Class SrpAuthHandler

Namespace

AppleTvControlLibrary.Auth

Assembly

AppleTv.Hap.dll

Handle SRP crypto routines for auth and key derivation.

public sealed class SrpAuthHandler

Inheritance

object

SrpAuthHandler

Inherited Members

object.ToString()

object.Equals(object)

object.Equals(object, object)

object.ReferenceEquals(object, object)

object.GetHashCode()

object.GetType()

Remarks

The SRP math here is ported from srptools (the library pyatv itself depends on for its SRP implementation), not from a generic SRP-6a reference, since the two differ in a few details (padding, hash argument ordering) that must match byte-for-byte for the HAP handshake to succeed.

Constructors

SrpAuthHandler()

Initializes a new instance of the SrpAuthHandler class.

public SrpAuthHandler()

Properties

PairingId

Gets or sets the pairing id used as the client identifier during pair-verify.

public byte[] PairingId { get; set; }

Property Value

byte[]

Remarks

pyatv exposes this as a mutable public attribute so callers can override it once credentials are known externally (i.e. not from a pairing performed by this instance); see MrpProtocol.start — pyatv/protocols/mrp/protocol.py line 137-140 as of pyatv 0.18.0.

SharedKey

Gets the shared secret (SRP session key) established during pair-setup.

public byte[] SharedKey { get; }

Property Value

byte[]

Methods

HkdfExpand(string, string, byte[])

Derives encryption keys from a shared secret using HKDF-SHA512.

public static byte[] HkdfExpand(string salt, string info, byte[] sharedSecret)

Parameters

salt string

The HKDF salt string.

info string

The HKDF info string.

sharedSecret byte[]

The shared secret to derive from.

Returns

byte[]

32 bytes of derived key material.

Initialize()

Initialize operation by generating new keys.

public (byte[] AuthPublic, byte[] PublicBytes) Initialize()

Returns

(byte[] AuthPublic, byte[] PublicBytes)

A tuple of (auth public key, verify public key).

Step1(int)

First pairing step. Sets up the SRP client session with the given PIN.

public void Step1(int pin)

Parameters

pin int

The PIN code entered by the user.

Step2(byte[], byte[])

Second pairing step. Processes the device's public key and salt.

public (byte[] PubKey, byte[] Proof) Step2(byte[] atvPubKey, byte[] atvSalt)

Parameters

atvPubKey byte[]

The device's SRP public key.

atvSalt byte[]

The device's SRP salt.

Returns

(byte[] AuthPublic, byte[] PublicBytes)

A tuple of (client public key, client proof).

Step3(string?)

Third pairing step. Builds the encrypted TLV payload with device identity.

public byte[] Step3(string? name = null)

Parameters

name string

An optional display name to include.

Returns

byte[]

The encrypted TLV8 payload.

Step4(byte[])

Last pairing step. Decrypts and parses the final device response.

public HapCredentials Step4(byte[] encryptedData)

Parameters

encryptedData byte[]

The encrypted TLV8 payload from the device.

Returns

HapCredentials

The resulting credentials for the paired device.

Verify1(HapCredentials, byte[], byte[])

First verification step.

public byte[] Verify1(HapCredentials credentials, byte[] sessionPubKey, byte[] encrypted)

Parameters

credentials HapCredentials

The credentials of the device being verified.

sessionPubKey byte[]

The device's X25519 public key.

encrypted byte[]

Encrypted TLV8 payload containing the device identifier and signature.

Returns

byte[]

Encrypted TLV8 payload to send back to the device.

Verify2(string, string, string)

Last verification step. Derives the output and input encryption keys.

public (byte[] OutputKey, byte[] InputKey) Verify2(string salt, string outputInfo, string inputInfo)

Parameters

salt string

The HKDF salt string.

outputInfo string

The HKDF info string for the output key.

inputInfo string

The HKDF info string for the input key.

Returns

(byte[] AuthPublic, byte[] PublicBytes)

A tuple of (output key, input key).