Hash and URL


To avoid UDP datagrams being fragmented, IKEv2 supports the Hash and URL encoding of X.509 certificates. Thus instead of transmitting the binary DER encoded certificates in the IKE_AUTH exchange, only a hash value and an URL of each certificate is submitted. This URL has to resolve to the DER encoded certificate and the hash is a SHA1 digest of that data.

The IKEv2 daemon charon supports both - sending and receiving - of certificates encoded in this manner. Receiving such payloads requires no special configuration from the user (except enabling the feature, see next section). Thus it is not specifically covered in this guide.


Figure 1. strongSwan Hash and URL example

In this guide we want to configure a connection between the road warriors carol and dave to the VPN gateway moon, using certificates issued by the strongSwan CA and served by the web server winnetou.

Enabling Hash and URL

Since there are some security concerns about accepting URLs supplied by unauthenticated peers and then blindly download whatever there may be, Hash and URL is currently disabled by default.

To enable Hash and URL support in charon the following option in strongswan.conf has to be set

charon {
  hash_and_url = yes

Configuring the Base URL

The URLs that will be sent by the IPsec peers are built by appending the SHA1 hash of the DER encoded certificate to a base URL configured in the authorities section of swanctl.conf. The option is cert_uri_base and it has to be added to the authorities section of the CA that issued the certificate we want to send as Hash and URL.

A peer that sends its certificates as Hash and URL still has to have these certificates locally available.

On carol the required CA section might look like this (likewise on moon and dave):

authorities {
  strongswan {
    cacert = strongswanCert.pem
    cert_uri_base = http://winnetou.strongswan.org/certs/

The URLs will then be built by concatenating the value of cert_uri_base and the SHA1 hash of the DER encoded certificate (see an example later on).

Preparing the Certificates

Next the certificates have to be prepared and uploaded to the web server winnetou. If the X.509 certificates provided, are PEM encoded they have to be converted to DER encoding first, e.g. using the openssl command

$ openssl x509 -in carolCert.pem -inform PEM -out carolCert.der -outform DER

As mentioned above the URL is just the base URL plus the SHA1 hash of the certificate data. To compute the hash, either openssl

$ openssl dgst -sha1 carolCert.der
SHA1(carolCert.der)= 2979610b950df52476a7b0f1cd2baa2bf8026d77

or sha1sum

$ sha1sum carolCert.der
2979610b950df52476a7b0f1cd2baa2bf8026d77  carolCert.der

can be used. The file carolCert.der is now renamed to 2979610b950df52476a7b0f1cd2baa2bf8026d77 and uploaded to the web server winnetou.

Hash and URL Use

In the example presented above carol sends her certificate in the form of a Hash and URL and moon fetches the actual certificate from winnetou

received cert request for "C=CH, O=strongSwan Project, CN=strongSwan Root CA"
received hash-and-url for end entity cert "http://winnetou.strongswan.org/certs/2979610b950df52476a7b0f1cd2baa2bf8026d77"
fetching certificate from 'http://winnetou.strongswan.org/certs/2979610b950df52476a7b0f1cd2baa2bf8026d77' ...
fetched certificate "C=CH, O=strongSwan Project, OU=Research, CN=carol@strongswan.org"

Multi-Level CA Example

Hash and URL also works in a multi-level CA setup where intermediate CA certificates are used.

Figure 2. strongSwan multi-level CA Hash and URL example