Kategorie:SSH/Kryptografie

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

SSH/Kryptografie/Konfiguration

Anhang

Siehe auch

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