Update man page for puttygen --random-device.

[PuTTY.git] / import.c

/*
 * Code for PuTTY to import and export private key files in other
 * SSH clients' formats.
 */

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <ctype.h>

#include "putty.h"
#include "ssh.h"
#include "misc.h"

int openssh_pem_encrypted(const Filename *filename);
int openssh_new_encrypted(const Filename *filename);
struct ssh2_userkey *openssh_pem_read(const Filename *filename,
                                      char *passphrase,
                                      const char **errmsg_p);
struct ssh2_userkey *openssh_new_read(const Filename *filename,
                                      char *passphrase,
                                      const char **errmsg_p);
int openssh_auto_write(const Filename *filename, struct ssh2_userkey *key,
                       char *passphrase);
int openssh_pem_write(const Filename *filename, struct ssh2_userkey *key,
                      char *passphrase);
int openssh_new_write(const Filename *filename, struct ssh2_userkey *key,
                      char *passphrase);

int sshcom_encrypted(const Filename *filename, char **comment);
struct ssh2_userkey *sshcom_read(const Filename *filename, char *passphrase,
                                 const char **errmsg_p);
int sshcom_write(const Filename *filename, struct ssh2_userkey *key,
                 char *passphrase);

/*
 * Given a key type, determine whether we know how to import it.
 */
int import_possible(int type)
{
    if (type == SSH_KEYTYPE_OPENSSH_PEM)
        return 1;
    if (type == SSH_KEYTYPE_OPENSSH_NEW)
        return 1;
    if (type == SSH_KEYTYPE_SSHCOM)
        return 1;
    return 0;
}

/*
 * Given a key type, determine what native key type
 * (SSH_KEYTYPE_SSH1 or SSH_KEYTYPE_SSH2) it will come out as once
 * we've imported it.
 */
int import_target_type(int type)
{
    /*
     * There are no known foreign SSH-1 key formats.
     */
    return SSH_KEYTYPE_SSH2;
}

/*
 * Determine whether a foreign key is encrypted.
 */
int import_encrypted(const Filename *filename, int type, char **comment)
{
    if (type == SSH_KEYTYPE_OPENSSH_PEM) {
        /* OpenSSH PEM format doesn't contain a key comment at all */
        *comment = dupstr(filename_to_str(filename));
        return openssh_pem_encrypted(filename);
    } else if (type == SSH_KEYTYPE_OPENSSH_NEW) {
        /* OpenSSH new format does, but it's inside the encrypted
         * section for some reason */
        *comment = dupstr(filename_to_str(filename));
        return openssh_new_encrypted(filename);
    } else if (type == SSH_KEYTYPE_SSHCOM) {
        return sshcom_encrypted(filename, comment);
    }
    return 0;
}

/*
 * Import an SSH-1 key.
 */
int import_ssh1(const Filename *filename, int type,
                struct RSAKey *key, char *passphrase, const char **errmsg_p)
{
    return 0;
}

/*
 * Import an SSH-2 key.
 */
struct ssh2_userkey *import_ssh2(const Filename *filename, int type,
                                 char *passphrase, const char **errmsg_p)
{
    if (type == SSH_KEYTYPE_OPENSSH_PEM)
        return openssh_pem_read(filename, passphrase, errmsg_p);
    else if (type == SSH_KEYTYPE_OPENSSH_NEW)
        return openssh_new_read(filename, passphrase, errmsg_p);
    if (type == SSH_KEYTYPE_SSHCOM)
        return sshcom_read(filename, passphrase, errmsg_p);
    return NULL;
}

/*
 * Export an SSH-1 key.
 */
int export_ssh1(const Filename *filename, int type, struct RSAKey *key,
                char *passphrase)
{
    return 0;
}

/*
 * Export an SSH-2 key.
 */
int export_ssh2(const Filename *filename, int type,
                struct ssh2_userkey *key, char *passphrase)
{
    if (type == SSH_KEYTYPE_OPENSSH_AUTO)
        return openssh_auto_write(filename, key, passphrase);
    if (type == SSH_KEYTYPE_OPENSSH_NEW)
        return openssh_new_write(filename, key, passphrase);
    if (type == SSH_KEYTYPE_SSHCOM)
        return sshcom_write(filename, key, passphrase);
    return 0;
}

/*
 * Strip trailing CRs and LFs at the end of a line of text.
 */
void strip_crlf(char *str)
{
    char *p = str + strlen(str);

    while (p > str && (p[-1] == '\r' || p[-1] == '\n'))
        *--p = '\0';
}

/* ----------------------------------------------------------------------
 * Helper routines. (The base64 ones are defined in sshpubk.c.)
 */

#define isbase64(c) (    ((c) >= 'A' && (c) <= 'Z') || \
                         ((c) >= 'a' && (c) <= 'z') || \
                         ((c) >= '0' && (c) <= '9') || \
                         (c) == '+' || (c) == '/' || (c) == '=' \
                         )

/*
 * Read an ASN.1/BER identifier and length pair.
 * 
 * Flags are a combination of the #defines listed below.
 * 
 * Returns -1 if unsuccessful; otherwise returns the number of
 * bytes used out of the source data.
 */

/* ASN.1 tag classes. */
#define ASN1_CLASS_UNIVERSAL        (0 << 6)
#define ASN1_CLASS_APPLICATION      (1 << 6)
#define ASN1_CLASS_CONTEXT_SPECIFIC (2 << 6)
#define ASN1_CLASS_PRIVATE          (3 << 6)
#define ASN1_CLASS_MASK             (3 << 6)

/* Primitive versus constructed bit. */
#define ASN1_CONSTRUCTED            (1 << 5)

static int ber_read_id_len(void *source, int sourcelen,
                           int *id, int *length, int *flags)
{
    unsigned char *p = (unsigned char *) source;

    if (sourcelen == 0)
        return -1;

    *flags = (*p & 0xE0);
    if ((*p & 0x1F) == 0x1F) {
        *id = 0;
        while (*p & 0x80) {
            p++, sourcelen--;
            if (sourcelen == 0)
                return -1;
            *id = (*id << 7) | (*p & 0x7F);
        }
        p++, sourcelen--;
    } else {
        *id = *p & 0x1F;
        p++, sourcelen--;
    }

    if (sourcelen == 0)
        return -1;

    if (*p & 0x80) {
        unsigned len;
        int n = *p & 0x7F;
        p++, sourcelen--;
        if (sourcelen < n)
            return -1;
        len = 0;
        while (n--)
            len = (len << 8) | (*p++);
        sourcelen -= n;
        *length = toint(len);
    } else {
        *length = *p;
        p++, sourcelen--;
    }

    return p - (unsigned char *) source;
}

/*
 * Write an ASN.1/BER identifier and length pair. Returns the
 * number of bytes consumed. Assumes dest contains enough space.
 * Will avoid writing anything if dest is NULL, but still return
 * amount of space required.
 */
static int ber_write_id_len(void *dest, int id, int length, int flags)
{
    unsigned char *d = (unsigned char *)dest;
    int len = 0;

    if (id <= 30) {
        /*
         * Identifier is one byte.
         */
        len++;
        if (d) *d++ = id | flags;
    } else {
        int n;
        /*
         * Identifier is multiple bytes: the first byte is 11111
         * plus the flags, and subsequent bytes encode the value of
         * the identifier, 7 bits at a time, with the top bit of
         * each byte 1 except the last one which is 0.
         */
        len++;
        if (d) *d++ = 0x1F | flags;
        for (n = 1; (id >> (7*n)) > 0; n++)
            continue;                  /* count the bytes */
        while (n--) {
            len++;
            if (d) *d++ = (n ? 0x80 : 0) | ((id >> (7*n)) & 0x7F);
        }
    }

    if (length < 128) {
        /*
         * Length is one byte.
         */
        len++;
        if (d) *d++ = length;
    } else {
        int n;
        /*
         * Length is multiple bytes. The first is 0x80 plus the
         * number of subsequent bytes, and the subsequent bytes
         * encode the actual length.
         */
        for (n = 1; (length >> (8*n)) > 0; n++)
            continue;                  /* count the bytes */
        len++;
        if (d) *d++ = 0x80 | n;
        while (n--) {
            len++;
            if (d) *d++ = (length >> (8*n)) & 0xFF;
        }
    }

    return len;
}

static int put_uint32(void *target, unsigned val)
{
    unsigned char *d = (unsigned char *)target;

    PUT_32BIT(d, val);
    return 4;
}

static int put_string(void *target, const void *data, int len)
{
    unsigned char *d = (unsigned char *)target;

    PUT_32BIT(d, len);
    memcpy(d+4, data, len);
    return len+4;
}

static int put_string_z(void *target, const char *string)
{
    return put_string(target, string, strlen(string));
}

static int put_mp(void *target, void *data, int len)
{
    unsigned char *d = (unsigned char *)target;
    unsigned char *i = (unsigned char *)data;

    if (*i & 0x80) {
        PUT_32BIT(d, len+1);
        d[4] = 0;
        memcpy(d+5, data, len);
        return len+5;
    } else {
        PUT_32BIT(d, len);
        memcpy(d+4, data, len);
        return len+4;
    }
}

/* Simple structure to point to an mp-int within a blob. */
struct mpint_pos { void *start; int bytes; };

static int ssh2_read_mpint(void *data, int len, struct mpint_pos *ret)
{
    int bytes;
    unsigned char *d = (unsigned char *) data;

    if (len < 4)
        goto error;
    bytes = toint(GET_32BIT(d));
    if (bytes < 0 || len-4 < bytes)
        goto error;

    ret->start = d + 4;
    ret->bytes = bytes;
    return bytes+4;

    error:
    ret->start = NULL;
    ret->bytes = -1;
    return len;                        /* ensure further calls fail as well */
}

/* ----------------------------------------------------------------------
 * Code to read and write OpenSSH private keys, in the old-style PEM
 * format.
 */

typedef enum {
    OP_DSA, OP_RSA, OP_ECDSA
} openssh_pem_keytype;
typedef enum {
    OP_E_3DES, OP_E_AES
} openssh_pem_enc;

struct openssh_pem_key {
    openssh_pem_keytype keytype;
    int encrypted;
    openssh_pem_enc encryption;
    char iv[32];
    unsigned char *keyblob;
    int keyblob_len, keyblob_size;
};

static struct openssh_pem_key *load_openssh_pem_key(const Filename *filename,
                                                    const char **errmsg_p)
{
    struct openssh_pem_key *ret;
    FILE *fp = NULL;
    char *line = NULL;
    const char *errmsg;
    char *p;
    int headers_done;
    char base64_bit[4];
    int base64_chars = 0;

    ret = snew(struct openssh_pem_key);
    ret->keyblob = NULL;
    ret->keyblob_len = ret->keyblob_size = 0;

    fp = f_open(filename, "r", FALSE);
    if (!fp) {
        errmsg = "unable to open key file";
        goto error;
    }

    if (!(line = fgetline(fp))) {
        errmsg = "unexpected end of file";
        goto error;
    }
    strip_crlf(line);
    if (!strstartswith(line, "-----BEGIN ") ||
        !strendswith(line, "PRIVATE KEY-----")) {
        errmsg = "file does not begin with OpenSSH key header";
        goto error;
    }
    /*
     * Parse the BEGIN line. For old-format keys, this tells us the
     * type of the key; for new-format keys, all it tells us is the
     * format, and we'll find out the key type once we parse the
     * base64.
     */
    if (!strcmp(line, "-----BEGIN RSA PRIVATE KEY-----")) {
        ret->keytype = OP_RSA;
    } else if (!strcmp(line, "-----BEGIN DSA PRIVATE KEY-----")) {
        ret->keytype = OP_DSA;
    } else if (!strcmp(line, "-----BEGIN EC PRIVATE KEY-----")) {
        ret->keytype = OP_ECDSA;
    } else if (!strcmp(line, "-----BEGIN OPENSSH PRIVATE KEY-----")) {
        errmsg = "this is a new-style OpenSSH key";
        goto error;
    } else {
        errmsg = "unrecognised key type";
        goto error;
    }
    smemclr(line, strlen(line));
    sfree(line);
    line = NULL;

    ret->encrypted = FALSE;
    memset(ret->iv, 0, sizeof(ret->iv));

    headers_done = 0;
    while (1) {
        if (!(line = fgetline(fp))) {
            errmsg = "unexpected end of file";
            goto error;
        }
        strip_crlf(line);
        if (strstartswith(line, "-----END ") &&
            strendswith(line, "PRIVATE KEY-----")) {
            sfree(line);
            line = NULL;
            break;                     /* done */
        }
        if ((p = strchr(line, ':')) != NULL) {
            if (headers_done) {
                errmsg = "header found in body of key data";
                goto error;
            }
            *p++ = '\0';
            while (*p && isspace((unsigned char)*p)) p++;
            if (!strcmp(line, "Proc-Type")) {
                if (p[0] != '4' || p[1] != ',') {
                    errmsg = "Proc-Type is not 4 (only 4 is supported)";
                    goto error;
                }
                p += 2;
                if (!strcmp(p, "ENCRYPTED"))
                    ret->encrypted = TRUE;
            } else if (!strcmp(line, "DEK-Info")) {
                int i, j, ivlen;

                if (!strncmp(p, "DES-EDE3-CBC,", 13)) {
                    ret->encryption = OP_E_3DES;
                    ivlen = 8;
                } else if (!strncmp(p, "AES-128-CBC,", 12)) {
                    ret->encryption = OP_E_AES;
                    ivlen = 16;
                } else {
                    errmsg = "unsupported cipher";
                    goto error;
                }
                p = strchr(p, ',') + 1;/* always non-NULL, by above checks */
                for (i = 0; i < ivlen; i++) {
                    if (1 != sscanf(p, "%2x", &j)) {
                        errmsg = "expected more iv data in DEK-Info";
                        goto error;
                    }
                    ret->iv[i] = j;
                    p += 2;
                }
                if (*p) {
                    errmsg = "more iv data than expected in DEK-Info";
                    goto error;
                }
            }
        } else {
            headers_done = 1;

            p = line;
            while (isbase64(*p)) {
                base64_bit[base64_chars++] = *p;
                if (base64_chars == 4) {
                    unsigned char out[3];
                    int len;

                    base64_chars = 0;

                    len = base64_decode_atom(base64_bit, out);

                    if (len <= 0) {
                        errmsg = "invalid base64 encoding";
                        goto error;
                    }

                    if (ret->keyblob_len + len > ret->keyblob_size) {
                        ret->keyblob_size = ret->keyblob_len + len + 256;
                        ret->keyblob = sresize(ret->keyblob, ret->keyblob_size,
                                               unsigned char);
                    }

                    memcpy(ret->keyblob + ret->keyblob_len, out, len);
                    ret->keyblob_len += len;

                    smemclr(out, sizeof(out));
                }

                p++;
            }
        }
        smemclr(line, strlen(line));
        sfree(line);
        line = NULL;
    }

    fclose(fp);
    fp = NULL;

    if (ret->keyblob_len == 0 || !ret->keyblob) {
        errmsg = "key body not present";
        goto error;
    }

    if (ret->encrypted && ret->keyblob_len % 8 != 0) {
        errmsg = "encrypted key blob is not a multiple of "
            "cipher block size";
        goto error;
    }

    smemclr(base64_bit, sizeof(base64_bit));
    if (errmsg_p) *errmsg_p = NULL;
    return ret;

    error:
    if (line) {
        smemclr(line, strlen(line));
        sfree(line);
        line = NULL;
    }
    smemclr(base64_bit, sizeof(base64_bit));
    if (ret) {
        if (ret->keyblob) {
            smemclr(ret->keyblob, ret->keyblob_size);
            sfree(ret->keyblob);
        }
        smemclr(ret, sizeof(*ret));
        sfree(ret);
    }
    if (errmsg_p) *errmsg_p = errmsg;
    if (fp) fclose(fp);
    return NULL;
}

int openssh_pem_encrypted(const Filename *filename)
{
    struct openssh_pem_key *key = load_openssh_pem_key(filename, NULL);
    int ret;

    if (!key)
        return 0;
    ret = key->encrypted;
    smemclr(key->keyblob, key->keyblob_size);
    sfree(key->keyblob);
    smemclr(key, sizeof(*key));
    sfree(key);
    return ret;
}

struct ssh2_userkey *openssh_pem_read(const Filename *filename,
                                      char *passphrase,
                                      const char **errmsg_p)
{
    struct openssh_pem_key *key = load_openssh_pem_key(filename, errmsg_p);
    struct ssh2_userkey *retkey;
    unsigned char *p, *q;
    int ret, id, len, flags;
    int i, num_integers;
    struct ssh2_userkey *retval = NULL;
    const char *errmsg;
    unsigned char *blob;
    int blobsize = 0, blobptr, privptr;
    char *modptr = NULL;
    int modlen = 0;

    blob = NULL;

    if (!key)
        return NULL;

    if (key->encrypted) {
        /*
         * Derive encryption key from passphrase and iv/salt:
         * 
         *  - let block A equal MD5(passphrase || iv)
         *  - let block B equal MD5(A || passphrase || iv)
         *  - block C would be MD5(B || passphrase || iv) and so on
         *  - encryption key is the first N bytes of A || B
         *
         * (Note that only 8 bytes of the iv are used for key
         * derivation, even when the key is encrypted with AES and
         * hence there are 16 bytes available.)
         */
        struct MD5Context md5c;
        unsigned char keybuf[32];

        MD5Init(&md5c);
        MD5Update(&md5c, (unsigned char *)passphrase, strlen(passphrase));
        MD5Update(&md5c, (unsigned char *)key->iv, 8);
        MD5Final(keybuf, &md5c);

        MD5Init(&md5c);
        MD5Update(&md5c, keybuf, 16);
        MD5Update(&md5c, (unsigned char *)passphrase, strlen(passphrase));
        MD5Update(&md5c, (unsigned char *)key->iv, 8);
        MD5Final(keybuf+16, &md5c);

        /*
         * Now decrypt the key blob.
         */
        if (key->encryption == OP_E_3DES)
            des3_decrypt_pubkey_ossh(keybuf, (unsigned char *)key->iv,
                                     key->keyblob, key->keyblob_len);
        else {
            void *ctx;
            assert(key->encryption == OP_E_AES);
            ctx = aes_make_context();
            aes128_key(ctx, keybuf);
            aes_iv(ctx, (unsigned char *)key->iv);
            aes_ssh2_decrypt_blk(ctx, key->keyblob, key->keyblob_len);
            aes_free_context(ctx);
        }

        smemclr(&md5c, sizeof(md5c));
        smemclr(keybuf, sizeof(keybuf));
    }

    /*
     * Now we have a decrypted key blob, which contains an ASN.1
     * encoded private key. We must now untangle the ASN.1.
     *
     * We expect the whole key blob to be formatted as a SEQUENCE
     * (0x30 followed by a length code indicating that the rest of
     * the blob is part of the sequence). Within that SEQUENCE we
     * expect to see a bunch of INTEGERs. What those integers mean
     * depends on the key type:
     *
     *  - For RSA, we expect the integers to be 0, n, e, d, p, q,
     *    dmp1, dmq1, iqmp in that order. (The last three are d mod
     *    (p-1), d mod (q-1), inverse of q mod p respectively.)
     *
     *  - For DSA, we expect them to be 0, p, q, g, y, x in that
     *    order.
     *
     *  - In ECDSA the format is totally different: we see the
     *    SEQUENCE, but beneath is an INTEGER 1, OCTET STRING priv
     *    EXPLICIT [0] OID curve, EXPLICIT [1] BIT STRING pubPoint
     */
    
    p = key->keyblob;

    /* Expect the SEQUENCE header. Take its absence as a failure to
     * decrypt, if the key was encrypted. */
    ret = ber_read_id_len(p, key->keyblob_len, &id, &len, &flags);
    p += ret;
    if (ret < 0 || id != 16 || len < 0 ||
        key->keyblob+key->keyblob_len-p < len) {
        errmsg = "ASN.1 decoding failure";
        retval = key->encrypted ? SSH2_WRONG_PASSPHRASE : NULL;
        goto error;
    }

    /* Expect a load of INTEGERs. */
    if (key->keytype == OP_RSA)
        num_integers = 9;
    else if (key->keytype == OP_DSA)
        num_integers = 6;
    else
        num_integers = 0;              /* placate compiler warnings */


    if (key->keytype == OP_ECDSA) {
        /* And now for something completely different */
        unsigned char *priv;
        int privlen;
        const struct ssh_signkey *alg;
        const struct ec_curve *curve;
        int algnamelen, curvenamelen;
        /* Read INTEGER 1 */
        ret = ber_read_id_len(p, key->keyblob+key->keyblob_len-p,
                              &id, &len, &flags);
        p += ret;
        if (ret < 0 || id != 2 || len != 1 ||
            key->keyblob+key->keyblob_len-p < len || p[0] != 1) {
            errmsg = "ASN.1 decoding failure";
            retval = key->encrypted ? SSH2_WRONG_PASSPHRASE : NULL;
            goto error;
        }
        p += 1;
        /* Read private key OCTET STRING */
        ret = ber_read_id_len(p, key->keyblob+key->keyblob_len-p,
                              &id, &len, &flags);
        p += ret;
        if (ret < 0 || id != 4 || len < 0 ||
            key->keyblob+key->keyblob_len-p < len) {
            errmsg = "ASN.1 decoding failure";
            retval = key->encrypted ? SSH2_WRONG_PASSPHRASE : NULL;
            goto error;
        }
        priv = p;
        privlen = len;
        p += len;
        /* Read curve OID */
        ret = ber_read_id_len(p, key->keyblob+key->keyblob_len-p,
                              &id, &len, &flags);
        p += ret;
        if (ret < 0 || id != 0 || len < 0 ||
            key->keyblob+key->keyblob_len-p < len) {
            errmsg = "ASN.1 decoding failure";
            retval = key->encrypted ? SSH2_WRONG_PASSPHRASE : NULL;
            goto error;
        }
        ret = ber_read_id_len(p, key->keyblob+key->keyblob_len-p,
                              &id, &len, &flags);
        p += ret;
        if (ret < 0 || id != 6 || len < 0 ||
            key->keyblob+key->keyblob_len-p < len) {
            errmsg = "ASN.1 decoding failure";
            retval = key->encrypted ? SSH2_WRONG_PASSPHRASE : NULL;
            goto error;
        }
        alg = ec_alg_by_oid(len, p, &curve);
        if (!alg) {
            errmsg = "Unsupported ECDSA curve.";
            retval = NULL;
            goto error;
        }
        p += len;
        /* Read BIT STRING point */
        ret = ber_read_id_len(p, key->keyblob+key->keyblob_len-p,
                              &id, &len, &flags);
        p += ret;
        if (ret < 0 || id != 1 || len < 0 ||
            key->keyblob+key->keyblob_len-p < len) {
            errmsg = "ASN.1 decoding failure";
            retval = key->encrypted ? SSH2_WRONG_PASSPHRASE : NULL;
            goto error;
        }
        ret = ber_read_id_len(p, key->keyblob+key->keyblob_len-p,
                              &id, &len, &flags);
        p += ret;
        if (ret < 0 || id != 3 || len < 0 ||
            key->keyblob+key->keyblob_len-p < len ||
            len != ((((curve->fieldBits + 7) / 8) * 2) + 2)) {
            errmsg = "ASN.1 decoding failure";
            retval = key->encrypted ? SSH2_WRONG_PASSPHRASE : NULL;
            goto error;
        }
        p += 1; len -= 1; /* Skip 0x00 before point */

        /* Construct the key */
        retkey = snew(struct ssh2_userkey);
        if (!retkey) {
            errmsg = "out of memory";
            goto error;
        }
        retkey->alg = alg;
        blob = snewn((4+19 + 4+8 + 4+len) + (4+1+privlen), unsigned char);
        if (!blob) {
            sfree(retkey);
            errmsg = "out of memory";
            goto error;
        }

        q = blob;

        algnamelen = strlen(alg->name);
        PUT_32BIT(q, algnamelen); q += 4;
        memcpy(q, alg->name, algnamelen); q += algnamelen;

        curvenamelen = strlen(curve->name);
        PUT_32BIT(q, curvenamelen); q += 4;
        memcpy(q, curve->name, curvenamelen); q += curvenamelen;

        PUT_32BIT(q, len); q += 4;
        memcpy(q, p, len); q += len;

        /*
         * To be acceptable to our createkey(), the private blob must
         * contain a valid mpint, i.e. without the top bit set. But
         * the input private string may have the top bit set, so we
         * prefix a zero byte to ensure createkey() doesn't fail for
         * that reason.
         */
        PUT_32BIT(q, privlen+1);
        q[4] = 0;
        memcpy(q+5, priv, privlen);

        retkey->data = retkey->alg->createkey(retkey->alg,
                                              blob, q-blob,
                                              q, 5+privlen);

        if (!retkey->data) {
            sfree(retkey);
            errmsg = "unable to create key data structure";
            goto error;
        }

    } else if (key->keytype == OP_RSA || key->keytype == OP_DSA) {

        /*
         * Space to create key blob in.
         */
        blobsize = 256+key->keyblob_len;
        blob = snewn(blobsize, unsigned char);
        PUT_32BIT(blob, 7);
        if (key->keytype == OP_DSA)
            memcpy(blob+4, "ssh-dss", 7);
        else if (key->keytype == OP_RSA)
            memcpy(blob+4, "ssh-rsa", 7);
        blobptr = 4+7;
        privptr = -1;

        for (i = 0; i < num_integers; i++) {
            ret = ber_read_id_len(p, key->keyblob+key->keyblob_len-p,
                                  &id, &len, &flags);
            p += ret;
            if (ret < 0 || id != 2 || len < 0 ||
                key->keyblob+key->keyblob_len-p < len) {
                errmsg = "ASN.1 decoding failure";
                retval = key->encrypted ? SSH2_WRONG_PASSPHRASE : NULL;
                goto error;
            }

            if (i == 0) {
                /*
                 * The first integer should be zero always (I think
                 * this is some sort of version indication).
                 */
                if (len != 1 || p[0] != 0) {
                    errmsg = "version number mismatch";
                    goto error;
                }
            } else if (key->keytype == OP_RSA) {
                /*
                 * Integers 1 and 2 go into the public blob but in the
                 * opposite order; integers 3, 4, 5 and 8 go into the
                 * private blob. The other two (6 and 7) are ignored.
                 */
                if (i == 1) {
                    /* Save the details for after we deal with number 2. */
                    modptr = (char *)p;
                    modlen = len;
                } else if (i != 6 && i != 7) {
                    PUT_32BIT(blob+blobptr, len);
                    memcpy(blob+blobptr+4, p, len);
                    blobptr += 4+len;
                    if (i == 2) {
                        PUT_32BIT(blob+blobptr, modlen);
                        memcpy(blob+blobptr+4, modptr, modlen);
                        blobptr += 4+modlen;
                        privptr = blobptr;
                    }
                }
            } else if (key->keytype == OP_DSA) {
                /*
                 * Integers 1-4 go into the public blob; integer 5 goes
                 * into the private blob.
                 */
                PUT_32BIT(blob+blobptr, len);
                memcpy(blob+blobptr+4, p, len);
                blobptr += 4+len;
                if (i == 4)
                    privptr = blobptr;
            }

            /* Skip past the number. */
            p += len;
        }

        /*
         * Now put together the actual key. Simplest way to do this is
         * to assemble our own key blobs and feed them to the createkey
         * functions; this is a bit faffy but it does mean we get all
         * the sanity checks for free.
         */
        assert(privptr > 0);          /* should have bombed by now if not */
        retkey = snew(struct ssh2_userkey);
        retkey->alg = (key->keytype == OP_RSA ? &ssh_rsa : &ssh_dss);
        retkey->data = retkey->alg->createkey(retkey->alg, blob, privptr,
                                              blob+privptr,
                                              blobptr-privptr);
        if (!retkey->data) {
            sfree(retkey);
            errmsg = "unable to create key data structure";
            goto error;
        }

    } else {
        assert(0 && "Bad key type from load_openssh_pem_key");
        errmsg = "Bad key type from load_openssh_pem_key";
        goto error;
    }

    /*
     * The old key format doesn't include a comment in the private
     * key file.
     */
    retkey->comment = dupstr("imported-openssh-key");

    errmsg = NULL;                     /* no error */
    retval = retkey;

    error:
    if (blob) {
        smemclr(blob, blobsize);
        sfree(blob);
    }
    smemclr(key->keyblob, key->keyblob_size);
    sfree(key->keyblob);
    smemclr(key, sizeof(*key));
    sfree(key);
    if (errmsg_p) *errmsg_p = errmsg;
    return retval;
}

int openssh_pem_write(const Filename *filename, struct ssh2_userkey *key,
                      char *passphrase)
{
    unsigned char *pubblob, *privblob, *spareblob;
    int publen, privlen, sparelen = 0;
    unsigned char *outblob;
    int outlen;
    struct mpint_pos numbers[9];
    int nnumbers, pos, len, seqlen, i;
    const char *header, *footer;
    char zero[1];
    unsigned char iv[8];
    int ret = 0;
    FILE *fp;

    /*
     * Fetch the key blobs.
     */
    pubblob = key->alg->public_blob(key->data, &publen);
    privblob = key->alg->private_blob(key->data, &privlen);
    spareblob = outblob = NULL;

    outblob = NULL;
    len = 0;

    /*
     * Encode the OpenSSH key blob, and also decide on the header
     * line.
     */
    if (key->alg == &ssh_rsa || key->alg == &ssh_dss) {
        /*
         * The RSA and DSS handlers share some code because the two
         * key types have very similar ASN.1 representations, as a
         * plain SEQUENCE of big integers. So we set up a list of
         * bignums per key type and then construct the actual blob in
         * common code after that.
         */
        if (key->alg == &ssh_rsa) {
            int pos;
            struct mpint_pos n, e, d, p, q, iqmp, dmp1, dmq1;
            Bignum bd, bp, bq, bdmp1, bdmq1;

            /*
             * These blobs were generated from inside PuTTY, so we needn't
             * treat them as untrusted.
             */
            pos = 4 + GET_32BIT(pubblob);
            pos += ssh2_read_mpint(pubblob+pos, publen-pos, &e);
            pos += ssh2_read_mpint(pubblob+pos, publen-pos, &n);
            pos = 0;
            pos += ssh2_read_mpint(privblob+pos, privlen-pos, &d);
            pos += ssh2_read_mpint(privblob+pos, privlen-pos, &p);
            pos += ssh2_read_mpint(privblob+pos, privlen-pos, &q);
            pos += ssh2_read_mpint(privblob+pos, privlen-pos, &iqmp);

            assert(e.start && iqmp.start); /* can't go wrong */

            /* We also need d mod (p-1) and d mod (q-1). */
            bd = bignum_from_bytes(d.start, d.bytes);
            bp = bignum_from_bytes(p.start, p.bytes);
            bq = bignum_from_bytes(q.start, q.bytes);
            decbn(bp);
            decbn(bq);
            bdmp1 = bigmod(bd, bp);
            bdmq1 = bigmod(bd, bq);
            freebn(bd);
            freebn(bp);
            freebn(bq);

            dmp1.bytes = (bignum_bitcount(bdmp1)+8)/8;
            dmq1.bytes = (bignum_bitcount(bdmq1)+8)/8;
            sparelen = dmp1.bytes + dmq1.bytes;
            spareblob = snewn(sparelen, unsigned char);
            dmp1.start = spareblob;
            dmq1.start = spareblob + dmp1.bytes;
            for (i = 0; i < dmp1.bytes; i++)
                spareblob[i] = bignum_byte(bdmp1, dmp1.bytes-1 - i);
            for (i = 0; i < dmq1.bytes; i++)
                spareblob[i+dmp1.bytes] = bignum_byte(bdmq1, dmq1.bytes-1 - i);
            freebn(bdmp1);
            freebn(bdmq1);

            numbers[0].start = zero; numbers[0].bytes = 1; zero[0] = '\0';
            numbers[1] = n;
            numbers[2] = e;
            numbers[3] = d;
            numbers[4] = p;
            numbers[5] = q;
            numbers[6] = dmp1;
            numbers[7] = dmq1;
            numbers[8] = iqmp;

            nnumbers = 9;
            header = "-----BEGIN RSA PRIVATE KEY-----\n";
            footer = "-----END RSA PRIVATE KEY-----\n";
        } else {                       /* ssh-dss */
            int pos;
            struct mpint_pos p, q, g, y, x;

            /*
             * These blobs were generated from inside PuTTY, so we needn't
             * treat them as untrusted.
             */
            pos = 4 + GET_32BIT(pubblob);
            pos += ssh2_read_mpint(pubblob+pos, publen-pos, &p);
            pos += ssh2_read_mpint(pubblob+pos, publen-pos, &q);
            pos += ssh2_read_mpint(pubblob+pos, publen-pos, &g);
            pos += ssh2_read_mpint(pubblob+pos, publen-pos, &y);
            pos = 0;
            pos += ssh2_read_mpint(privblob+pos, privlen-pos, &x);

            assert(y.start && x.start); /* can't go wrong */

            numbers[0].start = zero; numbers[0].bytes = 1; zero[0] = '\0';
            numbers[1] = p;
            numbers[2] = q;
            numbers[3] = g;
            numbers[4] = y;
            numbers[5] = x;

            nnumbers = 6;
            header = "-----BEGIN DSA PRIVATE KEY-----\n";
            footer = "-----END DSA PRIVATE KEY-----\n";
        }

        /*
         * Now count up the total size of the ASN.1 encoded integers,
         * so as to determine the length of the containing SEQUENCE.
         */
        len = 0;
        for (i = 0; i < nnumbers; i++) {
            len += ber_write_id_len(NULL, 2, numbers[i].bytes, 0);
            len += numbers[i].bytes;
        }
        seqlen = len;
        /* Now add on the SEQUENCE header. */
        len += ber_write_id_len(NULL, 16, seqlen, ASN1_CONSTRUCTED);

        /*
         * Now we know how big outblob needs to be. Allocate it.
         */
        outblob = snewn(len, unsigned char);

        /*
         * And write the data into it.
         */
        pos = 0;
        pos += ber_write_id_len(outblob+pos, 16, seqlen, ASN1_CONSTRUCTED);
        for (i = 0; i < nnumbers; i++) {
            pos += ber_write_id_len(outblob+pos, 2, numbers[i].bytes, 0);
            memcpy(outblob+pos, numbers[i].start, numbers[i].bytes);
            pos += numbers[i].bytes;
        }
    } else if (key->alg == &ssh_ecdsa_nistp256 ||
               key->alg == &ssh_ecdsa_nistp384 ||
               key->alg == &ssh_ecdsa_nistp521) {
        const unsigned char *oid;
        int oidlen;
        int pointlen;

        /*
         * Structure of asn1:
         * SEQUENCE
         *   INTEGER 1
         *   OCTET STRING (private key)
         *   [0]
         *     OID (curve)
         *   [1]
         *     BIT STRING (0x00 public key point)
         */
        oid = ec_alg_oid(key->alg, &oidlen);
        pointlen = (((struct ec_key *)key->data)->publicKey.curve->fieldBits
                    + 7) / 8 * 2;

        len = ber_write_id_len(NULL, 2, 1, 0);
        len += 1;
        len += ber_write_id_len(NULL, 4, privlen - 4, 0);
        len+= privlen - 4;
        len += ber_write_id_len(NULL, 0, oidlen +
                                ber_write_id_len(NULL, 6, oidlen, 0),
                                ASN1_CLASS_CONTEXT_SPECIFIC | ASN1_CONSTRUCTED);
        len += ber_write_id_len(NULL, 6, oidlen, 0);
        len += oidlen;
        len += ber_write_id_len(NULL, 1, 2 + pointlen +
                                ber_write_id_len(NULL, 3, 2 + pointlen, 0),
                                ASN1_CLASS_CONTEXT_SPECIFIC | ASN1_CONSTRUCTED);
        len += ber_write_id_len(NULL, 3, 2 + pointlen, 0);
        len += 2 + pointlen;

        seqlen = len;
        len += ber_write_id_len(NULL, 16, seqlen, ASN1_CONSTRUCTED);

        outblob = snewn(len, unsigned char);
        assert(outblob);

        pos = 0;
        pos += ber_write_id_len(outblob+pos, 16, seqlen, ASN1_CONSTRUCTED);
        pos += ber_write_id_len(outblob+pos, 2, 1, 0);
        outblob[pos++] = 1;
        pos += ber_write_id_len(outblob+pos, 4, privlen - 4, 0);
        memcpy(outblob+pos, privblob + 4, privlen - 4);
        pos += privlen - 4;
        pos += ber_write_id_len(outblob+pos, 0, oidlen +
                                ber_write_id_len(NULL, 6, oidlen, 0),
                                ASN1_CLASS_CONTEXT_SPECIFIC | ASN1_CONSTRUCTED);
        pos += ber_write_id_len(outblob+pos, 6, oidlen, 0);
        memcpy(outblob+pos, oid, oidlen);
        pos += oidlen;
        pos += ber_write_id_len(outblob+pos, 1, 2 + pointlen +
                                ber_write_id_len(NULL, 3, 2 + pointlen, 0),
                                ASN1_CLASS_CONTEXT_SPECIFIC | ASN1_CONSTRUCTED);
        pos += ber_write_id_len(outblob+pos, 3, 2 + pointlen, 0);
        outblob[pos++] = 0;
        memcpy(outblob+pos, pubblob+39, 1 + pointlen);
        pos += 1 + pointlen;

        header = "-----BEGIN EC PRIVATE KEY-----\n";
        footer = "-----END EC PRIVATE KEY-----\n";
    } else {
        assert(0);                     /* zoinks! */
        exit(1); /* XXX: GCC doesn't understand assert() on some systems. */
    }

    /*
     * Encrypt the key.
     *
     * For the moment, we still encrypt our OpenSSH keys using
     * old-style 3DES.
     */
    if (passphrase) {
        struct MD5Context md5c;
        unsigned char keybuf[32];

        /*
         * Round up to the cipher block size, ensuring we have at
         * least one byte of padding (see below).
         */
        outlen = (len+8) &~ 7;
        {
            unsigned char *tmp = snewn(outlen, unsigned char);
            memcpy(tmp, outblob, len);
            smemclr(outblob, len);
            sfree(outblob);
            outblob = tmp;
        }

        /*
         * Padding on OpenSSH keys is deterministic. The number of
         * padding bytes is always more than zero, and always at most
         * the cipher block length. The value of each padding byte is
         * equal to the number of padding bytes. So a plaintext that's
         * an exact multiple of the block size will be padded with 08
         * 08 08 08 08 08 08 08 (assuming a 64-bit block cipher); a
         * plaintext one byte less than a multiple of the block size
         * will be padded with just 01.
         *
         * This enables the OpenSSL key decryption function to strip
         * off the padding algorithmically and return the unpadded
         * plaintext to the next layer: it looks at the final byte, and
         * then expects to find that many bytes at the end of the data
         * with the same value. Those are all removed and the rest is
         * returned.
         */
        assert(pos == len);
        while (pos < outlen) {
            outblob[pos++] = outlen - len;
        }

        /*
         * Invent an iv. Then derive encryption key from passphrase
         * and iv/salt:
         * 
         *  - let block A equal MD5(passphrase || iv)
         *  - let block B equal MD5(A || passphrase || iv)
         *  - block C would be MD5(B || passphrase || iv) and so on
         *  - encryption key is the first N bytes of A || B
         */
        for (i = 0; i < 8; i++) iv[i] = random_byte();

        MD5Init(&md5c);
        MD5Update(&md5c, (unsigned char *)passphrase, strlen(passphrase));
        MD5Update(&md5c, iv, 8);
        MD5Final(keybuf, &md5c);

        MD5Init(&md5c);
        MD5Update(&md5c, keybuf, 16);
        MD5Update(&md5c, (unsigned char *)passphrase, strlen(passphrase));
        MD5Update(&md5c, iv, 8);
        MD5Final(keybuf+16, &md5c);

        /*
         * Now encrypt the key blob.
         */
        des3_encrypt_pubkey_ossh(keybuf, iv, outblob, outlen);

        smemclr(&md5c, sizeof(md5c));
        smemclr(keybuf, sizeof(keybuf));
    } else {
        /*
         * If no encryption, the blob has exactly its original
         * cleartext size.
         */
        outlen = len;
    }

    /*
     * And save it. We'll use Unix line endings just in case it's
     * subsequently transferred in binary mode.
     */
    fp = f_open(filename, "wb", TRUE);      /* ensure Unix line endings */
    if (!fp)
        goto error;
    fputs(header, fp);
    if (passphrase) {
        fprintf(fp, "Proc-Type: 4,ENCRYPTED\nDEK-Info: DES-EDE3-CBC,");
        for (i = 0; i < 8; i++)
            fprintf(fp, "%02X", iv[i]);
        fprintf(fp, "\n\n");
    }
    base64_encode(fp, outblob, outlen, 64);
    fputs(footer, fp);
    fclose(fp);
    ret = 1;

    error:
    if (outblob) {
        smemclr(outblob, outlen);
        sfree(outblob);
    }
    if (spareblob) {
        smemclr(spareblob, sparelen);
        sfree(spareblob);
    }
    if (privblob) {
        smemclr(privblob, privlen);
        sfree(privblob);
    }
    if (pubblob) {
        smemclr(pubblob, publen);
        sfree(pubblob);
    }
    return ret;
}

/* ----------------------------------------------------------------------
 * Code to read and write OpenSSH private keys in the new-style format.
 */

typedef enum {
    ON_E_NONE, ON_E_AES256CBC
} openssh_new_cipher;
typedef enum {
    ON_K_NONE, ON_K_BCRYPT
} openssh_new_kdf;

struct openssh_new_key {
    openssh_new_cipher cipher;
    openssh_new_kdf kdf;
    union {
        struct {
            int rounds;
            /* This points to a position within keyblob, not a
             * separately allocated thing */
            const unsigned char *salt;
            int saltlen;
        } bcrypt;
    } kdfopts;
    int nkeys, key_wanted;
    /* This too points to a position within keyblob */
    unsigned char *privatestr;
    int privatelen;

    unsigned char *keyblob;
    int keyblob_len, keyblob_size;
};

static struct openssh_new_key *load_openssh_new_key(const Filename *filename,
                                                    const char **errmsg_p)
{
    struct openssh_new_key *ret;
    FILE *fp = NULL;
    char *line = NULL;
    const char *errmsg;
    char *p;
    char base64_bit[4];
    int base64_chars = 0;
    const void *filedata;
    int filelen;
    const void *string, *kdfopts, *bcryptsalt, *pubkey;
    int stringlen, kdfoptlen, bcryptsaltlen, pubkeylen;
    unsigned bcryptrounds, nkeys, key_index;

    ret = snew(struct openssh_new_key);
    ret->keyblob = NULL;
    ret->keyblob_len = ret->keyblob_size = 0;

    fp = f_open(filename, "r", FALSE);
    if (!fp) {
        errmsg = "unable to open key file";
        goto error;
    }

    if (!(line = fgetline(fp))) {
        errmsg = "unexpected end of file";
        goto error;
    }
    strip_crlf(line);
    if (0 != strcmp(line, "-----BEGIN OPENSSH PRIVATE KEY-----")) {
        errmsg = "file does not begin with OpenSSH new-style key header";
        goto error;
    }
    smemclr(line, strlen(line));
    sfree(line);
    line = NULL;

    while (1) {
        if (!(line = fgetline(fp))) {
            errmsg = "unexpected end of file";
            goto error;
        }
        strip_crlf(line);
        if (0 == strcmp(line, "-----END OPENSSH PRIVATE KEY-----")) {
            sfree(line);
            line = NULL;
            break;                     /* done */
        }

        p = line;
        while (isbase64(*p)) {
            base64_bit[base64_chars++] = *p;
            if (base64_chars == 4) {
                unsigned char out[3];
                int len;

                base64_chars = 0;

                len = base64_decode_atom(base64_bit, out);

                if (len <= 0) {
                    errmsg = "invalid base64 encoding";
                    goto error;
                }

                if (ret->keyblob_len + len > ret->keyblob_size) {
                    ret->keyblob_size = ret->keyblob_len + len + 256;
                    ret->keyblob = sresize(ret->keyblob, ret->keyblob_size,
                                           unsigned char);
                }

                memcpy(ret->keyblob + ret->keyblob_len, out, len);
                ret->keyblob_len += len;

                smemclr(out, sizeof(out));
            }

            p++;
        }
        smemclr(line, strlen(line));
        sfree(line);
        line = NULL;
    }

    fclose(fp);
    fp = NULL;

    if (ret->keyblob_len == 0 || !ret->keyblob) {
        errmsg = "key body not present";
        goto error;
    }

    filedata = ret->keyblob;
    filelen = ret->keyblob_len;

    if (filelen < 15 || 0 != memcmp(filedata, "openssh-key-v1\0", 15)) {
        errmsg = "new-style OpenSSH magic number missing\n";
        goto error;
    }
    filedata = (const char *)filedata + 15;
    filelen -= 15;

    if (!(string = get_ssh_string(&filelen, &filedata, &stringlen))) {
        errmsg = "encountered EOF before cipher name\n";
        goto error;
    }
    if (match_ssh_id(stringlen, string, "none")) {
        ret->cipher = ON_E_NONE;
    } else if (match_ssh_id(stringlen, string, "aes256-cbc")) {
        ret->cipher = ON_E_AES256CBC;
    } else {
        errmsg = "unrecognised cipher name\n";
        goto error;
    }

    if (!(string = get_ssh_string(&filelen, &filedata, &stringlen))) {
        errmsg = "encountered EOF before kdf name\n";
        goto error;
    }
    if (match_ssh_id(stringlen, string, "none")) {
        ret->kdf = ON_K_NONE;
    } else if (match_ssh_id(stringlen, string, "bcrypt")) {
        ret->kdf = ON_K_BCRYPT;
    } else {
        errmsg = "unrecognised kdf name\n";
        goto error;
    }

    if (!(kdfopts = get_ssh_string(&filelen, &filedata, &kdfoptlen))) {
        errmsg = "encountered EOF before kdf options\n";
        goto error;
    }
    switch (ret->kdf) {
      case ON_K_NONE:
        if (kdfoptlen != 0) {
            errmsg = "expected empty options string for 'none' kdf";
            goto error;
        }
        break;
      case ON_K_BCRYPT:
        if (!(bcryptsalt = get_ssh_string(&kdfoptlen, &kdfopts,
                                          &bcryptsaltlen))) {
            errmsg = "bcrypt options string did not contain salt\n";
            goto error;
        }
        if (!get_ssh_uint32(&kdfoptlen, &kdfopts, &bcryptrounds)) {
            errmsg = "bcrypt options string did not contain round count\n";
            goto error;
        }
        ret->kdfopts.bcrypt.salt = bcryptsalt;
        ret->kdfopts.bcrypt.saltlen = bcryptsaltlen;
        ret->kdfopts.bcrypt.rounds = bcryptrounds;
        break;
    }

    /*
     * At this point we expect a uint32 saying how many keys are
     * stored in this file. OpenSSH new-style key files can
     * contain more than one. Currently we don't have any user
     * interface to specify which one we're trying to extract, so
     * we just bomb out with an error if more than one is found in
     * the file. However, I've put in all the mechanism here to
     * extract the nth one for a given n, in case we later connect
     * up some UI to that mechanism. Just arrange that the
     * 'key_wanted' field is set to a value in the range [0,
     * nkeys) by some mechanism.
     */
    if (!get_ssh_uint32(&filelen, &filedata, &nkeys)) {
        errmsg = "encountered EOF before key count\n";
        goto error;
    }
    if (nkeys != 1) {
        errmsg = "multiple keys in new-style OpenSSH key file "
            "not supported\n";
        goto error;
    }
    ret->nkeys = nkeys;
    ret->key_wanted = 0;

    for (key_index = 0; key_index < nkeys; key_index++) {
        if (!(pubkey = get_ssh_string(&filelen, &filedata, &pubkeylen))) {
            errmsg = "encountered EOF before kdf options\n";
            goto error;
        }
    }

    /*
     * Now we expect a string containing the encrypted part of the
     * key file.
     */
    if (!(string = get_ssh_string(&filelen, &filedata, &stringlen))) {
        errmsg = "encountered EOF before private key container\n";
        goto error;
    }
    ret->privatestr = (unsigned char *)string;
    ret->privatelen = stringlen;

    /*
     * And now we're done, until asked to actually decrypt.
     */

    smemclr(base64_bit, sizeof(base64_bit));
    if (errmsg_p) *errmsg_p = NULL;
    return ret;

    error:
    if (line) {
        smemclr(line, strlen(line));
        sfree(line);
        line = NULL;
    }
    smemclr(base64_bit, sizeof(base64_bit));
    if (ret) {
        if (ret->keyblob) {
            smemclr(ret->keyblob, ret->keyblob_size);
            sfree(ret->keyblob);
        }
        smemclr(ret, sizeof(*ret));
        sfree(ret);
    }
    if (errmsg_p) *errmsg_p = errmsg;
    if (fp) fclose(fp);
    return NULL;
}

int openssh_new_encrypted(const Filename *filename)
{
    struct openssh_new_key *key = load_openssh_new_key(filename, NULL);
    int ret;

    if (!key)
        return 0;
    ret = (key->cipher != ON_E_NONE);
    smemclr(key->keyblob, key->keyblob_size);
    sfree(key->keyblob);
    smemclr(key, sizeof(*key));
    sfree(key);
    return ret;
}

struct ssh2_userkey *openssh_new_read(const Filename *filename,
                                      char *passphrase,
                                      const char **errmsg_p)
{
    struct openssh_new_key *key = load_openssh_new_key(filename, errmsg_p);
    struct ssh2_userkey *retkey;
    int i;
    struct ssh2_userkey *retval = NULL;
    const char *errmsg;
    unsigned char *blob;
    int blobsize = 0;
    unsigned checkint0, checkint1;
    const void *priv, *string;
    int privlen, stringlen, key_index;
    const struct ssh_signkey *alg;

    blob = NULL;

    if (!key)
        return NULL;

    if (key->cipher != ON_E_NONE) {
        unsigned char keybuf[48];
        int keysize;

        /*
         * Construct the decryption key, and decrypt the string.
         */
        switch (key->cipher) {
          case ON_E_NONE:
            keysize = 0;
            break;
          case ON_E_AES256CBC:
            keysize = 48;              /* 32 byte key + 16 byte IV */
            break;
          default:
            assert(0 && "Bad cipher enumeration value");
        }
        assert(keysize <= sizeof(keybuf));
        switch (key->kdf) {
          case ON_K_NONE:
            memset(keybuf, 0, keysize);
            break;
          case ON_K_BCRYPT:
            openssh_bcrypt(passphrase,
                           key->kdfopts.bcrypt.salt,
                           key->kdfopts.bcrypt.saltlen,
                           key->kdfopts.bcrypt.rounds,
                           keybuf, keysize);
            break;
          default:
            assert(0 && "Bad kdf enumeration value");
        }
        switch (key->cipher) {
          case ON_E_NONE:
            break;
          case ON_E_AES256CBC:
            if (key->privatelen % 16 != 0) {
                errmsg = "private key container length is not a"
                    " multiple of AES block size\n";
                goto error;
            }
            {
                void *ctx = aes_make_context();
                aes256_key(ctx, keybuf);
                aes_iv(ctx, keybuf + 32);
                aes_ssh2_decrypt_blk(ctx, key->privatestr,
                                     key->privatelen);
                aes_free_context(ctx);
            }
            break;
          default:
            assert(0 && "Bad cipher enumeration value");
        }
    }

    /*
     * Now parse the entire encrypted section, and extract the key
     * identified by key_wanted.
     */
    priv = key->privatestr;
    privlen = key->privatelen;

    if (!get_ssh_uint32(&privlen, &priv, &checkint0) ||
        !get_ssh_uint32(&privlen, &priv, &checkint1) ||
        checkint0 != checkint1) {
        errmsg = "decryption check failed";
        goto error;
    }

    retkey = NULL;
    for (key_index = 0; key_index < key->nkeys; key_index++) {
        const unsigned char *thiskey;
        int thiskeylen;

        /*
         * Read the key type, which will tell us how to scan over
         * the key to get to the next one.
         */
        if (!(string = get_ssh_string(&privlen, &priv, &stringlen))) {
            errmsg = "expected key type in private string";
            goto error;
        }

        /*
         * Preliminary key type identification, and decide how
         * many pieces of key we expect to see. Currently
         * (conveniently) all key types can be seen as some number
         * of strings, so we just need to know how many of them to
         * skip over. (The numbers below exclude the key comment.)
         */
        {
            /* find_pubkey_alg needs a zero-terminated copy of the
             * algorithm name */
            char *name_zt = dupprintf("%.*s", stringlen, (char *)string);
            alg = find_pubkey_alg(name_zt);
            sfree(name_zt);
        }

        if (!alg) {
            errmsg = "private key type not recognised\n";
            goto error;
        }

        thiskey = priv;

        /*
         * Skip over the pieces of key.
         */
        for (i = 0; i < alg->openssh_private_npieces; i++) {
            if (!(string = get_ssh_string(&privlen, &priv, &stringlen))) {
                errmsg = "ran out of data in mid-private-key";
                goto error;
            }
        }

        thiskeylen = (int)((const unsigned char *)priv -
                           (const unsigned char *)thiskey);
        if (key_index == key->key_wanted) {
            retkey = snew(struct ssh2_userkey);
            retkey->alg = alg;
            retkey->data = alg->openssh_createkey(alg, &thiskey, &thiskeylen);
            if (!retkey->data) {
                sfree(retkey);
                errmsg = "unable to create key data structure";
                goto error;
            }
        }

        /*
         * Read the key comment.
         */
        if (!(string = get_ssh_string(&privlen, &priv, &stringlen))) {
            errmsg = "ran out of data at key comment";
            goto error;
        }
        if (key_index == key->key_wanted) {
            assert(retkey);
            retkey->comment = dupprintf("%.*s", stringlen,
                                        (const char *)string);
        }
    }

    if (!retkey) {
        errmsg = "key index out of range";
        goto error;
    }

    /*
     * Now we expect nothing left but padding.
     */
    for (i = 0; i < privlen; i++) {
        if (((const unsigned char *)priv)[i] != (unsigned char)(i+1)) {
            errmsg = "padding at end of private string did not match";
            goto error;
        }
    }

    errmsg = NULL;                     /* no error */
    retval = retkey;

    error:
    if (blob) {
        smemclr(blob, blobsize);
        sfree(blob);
    }
    smemclr(key->keyblob, key->keyblob_size);
    sfree(key->keyblob);
    smemclr(key, sizeof(*key));
    sfree(key);
    if (errmsg_p) *errmsg_p = errmsg;
    return retval;
}

int openssh_new_write(const Filename *filename, struct ssh2_userkey *key,
                      char *passphrase)
{
    unsigned char *pubblob, *privblob, *outblob, *p;
    unsigned char *private_section_start, *private_section_length_field;
    int publen, privlen, commentlen, maxsize, padvalue, i;
    unsigned checkint;
    int ret = 0;
    unsigned char bcrypt_salt[16];
    const int bcrypt_rounds = 16;
    FILE *fp;

    /*
     * Fetch the key blobs and find out the lengths of things.
     */
    pubblob = key->alg->public_blob(key->data, &publen);
    i = key->alg->openssh_fmtkey(key->data, NULL, 0);
    privblob = snewn(i, unsigned char);
    privlen = key->alg->openssh_fmtkey(key->data, privblob, i);
    assert(privlen == i);
    commentlen = strlen(key->comment);

    /*
     * Allocate enough space for the full binary key format. No need
     * to be absolutely precise here.
     */
    maxsize = (16 +                    /* magic number */
               32 +                    /* cipher name string */
               32 +                    /* kdf name string */
               64 +                    /* kdf options string */
               4 +                     /* key count */
               4+publen +              /* public key string */
               4 +                     /* string header for private section */
               8 +                     /* checkint x 2 */
               4+strlen(key->alg->name) + /* key type string */
               privlen +               /* private blob */
               4+commentlen +          /* comment string */
               16);                    /* padding at end of private section */
    outblob = snewn(maxsize, unsigned char);

    /*
     * Construct the cleartext version of the blob.
     */
    p = outblob;

    /* Magic number. */
    memcpy(p, "openssh-key-v1\0", 15);
    p += 15;

    /* Cipher and kdf names, and kdf options. */
    if (!passphrase) {
        memset(bcrypt_salt, 0, sizeof(bcrypt_salt)); /* prevent warnings */
        p += put_string_z(p, "none");
        p += put_string_z(p, "none");
        p += put_string_z(p, "");
    } else {
        unsigned char *q;
        for (i = 0; i < (int)sizeof(bcrypt_salt); i++)
            bcrypt_salt[i] = random_byte();
        p += put_string_z(p, "aes256-cbc");
        p += put_string_z(p, "bcrypt");
        q = p;
        p += 4;
        p += put_string(p, bcrypt_salt, sizeof(bcrypt_salt));
        p += put_uint32(p, bcrypt_rounds);
        PUT_32BIT_MSB_FIRST(q, (unsigned)(p - (q+4)));
    }

    /* Number of keys. */
    p += put_uint32(p, 1);

    /* Public blob. */
    p += put_string(p, pubblob, publen);

    /* Begin private section. */
    private_section_length_field = p;
    p += 4;
    private_section_start = p;

    /* checkint. */
    checkint = 0;
    for (i = 0; i < 4; i++)
        checkint = (checkint << 8) + random_byte();
    p += put_uint32(p, checkint);
    p += put_uint32(p, checkint);

    /* Private key. The main private blob goes inline, with no string
     * wrapper. */
    p += put_string_z(p, key->alg->name);
    memcpy(p, privblob, privlen);
    p += privlen;

    /* Comment. */
    p += put_string_z(p, key->comment);

    /* Pad out the encrypted section. */
    padvalue = 1;
    do {
        *p++ = padvalue++;
    } while ((p - private_section_start) & 15);

    assert(p - outblob < maxsize);

    /* Go back and fill in the length field for the private section. */
    PUT_32BIT_MSB_FIRST(private_section_length_field,
                        p - private_section_start);

    if (passphrase) {
        /*
         * Encrypt the private section. We need 48 bytes of key
         * material: 32 bytes AES key + 16 bytes iv.
         */
        unsigned char keybuf[48];
        void *ctx;

        openssh_bcrypt(passphrase,
                       bcrypt_salt, sizeof(bcrypt_salt), bcrypt_rounds,
                       keybuf, sizeof(keybuf));

        ctx = aes_make_context();
        aes256_key(ctx, keybuf);
        aes_iv(ctx, keybuf + 32);
        aes_ssh2_encrypt_blk(ctx, private_section_start,
                             p - private_section_start);
        aes_free_context(ctx);

        smemclr(keybuf, sizeof(keybuf));
    }

    /*
     * And save it. We'll use Unix line endings just in case it's
     * subsequently transferred in binary mode.
     */
    fp = f_open(filename, "wb", TRUE);      /* ensure Unix line endings */
    if (!fp)
        goto error;
    fputs("-----BEGIN OPENSSH PRIVATE KEY-----\n", fp);
    base64_encode(fp, outblob, p - outblob, 64);
    fputs("-----END OPENSSH PRIVATE KEY-----\n", fp);
    fclose(fp);
    ret = 1;

    error:
    if (outblob) {
        smemclr(outblob, maxsize);
        sfree(outblob);
    }
    if (privblob) {
        smemclr(privblob, privlen);
        sfree(privblob);
    }
    if (pubblob) {
        smemclr(pubblob, publen);
        sfree(pubblob);
    }
    return ret;
}

/* ----------------------------------------------------------------------
 * The switch function openssh_auto_write(), which chooses one of the
 * concrete OpenSSH output formats based on the key type.
 */
int openssh_auto_write(const Filename *filename, struct ssh2_userkey *key,
                       char *passphrase)
{
    /*
     * The old OpenSSH format supports a fixed list of key types. We
     * assume that anything not in that fixed list is newer, and hence
     * will use the new format.
     */
    if (key->alg == &ssh_dss ||
        key->alg == &ssh_rsa ||
        key->alg == &ssh_ecdsa_nistp256 ||
        key->alg == &ssh_ecdsa_nistp384 ||
        key->alg == &ssh_ecdsa_nistp521)
        return openssh_pem_write(filename, key, passphrase);
    else
        return openssh_new_write(filename, key, passphrase);
}

/* ----------------------------------------------------------------------
 * Code to read ssh.com private keys.
 */

/*
 * The format of the base64 blob is largely SSH-2-packet-formatted,
 * except that mpints are a bit different: they're more like the
 * old SSH-1 mpint. You have a 32-bit bit count N, followed by
 * (N+7)/8 bytes of data.
 * 
 * So. The blob contains:
 * 
 *  - uint32 0x3f6ff9eb       (magic number)
 *  - uint32 size             (total blob size)
 *  - string key-type         (see below)
 *  - string cipher-type      (tells you if key is encrypted)
 *  - string encrypted-blob
 * 
 * (The first size field includes the size field itself and the
 * magic number before it. All other size fields are ordinary SSH-2
 * strings, so the size field indicates how much data is to
 * _follow_.)
 * 
 * The encrypted blob, once decrypted, contains a single string
 * which in turn contains the payload. (This allows padding to be
 * added after that string while still making it clear where the
 * real payload ends. Also it probably makes for a reasonable
 * decryption check.)
 * 
 * The payload blob, for an RSA key, contains:
 *  - mpint e
 *  - mpint d
 *  - mpint n  (yes, the public and private stuff is intermixed)
 *  - mpint u  (presumably inverse of p mod q)
 *  - mpint p  (p is the smaller prime)
 *  - mpint q  (q is the larger)
 * 
 * For a DSA key, the payload blob contains:
 *  - uint32 0
 *  - mpint p
 *  - mpint g
 *  - mpint q
 *  - mpint y
 *  - mpint x
 * 
 * Alternatively, if the parameters are `predefined', that
 * (0,p,g,q) sequence can be replaced by a uint32 1 and a string
 * containing some predefined parameter specification. *shudder*,
 * but I doubt we'll encounter this in real life.
 * 
 * The key type strings are ghastly. The RSA key I looked at had a
 * type string of
 * 
 *   `if-modn{sign{rsa-pkcs1-sha1},encrypt{rsa-pkcs1v2-oaep}}'
 * 
 * and the DSA key wasn't much better:
 * 
 *   `dl-modp{sign{dsa-nist-sha1},dh{plain}}'
 * 
 * It isn't clear that these will always be the same. I think it
 * might be wise just to look at the `if-modn{sign{rsa' and
 * `dl-modp{sign{dsa' prefixes.
 * 
 * Finally, the encryption. The cipher-type string appears to be
 * either `none' or `3des-cbc'. Looks as if this is SSH-2-style
 * 3des-cbc (i.e. outer cbc rather than inner). The key is created
 * from the passphrase by means of yet another hashing faff:
 * 
 *  - first 16 bytes are MD5(passphrase)
 *  - next 16 bytes are MD5(passphrase || first 16 bytes)
 *  - if there were more, they'd be MD5(passphrase || first 32),
 *    and so on.
 */

#define SSHCOM_MAGIC_NUMBER 0x3f6ff9eb

struct sshcom_key {
    char comment[256];                 /* allowing any length is overkill */
    unsigned char *keyblob;
    int keyblob_len, keyblob_size;
};

static struct sshcom_key *load_sshcom_key(const Filename *filename,
                                          const char **errmsg_p)
{
    struct sshcom_key *ret;
    FILE *fp;
    char *line = NULL;
    int hdrstart, len;
    const char *errmsg;
    char *p;
    int headers_done;
    char base64_bit[4];
    int base64_chars = 0;

    ret = snew(struct sshcom_key);
    ret->comment[0] = '\0';
    ret->keyblob = NULL;
    ret->keyblob_len = ret->keyblob_size = 0;

    fp = f_open(filename, "r", FALSE);
    if (!fp) {
        errmsg = "unable to open key file";
        goto error;
    }
    if (!(line = fgetline(fp))) {
        errmsg = "unexpected end of file";
        goto error;
    }
    strip_crlf(line);
    if (0 != strcmp(line, "---- BEGIN SSH2 ENCRYPTED PRIVATE KEY ----")) {
        errmsg = "file does not begin with ssh.com key header";
        goto error;
    }
    smemclr(line, strlen(line));
    sfree(line);
    line = NULL;

    headers_done = 0;
    while (1) {
        if (!(line = fgetline(fp))) {
            errmsg = "unexpected end of file";
            goto error;
        }
        strip_crlf(line);
        if (!strcmp(line, "---- END SSH2 ENCRYPTED PRIVATE KEY ----")) {
            sfree(line);
            line = NULL;
            break;                     /* done */
        }
        if ((p = strchr(line, ':')) != NULL) {
            if (headers_done) {
                errmsg = "header found in body of key data";
                goto error;
            }
            *p++ = '\0';
            while (*p && isspace((unsigned char)*p)) p++;
            hdrstart = p - line;

            /*
             * Header lines can end in a trailing backslash for
             * continuation.
             */
            len = hdrstart + strlen(line+hdrstart);
            assert(!line[len]);
            while (line[len-1] == '\\') {
                char *line2;
                int line2len;

                line2 = fgetline(fp);
                if (!line2) {
                    errmsg = "unexpected end of file";
                    goto error;
                }
                strip_crlf(line2);

                line2len = strlen(line2);
                line = sresize(line, len + line2len + 1, char);
                strcpy(line + len - 1, line2);
                len += line2len - 1;
                assert(!line[len]);

                smemclr(line2, strlen(line2));
                sfree(line2);
                line2 = NULL;
            }
            p = line + hdrstart;
            strip_crlf(p);
            if (!strcmp(line, "Comment")) {
                /* Strip quotes in comment if present. */
                if (p[0] == '"' && p[strlen(p)-1] == '"') {
                    p++;
                    p[strlen(p)-1] = '\0';
                }
                strncpy(ret->comment, p, sizeof(ret->comment));
                ret->comment[sizeof(ret->comment)-1] = '\0';
            }
        } else {
            headers_done = 1;

            p = line;
            while (isbase64(*p)) {
                base64_bit[base64_chars++] = *p;
                if (base64_chars == 4) {
                    unsigned char out[3];

                    base64_chars = 0;

                    len = base64_decode_atom(base64_bit, out);

                    if (len <= 0) {
                        errmsg = "invalid base64 encoding";
                        goto error;
                    }

                    if (ret->keyblob_len + len > ret->keyblob_size) {
                        ret->keyblob_size = ret->keyblob_len + len + 256;
                        ret->keyblob = sresize(ret->keyblob, ret->keyblob_size,
                                               unsigned char);
                    }

                    memcpy(ret->keyblob + ret->keyblob_len, out, len);
                    ret->keyblob_len += len;
                }

                p++;
            }
        }
        smemclr(line, strlen(line));
        sfree(line);
        line = NULL;
    }

    if (ret->keyblob_len == 0 || !ret->keyblob) {
        errmsg = "key body not present";
        goto error;
    }

    fclose(fp);
    if (errmsg_p) *errmsg_p = NULL;
    return ret;

    error:
    if (fp)
        fclose(fp);

    if (line) {
        smemclr(line, strlen(line));
        sfree(line);
        line = NULL;
    }
    if (ret) {
        if (ret->keyblob) {
            smemclr(ret->keyblob, ret->keyblob_size);
            sfree(ret->keyblob);
        }
        smemclr(ret, sizeof(*ret));
        sfree(ret);
    }
    if (errmsg_p) *errmsg_p = errmsg;
    return NULL;
}

int sshcom_encrypted(const Filename *filename, char **comment)
{
    struct sshcom_key *key = load_sshcom_key(filename, NULL);
    int pos, len, answer;

    answer = 0;

    *comment = NULL;
    if (!key)
        goto done;

    /*
     * Check magic number.
     */
    if (GET_32BIT(key->keyblob) != 0x3f6ff9eb) {
        goto done;                     /* key is invalid */
    }

    /*
     * Find the cipher-type string.
     */
    pos = 8;
    if (key->keyblob_len < pos+4)
        goto done;                     /* key is far too short */
    len = toint(GET_32BIT(key->keyblob + pos));
    if (len < 0 || len > key->keyblob_len - pos - 4)
        goto done;                     /* key is far too short */
    pos += 4 + len;                    /* skip key type */
    len = toint(GET_32BIT(key->keyblob + pos)); /* find cipher-type length */
    if (len < 0 || len > key->keyblob_len - pos - 4)
        goto done;                     /* cipher type string is incomplete */
    if (len != 4 || 0 != memcmp(key->keyblob + pos + 4, "none", 4))
        answer = 1;

    done:
    if (key) {
        *comment = dupstr(key->comment);
        smemclr(key->keyblob, key->keyblob_size);
        sfree(key->keyblob);
        smemclr(key, sizeof(*key));
        sfree(key);
    } else {
        *comment = dupstr("");
    }
    return answer;
}

static int sshcom_read_mpint(void *data, int len, struct mpint_pos *ret)
{
    unsigned bits, bytes;
    unsigned char *d = (unsigned char *) data;

    if (len < 4)
        goto error;
    bits = GET_32BIT(d);

    bytes = (bits + 7) / 8;
    if (len < 4+bytes)
        goto error;

    ret->start = d + 4;
    ret->bytes = bytes;
    return bytes+4;

    error:
    ret->start = NULL;
    ret->bytes = -1;
    return len;                        /* ensure further calls fail as well */
}

static int sshcom_put_mpint(void *target, void *data, int len)
{
    unsigned char *d = (unsigned char *)target;
    unsigned char *i = (unsigned char *)data;
    int bits = len * 8 - 1;

    while (bits > 0) {
        if (*i & (1 << (bits & 7)))
            break;
        if (!(bits-- & 7))
            i++, len--;
    }

    PUT_32BIT(d, bits+1);
    memcpy(d+4, i, len);
    return len+4;
}

struct ssh2_userkey *sshcom_read(const Filename *filename, char *passphrase,
                                 const char **errmsg_p)
{
    struct sshcom_key *key = load_sshcom_key(filename, errmsg_p);
    const char *errmsg;
    int pos, len;
    const char prefix_rsa[] = "if-modn{sign{rsa";
    const char prefix_dsa[] = "dl-modp{sign{dsa";
    enum { RSA, DSA } type;
    int encrypted;
    char *ciphertext;
    int cipherlen;
    struct ssh2_userkey *ret = NULL, *retkey;
    const struct ssh_signkey *alg;
    unsigned char *blob = NULL;
    int blobsize = 0, publen, privlen;

    if (!key)
        return NULL;

    /*
     * Check magic number.
     */
    if (GET_32BIT(key->keyblob) != SSHCOM_MAGIC_NUMBER) {
        errmsg = "key does not begin with magic number";
        goto error;
    }

    /*
     * Determine the key type.
     */
    pos = 8;
    if (key->keyblob_len < pos+4 ||
        (len = toint(GET_32BIT(key->keyblob + pos))) < 0 ||
        len > key->keyblob_len - pos - 4) {
        errmsg = "key blob does not contain a key type string";
        goto error;
    }
    if (len > sizeof(prefix_rsa) - 1 &&
        !memcmp(key->keyblob+pos+4, prefix_rsa, sizeof(prefix_rsa) - 1)) {
        type = RSA;
    } else if (len > sizeof(prefix_dsa) - 1 &&
        !memcmp(key->keyblob+pos+4, prefix_dsa, sizeof(prefix_dsa) - 1)) {
        type = DSA;
    } else {
        errmsg = "key is of unknown type";
        goto error;
    }
    pos += 4+len;

    /*
     * Determine the cipher type.
     */
    if (key->keyblob_len < pos+4 ||
        (len = toint(GET_32BIT(key->keyblob + pos))) < 0 ||
        len > key->keyblob_len - pos - 4) {
        errmsg = "key blob does not contain a cipher type string";
        goto error;
    }
    if (len == 4 && !memcmp(key->keyblob+pos+4, "none", 4))
        encrypted = 0;
    else if (len == 8 && !memcmp(key->keyblob+pos+4, "3des-cbc", 8))
        encrypted = 1;
    else {
        errmsg = "key encryption is of unknown type";
        goto error;
    }
    pos += 4+len;

    /*
     * Get hold of the encrypted part of the key.
     */
    if (key->keyblob_len < pos+4 ||
        (len = toint(GET_32BIT(key->keyblob + pos))) < 0 ||
        len > key->keyblob_len - pos - 4) {
        errmsg = "key blob does not contain actual key data";
        goto error;
    }
    ciphertext = (char *)key->keyblob + pos + 4;
    cipherlen = len;
    if (cipherlen == 0) {
        errmsg = "length of key data is zero";
        goto error;
    }

    /*
     * Decrypt it if necessary.
     */
    if (encrypted) {
        /*
         * Derive encryption key from passphrase and iv/salt:
         * 
         *  - let block A equal MD5(passphrase)
         *  - let block B equal MD5(passphrase || A)
         *  - block C would be MD5(passphrase || A || B) and so on
         *  - encryption key is the first N bytes of A || B
         */
        struct MD5Context md5c;
        unsigned char keybuf[32], iv[8];

        if (cipherlen % 8 != 0) {
            errmsg = "encrypted part of key is not a multiple of cipher block"
                " size";
            goto error;
        }

        MD5Init(&md5c);
        MD5Update(&md5c, (unsigned char *)passphrase, strlen(passphrase));
        MD5Final(keybuf, &md5c);

        MD5Init(&md5c);
        MD5Update(&md5c, (unsigned char *)passphrase, strlen(passphrase));
        MD5Update(&md5c, keybuf, 16);
        MD5Final(keybuf+16, &md5c);

        /*
         * Now decrypt the key blob.
         */
        memset(iv, 0, sizeof(iv));
        des3_decrypt_pubkey_ossh(keybuf, iv, (unsigned char *)ciphertext,
                                 cipherlen);

        smemclr(&md5c, sizeof(md5c));
        smemclr(keybuf, sizeof(keybuf));

        /*
         * Hereafter we return WRONG_PASSPHRASE for any parsing
         * error. (But only if we've just tried to decrypt it!
         * Returning WRONG_PASSPHRASE for an unencrypted key is
         * automatic doom.)
         */
        if (encrypted)
            ret = SSH2_WRONG_PASSPHRASE;
    }

    /*
     * Strip away the containing string to get to the real meat.
     */
    len = toint(GET_32BIT(ciphertext));
    if (len < 0 || len > cipherlen-4) {
        errmsg = "containing string was ill-formed";
        goto error;
    }
    ciphertext += 4;
    cipherlen = len;

    /*
     * Now we break down into RSA versus DSA. In either case we'll
     * construct public and private blobs in our own format, and
     * end up feeding them to alg->createkey().
     */
    blobsize = cipherlen + 256;
    blob = snewn(blobsize, unsigned char);
    privlen = 0;
    if (type == RSA) {
        struct mpint_pos n, e, d, u, p, q;
        int pos = 0;
        pos += sshcom_read_mpint(ciphertext+pos, cipherlen-pos, &e);
        pos += sshcom_read_mpint(ciphertext+pos, cipherlen-pos, &d);
        pos += sshcom_read_mpint(ciphertext+pos, cipherlen-pos, &n);
        pos += sshcom_read_mpint(ciphertext+pos, cipherlen-pos, &u);
        pos += sshcom_read_mpint(ciphertext+pos, cipherlen-pos, &p);
        pos += sshcom_read_mpint(ciphertext+pos, cipherlen-pos, &q);
        if (!q.start) {
            errmsg = "key data did not contain six integers";
            goto error;
        }

        alg = &ssh_rsa;
        pos = 0;
        pos += put_string(blob+pos, "ssh-rsa", 7);
        pos += put_mp(blob+pos, e.start, e.bytes);
        pos += put_mp(blob+pos, n.start, n.bytes);
        publen = pos;
        pos += put_string(blob+pos, d.start, d.bytes);
        pos += put_mp(blob+pos, q.start, q.bytes);
        pos += put_mp(blob+pos, p.start, p.bytes);
        pos += put_mp(blob+pos, u.start, u.bytes);
        privlen = pos - publen;
    } else {
        struct mpint_pos p, q, g, x, y;
        int pos = 4;

        assert(type == DSA); /* the only other option from the if above */

        if (GET_32BIT(ciphertext) != 0) {
            errmsg = "predefined DSA parameters not supported";
            goto error;
        }
        pos += sshcom_read_mpint(ciphertext+pos, cipherlen-pos, &p);
        pos += sshcom_read_mpint(ciphertext+pos, cipherlen-pos, &g);
        pos += sshcom_read_mpint(ciphertext+pos, cipherlen-pos, &q);
        pos += sshcom_read_mpint(ciphertext+pos, cipherlen-pos, &y);
        pos += sshcom_read_mpint(ciphertext+pos, cipherlen-pos, &x);
        if (!x.start) {
            errmsg = "key data did not contain five integers";
            goto error;
        }

        alg = &ssh_dss;
        pos = 0;
        pos += put_string(blob+pos, "ssh-dss", 7);
        pos += put_mp(blob+pos, p.start, p.bytes);
        pos += put_mp(blob+pos, q.start, q.bytes);
        pos += put_mp(blob+pos, g.start, g.bytes);
        pos += put_mp(blob+pos, y.start, y.bytes);
        publen = pos;
        pos += put_mp(blob+pos, x.start, x.bytes);
        privlen = pos - publen;
    }

    assert(privlen > 0);               /* should have bombed by now if not */

    retkey = snew(struct ssh2_userkey);
    retkey->alg = alg;
    retkey->data = alg->createkey(alg, blob, publen, blob+publen, privlen);
    if (!retkey->data) {
        sfree(retkey);
        errmsg = "unable to create key data structure";
        goto error;
    }
    retkey->comment = dupstr(key->comment);

    errmsg = NULL; /* no error */
    ret = retkey;

    error:
    if (blob) {
        smemclr(blob, blobsize);
        sfree(blob);
    }
    smemclr(key->keyblob, key->keyblob_size);
    sfree(key->keyblob);
    smemclr(key, sizeof(*key));
    sfree(key);
    if (errmsg_p) *errmsg_p = errmsg;
    return ret;
}

int sshcom_write(const Filename *filename, struct ssh2_userkey *key,
                 char *passphrase)
{
    unsigned char *pubblob, *privblob;
    int publen, privlen;
    unsigned char *outblob;
    int outlen;
    struct mpint_pos numbers[6];
    int nnumbers, initial_zero, pos, lenpos, i;
    const char *type;
    char *ciphertext;
    int cipherlen;
    int ret = 0;
    FILE *fp;

    /*
     * Fetch the key blobs.
     */
    pubblob = key->alg->public_blob(key->data, &publen);
    privblob = key->alg->private_blob(key->data, &privlen);
    outblob = NULL;

    /*
     * Find the sequence of integers to be encoded into the OpenSSH
     * key blob, and also decide on the header line.
     */
    if (key->alg == &ssh_rsa) {
        int pos;
        struct mpint_pos n, e, d, p, q, iqmp;

        /*
         * These blobs were generated from inside PuTTY, so we needn't
         * treat them as untrusted.
         */
        pos = 4 + GET_32BIT(pubblob);
        pos += ssh2_read_mpint(pubblob+pos, publen-pos, &e);
        pos += ssh2_read_mpint(pubblob+pos, publen-pos, &n);
        pos = 0;
        pos += ssh2_read_mpint(privblob+pos, privlen-pos, &d);
        pos += ssh2_read_mpint(privblob+pos, privlen-pos, &p);
        pos += ssh2_read_mpint(privblob+pos, privlen-pos, &q);
        pos += ssh2_read_mpint(privblob+pos, privlen-pos, &iqmp);

        assert(e.start && iqmp.start); /* can't go wrong */

        numbers[0] = e;
        numbers[1] = d;
        numbers[2] = n;
        numbers[3] = iqmp;
        numbers[4] = q;
        numbers[5] = p;

        nnumbers = 6;
        initial_zero = 0;
        type = "if-modn{sign{rsa-pkcs1-sha1},encrypt{rsa-pkcs1v2-oaep}}";
    } else if (key->alg == &ssh_dss) {
        int pos;
        struct mpint_pos p, q, g, y, x;

        /*
         * These blobs were generated from inside PuTTY, so we needn't
         * treat them as untrusted.
         */
        pos = 4 + GET_32BIT(pubblob);
        pos += ssh2_read_mpint(pubblob+pos, publen-pos, &p);
        pos += ssh2_read_mpint(pubblob+pos, publen-pos, &q);
        pos += ssh2_read_mpint(pubblob+pos, publen-pos, &g);
        pos += ssh2_read_mpint(pubblob+pos, publen-pos, &y);
        pos = 0;
        pos += ssh2_read_mpint(privblob+pos, privlen-pos, &x);

        assert(y.start && x.start); /* can't go wrong */

        numbers[0] = p;
        numbers[1] = g;
        numbers[2] = q;
        numbers[3] = y;
        numbers[4] = x;

        nnumbers = 5;
        initial_zero = 1;
        type = "dl-modp{sign{dsa-nist-sha1},dh{plain}}";
    } else {
        assert(0);                     /* zoinks! */
        exit(1); /* XXX: GCC doesn't understand assert() on some systems. */
    }

    /*
     * Total size of key blob will be somewhere under 512 plus
     * combined length of integers. We'll calculate the more
     * precise size as we construct the blob.
     */
    outlen = 512;
    for (i = 0; i < nnumbers; i++)
        outlen += 4 + numbers[i].bytes;
    outblob = snewn(outlen, unsigned char);

    /*
     * Create the unencrypted key blob.
     */
    pos = 0;
    PUT_32BIT(outblob+pos, SSHCOM_MAGIC_NUMBER); pos += 4;
    pos += 4;                          /* length field, fill in later */
    pos += put_string(outblob+pos, type, strlen(type));
    {
        const char *ciphertype = passphrase ? "3des-cbc" : "none";
        pos += put_string(outblob+pos, ciphertype, strlen(ciphertype));
    }
    lenpos = pos;                      /* remember this position */
    pos += 4;                          /* encrypted-blob size */
    pos += 4;                          /* encrypted-payload size */
    if (initial_zero) {
        PUT_32BIT(outblob+pos, 0);
        pos += 4;
    }
    for (i = 0; i < nnumbers; i++)
        pos += sshcom_put_mpint(outblob+pos,
                                numbers[i].start, numbers[i].bytes);
    /* Now wrap up the encrypted payload. */
    PUT_32BIT(outblob+lenpos+4, pos - (lenpos+8));
    /* Pad encrypted blob to a multiple of cipher block size. */
    if (passphrase) {
        int padding = -(pos - (lenpos+4)) & 7;
        while (padding--)
            outblob[pos++] = random_byte();
    }
    ciphertext = (char *)outblob+lenpos+4;
    cipherlen = pos - (lenpos+4);
    assert(!passphrase || cipherlen % 8 == 0);
    /* Wrap up the encrypted blob string. */
    PUT_32BIT(outblob+lenpos, cipherlen);
    /* And finally fill in the total length field. */
    PUT_32BIT(outblob+4, pos);

    assert(pos < outlen);

    /*
     * Encrypt the key.
     */
    if (passphrase) {
        /*
         * Derive encryption key from passphrase and iv/salt:
         * 
         *  - let block A equal MD5(passphrase)
         *  - let block B equal MD5(passphrase || A)
         *  - block C would be MD5(passphrase || A || B) and so on
         *  - encryption key is the first N bytes of A || B
         */
        struct MD5Context md5c;
        unsigned char keybuf[32], iv[8];

        MD5Init(&md5c);
        MD5Update(&md5c, (unsigned char *)passphrase, strlen(passphrase));
        MD5Final(keybuf, &md5c);

        MD5Init(&md5c);
        MD5Update(&md5c, (unsigned char *)passphrase, strlen(passphrase));
        MD5Update(&md5c, keybuf, 16);
        MD5Final(keybuf+16, &md5c);

        /*
         * Now decrypt the key blob.
         */
        memset(iv, 0, sizeof(iv));
        des3_encrypt_pubkey_ossh(keybuf, iv, (unsigned char *)ciphertext,
                                 cipherlen);

        smemclr(&md5c, sizeof(md5c));
        smemclr(keybuf, sizeof(keybuf));
    }

    /*
     * And save it. We'll use Unix line endings just in case it's
     * subsequently transferred in binary mode.
     */
    fp = f_open(filename, "wb", TRUE);      /* ensure Unix line endings */
    if (!fp)
        goto error;
    fputs("---- BEGIN SSH2 ENCRYPTED PRIVATE KEY ----\n", fp);
    fprintf(fp, "Comment: \"");
    /*
     * Comment header is broken with backslash-newline if it goes
     * over 70 chars. Although it's surrounded by quotes, it
     * _doesn't_ escape backslashes or quotes within the string.
     * Don't ask me, I didn't design it.
     */
    {
        int slen = 60;                 /* starts at 60 due to "Comment: " */
        char *c = key->comment;
        while ((int)strlen(c) > slen) {
            fprintf(fp, "%.*s\\\n", slen, c);
            c += slen;
            slen = 70;                 /* allow 70 chars on subsequent lines */
        }
        fprintf(fp, "%s\"\n", c);
    }
    base64_encode(fp, outblob, pos, 70);
    fputs("---- END SSH2 ENCRYPTED PRIVATE KEY ----\n", fp);
    fclose(fp);
    ret = 1;

    error:
    if (outblob) {
        smemclr(outblob, outlen);
        sfree(outblob);
    }
    if (privblob) {
        smemclr(privblob, privlen);
        sfree(privblob);
    }
    if (pubblob) {
        smemclr(pubblob, publen);
        sfree(pubblob);
    }
    return ret;
}