Patch the go crypto libraries to enable gnu dummy values.

go/crypto.patch

@@ -0,0 +1,86 @@
+--- go/src/github.com/ProtonMail/gopenpgp/vendor/golang.org/x/crypto/openpgp/s2k/s2k.go	2019-07-20 15:43:48.000000000 -0700
++++ go/src/github.com/ProtonMail/gopenpgp/vendor/golang.org/x/crypto/openpgp/s2k/s2k.go	2019-07-20 15:53:58.000000000 -0700
+@@ -121,6 +121,53 @@
+ 	}
+ }
+ 
++func parseGNUExtensions(r io.Reader) (f func(out, in []byte), err error) {
++	var buf [9]byte
++
++	// A three-byte string identifier
++	_, err = io.ReadFull(r, buf[:3])
++	if err != nil {
++		return
++	}
++	gnuExt := string(buf[:3])
++
++	if gnuExt != "GNU" {
++		return nil, errors.UnsupportedError("Malformed GNU extension: " + gnuExt)
++	}
++	_, err = io.ReadFull(r, buf[:1])
++	if err != nil {
++		return
++	}
++	gnuExtType := int(buf[0])
++	switch gnuExtType {
++	case 1:
++		return nil, nil
++	case 2:
++		// Read a serial number, which is prefixed by a 1-byte length.
++		// The maximum length is 16.
++		var lenBuf [1]byte
++		_, err = io.ReadFull(r, lenBuf[:])
++		if err != nil {
++			return
++		}
++
++		maxLen := 16
++		ivLen := int(lenBuf[0])
++		if ivLen > maxLen {
++			ivLen = maxLen
++		}
++		ivBuf := make([]byte, ivLen)
++		// For now we simply discard the IV
++		_, err = io.ReadFull(r, ivBuf)
++		if err != nil {
++			return
++		}
++		return nil, nil
++	default:
++		return nil, errors.UnsupportedError("unknown S2K GNU protection mode: " + strconv.Itoa(int(gnuExtType)))
++	}
++}
++
+ // Iterated writes to out the result of computing the Iterated and Salted S2K
+ // function (RFC 4880, section 3.7.1.3) using the given hash, input passphrase,
+ // salt and iteration count.
+@@ -167,6 +214,12 @@
+ 		return
+ 	}
+ 
++	// GNU Extensions; handle them before we try to look for a hash, which won't
++	// be needed in most cases anyway.
++	if buf[0] == 101 {
++		return parseGNUExtensions(r)
++	}
++
+ 	hash, ok := HashIdToHash(buf[1])
+ 	if !ok {
+ 		return nil, errors.UnsupportedError("hash for S2K function: " + strconv.Itoa(int(buf[1])))
+
+--- go/src/github.com/ProtonMail/gopenpgp/vendor/golang.org/x/crypto/openpgp/packet/private_key.go	2019-07-20 15:43:48.000000000 -0700
++++ go/src/github.com/ProtonMail/gopenpgp/vendor/golang.org/x/crypto/openpgp/packet/private_key.go	2019-07-20 16:26:05.000000000 -0700
+@@ -154,6 +154,13 @@
+ 		if s2kType == 254 {
+ 			pk.sha1Checksum = true
+ 		}
++		// S2K == nil implies that we got a "GNU Dummy" S2K. For instance,
++		// because our master secret key is on a USB key in a vault somewhere.
++		// In that case, there is no further data to consume here.
++		if pk.s2k == nil {
++			pk.Encrypted = false
++			return
++		}
+ 	default:
+ 		return errors.UnsupportedError("deprecated s2k function in private key")
+ 	}

gopenpgp_build.sh

@@ -14,6 +14,7 @@ go get -u github.com/ProtonMail/gopenpgp || true
 PACKAGE_PATH="github.com/ProtonMail/gopenpgp"
 
 ( cd "$GOPATH/src/$PACKAGE_PATH" && GO111MODULE=on go mod vendor )
+patch -p0 < $GOPATH/crypto.patch
 
 OUTPUT_PATH="$GOPATH/dist"
 mkdir -p "$OUTPUT_PATH"