Revision as of 13:17, 25 September 2017

Last revision (mm/dd/yy): 09/25/2017

Introduction

This article is focused on providing clear and simple examples for the cipher string. They are based on different scenarios where you use the Transport Layer Security (TLS) protocol.

Recommendations for a cipher string

Secenarios

The cipher strings are based on the recommendation to setup your policy to get a whitelist for yours ciphers as described in the Transport Layer Protection Cheat Sheet (Rule - Only Support Strong Cryptographic Ciphers). The recommened cipher strings are based on the different scenarios:

• OWASP Cipher String 'A+' (Advanced+, limited compatibility, e.g. to more recent browser versions)

• Recommended if you control the server and the clients (e.g. by approvement) and if you check the compatibility before using it
• Includes solely the strongest perfect forward secrecy (PFS) ciphers
• Protocol: TLSv1.2 (and above)

• OWASP Cipher String 'A' (Advanced, wider compatibility, e.g. to most newer browser versions)

• Recommended if you control the server and the clients (e.g. by approvement) if the 'A+' string does not work, make sure to check the compatibility before using it
• includes solely the stronger PFS ciphers
• Protocol: TLSv1.2 (and above)

• OWASP Cipher String 'B' (Broad compatibility to browsers, check the compatibility to other protocols before using it, e.g. IMAPS)

• Recommended if you solely control the server and the clients use their browsers and if you check the compatibility before using it for other protocols than https
• Includes solely PFS ciphers
• Be aware of additional risks and of new vulnerabilities that may appear are more likely than above
• Plan to phase out SHA-1 and TLSv1/TLSv1.1 for https in middle-term
• Protocol: TLSv1.0/better TLSv1.1 (and above)

• OWASP Cipher String 'C' (Widest Compatibility, compatibility to most legacy browsers, legacy libraries (still patched) and other application protocols besides https, e.g. IMAPS)

• You may use this if you solely control the server and your clients use elder browsers and other elder libraries or if you use other protocols than https
• Be aware of the existing risks and of new vulnerabilities that may appear more likely
• PFS ciphers are preferred, except DHE with SHA-1 (to prevent possible incompatibility issues)
• Plan to move to 'A' for https or at least 'B' otherwise in middle-term
• Protocol: TLSv1.0 (and above)

• OWASP Cipher String 'C-' (Legacy, widest compatibility to real old browsers and legacy libraries and other application protocols like SMTP)

• Take care, use this cipher string only if you are forced to support 3DES(=TLS_RSA_WITH_3DES_EDE_CBC_SHA, =DES-CBC3-SHA) for real old clients with very old libraries or old libraries for other protocols besides https
• Be aware of the existing risks (e.g. ciphers without PFS or with 3DES) and of new vulnerabilities that may appear the most likely
• PFS ciphers are preferred, except DHE with SHA-1 (to prevent possible incompatibility issues)
• Plan to move at least to 'C' in a short-term
• Protocol: TLSv1.0 (and above)

Table of the ciphers (and their priority from high (1) to low (e.g. 19))

Remarks:
- Elder versions of Internet-Explorer- and Java do not support Diffie-Hellman parameters >1024 bit. So the ciphers 'TLS_DHE_RSA_WITH_AES_256_CBC_SHA' and 'TLS_DHE_RSA_WITH_AES_128_CBC_SHA' were moved to the end to prevent possible incompatibility issues. Other Option: Do not offer these Ciphers at all.

Examples for cipher strings

• OpenSSL

Cipher-String	OpennSSL-Syntax
Advanced+ (A+)	DHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256
Advanced (A)	DHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-SHA256:DHE-RSA-AES128-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA256
Broad Compatibility (B)	DHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-SHA256:DHE-RSA-AES128-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA256:ECDHE-RSA-AES256-SHA:ECDHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA
Widest Compatibility (C)	DHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-SHA256:DHE-RSA-AES128-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA256:ECDHE-RSA-AES256-SHA:ECDHE-RSA-AES128-SHA:AES256-GCM-SHA384:AES128-GCM-SHA256:AES256-SHA256:AES128-SHA256:AES256-SHA:AES128-SHA:DHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA
Legacy (C-)	DHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-SHA256:DHE-RSA-AES128-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA256:ECDHE-RSA-AES256-SHA:ECDHE-RSA-AES128-SHA:AES256-GCM-SHA384:AES128-GCM-SHA256:AES256-SHA256:AES128-SHA256:AES256-SHA:AES128-SHA:DES-CBC3-SHA:DHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA

At a glance: Hardening of other parts of the configuration of TLS/SSL for web servers

• Use solely secure and server initiated renegotiation
• Disable compression
• Check the configuration for all virtual hosts
• If you use 'Server Name Indication' (SNI), verify which virtual server is your default server. Ancient browsers, OSs or runtime environments without SNI support can only reach this server
• Use only the TLS/SSL extensions that you really need, e.g. deactivate heart beat (see Heartbleed), do not activate insecure or untested drafts for extensions e.g. additional random, opaque PRF input (see. DualECTLS)
• Set reasonable HTML Headers, e.g. HTTP Strict Transport Security (HSTS)

Example configs

Apache

• Cipher String 'A':

Remarks:
- The cipher string is compiled as a whitelist of individual ciphers to get a better compatibility even with old versions of OpenSSL.
- Monitor the performance of your server, e.g. the TLS handshake with DHE hinders the CPU abt 2.4 times more than ECDHE, cf. Vincent Bernat, 2011, nmav's Blog, 2011.

• Verify your cipher string using your crypto library, e.g. openssl using cipher string 'A':

0x00,0x9F - DHE-RSA-AES256-GCM-SHA384   TLSv1.2 Kx=DH     Au=RSA  Enc=AESGCM(256) Mac=AEAD
0x00,0x9E - DHE-RSA-AES128-GCM-SHA256   TLSv1.2 Kx=DH     Au=RSA  Enc=AESGCM(128) Mac=AEAD
0xC0,0x30 - ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH   Au=RSA  Enc=AESGCM(256) Mac=AEAD
0xC0,0x2F - ECDHE-RSA-AES128-GCM-SHA256 TLSv1.2 Kx=ECDH   Au=RSA  Enc=AESGCM(128) Mac=AEAD
0x00,0x6B - DHE-RSA-AES256-SHA256       TLSv1.2 Kx=DH     Au=RSA  Enc=AES(256)    Mac=SHA256
0x00,0x67 - DHE-RSA-AES128-SHA256       TLSv1.2 Kx=DH     Au=RSA  Enc=AES(128)    Mac=SHA256
0xC0,0x28 - ECDHE-RSA-AES256-SHA384     TLSv1.2 Kx=ECDH   Au=RSA  Enc=AES(256)    Mac=SHA384
0xC0,0x27 - ECDHE-RSA-AES128-SHA256     TLSv1.2 Kx=ECDH   Au=RSA  Enc=AES(128)    Mac=SHA256

CAUTION: You need a newer version of OpenSSL to use this cipher string!

Related Articles

• OWASP: Transport Layer Protection Cheat Sheet

Authors and Primary Editors

Torsten Gigler @
Achim Hoffmann @

Other Cheatsheets