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Cisco ASA 5500 Series Configuration Guide using the CLI
Chapter41 Configuring Digital Certificates
Information About Digital Certificates
•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.
Trustpoints
Trustpoints let you manage and track CAs and certificates. A trustpoint is a representation of a CA or
identity pair. A trustpoint includes the identity of the CA, CA-specific configuration parameters, and an
association with one, enrolled identity certificate.
After you have defined a trustpoint, you can reference it by name in commands requiring that you specify
a CA. You can configure many trustpoints.
Note If an ASA has multiple trustpoints that share the same CA, only one of these trustpoints sharing the CA
can be used to validate user certificates. To control which trustpoint sharing a CA is used for validation
of user certificates issued by that CA, use the support-user-cert-validation command.
For automatic enrollment, a trustpoint must be configured with an enrollment URL, and the CA that the
trustpoint represents must be available on the network and must support SCEP.
You can export and import the keypair and issued certificates associated with a trustpoint in PKCS12
format. This format is useful to manually duplicate a trustpoint configuration on a different ASA.

Certificate Enrollment

The ASA needs a CA certificate for each trustpoint and one or two certificates for itself, depending upon
the configuration of the keys used by the trustpoint. If the trustpoint uses separate RSA keys for signing
and encryption, the ASA needs two certificates, one for each purpose. In other key configurations, only
one certificate is needed.
The ASA supports automatic enrollment with SCEP and with manual enrollment, which lets you paste
a base-64-encoded certificate directly into the terminal. For site-to-site VPNs, you must enroll each
ASA. For remote access VPNs, you must enroll each ASA and each remote access VPN client.

Proxy for SCEP Requests

The ASA can proxy SCEP requests between AnyConnect and a third-party CA. The CA only needs to
be accessible to the ASA if it is acting as the proxy. For the ASA to provide this service, the user must
authenticate using any of the methods supported by AAA before the ASA sends an enrollment request.
You can also use host scan and dynamic access policies to enforce rules of eligibility to enroll.
The ASA supports this feature only with an AnyConnect SSL or IKEv2 VPN session. It supports all
SCEP-compliant CAs, including IOS CS, Windows Server 2003 CA, and Windows Server 2008 CA.
Clientless (browser-based) access does not support SCEP proxy, although
WebLaunch—clientless-initiated AnyConnect—does support it.