In our Solana tutorials thus far, we’ve only had one account initialize and write to the account.

In practice, this is very restrictive. For example, if user Alice is transferring points to Bob, Alice must be able to write to an account initialized by user Bob.

In this tutorial we will demonstrate initializing an account with one wallet and updating it with another.

The initialization step

The Rust code we’ve been using to initialize accounts doesn’t change:

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use anchor_lang::prelude::*;
use std::mem::size_of;

declare_id!("61As9Y8pREgvFZzps6rpFai8UkageeHT6kW1dnGRiefb");

#[program]
pub mod other_write {
    use super::*;

    pub fn initialize(ctx: Context<Initialize>) -> Result<()> {
        Ok(())
    }
}

#[derive(Accounts)]
pub struct Initialize<'info> {
    #[account(init,
              payer = signer,
              space=size_of::<MyStorage>() + 8,
              seeds = [],
              bump)]
    pub my_storage: Account<'info, MyStorage>,

    #[account(mut)]
    pub signer: Signer<'info>,

    pub system_program: Program<'info, System>,
}

#[account]
pub struct MyStorage {
    x: u64,
}

Doing the initialization transaction with an alternate wallet

However, there is an important change in the client code:

• For testing purposes, we create a new wallet called newKeypair. This is different from the one Anchor provides by default.
• We airdrop that new wallet 1 SOL so it can pay for transactions.
• Pay attention to the comment // THIS MUST BE EXPLICITLY SPECIFIED. We are passing the publicKey of that wallet for the Signer field. When we use the default signer built into Anchor, Anchor passes this in the background for us. However, when we use a different wallet, we need to provide this explicitly.
• We set the signer to be newKeypair with the .signers([newKeypair]) configuration.

We will explain after this code snippet why we are (seemingly) specifying the signer twice:

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import * as anchor from "@coral-xyz/anchor";
import { Program } from "@coral-xyz/anchor";
import { OtherWrite } from "../target/types/other_write";

// this airdrops sol to an address
async function airdropSol(publicKey, amount) {
  let airdropTx = await anchor.getProvider().connection.requestAirdrop(publicKey, amount);
  await confirmTransaction(airdropTx);
}

async function confirmTransaction(tx) {
  const latestBlockHash = await anchor.getProvider().connection.getLatestBlockhash();
  await anchor.getProvider().connection.confirmTransaction({
    blockhash: latestBlockHash.blockhash,
    lastValidBlockHeight: latestBlockHash.lastValidBlockHeight,
    signature: tx,
  });
}

describe("other_write", () => {
  anchor.setProvider(anchor.AnchorProvider.env());

  const program = anchor.workspace.OtherWrite as Program<OtherWrite>;

  it("Is initialized!", async () => {
    const newKeypair = anchor.web3.Keypair.generate();
    await airdropSol(newKeypair.publicKey, 1e9); // 1 SOL

    let seeds = [];
    const [myStorage, _bump] = anchor.web3.PublicKey.findProgramAddressSync(seeds, program.programId);
    
    await program.methods.initialize().accounts({
      myStorage: myStorage,
      signer: newKeypair.publicKey // ** THIS MUST BE EXPLICITLY SPECIFIED **
    }).signers([newKeypair]).rpc();
  });
});

It is not required that the key signer be called signer.

Exercise: In the Rust code, change payer = signer to payer = fren and pub signer: Signer<'info>, to pub fren: Signer<'info>, and change signer: newKeypair.publicKey to fren: newKeypair.publicKey in the test. The initialization should succeed and the test should pass.

Why does Anchor require specifying the Signer and the publicKey?

At first it might seem redundant that we are specifying the signer twice, but let’s take a closer look:

In the red box, we see the fren field specified to be a Signer account. The Signer type means Anchor will look at the signature of the transaction and make sure the signature matches the address passed here.

We will see later how we can use this to validate the Signer is authorized to conduct certain a transaction.

Anchor has been doing this the whole time behind the scenes, but since we passed in a Signer other than the one Anchor uses by default, we have to be explicit about what account the Signer is.

Error: unknown signer in Solana Anchor

The unknown signer error occurs when the signer of the transaction does not match the public key passed to Signer.

Suppose we modify the test to remove the .signers([newKeypair]) spec. Anchor will use the default signer instead, and the default signer will not match the publicKey of our newKeypair wallet:

We will get the following error:

Similarly, if we don’t pass in the publicKey explicitly, Anchor will silently use the default signer:

And we will get the following Error: unknown signer:

Somewhat misleadingly, Anchor isn’t saying the signer is unknown because it wasn’t specified per se. Anchor is able to figure out that if no signer is specified, then it will use the default signer. If we remove both the .signers([newKeypair]) code and the fren: newKeypair.publicKey code, then Anchor will use the default signer for both the public key to check against, and the signature of the signer to verify it matches the public key.

The following code will result in a successful initialization because both the Signer public key and the account that signs the transaction are the Anchor default signer.

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  await program.methods.initialize().accounts({
    myStorage: myStorage
  }).rpc();
});

Bob can write to an account Alice initialized

Below we show an Anchor program with functions to initialize an account and write to it.

This will be familiar from our Solana counter program tutorial, but pay attention to the small addition marked by the // THIS FIELD MUST BE INCLUDED comment near the bottom:

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use anchor_lang::prelude::*;
use std::mem::size_of;

declare_id!("61As9Y8pREgvFZzps6rpFai8UkageeHT6kW1dnGRiefb");

#[program]
pub mod other_write {
    use super::*;

    pub fn initialize(ctx: Context<Initialize>) -> Result<()> {
        Ok(())
    }

    pub fn update_value(ctx: Context<UpdateValue>, new_value: u64) -> Result<()> {
        ctx.accounts.my_storage.x = new_value;
        Ok(())
    }
}

#[derive(Accounts)]
pub struct Initialize<'info> {
    #[account(init,
              payer = fren,
              space=size_of::<MyStorage>() + 8,
              seeds = [],
              bump)]
    pub my_storage: Account<'info, MyStorage>,

    #[account(mut)]
    pub fren: Signer<'info>, // A public key is passed here

    pub system_program: Program<'info, System>,
}

#[derive(Accounts)]
pub struct UpdateValue<'info> {
    #[account(mut, seeds = [], bump)]
    pub my_storage: Account<'info, MyStorage>,

    // THIS FIELD MUST BE INCLUDED
    #[account(mut)]
    pub fren: Signer<'info>,
}

#[account]
pub struct MyStorage {
    x: u64,
}

The following client code will create a wallet for Alice and Bob and airdrop them 1 SOL each. Alice will initialize the account MyStorage, and Bob will write to it:

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import * as anchor from "@coral-xyz/anchor";
import { Program } from "@coral-xyz/anchor";
import { OtherWrite } from "../target/types/other_write";

// this airdrops sol to an address
async function airdropSol(publicKey, amount) {
  let airdropTx = await anchor.getProvider().connection.requestAirdrop(publicKey, amount);
  await confirmTransaction(airdropTx);
}

async function confirmTransaction(tx) {
  const latestBlockHash = await anchor.getProvider().connection.getLatestBlockhash();
  await anchor.getProvider().connection.confirmTransaction({
    blockhash: latestBlockHash.blockhash,
    lastValidBlockHeight: latestBlockHash.lastValidBlockHeight,
    signature: tx,
  });
}

describe("other_write", () => {
  // Configure the client to use the local cluster.
  anchor.setProvider(anchor.AnchorProvider.env());

  const program = anchor.workspace.OtherWrite as Program<OtherWrite>;

  it("Is initialized!", async () => {
    const alice = anchor.web3.Keypair.generate();
    const bob = anchor.web3.Keypair.generate();

    const airdrop_alice_tx = await anchor.getProvider().connection.requestAirdrop(alice.publicKey, 1 * anchor.web3.LAMPORTS_PER_SOL);
    await confirmTransaction(airdrop_alice_tx);

    const airdrop_alice_bob = await anchor.getProvider().connection.requestAirdrop(bob.publicKey, 1 * anchor.web3.LAMPORTS_PER_SOL);
    await confirmTransaction(airdrop_alice_bob);

    let seeds = [];
    const [myStorage, _bump] = anchor.web3.PublicKey.findProgramAddressSync(seeds, program.programId);
    
    // ALICE INITIALIZE ACCOUNT
    await program.methods.initialize().accounts({
      myStorage: myStorage,
      fren: alice.publicKey
    }).signers([alice]).rpc();

    // BOB WRITE TO ACCOUNT
    await program.methods.updateValue(new anchor.BN(3)).accounts({
      myStorage: myStorage,
      fren: bob.publicKey
    }).signers([bob]).rpc();

    let value = await program.account.myStorage.fetch(myStorage);
    console.log(`value stored is ${value.x}`);
  });
});

Restricting writes to Solana accounts

In real applications, we don’t want Bob writing arbitrary data to arbitrary accounts. Let’s create a basic example where users can initialize an account with 10 points and transfer those points to another account. (There is an obvious problem that a hacker can create as many accounts as they want using separate wallets, but that is outside of the scope of our example).

Building a proto-ERC20 program

Alice should be able to modify both her account and Bob’s account. That is, she should be able to deduct her points and credit Bob’s points. She should not be able to deduct Bob’s points — only Bob should be able to do that.

By convention, we call an address that can make privileged changes to an account an “authority” in Solana. It is a common pattern to store the “authority” field in the account struct to signify that only that account can conduct sensitive operations on that account (such as deducting points in our example).

This is somewhat analogous to the onlyOwner pattern in Solidity, except that instead of applying to the entire contract, it applies only to a single account:

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use anchor_lang::prelude::*;
use std::mem::size_of;

declare_id!("HFmGQX4wPgPYVMFe4WrBi925NKvGySrEG2LGyRXsXJ4Z");

const STARTING_POINTS: u32 = 10;

#[program]
pub mod points {
    use super::*;

    pub fn initialize(ctx: Context<Initialize>) -> Result<()> {
        ctx.accounts.player.points = STARTING_POINTS;
        ctx.accounts.player.authority = ctx.accounts.signer.key();
        Ok(())
    }

    pub fn transfer_points(ctx: Context<TransferPoints>,
                           amount: u32) -> Result<()> {
        require!(ctx.accounts.from.authority == ctx.accounts.signer.key(), Errors::SignerIsNotAuthority);
        require!(ctx.accounts.from.points >= amount, Errors::InsufficientPoints);
        
        ctx.accounts.from.points -= amount;
        ctx.accounts.to.points += amount;
        Ok(())
    }
}

#[error_code]
pub enum Errors {
    #[msg("SignerIsNotAuthority")]
    SignerIsNotAuthority,
    #[msg("InsufficientPoints")]
    InsufficientPoints
}

#[derive(Accounts)]
pub struct Initialize<'info> {
    #[account(init,
              payer = signer,
              space = size_of::<Player>() + 8,
              seeds = [&(signer.as_ref().key().to_bytes())],
              bump)]
    player: Account<'info, Player>,
    #[account(mut)]
    signer: Signer<'info>,
    system_program: Program<'info, System>,
}

#[derive(Accounts)]
pub struct TransferPoints<'info> {
    #[account(mut)]
    from: Account<'info, Player>,
    #[account(mut)]
    to: Account<'info, Player>,
    #[account(mut)]
    signer: Signer<'info>,
}

#[account]
pub struct Player {
    points: u32,
    authority: Pubkey
}

Note that we use the address of the signer (&(signer.as_ref().key().to_bytes())) to derive the address of the account where their points are stored. This behaves like a Solidity mapping in Solana, where the Solana “msg.sender / tx.origin” is the key.

In the initialize function, the program sets the initial points to 10 and the authority to the signer. The user does not have control over these initial values.

The transfer_points function uses Solana Anchor require macros and error code macros to ensure that 1) the Signer of the transaction is the authority of the account whose balance is getting deducted; and 2) the account has enough points balance to transfer.

The test codebase should be straightforward to understand. Alice and Bob initialize their accounts, then Alice transfers 5 points to Bob:

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import * as anchor from "@coral-xyz/anchor";
import { Program } from "@coral-xyz/anchor";
import { Points } from "../target/types/points";

// this airdrops sol to an address
async function airdropSol(publicKey, amount) {
  let airdropTx = await anchor.getProvider().connection.requestAirdrop(publicKey, amount);
  await confirmTransaction(airdropTx);
}

async function confirmTransaction(tx) {
  const latestBlockHash = await anchor.getProvider().connection.getLatestBlockhash();
  await anchor.getProvider().connection.confirmTransaction({
    blockhash: latestBlockHash.blockhash,
    lastValidBlockHeight: latestBlockHash.lastValidBlockHeight,
    signature: tx,
  });
}

describe("points", () => {
  anchor.setProvider(anchor.AnchorProvider.env());
  const program = anchor.workspace.Points as Program<Points>;


  it("Alice transfers points to Bob", async () => {
    const alice = anchor.web3.Keypair.generate();
    const bob = anchor.web3.Keypair.generate();

    const airdrop_alice_tx = await anchor.getProvider().connection.requestAirdrop(alice.publicKey, 1 * anchor.web3.LAMPORTS_PER_SOL);
    await confirmTransaction(airdrop_alice_tx);

    const airdrop_alice_bob = await anchor.getProvider().connection.requestAirdrop(bob.publicKey, 1 * anchor.web3.LAMPORTS_PER_SOL);
    await confirmTransaction(airdrop_alice_bob);

    let seeds_alice = [alice.publicKey.toBytes()];
    const [playerAlice, _bumpA] = anchor.web3.PublicKey.findProgramAddressSync(seeds_alice, program.programId);

    let seeds_bob = [bob.publicKey.toBytes()];
    const [playerBob, _bumpB] = anchor.web3.PublicKey.findProgramAddressSync(seeds_bob, program.programId);

    // Alice and Bob initialize their accounts
    await program.methods.initialize().accounts({
      player: playerAlice,
      signer: alice.publicKey,
    }).signers([alice]).rpc();

    await program.methods.initialize().accounts({
      player: playerBob,
      signer: bob.publicKey,
    }).signers([bob]).rpc();

    // Alice transfers 5 points to Bob. Note that this is a u32
    // so we don't need a BigNum
    await program.methods.transferPoints(5).accounts({
      from: playerAlice,
      to: playerBob,
      signer: alice.publicKey,
    }).signers([alice]).rpc();

    console.log(`Alice has ${(await program.account.player.fetch(playerAlice)).points} points`);
    console.log(`Bob has ${(await program.account.player.fetch(playerBob)).points} points`)
  });
});

Exercise: Create a keypair mallory and try to get mallory to steal points from Alice or Bob by using mallory as the signer in .signers([mallory]). Your attack should fail, but you should try anyway.

Using Anchor Constraints to replace require! macros

An alternative to writing require!(ctx.accounts.from.authority == ctx.accounts.signer.key(), Errors::SignerIsNotAuthority); is to use an Anchor constraint. The Anchor account docs give us a list of constraints available to us.

Anchor has_one constraint

The has_one constraint assumes that there is “shared key” between #[derive(Accounts)] and #[account] and checks that both of those keys have the same value. The best way to demonstrate this is with a picture:

Behind the scenes, Anchor will block the transaction if the authority account passed as part of the transaction (as the Signer) is not equal to the authority stored in the account.

In our implementation above, we used the key authority in the account and signer in the #[derive(Accounts)]. This mismatch of key names will prevent this macro from working, so the code above changes the key signer to authority. Authority is not a special keyword, merely a convention. You could, as an exercise, change all instances of authority to fren and the code will work the same.

Anchor constraint constraint

We can also replace the macro require!(ctx.accounts.from.points >= amount, Errors::InsufficientPoints); with an Anchor constraint.

The constraint macro allows us to place arbitrary constraints on accounts passed to the transactions and data in the account. In our case, we want to make sure the sender has enough points:

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#[derive(Accounts)]
#[instruction(amount: u32)] // amount must be passed as an instruction
pub struct TransferPoints<'info> {
    #[account(mut,
              has_one = authority,
              constraint = from.points >= amount)]
    from: Account<'info, Player>,
    #[account(mut)]
    to: Account<'info, Player>,
    authority: Signer<'info>,
}

#[account]
pub struct Player {
    points: u32,
    authority: Pubkey
}

The macro is smart enough to recognize that from is based on the account passed into the from key, and that account has a points field. The amount from the transfer_points function argument must be passed via the instruction macro so the constraint macro can compare amount to the point balance in the account.

Adding custom error messages to Anchor constraints

We can improve the readability of error messages when constraints are violated by adding custom errors, the same custom errors we passed to the require! macros using the @ notation:

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#[derive(Accounts)]
#[instruction(amount: u32)]
pub struct TransferPoints<'info> {
    #[account(mut,
              has_one = authority @ Errors::SignerIsNotAuthority,
              constraint = from.points >= amount @ Errors::InsufficientPoints)]
    from: Account<'info, Player>,
    #[account(mut)]
    to: Account<'info, Player>,
    authority: Signer<'info>,
}

#[account]
pub struct Player {
    points: u32,
    authority: Pubkey
}

The Errors enum was defined in the Rust code earlier that used them in the require! macros.

Exercise: modify the tests to violate the has_one and constraint macro and observe the error messages.

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Originally Published March, 5, 2024