Fix issue 25 (segmentio#26)

* fix panic on parsing invalid KSUID representations

* more tests

* cleanup

base62.go

@@ -1,5 +1,7 @@
 package ksuid
 
+import "errors"
+
 const (
 	// lexographic ordering (based on Unicode table) is 0-9A-Za-z
 	base62Characters = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
@@ -8,6 +10,10 @@ const (
 	offsetLowercase  = 36
 )
 
+var (
+	errShortBuffer = errors.New("the output buffer is too small to hold to decoded value")
+)
+
 // Converts a base 62 byte into the number value that it represents.
 func base62Value(digit byte) byte {
 	switch {
@@ -102,7 +108,7 @@ func fastAppendEncodeBase62(dst []byte, src []byte) []byte {
 // is 27 bytes long and dst is 20 bytes long.
 //
 // Any unused bytes in dst will be set to zero.
-func fastDecodeBase62(dst []byte, src []byte) {
+func fastDecodeBase62(dst []byte, src []byte) error {
 	const srcBase = 62
 	const dstBase = 4294967296
 
@@ -156,6 +162,10 @@ func fastDecodeBase62(dst []byte, src []byte) {
 			}
 		}
 
+		if n < 4 {
+			return errShortBuffer
+		}
+
 		dst[n-4] = byte(remainder >> 24)
 		dst[n-3] = byte(remainder >> 16)
 		dst[n-2] = byte(remainder >> 8)
@@ -166,6 +176,7 @@ func fastDecodeBase62(dst []byte, src []byte) {
 
 	var zero [20]byte
 	copy(dst[:n], zero[:])
+	return nil
 }
 
 // This function appends the base 62 decoded version of src into dst.

ksuid.go

@@ -30,6 +30,9 @@ const (
 	// The length of a KSUID when string (base62) encoded
 	stringEncodedLength = 27
 
+	// A string-encoded minimum value for a KSUID
+	minStringEncoded = "000000000000000000000000000"
+
 	// A string-encoded maximum value for a KSUID
 	maxStringEncoded = "aWgEPTl1tmebfsQzFP4bxwgy80V"
 )
@@ -46,6 +49,7 @@ var (
 
 	errSize        = fmt.Errorf("Valid KSUIDs are %v bytes", byteLength)
 	errStrSize     = fmt.Errorf("Valid encoded KSUIDs are %v characters", stringEncodedLength)
+	errStrValue    = fmt.Errorf("Valid encoded KSUIDs are bounded by %s and %s", minStringEncoded, maxStringEncoded)
 	errPayloadSize = fmt.Errorf("Valid KSUID payloads are %v bytes", payloadLengthInBytes)
 
 	// Represents a completely empty (invalid) KSUID
@@ -179,7 +183,11 @@ func Parse(s string) (KSUID, error) {
 	dst := [byteLength]byte{}
 
 	copy(src[:], s[:])
-	fastDecodeBase62(dst[:], src[:])
+
+	if err := fastDecodeBase62(dst[:], src[:]); err != nil {
+		return Nil, errStrValue
+	}
+
 	return FromBytes(dst[:])
 }
 

ksuid_test.go

@@ -97,6 +97,19 @@ func TestParse(t *testing.T) {
 	}
 }
 
+func TestIssue25(t *testing.T) {
+	// https://github.com/segmentio/ksuid/issues/25
+	for _, s := range []string{
+		"aaaaaaaaaaaaaaaaaaaaaaaaaaa",
+		"aWgEPTl1tmebfsQzFP4bxwgy80!",
+	} {
+		_, err := Parse(s)
+		if err != errStrValue {
+			t.Error("invalid KSUID representations cannot be successfully parsed, got err =", err)
+		}
+	}
+}
+
 func TestEncodeAndDecode(t *testing.T) {
 	x := New()
 	builtFromEncodedString, err := Parse(x.String())