Split off DTLS record header parsing in test runner.

The DTLS 1.3 record header is formatted differently than the old record
header, but the code to read/process a DTLS record mixes record header
parsing with other record processing code. This change provides a clear
delineation between processing the record header and processing the
record, which will assist in adding support for the DTLS 1.3 record
header.

Bug: 715
Change-Id: I13a0bb5c184e79b88f064e9ac8ecbc82eb56750a
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/69950
Commit-Queue: Bob Beck <bbe@google.com>
Reviewed-by: Bob Beck <bbe@google.com>

ssl/test/runner/dtls.go

diff --git a/ssl/test/runner/dtls.go b/ssl/test/runner/dtls.go
index 95e1a9a..1893afe 100644
--- a/ssl/test/runner/dtls.go
+++ b/ssl/test/runner/dtls.go
@@ -23,29 +23,13 @@
 	"net"
 )
 
-func (c *Conn) dtlsDoReadRecord(want recordType) (recordType, *block, error) {
-	recordHeaderLen := dtlsMaxRecordHeaderLen
-
-	if c.rawInput == nil {
-		c.rawInput = c.in.newBlock()
-	}
-	b := c.rawInput
-
-	// Read a new packet only if the current one is empty.
-	var newPacket bool
-	if len(b.data) == 0 {
-		// Pick some absurdly large buffer size.
-		b.resize(maxCiphertext + recordHeaderLen)
-		n, err := c.conn.Read(c.rawInput.data)
-		if err != nil {
-			return 0, nil, err
-		}
-		if c.config.Bugs.MaxPacketLength != 0 && n > c.config.Bugs.MaxPacketLength {
-			return 0, nil, fmt.Errorf("dtls: exceeded maximum packet length")
-		}
-		c.rawInput.resize(n)
-		newPacket = true
-	}
+// readDTLSRecordHeader reads the record header from the block. Based on the
+// header it reads, it checks the header's validity and sets appropriate state
+// as needed. This function returns the record header, the record type indicated
+// in the header (if it contains the type), and the sequence number to use for
+// record decryption.
+func (c *Conn) readDTLSRecordHeader(b *block) (headerLen int, recordLen int, typ recordType, seq []byte, err error) {
+	recordHeaderLen := 13
 
 	// Read out one record.
 	//
@@ -53,9 +37,9 @@
 	// but this is test code. We should not be tolerant of our
 	// peer sending garbage.
 	if len(b.data) < recordHeaderLen {
-		return 0, nil, errors.New("dtls: failed to read record header")
+		return 0, 0, 0, nil, errors.New("dtls: failed to read record header")
 	}
-	typ := recordType(b.data[0])
+	typ = recordType(b.data[0])
 	vers := uint16(b.data[1])<<8 | uint16(b.data[2])
 	// Alerts sent near version negotiation do not have a well-defined
 	// record-layer version prior to TLS 1.3. (In TLS 1.3, the record-layer
@@ -68,32 +52,61 @@
 			}
 			if vers != wireVersion {
 				c.sendAlert(alertProtocolVersion)
-				return 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: received record with version %x when expecting version %x", vers, c.wireVersion))
+				return 0, 0, 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: received record with version %x when expecting version %x", vers, c.wireVersion))
 			}
 		} else {
 			// Pre-version-negotiation alerts may be sent with any version.
 			if expect := c.config.Bugs.ExpectInitialRecordVersion; expect != 0 && vers != expect {
 				c.sendAlert(alertProtocolVersion)
-				return 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: received record with version %x when expecting version %x", vers, expect))
+				return 0, 0, 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: received record with version %x when expecting version %x", vers, expect))
 			}
 		}
 	}
 	epoch := b.data[3:5]
-	seq := b.data[5:11]
+	seq = b.data[5:11]
 	// For test purposes, require the sequence number be monotonically
 	// increasing, so c.in includes the minimum next sequence number. Gaps
 	// may occur if packets failed to be sent out. A real implementation
 	// would maintain a replay window and such.
 	if !bytes.Equal(epoch, c.in.seq[:2]) {
 		c.sendAlert(alertIllegalParameter)
-		return 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: bad epoch, want %x, got %x", c.in.seq[:2], epoch))
+		return 0, 0, 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: bad epoch"))
 	}
 	if bytes.Compare(seq, c.in.seq[2:]) < 0 {
 		c.sendAlert(alertIllegalParameter)
-		return 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: bad sequence number"))
+		return 0, 0, 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: bad sequence number"))
 	}
 	copy(c.in.seq[2:], seq)
-	n := int(b.data[11])<<8 | int(b.data[12])
+	recordLen = int(b.data[11])<<8 | int(b.data[12])
+	return recordHeaderLen, recordLen, typ, b.data[3:11], nil
+}
+
+func (c *Conn) dtlsDoReadRecord(want recordType) (recordType, *block, error) {
+	if c.rawInput == nil {
+		c.rawInput = c.in.newBlock()
+	}
+	b := c.rawInput
+
+	// Read a new packet only if the current one is empty.
+	var newPacket bool
+	if len(b.data) == 0 {
+		// Pick some absurdly large buffer size.
+		b.resize(maxCiphertext + dtlsMaxRecordHeaderLen)
+		n, err := c.conn.Read(c.rawInput.data)
+		if err != nil {
+			return 0, nil, err
+		}
+		if c.config.Bugs.MaxPacketLength != 0 && n > c.config.Bugs.MaxPacketLength {
+			return 0, nil, fmt.Errorf("dtls: exceeded maximum packet length")
+		}
+		c.rawInput.resize(n)
+		newPacket = true
+	}
+
+	recordHeaderLen, n, typ, seq, err := c.readDTLSRecordHeader(b)
+	if err != nil {
+		return 0, nil, err
+	}
 	if n > maxCiphertext || len(b.data) < recordHeaderLen+n {
 		c.sendAlert(alertRecordOverflow)
 		return 0, nil, c.in.setErrorLocked(fmt.Errorf("dtls: oversized record received with length %d", n))
@@ -101,7 +114,7 @@
 	b, c.rawInput = c.in.splitBlock(b, recordHeaderLen+n)
 
 	// Process message.
-	ok, off, _, alertValue := c.in.decrypt(b.data[3:11], recordHeaderLen, b)
+	ok, off, _, alertValue := c.in.decrypt(seq, recordHeaderLen, b)
 	if !ok {
 		// A real DTLS implementation would silently ignore bad records,
 		// but we want to notice errors from the implementation under