fix: let decode look for additional base32 padding

Update FromString and XID.TextUnmarshal so that it looks for discarded
bits in the final source character. This ensures that XIDs that have
been manually tampered with in a way that's ignored by base32 decode,
will not pass as valid.

id.go

@@ -238,7 +238,9 @@ func (id *ID) UnmarshalText(text []byte) error {
 			return ErrInvalidID
 		}
 	}
-	decode(id, text)
+	if !decode(id, text) {
+		return ErrInvalidID
+	}
 	return nil
 }
 
@@ -256,8 +258,8 @@ func (id *ID) UnmarshalJSON(b []byte) error {
 	return id.UnmarshalText(b[1 : len(b)-1])
 }
 
-// decode by unrolling the stdlib base32 algorithm + removing all safe checks
-func decode(id *ID, src []byte) {
+// decode by unrolling the stdlib base32 algorithm + customized safe check.
+func decode(id *ID, src []byte) bool {
 	_ = src[19]
 	_ = id[11]
 
@@ -273,6 +275,16 @@ func decode(id *ID, src []byte) {
 	id[2] = dec[src[3]]<<4 | dec[src[4]]>>1
 	id[1] = dec[src[1]]<<6 | dec[src[2]]<<1 | dec[src[3]]>>4
 	id[0] = dec[src[0]]<<3 | dec[src[1]]>>2
+
+	// Validate that there are no discarer bits (padding) in src that would
+	// cause the string-encoded id not to equal src.
+	var check [4]byte
+
+	check[3] = encoding[(id[11]<<4)&0x1F]
+	check[2] = encoding[(id[11]>>1)&0x1F]
+	check[1] = encoding[(id[11]>>6)&0x1F|(id[10]<<2)&0x1F]
+	check[0] = encoding[id[10]>>3]
+	return bytes.Equal([]byte(src[16:20]), check[:])
 }
 
 // Time returns the timestamp part of the id.