const (

    // MaxMissingMessages is the maximum number of missing messages that
    // we'll keep track of.
    MaxMissingMessages = 8

    // RatchetKeyMaxLifetime is the maximum lifetime of the ratchet
    RatchetKeyMaxLifetime = time.Hour * 672

    // DoubleRatchetOverhead is the number of bytes the ratchet adds in ciphertext overhead.
    DoubleRatchetOverhead = 120
)

var (
    ErrDuplicateOrDelayed                     = errors.New("Ratchet: duplicate message or message delayed longer than tolerance")
    ErrHandshakeAlreadyComplete               = errors.New("Ratchet: handshake already complete")
    ErrCannotDecrypt                          = errors.New("Ratchet: cannot decrypt")
    ErrIncorrectHeaderSize                    = errors.New("Ratchet: incorrect header size")
    ErrSerialisedKeyLength                    = errors.New("Ratchet: bad serialised key length")
    ErrNextEncryptedMessageWithoutRatchetFlag = errors.New("Ratchet: received message encrypted to next header key without ratchet flag set")
    ErrOldFormKeyExchange                     = errors.New("Ratchet: peer using old-form key exchange")
    ErrCorruptMessage                         = errors.New("Ratchet: corrupt message")
    ErrMessageExceedsReorderingLimit          = errors.New("Ratchet: message exceeds reordering limit")
    ErrEchoedDHValues                         = errors.New("Ratchet: peer echoed our own DH values back")
    ErrInvalidSignatureLength                 = errors.New("Ratchet: invalid signature length")
    ErrRatchetHeaderTooSmall                  = errors.New("Ratchet: header too small to be valid")
    ErrInvalidKeyExchange                     = errors.New("Ratchet: peer's key exchange is invalid")
    ErrFailedToInitializeRatchet              = errors.New("Ratchet: failed to initialize")
    ErrInvalidPubkey                          = errors.New("Ratchet: invalid public key")
    ErrInvalidPublicIdentityKey               = errors.New("Ratchet: invalid public identity key")
    ErrInvalidSignature                       = errors.New("Ratchet: invalid signature")
    ErrKeyExchangeKeysNotIsomorphicallyEqual  = errors.New("Ratchet: key exchange and identity public keys must be isomorphically equal")
)

type KeyExchange struct {
    PublicKey      []byte
    IdentityPublic []byte
    Dh             []byte
    Dh1            []byte
}

type MessageKey struct {
    Num          uint32
    Key          []byte
    CreationTime int64
}

type Ratchet struct {
    TheirSigningPublic  *memguard.LockedBuffer // 32 bytes long
    TheirIdentityPublic *memguard.LockedBuffer // 32 bytes long

    MySigningPublic  *memguard.LockedBuffer // 32 bytes long
    MySigningPrivate *memguard.LockedBuffer // 64 bytes long

    MyIdentityPrivate *memguard.LockedBuffer // 32 bytes long
    MyIdentityPublic  *memguard.LockedBuffer // 32 bytes long

    // Now is an optional function that will be used to get the current
    // time. If nil, time.Now is used.
    Now func() time.Time
    // contains filtered or unexported fields
}

type SavedKeys struct {
    HeaderKey   []byte
    MessageKeys []*MessageKey
}

type SignedKeyExchange struct {
    Signed    []byte
    Signature []byte
}

type State struct {
    TheirSigningPublic  []byte
    TheirIdentityPublic []byte
    MySigningPublic     []byte
    MySigningPrivate    []byte
    MyIdentityPrivate   []byte
    MyIdentityPublic    []byte
    SavedKeys           []*SavedKeys
    RootKey             []byte
    SendHeaderKey       []byte
    RecvHeaderKey       []byte
    NextSendHeaderKey   []byte
    NextRecvHeaderKey   []byte
    SendChainKey        []byte
    RecvChainKey        []byte
    SendRatchetPrivate  []byte
    RecvRatchetPublic   []byte
    SendCount           uint32
    RecvCount           uint32
    PrevSendCount       uint32
    Private0            []byte
    Private1            []byte
    Ratchet             bool
}