Kategorie:SSH/Kryptografie

Beschreibung

Key material handling

Key material identifies the cryptographic secrets that compose a key

All key material must be treated as RESTRICTED data, meaning that

• Only individual with specific training and need-to-know should have access to key material
• Key material must be encrypted on transmission
• Key material can be stored in clear text, but only with proper access control (limited access)

This includes

• OpenSSH server keys (/etc/ssh/ssh_host_*key)
• Client keys (~/.ssh/id_{rsa,dsa,ecdsa,ed25519} and ~/.ssh/identity).

Client key size and login latency

Figure out the impact on performance of using larger keys

• Such as RSA 4096 bytes keys - on the client side

Tests

Idle, i7 4500 intel CPU

• OpenSSH_6.7p1
• OpenSSL 1.0.1l
• ed25519 server keys

The following command is ran 10 times

time ssh localhost -i .ssh/id_thekey exit

Results

Client key	Minimum	Maximum	Average
RSA 4096	120ms	145ms	127ms
RSA 2048	120ms	129ms	127ms
ed25519	117ms	138ms	120ms

Slower Machines

These numbers may differ on a slower machine

• This contains the complete login sequence
• Therefore is subject to variations

Summery

• The latency differences are not significant
• It does not impact performance sufficiently

OpenSSH server

SSH is used to

• remotely manage computer systems
• secururly transfer files over untrusted networks
• create "ad-hoc" virtual-private networks

OpenSSH

• OpenSSH is the most popular implementation of the SSH protocol
• It is maintained by the OpenBSD project
• 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

Kompatibilität

• 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

1. 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.

Configuration

Different versions of OpenSSH support different options which are not always compatible.

• This guide shows settings for the most commonly deployed OpenSSH versions at Mozilla - however, using the latest version of OpenSSH is recommended.

Modern (OpenSSH 6.7+)

File: /etc/ssh/sshd_config

 # Supported HostKey algorithms by order of preference.
HostKey /etc/ssh/ssh_host_ed25519_key
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_ecdsa_key

KexAlgorithms curve25519-sha256@libssh.org,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256,diffie-hellman-group-exchange-sha256

Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-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,umac-128@openssh.com

# Password based logins are disabled - only public key based logins are allowed.

AuthenticationMethods publickey

# LogLevel VERBOSE logs user's key fingerprint on login. 

• Needed to have a clear audit track of which key was using to log in.

LogLevel VERBOSE

# Log sftp level file access (read/write/etc.) that would not be easily logged otherwise.

Subsystem sftp /usr/lib/ssh/sftp-server -f AUTHPRIV -l INFO

# Root login is not allowed for auditing reasons. 

• This is because it's difficult to track which process belongs to which root user:

#
# On Linux, user sessions are tracking using a kernel-side session id, however, this session id is not recorded by OpenSSH.
# Additionally, only tools such as systemd and auditd record the process session id.
# On other OSes, the user session id is not necessarily recorded at all kernel-side.
# Using regular users in combination with /bin/su or /usr/bin/sudo ensure a clear audit track.

PermitRootLogin No

# Use kernel sandbox mechanisms where possible in unprivileged processes
# Systrace on OpenBSD, Seccomp on Linux, seatbelt on MacOSX/Darwin, rlimit elsewhere.

UsePrivilegeSeparation sandbox

File: /etc/ssh/moduli

All Diffie-Hellman moduli in use should be at least 3072-bit-long (they are used for diffie-hellman-group-exchange-sha256) as per our Security/Guidelines/Key_Management recommendations.

• See also man moduli.

To deactivate short moduli in two commands: awk '$5 >= 3071' /etc/ssh/moduli > /etc/ssh/moduli.tmp && mv /etc/ssh/moduli.tmp /etc/ssh/moduli

Intermediate (OpenSSH 5.3)

This is mainly for use by RHEL6, CentOS6, etc.

• which run older versions of OpenSSH.

File: /etc/ssh/sshd_config

# Supported HostKey algorithms by order of preference.

HostKey /etc/ssh/ssh_host_rsa_key HostKey /etc/ssh/ssh_host_ecdsa_key

KexAlgorithms diffie-hellman-group-exchange-sha256 MACs hmac-sha2-512,hmac-sha2-256 Ciphers aes256-ctr,aes192-ctr,aes128-ctr

# Password based logins are disabled - only public key based logins are allowed.

RequiredAuthentications2 publickey

# RequiredAuthentications2 not work on official OpenSSH 5.3 portable.
# In this is your case, use this instead:
#PubkeyAuthentication yes
#PasswordAuthentication no

# LogLevel VERBOSE logs user's key fingerprint on login. 

• Needed to have a clear audit track of which key was using to log in.

LogLevel VERBOSE

# Log sftp level file access (read/write/etc.) that would not be easily logged otherwise.

Subsystem sftp /usr/lib/ssh/sftp-server -f AUTHPRIV -l INFO

# Root login is not allowed for auditing reasons. 

• This is because it's difficult to track which process belongs to which root user:

#
# On Linux, user sessions are tracking using a kernel-side session id, however, this session id is not recorded by OpenSSH.
# Additionally, only tools such as systemd and auditd record the process session id.
# On other OSes, the user session id is not necessarily recorded at all kernel-side.
# Using regular users in combination with /bin/su or /usr/bin/sudo ensure a clear audit track.

PermitRootLogin No

File: /etc/ssh/moduli

All Diffie-Hellman moduli in use should be at least 2048-bit-long.

• From the structure of moduli files, this means the fifth field of all lines in this file should be greater than or equal to 2047.

To deactivate weak moduli in two commands: awk '$5 >= 2047' /etc/ssh/moduli > /etc/ssh/moduli.tmp && mv /etc/ssh/moduli.tmp /etc/ssh/moduli

Multi-Factor Authentication (OpenSSH 6.3+)

Recent versions of OpenSSH support MFA (Multi-Factor Authentication).

• Using MFA is recommended where possible.

It requires additional setup, such as using the OATH Toolkit or DuoSecurity.

ATTENTION

In order to allow using one time passwords (OTPs) and any other text input, Keyboard-interactive is enabled in OpenSSH. This MAY allow for password authentication to work. It is therefore very important to check your PAM configuration so that PAM disallow password authentication for OpenSSH.

OpenSSH 6.3+ (default)

File: /etc/ssh/sshd_config

# IMPORTANT: you will have to ensure OpenSSH cannot authenticate with passwords with PAM in /etc/pam.d/sshd
# "PasswordAuthentication no" is not sufficient!
PubkeyAuthentication yes
PasswordAuthentication no
AuthenticationMethods publickey,keyboard-interactive:pam
KbdInteractiveAuthentication yes
UsePAM yes
# Ensure /bin/login is not used so that it cannot bypass PAM settings for sshd.
UseLogin no

OpenSSH 5.3+ w/ RedHat/CentOS patch (old)

File: /etc/ssh/sshd_config

# Allow keyboard-interactive.
# IMPORTANT: you will have to ensure OpenSSH cannot authenticate with passwords with PAM in /etc/pam.d/sshd
# "PasswordAuthentication no" is not sufficient!
RequiredAuthentications2 publickey,keyboard-interactive:skey
PasswordAuthentication no
ChallengeResponseAuthentication yes
UsePAM yes
# Ensure /bin/login is not used so that it cannot bypass PAM settings for sshd.
UseLogin no

PAM configuration for use with the OATH Toolkit or DuoSecurity as second authentication factor. 

File: /etc/pam.d/sshd

#%PAM-1.0
auth       required     pam_sepermit.so

# WARNING: make sure any password authentication module is disabled.
# Example: pam_unix.so, or "password-auth", "system-auth", etc.
#auth       include      password-auth

# Options to enable when using OATH toolkit
#auth       requisite     pam_oath.so usersfile=/etc/users.oath digits=6 window=20

# Options to enable when using DuoSecurity
#auth    sufficient      /lib64/security/pam_duo.so

account    required     pam_nologin.so

Ciphers and algorithms choice

• When CHACHA20 (OpenSSH 6.5+) is not available, AES-GCM (OpenSSH 6.1+) and any other algorithm using EtM (Encrypt then MAC) disclose the packet length - giving some information to the attacker.
• Only recent OpenSSH servers and client support CHACHA20.

• NIST curves (ecdh-sha2-nistp512,ecdh-sha2-nistp384,ecdh-sha2-nistp256) are listed for compatibility, but the use of curve25519 is generally preferred.

• SSH protocol 2 supports DH and ECDH key-exchange as well as forward secrecy.
• Regarding group sizes, please refer to Security/Guidelines/Key_Management.

The various algorithms supported by a particular OpenSSH version can be listed with the following commands:

$ ssh -Q cipher
$ ssh -Q cipher-auth
$ ssh -Q mac
$ ssh -Q kex
$ ssh -Q key

Reference documents

• Key Management
• Server Side TLS
• RFC4418 (umac)
• umac draft
• Safe curves
• DJM blog
• Stribika blog
• AES-GCM performance study
• CHACHA20 vs AES-GCM performance study
• PROTOCOL.certkeys
• rfc44880bis from GnuPG
• Weak Diffie-Hellman and the Logjam Attack
• On OpenSSH and Logjam, by Jethro Beekman