## Problem

Encrypting a wallet through JSON RPC or Qt *appears to* allow a user to include null characters in the passphrase, but silently ignores everything including and after the first null character.

For instance (on regtest), trying to set a passphrase of "`a{null character}b`":

`curl --user __cookie__ --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "encryptwallet", "params": ["a\u0000b"]}' -H 'content-type: text/plain;' http://127.0.0.1:18443/`

This will succeed, but allow the user to unlock with the passphrase "`a`", instead of the expected full passphrase (which also works):

`curl --user __cookie__ --data-binary '{"jsonrpc": "1.0", "id":"curltest", "method": "walletpassphrase", "params": ["a",10]}' -H 'content-type: text/plain;' http://127.0.0.1:18443/`

I am also able to replicate it in Qt on my macOS machine by running `printf 'a\0b' | pbcopy` and pasting the result into the passphrase dialog. 

My main concern is a user thinking that they're generating, say, 32 random bytes as a passphrase, and if they're unlucky and get a zero in the first few bytes, it unexpectedly cuts their security down to almost nothing. 

## Root Cause

The reason is due to our `SecureString` type. `SecureString` is a `std::string` specialization with a secure allocator. However, when assigned, it's treated like a C- string (no explicit length and null-terminated). See the [original PR](https://github.com/bitcoin/bitcoin/pull/666) for more details.

## Potential Solutions

I think there are two plausible approaches to take. The first (and my preference) is to allow and support null characters, and I will submit a PR that enables that (by making `SecureString` use the entire string). The second is to explicitly reject any passphrases that contain null characters.

One significant complication may be that, if anyone is *already* using a passphrase with a null, then my first solution would stop their wallet from unlocking. However, it would still be unlockable just by trimming the null and any subsequent characters.
{
    
    if (event->type() == QEvent::KeyPress) {
        QKeyEvent *ke = static_cast<QKeyEvent *>(event);
        if (ke->key() == Qt::Key_CapsLock) {
            fCapsLock = !fCapsLock;
        }
        if (fCapsLock) {
            ui->capsLabel->setText(tr("Warning: The Caps Lock key is on!"));
        } else {
            ui->capsLabel->clear();
        }
    }
    return QWidget::event(event);
}
void AskPassphraseDialog::toggleShowPassword(bool show)
{
    ui->toggleShowPasswordButton->setDown(show);
    const auto mode = show ? QLineEdit::Normal : QLineEdit::Password;
    ui->passEdit1->setEchoMode(mode);
    ui->passEdit2->setEchoMode(mode);
    ui->passEdit3->setEchoMode(mode);
}
bool AskPassphraseDialog::eventFilter(QObject *object, QEvent *event)