asn1_unwrap() function to parse ASN.1 objects with length/type

src/libstrongswan/asn1/asn1.c

@@ -311,6 +311,53 @@ size_t asn1_length(chunk_t *blob)
 	return len;
 }
 
+/*
+ * See header.
+ */
+int asn1_unwrap(chunk_t *blob, chunk_t *inner)
+{
+	chunk_t res;
+	u_char len;
+	int type;
+	
+	if (blob->len < 2)
+	{
+		return ASN1_INVALID;
+	}
+	type = blob->ptr[0];
+	len = blob->ptr[1];
+	*blob = chunk_skip(*blob, 2);
+	
+	if ((len & 0x80) == 0)
+	{	/* single length octet */
+		res.len = len;
+	}
+	else
+	{	/* composite length, determine number of length octets */
+		len &= 0x7f;
+		if (len == 0 || len > sizeof(res.len))
+		{
+			return ASN1_INVALID;
+		}
+		res.len = 0;
+		while (len-- > 0)
+		{
+			res.len = 256 * res.len + blob->ptr[0];
+			*blob = chunk_skip(*blob, 1);
+		}
+	}
+	if (res.len > blob->len)
+	{
+		return ASN1_INVALID;
+	}
+	res.ptr = blob->ptr;
+	*blob = chunk_skip(*blob, res.len);
+	/* updating inner not before we are finished allows a caller to pass
+	 * blob = inner */
+	*inner = res;
+	return type;
+}
+
 #define TIME_MAX	0x7fffffff
 
 static const int days[] = { 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 };

src/libstrongswan/asn1/asn1.h

@@ -74,7 +74,9 @@ typedef enum {
     ASN1_CONTEXT_C_2 =		0xA2,
     ASN1_CONTEXT_C_3 =		0xA3,
     ASN1_CONTEXT_C_4 =		0xA4,
-    ASN1_CONTEXT_C_5 =		0xA5
+    ASN1_CONTEXT_C_5 =		0xA5,
+
+    ASN1_INVALID = 			0x100,
 } asn1_t;
 
 #define ASN1_INVALID_LENGTH	0xffffffff
@@ -122,6 +124,15 @@ chunk_t asn1_build_known_oid(int n);
  */
 size_t asn1_length(chunk_t *blob);
 
+/**
+ * Unwrap the inner content of an ASN.1 type/length wrapped object.
+ *
+ * @param blob		blob to parse header from, moved behind parsed content
+ * @param content	inner content
+ * @return			parsed type, ASN1_INVALID if length parsing failed
+ */
+int asn1_unwrap(chunk_t *blob, chunk_t *content);
+
 /**
  * Parses an ASN.1 algorithmIdentifier object
  *