Sign and verify manifests in Go with OpenPGP instead of running gpg (closes #181)

mfer ran the gpg binary to sign, export keys and verify, so it failed
wherever gpg is missing. It now uses github.com/ProtonMail/go-crypto/openpgp.
--sign-key and MFER_SIGN_KEY name a file holding one version 4 OpenPGP
secret key; a protected key's passphrase comes from
MFER_SIGN_KEY_PASSPHRASE or a terminal prompt. gen and freshen check that
the key can sign before they read any file. Verification keeps the rules
of the --require-signature fix: one primary key in the embedded block,
counted from its packets, exactly one signature, made by that key or a
subkey, and signer equal to its fingerprint. The embedded block may hold
no DSA key and no secret key, and an armored field must be one
well-formed block.

Model: opus-5-5

mfer/builder.go

@@ -290,7 +290,7 @@ func (b *Builder) SetIncludeTimestamps(include bool) {
 	b.includeTimestamps = include
 }
 
-// SetSigningOptions sets the GPG signing options for the manifest.
+// SetSigningOptions sets the key the manifest is signed with.
 // If opts is non-nil, the manifest will be signed when Build() is called.
 func (b *Builder) SetSigningOptions(opts *SigningOptions) {
 	b.mu.Lock()
@@ -299,8 +299,8 @@ func (b *Builder) SetSigningOptions(opts *SigningOptions) {
 	b.signingOptions = opts
 }
 
-// Build finalizes the manifest and writes it to the writer. ctx bounds the
-// gpg runs that sign the manifest when signing options are set.
+// Build finalizes the manifest and writes it to the writer. When signing
+// options are set, it does not sign once ctx has ended.
 func (b *Builder) Build(ctx context.Context, w io.Writer) error {
 	b.mu.Lock()
 	defer b.mu.Unlock()