Program Derived Addresses

Problem

Programs cannot generate signatures when issuing instructions to other programs as defined in the Cross-Program Invocations design.

The lack of programmatic signature generation limits the kinds of programs that can be implemented in Solana. A program may be given the authority over an account and later want to transfer that authority to another. This is impossible today because the program cannot act as the signer in the transaction that gives authority.

For example, if two users want to make a wager on the outcome of a game in Solana, they must each transfer their wager's assets to some intermediary that will honor their agreement. Currently, there is no way to implement this intermediary as a program in Solana because the intermediary program cannot transfer the assets to the winner.

This capability is necessary for many DeFi applications since they require assets to be transferred to an escrow agent until some event occurs that determines the new owner.

  • Decentralized Exchanges that transfer assets between matching bid and ask orders.

  • Auctions that transfer assets to the winner.

  • Games or prediction markets that collect and redistribute prizes to the winners.

Proposed Solution

The key to the design is two-fold:

  1. Allow programs to control specific addresses, called Program-Addresses, in such a way that no external user can generate valid transactions with signatures for those addresses.

  2. Allow programs to programmatically sign for Program-Addresses that are present in instructions invoked via Cross-Program Invocations.

Given the two conditions, users can securely transfer or assign the authority of on-chain assets to Program-Addresses and the program can then assign that authority elsewhere at its discretion.

Private keys for Program Addresses

A Program -Address has no private key associated with it, and generating a signature for it is impossible. While it has no private key of its own, it can issue an instruction that includes the Program-Address as a signer.

Hash-based generated Program Addresses

All 256-bit values are valid ed25519 curve points and valid ed25519 public keys. All are equally secure and equally as hard to break. Based on this assumption, Program Addresses can be deterministically derived from a base seed using a 256-bit preimage resistant hash function.

Deterministic Program Addresses for programs follow a similar derivation path as Accounts created with SystemInstruction::CreateAccountWithSeed which is implemented with system_instruction::create_address_with_seed.

For reference that implementation is as follows:

pub fn create_address_with_seed(
base: &Pubkey,
seed: &str,
program_id: &Pubkey,
) -> Result<Pubkey, SystemError> {
if seed.len() > MAX_ADDRESS_SEED_LEN {
return Err(SystemError::MaxSeedLengthExceeded);
}
Ok(Pubkey::new(
hashv(&[base.as_ref(), seed.as_ref(), program_id.as_ref()]).as_ref(),
))
}

Programs can deterministically derive any number of addresses by using keywords. These keywords can symbolically identify how the addresses are used.

//! Generate a derived program address
//! * seeds, symbolic keywords used to derive the key
//! * owner, program that the key is derived for
pub fn create_program_address(seeds: &[&str], owner: &Pubkey) -> Result<Pubkey, PubkeyError> {
let mut hasher = Hasher::default();
for seed in seeds.iter() {
if seed.len() > MAX_SEED_LEN {
return Err(PubkeyError::MaxSeedLengthExceeded);
}
hasher.hash(seed.as_ref());
}
hasher.hashv(&[owner.as_ref(), "ProgramDerivedAddress".as_ref()]);
Ok(Pubkey::new(hashv(&[hasher.result().as_ref()]).as_ref()))
}

Using Program Addresses

Clients can use the create_program_address function to generate a destination address.

//deterministically derive the escrow key
let escrow_pubkey = create_program_address(&[&["escrow"]], &escrow_program_id);
let message = Message::new(vec![
token_instruction::transfer(&alice_pubkey, &escrow_pubkey, 1),
]);
//transfer 1 token to escrow
client.send_and_confirm_message(&[&alice_keypair], &message);

Programs can use the same function to generate the same address. In the function below the program issues a token_instruction::transfer from Program Address as if it had the private key to sign the transaction.

fn transfer_one_token_from_escrow(
program_id: &Pubkey,
keyed_accounts: &[KeyedAccount]
) -> Result<()> {
// User supplies the destination
let alice_pubkey = keyed_accounts[1].unsigned_key();
// Deterministically derive the escrow pubkey.
let escrow_pubkey = create_program_address(&[&["escrow"]], program_id);
// Create the transfer instruction
let instruction = token_instruction::transfer(&escrow_pubkey, &alice_pubkey, 1);
// The runtime deterministically derives the key from the currently
// executing program ID and the supplied keywords.
// If the derived key matches a key marked as signed in the instruction
// then that key is accepted as signed.
invoke_signed(&instruction, &[&["escrow"]])?
}

Instructions that require signers

The addresses generated with create_program_address are indistinguishable from any other public key. The only way for the runtime to verify that the address belongs to a program is for the program to supply the keywords used to generate the address.

The runtime will internally call create_program_address, and compare the result against the addresses supplied in the instruction.