Checking provisioning profile's developer certificate validity

I want to allow customers to upload their own provisioning profile, including icons, so that I can make them a custom version of my app on the fly, which they can then publish.

However, I'm having a little trouble validating the provisioning profile. In particular, I want to check whether the DeveloperCertificate is actually a valid certificate. The profile looks something like this:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>ApplicationIdentifierPrefix</key>
    <array>
        <string>ABCDEFGH</string>
    </array>
    <key>CreationDate</key>
    <date>2012-03-28T11:17:23Z</date>
    <key>DeveloperCertificates</key>
    <array>
        <data>
        MIIFajCCBFKgAwIBAgIIddUra9YprMQwDQYJKoZIhvcNAQEFBQAwgZYxCzAJ
        BgNVBAYTAlVTMRMwEQYDVQQKDApBcHBsZSBJbmMuMSwwKgYDVQQLDCNBcHBs
        ZSBXb3JsZHdpZGUgRGV2ZWxvcGVyIFJlbGF0aW9uczFEMEIGA1UEAww7QXBw
        ...     
        </data>
    </array>
    ...
</dict>

So, I extract the certificate(s) and then want to check them, preferably using an openssl command. What is the encryption used for these certificates, and how do I verify them using openssl? I would think that this uses pkcs12, but trying that gives me an error:

$ openssl pkcs12 -noout -in testcertificate
140653159306912:error:0D0680A8:asn1 encoding routines:ASN1_CHECK_TLEN:wrong tag:tasn_dec.c:1319:
140653159306912:error:0D07803A:asn1 encoding routines:ASN1_ITEM_EX_D2I:nested asn1 error:tasn_dec.c:381:Type=PKCS12

Can anyone point me in the right direction? It is essential that I can somehow verify developer certificates' validity.

Thanks