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134 lines
4.2 KiB
ReStructuredText
134 lines
4.2 KiB
ReStructuredText
libsecureboot
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*************
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This library depends one way or another on verifying digital signatures.
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To do that, the necessary trust anchors need to be available.
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The simplest (and most attractive for an embedded system) is to
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capture them in this library.
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The makefile ``local.trust.mk`` is responsible for doing that.
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The file provided is just an example and depends on the environment
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here at Juniper.
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Within Juniper we use signing servers, which apart from signing things
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provide access to the necessary trust anchors.
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That signing server is freely available - see
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http://www.crufty.net/sjg/docs/signing-server.htm
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X.509 certificates chains offer a lot of flexibility over time and are
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a great solution for an embedded vendor like Juniper or even
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FreeBSD.org, but are probably overkill for personal or small site use.
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Setting up a CA for this is rather involved so I'll just provide a
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link below to suitable tutorial below.
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Using OpenPGP is much simpler.
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OpenPGP
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========
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This is very simple to setup and use.
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An RSA key pair can be generated with::
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GNUPGHOME=$PWD/.gnupg gpg --openpgp \
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--quick-generate-key --batch --passphrase '' "keyname" RSA
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The use of ``GNUPGHOME=$PWD/.gnupg`` just avoids messing with personal
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keyrings.
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We can list the resulting key::
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GNUPGHOME=$PWD/.gnupg gpg --openpgp --list-keys
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gpg: WARNING: unsafe permissions on homedir
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'/h/sjg/openpgp/.gnupg'
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gpg: Warning: using insecure memory!
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/h/sjg/openpgp/.gnupg/pubring.kbx
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---------------------------------
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pub rsa2048 2018-03-26 [SC] [expires: 2020-03-25]
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AB39B111E40DD019E0E7C171ACA72B4719FD2523
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uid [ultimate] OpenPGPtest
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The ``keyID`` we want later will be the last 8 octets
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(``ACA72B4719FD2523``)
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This is what we will use for looking up the key.
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We can then export the private and public keys::
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GNUPGHOME=$PWD/.gnupg gpg --openpgp \
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--export --armor > ACA72B4719FD2523.pub.asc
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GNUPGHOME=$PWD/.gnupg gpg --openpgp \
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--export-secret-keys --armor > ACA72B4719FD2523.sec.asc
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The public key ``ACA72B4719FD2523.pub.asc`` is what we want to
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embed in this library.
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If you look at the ``ta_asc.h`` target in ``openpgp/Makefile.inc``
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we want the trust anchor in a file named ``t*.asc``
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eg. ``ta_openpgp.asc``.
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The ``ta_asc.h`` target will capture all such ``t*.asc`` into that
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header.
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Signatures
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----------
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We expect ascii armored (``.asc``) detached signatures.
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Eg. signature for ``manifest`` would be in ``manifest.asc``
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We only support version 4 signatures using RSA (the default for ``gpg``).
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OpenSSL
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========
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The basic idea here is to setup a private CA.
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There are lots of good tutorials on available on this topic;
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just google *setup openssl ca*.
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A good example is https://jamielinux.com/docs/openssl-certificate-authority/
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All we need for this library is a copy of the PEM encoded root CA
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certificate (trust anchor). This is expected to be in a file named
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``t*.pem`` eg. ``ta_rsa.pem``.
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The ``ta.h`` target in ``Makefile.inc`` will combine all such
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``t*.pem`` files into that header.
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Signatures
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----------
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For Junos we currently use EC DSA signatures with file extension
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``.esig`` so the signature for ``manifest`` would be ``manifest.esig``
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This was the first signature method we used with the remote signing
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servers and it ends up being a signature of a hash.
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Ie. client sends a hash which during signing gets hashed again.
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So for Junos we define VE_ECDSA_HASH_AGAIN which causes ``verify_ec``
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to hash again.
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Otherwise our EC DSA and RSA signatures are the default used by
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OpenSSL - an original design goal was that a customer could verify our
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signatures using nothing but an ``openssl`` binary.
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Self tests
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==========
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If you want the ``loader`` to perform self-test of a given signature
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verification method on startup (a must for FIPS 140-2 certification)
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you need to provide a suitable file signed by each supported trust
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anchor.
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These should be stored in files with names that start with ``v`` and
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have the same extension as the corresponding trust anchor.
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Eg. for ``ta_openpgp.asc`` we use ``vc_openpgp.asc``
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and for ``ta_rsa.pem`` we use ``vc_rsa.pem``.
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Note for the X.509 case we simply extract the 2nd last certificate
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from the relevant chain - which is sure to be a valid certificate
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signed by the corresponding trust anchor.
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--------------------
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