Support different verification options per image

Different images will require different verification options. This
commit adds configuration that allows you to define different
'verifiers' for specific image references, or image reference patterns.

At the moment it supports verification by public key, or the existing
options, but should be expanded to include all supported options.

Also modifies the response from the provider to include an error
per-image checked, rather than returning any error as a 'system' error.

I've also removed the _invalid suffix from the key returned in the
response when there's an error. The presence of the 'error' field
indicates this better, I think.

Signed-off-by: Rob Best <robertbest89@gmail.com>

Author: ribbybibby

Dockerfile

@@ -24,7 +24,7 @@ RUN go mod download
 
 COPY . .
 
-RUN go build -o provider provider.go
+RUN go build -o provider .
 
 FROM $BASEIMAGE
 

README.md

@@ -44,3 +44,32 @@ $ cosign sign --key cosign.key devopps/signed:latest
 ```
 
 So, once you are ready, let's apply these manifests one by one. It should allow deploying Pod for valid.yaml, and deny for the other one.
+
+## Configuration
+
+The provider can be configured with a configuration file passed with the
+`-config-file=<file>` flag.
+
+The verification options for specific image references can be configured by
+defining verifiers. 
+
+If a matching verifier can't be found then it will return an error for that image
+in the response.
+
+```yaml
+verifiers:
+  # Verify images in the my-project GCR registry with GCP KMS
+  - image: "eu.gcr.io/my-project/*"
+    options:
+      key: "gcpkms://projects/my-project/locations/global/keyRings/my-keyring/cryptoKeys/my-key"
+
+  # Verify images from my-registry with cosign.pub
+  - image: "my-registry:12345/*"
+    options:
+      key: "/cosign.pub"
+
+  # Verify any other image with the rekor server
+  - image: "*"
+    options:
+      rekorURL: "https://rekor.sigstore.dev"
+```

config.go

@@ -0,0 +1,93 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"gopkg.in/yaml.v3"
+)
+
+var (
+	// DefaultOptions are the default verification options
+	DefaultOptions = &CheckOptions{
+		RekorURL: "https://rekor.sigstore.dev",
+	}
+
+	// DefaultConfig is the configuration used when none is provided
+	DefaultConfig = &Config{
+		Verifiers: []Verifier{
+			{
+				Options: DefaultOptions,
+			},
+		},
+	}
+)
+
+// LoadConfig loads configuration from a file. If a file isn't provided then it
+// returns the default configuration.
+func LoadConfig(confFile string) (*Config, error) {
+	if confFile == "" {
+		return DefaultConfig, nil
+	}
+
+	var c *Config
+
+	yamlReader, err := os.Open(confFile)
+	if err != nil {
+		return c, fmt.Errorf("error reading config file: %s", err)
+	}
+	defer yamlReader.Close()
+	decoder := yaml.NewDecoder(yamlReader)
+	decoder.KnownFields(true)
+
+	if err = decoder.Decode(&c); err != nil {
+		return c, fmt.Errorf("error parsing config file: %s", err)
+	}
+
+	return c, nil
+}
+
+// Config configures the provider
+type Config struct {
+	// Verifiers is a list of verifiers. The validator will iterate over
+	// this list until it finds a verifier that matches the image its
+	// validating.
+	Verifiers []Verifier `yaml:"verifiers"`
+}
+
+// Verifier verifies an image
+type Verifier struct {
+	// Image is an image reference, either to a specific image or a pattern.
+	// Supports '*' and '?' in the pattern string.
+	Image string `yaml:"image,omitempty"`
+
+	// Options defines verification options
+	Options *CheckOptions `yaml:"options,omitempty"`
+}
+
+// UnmarshalYAML sets default options for the verifier config when they aren't
+// provided
+func (v *Verifier) UnmarshalYAML(unmarshal func(interface{}) error) error {
+	type rawVerifier Verifier
+	var raw rawVerifier
+	if err := unmarshal(&raw); err != nil {
+		return err
+	}
+
+	if raw.Options == nil {
+		raw.Options = DefaultOptions
+	}
+
+	*v = Verifier(raw)
+
+	return nil
+}
+
+// CheckOptions are the options used to verify the signature of an image
+type CheckOptions struct {
+	// Key is a path to a public key file, KMS URI or Kubernetes Secret
+	Key string `yaml:"key,omitempty"`
+
+	// RekorURL is the address of a rekor STL server
+	RekorURL string `yaml:"rekorURL,omitempty"`
+}