This section documents the binary format of a transaction.
A transaction contains a compact-array of signatures, followed by a message. Each item in the signatures array is a digital signature of the given message. The Solana runtime verifies that the number of signatures matches the number in the first 8 bits of the message header. It also verifies that each signature was signed by the private key corresponding to the public key at the same index in the message's account addresses array.
Each digital signature is in the ed25519 binary format and consumes 64 bytes.
The message header contains three unsigned 8-bit values. The first value is the number of required signatures in the containing transaction. The second value is the number of those corresponding account addresses that are read-only. The third value in the message header is the number of read-only account addresses not requiring signatures.
The addresses that require signatures appear at the beginning of the account address array, with addresses requesting write access first and read-only accounts following. The addresses that do not require signatures follow the addresses that do, again with read-write accounts first and read-only accounts following.
A blockhash contains a 32-byte SHA-256 hash. It is used to indicate when a client last observed the ledger. Validators will reject transactions when the blockhash is too old.
An instruction contains a program ID index, followed by a compact-array of account address indexes, followed by a compact-array of opaque 8-bit data. The program ID index is used to identify an on-chain program that can interpret the opaque data. The program ID index is an unsigned 8-bit index to an account address in the message's array of account addresses. The account address indexes are each an unsigned 8-bit index into that same array.
A compact-array is serialized as the array length, followed by each array item. The array length is a special multi-byte encoding called compact-u16.
A compact-u16 is a multi-byte encoding of 16 bits. The first byte contains the lower 7 bits of the value in its lower 7 bits. If the value is above 0x7f, the high bit is set and the next 7 bits of the value are placed into the lower 7 bits of a second byte. If the value is above 0x3fff, the high bit is set and the remaining 2 bits of the value are placed into the lower 2 bits of a third byte.
An account address is 32-bytes of arbitrary data. When the address requires a digital signature, the runtime interprets it as the public key of an ed25519 keypair.