## 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.
{
    return RPCHelpMan{"getwalletinfo",
                "Returns an object containing various wallet state info.\n",
                {},
                RPCResult{
                    RPCResult::Type::OBJ, "", "",
                    {
                        {
                        {RPCResult::Type::STR, "walletname", "the wallet name"},
                        {RPCResult::Type::NUM, "walletversion", "the wallet version"},
                        {RPCResult::Type::STR, "format", "the database format (bdb or sqlite)"},
                        {RPCResult::Type::STR_AMOUNT, "balance", "DEPRECATED. Identical to getbalances().mine.trusted"},
                        {RPCResult::Type::STR_AMOUNT, "unconfirmed_balance", "DEPRECATED. Identical to getbalances().mine.untrusted_pending"},
                        {RPCResult::Type::STR_AMOUNT, "immature_balance", "DEPRECATED. Identical to getbalances().mine.immature"},
                        {RPCResult::Type::NUM, "txcount", "the total number of transactions in the wallet"},
                        {RPCResult::Type::NUM_TIME, "keypoololdest", true, "the " + UNIX_EPOCH_TIME + " of the oldest pre-generated key in the key pool. Legacy wallets only."},
                        {RPCResult::Type::NUM, "keypoolsize", "how many new keys are pre-generated (only counts external keys)"},
                        {RPCResult::Type::NUM, "keypoolsize_hd_internal", true, "how many new keys are pre-generated for internal use (used for change outputs, only appears if the wallet is using this feature, otherwise external keys are used)"},
                        {RPCResult::Type::NUM_TIME, "unlocked_until", true, "the " + UNIX_EPOCH_TIME + " until which the wallet is unlocked for transfers, or 0 if the wallet is locked (only present for passphrase-encrypted wallets)"},
                        {RPCResult::Type::STR_AMOUNT, "paytxfee", "the transaction fee configuration, set in " + CURRENCY_UNIT + "/kvB"},
                        {RPCResult::Type::STR_HEX, "hdseedid", true, "the Hash160 of the HD seed (only present when HD is enabled)"},
                        {RPCResult::Type::BOOL, "private_keys_enabled", "false if privatekeys are disabled for this wallet (enforced watch-only wallet)"},
                        {RPCResult::Type::BOOL, "avoid_reuse", "whether this wallet tracks clean/dirty coins in terms of reuse"},
                        {RPCResult::Type::OBJ, "scanning", "current scanning details, or false if no scan is in progress",
                        {
                            {RPCResult::Type::NUM, "duration", "elapsed seconds since scan start"},
                            {RPCResult::Type::NUM, "progress", "scanning progress percentage [0.0, 1.0]"},
                        }, true},
                        {RPCResult::Type::BOOL, "descriptors", "whether this wallet uses descriptors for scriptPubKey management"},
                        {RPCResult::Type::BOOL, "external_signer", "whether this wallet is configured to use an external signer such as a hardware wallet"},