[RFC PATCH 1/4] lib/asn1_encoder: Add a function to encode many byte integer values.

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

An example is the encoding of ECC signatures used by the ECDSA
signature verification code.  A user is the new SPDM support where
raw signatures are returned by the responder.  These can then
be encoded so that we can pass them to signature_verify()

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
---

Needs considerably more testing with crafted examples to hit the corner
cases.  For now it's enough to show how this 'might' fit together.

 include/linux/asn1_encoder.h |  3 ++
 lib/asn1_encoder.c           | 54 ++++++++++++++++++++++++++++++++++++
 2 files changed, 57 insertions(+)

diff --git a/include/linux/asn1_encoder.h b/include/linux/asn1_encoder.h
index 08cd0c2ad34f..30c3ebacd46c 100644
--- a/include/linux/asn1_encoder.h
+++ b/include/linux/asn1_encoder.h
@@ -19,6 +19,9 @@ unsigned char *
 asn1_encode_tag(unsigned char *data, const unsigned char *end_data,
 		u32 tag, const unsigned char *string, int len);
 unsigned char *
+asn1_encode_integer_large_positive(unsigned char *data, const unsigned char *end_data,
+			  u32 tag, const unsigned char *integer, int len);
+unsigned char *
 asn1_encode_octet_string(unsigned char *data,
 			 const unsigned char *end_data,
 			 const unsigned char *string, u32 len);
diff --git a/lib/asn1_encoder.c b/lib/asn1_encoder.c
index 41e71aae3ef6..135c5cf2d77e 100644
--- a/lib/asn1_encoder.c
+++ b/lib/asn1_encoder.c
@@ -317,6 +317,60 @@ asn1_encode_tag(unsigned char *data, const unsigned char *end_data,
 }
 EXPORT_SYMBOL_GPL(asn1_encode_tag);
 
+unsigned char *
+asn1_encode_integer_large_positive(unsigned char *data, const unsigned char *end_data,
+				   u32 tag, const unsigned char *integer, int len)
+{
+	int data_len = end_data - data;
+	unsigned char *d = &data[2];
+	bool found = false;
+	int i;
+
+	if (WARN(tag > 30, "ASN.1 tag can't be > 30"))
+		return ERR_PTR(-EINVAL);
+
+	if (!integer && WARN(len > 127,
+			    "BUG: recode tag is too big (>127)"))
+		return ERR_PTR(-EINVAL);
+
+	if (IS_ERR(data))
+		return data;
+
+	if (data_len < 3)
+		return ERR_PTR(-EINVAL);
+
+
+	data[0] = _tagn(UNIV, PRIM, tag);
+	/* Leave space for length */
+	data_len -= 2;
+
+	for (i = 0; i < len; i++) {
+		int byte = integer[i];
+
+		if (!found && byte == 0)
+			continue;
+
+		/*
+		 * as per encode_integer
+		 */
+		if (!found && (byte & 0x80)) {
+			*d++ = 0;
+			data_len--;
+		}
+		found = true;
+		if (data_len == 0)
+			return ERR_PTR(-EINVAL);
+
+		*d++ = byte;
+		data_len--;
+	}
+
+	data[1] = d - data - 2;
+
+	return d;
+}
+EXPORT_SYMBOL_GPL(asn1_encode_integer_large_positive);
+
 /**
  * asn1_encode_octet_string() - encode an ASN.1 OCTET STRING
  * @data:	pointer to encode at
-- 
2.19.1