[libcalamares] Add printable-entropy (e.g. for password salt)

src/libcalamares/utils/Entropy.cpp

@@ -55,7 +55,6 @@ CalamaresUtils::getEntropy( int size, QByteArray& b )
     std::mt19937_64 twister( seed );
 
     std::uint64_t next = 0;
-
     do
     {
         next = twister();
@@ -78,3 +77,48 @@ CalamaresUtils::getEntropy( int size, QByteArray& b )
 
     return EntropySource::Twister;
 }
+
+CalamaresUtils::EntropySource
+CalamaresUtils::getPrintableEntropy( int size, QString& s )
+{
+    s.clear();
+    if ( size < 1 )
+    {
+        return EntropySource::None;
+    }
+
+    static const char salt_chars[] = { '.', '/', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D',
+                                       'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T',
+                                       'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
+                                       'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z' };
+    static_assert( sizeof( salt_chars ) == 64, "Missing salt_chars" );
+
+    // Number of bytes we're going to need
+    int byteSize = ( ( size * 6 ) / 8 ) + 1;
+    QByteArray b;
+    EntropySource r = getEntropy( byteSize, b );
+
+    int bitsLeft = 0;
+    int byteOffset = 0;
+    qint64 next = 0;
+    do
+    {
+        if ( bitsLeft < 6 )
+        {
+            next = ( next << 8 ) | b.at( byteOffset++ );
+            bitsLeft += 8;
+        }
+        char c = salt_chars[ next & 0b0111111 ];
+        next >>= 6;
+        bitsLeft -= 6;
+        s.append( c );
+    } while ( ( s.length() < size ) && ( byteOffset < b.size() ) );
+
+    if ( s.length() < size )
+    {
+        // It's incomplete, not really no-entropy
+        return EntropySource::None;
+    }
+
+    return r;
+}

src/libcalamares/utils/Entropy.h

@@ -40,6 +40,14 @@ enum class EntropySource
  * bytes from a suitable source. Returns which source was used.
  */
 DLLEXPORT EntropySource getEntropy( int size, QByteArray& b );
+
+/** @brief Fill string @p s with exactly @p size random printable ASCII characters
+ *
+ * The characters are picked from a set of 64 (2^6). The string
+ * contains 6 * size bits of entropy. * Returns which source was used.
+ * @see getEntropy
+ */
+DLLEXPORT EntropySource getPrintableEntropy( int size, QString& s );
 }  // namespace CalamaresUtils
 
 #endif

src/libcalamares/utils/Tests.cpp

@@ -185,18 +185,37 @@ LibCalamaresTests::testEntropy()
 
     auto r0 = CalamaresUtils::getEntropy( 0, data );
     QCOMPARE( CalamaresUtils::EntropySource::None, r0 );
-    QCOMPARE( 0, data.size() );
+    QCOMPARE( data.size(), 0 );
 
     auto r1 = CalamaresUtils::getEntropy( 16, data );
     QVERIFY( r1 != CalamaresUtils::EntropySource::None );
-    QCOMPARE( 16, data.size() );
+    QCOMPARE( data.size(), 16 );
     // This can randomly fail (but not often)
     QVERIFY( data.at( data.size() - 1 ) != char( 0xcb ) );
 
     auto r2 = CalamaresUtils::getEntropy( 8, data );
     QVERIFY( r2 != CalamaresUtils::EntropySource::None );
-    QCOMPARE( 8, data.size() );
+    QCOMPARE( data.size(), 8 );
     QCOMPARE( r1, r2 );
     // This can randomly fail (but not often)
     QVERIFY( data.at( data.size() - 1 ) != char( 0xcb ) );
 }
+
+void
+LibCalamaresTests::testPrintableEntropy()
+{
+    QString s;
+
+    auto r0 = CalamaresUtils::getPrintableEntropy( 0, s );
+    QCOMPARE( CalamaresUtils::EntropySource::None, r0 );
+    QCOMPARE( s.length(), 0 );
+
+    auto r1 = CalamaresUtils::getPrintableEntropy( 16, s );
+    QVERIFY( r1 != CalamaresUtils::EntropySource::None );
+    QCOMPARE( s.length(), 16 );
+    for ( QChar c : s )
+    {
+        QVERIFY( c.isPrint() );
+        QCOMPARE( c.cell(), 0 );
+    }
+}

src/libcalamares/utils/Tests.h

@@ -42,6 +42,7 @@ private Q_SLOTS:
 
     /** @brief Tests the entropy functions. */
     void testEntropy();
+    void testPrintableEntropy();
 };
 
 #endif