diff --git a/src/libopensc/compression.c b/src/libopensc/compression.c
index 3c8352d2..3dfd0ead 100644
--- a/src/libopensc/compression.c
+++ b/src/libopensc/compression.c
@@ -52,18 +52,28 @@ static int zerr_to_opensc(int err) {
 }
 static int detect_method(const u8* in, size_t inLen) {
 	if (in != NULL && inLen > 1) {
-		if (inLen > 2 && in[0] == 0x1f && in[1] == 0x8b)
+		if (in[0] == 0x1f && in[1] == 0x8b)
 			return COMPRESSION_GZIP;
-#if 0
-		if ((in[0] & 0x10) == Z_DEFLATED)
-			/* REALLY SIMPLE ZLIB TEST -- 
-			 * Check for the compression method to be set to 8...
-			 * many things can spoof this, but this is ok for now
-			 * */
+		/*
+		 * A zlib stream has the following structure:
+		 *   0   1
+		 * +---+---+
+		 * |CMF|FLG|   (more-->)
+		 * +---+---+
+		 *
+		 * FLG (FLaGs)
+		 * 	This flag byte is divided as follows:
+		 *
+		 * 	bits 0 to 4  FCHECK  (check bits for CMF and FLG)
+		 * 	bit  5       FDICT   (preset dictionary)
+		 * 	bits 6 to 7  FLEVEL  (compression level)
+		 *
+		 * 	The FCHECK value must be such that CMF and FLG, when viewed as
+		 * 	a 16-bit unsigned integer stored in MSB order (CMF*256 + FLG),
+		 * 	is a multiple of 31.
+		 */
+		if ((((uint16_t) in[0])*256 + in[1]) % 31 == 0)
 			return COMPRESSION_ZLIB;
-#else
-		return COMPRESSION_ZLIB;
-#endif
 	}
 	return COMPRESSION_UNKNOWN;
 }
@@ -105,8 +115,10 @@ static int sc_decompress_gzip(u8* out, size_t* outLen, const u8* in, size_t inLe
 	gz.next_out = out;
 	gz.avail_out = *outLen;
 
+	*outLen = 0;
+
 	err = inflateInit2(&gz, window_size);
-	if(err != Z_OK) return zerr_to_opensc(err);
+	if (err != Z_OK) return zerr_to_opensc(err);
 	err = inflate(&gz, Z_FINISH);
 	if(err != Z_STREAM_END) {
 		inflateEnd(&gz);
@@ -144,7 +156,8 @@ int sc_decompress(u8* out, size_t* outLen, const u8* in, size_t inLen, int metho
 
 	if(method == COMPRESSION_AUTO) {
 		method = detect_method(in, inLen);
-		if(method == COMPRESSION_UNKNOWN) {
+		if (method == COMPRESSION_UNKNOWN) {
+			*outLen = 0;
 			return SC_ERROR_UNKNOWN_DATA_RECEIVED;
 		}
 	}