Nmap update - SSL/TLS ciper strength indication

Question

Previously with nmap I found it easy to understand the strengths of ciphers noted by the tool (strong, weak...)

However this has now changed to a rating of A-F. What does this mean and why did they make that change?

Below is an example of the output of the tool

|  TLSv1.2: 
|     ciphers: 
|       TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA (dh 1024) - D
|       TLS_DHE_RSA_WITH_AES_128_CBC_SHA (dh 1024) - A
|       TLS_DHE_RSA_WITH_AES_128_CBC_SHA256 (dh 1024) - A
|       TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 (dh 1024) - A
|       TLS_DHE_RSA_WITH_AES_256_CBC_SHA (dh 1024) - A
|       TLS_DHE_RSA_WITH_AES_256_CBC_SHA256 (dh 1024) - A
|       TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 (dh 1024) - A
|       TLS_DHE_RSA_WITH_CAMELLIA_128_CBC_SHA (dh 1024) - A
|       TLS_DHE_RSA_WITH_CAMELLIA_256_CBC_SHA (dh 1024) - A
|       TLS_DHE_RSA_WITH_SEED_CBC_SHA (dh 1024) - A
|       TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA (secp384r1) - C
|       TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA (secp384r1) - A
|       TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256 (secp384r1) - A
|       TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (secp384r1) - A
|       TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA (secp384r1) - A
|       TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384 (secp384r1) - A
|       TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (secp384r1) - A
|       TLS_ECDHE_RSA_WITH_RC4_128_SHA (secp384r1) - A
|       TLS_ECDH_anon_WITH_3DES_EDE_CBC_SHA - F
|       TLS_ECDH_anon_WITH_AES_128_CBC_SHA - F
|       TLS_ECDH_anon_WITH_AES_256_CBC_SHA - F
|       TLS_ECDH_anon_WITH_RC4_128_SHA - F
|       TLS_RSA_WITH_3DES_EDE_CBC_SHA (rsa 4096) - C
|       TLS_RSA_WITH_AES_128_CBC_SHA (rsa 4096) - A
|       TLS_RSA_WITH_AES_128_CBC_SHA256 (rsa 4096) - A
|       TLS_RSA_WITH_AES_128_GCM_SHA256 (rsa 4096) - A
|       TLS_RSA_WITH_AES_256_CBC_SHA (rsa 4096) - A
|       TLS_RSA_WITH_AES_256_CBC_SHA256 (rsa 4096) - A
|       TLS_RSA_WITH_AES_256_GCM_SHA384 (rsa 4096) - A
|       TLS_RSA_WITH_CAMELLIA_128_CBC_SHA (rsa 4096) - A
|       TLS_RSA_WITH_CAMELLIA_256_CBC_SHA (rsa 4096) - A
|       TLS_RSA_WITH_IDEA_CBC_SHA (rsa 4096) - A
|       TLS_RSA_WITH_RC4_128_MD5 (rsa 4096) - A
|       TLS_RSA_WITH_RC4_128_SHA (rsa 4096) - A
|       TLS_RSA_WITH_SEED_CBC_SHA (rsa 4096) - A

Answer 1

Per the nmap documentation for the SSL-enum-ciphers script the A-F rating is intended to be an indication of the quality of the cipher (with A being the best rating and F being the worst.

As to the reason for their change, I think you'd need to ask the relevant developer about that (there's an e-mail address on that documentation page I linked) if you want to know that.

Answer 2

Under the old system, we had to keep a static file mapping cipher suites to strength ratings, and the mapping decision was not very transparent: you had to just trust us (the Nmap developers) on the strengths. Also, there was no way to incorporate critical information like DH parameter size or RSA key length. We decided to incorporate a dynamic scoring system, based on Qualys SSL Labs's scoring. The changelog entry for Nmap 6.49BETA1 says:

[NSE] Rework ssl-enum-ciphers to actually score the strength of the SSL/TLS handshake, including certificate key size and DH parameters if applicable. This is similar to Qualys's SSL Labs scanner, and means that we no longer maintain a list of scores per ciphersuite. [Daniel Miller]

If you have an F, then your configuration is essentially broken, and offers negligible security.

Other notable changes:

• Support for fragmented records, weird handshake size limitations, and non-typical port numbers.
• Reporting the elliptic curve name used.
• Ordering ciphers by server's preference, if relevant.
• Warnings for SSL misconfigurations such as MD5-signed certificates, low-quality ephemeral DH parameters, and the POODLE vulnerability.