Add alternative crypto implementation.

[paraslash.git] / gcrypt.c

/*
 * Copyright (C) 2011 Andre Noll <maan@systemlinux.org>
 *
 * Licensed under the GPL v2. For licencing details see COPYING.
 */

/** \file gcrypt.c Libgrcypt-based encryption/decryption routines. */

#include <regex.h>
#include <stdbool.h>
#include <gcrypt.h>

#include "para.h"
#include "error.h"
#include "string.h"
#include "crypt.h"
#include "crypt_backend.h"
#include "fd.h"

//#define GCRYPT_DEBUG 1

#ifdef GCRYPT_DEBUG
static void dump_buffer(const char *msg, unsigned char *buf, int len)
{
        int i;

        fprintf(stderr, "%s (%u bytes): ", msg, len);
        for (i = 0; i < len; i++)
                fprintf(stderr, "%02x ", buf[i]);
        fprintf(stderr, "\n");
}
#else
/** Empty. Define GCRYPT_DEBUG to dump buffers. */
#define dump_buffer(a, b, c)
#endif

void hash_function(const char *data, unsigned long len, unsigned char *hash)
{
        gcry_error_t gret;
        gcry_md_hd_t handle;
        unsigned char *md;

        gret = gcry_md_open(&handle, GCRY_MD_SHA1, 0);
        assert(gret == 0);
        gcry_md_write(handle, data, (size_t)len);
        gcry_md_final(handle);
        md = gcry_md_read(handle, GCRY_MD_SHA1);
        assert(md);
        memcpy(hash, md, HASH_SIZE);
        gcry_md_close(handle);
}

void get_random_bytes_or_die(unsigned char *buf, int num)
{
        gcry_randomize(buf, (size_t)num, GCRY_STRONG_RANDOM);
}

/*
 * This is called at the beginning of every program that uses libgcrypt.  We
 * don't have to initialize any random seed here, but the call to gcry_control
 * is necessary to avoid warnings of the form "missing initialization - please
 * fix the application".
 */
void init_random_seed_or_die(void)
{
        gcry_control(GCRYCTL_INITIALIZATION_FINISHED, 0);
}

/** S-expression for the public part of an RSA key. */
#define RSA_PUBKEY_SEXP  "(public-key (rsa (n %m) (e %m)))"
/** S-expression for a private RSA key. */
#define RSA_PRIVKEY_SEXP "(private-key (rsa (n %m) (e %m) (d %m) (p %m) (q %m) (u %m)))"

/* rfc 3447, appendix B.2 */
static void mgf1(unsigned char *seed, size_t seed_len, unsigned result_len,
                unsigned char *result)
{
        gcry_error_t gret;
        gcry_md_hd_t handle;
        size_t n;;
        unsigned char *md;
        unsigned char octet_string[4], *rp = result, *end = rp + result_len;

        assert(result_len / HASH_SIZE < 1ULL << 31);
        gret = gcry_md_open(&handle, GCRY_MD_SHA1, 0);
        assert(gret == 0);
        for (n = 0; rp < end; n++) {
                gcry_md_write(handle, seed, seed_len);
                octet_string[0] = (unsigned char)((n >> 24) & 255);
                octet_string[1] = (unsigned char)((n >> 16) & 255);
                octet_string[2] = (unsigned char)((n >> 8)) & 255;
                octet_string[3] = (unsigned char)(n & 255);
                gcry_md_write(handle, octet_string, 4);
                gcry_md_final(handle);
                md = gcry_md_read(handle, GCRY_MD_SHA1);
                memcpy(rp, md, PARA_MIN(HASH_SIZE, (int)(end - rp)));
                rp += HASH_SIZE;
                gcry_md_reset(handle);
        }
        gcry_md_close(handle);
}

/** The sha1 hash of an empty file. */
static const unsigned char empty_hash[HASH_SIZE] =
        "\xda" "\x39" "\xa3" "\xee" "\x5e"
        "\x6b" "\x4b" "\x0d" "\x32" "\x55"
        "\xbf" "\xef" "\x95" "\x60" "\x18"
        "\x90" "\xaf" "\xd8" "\x07" "\x09";

/* rfc3447, section 7.1.1 */
static void pad_oaep(unsigned char *in, size_t in_len, unsigned char *out,
                size_t out_len)
{
        size_t ps_len = out_len - in_len - 2 * HASH_SIZE - 2;
        size_t n, mask_len = out_len - HASH_SIZE - 1;
        unsigned char *seed = out + 1, *db = seed + HASH_SIZE,
                *ps = db + HASH_SIZE, *one = ps + ps_len;
        unsigned char *db_mask, seed_mask[HASH_SIZE];

        assert(in_len <= out_len - 2 - 2 * HASH_SIZE);
        assert(out_len > 2 * HASH_SIZE + 2);
        PARA_DEBUG_LOG("padding %zu byte input -> %zu byte output\n",
                in_len, out_len);
        dump_buffer("unpadded buffer", in, in_len);

        out[0] = '\0';
        get_random_bytes_or_die(seed, HASH_SIZE);
        memcpy(db, empty_hash, HASH_SIZE);
        memset(ps, 0, ps_len);
        *one = 0x01;
        memcpy(one + 1, in, in_len);
        db_mask = para_malloc(mask_len);
        mgf1(seed, HASH_SIZE, mask_len, db_mask);
        for (n = 0; n < mask_len; n++)
                db[n] ^= db_mask[n];
        mgf1(db, mask_len, HASH_SIZE, seed_mask);
        for (n = 0; n < HASH_SIZE; n++)
                seed[n] ^= seed_mask[n];
        free(db_mask);
        dump_buffer("padded buffer", out, out_len);
}

/* rfc 3447, section 7.1.2 */
static int unpad_oaep(unsigned char *in, size_t in_len, unsigned char *out,
                size_t *out_len)
{       int ret;
        unsigned char *masked_seed = in + 1;
        unsigned char *db = in + 1 + HASH_SIZE;
        unsigned char seed[HASH_SIZE], seed_mask[HASH_SIZE];
        unsigned char *db_mask, *p;
        size_t n, mask_len = in_len - HASH_SIZE - 1;

        mgf1(db, mask_len, HASH_SIZE, seed_mask);
        for (n = 0; n < HASH_SIZE; n++)
                seed[n] = masked_seed[n] ^ seed_mask[n];
        db_mask = para_malloc(mask_len);
        mgf1(seed, HASH_SIZE, mask_len, db_mask);
        for (n = 0; n < mask_len; n++)
                db[n] ^= db_mask[n];
        free(db_mask);
        if (memcmp(db, empty_hash, HASH_SIZE))
                return -E_OEAP;
        for (p = db + HASH_SIZE; p < in + in_len - 1; p++)
                if (*p != '\0')
                        break;
        if (p >= in + in_len - 1)
                return -E_OEAP;
        p++;
        *out_len = in + in_len - p;
        memcpy(out, p, *out_len);
        return ret;
}

struct asymmetric_key {
        gcry_sexp_t sexp;
        int num_bytes;
};

static const char *gcrypt_strerror(gcry_error_t gret)
{
        return gcry_strerror(gcry_err_code(gret));
}

static int decode_key(const char *key_file, const char *header_str,
                const char *footer_str, unsigned char **result)
{
        int ret, ret2, i, j;
        void *map;
        size_t map_size, key_size, blob_size;
        unsigned char *blob = NULL;
        char *begin, *footer, *key;

        ret = mmap_full_file(key_file, O_RDONLY, &map, &map_size, NULL);
        if (ret < 0)
                return ret;
        ret = -E_KEY_MARKER;
        if (strncmp(map, header_str, strlen(header_str)))
                goto unmap;
        footer = strstr(map, footer_str);
        ret = -E_KEY_MARKER;
        if (!footer)
                goto unmap;
        begin = map + strlen(header_str);
        /* skip whitespace at the beginning */
        for (; begin < footer; begin++) {
                if (para_isspace(*begin))
                        continue;
                break;
        }
        ret = -E_KEY_MARKER;
        if (begin >= footer)
                goto unmap;

        key_size = footer - begin;
        key = para_malloc(key_size + 1);
        for (i = 0, j = 0; begin + i < footer; i++) {
                if (para_isspace(begin[i]))
                        continue;
                key[j++] = begin[i];
        }
        key[j] = '\0';
        //PARA_CRIT_LOG("key: %s\n", key);
        blob_size = key_size * 2;
        blob = para_malloc(blob_size);
        ret = base64_decode(key, blob, blob_size);
        free(key);
        if (ret < 0)
                goto free_unmap;
        goto unmap;
free_unmap:
        free(blob);
        blob = NULL;
unmap:
        ret2 = para_munmap(map, map_size);
        if (ret >= 0 && ret2 < 0)
                ret = ret2;
        if (ret < 0) {
                free(blob);
                blob = NULL;
        }
        *result = blob;
        return ret;
}

/** ASN Types and their code. */
enum asn1_types {
        /** The next object is an integer. */
        ASN1_TYPE_INTEGER = 0x2,
        /** Bit string object. */
        ASN1_TYPE_BIT_STRING = 0x03,
        /** Keys start with one big type sequence. */
        ASN1_TYPE_SEQUENCE = 0x30,
};

/* bit 6 has value 0 */
static inline bool is_primitive(unsigned char c)
{
        return ((c & (1<<6)) == 0);
}

static inline bool is_primitive_integer(unsigned char c)
{
        if (!is_primitive(c))
                return false;
        return ((c & 0x1f) == ASN1_TYPE_INTEGER);
}

/* Bit 8 is zero (and bits 7-1 give the length) */
static inline bool is_short_form(unsigned char c)
{
        return (c & 0x80) == 0;
}

static inline int get_short_form_length(unsigned char c)
{
        return c & 0x7f;
}

static inline int get_long_form_num_length_bytes(unsigned char c)
{
        return c & 0x7f;
}

static int find_pubkey_bignum_offset(const unsigned char *data, int len)
{
        const unsigned char *p = data, *end = data + len;

        /* the whole thing istarts with one sequence */
        if (*p != ASN1_TYPE_SEQUENCE)
                return -E_ASN1_PARSE;
        p++;
        if (p >= end)
                return -E_ASN1_PARSE;
        if (is_short_form(*p))
                p++;
        else
                p += 1 + get_long_form_num_length_bytes(*p);
        if (p >= end)
                return -E_ASN1_PARSE;
        /* another sequence containing the object id, skip it */
        if (*p != ASN1_TYPE_SEQUENCE)
                return -E_ASN1_PARSE;
        p++;
        if (p >= end)
                return -E_ASN1_PARSE;
        if (!is_short_form(*p))
                return -E_ASN1_PARSE;
        p += 1 + get_short_form_length(*p);
        if (p >= end)
                return -E_ASN1_PARSE;
        /* all numbers are wrapped in a bit string object that follows */
        if (*p != ASN1_TYPE_BIT_STRING)
                return -E_ASN1_PARSE;
        p++;
        if (p >= end)
                return -E_ASN1_PARSE;
        if (is_short_form(*p))
                p++;
        else
                p += 1 + get_long_form_num_length_bytes(*p);
        p++; /* skip number of unused bits in the bit string */
        if (p >= end)
                return -E_ASN1_PARSE;

        /* next, we have a sequence of two integers (n and e) */
        if (*p != ASN1_TYPE_SEQUENCE)
                return -E_ASN1_PARSE;
        p++;
        if (p >= end)
                return -E_ASN1_PARSE;
        if (is_short_form(*p))
                p++;
        else
                p += 1 + get_long_form_num_length_bytes(*p);
        if (p >= end)
                return -E_ASN1_PARSE;
        if (*p != ASN1_TYPE_INTEGER)
                return -E_ASN1_PARSE;
        return p - data;
}

/*
 * Returns: Number of bytes scanned. This may differ from the value returned via
 * bn_bytes because the latter does not include the ASN.1 prefix and a leading
 * zero is not considered as an additional byte for bn_bytes.
 */
static int read_bignum(unsigned char *start, unsigned char *end, gcry_mpi_t *bn,
                int *bn_bytes)
{
        int i, bn_size;
        gcry_error_t gret;
        unsigned char *cp = start;

        if (!is_primitive_integer(*cp))
                return -E_BAD_PRIVATE_KEY;
        cp++;
        if (is_short_form(*cp)) {
                bn_size = get_short_form_length(*cp);
                cp++;
        } else {
                int num_bytes = get_long_form_num_length_bytes(*cp);
                if (cp + num_bytes > end)
                        return -E_BAD_PRIVATE_KEY;
                if (num_bytes > 4) /* nobody has such a large modulus */
                        return -E_BAD_PRIVATE_KEY;
                cp++;
                bn_size = 0;
                for (i = 0; i < num_bytes; i++, cp++)
                        bn_size = (bn_size << 8) + *cp;
        }
        PARA_DEBUG_LOG("bn_size %d (0x%x)\n", bn_size, bn_size);
        gret = gcry_mpi_scan(bn, GCRYMPI_FMT_STD, cp, bn_size, NULL);
        if (gret) {
                PARA_ERROR_LOG("%s while scanning n\n",
                        gcry_strerror(gcry_err_code(gret)));
                return-E_MPI_SCAN;
        }
        /*
         * Don't take the first leading zero into account for the size of the
         * bignum.
         */
        if (*cp == '\0') {
                cp++;
                bn_size--;
        }
        if (bn_bytes)
                *bn_bytes = bn_size;
        cp += bn_size;
//      unsigned char *buf;
//      gcry_mpi_aprint(GCRYMPI_FMT_HEX, &buf, NULL, *bn);
//      PARA_CRIT_LOG("bn: %s\n", buf);
        return cp - start;
}

static int find_privkey_bignum_offset(const unsigned char *data, int len)
{
        const unsigned char *p = data, *end = data + len;

        /* like the public key, the whole thing is contained in a sequence */
        if (*p != ASN1_TYPE_SEQUENCE)
                return -E_ASN1_PARSE;
        p++;
        if (p >= end)
                return -E_ASN1_PARSE;
        if (is_short_form(*p))
                p++;
        else
                p += 1 + get_long_form_num_length_bytes(*p);
        if (p >= end)
                return -E_ASN1_PARSE;

        /* Skip next integer  */
        if (*p != ASN1_TYPE_INTEGER)
                return -E_ASN1_PARSE;
        p++;
        if (p >= end)
                return -E_ASN1_PARSE;
        if (is_short_form(*p))
                p += 1 + get_short_form_length(*p);
        else
                p += 1 + get_long_form_num_length_bytes(*p);
        if (p >= end)
                return -E_ASN1_PARSE;
        return p - data;
}

/** Private keys start with this header. */
#define PRIVATE_KEY_HEADER "-----BEGIN RSA PRIVATE KEY-----"
/** Private keys end with this footer. */
#define PRIVATE_KEY_FOOTER "-----END RSA PRIVATE KEY-----"

static int get_private_key(const char *key_file, struct asymmetric_key **result)
{
        gcry_mpi_t n = NULL, e = NULL, d = NULL, p = NULL, q = NULL,
                u = NULL;
        unsigned char *blob, *cp, *end;
        int blob_size, ret, n_size;
        gcry_error_t gret;
        size_t erroff;
        gcry_sexp_t sexp;
        struct asymmetric_key *key;

        ret = decode_key(key_file, PRIVATE_KEY_HEADER, PRIVATE_KEY_FOOTER,
                &blob);
        if (ret < 0)
                return ret;
        blob_size = ret;
        end = blob + blob_size;
        ret = find_privkey_bignum_offset(blob, blob_size);
        if (ret < 0)
                goto free_blob;
        PARA_INFO_LOG("reading RSA params at offset %d\n", ret);
        cp = blob + ret;

        ret = read_bignum(cp, end, &n, &n_size);
        if (ret < 0)
                goto free_blob;
        cp += ret;

        ret = read_bignum(cp, end, &e, NULL);
        if (ret < 0)
                goto release_n;
        cp += ret;

        ret = read_bignum(cp, end, &d, NULL);
        if (ret < 0)
                goto release_e;
        cp += ret;

        ret = read_bignum(cp, end, &p, NULL);
        if (ret < 0)
                goto release_d;
        cp += ret;

        ret = read_bignum(cp, end, &q, NULL);
        if (ret < 0)
                goto release_p;
        cp += ret;
        ret = read_bignum(cp, end, &u, NULL);
        if (ret < 0)
                goto release_q;
        cp += ret;
        /*
         * OpenSSL uses slightly different parameters than gcrypt. To use these
         * parameters we need to swap the values of p and q and recompute u.
         */
        if (gcry_mpi_cmp(p, q) > 0) {
                gcry_mpi_swap(p, q);
                gcry_mpi_invm(u, p, q);
        }
        gret = gcry_sexp_build(&sexp, &erroff, RSA_PRIVKEY_SEXP,
                n, e, d, p, q, u);

        if (gret) {
                PARA_ERROR_LOG("offset %zu: %s\n", erroff,
                        gcry_strerror(gcry_err_code(gret)));
                ret = -E_SEXP_BUILD;
                goto release_u;
        }
        key = para_malloc(sizeof(*key));
        key->sexp = sexp;
        *result = key;
        ret = n_size * 8;
        PARA_INFO_LOG("succesfully read %d bit private key\n", ret);
release_u:
        gcry_mpi_release(u);
release_q:
        gcry_mpi_release(q);
release_p:
        gcry_mpi_release(p);
release_d:
        gcry_mpi_release(d);
release_e:
        gcry_mpi_release(e);
release_n:
        gcry_mpi_release(n);
free_blob:
        free(blob);
        return ret;
}

/** Public keys start with this header. */
#define PUBLIC_KEY_HEADER "-----BEGIN PUBLIC KEY-----"
/** Public keys end with this footer. */
#define PUBLIC_KEY_FOOTER "-----END PUBLIC KEY-----"

static int get_asn_public_key(const char *key_file, struct asymmetric_key **result)
{
        gcry_mpi_t n = NULL, e = NULL;
        unsigned char *blob, *cp, *end;
        int blob_size, ret, n_size;
        gcry_error_t gret;
        size_t erroff;
        gcry_sexp_t sexp;
        struct asymmetric_key *key;

        ret = decode_key(key_file, PUBLIC_KEY_HEADER, PUBLIC_KEY_FOOTER,
                &blob);
        if (ret < 0)
                return ret;
        blob_size = ret;
        end = blob + blob_size;
        ret = find_pubkey_bignum_offset(blob, blob_size);
        if (ret < 0)
                goto free_blob;
        PARA_DEBUG_LOG("decoding public RSA params at offset %d\n", ret);
        cp = blob + ret;

        ret = read_bignum(cp, end, &n, &n_size);
        if (ret < 0)
                goto free_blob;
        cp += ret;

        ret = read_bignum(cp, end, &e, NULL);
        if (ret < 0)
                goto release_n;
        cp += ret;

        gret = gcry_sexp_build(&sexp, &erroff, RSA_PUBKEY_SEXP, n, e);
        if (gret) {
                PARA_ERROR_LOG("offset %zu: %s\n", erroff,
                        gcry_strerror(gcry_err_code(gret)));
                ret = -E_SEXP_BUILD;
                goto release_e;
        }
        key = para_malloc(sizeof(*key));
        key->sexp = sexp;
        *result = key;
        ret = n_size * 8;
        PARA_INFO_LOG("successfully read %u bit asn public key\n", n_size * 8);

release_e:
        gcry_mpi_release(e);
release_n:
        gcry_mpi_release(n);
free_blob:
        free(blob);
        return ret;
}

static int get_ssh_public_key(unsigned char *data, int size, gcry_sexp_t *result)
{
        int ret;
        gcry_error_t gret;
        unsigned char *blob = NULL, *p, *end;
        size_t nr_scanned, erroff, decoded_size;
        gcry_mpi_t e = NULL, n = NULL;

        PARA_DEBUG_LOG("decoding %d byte  public rsa-ssh key\n", size);
        if (size > INT_MAX / 4)
                return -ERRNO_TO_PARA_ERROR(EOVERFLOW);
        blob = para_malloc(2 * size);
        ret = uudecode((char *)data, blob, 2 * size);
        if (ret < 0)
                goto free_blob;
        decoded_size = ret;
        end = blob + decoded_size;
        dump_buffer("decoded key", blob, decoded_size);
        ret = check_ssh_key_header(blob, decoded_size);
        if (ret < 0)
                goto free_blob;
        p = blob + ret;
        ret = -E_SSH_PARSE;
        if (p >= end)
                goto free_blob;
        PARA_DEBUG_LOG("scanning modulus and public exponent\n");
        gret = gcry_mpi_scan(&e, GCRYMPI_FMT_SSH, p, end - p, &nr_scanned);
        if (gret) {
                ret = -E_MPI_SCAN;
                PARA_CRIT_LOG("%s\n", gcry_strerror(gcry_err_code(gret)));
                goto free_blob;
        }
        PARA_DEBUG_LOG("scanned e (%zu bytes)\n", nr_scanned);
//      gcry_mpi_aprint(GCRYMPI_FMT_HEX, &buf, NULL, rsa_e);
//      PARA_CRIT_LOG("e: %s\n", buf);
        p += nr_scanned;
        if (p >= end)
                goto release_e;
        gret = gcry_mpi_scan(&n, GCRYMPI_FMT_SSH, p, end - p, &nr_scanned);
        if (gret) {
                ret = -E_MPI_SCAN;
                PARA_ERROR_LOG("%s\n", gcry_strerror(gcry_err_code(gret)));
                goto release_e;
        }
        PARA_DEBUG_LOG("scanned n (%zu bytes)\n", nr_scanned);
//      gcry_mpi_aprint(GCRYMPI_FMT_HEX, &buf, NULL, rsa_n);
//      PARA_CRIT_LOG("n: %s\n", buf);
        gret = gcry_sexp_build(result, &erroff, RSA_PUBKEY_SEXP, n, e);
        if (gret) {
                PARA_ERROR_LOG("offset %zu: %s\n", erroff,
                        gcry_strerror(gcry_err_code(gret)));
                ret = -E_SEXP_BUILD;
                goto release_n;
        }
        ret = nr_scanned / 32 * 32;
        PARA_INFO_LOG("successfully read %u bit ssh public key\n", ret * 8);
release_n:
        gcry_mpi_release(n);
release_e:
        gcry_mpi_release(e);
free_blob:
        free(blob);
        return ret;
}

int get_asymmetric_key(const char *key_file, int private,
                struct asymmetric_key **result)
{
        int ret, ret2;
        void *map;
        size_t map_size;
        unsigned char *start, *end;
        gcry_sexp_t sexp;
        struct asymmetric_key *key;

        if (private)
                return get_private_key(key_file, result);
        ret = mmap_full_file(key_file, O_RDONLY, &map, &map_size, NULL);
        if (ret < 0)
                return ret;
        ret = is_ssh_rsa_key(map, map_size);
        if (!ret) {
                ret = para_munmap(map, map_size);
                if (ret < 0)
                        return ret;
                return get_asn_public_key(key_file, result);
        }
        start = map + ret;
        end = map + map_size;
        ret = -E_SSH_PARSE;
        if (start >= end)
                goto unmap;
        ret = get_ssh_public_key(start, end - start, &sexp);
        if (ret < 0)
                goto unmap;
        key = para_malloc(sizeof(*key));
        key->num_bytes = ret;
        key->sexp = sexp;
        *result = key;
        ret = key->num_bytes;
unmap:
        ret2 = para_munmap(map, map_size);
        if (ret >= 0 && ret2 < 0)
                ret = ret2;
        return ret;
}

void free_asymmetric_key(struct asymmetric_key *key)
{
        if (!key)
                return;
        gcry_sexp_release(key->sexp);
        free(key);
}

int priv_decrypt(const char *key_file, unsigned char *outbuf,
                unsigned char *inbuf, int inlen)
{
        gcry_error_t gret;
        int ret, key_size;
        struct asymmetric_key *priv;
        gcry_mpi_t in_mpi = NULL, out_mpi = NULL;
        gcry_sexp_t in, out, priv_key;
        size_t nbytes;
        unsigned char oaep_buf[512];

        PARA_INFO_LOG("decrypting %d byte input\n", inlen);
        /* key_file -> asymmetric key priv */
        ret = get_private_key(key_file, &priv);
        if (ret < 0)
                return ret;
        key_size = ret / 8;

        /* asymmetric key priv -> sexp priv_key */
        ret = -E_SEXP_FIND;
        priv_key = gcry_sexp_find_token(priv->sexp, "private-key", 0);
        if (!priv_key)
                goto free_key;

        /* inbuf -> in_mpi */
        gret = gcry_mpi_scan(&in_mpi, GCRYMPI_FMT_USG, inbuf,
                inlen, NULL);
        if (gret) {
                PARA_ERROR_LOG("%s\n", gcrypt_strerror(gret));
                ret = -E_MPI_SCAN;
                goto key_release;
        }

        /* in_mpi -> in sexp */
        gret = gcry_sexp_build(&in, NULL, "(enc-val(rsa(a %m)))", in_mpi);
        if (gret) {
                PARA_ERROR_LOG("%s\n", gcrypt_strerror(gret));
                ret = -E_SEXP_BUILD;
                goto in_mpi_release;
        }

        /* rsa decryption: in sexp -> out sexp */
        gret = gcry_pk_decrypt(&out, in, priv_key);
        if (gret) {
                PARA_ERROR_LOG("decrypt: %s\n", gcrypt_strerror(gret));
                ret = -E_SEXP_DECRYPT;
                goto in_release;
        }
        out_mpi = gcry_sexp_nth_mpi(out, 0, GCRYMPI_FMT_USG);
        if (!out_mpi) {
                ret = -E_SEXP_FIND;
                goto out_release;
        }
        gret = gcry_mpi_print(GCRYMPI_FMT_USG, oaep_buf, sizeof(oaep_buf),
                &nbytes, out_mpi);
        if (gret) {
                PARA_ERROR_LOG("mpi_print: %s\n", gcrypt_strerror(gret));
                ret = -E_MPI_PRINT;
                goto out_mpi_release;
        }
        /*
         * An oaep-encoded buffer always starts with at least one zero byte.
         * However, leading zeroes in an mpi are omitted in the output of
         * gcry_mpi_print() when using the GCRYMPI_FMT_USG format. The
         * alternative, GCRYMPI_FMT_STD, does not work either because here the
         * leading zero(es) might also be omitted, depending on the value of
         * the second byte.
         *
         * To circumvent this, we shift the oaep buffer to the right. But first
         * we check that the buffer actually started with a zero byte, i.e. that
         * nbytes < key_size. Otherwise a decoding error occurred.
         */
        ret = -E_SEXP_DECRYPT;
        if (nbytes >= key_size)
                goto out_mpi_release;
        memmove(oaep_buf + key_size - nbytes, oaep_buf, nbytes);
        memset(oaep_buf, 0, key_size - nbytes);

        PARA_DEBUG_LOG("decrypted buffer before unpad (%d bytes):\n",
                key_size);
        dump_buffer("non-unpadded decrypted buffer", oaep_buf, key_size);;
        unpad_oaep(oaep_buf, key_size, outbuf, &nbytes);
        PARA_DEBUG_LOG("decrypted buffer after unpad (%zu bytes):\n",
                nbytes);
        dump_buffer("unpadded decrypted buffer", outbuf, nbytes);;
        ret = nbytes;
        PARA_INFO_LOG("successfully decrypted %u byte message\n", ret);
out_mpi_release:
        gcry_mpi_release(out_mpi);
out_release:
        gcry_sexp_release(out);
in_release:
        gcry_sexp_release(in);
in_mpi_release:
        gcry_mpi_release(in_mpi);
key_release:
        gcry_sexp_release(priv_key);
free_key:
        free_asymmetric_key(priv);
        return ret;
}

int pub_encrypt(struct asymmetric_key *pub, unsigned char *inbuf,
                unsigned len, unsigned char *outbuf)
{
        gcry_error_t gret;
        const size_t pad_size = 256;
        gcry_sexp_t pub_key, in, out, out_a;
        gcry_mpi_t out_mpi = NULL;
        size_t nbytes;
        int ret;
        unsigned char padded_input[256];

        PARA_INFO_LOG("encrypting %u byte input with %d-byte key\n", len, pub->num_bytes);

        /* get pub key */
        pub_key = gcry_sexp_find_token(pub->sexp, "public-key", 0);
        if (!pub_key)
                return -E_SEXP_FIND;

        /* inbuf -> padded inbuf */
        pad_oaep(inbuf, len, padded_input, pad_size);

        /* padded inbuf -> in sexp */
        gret = gcry_sexp_build(&in, NULL, "(data(flags raw)(value %b))",
                pad_size, padded_input);
        if (gret) {
                PARA_ERROR_LOG("%s\n", gcrypt_strerror(gret));
                ret = -E_SEXP_BUILD;
                goto key_release;
        }
        /* rsa sexp encryption: in -> out */
        gret = gcry_pk_encrypt(&out, in, pub_key);
        if (gret) {
                PARA_ERROR_LOG("%s\n", gcrypt_strerror(gret));
                ret = -E_SEXP_ENCRYPT;
                goto in_release;
        }
        /* extract a, an MPI with the result of the RSA operation */
        ret = -E_SEXP_FIND;
        out_a = gcry_sexp_find_token(out, "a", 0);
        if (!out_a)
                goto out_release;
        /* convert sexp out_a -> out_mpi */
        out_mpi = gcry_sexp_nth_mpi(out_a, 1, GCRYMPI_FMT_STD);
        if (!out_mpi) {
                ret = -E_SEXP_FIND;
                goto out_a_release;
        }
        gret = gcry_mpi_print(GCRYMPI_FMT_USG, outbuf, 512 /* FIXME */, &nbytes, out_mpi);
        if (gret) {
                PARA_ERROR_LOG("%s\n", gcrypt_strerror(gret));
                ret = -E_SEXP_ENCRYPT;
                goto out_mpi_release;
        }
        PARA_INFO_LOG("encrypted buffer is %zu bytes\n", nbytes);
        dump_buffer("enc buf", outbuf, nbytes);
        ret = nbytes;

out_mpi_release:
        gcry_mpi_release(out_mpi);
out_a_release:
        gcry_sexp_release(out_a);
out_release:
        gcry_sexp_release(out);
in_release:
        gcry_sexp_release(in);
key_release:
        gcry_sexp_release(pub_key);
        return ret;
}

struct stream_cipher {
        gcry_cipher_hd_t handle;
};

struct stream_cipher *sc_new(const unsigned char *data, int len)
{
        gcry_error_t gret;

        struct stream_cipher *sc = para_malloc(sizeof(*sc));
        gret = gcry_cipher_open(&sc->handle, GCRY_CIPHER_ARCFOUR,
                GCRY_CIPHER_MODE_STREAM, 0);
        if (gret) {
                PARA_ERROR_LOG("%s\n", gcrypt_strerror(gret));
                free(sc);
                return NULL;
        }
        gret = gcry_cipher_setkey(sc->handle, data, (size_t)len);
        assert(gret == 0);
        return sc;
}

void sc_free(struct stream_cipher *sc)
{
        if (!sc)
                return;
        gcry_cipher_close(sc->handle);
        free(sc);
}

int sc_send_bin_buffer(struct stream_cipher_context *scc, char *buf,
                size_t size)
{
        gcry_error_t gret;
        int ret;
        unsigned char *tmp = para_malloc(size);

        assert(size);
        gret = gcry_cipher_encrypt(scc->send->handle, tmp, size,
                (unsigned char *)buf, size);
        assert(gret == 0);
        ret = write_all(scc->fd, (char *)tmp, &size);
        free(tmp);
        return ret;
}

int sc_recv_bin_buffer(struct stream_cipher_context *scc, char *buf,
                size_t size)
{
        gcry_error_t gret;
        ssize_t ret = recv(scc->fd, buf, size, 0);

        if (ret < 0)
                ret = -ERRNO_TO_PARA_ERROR(errno);
        if (ret <= 0)
                return ret;
        /* perform in-place encryption */
        gret = gcry_cipher_encrypt(scc->recv->handle, (unsigned char *)buf, ret,
                NULL, 0);
        assert(gret == 0);
        return ret;
}