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Cisco ASA 5500 Series Configuration Guide using ASDM
Chapter44 Configuring Digital Certificates
Information About Digital Certificates
Obtaining the public key of a sender is normally handled externally or through an operation performed
at installation. For example, 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 each peer with which it
communicates; as a result, each new peer that you add to a network would require a configuration change
on each peer with which it needs to communicate securely.
When you use digital certificates, each peer is enrolled with a CA. When two peers try 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,
each certificate is authenticated by the CA, and all participating peers recognize the CA as an
authenticating authority. The process 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.
Key Pairs
Key pairs are RSA keys, which have the following characteristics:
•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 bits. The
default size is 1024. Many SSL connections using identity certificates with RSA key pairs that
exceed 1024 bits can cause a high CPU usage on the ASA and rejected clientless logins.
•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 help to
reduce exposure of the keys, because SSL uses a key for encryption but not signing. However, IKE
uses a key for signing but not encryption. By using separate keys for each, exposure of the keys is
minimized.