Resolve #2071 (openpgp.decrypt decrypted text was decoded twice)

dev/Stores/User/OpenPGP.js

@@ -268,13 +268,28 @@ export const OpenPGPUserStore = new class {
 			const decryptedKey = await decryptKey(privateKey, 'DECRYPT');
 			if (decryptedKey) {
 				const publicKey = findOpenPGPKey(this.publicKeys, sender/*, sign*/);
-				return await openpgp.decrypt({
+				// 'binary', not the default 'utf8'. What comes out is a MIME
+				// entity whose parts declare their own charset, and the MIME
+				// parser decodes each one. Letting openpgp decode to a string
+				// first meant every non ASCII character was decoded twice and
+				// arrived as U+FFFD: "Verschlüsselter" became "Verschl?sselter".
+				const result = await openpgp.decrypt({
 					message,
 					verificationKeys: publicKey?.key,
 //					expectSigned: true,
 //					signature: '', // Detached signature
-					decryptionKeys: decryptedKey
+					decryptionKeys: decryptedKey,
+					format: 'binary'
 				});
+				// One character per byte, which is what Mime/Encoding decodeText
+				// reads with charCodeAt. Chunked so a large message does not blow
+				// the argument limit of String.fromCharCode.
+				let text = '';
+				for (let j = 0; j < result.data.length; j += 0x8000) {
+					text += String.fromCharCode.apply(null, result.data.subarray(j, j + 0x8000));
+				}
+				result.data = text;
+				return result;
 			}
 		} catch (err) {
 			alert(err);