diff --git a/include/libg1m/format/std.h b/include/libg1m/format/std.h
index a60886f..004bbc7 100644
--- a/include/libg1m/format/std.h
+++ b/include/libg1m/format/std.h
@@ -60,10 +60,11 @@ struct standard_header {
 	uint8_t control2;
 
 	/* alignment */
-	uint8_t align[8];
+	uint8_t align[7];
 
 	/* is obfuscated - useful for G3P */
-	uint8_t obfuscated;
+	uint8_t obfuscated0;
+	uint8_t obfuscated1;
 
 	/* number of objects contained (useful for MCS filetype) */
 	uint16_t number;
diff --git a/include/libg1m/format/std/picture.h b/include/libg1m/format/std/picture.h
index 7276a0d..e21b8f0 100644
--- a/include/libg1m/format/std/picture.h
+++ b/include/libg1m/format/std/picture.h
@@ -83,12 +83,21 @@ struct g3p_subheader {
  * Before inflating, you should check if the image is obfuscated (Simon Lothar
  * says 'encrypted', I suppose that's more polite). It is if:
  *
- *		(std.obfuscated + 8) & 0xFF != (std.filesize & 0xff00) >> 8
+ *		std.obfuscated0 != std.subtype[0]
+ *			+ ((std.filesize & 0xFF00) >> 8) + (std.filesize & 0xFF)
  *
+ * Which is checked in `g1m_decode_std`.
  * If it is, then in order to de-obfuscate, you should do the following
- * operation:
+ * operations:
  *
- *		byte = ~((byte << 5) | (byte >> 3)) & 0xFF;
+ *		byte = ~byte;
+ *		byte = (byte << 5) | (byte >> 3);
+ *
+ * For completeness' sake, here are the operations you'll need to do to
+ * obfuscate while making a Prizm picture:
+ *
+ *		byte = (byte >> 5) | (byte << 3);
+ *		byte = ~byte;
  *
  * Then you can inflate.
  * Once this is done, you can get the next four bytes, also considered as
diff --git a/src/decode/std.c b/src/decode/std.c
index c073df6..504d69f 100644
--- a/src/decode/std.c
+++ b/src/decode/std.c
@@ -211,6 +211,17 @@ int g1m_decode_std(g1m_t **handle, const char *path, g1m_buffer_t *buffer,
 		if (find_decode_function(platform, type, &rd))
 			return (g1m_error_magic);
 
+		/* check the obfuscation bytes; imitates syscall 0x0C12
+		 * http://bible.planet-casio.com/simlo/chm/v20/fxCG20_Bfile.htm */
+		if (std->obfuscated0 == std->subtype[0]
+		+ ((std->filesize & 0xFF00) >> 8) + (std->filesize & 0xFF)) {
+			std->obfuscated0 = 0;
+			std->obfuscated1 = 0;
+		} else {
+			std->obfuscated0 = 1;
+			std->obfuscated1 = 1;
+		}
+
 		/* call it */
 		err = (*rd)(handle, buffer, std, &hd);
 		if (err) return (err);
diff --git a/src/decode/std/picture.c b/src/decode/std/picture.c
index c746600..3d5cc2d 100644
--- a/src/decode/std/picture.c
+++ b/src/decode/std/picture.c
@@ -33,8 +33,8 @@
 static void g3p_deobfuscate(uint8_t *buf, size_t n)
 {
 	while (n--) {
-		int byte = *buf;
-		*buf++ = ~((byte << 5) | (byte >> 3)) & 0xFF;
+		uint8_t byte = ~*buf;
+		*buf++ = (byte << 5) | (byte >> 3);
 	}
 }
 
@@ -86,9 +86,7 @@ int g1m_decode_std_g3p(g1m_t **h, g1m_buffer_t *buffer,
 	READ(defbuf, deflated_image_size)
 
 	/* unobfuscate if required */
-	int is_obfuscated =
-		((std->obfuscated + 8) & 0xff) != ((std->filesize & 0xff00) >> 8);
-	if (is_obfuscated) {
+	if (std->obfuscated0) {
 		log_info("Is obfuscated, let's deobfuscate!");
 		g3p_deobfuscate(defbuf, deflated_image_size);
 	}