GPG: check that the key found is a signing key

Throw an exception if not.

Change-Id: I60f36b271d5f44c6dc475302b169cb5b8a1e3945
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>

org.eclipse.jgit/resources/org/eclipse/jgit/internal/JGitText.properties

@@ -330,6 +330,7 @@ gpgNoKeyring=neither pubring.kbx nor secring.gpg files found
 gpgNoKeyInLegacySecring=no matching secret key found in legacy secring.gpg for key or user id: {0}
 gpgNoPublicKeyFound=Unable to find a public-key with key or user id: {0}
 gpgNoSecretKeyForPublicKey=unable to find associated secret key for public key: {0}
+gpgNotASigningKey=Secret key ({0}) is not suitable for signing
 gpgKeyInfo=GPG Key (fingerprint {0})
 gpgSigningCancelled=Signing was cancelled
 headRequiredToStash=HEAD required to stash local changes

org.eclipse.jgit/src/org/eclipse/jgit/internal/JGitText.java

@@ -391,6 +391,7 @@ public class JGitText extends TranslationBundle {
 	/***/ public String gpgNoKeyInLegacySecring;
 	/***/ public String gpgNoPublicKeyFound;
 	/***/ public String gpgNoSecretKeyForPublicKey;
+	/***/ public String gpgNotASigningKey;
 	/***/ public String gpgKeyInfo;
 	/***/ public String gpgSigningCancelled;
 	/***/ public String headRequiredToStash;

org.eclipse.jgit/src/org/eclipse/jgit/lib/internal/BouncyCastleGpgKeyLocator.java

@@ -261,6 +261,10 @@ class BouncyCastleGpgKeyLocator {
 					USER_PGP_LEGACY_SECRING_FILE);
 
 			if (secretKey != null) {
+				if (!secretKey.isSigningKey()) {
+					throw new PGPException(MessageFormat.format(
+							JGitText.get().gpgNotASigningKey, signingKey));
+				}
 				return new BouncyCastleGpgKey(secretKey, USER_PGP_LEGACY_SECRING_FILE);
 			}
 
@@ -294,6 +298,10 @@ class BouncyCastleGpgKeyLocator {
 				PGPSecretKey secretKey = attemptParseSecretKey(keyFile,
 						calculatorProvider, passphraseProvider, publicKey);
 				if (secretKey != null) {
+					if (!secretKey.isSigningKey()) {
+						throw new PGPException(MessageFormat.format(
+								JGitText.get().gpgNotASigningKey, signingKey));
+					}
 					return new BouncyCastleGpgKey(secretKey, userKeyboxPath);
 				}
 			}