secp256k1-zkp/src/modules/rangeproof/rangeproof_impl.h

/**********************************************************************
 * Copyright (c) 2015 Gregory Maxwell                                 *
 * Distributed under the MIT software license, see the accompanying   *
 * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
 **********************************************************************/

#ifndef _SECP256K1_RANGEPROOF_IMPL_H_
#define _SECP256K1_RANGEPROOF_IMPL_H_

#include "eckey.h"
#include "scalar.h"
#include "group.h"
#include "rangeproof.h"
#include "hash_impl.h"
#include "pedersen_impl.h"
#include "util.h"

#include "modules/rangeproof/pedersen.h"
#include "modules/rangeproof/borromean.h"

SECP256K1_INLINE static void secp256k1_rangeproof_pub_expand(secp256k1_gej *pubs,
 int exp, size_t *rsizes, size_t rings, const secp256k1_ge* genp) {
    secp256k1_gej base;
    size_t i;
    size_t j;
    size_t npub;
    VERIFY_CHECK(exp < 19);
    if (exp < 0) {
        exp = 0;
    }
    secp256k1_gej_set_ge(&base, genp);
    secp256k1_gej_neg(&base, &base);
    while (exp--) {
        /* Multiplication by 10 */
        secp256k1_gej tmp;
        secp256k1_gej_double_var(&tmp, &base, NULL);
        secp256k1_gej_double_var(&base, &tmp, NULL);
        secp256k1_gej_double_var(&base, &base, NULL);
        secp256k1_gej_add_var(&base, &base, &tmp, NULL);
    }
    npub = 0;
    for (i = 0; i < rings; i++) {
        for (j = 1; j < rsizes[i]; j++) {
            secp256k1_gej_add_var(&pubs[npub + j], &pubs[npub + j - 1], &base, NULL);
        }
        if (i < rings - 1) {
            secp256k1_gej_double_var(&base, &base, NULL);
            secp256k1_gej_double_var(&base, &base, NULL);
        }
        npub += rsizes[i];
    }
}

SECP256K1_INLINE static void secp256k1_rangeproof_serialize_point(unsigned char* data, const secp256k1_ge *point) {
    secp256k1_fe pointx;
    pointx = point->x;
    secp256k1_fe_normalize(&pointx);
    data[0] = !secp256k1_fe_is_quad_var(&point->y);
    secp256k1_fe_get_b32(data + 1, &pointx);
}

SECP256K1_INLINE static int secp256k1_rangeproof_genrand(secp256k1_scalar *sec, secp256k1_scalar *s, unsigned char *message,
 size_t *rsizes, size_t rings, const unsigned char *nonce, const secp256k1_ge *commit, const unsigned char *proof, size_t len, const secp256k1_ge* genp) {
    unsigned char tmp[32];
    unsigned char rngseed[32 + 33 + 33 + 10];
    secp256k1_rfc6979_hmac_sha256 rng;
    secp256k1_scalar acc;
    int overflow;
    int ret;
    size_t i;
    size_t j;
    int b;
    size_t npub;
    VERIFY_CHECK(len <= 10);
    memcpy(rngseed, nonce, 32);
    secp256k1_rangeproof_serialize_point(rngseed + 32, commit);
    secp256k1_rangeproof_serialize_point(rngseed + 32 + 33, genp);
    memcpy(rngseed + 33 + 33 + 32, proof, len);
    secp256k1_rfc6979_hmac_sha256_initialize(&rng, rngseed, 32 + 33 + 33 + len);
    secp256k1_scalar_clear(&acc);
    npub = 0;
    ret = 1;
    for (i = 0; i < rings; i++) {
        if (i < rings - 1) {
            secp256k1_rfc6979_hmac_sha256_generate(&rng, tmp, 32);
            do {
                secp256k1_rfc6979_hmac_sha256_generate(&rng, tmp, 32);
                secp256k1_scalar_set_b32(&sec[i], tmp, &overflow);
            } while (overflow || secp256k1_scalar_is_zero(&sec[i]));
            secp256k1_scalar_add(&acc, &acc, &sec[i]);
        } else {
            secp256k1_scalar_negate(&acc, &acc);
            sec[i] = acc;
        }
        for (j = 0; j < rsizes[i]; j++) {
            secp256k1_rfc6979_hmac_sha256_generate(&rng, tmp, 32);
            if (message) {
                for (b = 0; b < 32; b++) {
                    tmp[b] ^= message[(i * 4 + j) * 32 + b];
                    message[(i * 4 + j) * 32 + b] = tmp[b];
                }
            }
            secp256k1_scalar_set_b32(&s[npub], tmp, &overflow);
            ret &= !(overflow || secp256k1_scalar_is_zero(&s[npub]));
            npub++;
        }
    }
    secp256k1_rfc6979_hmac_sha256_finalize(&rng);
    secp256k1_scalar_clear(&acc);
    memset(tmp, 0, 32);
    return ret;
}

SECP256K1_INLINE static int secp256k1_range_proveparams(uint64_t *v, size_t *rings, size_t *rsizes, size_t *npub, size_t *secidx, uint64_t *min_value,
 int *mantissa, uint64_t *scale,  int *exp, int *min_bits, uint64_t value) {
    size_t i;
    *rings = 1;
    rsizes[0] = 1;
    secidx[0] = 0;
    *scale = 1;
    *mantissa = 0;
    *npub = 0;
    if (*min_value == UINT64_MAX) {
        /* If the minimum value is the maximal representable value, then we cannot code a range. */
        *exp = -1;
    }
    if (*exp >= 0) {
        int max_bits;
        uint64_t v2;
        if ((*min_value && value > INT64_MAX) || (value && *min_value >= INT64_MAX)) {
            /* If either value or min_value is >= 2^63-1 then the other must by zero to avoid overflowing the proven range. */
            return 0;
        }
        max_bits = *min_value ? secp256k1_clz64_var(*min_value) : 64;
        if (*min_bits > max_bits) {
            *min_bits = max_bits;
        }
        if (*min_bits > 61 || value > INT64_MAX) {
            /** Ten is not a power of two, so dividing by ten and then representing in base-2 times ten
             *   expands the representable range. The verifier requires the proven range is within 0..2**64.
             *   For very large numbers (all over 2**63) we must change our exponent to compensate.
             *  Rather than handling it precisely, this just disables use of the exponent for big values.
             */
            *exp = 0;
        }
        /* Mask off the least significant digits, as requested. */
        *v = value - *min_value;
        /* If the user has asked for more bits of proof then there is room for in the exponent, reduce the exponent. */
        v2 = *min_bits ? (UINT64_MAX>>(64-*min_bits)) : 0;
        for (i = 0; (int) i < *exp && (v2 <= UINT64_MAX / 10); i++) {
            *v /= 10;
            v2 *= 10;
        }
        *exp = i;
        v2 = *v;
        for (i = 0; (int) i < *exp; i++) {
            v2 *= 10;
            *scale *= 10;
        }
        /* If the masked number isn't precise, compute the public offset. */
        *min_value = value - v2;
        /* How many bits do we need to represent our value? */
        *mantissa = *v ? 64 - secp256k1_clz64_var(*v) : 1;
        if (*min_bits > *mantissa) {
            /* If the user asked for more precision, give it to them. */
            *mantissa = *min_bits;
        }
        /* Digits in radix-4, except for the last digit if our mantissa length is odd. */
        *rings = (*mantissa + 1) >> 1;
        for (i = 0; i < *rings; i++) {
            rsizes[i] = ((i < *rings - 1) | (!(*mantissa&1))) ? 4 : 2;
            *npub += rsizes[i];
            secidx[i] = (*v >> (i*2)) & 3;
        }
        VERIFY_CHECK(*mantissa>0);
        VERIFY_CHECK((*v & ~(UINT64_MAX>>(64-*mantissa))) == 0); /* Did this get all the bits? */
    } else {
        /* A proof for an exact value. */
        *exp = 0;
        *min_value = value;
        *v = 0;
        *npub = 2;
    }
    VERIFY_CHECK(*v * *scale + *min_value == value);
    VERIFY_CHECK(*rings > 0);
    VERIFY_CHECK(*rings <= 32);
    VERIFY_CHECK(*npub <= 128);
    return 1;
}

/* strawman interface, writes proof in proof, a buffer of plen, proves with respect to min_value the range for commit which has the provided blinding factor and value. */
SECP256K1_INLINE static int secp256k1_rangeproof_sign_impl(const secp256k1_ecmult_context* ecmult_ctx,
 const secp256k1_ecmult_gen_context* ecmult_gen_ctx,
 unsigned char *proof, size_t *plen, uint64_t min_value,
 const secp256k1_ge *commit, const unsigned char *blind, const unsigned char *nonce, int exp, int min_bits, uint64_t value,
 const unsigned char *message, size_t msg_len, const unsigned char *extra_commit, size_t extra_commit_len, const secp256k1_ge* genp){
    secp256k1_gej pubs[128];     /* Candidate digits for our proof, most inferred. */
    secp256k1_scalar s[128];     /* Signatures in our proof, most forged. */
    secp256k1_scalar sec[32];    /* Blinding factors for the correct digits. */
    secp256k1_scalar k[32];      /* Nonces for our non-forged signatures. */
    secp256k1_scalar stmp;
    secp256k1_sha256 sha256_m;
    unsigned char prep[4096];
    unsigned char tmp[33];
    unsigned char *signs;          /* Location of sign flags in the proof. */
    uint64_t v;
    uint64_t scale;                /* scale = 10^exp. */
    int mantissa;                  /* Number of bits proven in the blinded value. */
    size_t rings;                     /* How many digits will our proof cover. */
    size_t rsizes[32];                /* How many possible values there are for each place. */
    size_t secidx[32];                /* Which digit is the correct one. */
    size_t len;                       /* Number of bytes used so far. */
    size_t i;
    int overflow;
    size_t npub;
    len = 0;
    if (*plen < 65 || min_value > value || min_bits > 64 || min_bits < 0 || exp < -1 || exp > 18) {
        return 0;
    }
    if (!secp256k1_range_proveparams(&v, &rings, rsizes, &npub, secidx, &min_value, &mantissa, &scale, &exp, &min_bits, value)) {
        return 0;
    }
    proof[len] = (rsizes[0] > 1 ? (64 | exp) : 0) | (min_value ? 32 : 0);
    len++;
    if (rsizes[0] > 1) {
        VERIFY_CHECK(mantissa > 0 && mantissa <= 64);
        proof[len] = mantissa - 1;
        len++;
    }
    if (min_value) {
        for (i = 0; i < 8; i++) {
            proof[len + i] = (min_value >> ((7-i) * 8)) & 255;
        }
        len += 8;
    }
    /* Do we have enough room in the proof for the message? Each ring gives us 128 bytes, but the
     * final ring is used to encode the blinding factor and the value, so we can't use that. (Well,
     * technically there are 64 bytes available if we avoided the other data, but this is difficult
     * because it's not always in the same place. */
    if (msg_len > 0 && msg_len > 128 * (rings - 1)) {
        return 0;
    }
    /* Do we have enough room for the proof? */
    if (*plen - len < 32 * (npub + rings - 1) + 32 + ((rings+6) >> 3)) {
        return 0;
    }
    secp256k1_sha256_initialize(&sha256_m);
    secp256k1_rangeproof_serialize_point(tmp, commit);
    secp256k1_sha256_write(&sha256_m, tmp, 33);
    secp256k1_rangeproof_serialize_point(tmp, genp);
    secp256k1_sha256_write(&sha256_m, tmp, 33);
    secp256k1_sha256_write(&sha256_m, proof, len);

    memset(prep, 0, 4096);
    if (message != NULL) {
        memcpy(prep, message, msg_len);
    }
    /* Note, the data corresponding to the blinding factors must be zero. */
    if (rsizes[rings - 1] > 1) {
        size_t idx;
        /* Value encoding sidechannel. */
        idx = rsizes[rings - 1] - 1;
        idx -= secidx[rings - 1] == idx;
        idx = ((rings - 1) * 4 + idx) * 32;
        for (i = 0; i < 8; i++) {
            prep[8 + i + idx] = prep[16 + i + idx] = prep[24 + i + idx] = (v >> (56 - i * 8)) & 255;
            prep[i + idx] = 0;
        }
        prep[idx] = 128;
    }
    if (!secp256k1_rangeproof_genrand(sec, s, prep, rsizes, rings, nonce, commit, proof, len, genp)) {
        return 0;
    }
    memset(prep, 0, 4096);
    for (i = 0; i < rings; i++) {
        /* Sign will overwrite the non-forged signature, move that random value into the nonce. */
        k[i] = s[i * 4 + secidx[i]];
        secp256k1_scalar_clear(&s[i * 4 + secidx[i]]);
    }
    /** Genrand returns the last blinding factor as -sum(rest),
     *   adding in the blinding factor for our commitment, results in the blinding factor for
     *   the commitment to the last digit that the verifier can compute for itself by subtracting
     *   all the digits in the proof from the commitment. This lets the prover skip sending the
     *   blinded value for one digit.
     */
    secp256k1_scalar_set_b32(&stmp, blind, &overflow);
    secp256k1_scalar_add(&sec[rings - 1], &sec[rings - 1], &stmp);
    if (overflow || secp256k1_scalar_is_zero(&sec[rings - 1])) {
        return 0;
    }
    signs = &proof[len];
    /* We need one sign bit for each blinded value we send. */
    for (i = 0; i < (rings + 6) >> 3; i++) {
        signs[i] = 0;
        len++;
    }
    npub = 0;
    for (i = 0; i < rings; i++) {
        /*OPT: Use the precomputed gen2 basis?*/
        secp256k1_pedersen_ecmult(ecmult_gen_ctx, &pubs[npub], &sec[i], ((uint64_t)secidx[i] * scale) << (i*2), genp);
        if (secp256k1_gej_is_infinity(&pubs[npub])) {
            return 0;
        }
        if (i < rings - 1) {
            unsigned char tmpc[33];
            secp256k1_ge c;
            unsigned char quadness;
            /*OPT: split loop and batch invert.*/
            /*OPT: do not compute full pubs[npub] in ge form; we only need x */
            secp256k1_ge_set_gej_var(&c, &pubs[npub]);
            secp256k1_rangeproof_serialize_point(tmpc, &c);
            quadness = tmpc[0];
            secp256k1_sha256_write(&sha256_m, tmpc, 33);
            signs[i>>3] |= quadness << (i&7);
            memcpy(&proof[len], tmpc + 1, 32);
            len += 32;
        }
        npub += rsizes[i];
    }
    secp256k1_rangeproof_pub_expand(pubs, exp, rsizes, rings, genp);
    if (extra_commit != NULL) {
        secp256k1_sha256_write(&sha256_m, extra_commit, extra_commit_len);
    }
    secp256k1_sha256_finalize(&sha256_m, tmp);
    if (!secp256k1_borromean_sign(ecmult_ctx, ecmult_gen_ctx, &proof[len], s, pubs, k, sec, rsizes, secidx, rings, tmp, 32)) {
        return 0;
    }
    len += 32;
    for (i = 0; i < npub; i++) {
        secp256k1_scalar_get_b32(&proof[len],&s[i]);
        len += 32;
    }
    VERIFY_CHECK(len <= *plen);
    *plen = len;
    memset(prep, 0, 4096);
    return 1;
}

/* Computes blinding factor x given k, s, and the challenge e. */
SECP256K1_INLINE static void secp256k1_rangeproof_recover_x(secp256k1_scalar *x, const secp256k1_scalar *k, const secp256k1_scalar *e,
 const secp256k1_scalar *s) {
    secp256k1_scalar stmp;
    secp256k1_scalar_negate(x, s);
    secp256k1_scalar_add(x, x, k);
    secp256k1_scalar_inverse(&stmp, e);
    secp256k1_scalar_mul(x, x, &stmp);
}

/* Computes ring's nonce given the blinding factor x, the challenge e, and the signature s. */
SECP256K1_INLINE static void secp256k1_rangeproof_recover_k(secp256k1_scalar *k, const secp256k1_scalar *x, const secp256k1_scalar *e,
 const secp256k1_scalar *s) {
    secp256k1_scalar stmp;
    secp256k1_scalar_mul(&stmp, x, e);
    secp256k1_scalar_add(k, s, &stmp);
}

SECP256K1_INLINE static void secp256k1_rangeproof_ch32xor(unsigned char *x, const unsigned char *y) {
    int i;
    for (i = 0; i < 32; i++) {
        x[i] ^= y[i];
    }
}

SECP256K1_INLINE static int secp256k1_rangeproof_rewind_inner(secp256k1_scalar *blind, uint64_t *v,
 unsigned char *m, size_t *mlen, secp256k1_scalar *ev, secp256k1_scalar *s,
 size_t *rsizes, size_t rings, const unsigned char *nonce, const secp256k1_ge *commit, const unsigned char *proof, size_t len, const secp256k1_ge *genp) {
    secp256k1_scalar s_orig[128];
    secp256k1_scalar sec[32];
    secp256k1_scalar stmp;
    unsigned char prep[4096];
    unsigned char tmp[32];
    uint64_t value;
    size_t offset;
    size_t i;
    size_t j;
    int b;
    size_t skip1;
    size_t skip2;
    size_t npub;
    npub = ((rings - 1) << 2) + rsizes[rings-1];
    VERIFY_CHECK(npub <= 128);
    VERIFY_CHECK(npub >= 1);
    memset(prep, 0, 4096);
    /* Reconstruct the provers random values. */
    secp256k1_rangeproof_genrand(sec, s_orig, prep, rsizes, rings, nonce, commit, proof, len, genp);
    *v = UINT64_MAX;
    secp256k1_scalar_clear(blind);
    if (rings == 1 && rsizes[0] == 1) {
        /* With only a single proof, we can only recover the blinding factor. */
        secp256k1_rangeproof_recover_x(blind, &s_orig[0], &ev[0], &s[0]);
        if (v) {
            *v = 0;
        }
        if (mlen) {
            *mlen = 0;
        }
        return 1;
    }
    npub = (rings - 1) << 2;
    for (j = 0; j < 2; j++) {
        size_t idx;
        /* Look for a value encoding in the last ring. */
        idx = npub + rsizes[rings - 1] - 1 - j;
        secp256k1_scalar_get_b32(tmp, &s[idx]);
        secp256k1_rangeproof_ch32xor(tmp, &prep[idx * 32]);
        if ((tmp[0] & 128) && (memcmp(&tmp[16], &tmp[24], 8) == 0) && (memcmp(&tmp[8], &tmp[16], 8) == 0)) {
            value = 0;
            for (i = 0; i < 8; i++) {
                value = (value << 8) + tmp[24 + i];
            }
            if (v) {
                *v = value;
            }
            memcpy(&prep[idx * 32], tmp, 32);
            break;
        }
    }
    if (j > 1) {
        /* Couldn't extract a value. */
        if (mlen) {
            *mlen = 0;
        }
        return 0;
    }
    skip1 = rsizes[rings - 1] - 1 - j;
    skip2 = ((value >> ((rings - 1) << 1)) & 3);
    if (skip1 == skip2) {
        /*Value is in wrong position.*/
        if (mlen) {
            *mlen = 0;
        }
        return 0;
    }
    skip1 += (rings - 1) << 2;
    skip2 += (rings - 1) << 2;
    /* Like in the rsize[] == 1 case, Having figured out which s is the one which was not forged, we can recover the blinding factor. */
    secp256k1_rangeproof_recover_x(&stmp, &s_orig[skip2], &ev[skip2], &s[skip2]);
    secp256k1_scalar_negate(&sec[rings - 1], &sec[rings - 1]);
    secp256k1_scalar_add(blind, &stmp, &sec[rings - 1]);
    if (!m || !mlen || *mlen == 0) {
        if (mlen) {
            *mlen = 0;
        }
        /* FIXME: cleanup in early out/failure cases. */
        return 1;
    }
    offset = 0;
    npub = 0;
    for (i = 0; i < rings; i++) {
        size_t idx;
        idx = (value >> (i << 1)) & 3;
        for (j = 0; j < rsizes[i]; j++) {
            if (npub == skip1 || npub == skip2) {
                npub++;
                continue;
            }
            if (idx == j) {
                /** For the non-forged signatures the signature is calculated instead of random, instead we recover the prover's nonces.
                 *  this could just as well recover the blinding factors and messages could be put there as is done for recovering the
                 *  blinding factor in the last ring, but it takes an inversion to recover x so it's faster to put the message data in k.
                 */
                secp256k1_rangeproof_recover_k(&stmp, &sec[i], &ev[npub], &s[npub]);
            } else {
                stmp = s[npub];
            }
            secp256k1_scalar_get_b32(tmp, &stmp);
            secp256k1_rangeproof_ch32xor(tmp, &prep[npub * 32]);
            for (b = 0; b < 32 && offset < *mlen; b++) {
                m[offset] = tmp[b];
                offset++;
            }
            npub++;
        }
    }
    *mlen = offset;
    memset(prep, 0, 4096);
    for (i = 0; i < 128; i++) {
        secp256k1_scalar_clear(&s_orig[i]);
    }
    for (i = 0; i < 32; i++) {
        secp256k1_scalar_clear(&sec[i]);
    }
    secp256k1_scalar_clear(&stmp);
    return 1;
}

SECP256K1_INLINE static int secp256k1_rangeproof_getheader_impl(size_t *offset, int *exp, int *mantissa, uint64_t *scale,
 uint64_t *min_value, uint64_t *max_value, const unsigned char *proof, size_t plen) {
    int i;
    int has_nz_range;
    int has_min;
    if (plen < 65 || ((proof[*offset] & 128) != 0)) {
        return 0;
    }
    has_nz_range = proof[*offset] & 64;
    has_min = proof[*offset] & 32;
    *exp = -1;
    *mantissa = 0;
    if (has_nz_range) {
        *exp = proof[*offset] & 31;
        *offset += 1;
        if (*exp > 18) {
           return 0;
        }
        *mantissa = proof[*offset] + 1;
        if (*mantissa > 64) {
            return 0;
         }
        *max_value = UINT64_MAX>>(64-*mantissa);
    } else {
        *max_value = 0;
    }
    *offset += 1;
    *scale = 1;
    for (i = 0; i < *exp; i++) {
        if (*max_value > UINT64_MAX / 10) {
            return 0;
        }
        *max_value *= 10;
        *scale *= 10;
    }
    *min_value = 0;
    if (has_min) {
        if(plen - *offset < 8) {
            return 0;
        }
        /*FIXME: Compact minvalue encoding?*/
        for (i = 0; i < 8; i++) {
            *min_value = (*min_value << 8) | proof[*offset + i];
        }
        *offset += 8;
    }
    if (*max_value > UINT64_MAX - *min_value) {
        return 0;
    }
    *max_value += *min_value;
    return 1;
}

/* Verifies range proof (len plen) for commit, the min/max values proven are put in the min/max arguments; returns 0 on failure 1 on success.*/
SECP256K1_INLINE static int secp256k1_rangeproof_verify_impl(const secp256k1_ecmult_context* ecmult_ctx,
 const secp256k1_ecmult_gen_context* ecmult_gen_ctx,
 unsigned char *blindout, uint64_t *value_out, unsigned char *message_out, size_t *outlen, const unsigned char *nonce,
 uint64_t *min_value, uint64_t *max_value, const secp256k1_ge *commit, const unsigned char *proof, size_t plen, const unsigned char *extra_commit, size_t extra_commit_len, const secp256k1_ge* genp) {
    secp256k1_gej accj;
    secp256k1_gej pubs[128];
    secp256k1_ge c;
    secp256k1_scalar s[128];
    secp256k1_scalar evalues[128]; /* Challenges, only used during proof rewind. */
    secp256k1_sha256 sha256_m;
    size_t rsizes[32];
    int ret;
    size_t i;
    int exp;
    int mantissa;
    size_t offset;
    size_t rings;
    int overflow;
    size_t npub;
    int offset_post_header;
    uint64_t scale;
    unsigned char signs[31];
    unsigned char m[33];
    const unsigned char *e0;
    offset = 0;
    if (!secp256k1_rangeproof_getheader_impl(&offset, &exp, &mantissa, &scale, min_value, max_value, proof, plen)) {
        return 0;
    }
    offset_post_header = offset;
    rings = 1;
    rsizes[0] = 1;
    npub = 1;
    if (mantissa != 0) {
        rings = (mantissa >> 1);
        for (i = 0; i < rings; i++) {
            rsizes[i] = 4;
        }
        npub = (mantissa >> 1) << 2;
        if (mantissa & 1) {
            rsizes[rings] = 2;
            npub += rsizes[rings];
            rings++;
        }
    }
    VERIFY_CHECK(rings <= 32);
    if (plen - offset < 32 * (npub + rings - 1) + 32 + ((rings+6) >> 3)) {
        return 0;
    }
    secp256k1_sha256_initialize(&sha256_m);
    secp256k1_rangeproof_serialize_point(m, commit);
    secp256k1_sha256_write(&sha256_m, m, 33);
    secp256k1_rangeproof_serialize_point(m, genp);
    secp256k1_sha256_write(&sha256_m, m, 33);
    secp256k1_sha256_write(&sha256_m, proof, offset);
    for(i = 0; i < rings - 1; i++) {
        signs[i] = (proof[offset + ( i>> 3)] & (1 << (i & 7))) != 0;
    }
    offset += (rings + 6) >> 3;
    if ((rings - 1) & 7) {
        /* Number of coded blinded points is not a multiple of 8, force extra sign bits to 0 to reject mutation. */
        if ((proof[offset - 1] >> ((rings - 1) & 7)) != 0) {
            return 0;
        }
    }
    npub = 0;
    secp256k1_gej_set_infinity(&accj);
    if (*min_value) {
        secp256k1_pedersen_ecmult_small(&accj, *min_value, genp);
    }
    for(i = 0; i < rings - 1; i++) {
        secp256k1_fe fe;
        if (!secp256k1_fe_set_b32(&fe, &proof[offset]) ||
            !secp256k1_ge_set_xquad(&c, &fe)) {
            return 0;
        }
        if (signs[i]) {
            secp256k1_ge_neg(&c, &c);
        }
        /* Not using secp256k1_rangeproof_serialize_point as we almost have it
         * serialized form already. */
        secp256k1_sha256_write(&sha256_m, &signs[i], 1);
        secp256k1_sha256_write(&sha256_m, &proof[offset], 32);
        secp256k1_gej_set_ge(&pubs[npub], &c);
        secp256k1_gej_add_ge_var(&accj, &accj, &c, NULL);
        offset += 32;
        npub += rsizes[i];
    }
    secp256k1_gej_neg(&accj, &accj);
    secp256k1_gej_add_ge_var(&pubs[npub], &accj, commit, NULL);
    if (secp256k1_gej_is_infinity(&pubs[npub])) {
        return 0;
    }
    secp256k1_rangeproof_pub_expand(pubs, exp, rsizes, rings, genp);
    npub += rsizes[rings - 1];
    e0 = &proof[offset];
    offset += 32;
    for (i = 0; i < npub; i++) {
        secp256k1_scalar_set_b32(&s[i], &proof[offset], &overflow);
        if (overflow) {
            return 0;
        }
        offset += 32;
    }
    if (offset != plen) {
        /*Extra data found, reject.*/
        return 0;
    }
    if (extra_commit != NULL) {
        secp256k1_sha256_write(&sha256_m, extra_commit, extra_commit_len);
    }
    secp256k1_sha256_finalize(&sha256_m, m);
    ret = secp256k1_borromean_verify(ecmult_ctx, nonce ? evalues : NULL, e0, s, pubs, rsizes, rings, m, 32);
    if (ret && nonce) {
        /* Given the nonce, try rewinding the witness to recover its initial state. */
        secp256k1_scalar blind;
        uint64_t vv;
        if (!ecmult_gen_ctx) {
            return 0;
        }
        if (!secp256k1_rangeproof_rewind_inner(&blind, &vv, message_out, outlen, evalues, s, rsizes, rings, nonce, commit, proof, offset_post_header, genp)) {
            return 0;
        }
        /* Unwind apparently successful, see if the commitment can be reconstructed. */
        /* FIXME: should check vv is in the mantissa's range. */
        vv = (vv * scale) + *min_value;
        secp256k1_pedersen_ecmult(ecmult_gen_ctx, &accj, &blind, vv, genp);
        if (secp256k1_gej_is_infinity(&accj)) {
            return 0;
        }
        secp256k1_gej_neg(&accj, &accj);
        secp256k1_gej_add_ge_var(&accj, &accj, commit, NULL);
        if (!secp256k1_gej_is_infinity(&accj)) {
            return 0;
        }
        if (blindout) {
            secp256k1_scalar_get_b32(blindout, &blind);
        }
        if (value_out) {
            *value_out = vv;
        }
    }
    return ret;
}

#endif