C H A P T E R 39 Configuring Certificates - Cisco FirePOWER ASA 5500 series Configuration Manual

Public Key Cryptography
Obtaining the public key of a sender is normally handled out-of-band or through an operation done at
installation. For instance, most web browsers are configured with the root certificates of several CAs by
default. For VPN, the IKE protocol, a component of IPSec, can use digital signatures to authenticate peer
devices before setting up security associations.
Certificate Scalability
Without digital certificates, you must manually configure each IPSec peer for every peer with which it
communicates, and every new peer you add to a network would thus require a configuration change on
every peer with which you need it to communicate securely.
When you use digital certificates, each peer is enrolled with a CA. When two peers attempt to
communicate, they exchange certificates and digitally sign data to authenticate each other. When a new
peer is added to the network, you enroll that peer with a CA and none of the other peers need
modification. When the new peer attempts an IPSec connection, certificates are automatically exchanged
and the peer can be authenticated.
With a CA, a peer authenticates itself to the remote peer by sending a certificate to the remote peer and
performing some public key cryptography. Each peer sends its unique certificate which was issued by
the CA. This process works because each certificate encapsulates the public key for the associated peer
and each certificate is authenticated by the CA, and all participating peers recognize the CA as an
authenticating authority. This is called IKE with an RSA signature.
The peer can continue sending its certificate for multiple IPSec sessions, and to multiple IPSec peers,
until the certificate expires. When its certificate expires, the peer administrator must obtain a new one
from the CA.
CAs can also revoke certificates for peers that no longer participate in IPSec. Revoked certificates are
not recognized as valid by other peers. Revoked certificates are listed in a CRL, which each peer may
check before accepting a certificate from another peer.
Some CAs have an RA as part of their implementation. An RA is a server that acts as a proxy for the CA
so that CA functions can continue when the CA is unavailable.
About Key Pairs
Key pairs are RSA keys.
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Cisco Security Appliance Command Line Configuration Guide
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RSA keys can be used for SSH or SSL.
SCEP enrollment supports the certification of RSA keys.
For the purposes of generating keys, the maximum key modulus for RSA keys is 2048. The default
size is 1024.
For signature operations, the supported maximum key size is 4096 bits.
You can generate a general purpose RSA key pair, used for both signing and encryption, or you can
generate separate RSA key pairs for each purpose.
Separate signing and encryption keys helps reduce exposure of the keys. This is because SSL uses
a key for encryption but not signing but IKE uses a key for signing but not encryption. By using
separate keys for each, exposure of the keys is minimized.
Chapter 39 Configuring Certificates
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