This is about how to configure OpenSSH. Especially many configuration parameters becomes aged in recent version of OpenSSH, most notably DSA keys are obsoleted and disabled by default.

This summary mostly based on StackExchange and the famous github page.

Structure of OpenSSH

Establishing a connection via OpenSSH: All begins with a key exchange, then a public key encryption for host authentication and then client authentication. Afterwards, data stream is encrypted by symmetric ciphers. Encrypted data also comes with the message authentication code to allow integrity check.

Key exchange

There is only two ways to do key exchage: Diffie-Hellman and elliptic Diffie-Hellman. In OpenSSH, only 8 key exchange protocols supported: curve25519-sha256 (ECDH over Curve25519 + SHA2), diffie-hellman-group{1,14}-sha1 (1024/2048-bit DH + SHA1), diffie-hellman-group-exchange-sha{1,2} (Custom DH + SHA1/SHA2), ecdh-sha2-nistp{256,384,521} (ECDH over NIST P-256/384/521 with SHA2).

Elliptic curve implementation ECDSA is commonly based on NIST standard, which prescribes a few curves. However, it is believed that the curves are weak and suspicious for NSA backdoors. Curve25519 and Curve1174 are suggested alternatives.

SHA1 is broken, so SHA2 should be used in all application. An a key length of 1024 bit is not considered sufficient today. This leaves only two key exchange protocols as options: curve25519-sha256, diffie-hellman-group-exchange-sha2.

Public key encryption

Host authentication is always key-based. For SSHv2, there are four algorithms available: DSA with SHA1, ECDSA with SHA256/385/512 (depends on key size), Ed25519 with SHA512, RSA (with key size 1024 or 2048) with SHA1.

DSA is invented at the time when RSA was patented. RSA is based on integer factorization while DSA is based on discrete logarithm. It is reported that DSA is faster in signature generation and decryption but slower in signature validation and encryption.

There is a chance that RSA and DSA both become mathematically broken in a short future due to mutual research advance in the General Number Field Sieve algorithm to factor integers and the Function Field Sieve algorithm to solve discrete log. Therefore elliptic curve should be the direction for today.

Another issue with DSA is the implementation allows only 1024-bit key for the compliance to NIST FIPS 186-2, which is insufficient today. RSA allows 2048-bit alternative, but not DSA in recent version’s OpenSSH.

Both DSA and ECDSA assumes a good RNG. The cipher has a parameter that required to be random, secure, and unique. If the machine has a poor RNG, the algorithm may fail. For example, same used twice can help figure out private key.

Therefore, the accepted cipher are only ed25519 and rsa.

Symmetric cipher

OpenSSH supports quite a number of symmetric cipher: 3des-cbc, aes{128,192,256}-{cbc,ctr}, aes{128,256}-gcm@openssh.com, arcfour, arcfour{128,256}, blowfish-cbc, cast128-cbc, chacha20-poly1305@openssh.com.

3DES and arcfour are not considered secure. Block ciphers blowfish and cast128 are with only 64-bit block size, which is too short (128 bit or above should be recommended).

Cipher modes AE (authenticated encryption) is recommended but CTR should be allowed for compatibility, given CTR with encrypt-then-MAC is provably secure. AES-GCM has the weakness of SSH not encrypt the message size when GCM is in use, which may allow some traffic analysis without decrypting data.

This leaves the usable ciphers be chacha20 and the five AES.

Message authentication code

Options: hmac-md5, hmac-md5-96, hmac-ripemd160, hmac-sha1, hmac-sha1-96, hmac-sha2-{256,512}, umac-{64,128}, hmac-md5-etm@openssh.com, hmac-md5-96-etm@openssh.com, hmac-ripemd160-etm@openssh.com, hmac-sha1-etm@openssh.com, hmac-sha1-96-etm@openssh.com, hmac-sha2-{256,512}-etm@openssh.com, umac-{64,128}-etm@openssh.com

MAC is for data integrity while encryption is for confidentiality. If AE cipher mode is selected, integrity is already given. Otherwise we need a MAC.

There are 3 common ways to combine cipher and MAC:

  1. Encrypt-then-MAC: encrypt(message) + MAC(encrypt(message))
  2. MAC-then-encrypt: encrypt(message + MAC(message))
  3. Encrypt-and-MAC: encrypt(message) + MAC(message)

MAC-then-encrypt have lead to many attacks on TLS. Encrypt-and-MAC also lead to some attacks on SSH. Only encrypt-then-MAC (ETM) should be used, because otherwise the time to confirm decryption failure and MAC verification failure is different. CTR-and-HMAC can be considered sufficient even there is no security proof for it.

MD5 and SHA1 are considered weak. Tag size and key size both should be at least 128 bits. This leaves only the SHA2, umac-128, and ripemd160 in ETM mode as MAC.

Config files

Below are the snippet of configuration files for OpenSSH. PasswordAuthentication (RFC4252, section 8) should always be disabled. If password is allowed for client authentication, use ChallengeResponseAuthentication instead. The latter is the keyboard interactive (RFC4256) authentication that requires password to be type via tty device. This will forbid authentication in this way:

echo $PASSWORD | ssh $HOST $CMD

For /etc/ssh/sshd_config:

Protocol 2
HostKey /etc/ssh/ssh_host_ed25519_key
HostKey /etc/ssh/ssh_host_rsa_key
PubkeyAuthentication yes
PasswordAuthentication no
ChallengeResponseAuthentication no
KexAlgorithms curve25519-sha256@libssh.org,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,hmac-ripemd160-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,hmac-ripemd160,umac-128@openssh.com

For /etc/ssh/ssh_config:

Host *
    KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256
    PubkeyAuthentication yes
    PasswordAuthentication no
    ChallengeResponseAuthentication no
    HostKeyAlgorithms ssh-ed25519-cert-v01@openssh.com,ssh-rsa-cert-v01@openssh.com,ssh-ed25519,ssh-rsa
    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,hmac-ripemd160-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,hmac-ripemd160,umac-128@openssh.com

Further enhancing the security of SSH: consider fail2ban to keep off brute force attack by lock out IPs upon several failed attempt; and libpam-google-authenticator for 2nd factor authentication.

Reference:

  1. Information Security StackExchange (2012) RSA vs DSA for SSH
  2. Ptacek et al (2013) The factoring dead, BlackHat 2013 presentation
  3. Bernstein and Lange (2013) Security dangers of the NIST curves
  4. Stribika (2015) Secure Secure Shell
  5. NIST FIPS 186-4 (2016)