util.go

package dtls

import (
	"crypto"
	"crypto/ecdsa"
	"crypto/elliptic"
	"crypto/rand"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/binary"
	"encoding/hex"
	"math/big"
	"time"
)

// Parse a big endian uint24
func bigEndianUint24(raw []byte) uint32 {
	if len(raw) < 3 {
		return 0
	}

	rawCopy := make([]byte, 4)
	copy(rawCopy[1:], raw)
	return binary.BigEndian.Uint32(rawCopy)
}

func putBigEndianUint24(out []byte, in uint32) {
	tmp := make([]byte, 4)
	binary.BigEndian.PutUint32(tmp, in)
	copy(out, tmp[1:])
}

func putBigEndianUint48(out []byte, in uint64) {
	tmp := make([]byte, 8)
	binary.BigEndian.PutUint64(tmp, in)
	copy(out, tmp[2:])
}

// GenerateSelfSigned creates a self-signed certificate
func GenerateSelfSigned() (*x509.Certificate, crypto.PrivateKey, error) {
	priv, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
	if err != nil {
		return nil, nil, err
	}

	origin := make([]byte, 16)

	// Max random value, a 130-bits integer, i.e 2^130 - 1
	maxBigInt := new(big.Int)
	/* #nosec */
	maxBigInt.Exp(big.NewInt(2), big.NewInt(130), nil).Sub(maxBigInt, big.NewInt(1))
	serialNumber, err := rand.Int(rand.Reader, maxBigInt)
	if err != nil {
		return nil, nil, err
	}

	template := x509.Certificate{
		ExtKeyUsage: []x509.ExtKeyUsage{
			x509.ExtKeyUsageClientAuth,
			x509.ExtKeyUsageServerAuth,
		},
		BasicConstraintsValid: true,
		NotBefore:             time.Now(),
		KeyUsage:              x509.KeyUsageKeyEncipherment | x509.KeyUsageDigitalSignature,
		NotAfter:              time.Now().AddDate(0, 1, 0),
		SerialNumber:          serialNumber,
		Version:               2,
		Subject:               pkix.Name{CommonName: hex.EncodeToString(origin)},
		IsCA:                  true,
	}

	raw, err := x509.CreateCertificate(rand.Reader, &template, &template, &priv.PublicKey, priv)
	if err != nil {
		return nil, nil, err
	}

	cert, err := x509.ParseCertificate(raw)
	if err != nil {
		return nil, nil, err
	}

	return cert, priv, nil
}

func max(a, b int) int {
	if a > b {
		return a
	}
	return b
}

// examinePadding returns, in constant time, the length of the padding to remove
// from the end of payload. It also returns a byte which is equal to 255 if the
// padding was valid and 0 otherwise. See RFC 2246, Section 6.2.3.2.
//
// https://github.com/golang/go/blob/039c2081d1178f90a8fa2f4e6958693129f8de33/src/crypto/tls/conn.go#L245
func examinePadding(payload []byte) (toRemove int, good byte) {
	if len(payload) < 1 {
		return 0, 0
	}

	paddingLen := payload[len(payload)-1]
	t := uint(len(payload)-1) - uint(paddingLen)
	// if len(payload) >= (paddingLen - 1) then the MSB of t is zero
	good = byte(int32(^t) >> 31)

	// The maximum possible padding length plus the actual length field
	toCheck := 256
	// The length of the padded data is public, so we can use an if here
	if toCheck > len(payload) {
		toCheck = len(payload)
	}

	for i := 0; i < toCheck; i++ {
		t := uint(paddingLen) - uint(i)
		// if i <= paddingLen then the MSB of t is zero
		mask := byte(int32(^t) >> 31)
		b := payload[len(payload)-1-i]
		good &^= mask&paddingLen ^ mask&b
	}

	// We AND together the bits of good and replicate the result across
	// all the bits.
	good &= good << 4
	good &= good << 2
	good &= good << 1
	good = uint8(int8(good) >> 7)

	toRemove = int(paddingLen) + 1

	return toRemove, good
}

func findMatchingSRTPProfile(a, b []SRTPProtectionProfile) (SRTPProtectionProfile, bool) {
	for _, aProfile := range a {
		for _, bProfile := range b {
			if aProfile == bProfile {
				return aProfile, true
			}
		}
	}
	return 0, false
}