From b19332a69fbd7b82f0e08c18f55a6880487d55e9 Mon Sep 17 00:00:00 2001
From: Jeffrey Walton <noloader@gmail.com>
Date: Sat, 24 Dec 2016 04:55:21 -0500
Subject: [PATCH] Add additional validations based on X.690 rules

The library was a tad bit fast and loose with respect to parsing some of the ASN.1 presented to it. It was kind of like we used Alternate Encoding Rules (AER), which was more relaxed than BER, CER or DER. This commit closes most of the gaps.

The changes are distantly related to Issue 346. Issue 346 caught a CVE bcause of the transient DoS. These fixes did not surface with negative effcts. Rather, the library was a bit too accomodating to the point it was not conforming
---
 asn.cpp | 11 ++++++-----
 asn.h   | 32 +++++++++++++++++++++-----------
 2 files changed, 27 insertions(+), 16 deletions(-)

diff --git a/asn.cpp b/asn.cpp
index 2e923ef7..79b4cbc3 100644
--- a/asn.cpp
+++ b/asn.cpp
@@ -123,7 +123,7 @@ size_t BERDecodeOctetString(BufferedTransformation &bt, SecByteBlock &str)
 	size_t bc;
 	if (!BERLengthDecode(bt, bc))
 		BERDecodeError();
-	if (bc > bt.MaxRetrievable())
+	if (bc > bt.MaxRetrievable()) // Issue 346
 		BERDecodeError();
 
 	str.New(bc);
@@ -141,7 +141,7 @@ size_t BERDecodeOctetString(BufferedTransformation &bt, BufferedTransformation &
 	size_t bc;
 	if (!BERLengthDecode(bt, bc))
 		BERDecodeError();
-	if (bc > bt.MaxRetrievable())
+	if (bc > bt.MaxRetrievable()) // Issue 346
 		BERDecodeError();
 
 	bt.TransferTo(str, bc);
@@ -165,7 +165,7 @@ size_t BERDecodeTextString(BufferedTransformation &bt, std::string &str, byte as
 	size_t bc;
 	if (!BERLengthDecode(bt, bc))
 		BERDecodeError();
-	if (bc > bt.MaxRetrievable())
+	if (bc > bt.MaxRetrievable()) // Issue 346
 		BERDecodeError();
 
 	SecByteBlock temp(bc);
@@ -196,11 +196,12 @@ size_t BERDecodeBitString(BufferedTransformation &bt, SecByteBlock &str, unsigne
 		BERDecodeError();
 	if (bc == 0)
 		BERDecodeError();
-	if (bc > bt.MaxRetrievable())
+	if (bc > bt.MaxRetrievable()) // Issue 346
 		BERDecodeError();
 
+	// X.690, 8.6.2.2: "The number [of unused bits] shall be in the range zero to seven"
 	byte unused;
-	if (!bt.Get(unused))
+	if (!bt.Get(unused) || unused > 7)
 		BERDecodeError();
 	unusedBits = unused;
 	str.resize(bc-1);
diff --git a/asn.h b/asn.h
index 33f0dd09..002d849b 100644
--- a/asn.h
+++ b/asn.h
@@ -55,13 +55,14 @@ enum ASNTag
 //! \note These tags and flags are not complete
 enum ASNIdFlag
 {
-	UNIVERSAL			= 0x00,
-//	DATA				= 0x01,
-//	HEADER				= 0x02,
-	CONSTRUCTED 		= 0x20,
-	APPLICATION 		= 0x40,
-	CONTEXT_SPECIFIC	= 0x80,
-	PRIVATE 			= 0xc0
+	UNIVERSAL           = 0x00,
+//	DATA                = 0x01,
+//	HEADER              = 0x02,
+	PRIMITIVE           = 0x00,
+	CONSTRUCTED         = 0x20,
+	APPLICATION         = 0x40,
+	CONTEXT_SPECIFIC    = 0x80,
+	PRIVATE             = 0xc0
 };
 
 //! \brief Raises a BERDecodeErr
@@ -478,9 +479,9 @@ size_t DEREncodeUnsigned(BufferedTransformation &out, T w, byte asnTag = INTEGER
 }
 
 //! \brief BER Decode unsigned value
-//! \tparam T class or type
+//! \tparam T fundamental C++ type
 //! \param in BufferedTransformation object
-//! \param w unsigned value to encode
+//! \param w the decoded value
 //! \param asnTag the ASN.1 type
 //! \param minValue the minimum expected value
 //! \param maxValue the maximum expected value
@@ -488,7 +489,7 @@ size_t DEREncodeUnsigned(BufferedTransformation &out, T w, byte asnTag = INTEGER
 //! \details DEREncodeUnsigned() can be used with INTEGER, BOOLEAN, and ENUM
 template <class T>
 void BERDecodeUnsigned(BufferedTransformation &in, T &w, byte asnTag = INTEGER,
-					   T minValue = 0, T maxValue = ((std::numeric_limits<T>::max)()))
+					   T minValue = 0, T maxValue = T(0xffffffff))
 {
 	byte b;
 	if (!in.Get(b) || b != asnTag)
@@ -498,7 +499,11 @@ void BERDecodeUnsigned(BufferedTransformation &in, T &w, byte asnTag = INTEGER,
 	bool definite = BERLengthDecode(in, bc);
 	if (!definite)
 		BERDecodeError();
-	if (bc > in.MaxRetrievable())
+	if (bc > in.MaxRetrievable())  // Issue 346
+		BERDecodeError();
+	if (asnTag == BOOLEAN && bc != 1) // X.690, 8.2.1
+		BERDecodeError();
+	if ((asnTag == INTEGER || asnTag == ENUMERATED) && bc == 0) // X.690, 8.3.1 and 8.4
 		BERDecodeError();
 
 	SecByteBlock buf(bc);
@@ -506,6 +511,11 @@ void BERDecodeUnsigned(BufferedTransformation &in, T &w, byte asnTag = INTEGER,
 	if (bc != in.Get(buf, bc))
 		BERDecodeError();
 
+	// This consumes leading 0 octets. According to X.690, 8.3.2, it could be non-conforming behavior.
+	//  X.690, 8.3.2 says "the bits of the first octet and bit 8 of the second octet ... (a) shall
+	//  not all be ones and (b) shall not all be zeros ... These rules ensure that an integer value
+	//  is always encoded in the smallest possible number of octet".
+	// We invented AER (Alternate Encoding Rules), which is more relaxed than BER, CER, and DER.
 	const byte *ptr = buf;
 	while (bc > sizeof(w) && *ptr == 0)
 	{