Go: Begin working on abstracting server/organization list

Signed-off-by: jwijenbergh <jeroenwijenbergh@protonmail.com>

1 files changed, 200 insertions, 0 deletions

diff --git a/src/verify.go b/src/verify.go
new file mode 100644
index 0000000..336ba73
--- /dev/null
+++ b/src/verify.go
@@ -0,0 +1,200 @@
+package eduvpn_discovery
+
+import (
+	"fmt"
+	"github.com/jedisct1/go-minisign"
+	"os"
+)
+
+// getKeys returns keys taken from https://git.sr.ht/~eduvpn/disco.eduvpn.org#public-keys.
+func getKeys() []string {
+	return []string{
+		"RWRtBSX1alxyGX+Xn3LuZnWUT0w//B6EmTJvgaAxBMYzlQeI+jdrO6KF", // fkooman@tuxed.net, kolla@uninett.no
+		"RWQKqtqvd0R7rUDp0rWzbtYPA3towPWcLDCl7eY9pBMMI/ohCmrS0WiM", // RoSp
+	}
+}
+
+// Verify verifies the signature (.minisig file format) on signedJson.
+//
+// expectedFileName must be set to the file type to be verified, either "server_list.json" or "organization_list.json".
+// minSign must be set to the minimum UNIX timestamp (without milliseconds) for the file version.
+// This value should not be smaller than the time on the previous document verified.
+//
+// The return value will either be (true, nil) for a valid signature or (false, VerifyError) otherwise.
+//
+// Verify is a wrapper around verifyWithKeys where allowedPublicKeys is set to the list from https://git.sr.ht/~eduvpn/disco.eduvpn.org#public-keys.
+func Verify(signatureFileContent string, signedJson []byte, expectedFileName string, minSignTime uint64) (bool, error) {
+	keyStrs := getKeys()
+	if extraKey != "" {
+		keyStrs = append(keyStrs, extraKey)
+		_, err := fmt.Fprintf(os.Stderr, "INSECURE TEST MODE ENABLED WITH KEY %q\n", extraKey)
+		if err != nil {
+			panic(err)
+		}
+	}
+	valid, err := verifyWithKeys(signatureFileContent, signedJson, expectedFileName, minSignTime, keyStrs)
+	if err != nil {
+		if err.(detailedVerifyError).Code == errInvalidPublicKey {
+			panic(err) // This should not happen unless keyStrs has an invalid key
+		}
+		return valid, err.(detailedVerifyError).ToVerifyError()
+	}
+	return valid, nil
+}
+
+// extraKey is an extra allowed key for testing.
+var extraKey = ""
+
+// InsecureTestingSetExtraKey adds an extra allowed key for verification with Verify.
+// ONLY USE FOR TESTING. Applies to all threads. Probably not thread-safe. Do not call in parallel to Verify.
+//
+// keyString must be a Base64-encoded Minisign key, or empty to reset.
+func InsecureTestingSetExtraKey(keyString string) {
+	extraKey = keyString
+}
+
+// VerifyErrorCode Simplified error code for public interface.
+type VerifyErrorCode int8
+
+const (
+	ErrUnknownExpectedFileName    VerifyErrorCode = iota + 1 // Unknown expected file name specified. The signature has not been verified.
+	ErrInvalidSignature                                      // Signature is invalid (for the expected file type).
+	ErrInvalidSignatureUnknownKey                            // Signature was created with an unknown key and has not been verified.
+	ErrTooOld                                                // Signature timestamp smaller than specified minimum signing time (rollback).
+)
+
+type VerifyError struct {
+	Code     VerifyErrorCode
+	Detailed detailedVerifyError
+}
+
+func (err VerifyError) Error() string {
+	return err.Detailed.Error()
+}
+func (err VerifyError) Unwrap() error {
+	return err.Detailed
+}
+
+// verifyWithKeys verifies the Minisign signature in signatureFileContent (minisig file format) over the server_list/organization_list JSON in signedJson.
+//
+// Verification is performed using a matching key in allowedPublicKeys.
+// The signature is checked to be a Blake2b-prehashed Ed25519 Minisign signature with a valid trusted comment.
+// The file type that is verified is indicated by expectedFileName, which must be one of "server_list.json"/"organization_list.json".
+// The trusted comment is checked to be of the form "timestamp:<timestamp>\tfile:<expectedFileName>", optionally suffixed by something, e.g. "\thashed".
+// The signature is checked to have a timestamp with a value of at least minSignTime, which is a UNIX timestamp without milliseconds.
+//
+// The return value will either be (true, nil) on success or (false, detailedVerifyError) on failure.
+func verifyWithKeys(signatureFileContent string, signedJson []byte, expectedFileName string, minSignTime uint64, allowedPublicKeys []string) (bool, error) {
+	switch expectedFileName {
+	case "server_list.json", "organization_list.json":
+		break
+	default:
+		return false, detailedVerifyError{errUnknownExpectedFileName, "invalid expected file name", nil}
+	}
+
+	sig, err := minisign.DecodeSignature(signatureFileContent)
+	if err != nil {
+		return false, detailedVerifyError{errInvalidSignatureFormat, "invalid signature format", err}
+	}
+
+	// Check if signature is prehashed, see https://jedisct1.github.io/minisign/#signature-format
+	if sig.SignatureAlgorithm != [2]byte{'E', 'D'} {
+		return false, detailedVerifyError{errInvalidSignatureAlgorithm, "BLAKE2b-prehashed EdDSA signature required", nil}
+	}
+
+	// Find allowed key used for signature
+	for _, keyStr := range allowedPublicKeys {
+		key, err := minisign.NewPublicKey(keyStr)
+		if err != nil {
+			// Should only happen if Verify is wrong or extraKey is invalid
+			return false, detailedVerifyError{errInvalidPublicKey, "internal error: could not create public key", err}
+		}
+
+		if sig.KeyId != key.KeyId {
+			continue // Wrong key
+		}
+
+		valid, err := key.Verify(signedJson, sig)
+		if !valid {
+			return false, detailedVerifyError{errInvalidSignature, "invalid signature", err}
+		}
+
+		// Parse trusted comment
+		var signTime uint64
+		var sigFileName string
+		// sigFileName cannot have spaces
+		_, err = fmt.Sscanf(sig.TrustedComment, "trusted comment: timestamp:%d\tfile:%s", &signTime, &sigFileName)
+		if err != nil {
+			return false, detailedVerifyError{errInvalidTrustedComment, "failed to interpret trusted comment", err}
+		}
+
+		if sigFileName != expectedFileName {
+			return false, detailedVerifyError{errWrongFileName, "signature was created for wrong file", nil}
+		}
+
+		if signTime < minSignTime {
+			return false, detailedVerifyError{errTooOld, "signature was created a time earlier than the minimum time specified", nil}
+		}
+
+		return true, nil
+	}
+
+	// No matching allowed key found
+	return false, detailedVerifyError{errWrongKey, "signature was created with an unknown key", nil}
+}
+
+// detailedVerifyErrorCode used for unit tests.
+type detailedVerifyErrorCode int8
+
+const (
+	errUnknownExpectedFileName detailedVerifyErrorCode = iota + 1
+	errInvalidSignatureFormat
+	errInvalidSignatureAlgorithm
+	errInvalidPublicKey
+	errInvalidSignature
+	errInvalidTrustedComment
+	errWrongFileName
+	errTooOld
+	errWrongKey
+)
+
+func (code detailedVerifyErrorCode) ToVerifyErrorCode() VerifyErrorCode {
+	switch code {
+	case errUnknownExpectedFileName:
+		return ErrUnknownExpectedFileName
+	case errInvalidSignatureFormat:
+		return ErrInvalidSignature
+	case errInvalidSignatureAlgorithm:
+		return ErrInvalidSignature
+	case errInvalidPublicKey:
+		panic("errInvalidPublicKey cannot be converted to VerifyErrorCode")
+	case errInvalidSignature:
+		return ErrInvalidSignature
+	case errInvalidTrustedComment:
+		return ErrInvalidSignature
+	case errWrongFileName:
+		return ErrInvalidSignature
+	case errTooOld:
+		return ErrTooOld
+	case errWrongKey:
+		return ErrInvalidSignatureUnknownKey
+	}
+	panic("invalid detailedVerifyErrorCode")
+}
+
+type detailedVerifyError struct {
+	Code    detailedVerifyErrorCode
+	Message string
+	Cause   error
+}
+
+func (err detailedVerifyError) Error() string {
+	return err.Message
+}
+func (err detailedVerifyError) Unwrap() error {
+	return err.Cause
+}
+
+func (err detailedVerifyError) ToVerifyError() VerifyError {
+	return VerifyError{err.Code.ToVerifyErrorCode(), err}
+}