From ec57e983f7e797568d6c12d460497892be439313 Mon Sep 17 00:00:00 2001
From: Jeff Mitchell <jeffrey.mitchell@gmail.com>
Date: Wed, 26 Aug 2015 10:03:44 -0700
Subject: [PATCH] Don't allow duplicate x parts in Shamir. Add unit test for
 verification.

---
 shamir/shamir.go      | 11 +++++++++--
 shamir/shamir_test.go |  8 ++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/shamir/shamir.go b/shamir/shamir.go
index 5180202dd..da32f2c47 100644
--- a/shamir/shamir.go
+++ b/shamir/shamir.go
@@ -194,9 +194,16 @@ func Combine(parts [][]byte) ([]byte, error) {
 	x_samples := make([]uint8, len(parts))
 	y_samples := make([]uint8, len(parts))
 
-	// Set the x value for each sample
+	// Set the x value for each sample and ensure no x_sample values are the same,
+	// otherwise div() can be unhappy
+	checkMap := map[byte]bool{}
 	for i, part := range parts {
-		x_samples[i] = part[firstPartLen-1]
+		samp := part[firstPartLen-1]
+		if exists := checkMap[samp]; exists {
+			return nil, fmt.Errorf("duplicte part detected")
+		}
+		checkMap[samp] = true
+		x_samples[i] = samp
 	}
 
 	// Reconstruct each byte
diff --git a/shamir/shamir_test.go b/shamir/shamir_test.go
index e2ee3cf44..d2b2e68d1 100644
--- a/shamir/shamir_test.go
+++ b/shamir/shamir_test.go
@@ -71,6 +71,14 @@ func TestCombine_invalid(t *testing.T) {
 	if _, err := Combine(parts); err == nil {
 		t.Fatalf("should err")
 	}
+
+	parts = [][]byte{
+		[]byte("foo"),
+		[]byte("foo"),
+	}
+	if _, err := Combine(parts); err == nil {
+		t.Fatalf("should err")
+	}
 }
 
 func TestCombine(t *testing.T) {