oauth/io.github.kikin81.atproto.oauth/DpopSigner

DpopSigner

Signs DPoP proof JWTs using EC P-256 (ES256) per RFC 9449.

Each DpopSigner instance is bound to a single EC keypair for the lifetime of an OAuth session. The private key signs proofs; the public key is embedded in every proof's JWT header as a JWK so the server can verify possession.

AT Protocol DPoP proof JWT structure:

Header:

{ "typ": "dpop+jwt", "alg": "ES256", "jwk": { "kty": "EC", "crv": "P-256", "x": "...", "y": "..." } }

Payload:

{ "jti": "<unique>", "htm": "POST", "htu": "https://pds/oauth/par", "iat": 1234567890, "nonce": "..." }

For PDS resource requests (not PAR/token), the payload also includes:

{ "ath": "<base64url(sha256(access_token))>" }

Per the AT Protocol spec, the iss claim is deliberately omitted from all DPoP proofs.

Types

Companion

object Companion

ExportedKeyPair

data class ExportedKeyPair(val privateKeyEncoded: ByteArray, val publicKeyEncoded: ByteArray)

Properties

clockOffsetSeconds

var clockOffsetSeconds: Long

Clock offset in seconds between the device and the server. If the server's Date header says 12:00:05 and the device says 12:00:00, offset is +5. Applied to iat claims so DPoP proofs are accepted even when the device clock is off.

publicKey

val publicKey: ECPublicKey

Functions

calibrateClockFromHeader

fun calibrateClockFromHeader(dateHeader: String?)

Calibrates clockOffsetSeconds from a server response's Date header. Call this after the first response from any AT Protocol server.

exportKeyPair

fun exportKeyPair(): DpopSigner.ExportedKeyPair

Exports the keypair for session persistence. The caller is responsible for storing these securely (e.g. EncryptedSharedPreferences).

sign

fun sign(method: String, url: String, accessTokenHash: String? = null, nonce: String? = null): String

Signs a DPoP proof JWT for the given HTTP method + URL.