Add initial Steam TOTP support

* Add the concept of custom TOTP encoders, each with potential for custom
  code alphabet, length, step interval and code direction (i.e. reversed)
* Select custom encoder via overload of the digits field of a loaded entry
* Allow selection of custom encoders via the "TOTP Settings" field's
  size, as currently done by KeeTrayTOTP for Steam. Use "S" for the
  short name of the Steam custom encoder
* Allow selection of custom encoders via the "otp" field by appending
  a "&encoder=<name>" field to the URL query. For example,
  "&encoder=steam"
* Update TOTP set-up dialog to permit selection between (default,
  steam, custom) settings.

src/totp/totp.cpp

@@ -31,6 +31,39 @@
 const quint8 QTotp::defaultStep = 30;
 const quint8 QTotp::defaultDigits = 6;
 
+/**
+ * Custom encoder types. Each should be unique and >= 128 and < 255
+ * Values have no meaning outside of keepassxc
+ */
+/**
+ * Encoder for Steam Guard TOTP
+ */
+const quint8 QTotp::ENCODER_STEAM = 254;
+
+const QTotp::Encoder QTotp::defaultEncoder = { "", "", "0123456789", 0, 0, false };
+const QMap<quint8, QTotp::Encoder> QTotp::encoders{
+    { QTotp::ENCODER_STEAM, { "steam", "S", "23456789BCDFGHJKMNPQRTVWXY", 5, 30, true } },
+};
+
+/**
+ * These map the second field of the "TOTP Settings" field to our internal encoder number
+ * that overloads the digits field. Make sure that the key matches the shortName value
+ * in the corresponding Encoder
+ * NOTE: when updating this map, a corresponding edit to the settings regex must be made
+ *       in Entry::totpSeed()
+ */
+const QMap<QString, quint8> QTotp::shortNameToEncoder{
+    { "S", QTotp::ENCODER_STEAM },
+};
+/**
+ * These map the "encoder=" URL parameter of the "otp" field to our internal encoder number
+ * that overloads the digits field. Make sure that the key matches the name value
+ * in the corresponding Encoder
+ */
+const QMap<QString, quint8> QTotp::nameToEncoder{
+    { "steam", QTotp::ENCODER_STEAM },
+};
+
 QTotp::QTotp()
 {
 }
@@ -57,7 +90,10 @@ QString QTotp::parseOtpString(QString key, quint8& digits, quint8& step)
         if (q_step > 0 && q_step <= 60) {
             step = q_step;
         }
-
+        QString encName = query.queryItemValue("encoder");
+        if (!encName.isEmpty() && nameToEncoder.contains(encName)) {
+            digits = nameToEncoder[encName];
+        }
     } else {
         // Compatibility with "KeeOtp" plugin string format
         QRegExp rx("key=(.+)", Qt::CaseInsensitive, QRegExp::RegExp);
@@ -119,10 +155,24 @@ QString QTotp::generateTotp(const QByteArray key,
             | (hmac[offset + 3] & 0xff);
     // clang-format on
 
-    quint32 digitsPower = pow(10, numDigits);
+    const Encoder& encoder = encoders.value(numDigits, defaultEncoder);
+    // if encoder.digits is 0, we need to use the passed-in number of digits (default encoder)
+    quint8 digits = encoder.digits == 0 ? numDigits : encoder.digits;
+    int direction = -1;
+    int startpos = digits - 1;
+    if (encoder.reverse) {
+        direction = 1;
+        startpos = 0;
+    }
+    quint32 digitsPower = pow(encoder.alphabet.size(), digits);
 
     quint64 password = binary % digitsPower;
-    return QString("%1").arg(password, numDigits, 10, QChar('0'));
+    QString retval(int(digits), encoder.alphabet[0]);
+    for (quint8 pos = startpos; password > 0; pos += direction) {
+        retval[pos] = encoder.alphabet[int(password % encoder.alphabet.size())];
+        password /= encoder.alphabet.size();
+    }
+    return retval;
 }
 
 // See: https://github.com/google/google-authenticator/wiki/Key-Uri-Format
@@ -131,8 +181,8 @@ QUrl QTotp::generateOtpString(const QString& secret,
                               const QString& issuer,
                               const QString& username,
                               const QString& algorithm,
-                              const quint8& digits,
-                              const quint8& step)
+                              quint8 digits,
+                              quint8 step)
 {
     QUrl keyUri;
     keyUri.setScheme("otpauth");

src/totp/totp.h

@@ -20,6 +20,8 @@
 #define QTOTP_H
 
 #include <QtCore/qglobal.h>
+#include <QString>
+#include <QMap>
 
 class QUrl;
 
@@ -34,10 +36,25 @@ public:
                                   const QString& issuer,
                                   const QString& username,
                                   const QString& algorithm,
-                                  const quint8& digits,
-                                  const quint8& step);
+                                  quint8 digits,
+                                  quint8 step);
     static const quint8 defaultStep;
     static const quint8 defaultDigits;
+    struct Encoder
+    {
+        QString name;
+        QString shortName;
+        QString alphabet;
+        quint8 digits;
+        quint8 step;
+        bool reverse;
+    };
+    static const Encoder defaultEncoder;
+    // custom encoder values that overload the digits field
+    static const quint8 ENCODER_STEAM;
+    static const QMap<quint8, Encoder> encoders;
+    static const QMap<QString, quint8> shortNameToEncoder;
+    static const QMap<QString, quint8> nameToEncoder;
 };
 
 #endif // QTOTP_H