. Reflect user-input in HTTP response bodies
3. Reflect a secret (such as a CSRF token) in HTTP response bodies

more: http://breachattack.com/

SPDY

(see also http://en.wikipedia.org/wiki/SPDY)

SPDY is a protocol that incorporate TLS, which attempts to reduce latency when loading pages. It is currently not an HTTP standard (albeit it is being drafted for HTTP 2.0), but is widely supported.

SPDY version 3 is vulnerable to the CRIME attack (see also http://zoompf.com/2012/09/explaining-the-crime-weakness-in-spdy-and-ssl) - this is due to the use of compression. Clients currently implement a non-standard hack in with gzip in order to circumvent the vulnerability. SPDY version 4 is planned to include a proper fix.

TLS tickets (RFC 5077)

Once a TLS handshake has been negociated between the server and the client, both may exchange a session ticket, which contains an AES-CBC 128bit key which can decrypt the session. This key is generally static and only regenerated when the web server is restarted (with recent versions of Apache, it's stored in a file and also kept upon restarts).

The current work-around is to disable RFC 5077 support.

more: https://media.blackhat.com/us-13/US-13-Daigniere-TLS-Secrets-Slides.pdf

Nginx configuration details

Originally published on Julien Vehent's blog at https://jve.linuxwall.info/blog/index.php?post/2013/10/12/A-grade-SSL/TLS-with-Nginx-and-StartSSL

Building Nginx

To build Nginx from source, you will need a copy of the PCRE and OpenSSL libraries:

• PCRE can be found here: ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/
• OpenSSL can be found here: http://www.openssl.org/source/

Decompress both libraries next to the Nginx source code:

julien@sachiel:~/nginx_openssl$ ls
build_static_nginx.sh  nginx  openssl-1.0.1e  pcre-8.33

The script build_static_nginx.sh takes care of the rest. It should work out of the box, but you might have to edit the paths if you have different versions of the libraries. I builds a static version of OpenSSL into Nginx, so you don't have to install the openssl libs afterward.

#!/usr/bin/env bash
export BPATH=$(pwd)
export STATICLIBSSL="$BPATH/staticlibssl"

#-- Build static openssl
cd $BPATH/openssl-1.0.1e
rm -rf "$STATICLIBSSL"
mkdir "$STATICLIBSSL"
make clean
./config --prefix=$STATICLIBSSL no-shared enable-ec_nistp_64_gcc_128 \
&& make depend \
&& make \
&& make install_sw

#-- Build nginx
hg clone http://hg.nginx.org/nginx
cd $BPATH/nginx
mkdir -p $BPATH/opt/nginx
hg pull
./auto/configure --with-cc-opt="-I $STATICLIBSSL/include -I/usr/include" \
--with-ld-opt="-L $STATICLIBSSL/lib -Wl,-rpath -lssl -lcrypto -ldl -lz" \
--prefix=$BPATH/opt/nginx \
--with-pcre=$BPATH/pcre-8.33 \
--with-http_ssl_module \
--with-http_spdy_module \
--with-file-aio \
--with-ipv6 \
--with-http_gzip_static_module \
--with-http_stub_status_module \
--without-mail_pop3_module \
--without-mail_smtp_module \
--without-mail_imap_module \
&& make && make install
NGINXBIN=$BPATH/opt/nginx/sbin/nginx
if [ -x $NGINXBIN ]; then
    $NGINXBIN -V
    echo -e "\nNginx binary build in $BPATH/opt/nginx/sbin/nginx\n"
fi

Server Name Identification

Support for SNI is built into recent versions of nginx. Use nginx -V to check:

# /opt/nginx -V
...
TLS SNI support enabled
...

Configuration directives

ssl_certificate

This parameter points to file that contains the server and intermediate certificates, concatenated together. Nginx loads that file and sends its content in the SERVER HELLO message during the handshake.

ssl_certificate_key

This is the path to the private key.

ssl_dhparam

When DHE ciphers are used, a prime number is shared between server and client to perform the Diffie-Hellman Key Exchange. I won't get into the details of Perfect Forward Secrecy here, but do know that the larger the prime is, the better the security. Nginx lets you specify the prime number you want the server to send to the client in the ssl_dhparam directive. The prime number is sent by the server to the client in the Server Key Exchange message of the handshake. To generate the dhparam, use openssl dhparam 4096

A word of warning though, it appears that Java 6 does not support dhparam larger than 1024 bits. Clients that use Java 6 won't be able to connect to your site if you use a larger dhparam. (there might be issues with other libraries as well, I only know about the java one).

ssl_session_timeout

When a client connects multiple time to a server, the server uses session caching to accelerate the subsequent handshakes, effectively reusing the session key generated in the first handshake multiple times. This is called session resumption. This parameter sets the session timeout to 5 minutes, meaning that the session key will be deleted from the cache if not used for 5 minutes.

ssl_session_cache

The session cache is a shared memory that contains all the session keys. All the Nginx workers can access the shared memory. It is used for session resumption, and significantly reduces handshake latency when one client connects multiple times.

ssl_protocols

List the versions of TLS you wish to support. It's pretty much safe to disable SSLv3 these days, but TLSv1 is still required by a bunch of clients. Remember that clients are not only web browsers, but also libraries that might be used to crawl your site.

ssl_ciphers

The ciphersuite is truly the core of an SSL configuration. Mine is very long, and I spent a ridiculous amount of time researching it. I won't get into the details of its construction here, as I'll be writing more on this in the next few weeks.

ssl_prefer_server_ciphers

This parameter force nginx to pick the preferred cipher from its own ciphersuite, as opposed to using the one preferred by the client. This is an important option since most clients have unsafe or outdated preferences, and you'll most likely provide better security by enforcing a strong ciphersuite server-side.

HTTP Strict Transport Security

HSTS is a HTTP header that tells clients to connect to the site using HTTPS only. It enforces security, by telling clients that any HTTP URL to a given site should be ignored. The directive is cached on the client size for the duration of max-age. In this case, 182 days.

ssl_stapling

Nginx supports OCSP stapling in two modes. The OCSP file can be downloaded and made available to nginx, or nginx itself can retrieve the OCSP record and cache it. The second mode is recommended.

ssl_stapling_verify

Nginx has the ability to verify the OCSP record before caching it. But to enable it, a list of trusted certificate must be available in the ssl_trusted_certificate parameter.

ssl_trusted_certificate

This is a path to a file where CA certificates are concatenated. For ssl_stapling_verify to work, this file must contain the Root CA cert and the Intermediate CA certificates. In the case of StartSSL, the Root CA and Intermediate I use are here: https://jve.linuxwall.info/ressources/code/startssl_trust_chain.txt

resolver

Nginx needs a DNS resolver to obtain the IP address of the OCSP responder.