Postfix - Only use modern and secure ciphers

[Thomas2500]

With the change of tls_medium_cipherlist, the number of available ciphers for TLSv1.2 will be reduced from 59 to 9 (as per testssl.sh results). The smtpd_tls_mandatory_ciphers setting will be updated to 'medium' to accommodate the new custom list.

The cipher list will change as follows:

Current Cipher List:

TLSv1.2 (server order)
 xc02c   ECDHE-ECDSA-AES256-GCM-SHA384     ECDH 253   AESGCM      256      TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384            
 xc030   ECDHE-RSA-AES256-GCM-SHA384       ECDH 253   AESGCM      256      TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384              
 x9f     DHE-RSA-AES256-GCM-SHA384         DH 4096    AESGCM      256      TLS_DHE_RSA_WITH_AES_256_GCM_SHA384                
 xcca9   ECDHE-ECDSA-CHACHA20-POLY1305     ECDH 253   ChaCha20    256      TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256      
 xcca8   ECDHE-RSA-CHACHA20-POLY1305       ECDH 253   ChaCha20    256      TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256        
 xccaa   DHE-RSA-CHACHA20-POLY1305         DH 4096    ChaCha20    256      TLS_DHE_RSA_WITH_CHACHA20_POLY1305_SHA256          
 xc0af   ECDHE-ECDSA-AES256-CCM8           ECDH 253   AESCCM8     256      TLS_ECDHE_ECDSA_WITH_AES_256_CCM_8                 
 xc0ad   ECDHE-ECDSA-AES256-CCM            ECDH 253   AESCCM      256      TLS_ECDHE_ECDSA_WITH_AES_256_CCM                   
 xc0a3   DHE-RSA-AES256-CCM8               DH 4096    AESCCM8     256      TLS_DHE_RSA_WITH_AES_256_CCM_8                     
 xc09f   DHE-RSA-AES256-CCM                DH 4096    AESCCM      256      TLS_DHE_RSA_WITH_AES_256_CCM                       
 xc05d   ECDHE-ECDSA-ARIA256-GCM-SHA384    ECDH 253   ARIAGCM     256      TLS_ECDHE_ECDSA_WITH_ARIA_256_GCM_SHA384           
 xc061   ECDHE-ARIA256-GCM-SHA384          ECDH 253   ARIAGCM     256      TLS_ECDHE_RSA_WITH_ARIA_256_GCM_SHA384             
 xc053   DHE-RSA-ARIA256-GCM-SHA384        DH 4096    ARIAGCM     256      TLS_DHE_RSA_WITH_ARIA_256_GCM_SHA384               
 xc024   ECDHE-ECDSA-AES256-SHA384         ECDH 253   AES         256      TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384            
 xc028   ECDHE-RSA-AES256-SHA384           ECDH 253   AES         256      TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384              
 x6b     DHE-RSA-AES256-SHA256             DH 4096    AES         256      TLS_DHE_RSA_WITH_AES_256_CBC_SHA256                
 xc073   ECDHE-ECDSA-CAMELLIA256-SHA384    ECDH 253   Camellia    256      TLS_ECDHE_ECDSA_WITH_CAMELLIA_256_CBC_SHA384       
 xc077   ECDHE-RSA-CAMELLIA256-SHA384      ECDH 253   Camellia    256      TLS_ECDHE_RSA_WITH_CAMELLIA_256_CBC_SHA384         
 xc4     DHE-RSA-CAMELLIA256-SHA256        DH 4096    Camellia    256      TLS_DHE_RSA_WITH_CAMELLIA_256_CBC_SHA256           
 xc00a   ECDHE-ECDSA-AES256-SHA            ECDH 253   AES         256      TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA               
 xc014   ECDHE-RSA-AES256-SHA              ECDH 253   AES         256      TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA                 
 x39     DHE-RSA-AES256-SHA                DH 4096    AES         256      TLS_DHE_RSA_WITH_AES_256_CBC_SHA                   
 x88     DHE-RSA-CAMELLIA256-SHA           DH 4096    Camellia    256      TLS_DHE_RSA_WITH_CAMELLIA_256_CBC_SHA              
 x9d     AES256-GCM-SHA384                 RSA        AESGCM      256      TLS_RSA_WITH_AES_256_GCM_SHA384                    
 xc0a1   AES256-CCM8                       RSA        AESCCM8     256      TLS_RSA_WITH_AES_256_CCM_8                         
 xc09d   AES256-CCM                        RSA        AESCCM      256      TLS_RSA_WITH_AES_256_CCM                           
 xc051   ARIA256-GCM-SHA384                RSA        ARIAGCM     256      TLS_RSA_WITH_ARIA_256_GCM_SHA384                   
 x3d     AES256-SHA256                     RSA        AES         256      TLS_RSA_WITH_AES_256_CBC_SHA256                    
 xc0     CAMELLIA256-SHA256                RSA        Camellia    256      TLS_RSA_WITH_CAMELLIA_256_CBC_SHA256               
 x35     AES256-SHA                        RSA        AES         256      TLS_RSA_WITH_AES_256_CBC_SHA                       
 x84     CAMELLIA256-SHA                   RSA        Camellia    256      TLS_RSA_WITH_CAMELLIA_256_CBC_SHA                  
 xc02b   ECDHE-ECDSA-AES128-GCM-SHA256     ECDH 253   AESGCM      128      TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256            
 xc02f   ECDHE-RSA-AES128-GCM-SHA256       ECDH 253   AESGCM      128      TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256              
 x9e     DHE-RSA-AES128-GCM-SHA256         DH 4096    AESGCM      128      TLS_DHE_RSA_WITH_AES_128_GCM_SHA256                
 xc0ae   ECDHE-ECDSA-AES128-CCM8           ECDH 253   AESCCM8     128      TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8                 
 xc0ac   ECDHE-ECDSA-AES128-CCM            ECDH 253   AESCCM      128      TLS_ECDHE_ECDSA_WITH_AES_128_CCM                   
 xc0a2   DHE-RSA-AES128-CCM8               DH 4096    AESCCM8     128      TLS_DHE_RSA_WITH_AES_128_CCM_8                     
 xc09e   DHE-RSA-AES128-CCM                DH 4096    AESCCM      128      TLS_DHE_RSA_WITH_AES_128_CCM                       
 xc05c   ECDHE-ECDSA-ARIA128-GCM-SHA256    ECDH 253   ARIAGCM     128      TLS_ECDHE_ECDSA_WITH_ARIA_128_GCM_SHA256           
 xc060   ECDHE-ARIA128-GCM-SHA256          ECDH 253   ARIAGCM     128      TLS_ECDHE_RSA_WITH_ARIA_128_GCM_SHA256             
 xc052   DHE-RSA-ARIA128-GCM-SHA256        DH 4096    ARIAGCM     128      TLS_DHE_RSA_WITH_ARIA_128_GCM_SHA256               
 xc023   ECDHE-ECDSA-AES128-SHA256         ECDH 253   AES         128      TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256            
 xc027   ECDHE-RSA-AES128-SHA256           ECDH 253   AES         128      TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256              
 x67     DHE-RSA-AES128-SHA256             DH 4096    AES         128      TLS_DHE_RSA_WITH_AES_128_CBC_SHA256                
 xc072   ECDHE-ECDSA-CAMELLIA128-SHA256    ECDH 253   Camellia    128      TLS_ECDHE_ECDSA_WITH_CAMELLIA_128_CBC_SHA256       
 xc076   ECDHE-RSA-CAMELLIA128-SHA256      ECDH 253   Camellia    128      TLS_ECDHE_RSA_WITH_CAMELLIA_128_CBC_SHA256         
 xbe     DHE-RSA-CAMELLIA128-SHA256        DH 4096    Camellia    128      TLS_DHE_RSA_WITH_CAMELLIA_128_CBC_SHA256           
 xc009   ECDHE-ECDSA-AES128-SHA            ECDH 253   AES         128      TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA               
 xc013   ECDHE-RSA-AES128-SHA              ECDH 253   AES         128      TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA                 
 x33     DHE-RSA-AES128-SHA                DH 4096    AES         128      TLS_DHE_RSA_WITH_AES_128_CBC_SHA                   
 x45     DHE-RSA-CAMELLIA128-SHA           DH 4096    Camellia    128      TLS_DHE_RSA_WITH_CAMELLIA_128_CBC_SHA              
 x9c     AES128-GCM-SHA256                 RSA        AESGCM      128      TLS_RSA_WITH_AES_128_GCM_SHA256                    
 xc0a0   AES128-CCM8                       RSA        AESCCM8     128      TLS_RSA_WITH_AES_128_CCM_8                         
 xc09c   AES128-CCM                        RSA        AESCCM      128      TLS_RSA_WITH_AES_128_CCM                           
 xc050   ARIA128-GCM-SHA256                RSA        ARIAGCM     128      TLS_RSA_WITH_ARIA_128_GCM_SHA256                   
 x3c     AES128-SHA256                     RSA        AES         128      TLS_RSA_WITH_AES_128_CBC_SHA256                    
 xba     CAMELLIA128-SHA256                RSA        Camellia    128      TLS_RSA_WITH_CAMELLIA_128_CBC_SHA256               
 x2f     AES128-SHA                        RSA        AES         128      TLS_RSA_WITH_AES_128_CBC_SHA                       
 x41     CAMELLIA128-SHA                   RSA        Camellia    128      TLS_RSA_WITH_CAMELLIA_128_CBC_SHA                  
TLSv1.3 (server order)
 x1302   TLS_AES_256_GCM_SHA384            ECDH 253   AESGCM      256      TLS_AES_256_GCM_SHA384                             
 x1303   TLS_CHACHA20_POLY1305_SHA256      ECDH 253   ChaCha20    256      TLS_CHACHA20_POLY1305_SHA256                       
 x1301   TLS_AES_128_GCM_SHA256            ECDH 253   AESGCM      128      TLS_AES_128_GCM_SHA256

New Cipher List:

TLSv1.2 (server order)
 xc02b   ECDHE-ECDSA-AES128-GCM-SHA256     ECDH 253   AESGCM      128      TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256            
 xc02f   ECDHE-RSA-AES128-GCM-SHA256       ECDH 253   AESGCM      128      TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256              
 xc02c   ECDHE-ECDSA-AES256-GCM-SHA384     ECDH 253   AESGCM      256      TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384            
 xc030   ECDHE-RSA-AES256-GCM-SHA384       ECDH 253   AESGCM      256      TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384              
 xcca9   ECDHE-ECDSA-CHACHA20-POLY1305     ECDH 253   ChaCha20    256      TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305_SHA256      
 xcca8   ECDHE-RSA-CHACHA20-POLY1305       ECDH 253   ChaCha20    256      TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305_SHA256        
 x9e     DHE-RSA-AES128-GCM-SHA256         DH 4096    AESGCM      128      TLS_DHE_RSA_WITH_AES_128_GCM_SHA256                
 x9f     DHE-RSA-AES256-GCM-SHA384         DH 4096    AESGCM      256      TLS_DHE_RSA_WITH_AES_256_GCM_SHA384                
 xccaa   DHE-RSA-CHACHA20-POLY1305         DH 4096    ChaCha20    256      TLS_DHE_RSA_WITH_CHACHA20_POLY1305_SHA256          
TLSv1.3 (server order)
 x1302   TLS_AES_256_GCM_SHA384            ECDH 253   AESGCM      256      TLS_AES_256_GCM_SHA384                             
 x1303   TLS_CHACHA20_POLY1305_SHA256      ECDH 253   ChaCha20    256      TLS_CHACHA20_POLY1305_SHA256                       
 x1301   TLS_AES_128_GCM_SHA256            ECDH 253   AESGCM      128      TLS_AES_128_GCM_SHA256

Testing the change
To test the changes on your system, install testssl.sh and run:

testssl -t smtp mailcow.your.tld:25

During tests on my server, only scanning bots failed to connect to the SMTP server.