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package stun
import (
"errors"
"hash/crc32"
)
// FingerprintAttr represents FINGERPRINT attribute.
//
// RFC 5389 Section 15.5
type FingerprintAttr struct{}
// ErrFingerprintMismatch means that computed fingerprint differs from expected.
var ErrFingerprintMismatch = errors.New("fingerprint check failed")
// Fingerprint is shorthand for FingerprintAttr.
//
// Example:
//
// m := New()
// Fingerprint.AddTo(m)
var Fingerprint FingerprintAttr
const (
fingerprintXORValue uint32 = 0x5354554e //nolint:staticcheck
fingerprintSize = 4 // 32 bit
)
// FingerprintValue returns CRC-32 of b XOR-ed by 0x5354554e.
//
// The value of the attribute is computed as the CRC-32 of the STUN message
// up to (but excluding) the FINGERPRINT attribute itself, XOR'ed with
// the 32-bit value 0x5354554e (the XOR helps in cases where an
// application packet is also using CRC-32 in it).
func FingerprintValue(b []byte) uint32 {
return crc32.ChecksumIEEE(b) ^ fingerprintXORValue // XOR
}
// AddTo adds fingerprint to message.
func (FingerprintAttr) AddTo(m *Message) error {
l := m.Length
// length in header should include size of fingerprint attribute
m.Length += fingerprintSize + attributeHeaderSize // increasing length
m.WriteLength() // writing Length to Raw
b := make([]byte, fingerprintSize)
val := FingerprintValue(m.Raw)
bin.PutUint32(b, val)
m.Length = l
m.Add(AttrFingerprint, b)
return nil
}
// Check reads fingerprint value from m and checks it, returning error if any.
// Can return *AttrLengthErr, ErrAttributeNotFound, and *CRCMismatch.
func (FingerprintAttr) Check(m *Message) error {
b, err := m.Get(AttrFingerprint)
if err != nil {
return err
}
if err = CheckSize(AttrFingerprint, len(b), fingerprintSize); err != nil {
return err
}
val := bin.Uint32(b)
attrStart := len(m.Raw) - (fingerprintSize + attributeHeaderSize)
expected := FingerprintValue(m.Raw[:attrStart])
return checkFingerprint(val, expected)
}
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