Kategorie:SSH/Kryptografie: Unterschied zwischen den Versionen

Umgang mit Schlüsselmaterial

Schlüsselmaterial identifiziert die kryptografischen Geheimnisse, aus denen ein Schlüssel besteht.

Sämtliches Schlüsselmaterial muss als RESTRICTED-Daten behandelt werden

• Nur Personen mit spezieller Ausbildung und dem Bedarf an Wissen sollten Zugang zu Schlüsselmaterial haben.
• Das Schlüsselmaterial muss bei der Übertragung verschlüsselt werden.
• Schlüsselmaterial kann im Klartext gespeichert werden, aber nur mit einer angemessenen Zugangskontrolle (begrenzter Zugang).

Dazu gehören

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

Ciphers and algorithms choice

Recent OpenSSH servers and client support CHACHA20

• 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.
• 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

• DH
• ECDH key-exchange
• forward secrecy

Group sizes

• 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

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

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

Konfiguration

OpenSSH

Settings

OpenSSH 6.6

# 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 supports Curve25519

Tested 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

• Cisco SSH is a basic SSH reference for all routers and switches
• 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 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 Versions

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.

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.

Anhang

Siehe auch

• SSH/Kryptografie
• SSH/Kryptografie/Konfiguration

Sicherheit

Dokumentation

Links

Projekt

Weblinks

1. Key Management
2. Server Side TLS
3. RFC4418 (umac)
4. umac draft
5. Safe curves
6. DJM blog
7. Stribika blog
8. AES-GCM performance study
9. CHACHA20 vs AES-GCM performance study
10. PROTOCOL.certkeys
11. rfc44880bis from GnuPG
12. Weak Diffie-Hellman and the Logjam Attack
13. On OpenSSH and Logjam, by Jethro Beekman