Why is SHA256 a signature type option for a multisig transaction?

Question

In a multi signature transaction, we can define the Signer Type as SHA256 through the Set Options operation

(https://www.stellar.org/developers/guides/concepts/multi-sig.html#alternate-signature-types).

Since Stellar uses Ed25519, why not use Ed25519 only?

Answer 1

SHA-256 hash signer type is used for Hash(x) alternate signature type. Here is how it works:

• You pick a secret x - there are no restrictions on the value of x but for security reasons it should be a random 256 bit value.
• You calculate the SHA-256 hash of x and add it (the hash of x) as a signer to the account using set_options operation.

Let's say that master key weight is equal 1, SHA-256 hash signer weight is also 1 but thresholds on the account are set to 2. Normally (with a single ed25519 signer) you only need to attach a signature generated using the private key of the account. But with SHA-256 signer you also need to know the preimage of the hash (so basically you need to know x that when hashed: SHA-256(x) gives a signer on that account).

This allows you to implement interesting concepts on Stellar, including lighting network.

Answer 2

The SHA256 signature is one of the alternate signature types. You can safely use a public key of any Stellar account as a signer with a Set Options operation.

Once you add the additional signer account you will be able to use the corresponding private key to sign the transactions on behalf of the source account.