// SPDX-License-Identifier: GPL-2.0+
/*
 * Elliptic Curve (Russian) Digital Signature Algorithm for Cryptographic API
 *
 * Copyright (c) 2019 Vitaly Chikunov <vt@altlinux.org>
 *
 * References:
 * GOST 34.10-2018, GOST R 34.10-2012, RFC 7091, ISO/IEC 14888-3:2018.
 *
 * Historical references:
 * GOST R 34.10-2001, RFC 4357, ISO/IEC 14888-3:2006/Amd 1:2010.
 *
 * 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.
 */

#include <linux/module.h>
#include <linux/crypto.h>
#include <crypto/streebog.h>
#include <crypto/internal/akcipher.h>
#include <crypto/akcipher.h>
#include <linux/oid_registry.h>
#include <linux/scatterlist.h>
#include "ecrdsa_params.asn1.h"
#include "ecrdsa_pub_key.asn1.h"
#include "ecc.h"
#include "ecrdsa_defs.h"

#define ECRDSA_MAX_SIG_SIZE (2 * 512 / 8)
#define ECRDSA_MAX_DIGITS (512 / 64)

struct ecrdsa_ctx {
        enum OID algo_oid; /* overall public key oid */
        enum OID curve_oid; /* parameter */
        enum OID digest_oid; /* parameter */
        const struct ecc_curve *curve; /* curve from oid */
        unsigned int digest_len; /* parameter (bytes) */
        const char *digest; /* digest name from oid */
        unsigned int key_len; /* @key length (bytes) */
        const char *key; /* raw public key */
        struct ecc_point pub_key;
        u64 _pubp[2][ECRDSA_MAX_DIGITS]; /* point storage for @pub_key */
};

static const struct ecc_curve *get_curve_by_oid(enum OID oid)
{
        switch (oid) {
        case OID_gostCPSignA:
        case OID_gostTC26Sign256B:
                return &gost_cp256a;
        case OID_gostCPSignB:
        case OID_gostTC26Sign256C:
                return &gost_cp256b;
        case OID_gostCPSignC:
        case OID_gostTC26Sign256D:
                return &gost_cp256c;
        case OID_gostTC26Sign512A:
                return &gost_tc512a;
        case OID_gostTC26Sign512B:
                return &gost_tc512b;
        /* The following two aren't implemented: */
        case OID_gostTC26Sign256A:
        case OID_gostTC26Sign512C:
        default:
                return NULL;
        }
}

static int ecrdsa_verify(struct akcipher_request *req)
{
        struct crypto_akcipher *tfm = crypto_akcipher_reqtfm(req);
        struct ecrdsa_ctx *ctx = akcipher_tfm_ctx(tfm);
        unsigned char sig[ECRDSA_MAX_SIG_SIZE];
        unsigned char digest[STREEBOG512_DIGEST_SIZE];
        unsigned int ndigits = req->dst_len / sizeof(u64);
        u64 r[ECRDSA_MAX_DIGITS]; /* witness (r) */
        u64 _r[ECRDSA_MAX_DIGITS]; /* -r */
        u64 s[ECRDSA_MAX_DIGITS]; /* second part of sig (s) */
        u64 e[ECRDSA_MAX_DIGITS]; /* h \mod q */
        u64 *v = e;               /* e^{-1} \mod q */
        u64 z1[ECRDSA_MAX_DIGITS];
        u64 *z2 = _r;
        struct ecc_point cc = ECC_POINT_INIT(s, e, ndigits); /* reuse s, e */

        /*
         * Digest value, digest algorithm, and curve (modulus) should have the
         * same length (256 or 512 bits), public key and signature should be
         * twice bigger.
         */
        if (!ctx->curve ||
            !ctx->digest ||
            !req->src ||
            !ctx->pub_key.x ||
            req->dst_len != ctx->digest_len ||
            req->dst_len != ctx->curve->g.ndigits * sizeof(u64) ||
            ctx->pub_key.ndigits != ctx->curve->g.ndigits ||
            req->dst_len * 2 != req->src_len ||
            WARN_ON(req->src_len > sizeof(sig)) ||
            WARN_ON(req->dst_len > sizeof(digest)))
                return -EBADMSG;

        sg_copy_to_buffer(req->src, sg_nents_for_len(req->src, req->src_len),
                          sig, req->src_len);
        sg_pcopy_to_buffer(req->src,
                           sg_nents_for_len(req->src,
                                            req->src_len + req->dst_len),
                           digest, req->dst_len, req->src_len);

        vli_from_be64(s, sig, ndigits);
        vli_from_be64(r, sig + ndigits * sizeof(u64), ndigits);

        /* Step 1: verify that 0 < r < q, 0 < s < q */
        if (vli_is_zero(r, ndigits) ||
            vli_cmp(r, ctx->curve->n, ndigits) == 1 ||
            vli_is_zero(s, ndigits) ||
            vli_cmp(s, ctx->curve->n, ndigits) == 1)
                return -EKEYREJECTED;

        /* Step 2: calculate hash (h) of the message (passed as input) */
        /* Step 3: calculate e = h \mod q */
        vli_from_le64(e, digest, ndigits);
        if (vli_cmp(e, ctx->curve->n, ndigits) == 1)
                vli_sub(e, e, ctx->curve->n, ndigits);
        if (vli_is_zero(e, ndigits))
                e[0] = 1;

        /* Step 4: calculate v = e^{-1} \mod q */
        vli_mod_inv(v, e, ctx->curve->n, ndigits);

        /* Step 5: calculate z_1 = sv \mod q, z_2 = -rv \mod q */
        vli_mod_mult_slow(z1, s, v, ctx->curve->n, ndigits);
        vli_sub(_r, ctx->curve->n, r, ndigits);
        vli_mod_mult_slow(z2, _r, v, ctx->curve->n, ndigits);

        /* Step 6: calculate point C = z_1P + z_2Q, and R = x_c \mod q */
        ecc_point_mult_shamir(&cc, z1, &ctx->curve->g, z2, &ctx->pub_key,
                              ctx->curve);
        if (vli_cmp(cc.x, ctx->curve->n, ndigits) == 1)
                vli_sub(cc.x, cc.x, ctx->curve->n, ndigits);

        /* Step 7: if R == r signature is valid */
        if (!vli_cmp(cc.x, r, ndigits))
                return 0;
        else
                return -EKEYREJECTED;
}

int ecrdsa_param_curve(void *context, size_t hdrlen, unsigned char tag,
                       const void *value, size_t vlen)
{
        struct ecrdsa_ctx *ctx = context;

        ctx->curve_oid = look_up_OID(value, vlen);
        if (!ctx->curve_oid)
                return -EINVAL;
        ctx->curve = get_curve_by_oid(ctx->curve_oid);
        return 0;
}

/* Optional. If present should match expected digest algo OID. */
int ecrdsa_param_digest(void *context, size_t hdrlen, unsigned char tag,
                        const void *value, size_t vlen)
{
        struct ecrdsa_ctx *ctx = context;
        int digest_oid = look_up_OID(value, vlen);

        if (digest_oid != ctx->digest_oid)
                return -EINVAL;
        return 0;
}

int ecrdsa_parse_pub_key(void *context, size_t hdrlen, unsigned char tag,
                         const void *value, size_t vlen)
{
        struct ecrdsa_ctx *ctx = context;

        ctx->key = value;
        ctx->key_len = vlen;
        return 0;
}

static u8 *ecrdsa_unpack_u32(u32 *dst, void *src)
{
        memcpy(dst, src, sizeof(u32));
        return src + sizeof(u32);
}

/* Parse BER encoded subjectPublicKey. */
static int ecrdsa_set_pub_key(struct crypto_akcipher *tfm, const void *key,
                              unsigned int keylen)
{
        struct ecrdsa_ctx *ctx = akcipher_tfm_ctx(tfm);
        unsigned int ndigits;
        u32 algo, paramlen;
        u8 *params;
        int err;

        err = asn1_ber_decoder(&ecrdsa_pub_key_decoder, ctx, key, keylen);
        if (err < 0)
                return err;

        /* Key parameters is in the key after keylen. */
        params = ecrdsa_unpack_u32(&paramlen,
                          ecrdsa_unpack_u32(&algo, (u8 *)key + keylen));

        if (algo == OID_gost2012PKey256) {
                ctx->digest     = "streebog256";
                ctx->digest_oid = OID_gost2012Digest256;
                ctx->digest_len = 256 / 8;
        } else if (algo == OID_gost2012PKey512) {
                ctx->digest     = "streebog512";
                ctx->digest_oid = OID_gost2012Digest512;
                ctx->digest_len = 512 / 8;
        } else
                return -ENOPKG;
        ctx->algo_oid = algo;

        /* Parse SubjectPublicKeyInfo.AlgorithmIdentifier.parameters. */
        err = asn1_ber_decoder(&ecrdsa_params_decoder, ctx, params, paramlen);
        if (err < 0)
                return err;
        /*
         * Sizes of algo (set in digest_len) and curve should match
         * each other.
         */
        if (!ctx->curve ||
            ctx->curve->g.ndigits * sizeof(u64) != ctx->digest_len)
                return -ENOPKG;
        /*
         * Key is two 256- or 512-bit coordinates which should match
         * curve size.
         */
        if ((ctx->key_len != (2 * 256 / 8) &&
             ctx->key_len != (2 * 512 / 8)) ||
            ctx->key_len != ctx->curve->g.ndigits * sizeof(u64) * 2)
                return -ENOPKG;

        ndigits = ctx->key_len / sizeof(u64) / 2;
        ctx->pub_key = ECC_POINT_INIT(ctx->_pubp[0], ctx->_pubp[1], ndigits);
        vli_from_le64(ctx->pub_key.x, ctx->key, ndigits);
        vli_from_le64(ctx->pub_key.y, ctx->key + ndigits * sizeof(u64),
                      ndigits);

        if (ecc_is_pubkey_valid_partial(ctx->curve, &ctx->pub_key))
                return -EKEYREJECTED;

        return 0;
}

static unsigned int ecrdsa_max_size(struct crypto_akcipher *tfm)
{
        struct ecrdsa_ctx *ctx = akcipher_tfm_ctx(tfm);

        /*
         * Verify doesn't need any output, so it's just informational
         * for keyctl to determine the key bit size.
         */
        return ctx->pub_key.ndigits * sizeof(u64);
}

static void ecrdsa_exit_tfm(struct crypto_akcipher *tfm)
{
}

static struct akcipher_alg ecrdsa_alg = {
        .verify         = ecrdsa_verify,
        .set_pub_key    = ecrdsa_set_pub_key,
        .max_size       = ecrdsa_max_size,
        .exit           = ecrdsa_exit_tfm,
        .base = {
                .cra_name        = "ecrdsa",
                .cra_driver_name = "ecrdsa-generic",
                .cra_priority    = 100,
                .cra_module      = THIS_MODULE,
                .cra_ctxsize     = sizeof(struct ecrdsa_ctx),
        },
};

static int __init ecrdsa_mod_init(void)
{
        return crypto_register_akcipher(&ecrdsa_alg);
}

static void __exit ecrdsa_mod_fini(void)
{
        crypto_unregister_akcipher(&ecrdsa_alg);
}

module_init(ecrdsa_mod_init);
module_exit(ecrdsa_mod_fini);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Vitaly Chikunov <vt@altlinux.org>");
MODULE_DESCRIPTION("EC-RDSA generic algorithm");
MODULE_ALIAS_CRYPTO("ecrdsa-generic");