How Do We Generate a Base64-Encoded SHA256 Hash of SubjectPublicKeyInfo of an X.509 Certificate, for Android N Certificate Pinning?

Question

The documentation in the N Developer Preview for their network security configuration offers these instructions:

Certificate pinning is done by providing a set of certificates by hash of the public key (SubjectPublicKeyInfo of the X.509 certificate). A certificate chain is then only valid if the certificate chain contains at least one of the pinned public keys.

The XML that they show is broken (missing a closing tag), but otherwise suggests that the hash is SHA256 and encoded base64:

<?xml version="1.0" encoding="utf-8"?>
<network-security-config>
    <domain-config>
        <domain includeSubdomains="true">example.com</domain>
        <pin-set expiration="2018-01-01">
            <pin digest="SHA-256">7HIpactkIAq2Y49orFOOQKurWxmmSFZhBCoQYcRhJ3Y=</pin>
            <!-- backup pin -->
            <pin digest="SHA-256">fwza0LRMXouZHRC8Ei+4PyuldPDcf3UKgO/04cDM1oE=</pin>
    </domain-config>
</network-security-config>

How do we create such a hash?

I tried the approach in this gist, but openssl x509 -inform der -pubkey -noout is not liking my CRT file. I cannot readily determine if the problem is in the CRT file, the instructions, my version of openssl, or something else.

Does anyone have a known good recipe for creating this hash?

Answer 1

openssl x509 -in cert.crt -pubkey -noout | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64

You may need to add -inform der to the first command if cert.crt is in DER form rather than in PEM form.