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User Guide for Cisco Security Manager 4.4
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Chapter 25 Configuring IKE and IPsec Policies
Understanding IKE
The following options, which are even more secure, are available for IKEv2 configurations on ASA
8.4(2+) devices:
–
SHA512—A 512-bit key.
–
SHA384—A 384-bit key.
–
SHA256—A 256-bit key.
• MD5 (Message Digest 5) produces a 128-bit digest and uses less processing time for an overall
faster performance than SHA, but it is considered to be weaker than SHA.
Related Topics
• Understanding IKE, page 25-5
• Configuring an IKE Proposal, page 25-9
Deciding Which Diffie-Hellman Modulus Group to Use
Security Manager supports the following Diffie-Hellman key derivation algorithms to generate IPsec
security association (SA) keys. Each group has a different size modulus. A larger modulus provides
higher security, but requires more processing time. You must have a matching modulus group on both
peers.
Tip If you select AES encryption, to support the large key sizes required by AES, ISAKMP negotiation
should use Diffie-Hellman (DH) Group 5 or higher. For IKEv1, ASA devices support groups 2 and 5
only.
• Diffie-Hellman Group 1: 768-bit modulus. Use to generate IPsec SA keys where the prime and
generator numbers are 768 bits.
• Diffie-Hellman Group 2: 1024-bit modulus. Use to generate IPsec SA keys where the prime and
generator numbers are 1024 bits. Cisco VPN Client Version 3.x or higher requires a minimum of
Group 2.
• Diffie-Hellman Group 5: 1536-bit modulus. Use to generate IPsec SA keys where the prime and
generator numbers are 2048 bits. Considered good protection for 128-bit keys, but group 14 is better.
• Diffie-Hellman Group 7: Use to generate IPsec SA keys when the elliptical curve field size is 163
characters. Group 7 is not supported on a Catalyst 6500/7600 device with VPNSM or VPN SPA
configuration.
• Diffie-Hellman Group 14: 2048-bit modulus. Considered good protection for 128-bit keys. (ASA
9.0.1+ devices only).
• Diffie-Hellman Group 15: 3072-bit modulus. Considered good protection for 192-bit keys.
• Diffie-Hellman Group 16: 4096-bit modulus. Considered good protection for 256-bit keys.
• Diffie-Hellman Group 19: (256-bit elliptical curve field size). (ASA 9.0.1+ devices only).
• Diffie-Hellman Group 20: (384-bit elliptical curve field size). (ASA 9.0.1+ devices only).
• Diffie-Hellman Group 21: (521-bit elliptical curve field size). (ASA 9.0.1+ devices only).
• Diffie-Hellman Group 24: ( 2048-bit modulus and 256-bit prime order subgroup). (ASA 9.0.1+
devices only).
