Work around yaSSL bug.

yaSSL has a couple of bugs in their DH client implementation. This
change works around the worst of the two.

Firstly, they expect the the DH public value to be the same length as
the prime. This change pads the public value as needed to ensure this.

Secondly, although they handle the first byte of the shared key being
zero, they don't handle the case of the second, third, etc bytes being
zero. So whenever that happens the handshake fails. I don't think that
there's anything that we can do about that one.

Change-Id: I789c9e5739f19449473305d59fe5c3fb9b4a6167
Reviewed-on: https://boringssl-review.googlesource.com/6578
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/test/runner/key_agreement.go b/ssl/test/runner/key_agreement.go
index 3a9b899..4a07ed7 100644
--- a/ssl/test/runner/key_agreement.go
+++ b/ssl/test/runner/key_agreement.go
@@ -15,6 +15,7 @@
 	"crypto/x509"
 	"encoding/asn1"
 	"errors"
+	"fmt"
 	"io"
 	"math/big"
 )
@@ -652,6 +653,10 @@
 		return errServerKeyExchange
 	}
 
+	if l := config.Bugs.RequireDHPublicValueLen; l != 0 && l != yLen {
+		return fmt.Errorf("RequireDHPublicValueLen set to %d, but server's public value was %d bytes on the wire and %d bytes if minimal", l, yLen, (ka.yTheirs.BitLen()+7)/8)
+	}
+
 	sig := k
 	serverDHParams := skx.key[:len(skx.key)-len(sig)]