[tls] Add full X.509 certificate parsing

Signed-off-by: Michael Brown <mcb30@ipxe.org>

1 files changed, 1156 insertions, 23 deletions

diff --git a/src/crypto/x509.c b/src/crypto/x509.c
index a13b671a..2a5e72ba 100644
--- a/src/crypto/x509.c
+++ b/src/crypto/x509.c
@@ -18,48 +18,1181 @@
 
 FILE_LICENCE ( GPL2_OR_LATER );
 
-#include <stdlib.h>
 #include <string.h>
+#include <ctype.h>
 #include <errno.h>
+#include <assert.h>
 #include <ipxe/asn1.h>
+#include <ipxe/crypto.h>
+#include <ipxe/md5.h>
+#include <ipxe/sha1.h>
+#include <ipxe/sha256.h>
+#include <ipxe/rsa.h>
+#include <ipxe/rootcert.h>
 #include <ipxe/x509.h>
 
 /** @file
  *
  * X.509 certificates
  *
- * The structure of X.509v3 certificates is concisely documented in
- * RFC5280 section 4.1.  The structure of RSA public keys is
- * documented in RFC2313.
+ * The structure of X.509v3 certificates is documented in RFC 5280
+ * section 4.1.
  */
 
+/* Disambiguate the various error causes */
+#define ENOTSUP_ALGORITHM \
+	__einfo_error ( EINFO_ENOTSUP_ALGORITHM )
+#define EINFO_ENOTSUP_ALGORITHM \
+	__einfo_uniqify ( EINFO_ENOTSUP, 0x01, "Unsupported algorithm" )
+#define ENOTSUP_EXTENSION \
+	__einfo_error ( EINFO_ENOTSUP_EXTENSION )
+#define EINFO_ENOTSUP_EXTENSION \
+	__einfo_uniqify ( EINFO_ENOTSUP, 0x02, "Unsupported extension" )
+#define EINVAL_NON_SIGNATURE \
+	__einfo_error ( EINFO_EINVAL_NON_SIGNATURE )
+#define EINFO_EINVAL_NON_SIGNATURE \
+	__einfo_uniqify ( EINFO_EINVAL, 0x01, "Not a signature algorithm" )
+#define EINVAL_BIT_STRING \
+	__einfo_error ( EINFO_EINVAL_BIT_STRING )
+#define EINFO_EINVAL_BIT_STRING \
+	__einfo_uniqify ( EINFO_EINVAL, 0x02, "Invalid bit string" )
+#define EINVAL_TIME \
+	__einfo_error ( EINFO_EINVAL_TIME )
+#define EINFO_EINVAL_TIME \
+	__einfo_uniqify ( EINFO_EINVAL, 0x03, "Invalid time" )
+#define EINVAL_ALGORITHM_MISMATCH \
+	__einfo_error ( EINFO_EINVAL_ALGORITHM_MISMATCH )
+#define EINFO_EINVAL_ALGORITHM_MISMATCH \
+	__einfo_uniqify ( EINFO_EINVAL, 0x04, "Signature algorithm mismatch" )
+#define EINVAL_PATH_LEN \
+	__einfo_error ( EINFO_EINVAL_PATH_LEN )
+#define EINFO_EINVAL_PATH_LEN \
+	__einfo_uniqify ( EINFO_EINVAL, 0x05, "Invalid pathLenConstraint" )
+#define EINVAL_VERSION \
+	__einfo_error ( EINFO_EINVAL_VERSION )
+#define EINFO_EINVAL_VERSION \
+	__einfo_uniqify ( EINFO_EINVAL, 0x06, "Invalid version" )
+#define EACCES_WRONG_ISSUER \
+	__einfo_error ( EINFO_EACCES_WRONG_ISSUER )
+#define EINFO_EACCES_WRONG_ISSUER \
+	__einfo_uniqify ( EINFO_EACCES, 0x01, "Wrong issuer" )
+#define EACCES_NOT_CA \
+	__einfo_error ( EINFO_EACCES_NOT_CA )
+#define EINFO_EACCES_NOT_CA \
+	__einfo_uniqify ( EINFO_EACCES, 0x02, "Not a CA certificate" )
+#define EACCES_KEY_USAGE \
+	__einfo_error ( EINFO_EACCES_KEY_USAGE )
+#define EINFO_EACCES_KEY_USAGE \
+	__einfo_uniqify ( EINFO_EACCES, 0x03, "Incorrect key usage" )
+
+/** "commonName" object identifier */
+static uint8_t oid_common_name[] = { ASN1_OID_COMMON_NAME };
+
+/** "commonName" object identifier cursor */
+static struct asn1_cursor oid_common_name_cursor =
+	ASN1_OID_CURSOR ( oid_common_name );
+
+/** "rsaEncryption" object identifier */
+static uint8_t oid_rsa_encryption[] = { ASN1_OID_RSAENCRYPTION };
+
+/** "md5WithRSAEncryption" object identifier */
+static uint8_t oid_md5_with_rsa_encryption[] =
+	{ ASN1_OID_MD5WITHRSAENCRYPTION };
+
+/** "sha1WithRSAEncryption" object identifier */
+static uint8_t oid_sha1_with_rsa_encryption[] =
+	{ ASN1_OID_SHA1WITHRSAENCRYPTION };
+
+/** "sha256WithRSAEncryption" object identifier */
+static uint8_t oid_sha256_with_rsa_encryption[] =
+	{ ASN1_OID_SHA256WITHRSAENCRYPTION };
+
+/** Supported algorithms */
+static struct x509_algorithm x509_algorithms[] = {
+	{
+		.name = "rsaEncryption",
+		.pubkey = &rsa_algorithm,
+		.digest = NULL,
+		.oid = ASN1_OID_CURSOR ( oid_rsa_encryption ),
+	},
+	{
+		.name = "md5WithRSAEncryption",
+		.pubkey = &rsa_algorithm,
+		.digest = &md5_algorithm,
+		.oid = ASN1_OID_CURSOR ( oid_md5_with_rsa_encryption ),
+	},
+	{
+		.name = "sha1WithRSAEncryption",
+		.pubkey = &rsa_algorithm,
+		.digest = &sha1_algorithm,
+		.oid = ASN1_OID_CURSOR ( oid_sha1_with_rsa_encryption ),
+	},
+	{
+		.name = "sha256WithRSAEncryption",
+		.pubkey = &rsa_algorithm,
+		.digest = &sha256_algorithm,
+		.oid = ASN1_OID_CURSOR ( oid_sha256_with_rsa_encryption ),
+	},
+};
+
+/**
+ * Identify X.509 algorithm by OID
+ *
+ * @v oid		OID
+ * @ret algorithm	Algorithm, or NULL
+ */
+static struct x509_algorithm *
+x509_find_algorithm ( const struct asn1_cursor *oid ) {
+	struct x509_algorithm *algorithm;
+	unsigned int i;
+
+	for ( i = 0 ; i < ( sizeof ( x509_algorithms ) /
+			    sizeof ( x509_algorithms[0] ) ) ; i++ ) {
+		algorithm = &x509_algorithms[i];
+		if ( asn1_compare ( &algorithm->oid, oid ) == 0 )
+			return algorithm;
+	}
+
+	return NULL;
+}
+
+/**
+ * Parse X.509 certificate algorithm
+ *
+ * @v cert		X.509 certificate
+ * @v algorithm		Algorithm to fill in
+ * @v raw		ASN.1 cursor
+ * @ret rc		Return status code
+ */
+static int x509_parse_algorithm ( struct x509_certificate *cert,
+				  struct x509_algorithm **algorithm,
+				  const struct asn1_cursor *raw ) {
+	struct asn1_cursor cursor;
+	int rc;
+
+	/* Enter signatureAlgorithm */
+	memcpy ( &cursor, raw, sizeof ( cursor ) );
+	asn1_enter ( &cursor, ASN1_SEQUENCE );
+
+	/* Enter algorithm */
+	if ( ( rc = asn1_enter ( &cursor, ASN1_OID ) ) != 0 ) {
+		DBGC ( cert, "X509 %p cannot locate algorithm:\n", cert );
+		DBGC_HDA ( cert, 0, raw->data, raw->len );
+		return rc;
+	}
+
+	/* Identify algorithm */
+	*algorithm = x509_find_algorithm ( &cursor );
+	if ( ! *algorithm ) {
+		DBGC ( cert, "X509 %p unsupported algorithm:\n", cert );
+		DBGC_HDA ( cert, 0, cursor.data, cursor.len );
+		return -ENOTSUP_ALGORITHM;
+	}
+
+	return 0;
+}
+
+/**
+ * Parse X.509 certificate signature algorithm
+ *
+ * @v cert		X.509 certificate
+ * @v algorithm		Algorithm to fill in
+ * @v raw		ASN.1 cursor
+ * @ret rc		Return status code
+ */
+static int x509_parse_signature_algorithm ( struct x509_certificate *cert,
+					    struct x509_algorithm **algorithm,
+					    const struct asn1_cursor *raw ) {
+	int rc;
+
+	/* Parse algorithm */
+	if ( ( rc = x509_parse_algorithm ( cert, algorithm, raw ) ) != 0 )
+		return rc;
+
+	/* Check algorithm is a signature algorithm */
+	if ( ! x509_is_signature_algorithm ( *algorithm ) ) {
+		DBGC ( cert, "X509 %p algorithm %s is not a signature "
+		       "algorithm:\n", cert, (*algorithm)->name );
+		DBGC_HDA ( cert, 0, raw->data, raw->len );
+		return -EINVAL_NON_SIGNATURE;
+	}
+
+	return 0;
+}
+
+/**
+ * Parse X.509 certificate bit string
+ *
+ * @v cert		X.509 certificate
+ * @v bits		Bit string to fill in
+ * @v raw		ASN.1 cursor
+ * @ret rc		Return status code
+ */
+static int x509_parse_bit_string ( struct x509_certificate *cert,
+				   struct x509_bit_string *bits,
+				   const struct asn1_cursor *raw ) {
+	struct asn1_cursor cursor;
+	const struct asn1_bit_string *bit_string;
+	size_t len;
+	unsigned int unused;
+	uint8_t unused_mask;
+	const uint8_t *last;
+	int rc;
+
+	/* Enter bit string */
+	memcpy ( &cursor, raw, sizeof ( cursor ) );
+	if ( ( rc = asn1_enter ( &cursor, ASN1_BIT_STRING ) ) != 0 ) {
+		DBGC ( cert, "X509 %p cannot locate bit string:\n", cert );
+		DBGC_HDA ( cert, 0, raw->data, raw->len );
+		return rc;
+	}
+
+	/* Validity checks */
+	if ( cursor.len < sizeof ( *bit_string ) ) {
+		DBGC ( cert, "X509 %p invalid bit string:\n", cert );
+		DBGC_HDA ( cert, 0, raw->data, raw->len );
+		return -EINVAL_BIT_STRING;
+	}
+	bit_string = cursor.data;
+	len = ( cursor.len - offsetof ( typeof ( *bit_string ), data ) );
+	unused = bit_string->unused;
+	unused_mask = ( 0xff >> ( 8 - unused ) );
+	last = ( bit_string->data + len - 1 );
+	if ( ( unused >= 8 ) ||
+	     ( ( unused > 0 ) && ( len == 0 ) ) ||
+	     ( ( *last & unused_mask ) != 0 ) ) {
+		DBGC ( cert, "X509 %p invalid bit string:\n", cert );
+		DBGC_HDA ( cert, 0, raw->data, raw->len );
+		return -EINVAL_BIT_STRING;
+	}
+
+	/* Populate bit string */
+	bits->data = &bit_string->data;
+	bits->len = len;
+	bits->unused = unused;
+
+	return 0;
+}
+
+/**
+ * Parse X.509 certificate bit string that must be an integral number of bytes
+ *
+ * @v cert		X.509 certificate
+ * @v bits		Bit string to fill in
+ * @v raw		ASN.1 cursor
+ * @ret rc		Return status code
+ */
+static int x509_parse_integral_bit_string ( struct x509_certificate *cert,
+					    struct x509_bit_string *bits,
+					    const struct asn1_cursor *raw ) {
+	int rc;
+
+	/* Parse bit string */
+	if ( ( rc = x509_parse_bit_string ( cert, bits, raw ) ) != 0 )
+		return rc;
+
+	/* Check that there are no unused bits at end of string */
+	if ( bits->unused ) {
+		DBGC ( cert, "X509 %p invalid integral bit string:\n", cert );
+		DBGC_HDA ( cert, 0, raw->data, raw->len );
+		return -EINVAL_BIT_STRING;
+	}
+
+	return 0;
+}
+
+/**
+ * Parse X.509 certificate time
+ *
+ * @v cert		X.509 certificate
+ * @v time		Time to fill in
+ * @v raw		ASN.1 cursor
+ * @ret rc		Return status code
+ *
+ * RFC 5280 section 4.1.2.5 places several restrictions on the allowed
+ * formats for UTCTime and GeneralizedTime, and mandates the
+ * interpretation of centuryless year values.
+ */
+static int x509_parse_time ( struct x509_certificate *cert,
+			     struct x509_time *time,
+			     const struct asn1_cursor *raw ) {
+	struct asn1_cursor cursor;
+	unsigned int have_century;
+	unsigned int type;
+	union {
+		struct {
+			uint8_t century;
+			uint8_t year;
+			uint8_t month;
+			uint8_t day;
+			uint8_t hour;
+			uint8_t minute;
+			uint8_t second;
+		} __attribute__ (( packed )) named;
+		uint8_t raw[7];
+	} pairs;
+	const uint8_t *data;
+	size_t remaining;
+	unsigned int tens;
+	unsigned int units;
+	unsigned int i;
+	int rc;
+
+	/* Determine time format utcTime/generalizedTime */
+	memcpy ( &cursor, raw, sizeof ( cursor ) );
+	type = asn1_type ( &cursor );
+	switch ( type ) {
+	case ASN1_UTC_TIME:
+		have_century = 0;
+		break;
+	case ASN1_GENERALIZED_TIME:
+		have_century = 1;
+		break;
+	default:
+		DBGC ( cert, "X509 %p invalid time type %02x\n", cert, type );
+		DBGC_HDA ( cert, 0, raw->data, raw->len );
+		return -EINVAL_TIME;
+	}
+
+	/* Enter utcTime/generalizedTime */
+	if ( ( rc = asn1_enter ( &cursor, type ) ) != 0 ) {
+		DBGC ( cert, "X509 %p cannot locate %s time:\n", cert,
+		       ( ( type == ASN1_UTC_TIME ) ? "UTC" : "generalized" ) );
+		DBGC_HDA ( cert, 0, raw->data, raw->len );
+		return rc;
+	}
+
+	/* Parse digit string a pair at a time */
+	data = cursor.data;
+	remaining = cursor.len;
+	for ( i = ( have_century ? 0 : 1 ) ; i < sizeof ( pairs.raw ) ; i++ ) {
+		if ( remaining < 2 ) {
+			DBGC ( cert, "X509 %p invalid time:\n", cert );
+			DBGC_HDA ( cert, 0, raw->data, raw->len );
+			return -EINVAL_TIME;
+		}
+		tens = data[0];
+		units = data[1];
+		if ( ! ( isdigit ( tens ) && isdigit ( units ) ) ) {
+			DBGC ( cert, "X509 %p invalid time:\n", cert );
+			DBGC_HDA ( cert, 0, raw->data, raw->len );
+			return -EINVAL_TIME;
+		}
+		pairs.raw[i] = ( ( 10 * ( tens - '0' ) ) + ( units - '0' ) );
+		data += 2;
+		remaining -= 2;
+	}
+
+	/* Determine century if applicable */
+	if ( ! have_century )
+		pairs.named.century = ( ( pairs.named.year >= 50 ) ? 19 : 20 );
+
+	/* Check for trailing "Z" */
+	if ( ( remaining != 1 ) || ( data[0] != 'Z' ) ) {
+		DBGC ( cert, "X509 %p invalid time:\n", cert );
+		DBGC_HDA ( cert, 0, raw->data, raw->len );
+		return -EINVAL_TIME;
+	}
+
+	/* Fill in time */
+	time->year = ( ( pairs.named.century * 100 ) + pairs.named.year );
+	time->month = pairs.named.month;
+	time->day = pairs.named.day;
+	time->hour = pairs.named.hour;
+	time->minute = pairs.named.minute;
+	time->second = pairs.named.second;
+
+	return 0;
+}
+
+/**
+ * Parse X.509 certificate version
+ *
+ * @v cert		X.509 certificate
+ * @v raw		ASN.1 cursor
+ * @ret rc		Return status code
+ */
+static int x509_parse_version ( struct x509_certificate *cert,
+				const struct asn1_cursor *raw ) {
+	struct asn1_cursor cursor;
+	int version;
+	int rc;
+
+	/* Enter version */
+	memcpy ( &cursor, raw, sizeof ( cursor ) );
+	asn1_enter ( &cursor, ASN1_EXPLICIT_TAG ( 0 ) );
+
+	/* Parse integer */
+	if ( ( rc = asn1_integer ( &cursor, &version ) ) != 0 ) {
+		DBGC ( cert, "X509 %p cannot parse version: %s\n",
+		       cert, strerror ( rc ) );
+		DBGC_HDA ( cert, 0, raw->data, raw->len );
+		return rc;
+	}
+
+	/* Sanity check */
+	if ( version < 0 ) {
+		DBGC ( cert, "X509 %p invalid version %d\n", cert, version );
+		DBGC_HDA ( cert, 0, raw->data, raw->len );
+		return -EINVAL_VERSION;
+	}
+
+	/* Record version */
+	cert->version = version;
+	DBGC ( cert, "X509 %p is a version %d certificate\n",
+	       cert, ( cert->version + 1 ) );
+
+	return 0;
+}
+
+/**
+ * Parse X.509 certificate issuer
+ *
+ * @v cert		X.509 certificate
+ * @v raw		ASN.1 cursor
+ * @ret rc		Return status code
+ */
+static int x509_parse_issuer ( struct x509_certificate *cert,
+			       const struct asn1_cursor *raw ) {
+	struct x509_issuer *issuer = &cert->issuer;
+	int rc;
+
+	/* Record raw issuer */
+	memcpy ( &issuer->raw, raw, sizeof ( issuer->raw ) );
+	if ( ( rc = asn1_shrink ( &issuer->raw, ASN1_SEQUENCE ) ) != 0 ) {
+		DBGC ( cert, "X509 %p cannot shrink issuer: %s\n",
+		       cert, strerror ( rc ) );
+		return rc;
+	}
+	DBGC ( cert, "X509 %p issuer is:\n", cert );
+	DBGC_HDA ( cert, 0, issuer->raw.data, issuer->raw.len );
+
+	return 0;
+}
+
+/**
+ * Parse X.509 certificate validity
+ *
+ * @v cert		X.509 certificate
+ * @v raw		ASN.1 cursor
+ * @ret rc		Return status code
+ */
+static int x509_parse_validity ( struct x509_certificate *cert,
+				 const struct asn1_cursor *raw ) {
+	struct x509_validity *validity = &cert->validity;
+	struct x509_time *not_before = &validity->not_before;
+	struct x509_time *not_after = &validity->not_after;
+	struct asn1_cursor cursor;
+	int rc;
+
+	/* Enter validity */
+	memcpy ( &cursor, raw, sizeof ( cursor ) );
+	asn1_enter ( &cursor, ASN1_SEQUENCE );
+
+	/* Parse notBefore */
+	if ( ( rc = x509_parse_time ( cert, not_before, &cursor ) ) != 0 )
+		return rc;
+	DBGC ( cert, "X509 %p valid from %04d-%02d-%02d %02d:%02d:%02d\n",
+	       cert, not_before->year, not_before->month, not_before->day,
+	       not_before->hour, not_before->minute, not_before->second );
+	asn1_skip_any ( &cursor );
+
+	/* Parse notAfter */
+	if ( ( rc = x509_parse_time ( cert, not_after, &cursor ) ) != 0 )
+		return rc;
+	DBGC ( cert, "X509 %p valid until %04d-%02d-%02d %02d:%02d:%02d\n",
+	       cert, not_after->year, not_after->month, not_after->day,
+	       not_after->hour, not_after->minute, not_after->second );
+
+	return 0;
+}
+
+/**
+ * Parse X.509 certificate common name
+ *
+ * @v cert		X.509 certificate
+ * @v name		Common name to fill in
+ * @v raw		ASN.1 cursor
+ * @ret rc		Return status code
+ */
+static int x509_parse_common_name ( struct x509_certificate *cert,
+				    struct x509_name *name,
+				    const struct asn1_cursor *raw ) {
+	struct asn1_cursor cursor;
+	struct asn1_cursor oid_cursor;
+	struct asn1_cursor name_cursor;
+	int rc;
+
+	/* Enter name */
+	memcpy ( &cursor, raw, sizeof ( cursor ) );
+	asn1_enter ( &cursor, ASN1_SEQUENCE );
+
+	/* Scan through name list */
+	for ( ; cursor.len ; asn1_skip_any ( &cursor ) ) {
+		memcpy ( &oid_cursor, &cursor, sizeof ( oid_cursor ) );
+		asn1_enter ( &oid_cursor, ASN1_SET );
+		asn1_enter ( &oid_cursor, ASN1_SEQUENCE );
+		memcpy ( &name_cursor, &oid_cursor, sizeof ( name_cursor ) );
+		asn1_enter ( &oid_cursor, ASN1_OID );
+		if ( asn1_compare ( &oid_common_name_cursor, &oid_cursor ) != 0)
+			continue;
+		asn1_skip_any ( &name_cursor );
+		if ( ( rc = asn1_enter_any ( &name_cursor ) ) != 0 ) {
+			DBGC ( cert, "X509 %p cannot locate name:\n", cert );
+			DBGC_HDA ( cert, 0, raw->data, raw->len );
+			return rc;
+		}
+		name->data = name_cursor.data;
+		name->len = name_cursor.len;
+		return 0;
+	}
+
+	DBGC ( cert, "X509 %p no commonName found:\n", cert );
+	DBGC_HDA ( cert, 0, raw->data, raw->len );
+	return -ENOENT;
+}
+
+/**
+ * Parse X.509 certificate subject
+ *
+ * @v cert		X.509 certificate
+ * @v raw		ASN.1 cursor
+ * @ret rc		Return status code
+ */
+static int x509_parse_subject ( struct x509_certificate *cert,
+				const struct asn1_cursor *raw ) {
+	struct x509_subject *subject = &cert->subject;
+	struct x509_name *name = &subject->name;
+	int rc;
+
+	/* Record raw subject */
+	memcpy ( &subject->raw, raw, sizeof ( subject->raw ) );
+	asn1_shrink_any ( &subject->raw );
+	DBGC ( cert, "X509 %p subject is:\n", cert );
+	DBGC_HDA ( cert, 0, subject->raw.data, subject->raw.len );
+
+	/* Parse common name */
+	if ( ( rc = x509_parse_common_name ( cert, name, raw ) ) != 0 )
+		return rc;
+	DBGC ( cert, "X509 %p common name is:\n", cert );
+	DBGC_HDA ( cert, 0, name->data, name->len );
+
+	return 0;
+}
+
+/**
+ * Parse X.509 certificate public key information
+ *
+ * @v cert		X.509 certificate
+ * @v raw		ASN.1 cursor
+ * @ret rc		Return status code
+ */
+static int x509_parse_public_key ( struct x509_certificate *cert,
+				   const struct asn1_cursor *raw ) {
+	struct x509_public_key *public_key = &cert->subject.public_key;
+	struct x509_algorithm **algorithm = &public_key->algorithm;
+	struct asn1_cursor cursor;
+	int rc;
+
+	/* Record raw subjectPublicKeyInfo */
+	memcpy ( &cursor, raw, sizeof ( cursor ) );
+	asn1_shrink_any ( &cursor );
+	memcpy ( &public_key->raw, &cursor, sizeof ( public_key->raw ) );
+
+	/* Enter subjectPublicKeyInfo */
+	asn1_enter ( &cursor, ASN1_SEQUENCE );
+
+	/* Parse algorithm */
+	if ( ( rc = x509_parse_algorithm ( cert, algorithm, &cursor ) ) != 0 )
+		return rc;
+	DBGC ( cert, "X509 %p public key algorithm is %s\n",
+	       cert, (*algorithm)->name );
+	DBGC ( cert, "X509 %p public key is:\n", cert );
+	DBGC_HDA ( cert, 0, public_key->raw.data, public_key->raw.len );
+
+	return 0;
+}
+
+/**
+ * Parse X.509 certificate basic constraints
+ *
+ * @v cert		X.509 certificate
+ * @v raw		ASN.1 cursor
+ * @ret rc		Return status code
+ */
+static int x509_parse_basic_constraints ( struct x509_certificate *cert,
+					  const struct asn1_cursor *raw ) {
+	struct x509_basic_constraints *basic = &cert->extensions.basic;
+	struct asn1_cursor cursor;
+	int ca = 0;
+	int path_len;
+	int rc;
+
+	/* Enter basicConstraints */
+	memcpy ( &cursor, raw, sizeof ( cursor ) );
+	asn1_enter ( &cursor, ASN1_SEQUENCE );
+
+	/* Parse "cA", if present */
+	if ( asn1_type ( &cursor ) == ASN1_BOOLEAN ) {
+		ca = asn1_boolean ( &cursor );
+		if ( ca < 0 ) {
+			rc = ca;
+			DBGC ( cert, "X509 %p cannot parse cA: %s\n",
+			       cert, strerror ( rc ) );
+			DBGC_HDA ( cert, 0, raw->data, raw->len );
+			return rc;
+		}
+		asn1_skip_any ( &cursor );
+	}
+	basic->ca = ca;
+	DBGC ( cert, "X509 %p is %sa CA certificate\n",
+	       cert, ( basic->ca ? "" : "not " ) );
+
+	/* Ignore everything else unless "cA" is true */
+	if ( ! ca )
+		return 0;
+
+	/* Parse "pathLenConstraint", if present and applicable */
+	basic->path_len = -1U; /* Default is unlimited */
+	if ( asn1_type ( &cursor ) == ASN1_INTEGER ) {
+		if ( ( rc = asn1_integer ( &cursor, &path_len ) ) != 0 ) {
+			DBGC ( cert, "X509 %p cannot parse pathLenConstraint: "
+			       "%s\n", cert, strerror ( rc ) );
+			DBGC_HDA ( cert, 0, raw->data, raw->len );
+			return rc;
+		}
+		if ( path_len < 0 ) {
+			DBGC ( cert, "X509 %p invalid pathLenConstraint %d\n",
+			       cert, path_len );
+			DBGC_HDA ( cert, 0, raw->data, raw->len );
+			return -EINVAL;
+		}
+		basic->path_len = path_len;
+		DBGC ( cert, "X509 %p path length constraint is %u\n",
+		       cert, basic->path_len );
+	}
+
+	return 0;
+}
+
 /**
- * Identify X.509 certificate RSA public key
+ * Parse X.509 certificate key usage
  *
- * @v certificate	Certificate
- * @v rsa		RSA public key to fill in
+ * @v cert		X.509 certificate
+ * @v raw		ASN.1 cursor
  * @ret rc		Return status code
  */
-int x509_rsa_public_key ( const struct asn1_cursor *certificate,
-			  struct x509_rsa_public_key *key ) {
-	struct asn1_cursor *cursor = &key->raw;
+static int x509_parse_key_usage ( struct x509_certificate *cert,
+				  const struct asn1_cursor *raw ) {
+	struct x509_key_usage *usage = &cert->extensions.usage;
+	struct x509_bit_string bit_string;
+	const uint8_t *bytes;
+	size_t len;
+	unsigned int i;
 	int rc;
 
-	/* Locate subjectPublicKeyInfo */
-	memcpy ( cursor, certificate, sizeof ( *cursor ) );
-	rc = ( asn1_enter ( cursor, ASN1_SEQUENCE ), /* Certificate */
-	       asn1_enter ( cursor, ASN1_SEQUENCE ), /* tbsCertificate */
-	       asn1_skip_if_exists ( cursor, ASN1_EXPLICIT_TAG(0) ),/*version*/
-	       asn1_skip ( cursor, ASN1_INTEGER ), /* serialNumber */
-	       asn1_skip ( cursor, ASN1_SEQUENCE ), /* signature */
-	       asn1_skip ( cursor, ASN1_SEQUENCE ), /* issuer */
-	       asn1_skip ( cursor, ASN1_SEQUENCE ), /* validity */
-	       asn1_skip ( cursor, ASN1_SEQUENCE ) /* name */ );
-	if ( rc != 0 ) {
-		DBG ( "Cannot locate subjectPublicKeyInfo in:\n" );
-		DBG_HDA ( 0, certificate->data, certificate->len );
+	/* Mark extension as present */
+	usage->present = 1;
+
+	/* Parse bit string */
+	if ( ( rc = x509_parse_bit_string ( cert, &bit_string, raw ) ) != 0 )
+		return rc;
+
+	/* Parse key usage bits */
+	bytes = bit_string.data;
+	len = bit_string.len;
+	if ( len > sizeof ( usage->bits ) )
+		len = sizeof ( usage->bits );
+	for ( i = 0 ; i < len ; i++ ) {
+		usage->bits |= ( *(bytes++) << ( 8 * i ) );
+	}
+	DBGC ( cert, "X509 %p key usage is %08x\n", cert, usage->bits );
+
+	return 0;
+}
+
+/** "id-ce-basicConstraints" object identifier */
+static uint8_t oid_ce_basic_constraints[] = { ASN1_OID_BASICCONSTRAINTS };
+
+/** "id-ce-keyUsage" object identifier */
+static uint8_t oid_ce_key_usage[] = { ASN1_OID_KEYUSAGE };
+
+/** Supported certificate extensions */
+static struct x509_extension x509_extensions[] = {
+	{
+		.name = "basicConstraints",
+		.oid = ASN1_OID_CURSOR ( oid_ce_basic_constraints ),
+		.parse = x509_parse_basic_constraints,
+	},
+	{
+		.name = "keyUsage",
+		.oid = ASN1_OID_CURSOR ( oid_ce_key_usage ),
+		.parse = x509_parse_key_usage,
+	},
+};
+
+/**
+ * Identify X.509 extension by OID
+ *
+ * @v oid		OID
+ * @ret extension	Extension, or NULL
+ */
+static struct x509_extension *
+x509_find_extension ( const struct asn1_cursor *oid ) {
+	struct x509_extension *extension;
+	unsigned int i;
+
+	for ( i = 0 ; i < ( sizeof ( x509_extensions ) /
+			    sizeof ( x509_extensions[0] ) ) ; i++ ) {
+		extension = &x509_extensions[i];
+		if ( asn1_compare ( &extension->oid, oid ) == 0 )
+			return extension;
+	}
+
+	return NULL;
+}
+
+/**
+ * Parse X.509 certificate extension
+ *
+ * @v cert		X.509 certificate
+ * @v raw		ASN.1 cursor
+ * @ret rc		Return status code
+ */
+static int x509_parse_extension ( struct x509_certificate *cert,
+				  const struct asn1_cursor *raw ) {
+	struct asn1_cursor cursor;
+	struct asn1_cursor subcursor;
+	struct x509_extension *extension;
+	int is_critical = 0;
+	int rc;
+
+	/* Enter extension */
+	memcpy ( &cursor, raw, sizeof ( cursor ) );
+	asn1_enter ( &cursor, ASN1_SEQUENCE );
+
+	/* Try to identify extension */
+	memcpy ( &subcursor, &cursor, sizeof ( subcursor ) );
+	asn1_enter ( &subcursor, ASN1_OID );
+	extension = x509_find_extension ( &subcursor );
+	asn1_skip_any ( &cursor );
+	DBGC ( cert, "X509 %p found extension %s\n",
+	       cert, ( extension ? extension->name : "<unknown>" ) );
+
+	/* Identify criticality */
+	if ( asn1_type ( &cursor ) == ASN1_BOOLEAN ) {
+		is_critical = asn1_boolean ( &cursor );
+		if ( is_critical < 0 ) {
+			rc = is_critical;
+			DBGC ( cert, "X509 %p cannot parse extension "
+			       "criticality: %s\n", cert, strerror ( rc ) );
+			DBGC_HDA ( cert, 0, raw->data, raw->len );
+			return rc;
+		}
+		asn1_skip_any ( &cursor );
+	}
+
+	/* Handle unknown extensions */
+	if ( ! extension ) {
+		if ( is_critical ) {
+			/* Fail if we cannot handle a critical extension */
+			DBGC ( cert, "X509 %p cannot handle critical "
+			       "extension:\n", cert );
+			DBGC_HDA ( cert, 0, raw->data, raw->len );
+			return -ENOTSUP_EXTENSION;
+		} else {
+			/* Ignore unknown non-critical extensions */
+			return 0;
+		}
+	};
+
+	/* Extract extnValue */
+	if ( ( rc = asn1_enter ( &cursor, ASN1_OCTET_STRING ) ) != 0 ) {
+		DBGC ( cert, "X509 %p extension missing extnValue:\n", cert );
+		DBGC_HDA ( cert, 0, raw->data, raw->len );
+		return rc;
+	}
+
+	/* Parse extension */
+	if ( ( rc = extension->parse ( cert, &cursor ) ) != 0 )
 		return rc;
+
+	return 0;
+}
+
+/**
+ * Parse X.509 certificate extensions, if present
+ *
+ * @v cert		X.509 certificate
+ * @v raw		ASN.1 cursor
+ * @ret rc		Return status code
+ */
+static int x509_parse_extensions ( struct x509_certificate *cert,
+				   const struct asn1_cursor *raw ) {
+	struct asn1_cursor cursor;
+	int rc;
+
+	/* Enter extensions, if present */
+	memcpy ( &cursor, raw, sizeof ( cursor ) );
+	asn1_enter ( &cursor, ASN1_EXPLICIT_TAG ( 3 ) );
+	asn1_enter ( &cursor, ASN1_SEQUENCE );
+
+	/* Parse each extension in turn */
+	while ( cursor.len ) {
+		if ( ( rc = x509_parse_extension ( cert, &cursor ) ) != 0 )
+			return rc;
+		asn1_skip_any ( &cursor );
 	}
 
 	return 0;
 }
+
+/**
+ * Parse X.509 certificate tbsCertificate
+ *
+ * @v cert		X.509 certificate
+ * @v raw		ASN.1 cursor
+ * @ret rc		Return status code
+ */
+static int x509_parse_tbscertificate ( struct x509_certificate *cert,
+				       const struct asn1_cursor *raw ) {
+	struct x509_algorithm **algorithm = &cert->signature_algorithm;
+	struct asn1_cursor cursor;
+	int rc;
+
+	/* Record raw tbsCertificate */
+	memcpy ( &cursor, raw, sizeof ( cursor ) );
+	asn1_shrink_any ( &cursor );
+	memcpy ( &cert->tbs, &cursor, sizeof ( cert->tbs ) );
+
+	/* Enter tbsCertificate */
+	asn1_enter ( &cursor, ASN1_SEQUENCE );
+
+	/* Parse version, if present */
+	if ( asn1_type ( &cursor ) == ASN1_EXPLICIT_TAG ( 0 ) ) {
+		if ( ( rc = x509_parse_version ( cert, &cursor ) ) != 0 )
+			return rc;
+		asn1_skip_any ( &cursor );
+	}
+
+	/* Skip serialNumber */
+	asn1_skip ( &cursor, ASN1_INTEGER );
+
+	/* Parse signature */
+	if ( ( rc = x509_parse_signature_algorithm ( cert, algorithm,
+						     &cursor ) ) != 0 )
+		return rc;
+	DBGC ( cert, "X509 %p tbsCertificate signature algorithm is %s\n",
+	       cert, (*algorithm)->name );
+	asn1_skip_any ( &cursor );
+
+	/* Parse issuer */
+	if ( ( rc = x509_parse_issuer ( cert, &cursor ) ) != 0 )
+		return rc;
+	asn1_skip_any ( &cursor );
+
+	/* Parse validity */
+	if ( ( rc = x509_parse_validity ( cert, &cursor ) ) != 0 )
+		return rc;
+	asn1_skip_any ( &cursor );
+
+	/* Parse subject */
+	if ( ( rc = x509_parse_subject ( cert, &cursor ) ) != 0 )
+		return rc;
+	asn1_skip_any ( &cursor );
+
+	/* Parse subjectPublicKeyInfo */
+	if ( ( rc = x509_parse_public_key ( cert, &cursor ) ) != 0 )
+		return rc;
+	asn1_skip_any ( &cursor );
+
+	/* Parse extensions, if present */
+	if ( ( rc = x509_parse_extensions ( cert, &cursor ) ) != 0 )
+		return rc;
+
+	return 0;
+}
+
+/**
+ * Parse X.509 certificate from ASN.1 data
+ *
+ * @v cert		X.509 certificate
+ * @v data		Raw certificate data
+ * @v len		Length of raw data
+ * @ret rc		Return status code
+ */
+int x509_parse ( struct x509_certificate *cert, const void *data, size_t len ) {
+	struct x509_signature *signature = &cert->signature;
+	struct x509_algorithm **signature_algorithm = &signature->algorithm;
+	struct x509_bit_string *signature_value = &signature->value;
+	struct asn1_cursor cursor;
+	int rc;
+
+	/* Initialise certificate */
+	memset ( cert, 0, sizeof ( *cert ) );
+	cert->raw.data = data;
+	cert->raw.len = len;
+
+	/* Enter certificate */
+	memcpy ( &cursor, &cert->raw, sizeof ( cursor ) );
+	asn1_enter ( &cursor, ASN1_SEQUENCE );
+
+	/* Parse tbsCertificate */
+	if ( ( rc = x509_parse_tbscertificate ( cert, &cursor ) ) != 0 )
+		return rc;
+	asn1_skip_any ( &cursor );
+
+	/* Parse signatureAlgorithm */
+	if ( ( rc = x509_parse_signature_algorithm ( cert, signature_algorithm,
+						     &cursor ) ) != 0 )
+		return rc;
+	DBGC ( cert, "X509 %p signatureAlgorithm is %s\n",
+	       cert, (*signature_algorithm)->name );
+	asn1_skip_any ( &cursor );
+
+	/* Parse signatureValue */
+	if ( ( rc = x509_parse_integral_bit_string ( cert, signature_value,
+						     &cursor ) ) != 0 )
+		return rc;
+	DBGC ( cert, "X509 %p signatureValue is:\n", cert );
+	DBGC_HDA ( cert, 0, signature_value->data, signature_value->len );
+
+	/* Check that algorithm in tbsCertificate matches algorithm in
+	 * signature
+	 */
+	if ( signature->algorithm != (*signature_algorithm) ) {
+		DBGC ( cert, "X509 %p signature algorithm %s does not match "
+		       "signatureAlgorithm %s\n",
+		       cert, signature->algorithm->name,
+		       (*signature_algorithm)->name );
+		return -EINVAL_ALGORITHM_MISMATCH;
+	}
+
+	return 0;
+}
+
+/**
+ * Verify X.509 certificate signature
+ *
+ * @v cert		X.509 certificate
+ * @v public_key	X.509 public key
+ * @ret rc		Return status code
+ */
+static int x509_check_signature ( struct x509_certificate *cert,
+				  struct x509_public_key *public_key ) {
+	struct x509_signature *signature = &cert->signature;
+	struct x509_algorithm *algorithm = signature->algorithm;
+	struct digest_algorithm *digest = algorithm->digest;
+	struct pubkey_algorithm *pubkey = algorithm->pubkey;
+	uint8_t digest_ctx[ digest->ctxsize ];
+	uint8_t digest_out[ digest->digestsize ];
+	uint8_t pubkey_ctx[ pubkey->ctxsize ];
+	int rc;
+
+	/* Sanity check */
+	assert ( cert->signature_algorithm == cert->signature.algorithm );
+
+	/* Calculate certificate digest */
+	digest_init ( digest, digest_ctx );
+	digest_update ( digest, digest_ctx, cert->tbs.data, cert->tbs.len );
+	digest_final ( digest, digest_ctx, digest_out );
+	DBGC ( cert, "X509 %p digest:\n", cert );
+	DBGC_HDA ( cert, 0, digest_out, sizeof ( digest_out ) );
+
+	/* Check that signature public key algorithm matches signer */
+	if ( public_key->algorithm->pubkey != pubkey ) {
+		DBGC ( cert, "X509 %p signature algorithm %s does not match "
+		       "signer's algorithm %s\n",
+		       cert, algorithm->name, public_key->algorithm->name );
+		rc = -EINVAL_ALGORITHM_MISMATCH;
+		goto err_mismatch;
+	}
+
+	/* Verify signature using signer's public key */
+	if ( ( rc = pubkey_init ( pubkey, pubkey_ctx, public_key->raw.data,
+				  public_key->raw.len ) ) != 0 ) {
+		DBGC ( cert, "X509 %p cannot initialise public key: %s\n",
+		       cert, strerror ( rc ) );
+		goto err_pubkey_init;
+	}
+	if ( ( rc = pubkey_verify ( pubkey, pubkey_ctx, digest, digest_out,
+				    signature->value.data,
+				    signature->value.len ) ) != 0 ) {
+		DBGC ( cert, "X509 %p signature verification failed: %s\n",
+		       cert, strerror ( rc ) );
+		goto err_pubkey_verify;
+	}
+
+	/* Success */
+	rc = 0;
+
+ err_pubkey_verify:
+	pubkey_final ( pubkey, pubkey_ctx );
+ err_pubkey_init:
+ err_mismatch:
+	return rc;
+}
+
+/**
+ * Validate X.509 certificate against signing certificate
+ *
+ * @v cert		X.509 certificate
+ * @v issuer		X.509 issuer certificate
+ * @ret rc		Return status code
+ */
+int x509_validate ( struct x509_certificate *cert,
+		    struct x509_certificate *issuer ) {
+	struct x509_public_key *public_key = &issuer->subject.public_key;
+	int rc;
+
+	/* Check issuer.  In theory, this should be a full X.500 DN
+	 * comparison, which would require support for a plethora of
+	 * abominations such as TeletexString (which allows the
+	 * character set to be changed mid-string using escape codes).
+	 * In practice, we assume that anyone who deliberately changes
+	 * the encoding of the issuer DN is probably a masochist who
+	 * will rather enjoy the process of figuring out exactly why
+	 * their certificate doesn't work.
+	 *
+	 * See http://www.cs.auckland.ac.nz/~pgut001/pubs/x509guide.txt
+	 * for some enjoyable ranting on this subject.
+	 */
+	if ( asn1_compare ( &cert->issuer.raw, &issuer->subject.raw ) != 0 ) {
+		DBGC ( cert, "X509 %p issuer does not match X509 %p subject\n",
+		       cert, issuer );
+		DBGC_HDA ( cert, 0, cert->issuer.raw.data,
+			   cert->issuer.raw.len );
+		DBGC_HDA ( issuer, 0, issuer->subject.raw.data,
+			   issuer->subject.raw.len );
+		return -EACCES_WRONG_ISSUER;
+	}
+
+	/* Check that issuer is allowed to sign certificates */
+	if ( ! issuer->extensions.basic.ca ) {
+		DBGC ( issuer, "X509 %p cannot sign X509 %p: not a CA "
+		       "certificate\n", issuer, cert );
+		return -EACCES_NOT_CA;
+	}
+	if ( issuer->extensions.usage.present &&
+	     ( ! ( issuer->extensions.usage.bits & X509_KEY_CERT_SIGN ) ) ) {
+		DBGC ( issuer, "X509 %p cannot sign X509 %p: no keyCertSign "
+		       "usage\n", issuer, cert );
+		return -EACCES_KEY_USAGE;
+	}
+
+	/* Check signature */
+	if ( ( rc = x509_check_signature ( cert, public_key ) ) != 0 )
+		return rc;
+
+	DBGC ( cert, "X509 %p successfully validated using X509 %p\n",
+	       cert, issuer );
+	return 0;
+}
+
+/**
+ * Calculate X.509 certificate fingerprint
+ *
+ * @v cert		X.509 certificate
+ * @v digest		Digest algorithm
+ * @v fingerprint	Fingerprint buffer
+ */
+void x509_fingerprint ( struct x509_certificate *cert,
+			struct digest_algorithm *digest, void *fingerprint ) {
+	uint8_t ctx[ digest->ctxsize ];
+
+	/* Calculate fingerprint */
+	digest_init ( digest, ctx );
+	digest_update ( digest, ctx, cert->raw.data, cert->raw.len );
+	digest_final ( digest, ctx, fingerprint );
+}
+
+/**
+ * Validate X.509 root certificate
+ *
+ * @v cert		X.509 certificate
+ * @v root		X.509 root certificate store
+ * @ret rc		Return status code
+ */
+int x509_validate_root ( struct x509_certificate *cert,
+			 struct x509_root *root ) {
+	struct digest_algorithm *digest = root->digest;
+	uint8_t fingerprint[ digest->digestsize ];
+	const uint8_t *root_fingerprint = root->fingerprints;
+	unsigned int i;
+
+	/* Calculate certificate fingerprint */
+	x509_fingerprint ( cert, digest, fingerprint );
+
+	/* Check fingerprint against all root certificates */
+	for ( i = 0 ; i < root->count ; i++ ) {
+		if ( memcmp ( fingerprint, root_fingerprint,
+			      sizeof ( fingerprint ) ) == 0 ) {
+			DBGC ( cert, "X509 %p is a root certificate\n", cert );
+			return 0;
+		}
+		root_fingerprint += sizeof ( fingerprint );
+	}
+
+	DBGC ( cert, "X509 %p is not a root certificate\n", cert );
+	return -ENOENT;
+}
+
+/**
+ * Validate X.509 certificate chain
+ *
+ * @v parse_next	Parse next X.509 certificate in chain
+ * @v context		Context for parse_next()
+ * @v root		Root certificate store, or NULL to use default
+ * @v first		Initial X.509 certificate to fill in, or NULL
+ * @ret rc		Return status code
+ */
+int x509_validate_chain ( int ( * parse_next ) ( struct x509_certificate *cert,
+						 void *context ),
+			  void *context,
+			  struct x509_root *root,
+			  struct x509_certificate *first ) {
+	struct x509_certificate temp[2];
+	struct x509_certificate *current = &temp[0];
+	struct x509_certificate *next = &temp[1];
+	struct x509_certificate *swap;
+	int rc;
+
+	/* Use default root certificate store if none specified */
+	if ( ! root )
+		root = &root_certificates;
+
+	/* Get first certificate in chain */
+	if ( ( rc = parse_next ( current, context ) ) != 0 ) {
+		DBGC ( context, "X509 chain %p could not get first "
+		       "certificate: %s\n", context, strerror ( rc ) );
+		return rc;
+	}
+
+	/* Record first certificate, if applicable */
+	if ( first )
+		memcpy ( first, current, sizeof ( *first ) );
+
+	/* Process chain */
+	while ( 1 ) {
+
+		/* Succeed if we have reached a root certificate */
+		if ( x509_validate_root ( current, root ) == 0 )
+			return 0;
+
+		/* Get next certificate in chain */
+		if ( ( rc = parse_next ( next, context ) ) != 0 ) {
+			DBGC ( context, "X509 chain %p could not get next "
+			       "certificate: %s\n", context, strerror ( rc ) );
+			return rc;
+		}
+
+		/* Validate current certificate */
+		if ( ( rc = x509_validate ( current, next ) ) != 0 )
+			return rc;
+
+		/* Move to next certificate in chain */
+		swap = current;
+		current = next;
+		next = swap;
+	}
+}