Add check for maximum signature length in ecdsa.ParseDERSignature

* Updated the `ecdsa.ParseDERSignature` to include the MaxSigLen constant, which defines the maximum length of a DER encoded signature.
* The MaxSigLen constant is set to 72 bytes, as per the explanation in the comment.
* A new test case has been added to test the functionality with a long signature.
* The test case checks if signatures longer than the specified maximum length are properly handled.
* The test ensures that signatures exceeding the maximum length are correctly identified as invalid.

btcec/ecdsa/signature.go

@@ -37,10 +37,20 @@ var (
 	oneInitializer = []byte{0x01}
 )
 
-// MinSigLen is the minimum length of a DER encoded signature and is when both R
-// and S are 1 byte each.
-// 0x30 + <1-byte> + 0x02 + 0x01 + <byte> + 0x2 + 0x01 + <byte>
-const MinSigLen = 8
+const (
+	// MinSigLen is the minimum length of a DER encoded signature and is when both R
+	// and S are 1 byte each.
+	// 0x30 + <1-byte> + 0x02 + 0x01 + <byte> + 0x2 + 0x01 + <byte>
+	MinSigLen = 8
+
+	// MaxSigLen is the maximum length of a DER encoded signature and is
+	// when both R and S are 33 bytes each.  It is 33 bytes because a
+	// 256-bit integer requires 32 bytes and an additional leading null byte
+	// might be required if the high bit is set in the value.
+	//
+	// 0x30 + <1-byte> + 0x02 + 0x21 + <33 bytes> + 0x2 + 0x21 + <33 bytes>
+	MaxSigLen = 72
+)
 
 // canonicalPadding checks whether a big-endian encoded integer could
 // possibly be misinterpreted as a negative number (even though OpenSSL
@@ -68,9 +78,15 @@ func parseSig(sigStr []byte, der bool) (*Signature, error) {
 	// 0x30 <length of whole message> <0x02> <length of R> <R> 0x2
 	// <length of S> <S>.
 
-	if len(sigStr) < MinSigLen {
+	// The signature must adhere to the minimum and maximum allowed length.
+	totalSigLen := len(sigStr)
+	if totalSigLen < MinSigLen {
 		return nil, errors.New("malformed signature: too short")
 	}
+	if der && totalSigLen > MaxSigLen {
+		return nil, errors.New("malformed signature: too long")
+	}
+
 	// 0x30
 	index := 0
 	if sigStr[index] != 0x30 {

btcec/ecdsa/signature_test.go

@@ -333,6 +333,21 @@ var signatureTests = []signatureTest{
 		der:     false,
 		isValid: false,
 	},
+	{
+		name: "Long signature.",
+		sig: []byte{0x30, 0x44, 0x02, 0x20, 0x4e, 0x45, 0xe1, 0x69,
+			0x32, 0xb8, 0xaf, 0x51, 0x49, 0x61, 0xa1, 0xd3, 0xa1,
+			0xa2, 0x5f, 0xdf, 0x3f, 0x4f, 0x77, 0x32, 0xe9, 0xd6,
+			0x24, 0xc6, 0xc6, 0x15, 0x48, 0xab, 0x5f, 0xb8, 0xcd,
+			0x41, 0x02, 0x20, 0x18, 0x15, 0x22, 0xec, 0x8e, 0xca,
+			0x07, 0xde, 0x48, 0x60, 0xa4, 0xac, 0xdd, 0x12, 0x90,
+			0x9d, 0x83, 0x1c, 0xc5, 0x6c, 0xbb, 0xac, 0x46, 0x22,
+			0x08, 0x22, 0x21, 0xa8, 0x76, 0x8d, 0x1d, 0x09, 0x91,
+			0x17, 0x90, 0xda, 0x42, 0xca, 0xaf, 0x19, 0x7d, 0xb4,
+		},
+		der:     true,
+		isValid: false,
+	},
 }
 
 func TestSignatures(t *testing.T) {