Salt: improve saltiness.

src/modules/users/SetPasswordJob.cpp

@@ -25,6 +25,7 @@
 
 #include <QDir>
 
+#include <random>
 #include <crypt.h>
 
 
@@ -50,6 +51,53 @@ SetPasswordJob::prettyStatusMessage() const
 }
 
 
+/// Returns a modular hashing salt for method 6 (SHA512) with a 16 character random salt.
+QString
+make_salt(size_t length)
+{
+    Q_ASSERT(length >= 8);
+    Q_ASSERT(length <= 128);
+
+    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");
+
+    std::random_device r;
+    std::seed_seq seed{r(), r(), r(), r(), r(), r(), r(), r()};
+    std::mt19937_64 twister(seed);
+
+    std::uint64_t next;
+    size_t current_length = 0;
+
+    QString salt_string;
+    salt_string.reserve(length + 10);
+
+    while ( current_length < length )
+    {
+        next = twister();
+        // In 64 bits, we have 10 blocks of 6 bits; map each block of 6 bits
+        // to a single salt character.
+        for ( unsigned int char_count = 0; char_count < 10; ++char_count )
+        {
+            char c = salt_chars[next & 0b0111111];
+            next >>= 6;
+            salt_string.append( c );
+            if (++current_length >= length)
+                break;
+        }
+    }
+
+    salt_string.truncate( length );
+    salt_string.insert( 0, "$6$" );
+    salt_string.append( '$' );
+    return salt_string;
+}
+
 Calamares::JobResult
 SetPasswordJob::exec()
 {
@@ -75,8 +123,7 @@ SetPasswordJob::exec()
 
     QString encrypted = QString::fromLatin1(
                             crypt( m_newPassword.toUtf8(),
-                                   QString( "$6$%1$" )
-                                .arg( m_userName ).toUtf8() ) );
+                                   make_salt( 16 ).toUtf8() ) );
 
     int ec = CalamaresUtils::System::instance()->
                           targetEnvCall( { "usermod",