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

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?

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.

For setting up Android network-security-config pinning for a host that is already live, I prefer gnutls-cli (GnuTLS Client). It outputs a host's certificate info in a form where the sha256 is readibly copy-pasteable as base64 encoded. For example:

$ gnutls-cli stackoverflow.com </dev/null
<...>
 - subject `CN=*.stackexchange.com,O=Stack Exchange\, Inc.,L=New York,ST=NY,C=US', issuer `CN=DigiCert SHA2 High Assurance Server CA,OU=www.digicert.com,O=DigiCert Inc,C=US', serial 0x0e11bbd70d54b710d0c6f540b6b52ca4, RSA key 2048 bits, signed using RSA-SHA256, activated `2016-05-21 00:00:00 UTC', expires `2019-08-14 12:00:00 UTC', pin-sha256="2zKehMv7KtnGBz1d2U0bFrAOKb1aWWlrG9a0BzrOvwA="

Use this to get base64 sha256 of online https url:

openssl s_client -connect google.com:443 < /dev/null 2>/dev/null | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64