ocsp.c

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/*
 * Copyright (C) 2012 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 (at your option) 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., 51 Franklin Street, Fifth Floor, Boston, MA
 * 02110-1301, USA.
 */

FILE_LICENCE ( GPL2_OR_LATER );
FILE_SECBOOT ( PERMITTED );

#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <ipxe/asn1.h>
#include <ipxe/x509.h>
#include <ipxe/sha1.h>
#include <ipxe/base64.h>
#include <ipxe/uri.h>
#include <ipxe/ocsp.h>
#include <config/crypto.h>

/** @file
 *
 * Online Certificate Status Protocol
 *
 */

/* Disambiguate the various error causes */
#define EACCES_CERT_STATUS                                              \
        __einfo_error ( EINFO_EACCES_CERT_STATUS )
#define EINFO_EACCES_CERT_STATUS                                        \
        __einfo_uniqify ( EINFO_EACCES, 0x01,                           \
                          "Certificate status not good" )
#define EACCES_CERT_MISMATCH                                            \
        __einfo_error ( EINFO_EACCES_CERT_MISMATCH )
#define EINFO_EACCES_CERT_MISMATCH                                      \
        __einfo_uniqify ( EINFO_EACCES, 0x02,                           \
                          "Certificate ID mismatch" )
#define EACCES_NON_OCSP_SIGNING                                         \
        __einfo_error ( EINFO_EACCES_NON_OCSP_SIGNING )
#define EINFO_EACCES_NON_OCSP_SIGNING                                   \
        __einfo_uniqify ( EINFO_EACCES, 0x03,                           \
                          "Not an OCSP signing certificate" )
#define EACCES_STALE                                                    \
        __einfo_error ( EINFO_EACCES_STALE )
#define EINFO_EACCES_STALE                                              \
        __einfo_uniqify ( EINFO_EACCES, 0x04,                           \
                          "Stale (or premature) OCSP repsonse" )
#define EACCES_NO_RESPONDER                                             \
        __einfo_error ( EINFO_EACCES_NO_RESPONDER )
#define EINFO_EACCES_NO_RESPONDER                                       \
        __einfo_uniqify ( EINFO_EACCES, 0x05,                           \
                          "Missing OCSP responder certificate" )
#define ENOTSUP_RESPONSE_TYPE                                           \
        __einfo_error ( EINFO_ENOTSUP_RESPONSE_TYPE )
#define EINFO_ENOTSUP_RESPONSE_TYPE                                     \
        __einfo_uniqify ( EINFO_ENOTSUP, 0x01,                          \
                          "Unsupported OCSP response type" )
#define ENOTSUP_RESPONDER_ID                                            \
        __einfo_error ( EINFO_ENOTSUP_RESPONDER_ID )
#define EINFO_ENOTSUP_RESPONDER_ID                                      \
        __einfo_uniqify ( EINFO_ENOTSUP, 0x02,                          \
                          "Unsupported OCSP responder ID" )
#define EPROTO_MALFORMED_REQUEST                                        \
        __einfo_error ( EINFO_EPROTO_MALFORMED_REQUEST )
#define EINFO_EPROTO_MALFORMED_REQUEST                                  \
        __einfo_uniqify ( EINFO_EPROTO, OCSP_STATUS_MALFORMED_REQUEST,  \
                          "Illegal confirmation request" )
#define EPROTO_INTERNAL_ERROR                                           \
        __einfo_error ( EINFO_EPROTO_INTERNAL_ERROR )
#define EINFO_EPROTO_INTERNAL_ERROR                                     \
        __einfo_uniqify ( EINFO_EPROTO, OCSP_STATUS_INTERNAL_ERROR,     \
                          "Internal error in issuer" )
#define EPROTO_TRY_LATER                                                \
        __einfo_error ( EINFO_EPROTO_TRY_LATER )
#define EINFO_EPROTO_TRY_LATER                                          \
        __einfo_uniqify ( EINFO_EPROTO, OCSP_STATUS_TRY_LATER,          \
                          "Try again later" )
#define EPROTO_SIG_REQUIRED                                             \
        __einfo_error ( EINFO_EPROTO_SIG_REQUIRED )
#define EINFO_EPROTO_SIG_REQUIRED                                       \
        __einfo_uniqify ( EINFO_EPROTO, OCSP_STATUS_SIG_REQUIRED,       \
                          "Must sign the request" )
#define EPROTO_UNAUTHORIZED                                             \
        __einfo_error ( EINFO_EPROTO_UNAUTHORIZED )
#define EINFO_EPROTO_UNAUTHORIZED                                       \
        __einfo_uniqify ( EINFO_EPROTO, OCSP_STATUS_UNAUTHORIZED,       \
                          "Request unauthorized" )
#define EPROTO_STATUS( status )                                         \
        EUNIQ ( EINFO_EPROTO, (status), EPROTO_MALFORMED_REQUEST,       \
                EPROTO_INTERNAL_ERROR, EPROTO_TRY_LATER,                \
                EPROTO_SIG_REQUIRED, EPROTO_UNAUTHORIZED )

/** OCSP digest algorithm */
#define ocsp_digest_algorithm sha1_algorithm

/** OCSP digest algorithm identifier */
static const uint8_t ocsp_algorithm_id[] =
        { OCSP_ALGORITHM_IDENTIFIER ( ASN1_OID_SHA1 ) };

/** OCSP basic response type */
static const uint8_t oid_basic_response_type[] = { ASN1_OID_OCSP_BASIC };

/** OCSP basic response type cursor */
static struct asn1_cursor oid_basic_response_type_cursor =
        ASN1_CURSOR ( oid_basic_response_type );

/**
 * Free OCSP check
 *
 * @v refcnt            Reference count
 */
static void ocsp_free ( struct refcnt *refcnt ) {
        struct ocsp_check *ocsp =
                container_of ( refcnt, struct ocsp_check, refcnt );

        x509_put ( ocsp->cert );
        x509_put ( ocsp->issuer );
        free ( ocsp->uri_string );
        free ( ocsp->request.builder.data );
        free ( ocsp->response.data );
        x509_put ( ocsp->response.signer );
        free ( ocsp );
}

/**
 * Build OCSP request
 *
 * @v ocsp              OCSP check
 * @ret rc              Return status code
 */
static int ocsp_request ( struct ocsp_check *ocsp ) {
        struct digest_algorithm *digest = &ocsp_digest_algorithm;
        struct asn1_builder *builder = &ocsp->request.builder;
        struct asn1_cursor *cert_id_tail = &ocsp->request.cert_id_tail;
        uint8_t digest_ctx[digest->ctxsize];
        uint8_t name_digest[digest->digestsize];
        uint8_t pubkey_digest[digest->digestsize];
        int rc;

        /* Generate digests */
        digest_init ( digest, digest_ctx );
        digest_update ( digest, digest_ctx, ocsp->cert->issuer.raw.data,
                        ocsp->cert->issuer.raw.len );
        digest_final ( digest, digest_ctx, name_digest );
        digest_init ( digest, digest_ctx );
        digest_update ( digest, digest_ctx,
                        ocsp->issuer->subject.public_key.value.data,
                        ocsp->issuer->subject.public_key.value.len );
        digest_final ( digest, digest_ctx, pubkey_digest );

        /* Construct request */
        if ( ( rc = ( asn1_prepend_raw ( builder, ocsp->cert->serial.raw.data,
                                         ocsp->cert->serial.raw.len ),
                      asn1_prepend ( builder, ASN1_OCTET_STRING,
                                     pubkey_digest, sizeof ( pubkey_digest ) ),
                      asn1_prepend ( builder, ASN1_OCTET_STRING,
                                     name_digest, sizeof ( name_digest ) ),
                      asn1_prepend ( builder, ASN1_SEQUENCE,
                                     ocsp_algorithm_id,
                                     sizeof ( ocsp_algorithm_id ) ),
                      asn1_wrap ( builder, ASN1_SEQUENCE ),
                      asn1_wrap ( builder, ASN1_SEQUENCE ),
                      asn1_wrap ( builder, ASN1_SEQUENCE ),
                      asn1_wrap ( builder, ASN1_SEQUENCE ),
                      asn1_wrap ( builder, ASN1_SEQUENCE ) ) ) != 0 ) {
                DBGC ( ocsp, "OCSP %p \"%s\" could not build request: %s\n",
                       ocsp, x509_name ( ocsp->cert ), strerror ( rc ) );
                return rc;
        }
        DBGC2 ( ocsp, "OCSP %p \"%s\" request is:\n",
                ocsp, x509_name ( ocsp->cert ) );
        DBGC2_HDA ( ocsp, 0, builder->data, builder->len );

        /* Parse certificate ID for comparison with response */
        cert_id_tail->data = builder->data;
        cert_id_tail->len = builder->len;
        if ( ( rc = ( asn1_enter ( cert_id_tail, ASN1_SEQUENCE ),
                      asn1_enter ( cert_id_tail, ASN1_SEQUENCE ),
                      asn1_enter ( cert_id_tail, ASN1_SEQUENCE ),
                      asn1_enter ( cert_id_tail, ASN1_SEQUENCE ),
                      asn1_enter ( cert_id_tail, ASN1_SEQUENCE ),
                      asn1_skip ( cert_id_tail, ASN1_SEQUENCE ) ) ) != 0 ) {
                DBGC ( ocsp, "OCSP %p \"%s\" could not locate certID: %s\n",
                       ocsp, x509_name ( ocsp->cert ), strerror ( rc ) );
                return rc;
        }

        return 0;
}

/**
 * Build OCSP URI string
 *
 * @v ocsp              OCSP check
 * @ret rc              Return status code
 */
static int ocsp_uri_string ( struct ocsp_check *ocsp ) {
        struct x509_ocsp_responder *responder =
                &ocsp->cert->extensions.auth_info.ocsp;
        char *base64;
        char *sep;
        size_t base64_len;
        size_t uri_len;
        size_t len;
        int rc;

        /* Sanity check */
        if ( ! responder->uri.len ) {
                DBGC ( ocsp, "OCSP %p \"%s\" has no OCSP URI\n",
                       ocsp, x509_name ( ocsp->cert ) );
                rc = -ENOTTY;
                goto err_no_uri;
        }

        /* Calculate base64-encoded request length */
        base64_len = ( base64_encoded_len ( ocsp->request.builder.len )
                       + 1 /* NUL */ );

        /* Allocate and construct the base64-encoded request */
        base64 = malloc ( base64_len );
        if ( ! base64 ) {
                rc = -ENOMEM;
                goto err_alloc_base64;
        }
        base64_encode ( ocsp->request.builder.data, ocsp->request.builder.len,
                        base64, base64_len );

        /* Calculate URI-encoded base64-encoded request length */
        uri_len = ( uri_encode ( URI_PATH, base64, ( base64_len - 1 /* NUL */ ),
                                 NULL, 0 ) + 1 /* NUL */ );

        /* Allocate and construct the URI string */
        len = ( responder->uri.len + 1 /* possible "/" */ + uri_len );
        ocsp->uri_string = zalloc ( len );
        if ( ! ocsp->uri_string ) {
                rc = -ENOMEM;
                goto err_alloc_uri;
        }
        memcpy ( ocsp->uri_string, responder->uri.data, responder->uri.len );
        sep = &ocsp->uri_string[ responder->uri.len - 1 ];
        if ( *sep != '/' )
                *(++sep) = '/';
        uri_encode ( URI_PATH, base64, base64_len, ( sep + 1 ), uri_len );
        DBGC2 ( ocsp, "OCSP %p \"%s\" URI is %s\n",
                ocsp, x509_name ( ocsp->cert ), ocsp->uri_string );

        /* Success */
        rc = 0;

 err_alloc_uri:
        free ( base64 );
 err_alloc_base64:
 err_no_uri:
        return rc;
}

/**
 * Create OCSP check
 *
 * @v cert              Certificate to check
 * @v issuer            Issuing certificate
 * @ret ocsp            OCSP check
 * @ret rc              Return status code
 */
int ocsp_check ( struct x509_certificate *cert,
                 struct x509_certificate *issuer,
                 struct ocsp_check **ocsp ) {
        int rc;

        /* Sanity checks */
        assert ( cert != NULL );
        assert ( issuer != NULL );
        assert ( issuer->root != NULL );

        /* Allocate and initialise check */
        *ocsp = zalloc ( sizeof ( **ocsp ) );
        if ( ! *ocsp ) {
                rc = -ENOMEM;
                goto err_alloc;
        }
        ref_init ( &(*ocsp)->refcnt, ocsp_free );
        (*ocsp)->cert = x509_get ( cert );
        (*ocsp)->issuer = x509_get ( issuer );

        /* Build request */
        if ( ( rc = ocsp_request ( *ocsp ) ) != 0 )
                goto err_request;

        /* Build URI string */
        if ( ( rc = ocsp_uri_string ( *ocsp ) ) != 0 )
                goto err_uri_string;

        return 0;

 err_uri_string:
 err_request:
        ocsp_put ( *ocsp );
 err_alloc:
        *ocsp = NULL;
        return rc;
}

/**
 * Parse OCSP response status
 *
 * @v ocsp              OCSP check
 * @v raw               ASN.1 cursor
 * @ret rc              Return status code
 */
static int ocsp_parse_response_status ( struct ocsp_check *ocsp,
                                        const struct asn1_cursor *raw ) {
        struct asn1_cursor cursor;
        uint8_t status;
        int rc;

        /* Enter responseStatus */
        memcpy ( &cursor, raw, sizeof ( cursor ) );
        if ( ( rc = asn1_enter ( &cursor, ASN1_ENUMERATED ) ) != 0 ) {
                DBGC ( ocsp, "OCSP %p \"%s\" could not locate responseStatus: "
                       "%s\n", ocsp, x509_name ( ocsp->cert ), strerror ( rc ));
                return rc;
        }

        /* Extract response status */
        if ( cursor.len != sizeof ( status ) ) {
                DBGC ( ocsp, "OCSP %p \"%s\" invalid status:\n",
                       ocsp, x509_name ( ocsp->cert ) );
                DBGC_HDA ( ocsp, 0, cursor.data, cursor.len );
                return -EINVAL;
        }
        memcpy ( &status, cursor.data, sizeof ( status ) );

        /* Check response status */
        if ( status != OCSP_STATUS_SUCCESSFUL ) {
                DBGC ( ocsp, "OCSP %p \"%s\" response status %d\n",
                       ocsp, x509_name ( ocsp->cert ), status );
                return EPROTO_STATUS ( status );
        }

        return 0;
}

/**
 * Parse OCSP response type
 *
 * @v ocsp              OCSP check
 * @v raw               ASN.1 cursor
 * @ret rc              Return status code
 */
static int ocsp_parse_response_type ( struct ocsp_check *ocsp,
                                      const struct asn1_cursor *raw ) {
        struct asn1_cursor cursor;

        /* Enter responseType */
        memcpy ( &cursor, raw, sizeof ( cursor ) );
        asn1_enter ( &cursor, ASN1_OID );

        /* Check responseType is "basic" */
        if ( asn1_compare ( &oid_basic_response_type_cursor, &cursor ) != 0 ) {
                DBGC ( ocsp, "OCSP %p \"%s\" response type not supported:\n",
                       ocsp, x509_name ( ocsp->cert ) );
                DBGC_HDA ( ocsp, 0, cursor.data, cursor.len );
                return -ENOTSUP_RESPONSE_TYPE;
        }

        return 0;
}

/**
 * Compare responder's certificate name
 *
 * @v ocsp              OCSP check
 * @v cert              Certificate
 * @ret difference      Difference as returned by memcmp()
 */
static int ocsp_compare_responder_name ( struct ocsp_check *ocsp,
                                         struct x509_certificate *cert ) {
        struct ocsp_responder *responder = &ocsp->response.responder;

        /* Compare responder ID with certificate's subject */
        return asn1_compare ( &responder->id, &cert->subject.raw );
}

/**
 * Compare responder's certificate public key hash
 *
 * @v ocsp              OCSP check
 * @v cert              Certificate
 * @ret difference      Difference as returned by memcmp()
 */
static int ocsp_compare_responder_key_hash ( struct ocsp_check *ocsp,
                                             struct x509_certificate *cert ) {
        struct ocsp_responder *responder = &ocsp->response.responder;
        struct asn1_cursor key_hash;
        uint8_t ctx[SHA1_CTX_SIZE];
        uint8_t digest[SHA1_DIGEST_SIZE];
        int difference;

        /* Enter responder key hash */
        memcpy ( &key_hash, &responder->id, sizeof ( key_hash ) );
        asn1_enter ( &key_hash, ASN1_OCTET_STRING );

        /* Sanity check */
        difference = ( sizeof ( digest ) - key_hash.len );
        if ( difference )
                return difference;

        /* Generate SHA1 hash of certificate's public key */
        digest_init ( &sha1_algorithm, ctx );
        digest_update ( &sha1_algorithm, ctx,
                        cert->subject.public_key.value.data,
                        cert->subject.public_key.value.len );
        digest_final ( &sha1_algorithm, ctx, digest );

        /* Compare responder key hash with hash of certificate's public key */
        return memcmp ( digest, key_hash.data, sizeof ( digest ) );
}

/**
 * Parse OCSP responder ID
 *
 * @v ocsp              OCSP check
 * @v raw               ASN.1 cursor
 * @ret rc              Return status code
 */
static int ocsp_parse_responder_id ( struct ocsp_check *ocsp,
                                     const struct asn1_cursor *raw ) {
        struct ocsp_responder *responder = &ocsp->response.responder;
        struct asn1_cursor *responder_id = &responder->id;
        unsigned int type;

        /* Enter responder ID */
        memcpy ( responder_id, raw, sizeof ( *responder_id ) );
        type = asn1_type ( responder_id );
        asn1_enter_any ( responder_id );

        /* Identify responder ID type */
        switch ( type ) {
        case ASN1_EXPLICIT_TAG ( 1 ) :
                DBGC2 ( ocsp, "OCSP %p \"%s\" responder identified by name\n",
                        ocsp, x509_name ( ocsp->cert ) );
                responder->compare = ocsp_compare_responder_name;
                return 0;
        case ASN1_EXPLICIT_TAG ( 2 ) :
                DBGC2 ( ocsp, "OCSP %p \"%s\" responder identified by key "
                        "hash\n", ocsp, x509_name ( ocsp->cert ) );
                responder->compare = ocsp_compare_responder_key_hash;
                return 0;
        default:
                DBGC ( ocsp, "OCSP %p \"%s\" unsupported responder ID type "
                       "%d\n", ocsp, x509_name ( ocsp->cert ), type );
                return -ENOTSUP_RESPONDER_ID;
        }
}

/**
 * Parse OCSP certificate ID
 *
 * @v ocsp              OCSP check
 * @v raw               ASN.1 cursor
 * @ret rc              Return status code
 */
static int ocsp_parse_cert_id ( struct ocsp_check *ocsp,
                                const struct asn1_cursor *raw ) {
        static struct asn1_cursor algorithm = {
                .data = ocsp_algorithm_id,
                .len = sizeof ( ocsp_algorithm_id ),
        };
        struct asn1_cursor cert_id;
        struct asn1_cursor cursor;
        int rc;

        /* Enter cert ID */
        memcpy ( &cert_id, raw, sizeof ( cert_id ) );
        asn1_enter ( &cert_id, ASN1_SEQUENCE );

        /* Check certID algorithm (but not parameters) */
        memcpy ( &cursor, &cert_id, sizeof ( cursor ) );
        if ( ( rc = ( asn1_enter ( &cursor, ASN1_SEQUENCE ),
                      asn1_shrink ( &cursor, ASN1_OID ),
                      asn1_shrink ( &algorithm, ASN1_OID ) ) ) != 0 ) {
                DBGC ( ocsp, "OCSP %p \"%s\" certID missing algorithm:\n",
                       ocsp, x509_name ( ocsp->cert ) );
                DBGC_HDA ( ocsp, 0, cursor.data, cursor.len );
                return -EACCES_CERT_MISMATCH;
        }
        if ( asn1_compare ( &cursor, &algorithm ) != 0 ) {
                DBGC ( ocsp, "OCSP %p \"%s\" certID wrong algorithm:\n",
                       ocsp, x509_name ( ocsp->cert ) );
                DBGC_HDA ( ocsp, 0, cursor.data, cursor.len );
                return -EACCES_CERT_MISMATCH;
        }

        /* Check remaining certID fields */
        asn1_skip ( &cert_id, ASN1_SEQUENCE );
        if ( asn1_compare ( &cert_id, &ocsp->request.cert_id_tail ) != 0 ) {
                DBGC ( ocsp, "OCSP %p \"%s\" certID mismatch:\n",
                       ocsp, x509_name ( ocsp->cert ) );
                DBGC_HDA ( ocsp, 0, ocsp->request.cert_id_tail.data,
                           ocsp->request.cert_id_tail.len );
                DBGC_HDA ( ocsp, 0, cert_id.data, cert_id.len );
                return -EACCES_CERT_MISMATCH;
        }

        return 0;
}

/**
 * Parse OCSP responses
 *
 * @v ocsp              OCSP check
 * @v raw               ASN.1 cursor
 * @ret rc              Return status code
 */
static int ocsp_parse_responses ( struct ocsp_check *ocsp,
                                  const struct asn1_cursor *raw ) {
        struct ocsp_response *response = &ocsp->response;
        struct asn1_cursor cursor;
        int rc;

        /* Enter responses */
        memcpy ( &cursor, raw, sizeof ( cursor ) );
        asn1_enter ( &cursor, ASN1_SEQUENCE );

        /* Enter first singleResponse */
        asn1_enter ( &cursor, ASN1_SEQUENCE );

        /* Parse certID */
        if ( ( rc = ocsp_parse_cert_id ( ocsp, &cursor ) ) != 0 )
                return rc;
        asn1_skip_any ( &cursor );

        /* Check certStatus */
        if ( asn1_type ( &cursor ) != ASN1_IMPLICIT_TAG ( 0 ) ) {
                DBGC ( ocsp, "OCSP %p \"%s\" non-good certStatus:\n",
                       ocsp, x509_name ( ocsp->cert ) );
                DBGC_HDA ( ocsp, 0, cursor.data, cursor.len );
                return -EACCES_CERT_STATUS;
        }
        asn1_skip_any ( &cursor );

        /* Parse thisUpdate */
        if ( ( rc = asn1_generalized_time ( &cursor,
                                            &response->this_update ) ) != 0 ) {
                DBGC ( ocsp, "OCSP %p \"%s\" could not parse thisUpdate: %s\n",
                       ocsp, x509_name ( ocsp->cert ), strerror ( rc ) );
                return rc;
        }
        DBGC2 ( ocsp, "OCSP %p \"%s\" this update was at time %lld\n",
                ocsp, x509_name ( ocsp->cert ), response->this_update );
        asn1_skip_any ( &cursor );

        /* Parse nextUpdate, if present */
        if ( asn1_type ( &cursor ) == ASN1_EXPLICIT_TAG ( 0 ) ) {
                asn1_enter ( &cursor, ASN1_EXPLICIT_TAG ( 0 ) );
                if ( ( rc = asn1_generalized_time ( &cursor,
                                             &response->next_update ) ) != 0 ) {
                        DBGC ( ocsp, "OCSP %p \"%s\" could not parse "
                               "nextUpdate: %s\n", ocsp,
                               x509_name ( ocsp->cert ), strerror ( rc ) );
                        return rc;
                }
                DBGC2 ( ocsp, "OCSP %p \"%s\" next update is at time %lld\n",
                        ocsp, x509_name ( ocsp->cert ), response->next_update );
        } else {
                /* If no nextUpdate is present, this indicates that
                 * "newer revocation information is available all the
                 * time".  Actually, this indicates that there is no
                 * point to performing the OCSP check, since an
                 * attacker could replay the response at any future
                 * time and it would still be valid.
                 */
                DBGC ( ocsp, "OCSP %p \"%s\" responder is a moron\n",
                       ocsp, x509_name ( ocsp->cert ) );
                response->next_update = time ( NULL );
        }

        return 0;
}

/**
 * Parse OCSP response data
 *
 * @v ocsp              OCSP check
 * @v raw               ASN.1 cursor
 * @ret rc              Return status code
 */
static int ocsp_parse_tbs_response_data ( struct ocsp_check *ocsp,
                                          const struct asn1_cursor *raw ) {
        struct ocsp_response *response = &ocsp->response;
        struct asn1_cursor cursor;
        int rc;

        /* Record raw tbsResponseData */
        memcpy ( &cursor, raw, sizeof ( cursor ) );
        asn1_shrink_any ( &cursor );
        memcpy ( &response->tbs, &cursor, sizeof ( response->tbs ) );

        /* Enter tbsResponseData */
        asn1_enter ( &cursor, ASN1_SEQUENCE );

        /* Skip version, if present */
        asn1_skip_if_exists ( &cursor, ASN1_EXPLICIT_TAG ( 0 ) );

        /* Parse responderID */
        if ( ( rc = ocsp_parse_responder_id ( ocsp, &cursor ) ) != 0 )
                return rc;
        asn1_skip_any ( &cursor );

        /* Skip producedAt */
        asn1_skip_any ( &cursor );

        /* Parse responses */
        if ( ( rc = ocsp_parse_responses ( ocsp, &cursor ) ) != 0 )
                return rc;

        return 0;
}

/**
 * Parse OCSP certificates
 *
 * @v ocsp              OCSP check
 * @v raw               ASN.1 cursor
 * @ret rc              Return status code
 */
static int ocsp_parse_certs ( struct ocsp_check *ocsp,
                              const struct asn1_cursor *raw ) {
        struct ocsp_response *response = &ocsp->response;
        struct asn1_cursor cursor;
        struct x509_certificate *cert;
        int rc;

        /* Enter certs */
        memcpy ( &cursor, raw, sizeof ( cursor ) );
        asn1_enter ( &cursor, ASN1_EXPLICIT_TAG ( 0 ) );
        asn1_enter ( &cursor, ASN1_SEQUENCE );

        /* Parse certificate, if present.  The data structure permits
         * multiple certificates, but the protocol requires that the
         * OCSP signing certificate must either be the issuer itself,
         * or must be directly issued by the issuer (see RFC2560
         * section 4.2.2.2 "Authorized Responders").  We therefore
         * need to identify only the single certificate matching the
         * Responder ID.
         */
        while ( cursor.len ) {

                /* Parse certificate */
                if ( ( rc = x509_certificate ( cursor.data, cursor.len,
                                               &cert ) ) != 0 ) {
                        DBGC ( ocsp, "OCSP %p \"%s\" could not parse "
                               "certificate: %s\n", ocsp,
                               x509_name ( ocsp->cert ), strerror ( rc ) );
                        DBGC_HDA ( ocsp, 0, cursor.data, cursor.len );
                        return rc;
                }

                /* Use if this certificate matches the responder ID */
                if ( response->responder.compare ( ocsp, cert ) == 0 ) {
                        response->signer = cert;
                        DBGC2 ( ocsp, "OCSP %p \"%s\" response is signed by ",
                                ocsp, x509_name ( ocsp->cert ) );
                        DBGC2 ( ocsp, "\"%s\"\n",
                                x509_name ( response->signer ) );
                        return 0;
                }

                /* Otherwise, discard this certificate */
                x509_put ( cert );
                asn1_skip_any ( &cursor );
        }

        DBGC ( ocsp, "OCSP %p \"%s\" missing responder certificate\n",
               ocsp, x509_name ( ocsp->cert ) );
        return -EACCES_NO_RESPONDER;
}

/**
 * Parse OCSP basic response
 *
 * @v ocsp              OCSP check
 * @v raw               ASN.1 cursor
 * @ret rc              Return status code
 */
static int ocsp_parse_basic_response ( struct ocsp_check *ocsp,
                                       const struct asn1_cursor *raw ) {
        struct ocsp_response *response = &ocsp->response;
        struct asn1_algorithm **algorithm = &response->algorithm;
        struct asn1_cursor *signature = &response->signature;
        struct asn1_cursor cursor;
        int rc;

        /* Enter BasicOCSPResponse */
        memcpy ( &cursor, raw, sizeof ( cursor ) );
        asn1_enter ( &cursor, ASN1_SEQUENCE );

        /* Parse tbsResponseData */
        if ( ( rc = ocsp_parse_tbs_response_data ( ocsp, &cursor ) ) != 0 )
                return rc;
        asn1_skip_any ( &cursor );

        /* Parse signatureAlgorithm */
        if ( ( rc = asn1_signature_algorithm ( &cursor, algorithm ) ) != 0 ) {
                DBGC ( ocsp, "OCSP %p \"%s\" cannot parse signature "
                       "algorithm: %s\n",
                       ocsp, x509_name ( ocsp->cert ), strerror ( rc ) );
                return rc;
        }
        DBGC2 ( ocsp, "OCSP %p \"%s\" signature algorithm is %s\n",
                ocsp, x509_name ( ocsp->cert ), (*algorithm)->name );
        asn1_skip_any ( &cursor );

        /* Parse signature */
        memcpy ( signature, &cursor, sizeof ( *signature ) );
        if ( ( rc = asn1_enter_bits ( signature, NULL ) ) != 0 ) {
                DBGC ( ocsp, "OCSP %p \"%s\" cannot parse signature: %s\n",
                       ocsp, x509_name ( ocsp->cert ), strerror ( rc ) );
                return rc;
        }
        asn1_skip_any ( &cursor );

        /* Parse certs, if present */
        if ( ( asn1_type ( &cursor ) == ASN1_EXPLICIT_TAG ( 0 ) ) &&
             ( ( rc = ocsp_parse_certs ( ocsp, &cursor ) ) != 0 ) )
                return rc;

        return 0;
}

/**
 * Parse OCSP response bytes
 *
 * @v ocsp              OCSP check
 * @v raw               ASN.1 cursor
 * @ret rc              Return status code
 */
static int ocsp_parse_response_bytes ( struct ocsp_check *ocsp,
                                       const struct asn1_cursor *raw ) {
        struct asn1_cursor cursor;
        int rc;

        /* Enter responseBytes */
        memcpy ( &cursor, raw, sizeof ( cursor ) );
        asn1_enter ( &cursor, ASN1_EXPLICIT_TAG ( 0 ) );
        asn1_enter ( &cursor, ASN1_SEQUENCE );

        /* Parse responseType */
        if ( ( rc = ocsp_parse_response_type ( ocsp, &cursor ) ) != 0 )
                return rc;
        asn1_skip_any ( &cursor );

        /* Enter response */
        asn1_enter ( &cursor, ASN1_OCTET_STRING );

        /* Parse response */
        if ( ( rc = ocsp_parse_basic_response ( ocsp, &cursor ) ) != 0 )
                return rc;

        return 0;
}

/**
 * Parse OCSP response
 *
 * @v ocsp              OCSP check
 * @v raw               ASN.1 cursor
 * @ret rc              Return status code
 */
static int ocsp_parse_response ( struct ocsp_check *ocsp,
                                 const struct asn1_cursor *raw ) {
        struct asn1_cursor cursor;
        int rc;

        /* Enter OCSPResponse */
        memcpy ( &cursor, raw, sizeof ( cursor ) );
        asn1_enter ( &cursor, ASN1_SEQUENCE );

        /* Parse responseStatus */
        if ( ( rc = ocsp_parse_response_status ( ocsp, &cursor ) ) != 0 )
                return rc;
        asn1_skip_any ( &cursor );

        /* Parse responseBytes */
        if ( ( rc = ocsp_parse_response_bytes ( ocsp, &cursor ) ) != 0 )
                return rc;

        return 0;
}

/**
 * Receive OCSP response
 *
 * @v ocsp              OCSP check
 * @v data              Response data
 * @v len               Length of response data
 * @ret rc              Return status code
 */
int ocsp_response ( struct ocsp_check *ocsp, const void *data, size_t len ) {
        struct ocsp_response *response = &ocsp->response;
        struct asn1_cursor cursor;
        int rc;

        /* Duplicate data */
        x509_put ( response->signer );
        response->signer = NULL;
        free ( response->data );
        response->data = malloc ( len );
        if ( ! response->data )
                return -ENOMEM;
        memcpy ( response->data, data, len );
        cursor.data = response->data;
        cursor.len = len;

        /* Parse response */
        if ( ( rc = ocsp_parse_response ( ocsp, &cursor ) ) != 0 )
                return rc;

        return 0;
}

/**
 * Check OCSP response signature
 *
 * @v ocsp              OCSP check
 * @v signer            Signing certificate
 * @ret rc              Return status code
 */
static int ocsp_check_signature ( struct ocsp_check *ocsp,
                                  struct x509_certificate *signer ) {
        struct ocsp_response *response = &ocsp->response;
        struct digest_algorithm *digest = response->algorithm->digest;
        struct pubkey_algorithm *pubkey = response->algorithm->pubkey;
        struct asn1_cursor *key = &signer->subject.public_key.raw;
        uint8_t digest_ctx[ digest->ctxsize ];
        uint8_t digest_out[ digest->digestsize ];
        int rc;

        /* Generate digest */
        digest_init ( digest, digest_ctx );
        digest_update ( digest, digest_ctx, response->tbs.data,
                        response->tbs.len );
        digest_final ( digest, digest_ctx, digest_out );

        /* Verify digest */
        if ( ( rc = pubkey_verify ( pubkey, key, digest, digest_out,
                                    &response->signature ) ) != 0 ) {
                DBGC ( ocsp, "OCSP %p \"%s\" signature verification failed: "
                       "%s\n", ocsp, x509_name ( ocsp->cert ), strerror ( rc ));
                return rc;
        }

        DBGC2 ( ocsp, "OCSP %p \"%s\" signature is correct\n",
                ocsp, x509_name ( ocsp->cert ) );
        return 0;
}

/**
 * Validate OCSP response
 *
 * @v ocsp              OCSP check
 * @v time              Time at which to validate response
 * @ret rc              Return status code
 */
int ocsp_validate ( struct ocsp_check *ocsp, time_t time ) {
        struct ocsp_response *response = &ocsp->response;
        struct x509_certificate *signer;
        int rc;

        /* Sanity checks */
        assert ( response->data != NULL );

        /* The response may include a signer certificate; if this is
         * not present then the response must have been signed
         * directly by the issuer.
         */
        signer = ( response->signer ? response->signer : ocsp->issuer );

        /* Validate signer, if applicable.  If the signer is not the
         * issuer, then it must be signed directly by the issuer.
         */
        if ( signer != ocsp->issuer ) {
                /* Forcibly invalidate the signer, since we need to
                 * ensure that it was signed by our issuer (and not
                 * some other issuer).  This prevents a sub-CA's OCSP
                 * certificate from fraudulently signing OCSP
                 * responses from the parent CA.
                 */
                x509_invalidate ( signer );
                if ( ( rc = x509_validate ( signer, ocsp->issuer, time,
                                            ocsp->issuer->root ) ) != 0 ) {
                        DBGC ( ocsp, "OCSP %p \"%s\" could not validate ",
                               ocsp, x509_name ( ocsp->cert ) );
                        DBGC ( ocsp, "signer \"%s\": %s\n",
                               x509_name ( signer ), strerror ( rc ) );
                        return rc;
                }

                /* If signer is not the issuer, then it must have the
                 * extendedKeyUsage id-kp-OCSPSigning.
                 */
                if ( ! ( signer->extensions.ext_usage.bits &
                         X509_OCSP_SIGNING ) ) {
                        DBGC ( ocsp, "OCSP %p \"%s\" ",
                               ocsp, x509_name ( ocsp->cert ) );
                        DBGC ( ocsp, "signer \"%s\" is not an OCSP-signing "
                               "certificate\n", x509_name ( signer ) );
                        return -EACCES_NON_OCSP_SIGNING;
                }
        }

        /* Check OCSP response signature */
        if ( ( rc = ocsp_check_signature ( ocsp, signer ) ) != 0 )
                return rc;

        /* Check OCSP response is valid at the specified time
         * (allowing for some margin of error).
         */
        if ( response->this_update > ( time + TIMESTAMP_ERROR_MARGIN ) ) {
                DBGC ( ocsp, "OCSP %p \"%s\" response is not yet valid (at "
                       "time %lld)\n", ocsp, x509_name ( ocsp->cert ), time );
                return -EACCES_STALE;
        }
        if ( response->next_update < ( time - TIMESTAMP_ERROR_MARGIN ) ) {
                DBGC ( ocsp, "OCSP %p \"%s\" response is stale (at time "
                       "%lld)\n", ocsp, x509_name ( ocsp->cert ), time );
                return -EACCES_STALE;
        }
        DBGC2 ( ocsp, "OCSP %p \"%s\" response is valid (at time %lld)\n",
                ocsp, x509_name ( ocsp->cert ), time );

        /* Mark certificate as passing OCSP verification */
        ocsp->cert->extensions.auth_info.ocsp.good = 1;

        /* Validate certificate against issuer */
        if ( ( rc = x509_validate ( ocsp->cert, ocsp->issuer, time,
                                    ocsp->issuer->root ) ) != 0 ) {
                DBGC ( ocsp, "OCSP %p \"%s\" could not validate certificate: "
                       "%s\n", ocsp, x509_name ( ocsp->cert ), strerror ( rc ));
                return rc;
        }
        DBGC ( ocsp, "OCSP %p \"%s\" successfully validated ",
               ocsp, x509_name ( ocsp->cert ) );
        DBGC ( ocsp, "using \"%s\"\n", x509_name ( signer ) );

        return 0;
}