$$ \newcommand \Hash {\mathrm{Hash}} \newcommand \pk {\mathrm{pk}} \newcommand \MSigPrefix {\texttt{MultisigAddr}} $$

Authorization and Signatures

Transactions are not valid unless they are somehow authorized by the sender account (for example, with a signature).

The authorization information is not considered part of the transaction and does not affect the transaction ID (TXID).

Rather, when serializing a transaction for submitting to a node or including in a block, the transaction and its authorization appear together in a structure called a SignedTxn.

The SignedTxn struct contains:

• The transaction (in msgpack field txn);

• An OPTIONAL authorizer address (field sgnr);

• Exactly one of a signature (field sig), multisignature (field msig), or logic signature (field lsig).

The authorizer address, a 32-byte address, determines against what to verify the sig / msig / lsig, as described below.

If the sgnr field is omitted (or zero), then the authorizer address defaults to the transaction sender address.

At the time the transaction is applied to the Ledger, the authorizer address MUST match the transaction sender account’s spending key (or the sender address, if the account’s spending key is zero). If it does not match, then the transaction was improperly authorized and is invalid.

Signatures

• A valid signature (sig) is a (64-byte) valid Ed25519 signature of the transaction (encoded in canonical msgpack and with domain separation prefix TX) where the public key is the authorizer address (interpreted as an Ed25519 public key).

• A valid multisignature (msig) is an object containing the following fields and which hashes to the authorizer address as described in the Multisignature section:

  • The subsig array of subsignatures, each consisting of a signer address and a 64-byte signature of the transaction. Note that transaction signed by a multisignature account MUST contain all signer’s addresses in the subsig array even if the transaction has not been signed by that address.

  • The threshold thr that is a minimum number of REQUIRED signatures.

  • The multisignature version v (current value is \( 1 \)).

• A valid logic signature (lsig) is an object containing the following fields:

  • The logic l which is versioned bytecode (see AVM specifications).

  • An OPTIONAL single signature sig of 64-bytes valid from the authorizer address of the transaction which has signed the bytes in l.

  • An OPTIONAL multisignature lmsig from the authorizer address of the transaction over the bytes of the authorizer address and the bytes in l.

  • An OPTIONAL array of byte strings arg which are arguments supplied to the program in l (arg bytes are not covered by sig or msig).

The logic signature is valid if exactly one of sig or msig is a valid signature of the program by the authorizer address of the transaction, or if neither sig nor msig is set and the hash of the program is equal to the authorizer address.

Also the program MUST execute and finish with a single non-zero value on the AVM stack (see AVM specifications for details on program execution semantics).

Multisignature

Multisignature term describes a special multisignature address, signing and validation procedures.

In contrast with a single signature address that may be understood as a public key, multisignature address is a hash of a constant string identifier for:

• The \( \MSigPrefix \) prefix,

• A version \( v \),

• The multisignature authorization threshold \( t \),

• All \( n \) addresses (\( \pk \)) used for multisignature address creation.

$$ \mathrm{MSig} = \Hash(\MSigPrefix, v, t, \pk_1, \ldots \pk_n) $$

One address MAY be specified multiple times in multisignature address creation. In this case, every occurrence is counted independently in validation.

The repetition of the same address in the multisignature defines the “weight” of the address.

The multisignature validation process checks that:

1. All non-empty signatures are valid;

2. The valid signatures count is not less than the threshold.

Validation fails if any of the signatures is invalid, even if the count of all remaining correct signatures is greater or equals than the threshold.