WebAuthn only hands you the public key at creation time, which makes it look private. It isn't. ECDSA allows public key recovery: from one signature and the signed bytes you get at most two candidate keys, and a second signature leaves exactly one. Combined with the vault derivation, a single leaked assertion can reveal your vault. On Solana the key is in the instruction anyway. This matters for assertions you send off-chain, for example to a backend for login, or to a relayer before submission.

One signature plus its message is enough to recover the passkey's public key.

Signatures reveal the public key

Solana's fee payer must be the first <code>ed25519</code> signer of the transaction, so a passkey alone can never submit anything. In practice a relayer run by the wallet provider adds its signature and pays. The relayer sees your IP address, the time of each request, and the full transaction before it lands. On-chain, its address appears as the fee payer of every transaction your vault makes. If many users share one relayer this adds some cover, but a relayer used by only a few vaults links them all.

Someone else's ed25519 key signs and pays for every passkey transaction.

The fee payer sees, and links, your activity

The SPL Memo program, payment references from Solana Pay, invoice ids, and custom instruction data are stored unencrypted in the ledger. Exchanges and merchants often require a memo or reference that maps directly to your account with them. Once one such memo sits next to your vault, the vault is tied to your identity at that service.

Anything a dApp writes into a memo or instruction is public forever.