GNUTLS

Match the required ciphers for the German BSI

• BSI TR-02102-2 (german)

For example the ciphers that are defined secure for perfect forward secrecy in that document are:

Cipher-Suite	IANA-No.	Refenrenced	Usable until
TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256	0xC0,0x23	[RFC5289]	2027+
TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384	0xC0,0x24	[RFC5289]	2027+
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256	0xC0,0x2B	[RFC5289]	2027+
TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384	0xC0,0x2C	[RFC5289]	2027+
TLS_ECDHE_ECDSA_WITH_AES_128_CCM	0xC0,0xAC	[RFC7251]	2027+
TLS_ECDHE_ECDSA_WITH_AES_256_CCM	0xC0,0xAD	[RFC7251]	2027+
TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256	0xC0,0x27	[RFC5289]	2027+
TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384	0xC0,0x28	[RFC5289]	2027+
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256	0xC0,0x2F	[RFC5289]	2027+
TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384	0xC0,0x30	[RFC5289]	2027+
TLS_DHE_DSS_WITH_AES_128_CBC_SHA256	0x00,0x40	[RFC5246]	2027+
TLS_DHE_DSS_WITH_AES_256_CBC_SHA256	0x00,0x6A	[RFC5246]	2027+
TLS_DHE_DSS_WITH_AES_128_GCM_SHA256	0x00,0xA2	[RFC5288]	2027+
TLS_DHE_DSS_WITH_AES_256_GCM_SHA384	0x00,0xA3	[RFC5288]	2027+
TLS_DHE_RSA_WITH_AES_128_CBC_SHA256	0x00,0x67	[RFC5246]	2027+
TLS_DHE_RSA_WITH_AES_256_CBC_SHA256	0x00,0x6B	[RFC5246]	2027+
TLS_DHE_RSA_WITH_AES_128_GCM_SHA256	0x00,0x9E	[RFC5288]	2027+
TLS_DHE_RSA_WITH_AES_256_GCM_SHA384	0x00,0x9F	[RFC5288]	2027+
TLS_DHE_RSA_WITH_AES_128_CCM	0xC0,0x9E	[RFC6655]	2027+
TLS_DHE_RSA_WITH_AES_256_CCM	0xC0,0x9F	[RFC6655]	2027+

The key to the table is: TLS_(key exchange algorithms)_WITH_(ciphers)_(hash algorithms)

So to build a definition for GnuTLS that matches this requirements is:

1. Some basic security settings: %SERVER_PRECEDENCE:%LATEST_RECORD_VERSION:PFS
2. Disable defaults, enable only TLSv1.2: -VERS-TLS-ALL:+VERS-TLS1.2:-VERS-DTLS-ALL:-CIPHER-ALL:-KX-ALL:-MAC-ALL:-CURVE-ALL
3. Set the key exchange algorithms: +ECDHE-RSA:+ECDHE-ECDSA:+DHE-DSS:+DHE-RSA
4. Set the ciphers: +AES-256-CBC:+AES-128-CBC:+AES-256-GCM:+AES-128-GCM:+CHACHA20-POLY1305(CHACHA20-POLY1305 is used by google mail, so I needed it as well)
5. Set the hash algorithms: +SHA256:+SHA384:+AEAD (The +AEAD is something that is not directly seen be the list above, but you need it for GCM)
6. Set the wanted curves from the document above: +CURVE-SECP256R1:+CURVE-SECP384R1
7. Set the signature algorithm used in your certificate: +SIGN-RSA-SHA256 for me

And now put ist all together and let us see what happens:

$ gnutls-cli --list CIPHER --priority '%SERVER_PRECEDENCE:%LATEST_RECORD_VERSION:PFS:-VERS-TLS-ALL:+VERS-TLS1.2:-VERS-DTLS-ALL:-KX-ALL:-CIPHER-ALL:-MAC-ALL:-CURVE-ALL:-SIGN-ALL:+ECDHE-RSA:+ECDHE-ECDSA:+DHE-DSS:+DHE-RSA:+AES-256-CBC:+AES-128-CBC:+AES-256-GCM:+AES-128-GCM:+CHACHA20-POLY1305:+SHA256:+SHA384:+AEAD:+CURVE-SECP256R1:+CURVE-SECP384R1:+SIGN-RSA-SHA256'
Cipher suites for %SERVER_PRECEDENCE:%LATEST_RECORD_VERSION:PFS:-VERS-TLS-ALL:+VERS-TLS1.2:-VERS-DTLS-ALL:-KX-ALL:-CIPHER-ALL:-MAC-ALL:-CURVE-ALL:-SIGN-ALL:+ECDHE-RSA:+ECDHE-ECDSA:+DHE-DSS:+DHE-RSA:+AES-256-CBC:+AES-128-CBC:+AES-256-GCM:+AES-128-GCM:+CHACHA20-POLY1305:+SHA256:+SHA384:+AEAD:+CURVE-SECP256R1:+CURVE-SECP384R1:+SIGN-RSA-SHA256
TLS_ECDHE_RSA_AES_256_CBC_SHA384                  	0xc0, 0x28	TLS1.2
TLS_ECDHE_RSA_AES_128_CBC_SHA256                  	0xc0, 0x27	TLS1.2
TLS_ECDHE_RSA_AES_256_GCM_SHA384                  	0xc0, 0x30	TLS1.2
TLS_ECDHE_RSA_AES_128_GCM_SHA256                  	0xc0, 0x2f	TLS1.2
TLS_ECDHE_RSA_CHACHA20_POLY1305                   	0xcc, 0xa8	TLS1.2
TLS_ECDHE_ECDSA_AES_256_CBC_SHA384                	0xc0, 0x24	TLS1.2
TLS_ECDHE_ECDSA_AES_128_CBC_SHA256                	0xc0, 0x23	TLS1.2
TLS_ECDHE_ECDSA_AES_256_GCM_SHA384                	0xc0, 0x2c	TLS1.2
TLS_ECDHE_ECDSA_AES_128_GCM_SHA256                	0xc0, 0x2b	TLS1.2
TLS_ECDHE_ECDSA_CHACHA20_POLY1305                 	0xcc, 0xa9	TLS1.2
TLS_DHE_DSS_AES_256_CBC_SHA256                    	0x00, 0x6a	TLS1.2
TLS_DHE_DSS_AES_128_CBC_SHA256                    	0x00, 0x40	TLS1.2
TLS_DHE_DSS_AES_256_GCM_SHA384                    	0x00, 0xa3	TLS1.2
TLS_DHE_DSS_AES_128_GCM_SHA256                    	0x00, 0xa2	TLS1.2
TLS_DHE_RSA_AES_256_CBC_SHA256                    	0x00, 0x6b	TLS1.2
TLS_DHE_RSA_AES_128_CBC_SHA256                    	0x00, 0x67	TLS1.2
TLS_DHE_RSA_AES_256_GCM_SHA384                    	0x00, 0x9f	TLS1.2
TLS_DHE_RSA_AES_128_GCM_SHA256                    	0x00, 0x9e	TLS1.2
TLS_DHE_RSA_CHACHA20_POLY1305                     	0xcc, 0xaa	TLS1.2

Certificate types: CTYPE-X.509
Protocols: VERS-TLS1.2
Compression: COMP-NULL
Elliptic curves: CURVE-SECP256R1, CURVE-SECP384R1
PK-signatures: SIGN-RSA-SHA256

As you can see it is not all what we would expect, but is faces all what is implemented and can be used with our restrictions in GnuTLS. As far as I know: that's it!

A set of good and secure ciphers&co

%SERVER_PRECEDENCE:%LATEST_RECORD_VERSION:-VERS-DTLS-ALL:-KX-ALL:-CIPHER-ALL:-MAC-ALL:-CURVE-ALL:-SIGN-ALL:-VERS-TLS-ALL:+VERS-TLS1.3:+VERS-TLS1.2:+PFS:+SECURE256:-AES-256-CBC:-AES-128-CBC:-SIGN-RSA-SHA1

• %SERVER_PRECEDENCE : The ciphersuite will be selected according to server priorities and not the client’s.
• %LATEST_RECORD_VERSION : will use the latest TLS version record version in client hello.
• -... : remove anything
• +VERS-TLS1.3:+VERS-TLS1.2 : enable TLS versions 1.2 and 1.3
• +PFS : Means all the known to be secure ciphersuites that support perfect forward secrecy (ECDHE and DHE). The ciphers are sorted by security margin, although the 256-bit ciphers are included as a fallback only. The message authenticity security level is of 80 bits or more, and the certificate verification profile is set to GNUTLS_PROFILE_LOW (80-bits). This option is available since 3.2.4 or later.
• +SECURE256 : Currently alias for SECURE192. This option, will enable ciphers which use a 256-bit key but, due to limitations of the TLS protocol, the overall security level will be 192-bits (the security level depends on more factors than cipher key size).
• -AES-256-CBC:-AES-128-CBC : remove cbc ciphers again that are coming with +PFS
• -SIGN-RSA-SHA1 : remove rsa-sha1 signature algorithm which is also coming with +PFS