Kategorie:Kryptografie/Best Practice: Unterschied zwischen den Versionen

SSH

This section documents the Secure Shell (SSH) protocol. SSH is used to remotely manage computer systems, secururly transfer files over untrusted networks and to create "ad-hoc" virtual-private networks. SSH is defined by the following Internet Standards (RFCs): Table 2. SSH Standards

RFC	Title	Link
4251	The Secure Shell (SSH) Protocol Architecture	https://tools.ietf.org/html/rfc4251
4252	The Secure Shell (SSH) Authentication Protocol	https://tools.ietf.org/html/rfc4252
4253	The Secure Shell (SSH) Transport Layer Protocol	https://tools.ietf.org/html/rfc4253
6668	SHA-2 Data Integrity Verification for the Secure Shell (SSH) Transport Layer Protocol	https://tools.ietf.org/html/rfc6668
8268	More Modular Exponentiation (MODP) Diffie-Hellman (DH) Key Exchange (KEX) Groups for Secure Shell (SSH)	https://tools.ietf.org/html/rfc8268
8308	Extension Negotiation in the Secure Shell (SSH) Protocol	https://tools.ietf.org/html/rfc8308
8332	Use of RSA Keys with SHA-256 and SHA-512 in the Secure Shell (SSH) Protocol	https://tools.ietf.org/html/rfc8332

OpenSSH

• OpenSSH is the most popular implementation of the SSH protocol.
• It is maintained by the OpenBSD project and portable versions are disitributed with many unix-like operating-systems and Windows Server.

Tested with Version

• OpenSSH 6.6p1 (Gentoo)
• OpenSSH 6.6p1-2 on Ubuntu 14.04.2 LTS
• OpenSSH 7.2p2 on Ubuntu 16.04.3 LTS

Settings

Important OpenSSH 6.6 security settings

# Package generated configuration file
# See the sshd_config(5) manpage for details
# What ports, IPs and protocols we listen for
Port 22
# Use these options to restrict which interfaces/protocols sshd will bind to
#ListenAddress ::
#ListenAddress 0.0.0.0
Protocol 2
# HostKeys for protocol version 2
HostKey /etc/ssh/ssh_host_rsa_key
#HostKey /etc/ssh/ssh_host_dsa_key
#HostKey /etc/ssh/ssh_host_ecdsa_key
HostKey /etc/ssh/ssh_host_ed25519_key
#Privilege Separation is turned on for security
UsePrivilegeSeparation yes
# Lifetime and size of ephemeral version 1 server key
KeyRegenerationInterval 3600
ServerKeyBits 1024
# Logging
SyslogFacility AUTH
LogLevel INFO
# Authentication:
LoginGraceTime 120
PermitRootLogin no # or 'without-password' to allow SSH key based login
StrictModes yes
RSAAuthentication yes
PubkeyAuthentication yes
#AuthorizedKeysFile     %h/.ssh/authorized_keys
# Don't read the user's ~/.rhosts and ~/.shosts files
IgnoreRhosts yes
# For this to work you will also need host keys in /etc/ssh_known_hosts
RhostsRSAAuthentication no
# similar for protocol version 2
HostbasedAuthentication no
# Uncomment if you don't trust ~/.ssh/known_hosts for RhostsRSAAuthentication
#IgnoreUserKnownHosts yes
# To enable empty passwords, change to yes (NOT RECOMMENDED)
PermitEmptyPasswords no
# Change to yes to enable challenge-response passwords (beware issues with
# some PAM modules and threads)
ChallengeResponseAuthentication no
# Change to no to disable tunnelled clear text passwords
#PasswordAuthentication yes
# Kerberos options
#KerberosAuthentication no
#KerberosGetAFSToken no
#KerberosOrLocalPasswd yes
#KerberosTicketCleanup yes
# GSSAPI options
#GSSAPIAuthentication no
#GSSAPICleanupCredentials yes
# Cipher selection
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes128-ctr
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,hmac-ripemd160
KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1,diffie-hellman-group-exchange-sha1
X11Forwarding yes
X11DisplayOffset 10
PrintMotd no
PrintLastLog yes
TCPKeepAlive yes
#UseLogin no
#MaxStartups 10:30:60
#Banner /etc/issue.net
# Allow client to pass locale environment variables
AcceptEnv LANG LC_*
Subsystem sftp /usr/lib/openssh/sftp-server
# Set this to 'yes' to enable PAM authentication, account processing,
# and session processing. If this is enabled, PAM authentication will
# be allowed through the ChallengeResponseAuthentication and
# PasswordAuthentication.  Depending on your PAM configuration,
# PAM authentication via ChallengeResponseAuthentication may bypass
# the setting of "PermitRootLogin without-password".
# If you just want the PAM account and session checks to run without
# PAM authentication, then enable this but set PasswordAuthentication
# and ChallengeResponseAuthentication to 'no'.
UsePAM yes

Curve25519 OpenSSH 6.6p1 now supports Curve25519.

Tested with Version

• OpenSSH 6.5 (Debian Jessie)

Settings

Important OpenSSH 6.5 security settings

# Package generated configuration file
# See the sshd_config(5) manpage for details
# What ports, IPs and protocols we listen for
Port 22
# Use these options to restrict which interfaces/protocols sshd will bind to
#ListenAddress ::
#ListenAddress 0.0.0.0
Protocol 2
# HostKeys for protocol version 2
HostKey /etc/ssh/ssh_host_rsa_key
#HostKey /etc/ssh/ssh_host_dsa_key
#HostKey /etc/ssh/ssh_host_ecdsa_key
HostKey /etc/ssh/ssh_host_ed25519_key
#Privilege Separation is turned on for security
UsePrivilegeSeparation yes
# Lifetime and size of ephemeral version 1 server key
KeyRegenerationInterval 3600
ServerKeyBits 1024
# Logging
SyslogFacility AUTH
LogLevel INFO
# Authentication:
LoginGraceTime 120
PermitRootLogin no # or 'without-password' to allow SSH key based login
StrictModes yes
RSAAuthentication yes
PubkeyAuthentication yes
#AuthorizedKeysFile     %h/.ssh/authorized_keys
# Don't read the user's ~/.rhosts and ~/.shosts files
IgnoreRhosts yes
# For this to work you will also need host keys in /etc/ssh_known_hosts
RhostsRSAAuthentication no
# similar for protocol version 2
HostbasedAuthentication no
# Uncomment if you don't trust ~/.ssh/known_hosts for RhostsRSAAuthentication
#IgnoreUserKnownHosts yes
# To enable empty passwords, change to yes (NOT RECOMMENDED)
PermitEmptyPasswords no
# Change to yes to enable challenge-response passwords (beware issues with
# some PAM modules and threads)
ChallengeResponseAuthentication no
# Change to no to disable tunnelled clear text passwords
#PasswordAuthentication yes
# Kerberos options
#KerberosAuthentication no
#KerberosGetAFSToken no
#KerberosOrLocalPasswd yes
#KerberosTicketCleanup yes
# GSSAPI options
#GSSAPIAuthentication no
#GSSAPICleanupCredentials yes
# Cipher selection
Ciphers aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes128-ctr
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,hmac-ripemd160
KexAlgorithms diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1,diffie-hellman-group-exchange-sha1
X11Forwarding yes
X11DisplayOffset 10
PrintMotd no
PrintLastLog yes
TCPKeepAlive yes
#UseLogin no
#MaxStartups 10:30:60
#Banner /etc/issue.net
# Allow client to pass locale environment variables
AcceptEnv LANG LC_*
Subsystem sftp /usr/lib/openssh/sftp-server
# Set this to 'yes' to enable PAM authentication, account processing,
# and session processing. If this is enabled, PAM authentication will
# be allowed through the ChallengeResponseAuthentication and
# PasswordAuthentication.  Depending on your PAM configuration,
# PAM authentication via ChallengeResponseAuthentication may bypass
# the setting of "PermitRootLogin without-password".
# If you just want the PAM account and session checks to run without
# PAM authentication, then enable this but set PasswordAuthentication
# and ChallengeResponseAuthentication to 'no'.
UsePAM yes

Tested with Version

• OpenSSH 6.0p1 (Debian wheezy)

Settings

Important OpenSSH 6.0 security settings

# Package generated configuration file
# See the sshd_config(5) manpage for details
# What ports, IPs and protocols we listen for
Port 22
# Use these options to restrict which interfaces/protocols sshd will bind to
#ListenAddress ::
#ListenAddress 0.0.0.0
Protocol 2
# HostKeys for protocol version 2
HostKey /etc/ssh/ssh_host_rsa_key
#HostKey /etc/ssh/ssh_host_dsa_key
#HostKey /etc/ssh/ssh_host_ecdsa_key
#Privilege Separation is turned on for security
UsePrivilegeSeparation yes
# Lifetime and size of ephemeral version 1 server key
KeyRegenerationInterval 3600
ServerKeyBits 768
# Logging
SyslogFacility AUTH
LogLevel INFO
# Authentication:
LoginGraceTime 120
PermitRootLogin no # or 'without-password' to allow SSH key based login
StrictModes yes
RSAAuthentication yes
PubkeyAuthentication yes
#AuthorizedKeysFile     %h/.ssh/authorized_keys
# Don't read the user's ~/.rhosts and ~/.shosts files
IgnoreRhosts yes
# For this to work you will also need host keys in /etc/ssh_known_hosts
RhostsRSAAuthentication no
# similar for protocol version 2
HostbasedAuthentication no
# Uncomment if you don't trust ~/.ssh/known_hosts for RhostsRSAAuthentication
#IgnoreUserKnownHosts yes
# To enable empty passwords, change to yes (NOT RECOMMENDED)
PermitEmptyPasswords no
# Change to yes to enable challenge-response passwords (beware issues with
# some PAM modules and threads)
ChallengeResponseAuthentication no
# Change to no to disable tunnelled clear text passwords
#PasswordAuthentication yes
# Kerberos options
#KerberosAuthentication no
#KerberosGetAFSToken no
#KerberosOrLocalPasswd yes
#KerberosTicketCleanup yes
# GSSAPI options
#GSSAPIAuthentication no
#GSSAPICleanupCredentials yes
# Cipher selection
Ciphers aes256-ctr,aes128-ctr
MACs hmac-sha2-512,hmac-sha2-256,hmac-ripemd160
KexAlgorithms diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha1,diffie-hellman-group-exchange-sha1
X11Forwarding yes
X11DisplayOffset 10
PrintMotd no
PrintLastLog yes
TCPKeepAlive yes
#UseLogin no
#MaxStartups 10:30:60
#Banner /etc/issue.net
# Allow client to pass locale environment variables
AcceptEnv LANG LC_*
Subsystem sftp /usr/lib/openssh/sftp-server
# Set this to 'yes' to enable PAM authentication, account processing,
# and session processing. If this is enabled, PAM authentication will
# be allowed through the ChallengeResponseAuthentication and
# PasswordAuthentication.  Depending on your PAM configuration,
# PAM authentication via ChallengeResponseAuthentication may bypass
# the setting of "PermitRootLogin without-password".
# If you just want the PAM account and session checks to run without
# PAM authentication, then enable this but set PasswordAuthentication
# and ChallengeResponseAuthentication to 'no'.
UsePAM yes

Older Linux systems won’t support SHA2. PuTTY (Windows) does not support RIPE-MD160. Curve25519, AES-GCM and UMAC are only available upstream (OpenSSH 6.6p1). DSA host keys have been removed on purpose, the DSS standard does not support for DSA keys stronger than 1024bit [5] which is far below current standards (see section #section:keylengths). Legacy systems can use this configuration and simply omit unsupported ciphers, key exchange algorithms and MACs.

References

The OpenSSH sshd_config — OpenSSH SSH daemon configuration file man page is the best reference:

How to test

Connect a client with verbose logging enabled to the SSH server $ ssh -vvv myserver.com and observe the key exchange in the output.

Cisco ASA

Tested with Versions

• 9.1(3)

Settings

crypto key generate rsa modulus 2048 ssh version 2 ssh key-exchange group dh-group14-sha1

When the ASA is configured for SSH, by default both SSH versions 1 and 2 are allowed. In addition to that, only a group1 DH-key-exchange is used. This should be changed to allow only SSH version 2 and to use a key-exchange with group14. The generated RSA key should be 2048 bit (the actual supported maximum). A non-cryptographic best practice is to reconfigure the lines to only allow SSH-logins.

References

CLI Book 1: Cisco ASA Series General Operations CLI Configuration Guide, 9.1

How to test

Connect a client with verbose logging enabled to the SSH server $ ssh -vvv myserver.com and observe the key exchange in the output.

Cisco IOS

Tested with Versions

Table 3. Tested Myservice Versions

Program Version	OS/Distribution/Version	Comment
15.0	IOS
15.1	IOS
15.2	IOS

Settings

crypto key generate rsa modulus 4096 label SSH-KEYS ip ssh rsa keypair-name SSH-KEYS ip ssh version 2 ip ssh dh min size 2048 line vty 0 15 transport input ssh

Same as with the ASA, also on IOS by default both SSH versions 1 and 2 are allowed and the DH-key-exchange only use a DH-group of 768 Bit. In IOS, a dedicated Key-pair can be bound to SSH to reduce the usage of individual keys-pairs. From IOS Version 15.0 onwards, 4096 Bit rsa keys are supported and should be used according to the paradigm "use longest supported key". Also, do not forget to disable telnet vty access.

References

Cisco SSH

This guide is a basic SSH reference for all routers and switches. Pleaes refer to the specific documentation of the device and IOS version that you are configuring.

How to test

Connect a client with verbose logging enabled to the SSH server $ ssh -vvv switch.example.net and observe the key exchange in the output.

Mailservers

This section documents the most common mail servers. Mail servers may usually be grouped into three categories:* Mail Submission Agent (MSA)

• Mail Transfer Agent (MTA) / Mail Exchanger (MX)
• Mail Delivery Agent (MDA)

An email client (mail user agent, MUA) submits mail to the MSA. This is usually been done using the Simple Mail Transfer Protocol (SMTP). Afterwards, the mail is transmitted by the MTA over the Internet to the MTA of the receiver. This happens again via SMTP. Finally, the mail client of the receiver will fetch mail from an MDA usually via the Internet Message Access Protocol (IMAP) or the Post Office Protocol (POP). As MSAs and MTAs both use SMTP as transfer protocols, both functionalities may often be implemented with the same software. On the other hand, MDA software might or might not implement both IMAP and POP.

TLS usage in mail server protocols

Email protocols support TLS in two different ways. It may be added as a protocol wrapper on a different port. This method is referred to as Implicit TLS or as protocol variants SMTPS, IMAPS and POP3S. The other method is to establish a cleartext session first and switch to TLS afterwards by issuing the STARTTLS command. SMTP between MTAs usually makes use of opportunistic TLS. This means that an MTA will accept TLS connections when asked for it but will not require it. MTAs should always try opportunistic TLS handshakes outgoing and always accept incoming opportunistic TLS.

Recommended configuration

We recommend to use the following settings for Mail Transfer Agents:* correctly setup MX, A and PTR RRs without using CNAMEs at all

• the hostname used as HELO/EHLO in outgoing mail shall match the PTR RR
• enable opportunistic TLS, using the STARTTLS mechanism on port 25
• Implicit TLS on port 465 may be offered additionally
• use server and client certificates (most server certificates are client certificates as well)
• either the common name or at least an alternate subject name of the certificate shall match the PTR RR (client mode) or the MX RR (server mode)
• do not use self signed certificates
• accept all cipher suites, as the alternative would be to fall back to cleartext transmission
• an execption to the last sentence is that MTAs MUST NOT enable SSLv2 protocol support, due to the DROWN attack.

For MSA operation we recommend:* listen on submission port 587 with mandatory STARTTLS

• optionally listen on port 465 with Implicit TLS
• enforce SMTP AUTH even for local networks
• ensure that SMTP AUTH is not allowed on unencrypted connections
• only use the recommended cipher suites if all connecting MUAs support them

For MDA operation we recommend:* listen on the protocol port (143 for IMAP, 110 for POP3) with mandatory STARTTLS

• optionally listen on Implicit TLS ports (993 for IMAPS, 995 for POP3S)
• enforce authentication even for local networks
• make sure that authentication is not allowed on unencrypted connections
• use the recommended cipher suites if all connecting MUAs support them
• turn off SSLv2 (see: DROWN attack)

Dovecot

Tested with Versions

Table 4. Tested Dovecot Versions

Program Version	OS/Distribution/Version	Comment
2.1.7	Debian Wheezy	without ssl_prefer_server_ciphers
2.2.9	Debian Jessie
2.2.13	Debian 8.2 Jessie
2.0.19apple1	OS X Server 10.8.5	without ssl_prefer_server_ciphers
2.2.9	Ubuntu 14.04 Trusty
2.2.31	Ubuntu 16.04.3 LTS

Settings

Dovecot SSL Configuration

# SSL protocols to use, disable SSL, use TLS only

ssl_protocols = !SSLv3 !SSLv2

# SSL ciphers to use

ssl_cipher_list = EDH+CAMELLIA:EDH+aRSA:EECDH+aRSA+AESGCM:EECDH+aRSA+SHA256:EECDH:+CAMELLIA128:+AES128:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!DSS:!RC4:!SEED:!IDEA:!ECDSA:kEDH:CAMELLIA128-SHA:AES128-SHA

# Prefer the server's order of ciphers over client's. (Dovecot >=2.2.6 Required)

ssl_prefer_server_ciphers = yes

# Diffie-Hellman parameters length (Default is 1024, Dovecot >=2.2.7 Required)
# ToDo: for ReGenerating DH-Parameters:
# manually delete /var/lib/dovecot/ssl-parameters.dat and restart
# Dovecot to regenerate /var/lib/dovecot/ssl-parameters.dat

ssl_dh_parameters_length = 2048

# Disable Compression (Dovecot >= 2.2.14 Required)
# ssl_options = no_compression

Dovecot 2.0, 2.1: Almost as good as dovecot 2.2. Dovecot does not ignore unknown configuration parameters. Does not support ssl_prefer_server_ciphers.

Limitations

• Dovecot <2.2.14 does not support disabling TLS compression.In >2.2.14 [6] use: ssl_options = no_compression
• Dovecot <2.2.7 uses fixed DH parameters.In >2.2.7 [7] greater DH-Parameters are supported: ssl_dh_parameters_length = 2048.

References

• https://wiki2.dovecot.org/SSL

How to test

$ openssl s_client -crlf -connect example.com:993 $ openssl s_client -crlf -connect example.com:995 $ openssl s_client -crlf -starttls imap -connect example.com:143 $ openssl s_client -crlf -starttls pop3 -connect example.com:110 SSLyze offers scanning for common vulnerabilities and displays Protocols and Cipher-Suites. $ sslyze.exe --regular example.com:993 $ sslyze.exe --regular example.com:995 $ sslyze.exe --regular --starttls=imap example.com:143 $ sslyze.exe --regular --starttls=pop3 example.com:110

cyrus-imapd

Tested with Versions

Table 5. Tested cyrus-imapd Versions

Program Version	OS/Distribution/Version	Comment
2.4.17

Settings

To activate SSL/TLS configure your certificate with

Activating TLS in cyrus

tls_cert_file: /etc/ssl/certs/ssl-cert-snakeoil.pem tls_key_file: /etc/ssl/private/ssl-cert-snakeoil.key Do not forget to add necessary intermediate certificates to the .pem file. Limiting the ciphers provided may force (especially older) clients to connect without encryption at all! Sticking to the defaults is recommended. If you still want to force strong encryption use

TLS cipher selection in cyrus

tls_cipher_list: EDH+CAMELLIA:EDH+aRSA:EECDH+aRSA+AESGCM:EECDH+aRSA+SHA256:EECDH:+CAMELLIA128:+AES128:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!DSS:!RC4:!SEED:!IDEA:!ECDSA:kEDH:CAMELLIA128-SHA:AES128-SHA cyrus-imapd loads hardcoded 1024 bit DH parameters using get_rfc2409_prime_1024() by default. If you want to load your own DH parameters add them PEM encoded to the certificate file given in tls_cert_file. Do not forget to re-add them after updating your certificate. To prevent unencrypted connections on the STARTTLS ports you can set

Force encrypted connections in cyrus

allowplaintext: no This way MUAs can only authenticate with plain text authentication schemes after issuing the STARTTLS command. Providing CRAM-MD5 or DIGEST-MD5 methods is not recommended. To support POP3/IMAP on ports 110/143 with STARTTLS and POP3S/IMAPS on ports 995/993 check the SERVICES section in cyrus.conf

STARTTLS for POP3/IMAP and POP3S/IMAPS in cyrus

SERVICES {

   imap  cmd="imapd -U 30"    listen="imap"  prefork=0 maxchild=100
   imaps cmd="imapd -s -U 30" listen="imaps" prefork=0 maxchild=100
   pop3  cmd="pop3d -U 30"    listen="pop3"  prefork=0 maxchild=50
   pop3s cmd="pop3d -s -U 30" listen="pop3s" prefork=0 maxchild=50

}

Limitations

cyrus-imapd currently (2.4.17, trunk) does not support elliptic curve cryptography. Hence, ECDHE will not work even if defined in your cipher list. Currently there is no way to prefer server ciphers or to disable compression. There is a working patch for all three features.

How to test

$ openssl s_client -crlf -connect example.com:993

Postfix

Tested with Versions

Table 6. Tested Postfix Versions

Program Version	OS/Distribution/Version	Comment
2.9.6	Debian Wheezy	with OpenSSL 1.0.1e
2.11.0	Ubuntu 14.04.02	with OpenSSL 1.0.1f
3.1.0	Ubuntu 16.04.3 LTS

Settings

Postfix has five internal lists of ciphers, and the possibility to switch between those with smtpd_tls_ciphers. However, we leave this at its default value for server to server connections, as many mail servers only support outdated protocols and ciphers. We consider bad encryption still better than plain text transmission. For connections to MUAs, TLS is mandatory and the ciphersuite is modified.

MX and SMTP client configuration:

As discussed in section [smtp_general], because of opportunistic encryption we do not restrict the list of ciphers or protocols for communication with other mail servers to avoid transmission in plain text. There are still some steps needed to enable TLS, all in main.cf:

Opportunistic TLS in Postfix

# TLS parameters

smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key

# log TLS connection info

smtpd_tls_loglevel = 1 smtp_tls_loglevel = 1

# enable opportunistic TLS support in the SMTP server and client

smtpd_tls_security_level = may smtp_tls_security_level = may

# if you have authentication enabled, only offer it after STARTTLS

smtpd_tls_auth_only = yes tls_ssl_options = NO_COMPRESSION

MSA:

For the MSA smtpd process which communicates with mail clients, we first define the ciphers that are acceptable for the “mandatory” security level, again in main.cf:

MSA TLS configuration in Postfix

smtp_tls_mandatory_protocols = !SSLv2, !SSLv3 smtp_tls_protocols = !SSLv2, !SSLv3 lmtp_tls_mandatory_protocols = !SSLv2, !SSLv3 lmtp_tls_protocols = !SSLv2, !SSLv3 smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3 smtpd_tls_protocols = !SSLv2, !SSLv3 smtpd_tls_mandatory_ciphers=high tls_high_cipherlist=EDH+CAMELLIA:EDH+aRSA:EECDH+aRSA+AESGCM:EECDH+aRSA+SHA256:EECDH:+CAMELLIA128:+AES128:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!DSS:!RC4:!SEED:!IDEA:!ECDSA:kEDH:CAMELLIA128-SHA:AES128-SHA Then, we configure the MSA smtpd in master.cf with two additional options that are only used for this instance of smtpd:

MSA smtpd service configuration in Postfix

# ==========================================================================
# service type  private unpriv  chroot  wakeup  maxproc command + args
#               (yes)   (yes)   (no)    (never) (100)
# ==========================================================================
# ...

submission inet n - - - - smtpd

   -o smtpd_tls_security_level=encrypt
   -o tls_preempt_cipherlist=yes
# ...

For those users who want to use EECDH key exchange, it is possible to customize this via: The default value since Postfix 2.8 is “strong”.

EECDH customization in Postfix

smtpd_tls_eecdh_grade = ultra

Limitations

tls_ssl_options is supported from Postfix 2.11 onwards. You can leave the statement in the configuration for older versions, it will be ignored. tls_preempt_cipherlist is supported from Postfix 2.8 onwards. Again, you can leave the statement in for older versions.

References

• Refer to https://www.postfix.org/TLS_README.html for an in-depth discussion.

Additional settings

Postfix has two sets of built-in DH parameters that can be overridden with the smtpd_tls_dh512_param_file and smtpd_tls_dh1024_param_file options. The “dh512” parameters are used for export ciphers, while the “dh1024” ones are used for all other ciphers. The “bit length” in those parameter names is just a name, so one could use stronger parameter sets; it should be possible to e.g. use the IKE Group14 parameters (see section [DH] without much interoperability risk, but we have not tested this yet.

How to test

You can check the effect of the settings with the following command: $ zegrep "TLS connection established from.*with cipher" /var/log/mail.log | awk '{printf("%s %s %s %s\n", $12, $13, $14, $15)}' | sort | uniq -c | sort -n

     1 SSLv3 with cipher DHE-RSA-AES256-SHA
    23 TLSv1.2 with cipher DHE-RSA-AES256-GCM-SHA384
    60 TLSv1 with cipher ECDHE-RSA-AES256-SHA
   270 TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384
   335 TLSv1 with cipher DHE-RSA-AES256-SHA

$ openssl s_client -starttls smtp -crlf -connect example.com:25

Exim

Tested with Versions

Table 7. Tested Exim Versions

Program Version	OS/Distribution/Version	Comment
4.82	Debian Jessie
4.82	Ubuntu 14.04.2	with OpenSSL 1.0.1e

It is highly recommended to read Encrypted SMTP connections using TLS/SSL first.

MSA mode (submission):

In the main config section of Exim add:

Certificate selection in Exim (MSA)

tls_certificate = /etc/ssl/exim.crt tls_privatekey = /etc/ssl/exim.pem Don’t forget to add intermediate certificates to the .pem file if needed. Tell Exim to advertise STARTTLS in the EHLO answer to everyone:

TLS advertise in Exim (MSA)

tls_advertise_hosts = * If you want to support legacy SMTPS on port 465, and STARTTLS on smtp(25)/submission(587) ports set

STARTTLS and SMTPS in Exim (MSA)

daemon_smtp_ports = smtp : smtps : submission tls_on_connect_ports = 465 It is highly recommended to limit SMTP AUTH to SSL connections only. To do so add

SSL-only authentication in Exim (MSA)

server_advertise_condition = ${if eq{$tls_cipher}{}{no}{yes}} to every authenticator defined. Add the following rules on top of your acl_smtp_mail:

Submission mode in Exim (MSA)

acl_smtp_mail = acl_check_mail acl_check_mail:

 warn hosts = *
   control = submission/sender_retain
 accept

This switches Exim to submission mode and allows addition of missing “Message-ID” and “Date” headers. It is not advisable to restrict the default cipher list for MSA mode if you don’t know all connecting MUAs. If you still want to define one please consult the Exim documentation or ask on the exim-users mailinglist. The cipher used is written to the logfiles by default. You may want to add log_selector = <whatever your log_selector already contains> +tls_certificate_verified +tls_peerdn +tls_sni to get even more TLS information logged.

Server mode (incoming):

In the main config section of Exim add:

Certificate selection in Exim (Server)

tls_certificate = /etc/ssl/exim.crt tls_privatekey = /etc/ssl/exim.pem Don’t forget to add intermediate certificates to the .pem file if needed. Tell Exim to advertise STARTTLS in the EHLO answer to everyone:

TLS advertise in Exim (Server)

tls_advertise_hosts = * Listen on smtp(25) port only:

STARTTLS on SMTP in Exim (Server)

daemon_smtp_ports = smtp It is not advisable to restrict the default cipher list for opportunistic encryption as used by SMTP. Do not use cipher lists recommended for HTTPS! If you still want to define one please consult the Exim documentation or ask on the exim-users mailinglist. If you want to request and verify client certificates from sending hosts set

TLS certificate verification in Exim (Server)

tls_verify_certificates = /etc/pki/tls/certs/ca-bundle.crt tls_try_verify_hosts = * tls_try_verify_hosts only reports the result to your logfile. If you want to disconnect such clients you have to use tls_verify_hosts = * The cipher used is written to the logfiles by default. You may want to add log_selector = <whatever your log_selector already contains> +tls_certificate_verified +tls_peerdn +tls_sni to get even more TLS information logged.

Client mode (outgoing):

Exim uses opportunistic encryption in the SMTP transport by default. Client mode settings have to be done in the configuration section of the smtp transport (driver = smtp). If you want to use a client certificate (most server certificates can be used as client certificate, too) set

Certificate selection in Exim (Client)

tls_certificate = /etc/ssl/exim.crt tls_privatekey = /etc/ssl/exim.pem This is recommended for MTA-MTA traffic. Do not limit ciphers without a very good reason. In the worst case you end up without encryption at all instead of some weak encryption. Please consult the Exim documentation if you really need to define ciphers.

OpenSSL:

Exim already disables SSLv2 by default. We recommend to add openssl_options = +all +no_sslv2 +no_sslv3 +no_compression +cipher_server_preference to the main configuration.

+all is misleading here since OpenSSL only activates the most common workarounds. But that’s how SSL_OP_ALL is defined.

You do not need to set dh_parameters. Exim with OpenSSL by default uses parameter initialization with the “2048-bit MODP Group with 224-bit Prime Order Subgroup” defined in section 2.2 of RFC 5114 (ike23). If you want to set your own DH parameters please read the TLS documentation of exim.

GnuTLS:

GnuTLS is different in only some respects to OpenSSL:* tls_require_ciphers needs a GnuTLS priority string instead of a cipher list. It is recommended to use the defaults by not defining this option. It highly depends on the version of GnuTLS used. Therefore it is not advisable to change the defaults.

• There is no option like openssl_options

Exim string expansion Most of the options accept expansion strings. This way you can e.g. set cipher lists or STARTTLS advertisement conditionally. Please follow the link to the official Exim documentation to get more information.

Limitations:

Exim currently (4.82) does not support elliptic curves with OpenSSL. This means that ECDHE is not used even if defined in your cipher list. There already is a working patch to provide support.

How to test

$ openssl s_client -starttls smtp -crlf -connect example.com:25

Cisco ESA/IronPort

Tested with Versions

Table 8. Tested Cisco ESA/IronPort Versions

Program Version	OS/Distribution/Version	Comment
	AsyncOS 7.6.1
	AsyncOS 8.5.6
	AsyncOS 9.0.0
	AsyncOS 9.5.0
	AsyncOS 9.6.0
	AsyncOS 9.7.0

Settings

Import your certificate(s) using the WEBUI (Network → Certificates). From AsyncOS 9.0 and up, SSL parameters for inbound SMTP, outbound SMTP and GUI access can be configured in one step via the WEBUI (System Administration → SSL Configuration, see figure IronPort Default SSL Settings). For all versions prior to 9.0, you have to connect to the CLI and configure the SSL parameters separately, as shown below using inbound SMTP as example. ironport.example.com> sslconfig sslconfig settings:

 GUI HTTPS method:  sslv3tlsv1
 GUI HTTPS ciphers: RC4-SHA:RC4-MD5:ALL
 Inbound SMTP method:  sslv3tlsv1
 Inbound SMTP ciphers: RC4-SHA:RC4-MD5:ALL
 Outbound SMTP method:  sslv3tlsv1
 Outbound SMTP ciphers: RC4-SHA:RC4-MD5:ALL

Choose the operation you want to perform: - GUI - Edit GUI HTTPS ssl settings. - INBOUND - Edit Inbound SMTP ssl settings. - OUTBOUND - Edit Outbound SMTP ssl settings. - VERIFY - Verify and show ssl cipher list. []> inbound Enter the inbound SMTP ssl method you want to use. 1. SSL v2. 2. SSL v3 3. TLS v1 4. SSL v2 and v3 5. SSL v3 and TLS v1 6. SSL v2, v3 and TLS v1 [5]> 3 Enter the inbound SMTP ssl cipher you want to use. [RC4-SHA:RC4-MD5:ALL]> EDH+CAMELLIA:EDH+aRSA:EECDH+aRSA+AESGCM:EECDH+aRSA+SHA256:EECDH:+CAMELLIA128:+AES128:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!DSS:!RC4:!SEED:!IDEA:!ECDSA:kEDH:CAMELLIA128-SHA:AES128-SHA sslconfig settings:

 GUI HTTPS method:  sslv3tlsv1
 GUI HTTPS ciphers: RC4-SHA:RC4-MD5:ALL
 Inbound SMTP method:  tlsv1
 Inbound SMTP ciphers: EDH+CAMELLIA:EDH+aRSA:EECDH+aRSA+AESGCM:EECDH+aRSA+SHA384:EECDH+aRSA+SHA256:EECDH:+CAMELLIA256:+AES256:+CAMELLIA128:+AES128:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!RC4:!SEED:!ECDSA:CAMELLIA256-SHA:AES256-SHA:CAMELLIA128-SHA:AES128-SHA
 Outbound SMTP method:  sslv3tlsv1
 Outbound SMTP ciphers: RC4-SHA:RC4-MD5:ALL

Starting with AsyncOS 9.0 SSLv3 is disabled by default, whereas the default cipher set is still RC4-SHA:RC4-MD5:ALL (see figure IronPort Default SSL Settings).

"IronPort Default SSL Settings" Figure 1. IronPort Default SSL Settings After committing these changes in the CLI, you have to activate the use of TLS in several locations. For inbound connections, first select the appropriate certificate in the settings of each listener you want to have TLS enabled on (Network → Listeners, see figure IronPort Default SSL Settings). Afterwards, for each listener, configure all Mail Flow Policies which have their Connection Behavior set to “Accept” or “Relay” to at least prefer TLS (Mail Policies → Mail Flow Policies, see figure IronPort Default SSL Settings). It is recommended to also enable TLS in the default Mail Flow Policy, because these settings will be inherited by newly created policies, unless specifically overwritten. + TLS can be enforced by creating a new Mail Flow Policy with TLS set to “required”, creating a new Sender Group defining the addresses of the sending mail servers for which you want to enforce encryption (Mail Policies → HAT Overview) and using this new Sender Group in conjunction with the newly created Mail Flow Policy. "IronPort Listener Settings" Figure 2. IronPort Listener Settings "IronPort Mail Flow Policy Security Features" Figure 3. IronPort Mail Flow Policy Security Features TLS settings for outbound connections have to be configured within the Destination Controls (Mail Policies → Destination Controls). Choose the appropriate SSL certificate within the global settings and configure TLS to be preferred in the default profile to enable it for all outbound connections. After these two steps the Destination Control overview page should look like figure IronPort Destination Control overview on page . To enforce TLS for a specific destination domain, add an entry to the Destination Control Table and set “TLS Support” to “required”. "Destination Control overview" Figure 4. IronPort Destination Control overview

Limitations

All AsyncOS releases prior to version 9.5 use OpenSSL 0.9.8. Therefore TLS 1.2 is not supported in these versions and some of the suggested ciphers won’t work. Starting with AsyncOS 9.5 TLS 1.2 is fully supported. [8] You can check the supported ciphers on the CLI by using the option verify from within the sslconfig command: []> verify Enter the ssl cipher you want to verify. []> EDH+CAMELLIA:EDH+aRSA:EECDH+aRSA+AESGCM:EECDH+aRSA+SHA256:EECDH:+CAMELLIA128:+AES128:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!DSS:!RC4:!SEED:!IDEA:!ECDSA:kEDH:CAMELLIA128-SHA:AES128-SHA DHE-RSA-CAMELLIA256-SHA SSLv3 Kx=DH Au=RSA Enc=Camellia(256) Mac=SHA1 DHE-RSA-CAMELLIA128-SHA SSLv3 Kx=DH Au=RSA Enc=Camellia(128) Mac=SHA1 DHE-RSA-AES256-SHA SSLv3 Kx=DH Au=RSA Enc=AES(256) Mac=SHA1 DHE-RSA-AES128-SHA SSLv3 Kx=DH Au=RSA Enc=AES(128) Mac=SHA1 CAMELLIA128-SHA SSLv3 Kx=RSA Au=RSA Enc=Camellia(128) Mac=SHA1 AES128-SHA SSLv3 Kx=RSA Au=RSA Enc=AES(128) Mac=SHA1

How to test

$ openssl s_client -starttls smtp -crlf -connect example.com:25

Virtual Private Networks

Virtual Private Networks (VPN) provide means of connecting private networks over external / public transport networks. Since we do not control the transport networks we have to to take care about data integrity and privacy.

Some providers also offer VPNs for connecting company networks via MPLS or simular technologies. The provider promises you that nobody else has access to your data trnfserred over their lines. You have to decide if this promise does fit to your information security requirements.

Basically there are two ways to ensure the privacy and integrity of the data sent via external lines. First you could utilize the built-in security of the Internet Protocol (IPsec). The other way are specific applications that encrypt the data before sending it over the line. IPsec is an Internet standard (TODO RFC 6071). This guarantees the interoperability between implementations of different vendors. You can establish a secure connection with your customer or your supplier. On the other hand VPN applications mostly utilize TLS to secury the communication between the partners. TODO: Descision criteria IPsec vs. VPN application.

IPsec

IPsec is the Internet standard to encrypt the transport of data between private networks.

Technology

We describe only the use the Internet Key Exchange (IKE) version 2 in this dodument. IKEv1 has some usage and security problems and should not be used any more. (TODO: reference).

Authentication

IPsec authentication should optimally be performed via RSA signatures, with a key size of 2048 bits or more. Configuring only the trusted CA that issued the peer certificate provides for additional protection against fake certificates. If you need to use Pre-Shared Key (PSK) authentication:* Choose a random, long enough PSK (see below)

• Use a separate PSK for any IPsec connection
• Change the PSKs regularly
• Think aboute a secure way to exchange the PSK. Sending an SMS it not secure.

The size of the PSK should not be shorter than the output size of the hash algorithm used in IKE.footnote[It is used in a HMAC, see RFC2104  and the discussion starting in TODO: verify http://www.vpnc.org/ietf-ipsec/02.ipsec/msg00268.html.]. For a key composed only of upper- and lowercase letters, numbers, and two additional characters[9], table #tab:IPSEC_psk_len gives the minimum lengths in characters.

Cryptographic Suites:

IPSEC Cryptographic Suites are pre-defined settings for all the items of a configuration; they try to provide a balanced security level and make setting up VPNs easier. [10] When using any of those suites, make sure to enable “Perfect Forward Secrecy“ for Phase 2, as this is not specified in the suites. The equivalents to the recommended ciphers suites in section Recommended cipher suites are shown in table #tab:IPSEC_suites.

Phase 1:

TODO: Phase 1 and 2 are phrases from IKEv1. Re-write for IKEv2. Alternatively to the pre-defined cipher suites, you can define your own, as described in this and the next section. Phase 1 is the mutual authentication and key exchange phase; table #tab:IPSEC_ph1_params shows the parameters. Use only “main mode“, as “aggressive mode“ has known security vulnerabilities [11].

Phase 2:

Phase 2 is where the parameters that protect the actual data are negotiated; recommended parameters are shown in table #tab:IPSEC_ph2_params.

References

• Fergusen, N. and Schneier, B.: A Cryptographic Evaluation of IPsec: https://www.schneier.com/paper-ipsec.pdf

Check Point Next Generation Firewall

Tested with Versions

Table 9. Tested CheckPoint Versions

Program Version	OS/Distribution/Version	Comment
R77	Secure Platform

Settings

Please see section IPsec for guidance on parameter choice. In this section, we will configure a strong setup according to Configuration A. This is based on the concept of a VPN Community, which has all the settings for the gateways that are included in that community. Communities can be found in the IPSEC VPN tab of SmartDashboard. "VPN Community encryption properties" [fig:checkpoint_1] Either choose one of the encryption suites in the properties dialog (figure [checkpoint_1], or proceed to Custom Encryption..., where you can set encryption and hash for Phase 1 and 2 (figure [checkpoint_2]. "Custom Encryption Suite Properties" [checkpoint_2] The Diffie-Hellman groups and Perfect Forward Secrecy Settings can be found under Advanced Settings / Advanced VPN Properties (figure [checkpoint_3]. "Advanced VPN Properties" [checkpoint_3]

Additional settings

For remote Dynamic IP Gateways, the settings are not taken from the community, but set in the Global Properties dialog under Remote Access / VPN Authentication and Encryption. Via the Edit... button, you can configure sets of algorithms that all gateways support (figure [checkpoint_4]). "Remote Access Encryption Properties" [checkpoint_4] Please note that these settings restrict the available algorithms for all gateways, and also influence the VPN client connections.

References

• Check Point VPN R77 Administration Guide (may require a UserCenter account to access)

TLS Based Applications

OpenVPN

Tested with Versions

Table 10. Tested OpenVPN Versions

Program Version	OS/Distribution/Version	Comment
2.3.10	Ubuntu Xenial 16.04.1 LTS	linked against openssl (libssl.so.1.0.0)
2.3.2	Debian Wheezy (backports)	linked against openssl (libssl.so.1.0.0)
2.2.1	Debian Wheezy	linked against openssl (libssl.so.1.0.0)
2.3.2	Windows

Settings

General

We describe a configuration with certificate-based authentication; see below for details on the easyrsa tool to help you with that. OpenVPN uses TLS only for authentication and key exchange. The bulk traffic is then encrypted and authenticated with the OpenVPN protocol using those keys. Note that while the tls-cipher option takes a list of ciphers that is then negotiated as usual with TLS, the cipher and auth options both take a single argument that must match on client and server. OpenVPN duplexes the tunnel into a data and a control channel. The control channel is a usual TLS connection, the data channel currently uses encrypt-then-mac CBC, see https://github.com/BetterCrypto/Applied-Crypto-Hardening/pull/91#issuecomment-75365286

Server Configuration

Client Configuration

Client and server have to use compatible configurations, otherwise they can’t communicate. The cipher and auth directives have to be identical.

Justification for special settings

OpenVPN 2.3.1 changed the values that the tls-cipher option expects from OpenSSL to IANA cipher names. That means from that version on you will get Deprecated TLS cipher name warnings for the configurations above. You cannot use the selection strings from section Recommended cipher suites directly from 2.3.1 on, which is why we give an explicit cipher list here. In addition, there is a 256 character limit on configuration file line lengths; that limits the size of cipher suites, so we dropped all ECDHE suites. The configuration shown above is compatible with all tested versions.

References

• OpenVPN Documentation: Security Overview https://openvpn.net/index.php/open-source/documentation/security-overview.html

Additional settings

Key renegotiation interval

The default for renegotiation of encryption keys is one hour (reneg-sec 3600). If you transfer huge amounts of data over your tunnel, you might consider configuring a shorter interval, or switch to a byte- or packet-based interval (reneg-bytes or reneg-pkts).

Insecure ciphers

Sweet32[12] is an attack on 64-bit block ciphers, such as 3DES and Blowfish in OpenVPN. The following ciphers are affected, and should no longer be used:* BF-*

• DES* (including 3DES variants)
• RC2-*

The following ciphers are not affected:* AES-*

• CAMELLIA-*
• SEED-*

According to mitigation section on Sweet32 website[13] users users should change the cipher from the DES or Blowfish to AES (cipher AES-128-CBC). If cipher change is not possible users can mitigate the attack by forcing frequent rekeying (reneg-bytes 64000000).

Fixing “easy-rsa”

When installing an OpenVPN server instance, you are probably using easy-rsa to generate keys and certificates. The file vars in the easyrsa installation directory has a number of settings that should be changed to secure values: This will enhance the security of the key generation by using RSA keys with a length of 4096 bits, and set a lifetime of one year for the server/client certificates and five years for the CA certificate.

4096 bits is only an example of how to do this with easy-rsa. See also section Keylengths for a discussion on keylengths.

In addition, edit the pkitool script and replace all occurrences of sha1 with sha256, to sign the certificates with SHA256.

Limitations

Note that the ciphersuites shown by openvpn --show-tls are known, but not necessarily supported [14]. Which cipher suite is actually used can be seen in the logs: Control Channel: TLSv1, cipher TLSv1/SSLv3 DHE-RSA-CAMELLIA256-SHA, 2048 bit RSA

PPTP

PPTP is considered insecure, Microsoft recommends to _use a more secure VPN tunnel_[15]. There is a cloud service that cracks the underlying MS-CHAPv2 authentication protocol for the price of USD 200[16], and given the resulting MD4 hash, all PPTP traffic for a user can be decrypted.

Cisco ASA

The following settings reflect our recommendations as best as possible on the Cisco ASA platform. These are - of course - just settings regarding SSL/TLS (i.e. Cisco AnyConnect) and IPsec. For further security settings regarding this platform the appropriate Cisco guides should be followed.

Tested with Versions

Table 11. Tested Cisco ASA Versions

Program Version	OS/Distribution/Version	Comment
9.1(3)	X-series model

Settings

crypto ipsec ikev2 ipsec-proposal AES-Fallback

protocol esp encryption aes-256 aes-192 aes
protocol esp integrity sha-512 sha-384 sha-256

crypto ipsec ikev2 ipsec-proposal AES-GCM-Fallback

protocol esp encryption aes-gcm-256 aes-gcm-192 aes-gcm
protocol esp integrity sha-512 sha-384 sha-256

crypto ipsec ikev2 ipsec-proposal AES128-GCM

protocol esp encryption aes-gcm
protocol esp integrity sha-512

crypto ipsec ikev2 ipsec-proposal AES192-GCM

protocol esp encryption aes-gcm-192
protocol esp integrity sha-512

crypto ipsec ikev2 ipsec-proposal AES256-GCM

protocol esp encryption aes-gcm-256
protocol esp integrity sha-512

crypto ipsec ikev2 ipsec-proposal AES

protocol esp encryption aes
protocol esp integrity sha-1 md5

crypto ipsec ikev2 ipsec-proposal AES192

protocol esp encryption aes-192
protocol esp integrity sha-1 md5

crypto ipsec ikev2 ipsec-proposal AES256

protocol esp encryption aes-256
protocol esp integrity sha-1 md5

crypto ipsec ikev2 sa-strength-enforcement crypto ipsec security-association pmtu-aging infinite crypto dynamic-map SYSTEM_DEFAULT_CRYPTO_MAP 65535 set pfs group14 crypto dynamic-map SYSTEM_DEFAULT_CRYPTO_MAP 65535 set ikev2 ipsec-proposal AES256-GCM AES192-GCM AES128-GCM AES-GCM-Fallback AES-Fallback crypto map Outside-DMZ_map 65535 ipsec-isakmp dynamic SYSTEM_DEFAULT_CRYPTO_MAP crypto map Outside-DMZ_map interface Outside-DMZ crypto ikev2 policy 1

encryption aes-gcm-256
integrity null
group 14
prf sha512 sha384 sha256 sha
lifetime seconds 86400

crypto ikev2 policy 2

encryption aes-gcm-256 aes-gcm-192 aes-gcm
integrity null
group 14
prf sha512 sha384 sha256 sha
lifetime seconds 86400

crypto ikev2 policy 3

encryption aes-256 aes-192 aes
integrity sha512 sha384 sha256
group 14
prf sha512 sha384 sha256 sha
lifetime seconds 86400

crypto ikev2 policy 4

encryption aes-256 aes-192 aes
integrity sha512 sha384 sha256 sha
group 14
prf sha512 sha384 sha256 sha
lifetime seconds 86400

crypto ikev2 enable Outside-DMZ client-services port 443 crypto ikev2 remote-access trustpoint ASDM_TrustPoint0 ssl server-version tlsv1-only ssl client-version tlsv1-only ssl encryption dhe-aes256-sha1 dhe-aes128-sha1 aes256-sha1 aes128-sha1 ssl trust-point ASDM_TrustPoint0 Outside-DMZ

Justification for special settings

New IPsec policies have been defined which do not make use of ciphers that may be cause for concern. Policies have a "Fallback" option to support legacy devices. 3DES has been completely disabled as such Windows XP AnyConnect Clients will no longer be able to connect. The Cisco ASA platform does not currently support RSA Keys above 2048bits. Legacy ASA models (e.g. 5505, 5510, 5520, 5540, 5550) do not offer the possibility to configure for SHA256/SHA384/SHA512 nor AES-GCM for IKEv2 proposals.

References

• http://www.cisco.com/en/US/docs/security/asa/roadmap/asaroadmap.html
• http://www.cisco.com/web/about/security/intelligence/nextgen_crypto.html

Openswan

Tested with Version

Table 12. Tested OpenS/WAN Versions

Program Version	OS/Distribution/Version	Comment
2.6.39	Gentoo

Settings

The available algorithms depend on your kernel configuration (when using protostack=netkey) and/or build-time options.

To list the supported algorithms $ ipsec auto --status | less and look for ’algorithm ESP/IKE’ at the beginning. aggrmode=no

# ike format: cipher-hash;dhgroup
# recommended ciphers:
# - aes
# recommended hashes:
# - sha2_256 with at least 43 byte PSK
# - sha2_512 with at least 86 byte PSK
# recommended dhgroups:
# - modp2048 = DH14
# - modp3072 = DH15
# - modp4096 = DH16
# - modp6144 = DH17
# - modp8192 = DH18

ike=aes-sha2_256;modp2048 type=tunnel phase2=esp

# esp format: cipher-hash;dhgroup
# recommended ciphers configuration A:
# - aes_gcm_c-256 = AES_GCM_16
# - aes_ctr-256
# - aes_ccm_c-256 = AES_CCM_16
# - aes-256
# additional ciphers configuration B:
# - camellia-256
# - aes-128
# - camellia-128
# recommended hashes configuration A:
# - sha2-256
# - sha2-384
# - sha2-512
# - null (only with GCM/CCM ciphers)
# additional hashes configuration B:
# - sha1
# recommended dhgroups: same as above

phase2alg=aes_gcm_c-256-sha2_256;modp2048 salifetime=8h pfs=yes auto=ignore

How to test

Start the vpn and using $ ipsec auto --status | less and look for ’IKE algorithms wanted/found’ and ’ESP algorithms wanted/loaded’.

References

• https://www.openswan.org/

tinc

Tested with Versions

Table 13. Tested tinc Versions

Program Version	OS/Distribution/Version	Comment
1.0.23	Gentoo	linked against OpenSSL 1.0.1e
1.0.23	Sabayon	linked against OpenSSL 1.0.1e

Defaults

tinc uses 2048 bit RSA keys, Blowfish-CBC, and SHA1 as default settings and suggests the usage of CBC mode ciphers. Any key length up to 8192 is supported and it does not need to be a power of two. OpenSSL Ciphers and Digests are supported by tinc.

Settings

Generate keys with $ tincd -n NETNAME -K8192 Old keys will not be deleted (but disabled), you have to delete them manually. Add the following lines to your tinc.conf on all machines

References

• tincd(8) man page
• tinc.conf(5) man page
• tinc mailinglist http://www.tinc-vpn.org/pipermail/tinc/2014-January/003538.html

PGP/GPG - Pretty Good Privacy

The OpenPGP protocol defines a set of asymmetric- and symmetric encryption algorithms, signature methods and compression protocols. GnuPG, a FOSS implementation of the OpenPGP standard, is widely used for mail encryption. GnuPG signs a message, encrypts it symmetrically and encrypts the symmetric key and the hash with Bob’s public key asymmetrically. Research on SHA-1 conducted back in 2005 (see: SHA-1 Broken) as well as the first practical successful collision in early 2017 (see: SHAttered) has made clear that collision attacks are a real threat to the security of the SHA-1 hash function. Since SHA-1 is defined as a must implementation by the OpenPGP specification, GnuPG is still using it. Currently settings should be adapted to preferably avoid using SHA-1. When using GnuPG, there are a couple of things to take care of:* keylengths (see: Keylengths)

• randomness (see: Random Number Generators)
• preference of symmetric encryption algorithm (see: Architectural overview)
• preference of hash function (see: Architectural overview)

Properly dealing with key material, passphrases and the web-of-trust is outside of the scope of this document. The GnuPG website has a good tutorial on GnuPG. After 31 December 2017 GnuPG version 2.0.x is no longer supported and shall not be used anymore. Use the new long term version 2.1 instead.

Hashing

Avoid SHA-1 by preferring better hashing methods. GnuPG. Edit $HOME/.gnupg/gpg.conf:

Digest selection in GnuPG

personal-digest-preferences SHA512 cert-digest-algo SHA512 default-preference-list AES256 CAMELLIA256 AES192 CAMELLIA192 AES CAMELLIA128 TWOFISH SHA512 SHA384 SHA256 BZIP2 ZLIB ZIP

Key Generation

Because of lack of forward secrecy (see: [pfs]) in OpenPGP it is preferable to use large asymmetric keys for long term communication protection. A RSA key of 4096 bits should provide enough confidentiality for the next 10 years (see: Cryptographic Key Length Recommendation).

New key generation with GnuPG version 2.1

$ gpg --batch --full-gen-key $HOME/Desktop/params.txt`

Parameters for key generation with GnuPG version 2.1

Key-Type: RSA Key-Length: 4096 Subkey-Type: RSA Subkey-Length: 4096 Name-Real: <your-name> Name-Email: <your-email-address> Passphrase: <password> Expires: 2y

# My preferences: AES256, CAMELLIA256, AES192, CAMELLIA192, AES128, CAMELLIA128, TWOFISH, SHA512, SHA384, SHA256, BZIP2, ZLIB and ZIP

Preferences: S9 S13 S8 S12 S7 S11 S10 H10 H9 H8 Z3 Z2 Z1

The preferences parameters S9 to Z1 correspond to AES256, CAMELLIA256, AES192, CAMELLIA192, AES, CAMELLIA128, TWOFISH, SHA512, SHA384, SHA256, BZIP2, ZLIB and ZIP. The parameters 3DES, SHA-1 and uncompressed are set automatically by GnuPG.

ECC - Elliptic Curve Cryptography

Since the release of GnuPG version 2.1 end-2014 ECC is supported. Older versions though are still widely used therefore ECC is not yet applicable in practice.

IPMI, ILO and other lights out management solutions

Consider creating an unrouted management VLAN and access that only via VPN.

We strongly recommend that any remote management system for servers such as ILO, iDRAC, IPMI based solutions and similar systems never be connected to the public internet.

Instant Messaging Systems

General server configuration recommendations

For servers, we mostly recommend to apply what’s proposed by the Peter’s manifesto. In short:* require the use of TLS for both client-to-server and server-to-server connections

• prefer or require TLS cipher suites that enable forward secrecy
• deploy certificates issued by well-known and widely-deployed certification authorities (CAs)

The last point being out-of-scope for this section, we will only cover the first two points.

ejabberd

Tested with Versions

• ejabberd 14.12, Debian 7 Wheezy
• ejabberd 14.12, Ubuntu 14.04 Trusty
• ejabberd 15.03, Ubuntu 14.04 Trusty
• ejabberd 16.01, Ubuntu 14.04 Trusty

Settings

ejabberd is one of the popular Jabber servers. In order to be compliant with the manifesto, you should adapt your configuration:

TLS setup for ejabberd

listen:

   -
       port: 5222
       module: ejabberd_c2s
       certfile: "/path/to/ssl.pem"
       starttls: true
       starttls_required: true
       protocol_options:
           - "no_sslv3"
           - "no_tlsv1"
           - "cipher_server_preference"
       max_stanza_size: 65536
       dhfile: "/path/to/dhparams.pem"
       shaper: c2s_shaper
       access: c2s
   -
       port: 5269
       module: ejabberd_s2s_in

s2s_use_starttls: required_trusted s2s_certfile: "/path/to/ssl.pem" s2s_dhfile: "/etc/ejabberd/dhparams.pem" # Available from version 15.03 s2s_protocol_options:

   - "no_sslv3"
   - "no_tlsv1"
   - "cipher_server_preference"

Available from version 15.06

Additional settings

It is possible to explicitly specify a cipher string for TLS connections.

Specifying a cipher order and enforcing it

listen:

   -
       port: 5222
       module: ejabberd_c2s
       certfile: "/path/to/ssl.pem"
       starttls: true
       starttls_required: true
       protocol_options:
           - "no_sslv3"
           - "no_tlsv1"
        - "cipher_server_preference"
       max_stanza_size: 65536
       dhfile: "/path/to/dhparams.pem"
       ciphers: "EDH+CAMELLIA:EDH+aRSA:EECDH+aRSA+AESGCM:EECDH+aRSA+SHA256:EECDH:+CAMELLIA128:+AES128:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!DSS:!RC4:!SEED:!IDEA:!ECDSA:kEDH:CAMELLIA128-SHA:AES128-SHA"
   -
       port: 5269
       module: ejabberd_s2s_in

s2s_use_starttls: required_trusted s2s_certfile: "/path/to/ssl.pem" s2s_dhfile: "/etc/ejabberd/dhparams.pem" s2s_protocol_options:

   - "no_sslv3"
   - "no_tlsv1"
   - "cipher_server_preference"

s2s_ciphers: "EDH+CAMELLIA:EDH+aRSA:EECDH+aRSA+AESGCM:EECDH+aRSA+SHA256:EECDH:+CAMELLIA128:+AES128:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!DSS:!RC4:!SEED:!IDEA:!ECDSA:kEDH:CAMELLIA128-SHA:AES128-SHA"

	Available from version 15.06
	Available from version 15.03

Weak Ciphers Note that we are setting the SSL option cipher_server_preference. This enforces our cipher order when negotiating which ciphers are used, as the cipher order of some clients chooses weak ciphers over stronger ciphers.

Starting with version 15.03[17], it is possible to use custom Diffie-Hellman-Parameters. This allows us to negotiate stronger Diffie-Hellman-keys, and also helps us avoid problems with using common weak Diffie-Hellman-Parameters. You can generate your own parameter file by running: $ openssl dhparam -out dhparams.pem 4096 By default, ejabberd provides an administration website (look for the ejabberd_http module). Enable TLS protection for it like this:

   port: 5280
   module: ejabberd_http
   web_admin: true
   http_poll: true
   http_bind: true
   captcha: true
   certfile: "/path/to/ssl.pem"
   tls: true

Tested with Versions

• Debian Wheezy 2.1.10-4+deb7u1

Settings

Older versions of ejabberd use a different configuration file syntax. In order to be compliant with the manifesto, you should adapt your configuration[18] as follows:

{listen,

   [
       {5222, ejabberd_c2s, [
       {access, c2s},
       {shaper, c2s_shaper},
       {max_stanza_size, 65536},
       starttls,
       starttls_required,
       {certfile, "/etc/ejabberd/ejabberd.pem"}
       ]},
   ]}.

{s2s_use_starttls, required_trusted}.

{s2s_certfile, "/etc/ejabberd/ejabberd.pem"}.

Additional settings

Older versions of ejabberd (< 2.0.0) need to be patched to be able to parse all of the certificates in the CA chain. Specifying a custom cipher string is only possible starting with version 13.12 (see configuration for version 14.12 above).

References

• ejabberd, your superpowerful messaging framework

How to test

IM Observatory is a useful website to test Jabber server configurations.

Chat privacy - Off-the-Record Messaging (OTR)

Off-the-Record Messaging Protocol works on top of the Jabber protocol. It adds to popular chat clients (Adium, Pidgin…​) the following properties for encrypted chats:* Authentication

• Integrity
• Confidentiality
• Forward secrecy

It basically uses Diffie-Hellman, AES and SHA1. Communicating over an insecure instant messaging network, OTR can be used for end to end encryption. There are no specific configurations required but the protocol itself is worth to be mentioned.

Charybdis

There are numerous implementations of IRC servers. In this section, we choose Charybdis which serves as basis for ircd-seven, developed and used by freenode. Freenode is actually the biggest IRC network[19]. Charybdis is part of the Debian & Ubuntu distributions.

SSL relevant configuration for Charybdis/ircd-seven

/* Extensions */

#loadmodule "extensions/chm_sslonly_compat.so";

loadmodule "extensions/extb_ssl.so"; serverinfo {

   ssl_private_key = "etc/test.key";
   ssl_cert = "etc/test.cert";
   ssl_dh_params = "etc/dh.pem";
    # set ssld_count as number of cores - 1
   ssld_count = 1;

}; listen {

   sslport = 6697;

};

SILC

SILC is instant messaging protocol publicly released in 2000. SILC is a per-default secure chat protocol thanks to a generalized usage of symmetric encryption. Keys are generated by the server meaning that if compromised, communication could be compromised. The protocol is not really popular anymore.

Databases

Oracle

No information available / known.

MySQL

Tested with Versions

• MySQL 5.5 on Debian Wheezy
• MySQL 5.7.20 on Ubuntu 16.04.3

Settings

References

MySQL Documentation on Configuring MySQL to Use Encrypted Connections.

How to test

After restarting the server run the following query to see if the ssl settings are correct: show variables like '%ssl%';

DB2

Tested with Version

We do not test this here, since we only reference other papers for DB2 so far.

Settings

ssl_cipherspecs:

In the link above the whole SSL-configuration is described in-depth. The following command shows only how to set the recommended ciphersuites.

Recommended and supported ciphersuites

db2 update dbm cfg using SSL_CIPHERSPECS TLS_RSA_WITH_AES_256_CBC_SHA256, TLS_RSA_WITH_AES_128_GCM_SHA256, TLS_RSA_WITH_AES_128_CBC_SHA256, TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256, TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256, TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256, TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256, TLS_RSA_WITH_AES_256_GCM_SHA384, TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384, TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384, TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384, TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384, TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA, TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA, TLS_RSA_WITH_AES_256_CBC_SHA, TLS_RSA_WITH_AES_128_CBC_SHA, TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA, TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA

References

IBM DB2 Documentation on Supported cipher suites.https://www.ibm.com/support/knowledgecenter/SSEPGG_9.7.0/com.ibm.db2.luw.admin.sec.doc/doc/c0053544.html

PostgreSQL

Tested with Versions

• Debian Wheezy and PostgreSQL 9.1
• Linux Mint 14 nadia / Ubuntu 12.10 quantal with PostgreSQL 9.1+136 and OpenSSL 1.0.1c

Settings

To start in SSL mode the server.crt and server.key must exist in the servers data directory $PGDATA.

Starting with version 9.2, you have the possibility to set the path manually.

References

It’s recommended to read Security and Authentication in the manual. PostgreSQL Documentation on Secure TCP/IP Connections with SSL. PostgreSQL Documentation on Client Authentication.

How to test

To test your ssl settings, run psql with the sslmode parameter: $ psql "sslmode=require host=postgres-server dbname=database" your-username

Proxy Solutions

Within enterprise networks and corporations with increased levels of paranoia or at least some defined security requirements it is common not to allow direct connections to the public internet. For this reason proxy solutions are deployed on corporate networks to intercept and scan the traffic for potential threats within sessions. For encrypted traffic there are four options:* Block the connection because it cannot be scanned for threats.

• Bypass the threat-mitigation and pass the encrypted session to the client, which results in a situation where malicious content is transferred directly to the client without visibility to the security system.
• Intercept (i.e. terminate) the session at the proxy, scan there and re-encrypt the session towards the client (effectively MITM).
• Deploy special Certificate Authorities to enable Deep Packet Inspection on the wire.

While the latest solution might be the most "up to date", it arises a new front in the context of this paper, because the most secure part of a client’s connection could only be within the corporate network, if the proxy-server handles the connection to the destination server in an insecure manner. Conclusion: Don’t forget to check your proxy solutions SSL-capabilities. Also do so for your reverse proxies!

Bluecoat / Symantec

Blue Coat Systems was a well-known manufacturer of enterprise proxy appliances. In 2016 it was acquired by Symantec. The products are now known as Symantec ProxySG and Advanced Secure Gateway (ASG). The description below is for the original Blue Coat SG Operating System (SGOS). BlueCoat Proxy SG Appliances can be used as forward and reverse proxies. The reverse proxy feature is rather under-developed, and while it is possible and supported, there only seems to be limited use of this feature "in the wild" - nonetheless there are a few cipher suites to choose from, when enabling SSL features.

Tested with Versions

Proxy Appliance	SGOS 6.5.x	Blue Coat, now Symantec

Only allow TLS 1.0,1.1 and 1.2 protocols:

$conf t $(config)ssl $(config ssl)edit ssl-device-profile default $(config device-profile default)protocol tlsv1 tlsv1.1 tlsv1.2

 ok

Select your accepted cipher-suites:

$conf t Enter configuration commands, one per line. End with CTRL-Z. $(config)proxy-services $(config proxy-services)edit ReverseProxyHighCipher $(config ReverseProxyHighCipher)attribute cipher-suite Cipher# Use Description Strength

-------  ---   -----------------------   --------
     1  yes            AES128-SHA256      High
     2  yes            AES256-SHA256      High
     3  yes               AES128-SHA    Medium
     4  yes               AES256-SHA      High
     5  yes       DHE-RSA-AES128-SHA      High
     6  yes       DHE-RSA-AES256-SHA      High
              [...]
    13  yes          EXP-RC2-CBC-MD5    Export

Select cipher numbers to use, separated by commas: 2,5,6

 ok

The same protocols are available for forward proxy settings and should be adjusted accordingly: In your local policy file add the following section: <ssl>

   DENY server.connection.negotiated_ssl_version=(SSLV2, SSLV3)

Disabling protocols and ciphers in a forward proxy environment could lead to unexpected results on certain (misconfigured?) webservers (i.e. ones accepting only SSLv2/3 protocol connections)

HAProxy

See https://www.haproxy.org/ See https://timtaubert.de/blog/2014/11/the-sad-state-of-server-side-tls-session-resumption-implementations/ See https://cbonte.github.io/haproxy-dconv/configuration-1.5.html#5.1-npn See https://cbonte.github.io/haproxy-dconv/configuration-1.5.html#3.2-tune.ssl.cachesize See https://kura.io/2014/07/02/haproxy-ocsp-stapling/ See https://kura.io/2015/01/27/hpkp-http-public-key-pinning-with-haproxy/ HAProxy can be used as loadbalancer and proxy for TCP and HTTP-based applications. Since version 1.5 it supports SSL and IPv6.

Tested with Versions

HAProxy 1.5.11 with OpenSSL 1.0.1e on Debian Wheezy

Settings

global configuration

global

   ssl-default-bind-ciphers EDH+CAMELLIA:EDH+aRSA:EECDH+aRSA+AESGCM:EECDH+aRSA+SHA384:EECDH+aRSA+SHA256:EECDH:+CAMELLIA256:+AES256:+CAMELLIA128:+AES128:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!DSS:!RC4:!SEED:!ECDSA:CAMELLIA256-SHA:AES256-SHA:CAMELLIA128-SHA:AES128-SHA
   ssl-default-bind-options no-sslv3 no-tls-tickets #disable SSLv3
   tune.ssl.default-dh-param 2048 #tune DH to 2048

frontend configuration

frontend public

   bind *:80
   bind *:443 ssl crt server.pem
   mode http
   redirect scheme https code 301 if !{ ssl_fc } # redirect HTTP to HTTPS

backend configuration

backend backend

   mode http
   server server 192.168.1.1:80 check
   http-request set-header X-Forwarded-Port %[dst_port]
   http-request add-header X-Forwarded-Proto https if { ssl_fc }
   rspadd Strict-Transport-Security:\ max-age=15768000;\ includeSubDomains #enable HSTS header for this backend

Additional Settings

Enable NPN Support:

   bind *:443 ssl crt server.pem npn "http/1.1,http/1.0"

Append the npn command in the frontend configuration of HAProxy.

Enable OCSP stapling:

HAProxy supports since version 1.5.0 OCSP stapling. To enable it you have to generate the OCSP singing file in the same folder, with the same name as your certificate file plus the extension .ocsp. (e.g. your certificate file is named server.crt then the OCSP file have to be named server.crt.oscp)To generate the OCSP file use these commands: $ openssl x509 -in your.certificate.crt -noout -ocsp_uri # <- get your ocsp uri $ openssl ocsp -noverify -issuer ca.root.cert.crt -cert your.certificate.crt -url "YOUR OCSP URI" -respout your.certificate.crt.ocsp Reload HAProxy and now OCSP stapling should be enabled.Note: This OCSP signature file is only valid for a limited time. The simplest way of updating this file is by using cron.daily or something similar.

Enable HPKP:

Get certificate informations: $ openssl x509 -in server.crt -pubkey -noout | openssl rsa -pubin -outform der | openssl dgst -sha256 -binary | base64 Then you append the returned string in the HAProxy configuration. Add the following line to the backend configuration: rspadd Public-Key-Pins:\ pin-sha256="YOUR_KEY";\ max-age=15768000;\ includeSubDomains Reload HAProxy and HPKP should now be enabled.Note: Keep in mind to generate a backup key in case of problems with your primary key file.

How to test

See appendix Tools

Pound

Tested with Versions

Pound 2.6 See http://www.apsis.ch/pound See https://help.ubuntu.com/community/Pound

Settings

HTTPS Listener in Pound

# HTTP Listener, redirects to HTTPS

ListenHTTP

   Address 10.10.0.10
   Port    80
   Service
       Redirect "https://some.site.tld"
   End

End

## HTTPS Listener

ListenHTTPS

   Address      10.10.0.10
   Port         443
   AddHeader    "Front-End-Https: on"
   Cert         "/path/to/your/cert.pem"
    ## See 'man ciphers'.
   Ciphers      "TLSv1.2:TLSv1.1:!SSLv3:!SSLv2:EDH+CAMELLIA:EDH+aRSA:EECDH+aRSA+AESGCM:EECDH+aRSA+SHA384:EECDH+aRSA+SHA256:EECDH:+CAMELLIA256:+AES256:+CAMELLIA128:+AES128:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!DSS:!RC4:!SEED:!ECDSA:CAMELLIA256-SHA:AES256-SHA:CAMELLIA128-SHA:AES128-SHA"
   Service
       BackEnd
           Address 10.20.0.10
           Port 80
       End
   End

End

stunnel

Tested with Versions

• stunnel 4.53-1.1ubuntu1 on Ubuntu 14.04 Trusty with OpenSSL 1.0.1f, without disabling Secure Client-Initiated Renegotiation
• stunnel 5.02-1 on Ubuntu 14.04 Trusty with OpenSSL 1.0.1f
• stunnel 4.53-1.1 on Debian Wheezy with OpenSSL 1.0.1e, without disabling Secure Client-Initiated Renegotiation

Settings

HTTPS Listener in stunnel

ciphers = EDH+CAMELLIA:EDH+aRSA:EECDH+aRSA+AESGCM:EECDH+aRSA+SHA384:EECDH+aRSA+SHA256:EECDH:+CAMELLIA256:+AES256:+CAMELLIA128:+AES128:+SSLv3:!aNULL!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!DSS:!RC4:!SEED:!ECDSA:CAMELLIA256-SHA:AES256-SHA:CAMELLIA128-SHA:AES128-SHA curve = secp384r1 options = NO_SSLv2 options = NO_SSLv3 options = cipher_server_preference

; Secure Client-Initiated Renegotiation can only be disabled wit stunnel >= 4.54
;renegotiation = no

Additional information

Secure Client-Initiated Renegotiation can only be disabled for stunnel versions >= 4.54, when the renegotiation parameter has been added (See changelog).

References

stunnel documentation: https://www.stunnel.org/static/stunnel.html stunnel changelog: https://www.stunnel.org/sdf_ChangeLog.html

How to test

See appendix Tools

Kerberos

This section discusses various implementations of the Kerberos 5 authentication protocol on Unix and Unix-like systems as well as on Microsoft Windows.

Overview

Kerberos provides mutual authentication of two communicating parties, e.g. a user using a network service. The authentication process is mediated by a trusted third party, the Kerberos key distribution centre (KDC). Kerberos implements secure single-sign-on across a large number of network protocols and operating systems. Optionally, Kerberos can be used to create encrypted communications channels between the user and service.

Recommended reading

An understanding of the Kerberos protocol is necessary for properly implementing a Kerberos setup. Also, in the following section some knowledge about the inner workings of Kerberos is assumed. Therefore we strongly recommend reading the excellent introduction Kerberos: An Authentication Service for Computer Networks first. No further overview over Kerberos terminology and functions will be provided, for a discussion and a selection of relevant papers refer to Kerberos Papers and Documentation. The Kerberos protocol over time has been extended with a variety of extensions and Kerberos implementations provide additional services in addition to the aforementioned KDC. All discussed implementations provide support for trust relations between multiple realms, an administrative network service (kerberos-adm, kadmind) as well as a password changing service (kpasswd). Sometimes, alternative database backends for ticket storage, X.509 and SmartCard authentication are provided. Of those, only administrative and password changing services will be discussed.

Protocol versions Only the Kerberos 5 protocol and implementation will be discussed. Kerberos 4 is obsolete, insecure and its use is strongly discouraged.

Providing a suitable Setup for secure Kerberos Operations

The aim of Kerberos is to unify authentication across a wide range of services, for many different users and use cases and on many computer platforms. The resulting complexity and attack surface make it necessary to carefully plan and continuously evaluate the security of the overall ecosystem in which Kerberos is deployed. Several assumptions are made on which the security of a Kerberos infrastructure relies:* Every KDC in a realm needs to be trustworthy. The KDC’s principal database must not become known to or changed by an attacker. The contents of the principal database enables the attacker to impersonate any user or service in the realm.

• Synchronisation between KDCs must be secure, reliable and frequent. An attacker that is able to intercept or influence synchronisation messages obtains or influences parts of the principal database, enabling impersonation of affected principals. Unreliable or infrequent synchronisation enlarges the window of vulnerability after disabling principals or changing passwords that have been compromised or lost.
• KDCs must be available. An attacker is able to inhibit authentication for services and users by cutting off their access to a KDC.
• Users’ passwords must be secure. Since Kerberos is a single-sign-on mechanism, a single password may enable an attacker to access a large number of services.
• Service keytabs need to be secured against unauthorized access similarly to SSL/TLS server certificates. Obtaining a service keytab enables an attacker to impersonate a service.
• DNS infrastructure must be secure and reliable. Hosts that provide services need consistent forward and reverse DNS entries. The identity of a service is tied to its DNS name, similarly the realm a client belongs to as well as the KDC, kpasswd and kerberos-adm servers may be specified in DNS TXT and SRV records. Spoofed DNS entries will cause denial-of-service situations and might endanger (i_mit_Realm configuration decisions_, 2013) the security of a Kerberos realm.
• Clients and servers in Kerberos realms need to have synchronized clocks. Tickets in Kerberos are created with a limited, strictly enforced lifetime. This limits an attacker’s window of opportunity for various attacks such as the decryption of tickets in sniffed network traffic or the use of tickets read from a client computer’s memory. Kerberos will refuse tickets with old timestamps or timestamps in the future. This would enable an attacker with access to a systems clock to deny access to a service or all users logging in from a specific host.

Therefore we suggest:* Secure all KDCs at least as strongly as the most secure service in the realm.

• Dedicate physical (i.e. non-VM) machines to be KDCs. Do not run any services on those machines beyond the necessary KDC, kerberos-adm, kpasswd and kprop services.
• Restrict physical and administrative access to the KDCs as severely as possible. E.g. ssh access should be limited to responsible adminstrators and trusted networks.
• Encrypt and secure the KDCs backups.
• Replicate your primary KDC to at least one secondary KDC.
• Prefer easy-to-secure replication (propagation in Kerberos terms) methods.Especially avoid LDAP replication and database backends. LDAP enlarges the attack surface of your KDC and facilitates unauthorized access to the principal database e.g. by ACL misconfiguration.
• Use DNSSEC. If that is not possible, at least ensure that all servers and clients in a realm use a trustworthy DNS server contacted via secure network links.
• Use NTP on a trustworthy server via secure network links.
• Avoid services that require the user to enter a password which is then checked against Kerberos. Prefer services that are able to use authentication via service tickets, usually not requiring the user to enter a password except for the initial computer login to obtain a ticket-granting-ticket (TGT). This limits the ability of attackers to spy out passwords through compromised services.

Implementations

Cryptographic Algorithms in Kerberos Implementations

The encryption algorithms (commonly abbreviated ’etypes’ or ’enctypes’) in Kerberos exchanges are subject to negotiation between both sides of an exchange. Similarly, a ticket granting ticket (TGT), which is usually obtained on initial login, can only be issued if the principal contains a version of the password encrypted with an etype that is available both on the KDC and on the client where the login happens. Therefore, to ensure interoperability among components using different implementations as shown in Commonly supported Kerberos encryption types by implementation. Algorithm names, a selection of available etypes is necessary. However, the negotiation process may be subject to downgrade attacks and weak hashing algorithms endanger integrity protection and password security.

This means that the des3-cbc-sha1-kd or rc4-hmac algorithms should not be used, except if there is a concrete and unavoidable need to do so. Other des3-*, des-* and rc4-hmac-exp algorithms should never be used.

Along the lines of cipher string B, the following etypes are recommended: aes256-cts-hmac-sha1-96 camellia256-cts-cmac aes128-cts-hmac-sha1-96 camellia128-cts-cmac. Commonly supported Kerberos encryption types by implementation. Algorithm names according to RFC3961, except where aliases can be used or the algorithm is named differently altogether as stated (Raeburn, 2005)

ID	Algorithm	MIT	Heimdal	GNU Shishi	MS ActiveDirectory
1	des-cbc-crc	yes	yes	yes	yes
2	des-cbc-md4	yes	yes	yes	no
3	des-cbc-md5	yes	yes	yes	yes
6	des3-cbc-none	no	yes	yes	no
7	des3-cbc-sha1	no	yes[20]	no	no
16	des3-cbc-sha1-kd	yes[21]	yesfoonote:[named des3-cbc-sha1]	yes	no
17	aes128-cts-hmac-sha1-96	yes	yes	yes	yesfoonote:[since Vista, Server 2008]
18	aes256-cts-hmac-sha1-96	yes	yes	yes	yes[22]
23	rc4-hmac	yes	yes	yes	yes
24	rc4-hmac-exp	yes	no	yes	yes
25	camellia128-cts-cmac	yes[23]	no	no	no
26	camellia256-cts-cmac	yes[24]	no	no	no

Existing installations

The configuration samples below assume new installations without preexisting principals. For existing installations:* Existing setups should be migrated to a new master key if the current master key is using a weak enctype.

• When changing the list of supported_enctypes, principals where all enctypes are no longer supported will cease to work.
• Be aware that Kerberos 4 is obsolete and should not be used.
• Principals with weak enctypes pose an increased risk for password bruteforce attacks if an attacker gains access to the database.

To get rid of principals with unsupported or weak enctypes, a password change is usually the easiest way. Service principals can simply be recreated.

MIT krb5

KDC configuration

In /etc/krb5kdc/kdc.conf set the following in your realm’s configuration:

Encryption flags for MIT krb5 KDC

supported_enctypes = aes256-cts-hmac-sha1-96:normal camellia256-cts-cmac:normal aes128-cts-hmac-sha1-96:normal camellia128-cts-cmac:normal default_principal_flags = +preauth In /etc/krb5.conf set in the [libdefaults] section:

Encryption flags for MIT krb5 client

[libdefaults]
allow_weak_crypto = false
permitted_enctypes= aes256-cts-hmac-sha1-96 camellia256-cts-cmac aes128-cts-hmac -sha1-96 camellia128-cts-cmac
default_tkt_enctypes= aes256-cts-hmac-sha1-96 camellia256-cts-cmac aes128-cts-hmac-sha1-96 camellia128-cts-cmac
default_tgs_enctypes= aes256-cts-hmac-sha1-96 camellia256-cts-cmac aes128-cts-hmac-sha1-96 camellia128-cts-cmac

Upgrading a MIT krb5 database to a new enctype

To check if an upgrade is necessary, execute the following on the KDC in question: root@kdc.example.com:~# kdb5_util list_mkeys Master keys for Principal: K/M@EXAMPLE.COM KVNO: 1, Enctype: des-cbc-crc, Active on: Thu Jan 01 00:00:00 UTC 1970 *

In this case, an old unsafe enctype is in use as indicated by the star following the key line.

To upgrade, proceed as follows. First create a new master key for the database with the appropriate enctype. You will be prompted for a master password that can later be used to decrypt the database. A stash-file containing this encryption key will also be written. root@kdc.example.com:~# kdb5_util add_mkey -s -e aes256-cts-hmac-sha1-96 Creating new master key for master key principal 'K/M@EXAMPLE.COM' You will be prompted for a new database Master Password. It is important that you NOT FORGET this password. Enter KDC database master key: Re-enter KDC database master key to verify: Verify that the new master key has been successfully created. Note the key version number (KVNO) of the new master key, in this case 2. root@kdc.example.com:~# kdb5_util list_mkeys Master keys for Principal: K/M@EXAMPLE.COM KVNO: 2, Enctype: aes256-cts-hmac-sha1-96, No activate time set KVNO: 1, Enctype: des-cbc-crc, Active on: Thu Jan 01 00:00:00 UTC 1970 * Set the new master key as the active master key by giving its KVNO. The active master key will be indicated by an asterisk in the master key list. root@kdc.example.com:~# kdb5_util use_mkey 2 root@kdc.example.com:~# kdb5_util list_mkeys Master keys for Principal: K/M@EXAMPLE.COM KVNO: 2, Enctype: aes256-cts-hmac-sha1-96, Active on: Wed May 13 14:14:18 UTC 2015 * KVNO: 1, Enctype: des-cbc-crc, Active on: Thu Jan 01 00:00:00 UTC 1970 Reencrypt all principals to the new master key. root@kdc.example.com:~# kdb5_util update_princ_encryption Re-encrypt all keys not using master key vno 2? (type 'yes' to confirm)? yes 504 principals processed: 504 updated, 0 already current After verifying that everything still works as desired it is possible to remove unused master keys. root@kdc.example.com:~# kdb5_util purge_mkeys Will purge all unused master keys stored in the 'K/M@EXAMPLE.COM' principal, are you sure? (type 'yes' to confirm)? yes OK, purging unused master keys from 'K/M@EXAMPLE.COM'... Purging the following master key(s) from K/M@EXAMPLE.COM: KVNO: 1 1 key(s) purged.

Weblinks

1. https://bettercrypto.org/