50 lines
1.5 KiB
Go
50 lines
1.5 KiB
Go
package udp
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import (
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"crypto/hmac"
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"crypto/sha256"
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"encoding/binary"
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"net"
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"time"
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)
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// ttl is the number of seconds a connection ID should be valid according to
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// BEP 15.
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const ttl = 2 * time.Minute
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// NewConnectionID creates a new 8 byte connection identifier for UDP packets
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// as described by BEP 15.
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//
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// The first 4 bytes of the connection identifier is a unix timestamp and the
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// last 4 bytes are a truncated HMAC token created from the aforementioned
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// unix timestamp and the source IP address of the UDP packet.
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//
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// Truncated HMAC is known to be safe for 2^(-n) where n is the size in bits
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// of the truncated HMAC token. In this use case we have 32 bits, thus a
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// forgery probability of approximately 1 in 4 billion.
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func NewConnectionID(ip net.IP, now time.Time, key string) []byte {
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buf := make([]byte, 8)
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binary.BigEndian.PutUint32(buf, uint32(now.UTC().Unix()))
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mac := hmac.New(sha256.New, []byte(key))
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mac.Write(buf[:4])
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mac.Write(ip)
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macBytes := mac.Sum(nil)[:4]
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copy(buf[4:], macBytes)
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return buf
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}
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// ValidConnectionID determines whether a connection identifier is legitimate.
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func ValidConnectionID(connectionID []byte, ip net.IP, now time.Time, maxClockSkew time.Duration, key string) bool {
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ts := time.Unix(int64(binary.BigEndian.Uint32(connectionID[:4])), 0)
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if now.After(ts.Add(ttl)) || ts.After(now.Add(maxClockSkew)) {
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return false
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}
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mac := hmac.New(sha256.New, []byte(key))
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mac.Write(connectionID[:4])
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mac.Write(ip)
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expectedMAC := mac.Sum(nil)[:4]
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return hmac.Equal(expectedMAC, connectionID[4:])
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}
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