feat: add "env://" scheme for key lookup

Tested manually as well:

```shell
$ export COSIGN_PASSWORD=foo
$ cosign generate-key-pair
Enter password for private key:
Enter password for private key again:
Private key written to cosign.key
Public key written to cosign.pub
$ export MYPRIVKEY="$(cat cosign.key)"
$ export MYPUBKEY="$(cat cosign.pub)"
$ cosign verify-blob --key env://MYPUBKEY /dev/null --signature <(cosign sign-blob --key env://MYPRIVKEY /dev/null)
Using payload from: /dev/null
tlog entry created with index: 2095539
tlog entry verified with uuid: dd55086556d7ac0cded8f50961b68f7740e1435fbc5bb47460a8d78321313c7d index: 2095539
Verified OK
```

Signed-off-by: Zachary Newman <[email protected]>

cmd/cosign/cli/sign.go

@@ -47,6 +47,9 @@ func Sign() *cobra.Command {
   # sign a container image and add annotations
   cosign sign --key cosign.key -a key1=value1 -a key2=value2 <IMAGE>
 
+  # sign a container image with a key stored in an environment variable
+  cosign sign --key env://[ENV_VAR] <IMAGE>
+
   # sign a container image with a key pair stored in Azure Key Vault
   cosign sign --key azurekms://[VAULT_NAME][VAULT_URI]/[KEY] <IMAGE>
 

cmd/cosign/cli/verify.go

@@ -61,6 +61,9 @@ against the transparency log.`,
   # verify image with public key provided by URL
   cosign verify --key https://host.for/[FILE] <IMAGE>
 
+  # verify image with a key stored in an environment variable
+  cosign verify --key env://[ENV_VAR] <IMAGE>
+
   # verify image with public key stored in Google Cloud KMS
   cosign verify --key gcpkms://projects/[PROJECT]/locations/global/keyRings/[KEYRING]/cryptoKeys/[KEY] <IMAGE>
 

pkg/signature/keys.go

@@ -19,8 +19,6 @@ import (
 	"crypto"
 	"crypto/x509"
 	"fmt"
-	"os"
-	"path/filepath"
 	"strings"
 
 	"github.com/pkg/errors"
@@ -86,7 +84,7 @@ func VerifierForKeyRef(ctx context.Context, keyRef string, hashAlgorithm crypto.
 }
 
 func loadKey(keyPath string, pf cosign.PassFunc) (signature.SignerVerifier, error) {
-	kb, err := os.ReadFile(filepath.Clean(keyPath))
+	kb, err := blob.LoadFileOrURL(keyPath)
 	if err != nil {
 		return nil, err
 	}
@@ -169,10 +167,14 @@ func SignerVerifierFromKeyRef(ctx context.Context, keyRef string, pf cosign.Pass
 
 	if strings.Contains(keyRef, "://") {
 		sv, err := kms.Get(ctx, keyRef, crypto.SHA256)
-		if err != nil {
+		if err == nil {
+			return sv, nil
+		}
+		var e *kms.ProviderNotFoundError
+		if !errors.As(err, &e) {
 			return nil, fmt.Errorf("kms get: %w", err)
 		}
-		return sv, nil
+		// ProviderNotFoundError is okay; loadKey handles other URL schemes
 	}
 
 	return loadKey(keyRef, pf)

pkg/signature/keys_test.go

@@ -108,6 +108,35 @@ func TestPublicKeyFromFileRef(t *testing.T) {
 	}
 }
 
+func TestPublicKeyFromEnvVar(t *testing.T) {
+	keys, err := cosign.GenerateKeyPair(pass("whatever"))
+	if err != nil {
+		t.Fatalf("failed to generate keypair: %v", err)
+	}
+	ctx := context.Background()
+
+	os.Setenv("MY_ENV_VAR", string(keys.PublicBytes))
+	defer os.Unsetenv("MY_ENV_VAR")
+	if _, err := PublicKeyFromKeyRef(ctx, "env://MY_ENV_VAR"); err != nil {
+		t.Fatalf("PublicKeyFromKeyRef returned error: %v", err)
+	}
+}
+
+func TestSignerVerifierFromEnvVar(t *testing.T) {
+	passFunc := pass("whatever")
+	keys, err := cosign.GenerateKeyPair(passFunc)
+	if err != nil {
+		t.Fatalf("failed to generate keypair: %v", err)
+	}
+	ctx := context.Background()
+
+	os.Setenv("MY_ENV_VAR", string(keys.PrivateBytes))
+	defer os.Unsetenv("MY_ENV_VAR")
+	if _, err := SignerVerifierFromKeyRef(ctx, "env://MY_ENV_VAR", passFunc); err != nil {
+		t.Fatalf("SignerVerifierFromKeyRef returned error: %v", err)
+	}
+}
+
 func pass(s string) cosign.PassFunc {
 	return func(_ bool) ([]byte, error) {
 		return []byte(s), nil