Provably fair blackjack: how the cards are drawn, the edge and how to verify a hand

Provably fair blackjack looks like any other online blackjack table, but every card comes from seeds you can check after the hand. That changes more than the fairness question. Most originals deal from an infinite deck, which alters the odds slightly, makes card counting pointless and leaves basic strategy almost untouched. This guide explains how the cards are drawn, what the rules published by Shuffle, Thrill and Stake mean for the house edge, how to recompute a hand in our verifier, and which audited casinos run their own blackjack.

How provably fair blackjack draws a card

Before the hand, the casino shows you the hash of a secret server seed. You hold a client seed, and each bet has a nonce. The game feeds these into a keyed hash (HMAC) and turns the output into numbers, one per card. Each number picks one of 52 cards.

There are two common ways to get from the hash to a card:

• Stake-style floats. HMAC-SHA256 with the server seed as the key and client seed:nonce:round as the message. Each hash gives 32 bytes, which become eight random floats of 4 bytes each. A card is the float times 52, rounded down, as an index into a fixed card list. Cards 1 to 8 come from round 0, cards 9 to 16 from round 1, and so on. Shuffle's help centre publishes the same method and the same card list: diamonds, hearts, spades and clubs for each rank, starting at the 2s and ending with the aces.
• One hash per card. Thrill uses HMAC-SHA512 over client seed:nonce:cursor, and the cursor moves on by one for every draw. Each hash gives a whole number from 0 to 51 by rejection sampling, which avoids the small bias a plain remainder would add. Thrill's list runs Ace to King in hearts, diamonds, clubs and spades.

The two lists are in different orders, so the same index means a different card at Stake and at Thrill. That is why our verifier asks you to pick the casino: the algorithm and the card order both have to match.

Infinite deck or shoe?

A land-based table deals from a shoe of six or eight decks, so every card that comes out changes what is left.

Most provably fair blackjack does not work that way. Shuffle's rules say "Infinite decks in use", and its fairness article says the game is played "with an unlimited card deck". Thrill's rules say "Infinite decks: Each hand is independent and fair." Stake's card games use the same infinite-deck draw, as recorded in our Stake audit. Each card has the same 1 in 52 chance, whatever came before, even within the same hand. You can be dealt the same card twice in one hand.

Some casinos are vaguer. Duelbits says its hash decides "the card order" in Blackjack, without saying whether that order comes from one deck or an infinite one.

What the infinite deck changes for your strategy

Card counting

Counting works because removing cards from a shoe shifts the odds of what is left. An infinite deck removes nothing. A hand that used five tens leaves the next card with exactly the same 4 in 13 chance of being a ten. A game that reshuffles a finite deck before every hand gives the same result: the count resets every time.

Basic strategy

Basic strategy still applies, and it is close to the standard multi-deck chart. We solved the infinite-deck game in Python for the rule set Shuffle publishes (dealer stands on 17, double on any two cards, double after split, one split, one card to each split ace, dealer peeks). A few decisions worth knowing:

If a table hits soft 17 instead, hard 11 against an ace becomes a double, by a margin of less than 0.1% of the bet. Single-deck charts, which change some plays because of the exact cards in your hand, do not apply to an infinite deck.

Insurance is a side bet that the dealer has blackjack, paid at 2:1. With an ace showing, the chance of a ten underneath is 4 in 13, about 30.8%, below the 33.3% insurance needs to break even. Skip it.

House edge under common originals rules

The operators' own rule pages give the rules, but most do not give an edge for the main bet. Here is what they publish:

Neither page says whether "stands on 17" covers soft 17. We read it as standing on all 17s. Under those rules, with perfect basic strategy, our solver gives a house edge of 0.57%: about $5.70 lost per $1,000 wagered. The same game would cost 0.69% without double after split, 0.79% if the dealer hit soft 17, and 1.92% if blackjack paid 6:5 instead of 3:2. Those figures are ours, computed for an infinite deck, not numbers the casinos publish.

Two operators do put a number on it. MetaWin's Originals hub lists Blackjack at a 99.5% RTP, a 0.5% edge, without giving its rules or formula on that page. Stake's blog says blackjack's edge can drop to about 0.5% (an RTP of 99.5%) with good strategy, and about 2% without it. That is a general claim, not a figure for Stake's own table. Either way, blackjack is only one of the cheapest originals if you play basic strategy.

Side bets are a different matter. Shuffle publishes their edges: 2.00% for Perfect Pairs and 3.68% for 21+3, several times the main bet. Thrill lists the side bet payouts (Perfect Pair 25x, 21+3 Suited Trips 100x) but no edge.

How to verify a hand with our verifier

Here is a worked example with made-up seeds on a Stake-style game:

1. SHA-256 of the server seed gives 59ba46e6...71ee951a, which matches the hash, so the seed was fixed before the hand.
2. HMAC-SHA256 over quiet-maple-42:0:0 starts 92529c0f, the bytes 146, 82, 156 and 15. As a float that is 0.571573.
3. 0.571573 x 52 = 29.7. Index 29 in the Stake card list is the 9 of hearts.
4. The next floats give indexes 16, 23, 15, 13 and 43: 6♦, 7♣, 5♣, 5♥ and Q♣.

So the first six cards drawn for nonce 0 are 9♥ 6♦ 7♣ 5♣ 5♥ Q♣. Open this check in our verifier to see it run.

For your own hands:

1. Note the hashed server seed in the casino's fairness panel before you play.
2. Play the hand, then rotate your seed pair to reveal the server seed.
3. From your bet history, copy the client seed and the nonce for that hand.
4. Open the verifier, pick the casino (Stake, Shuffle or Thrill set the right algorithm) and choose Cards.
5. Paste the seeds and nonce and set how many cards to show. Splits and hits use more cards, so ask for enough.
6. Compare the sequence with the cards in your hand history.

The verifier lists cards in the order they were drawn. The rule pages we read do not say which draw goes to you and which to the dealer, so match the sequence against your history rather than assuming the first card is yours. At Thrill the nonce starts at 1, not 0. For the general method, see how to verify a provably fair bet.

Where to play provably fair blackjack

Of the casinos we have published audits for, these list blackjack among their provably fair originals. They differ a lot in what you can check:

Limits vary too. Thrill caps the main bet at $50,000 per seat; Shuffle's published limit for Blackjack is $2,000,000. Live dealer blackjack at the same casinos uses real cards from a real shoe and is not provably fair. For the rest of each casino's originals, see which casino games are provably fair and, for one full set worked through, Stake Originals explained.

Bottom line

Provably fair blackjack proves the cards were fixed before you acted. The infinite deck means counting is useless and basic strategy is the standard multi-deck chart. Under the rules Shuffle and Thrill publish, the edge is about 0.57% if you play perfectly, and side bets cost far more. Learn the chart, skip insurance, check a hand now and then, and set a loss limit before you sit down; our responsible gambling page has the tools.

Sources

• Shuffle: Original Blackjack game information (rules, side bet edges)
• Shuffle: Provably fair game events (infinite deck, card list)
• Thrill: Thrill Original Blackjack (rules and payouts)
• Thrill: Thrill Originals (bet and payout limits)
• Stake blog: how to play blackjack
• MetaWin: Originals hub (stated RTPs)
• Shuffle: maximum bet and win limits
• Card schemes as recorded in our casino audits and implemented in our verifier; house edges and strategy computed by us in Python for an infinite deck