Signed-off-by: Daiki Ueno <dueno@redhat.com>

Suggested in:
ostreedev#3278 (comment)

Signed-off-by: Daiki Ueno <dueno@redhat.com>

This defines a new interface OstreeBlobReader, which encapsulates the
key file parsing logic. This would make it easy to support custom file
formats such as PEM.

Signed-off-by: Daiki Ueno <dueno@redhat.com>

This adds a new class OstreePemReader, which reads PEM blocks from an
input stream.  This would be useful for the "x509" signing backend, as
the keys are typically stored in the PEM format.

Signed-off-by: Daiki Ueno <dueno@redhat.com>

Signed-off-by: Daiki Ueno <dueno@redhat.com>

The current "ed25519" signing type assumes raw Ed25519 key format for
both public and private keys. That requires custom processing of keys
after generated with openssl tools, and also lacks cryptographic
agility[1]; when Ed25519 becomes vulnerable, it would not be
straightforward to migrate to other algorithms, such as post-quantum
signature algorithms.

This patch adds a new signature type "spki" which uses the X.509
SubjectPublicKeyInfo format for public keys. Keys in this format can
easily be created with openssl tools and provide crypto agility as the
format embeds algorithm identifier.

Currently, the corresponding private keys shall be in the PKCS#8
format, while future extensions may support other format such as
opaque key handles on a hardware token.

The "spki" signature type prefers keys to be encoded in the PEM
format on disk, while it still accepts base64 encoded keys when given
through the command-line.

1. https://en.wikipedia.org/wiki/Cryptographic_agility

Signed-off-by: Daiki Ueno <dueno@redhat.com>