Kategorie:SSH/Kryptografie

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).

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

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

SSH/Kryptografie/Konfiguration

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