Trivial ASN.1 decoding functions.

src/crypto/asn1.c

+/*
+ * Copyright (C) 2007 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+#include <stdint.h>
+#include <stddef.h>
+#include <errno.h>
+#include <gpxe/asn1.h>
+
+/** @file
+ *
+ * ASN.1 encoding
+ *
+ */
+
+/**
+ * Start parsing ASN.1 object
+ *
+ * @v cursor		ASN.1 object cursor
+ * @v type		Expected type
+ * @ret len		Length of object body, or -1 on error
+ *
+ * The object cursor will be updated to point to the start of the
+ * object body (i.e. the first byte following the length byte(s)), and
+ * the length of the object body (i.e. the number of bytes until the
+ * following object tag, if any) is returned.
+ *
+ * If any error occurs (i.e. if the object is not of the expected
+ * type, or if we overflow beyond the end of the ASN.1 object), then
+ * the cursor will be invalidated and a negative value will be
+ * returned.
+ */
+static int asn1_start_object ( struct asn1_cursor *cursor,
+			       unsigned int type ) {
+	unsigned int len_len;
+	unsigned int len;
+
+	/* Sanity check */
+	if ( cursor->len < 2 /* Tag byte and first length byte */ ) {
+		if ( cursor->len )
+			DBGC ( cursor, "ASN1 %p too short\n", cursor );
+		goto notfound;
+	}
+
+	/* Check the tag byte */
+	if ( cursor->data[0] != type ) {
+		DBGC ( cursor, "ASN1 %p type mismatch (expected %d, got %d)\n",
+		       cursor, type, cursor->data[0] );
+		goto notfound;
+	}
+	cursor->data++;
+	cursor->len--;
+
+	/* Extract length of the length field and sanity check */
+	len_len = cursor->data[0];
+	if ( len_len & 0x80 ) {
+		len_len = ( len_len & 0x7f );
+		cursor->data++;
+		cursor->len--;
+	} else {
+		len_len = 1;
+	}
+	if ( cursor->len < len_len ) {
+		DBGC ( cursor, "ASN1 %p bad length field length %d (max %d)\n",
+		       cursor, len_len, cursor->len );
+		goto notfound;
+	}
+
+	/* Extract the length and sanity check */
+	for ( len = 0 ; len_len ; len_len-- ) {
+		len <<= 8;
+		len |= cursor->data[0];
+		cursor->data++;
+		cursor->len--;
+	}
+	if ( cursor->len < len ) {
+		DBGC ( cursor, "ASN1 %p bad length %d (max %d)\n",
+		       cursor, len, cursor->len );
+		goto notfound;
+	}
+
+	return len;
+
+ notfound:
+	cursor->data = NULL;
+	cursor->len = 0;
+	return -1;
+}
+
+/**
+ * Enter ASN.1 object
+ *
+ * @v cursor		ASN.1 object cursor
+ * @v type		Expected type
+ * @ret rc		Return status code
+ *
+ * The object cursor will be updated to point to the body of the
+ * current ASN.1 object.  If any error occurs, the object cursor will
+ * be invalidated.
+ */
+int asn1_enter_object ( struct asn1_cursor *cursor, unsigned int type ) {
+	int len;
+
+	len = asn1_start_object ( cursor, type );
+	if ( len < 0 )
+		return -ENOENT;
+
+	cursor->len = len;
+	DBGC ( cursor, "ASN1 %p entered object type %02x (len %x)\n",
+	       cursor, type, len );
+
+	return 0;
+}
+
+/**
+ * Skip ASN.1 object
+ *
+ * @v cursor		ASN.1 object cursor
+ * @v type		Expected type
+ * @ret rc		Return status code
+ *
+ * The object cursor will be updated to point to the next ASN.1
+ * object.  If any error occurs, the object cursor will be
+ * invalidated.
+ */
+int asn1_skip_object ( struct asn1_cursor *cursor, unsigned int type ) {
+	int len;
+
+	len = asn1_start_object ( cursor, type );
+	if ( len < 0 )
+		return -ENOENT;
+
+	cursor->data += len;
+	cursor->len -= len;
+	DBGC ( cursor, "ASN1 %p skipped object type %02x (len %x)\n",
+	       cursor, type, len );
+
+	if ( ! cursor->len ) {
+		DBGC ( cursor, "ASN1 %p reached end of object\n", cursor );
+		cursor->data = NULL;
+		return -ENOENT;
+	}
+
+	return 0;
+}

src/include/gpxe/asn1.h

+#ifndef _GPXE_ASN1_H
+#define _GPXE_ASN1_H
+
+/** @file
+ *
+ * ASN.1 encoding
+ *
+ */
+
+#define ASN1_INTEGER 0x02
+#define ASN1_BIT_STRING 0x03
+#define ASN1_OCTET_STRING 0x04
+#define ASN1_NULL 0x05
+#define ASN1_OID 0x06
+#define ASN1_SEQUENCE 0x30
+#define ASN1_IP_ADDRESS 0x40
+#define ASN1_EXPLICIT_TAG 0xa0
+
+/**
+ * A DER-encoded ASN.1 object cursor
+ */
+struct asn1_cursor {
+	/** Start of data */
+	uint8_t *data;
+	/** Length of data */
+	size_t len;
+};
+
+extern int asn1_enter_object ( struct asn1_cursor *cursor, unsigned int type );
+extern int asn1_skip_object ( struct asn1_cursor *cursor, unsigned int type );
+
+#endif /* _GPXE_ASN1_H */