SSI is a specification for gasless solana instructions, allowing any compatible wallet to securely execute instructions on Solana without paying gas, and without actually needing to use a solana wallet (:pogchamp: :wot:).
The gasless instruction functionality is obtained by using a secp256k1 private key to sign the instruction data you would normally provide to a solana program.
With some minor modification, any solana program that has 129 bytes of free space in the transaction itself can utilize gasless instructions.
Any compatible blockchain (currently solana + ethereum) which utilizes a secp256k1 public/private key can execute instructions on Solana without actually needing to have a solana wallet.
All you need is a way to construct a signed message in the SSI format, and relaying it to a program on Solana.
For simplicity a general purpose implementation of SSI is provided as ByteSignedIx, which acts as a wrapper around any existing instruction.
You can read the tests in byte_signed_ix.rs for an example PoC implementing the byte_signed_ix.