Provably fair

Coinflip

vs House uses your seed pair and the bet's nonce. A 1v1 game has its own server seed, the client seed creator:joiner (both players' client seeds) and nonce 0. The result is the lowest bit of the first byte of HMAC-SHA256(server seed, "client seed:nonce:0"): 0 is 2X, 1 is Donut.

Rock Paper Scissors

vs House uses your seed pair and the bet's nonce; the house's hand is the thrown hand. A 1v1 game has its own server seed, the client seed creator:joiner and nonce 0; the thrown hand is the joiner's only when their clock ran out. The throw reads HMAC-SHA256(server seed, "client seed:nonce:i") for i = 0, 1, ... as big-endian 32-bit numbers, skips any at or above 4294967295 (the largest multiple of 3 below 2^32), and takes the first one mod 3. The hand commitment, shown while the game was open, is SHA-256 of "server seed:creator hand".

Blackjack

Every card is its own draw from an infinite deck. The cards read HMAC-SHA256(server seed, "client seed:nonce:i") for i = 0, 1, ... one byte at a time, skip any byte of 208 or more and take the rest mod 52: the rank is the card mod 13 (A, 2 ... 10, J, Q, K) and the suit the card divided by 13 (♠ ♥ ♦ ♣). vs House uses your seed pair and the round's nonce, dealt player, dealer, player, dealer (the hole card), then every card in the order it was drawn: your hits first, the dealer's after you stand. A 1v1 game has its own server seed and the client seed creator:joiner; the creator's hand is nonce 0 and the joiner's nonce 1, each in the order its cards were dealt.