INCOMPATIBLE CHANGE to the SSH2 private key file format. There is

[u/mdw/putty] / sshpubk.c

/*
 * Generic SSH public-key handling operations. In particular,
 * reading of SSH public-key files, and also the generic `sign'
 * operation for ssh2 (which checks the type of the key and
 * dispatches to the appropriate key-type specific function).
 */

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

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

#define PUT_32BIT(cp, value) do { \
  (cp)[3] = (value); \
  (cp)[2] = (value) >> 8; \
  (cp)[1] = (value) >> 16; \
  (cp)[0] = (value) >> 24; } while (0)

#define GET_32BIT(cp) \
    (((unsigned long)(unsigned char)(cp)[0] << 24) | \
    ((unsigned long)(unsigned char)(cp)[1] << 16) | \
    ((unsigned long)(unsigned char)(cp)[2] << 8) | \
    ((unsigned long)(unsigned char)(cp)[3]))

#define rsa_signature "SSH PRIVATE KEY FILE FORMAT 1.1\n"

#define BASE64_TOINT(x) ( (x)-'A'<26 ? (x)-'A'+0 :\
                          (x)-'a'<26 ? (x)-'a'+26 :\
                          (x)-'0'<10 ? (x)-'0'+52 :\
                          (x)=='+' ? 62 : \
                          (x)=='/' ? 63 : 0 )

static int loadrsakey_main(FILE * fp, struct RSAKey *key,
                           char **commentptr, char *passphrase)
{
    unsigned char buf[16384];
    unsigned char keybuf[16];
    int len;
    int i, j, ciphertype;
    int ret = 0;
    struct MD5Context md5c;
    char *comment;

    /* Slurp the whole file (minus the header) into a buffer. */
    len = fread(buf, 1, sizeof(buf), fp);
    fclose(fp);
    if (len < 0 || len == sizeof(buf))
        goto end;                      /* file too big or not read */

    i = 0;

    /*
     * A zero byte. (The signature includes a terminating NUL.)
     */
    if (len - i < 1 || buf[i] != 0)
        goto end;
    i++;

    /* One byte giving encryption type, and one reserved uint32. */
    if (len - i < 1)
        goto end;
    ciphertype = buf[i];
    if (ciphertype != 0 && ciphertype != SSH_CIPHER_3DES)
        goto end;
    i++;
    if (len - i < 4)
        goto end;                      /* reserved field not present */
    if (buf[i] != 0 || buf[i + 1] != 0 || buf[i + 2] != 0
        || buf[i + 3] != 0) goto end;  /* reserved field nonzero, panic! */
    i += 4;

    /* Now the serious stuff. An ordinary SSH 1 public key. */
    i += makekey(buf + i, key, NULL, 1);
    if (len - i < 0)
        goto end;                      /* overran */

    /* Next, the comment field. */
    j = GET_32BIT(buf + i);
    i += 4;
    if (len - i < j)
        goto end;
    comment = smalloc(j + 1);
    if (comment) {
        memcpy(comment, buf + i, j);
        comment[j] = '\0';
    }
    i += j;
    if (commentptr)
        *commentptr = comment;
    if (key)
        key->comment = comment;
    if (!key) {
        return ciphertype != 0;
    }

    /*
     * Decrypt remainder of buffer.
     */
    if (ciphertype) {
        MD5Init(&md5c);
        MD5Update(&md5c, passphrase, strlen(passphrase));
        MD5Final(keybuf, &md5c);
        des3_decrypt_pubkey(keybuf, buf + i, (len - i + 7) & ~7);
        memset(keybuf, 0, sizeof(keybuf));      /* burn the evidence */
    }

    /*
     * We are now in the secret part of the key. The first four
     * bytes should be of the form a, b, a, b.
     */
    if (len - i < 4)
        goto end;
    if (buf[i] != buf[i + 2] || buf[i + 1] != buf[i + 3]) {
        ret = -1;
        goto end;
    }
    i += 4;

    /*
     * After that, we have one further bignum which is our
     * decryption exponent, and then the three auxiliary values
     * (iqmp, q, p).
     */
    i += makeprivate(buf + i, key);
    if (len - i < 0)
        goto end;
    i += ssh1_read_bignum(buf + i, &key->iqmp);
    if (len - i < 0)
        goto end;
    i += ssh1_read_bignum(buf + i, &key->q);
    if (len - i < 0)
        goto end;
    i += ssh1_read_bignum(buf + i, &key->p);
    if (len - i < 0)
        goto end;

    if (!rsa_verify(key)) {
        freersakey(key);
        ret = 0;
    } else
        ret = 1;

  end:
    memset(buf, 0, sizeof(buf));       /* burn the evidence */
    return ret;
}

int loadrsakey(char *filename, struct RSAKey *key, char *passphrase)
{
    FILE *fp;
    unsigned char buf[64];

    fp = fopen(filename, "rb");
    if (!fp)
        return 0;                      /* doesn't even exist */

    /*
     * Read the first line of the file and see if it's a v1 private
     * key file.
     */
    if (fgets(buf, sizeof(buf), fp) && !strcmp(buf, rsa_signature)) {
        return loadrsakey_main(fp, key, NULL, passphrase);
    }

    /*
     * Otherwise, we have nothing. Return empty-handed.
     */
    fclose(fp);
    return 0;
}

/*
 * See whether an RSA key is encrypted. Return its comment field as
 * well.
 */
int rsakey_encrypted(char *filename, char **comment)
{
    FILE *fp;
    unsigned char buf[64];

    fp = fopen(filename, "rb");
    if (!fp)
        return 0;                      /* doesn't even exist */

    /*
     * Read the first line of the file and see if it's a v1 private
     * key file.
     */
    if (fgets(buf, sizeof(buf), fp) && !strcmp(buf, rsa_signature)) {
        return loadrsakey_main(fp, NULL, comment, NULL);
    }
    fclose(fp);
    return 0;                          /* wasn't the right kind of file */
}

/*
 * Save an RSA key file. Return nonzero on success.
 */
int saversakey(char *filename, struct RSAKey *key, char *passphrase)
{
    unsigned char buf[16384];
    unsigned char keybuf[16];
    struct MD5Context md5c;
    unsigned char *p, *estart;
    FILE *fp;

    /*
     * Write the initial signature.
     */
    p = buf;
    memcpy(p, rsa_signature, sizeof(rsa_signature));
    p += sizeof(rsa_signature);

    /*
     * One byte giving encryption type, and one reserved (zero)
     * uint32.
     */
    *p++ = (passphrase ? SSH_CIPHER_3DES : 0);
    PUT_32BIT(p, 0);
    p += 4;

    /*
     * An ordinary SSH 1 public key consists of: a uint32
     * containing the bit count, then two bignums containing the
     * modulus and exponent respectively.
     */
    PUT_32BIT(p, bignum_bitcount(key->modulus));
    p += 4;
    p += ssh1_write_bignum(p, key->modulus);
    p += ssh1_write_bignum(p, key->exponent);

    /*
     * A string containing the comment field.
     */
    if (key->comment) {
        PUT_32BIT(p, strlen(key->comment));
        p += 4;
        memcpy(p, key->comment, strlen(key->comment));
        p += strlen(key->comment);
    } else {
        PUT_32BIT(p, 0);
        p += 4;
    }

    /*
     * The encrypted portion starts here.
     */
    estart = p;

    /*
     * Two bytes, then the same two bytes repeated.
     */
    *p++ = random_byte();
    *p++ = random_byte();
    p[0] = p[-2];
    p[1] = p[-1];
    p += 2;

    /*
     * Four more bignums: the decryption exponent, then iqmp, then
     * q, then p.
     */
    p += ssh1_write_bignum(p, key->private_exponent);
    p += ssh1_write_bignum(p, key->iqmp);
    p += ssh1_write_bignum(p, key->q);
    p += ssh1_write_bignum(p, key->p);

    /*
     * Now write zeros until the encrypted portion is a multiple of
     * 8 bytes.
     */
    while ((p - estart) % 8)
        *p++ = '\0';

    /*
     * Now encrypt the encrypted portion.
     */
    if (passphrase) {
        MD5Init(&md5c);
        MD5Update(&md5c, passphrase, strlen(passphrase));
        MD5Final(keybuf, &md5c);
        des3_encrypt_pubkey(keybuf, estart, p - estart);
        memset(keybuf, 0, sizeof(keybuf));      /* burn the evidence */
    }

    /*
     * Done. Write the result to the file.
     */
    fp = fopen(filename, "wb");
    if (fp) {
        int ret = (fwrite(buf, 1, p - buf, fp) == (size_t) (p - buf));
        ret = ret && (fclose(fp) == 0);
        return ret;
    } else
        return 0;
}

/* ----------------------------------------------------------------------
 * SSH2 private key load/store functions.
 */

/*
 * PuTTY's own format for SSH2 keys is as follows:
 *
 * The file is text. Lines are terminated by CRLF, although CR-only
 * and LF-only are tolerated on input.
 *
 * The first line says "PuTTY-User-Key-File-2: " plus the name of the
 * algorithm ("ssh-dss", "ssh-rsa" etc).
 *
 * The next line says "Encryption: " plus an encryption type.
 * Currently the only supported encryption types are "aes256-cbc"
 * and "none".
 *
 * The next line says "Comment: " plus the comment string.
 *
 * Next there is a line saying "Public-Lines: " plus a number N.
 * The following N lines contain a base64 encoding of the public
 * part of the key. This is encoded as the standard SSH2 public key
 * blob (with no initial length): so for RSA, for example, it will
 * read
 *
 *    string "ssh-rsa"
 *    mpint  exponent
 *    mpint  modulus
 *
 * Next, there is a line saying "Private-Lines: " plus a number N,
 * and then N lines containing the (potentially encrypted) private
 * part of the key. For the key type "ssh-rsa", this will be
 * composed of
 *
 *    mpint  private_exponent
 *    mpint  p                  (the larger of the two primes)
 *    mpint  q                  (the smaller prime)
 *    mpint  iqmp               (the inverse of q modulo p)
 *    data   padding            (to reach a multiple of the cipher block size)
 *
 * And for "ssh-dss", it will be composed of
 *
 *    mpint  x                  (the private key parameter)
 *  [ string hash   20-byte hash of mpints p || q || g   only in old format ]
 * 
 * Finally, there is a line saying "Private-MAC: " plus a hex
 * representation of a HMAC-SHA-1 of:
 *
 *    string  name of algorithm ("ssh-dss", "ssh-rsa")
 *    string  encryption type
 *    string  comment
 *    string  public-blob
 *    string  private-plaintext (the plaintext version of the
 *                               private part, including the final
 *                               padding)
 * 
 * The key to the MAC is itself a SHA-1 hash of:
 * 
 *    data    "putty-private-key-file-mac-key"
 *    data    passphrase
 *
 * Encrypted keys should have a MAC, whereas unencrypted ones must
 * have a hash.
 *
 * If the key is encrypted, the encryption key is derived from the
 * passphrase by means of a succession of SHA-1 hashes. Each hash
 * is the hash of:
 *
 *    uint32  sequence-number
 *    data    passphrase
 *
 * where the sequence-number increases from zero. As many of these
 * hashes are used as necessary.
 *
 * For backwards compatibility with snapshots between 0.51 and
 * 0.52, we also support the older key file format, which begins
 * with "PuTTY-User-Key-File-1" (version number differs). In this
 * format the Private-MAC: field only covers the private-plaintext
 * field and nothing else (and without the 4-byte string length on
 * the front too). Moreover, for RSA keys the Private-MAC: field
 * can be replaced with a Private-Hash: field which is a plain
 * SHA-1 hash instead of an HMAC. This is not allowable in DSA
 * keys. (Yes, the old format was a mess. Guess why it changed :-)
 */

static int read_header(FILE * fp, char *header)
{
    int len = 39;
    int c;

    while (len > 0) {
        c = fgetc(fp);
        if (c == '\n' || c == '\r' || c == EOF)
            return 0;                  /* failure */
        if (c == ':') {
            c = fgetc(fp);
            if (c != ' ')
                return 0;
            *header = '\0';
            return 1;                  /* success! */
        }
        if (len == 0)
            return 0;                  /* failure */
        *header++ = c;
        len--;
    }
    return 0;                          /* failure */
}

static char *read_body(FILE * fp)
{
    char *text;
    int len;
    int size;
    int c;

    size = 128;
    text = smalloc(size);
    len = 0;
    text[len] = '\0';

    while (1) {
        c = fgetc(fp);
        if (c == '\r' || c == '\n') {
            c = fgetc(fp);
            if (c != '\r' && c != '\n' && c != EOF)
                ungetc(c, fp);
            return text;
        }
        if (c == EOF) {
            sfree(text);
            return NULL;
        }
        if (len + 1 > size) {
            size += 128;
            text = srealloc(text, size);
        }
        text[len++] = c;
        text[len] = '\0';
    }
}

int base64_decode_atom(char *atom, unsigned char *out)
{
    int vals[4];
    int i, v, len;
    unsigned word;
    char c;

    for (i = 0; i < 4; i++) {
        c = atom[i];
        if (c >= 'A' && c <= 'Z')
            v = c - 'A';
        else if (c >= 'a' && c <= 'z')
            v = c - 'a' + 26;
        else if (c >= '0' && c <= '9')
            v = c - '0' + 52;
        else if (c == '+')
            v = 62;
        else if (c == '/')
            v = 63;
        else if (c == '=')
            v = -1;
        else
            return 0;                  /* invalid atom */
        vals[i] = v;
    }

    if (vals[0] == -1 || vals[1] == -1)
        return 0;
    if (vals[2] == -1 && vals[3] != -1)
        return 0;

    if (vals[3] != -1)
        len = 3;
    else if (vals[2] != -1)
        len = 2;
    else
        len = 1;

    word = ((vals[0] << 18) |
            (vals[1] << 12) | ((vals[2] & 0x3F) << 6) | (vals[3] & 0x3F));
    out[0] = (word >> 16) & 0xFF;
    if (len > 1)
        out[1] = (word >> 8) & 0xFF;
    if (len > 2)
        out[2] = word & 0xFF;
    return len;
}

static char *read_blob(FILE * fp, int nlines, int *bloblen)
{
    unsigned char *blob;
    char *line;
    int linelen, len;
    int i, j, k;

    /* We expect at most 64 base64 characters, ie 48 real bytes, per line. */
    blob = smalloc(48 * nlines);
    len = 0;
    for (i = 0; i < nlines; i++) {
        line = read_body(fp);
        if (!line) {
            sfree(blob);
            return NULL;
        }
        linelen = strlen(line);
        if (linelen % 4 != 0 || linelen > 64) {
            sfree(blob);
            sfree(line);
            return NULL;
        }
        for (j = 0; j < linelen; j += 4) {
            k = base64_decode_atom(line + j, blob + len);
            if (!k) {
                sfree(line);
                sfree(blob);
                return NULL;
            }
            len += k;
        }
        sfree(line);
    }
    *bloblen = len;
    return blob;
}

/*
 * Magic error return value for when the passphrase is wrong.
 */
struct ssh2_userkey ssh2_wrong_passphrase = {
    NULL, NULL, NULL
};

struct ssh2_userkey *ssh2_load_userkey(char *filename, char *passphrase)
{
    FILE *fp;
    char header[40], *b, *encryption, *comment, *mac;
    const struct ssh_signkey *alg;
    struct ssh2_userkey *ret;
    int cipher, cipherblk;
    unsigned char *public_blob, *private_blob;
    int public_blob_len, private_blob_len;
    int i, is_mac, old_fmt;
    int passlen = passphrase ? strlen(passphrase) : 0;

    ret = NULL;                        /* return NULL for most errors */
    comment = mac = NULL;
    public_blob = private_blob = NULL;

    fp = fopen(filename, "rb");
    if (!fp)
        goto error;

    /* Read the first header line which contains the key type. */
    if (!read_header(fp, header))
        goto error;
    if (0 == strcmp(header, "PuTTY-User-Key-File-2")) {
        old_fmt = 0;
    } else if (0 == strcmp(header, "PuTTY-User-Key-File-1")) {
        /* this is an old key file; warn and then continue */
        old_keyfile_warning();
        old_fmt = 1;
    } else
        goto error;
    if ((b = read_body(fp)) == NULL)
        goto error;
    /* Select key algorithm structure. */
    if (!strcmp(b, "ssh-rsa"))
        alg = &ssh_rsa;
    else if (!strcmp(b, "ssh-dss"))
        alg = &ssh_dss;
    else {
        sfree(b);
        goto error;
    }
    sfree(b);

    /* Read the Encryption header line. */
    if (!read_header(fp, header) || 0 != strcmp(header, "Encryption"))
        goto error;
    if ((encryption = read_body(fp)) == NULL)
        goto error;
    if (!strcmp(encryption, "aes256-cbc")) {
        cipher = 1;
        cipherblk = 16;
    } else if (!strcmp(encryption, "none")) {
        cipher = 0;
        cipherblk = 1;
    } else {
        sfree(encryption);
        goto error;
    }

    /* Read the Comment header line. */
    if (!read_header(fp, header) || 0 != strcmp(header, "Comment"))
        goto error;
    if ((comment = read_body(fp)) == NULL)
        goto error;

    /* Read the Public-Lines header line and the public blob. */
    if (!read_header(fp, header) || 0 != strcmp(header, "Public-Lines"))
        goto error;
    if ((b = read_body(fp)) == NULL)
        goto error;
    i = atoi(b);
    sfree(b);
    if ((public_blob = read_blob(fp, i, &public_blob_len)) == NULL)
        goto error;

    /* Read the Private-Lines header line and the Private blob. */
    if (!read_header(fp, header) || 0 != strcmp(header, "Private-Lines"))
        goto error;
    if ((b = read_body(fp)) == NULL)
        goto error;
    i = atoi(b);
    sfree(b);
    if ((private_blob = read_blob(fp, i, &private_blob_len)) == NULL)
        goto error;

    /* Read the Private-MAC or Private-Hash header line. */
    if (!read_header(fp, header))
        goto error;
    if (0 == strcmp(header, "Private-MAC")) {
        if ((mac = read_body(fp)) == NULL)
            goto error;
        is_mac = 1;
    } else if (0 == strcmp(header, "Private-Hash") &&
                           alg == &ssh_rsa && old_fmt) {
        if ((mac = read_body(fp)) == NULL)
            goto error;
        is_mac = 0;
    } else
        goto error;

    fclose(fp);
    fp = NULL;

    /*
     * Decrypt the private blob.
     */
    if (cipher) {
        unsigned char key[40];
        SHA_State s;

        if (!passphrase)
            goto error;
        if (private_blob_len % cipherblk)
            goto error;

        SHA_Init(&s);
        SHA_Bytes(&s, "\0\0\0\0", 4);
        SHA_Bytes(&s, passphrase, passlen);
        SHA_Final(&s, key + 0);
        SHA_Init(&s);
        SHA_Bytes(&s, "\0\0\0\1", 4);
        SHA_Bytes(&s, passphrase, passlen);
        SHA_Final(&s, key + 20);
        aes256_decrypt_pubkey(key, private_blob, private_blob_len);
    }

    /*
     * Verify the MAC.
     */
    {
        char realmac[41];
        unsigned char binary[20];
        unsigned char *macdata;
        int maclen;
        int free_macdata;

        if (old_fmt) {
            /* MAC (or hash) only covers the private blob. */
            macdata = private_blob;
            maclen = private_blob_len;
            free_macdata = 0;
        } else {
            unsigned char *p;
            int namelen = strlen(alg->name);
            int enclen = strlen(encryption);
            int commlen = strlen(comment);
            maclen = (4 + namelen +
                      4 + enclen +
                      4 + commlen +
                      4 + public_blob_len +
                      4 + private_blob_len);
            macdata = smalloc(maclen);
            p = macdata;
#define DO_STR(s,len) PUT_32BIT(p,(len));memcpy(p+4,(s),(len));p+=4+(len)
            DO_STR(alg->name, namelen);
            DO_STR(encryption, enclen);
            DO_STR(comment, commlen);
            DO_STR(public_blob, public_blob_len);
            DO_STR(private_blob, private_blob_len);

            free_macdata = 1;
        }

        if (is_mac) {
            SHA_State s;
            unsigned char mackey[20];
            char header[] = "putty-private-key-file-mac-key";

            SHA_Init(&s);
            SHA_Bytes(&s, header, sizeof(header)-1);
            if (passphrase)
                SHA_Bytes(&s, passphrase, passlen);
            SHA_Final(&s, mackey);

            hmac_sha1_simple(mackey, 20, macdata, maclen, binary);

            memset(mackey, 0, sizeof(mackey));
            memset(&s, 0, sizeof(s));
        } else {
            SHA_Simple(macdata, maclen, binary);
        }

        if (free_macdata) {
            memset(macdata, 0, maclen);
            sfree(macdata);
        }

        for (i = 0; i < 20; i++)
            sprintf(realmac + 2 * i, "%02x", binary[i]);

        if (strcmp(mac, realmac)) {
            /* An incorrect MAC is an unconditional Error if the key is
             * unencrypted. Otherwise, it means Wrong Passphrase. */
            ret = cipher ? SSH2_WRONG_PASSPHRASE : NULL;
            goto error;
        }
    }
    sfree(mac);

    /*
     * Create and return the key.
     */
    ret = smalloc(sizeof(struct ssh2_userkey));
    ret->alg = alg;
    ret->comment = comment;
    ret->data = alg->createkey(public_blob, public_blob_len,
                               private_blob, private_blob_len);
    if (!ret->data) {
        sfree(ret->comment);
        sfree(ret);
        ret = NULL;
    }
    sfree(public_blob);
    sfree(private_blob);
    sfree(encryption);
    return ret;

    /*
     * Error processing.
     */
  error:
    if (fp)
        fclose(fp);
    if (comment)
        sfree(comment);
    if (encryption)
        sfree(encryption);
    if (mac)
        sfree(mac);
    if (public_blob)
        sfree(public_blob);
    if (private_blob)
        sfree(private_blob);
    return ret;
}

char *ssh2_userkey_loadpub(char *filename, char **algorithm,
                           int *pub_blob_len)
{
    FILE *fp;
    char header[40], *b;
    const struct ssh_signkey *alg;
    unsigned char *public_blob;
    int public_blob_len;
    int i;

    public_blob = NULL;

    fp = fopen(filename, "rb");
    if (!fp)
        goto error;

    /* Read the first header line which contains the key type. */
    if (!read_header(fp, header)
        || (0 != strcmp(header, "PuTTY-User-Key-File-2") &&
            0 != strcmp(header, "PuTTY-User-Key-File-1")))
        goto error;
    if ((b = read_body(fp)) == NULL)
        goto error;
    /* Select key algorithm structure. Currently only ssh-rsa. */
    if (!strcmp(b, "ssh-rsa"))
        alg = &ssh_rsa;
    else if (!strcmp(b, "ssh-dss"))
        alg = &ssh_dss;
    else {
        sfree(b);
        goto error;
    }
    sfree(b);

    /* Read the Encryption header line. */
    if (!read_header(fp, header) || 0 != strcmp(header, "Encryption"))
        goto error;
    if ((b = read_body(fp)) == NULL)
        goto error;
    sfree(b);                          /* we don't care */

    /* Read the Comment header line. */
    if (!read_header(fp, header) || 0 != strcmp(header, "Comment"))
        goto error;
    if ((b = read_body(fp)) == NULL)
        goto error;
    sfree(b);                          /* we don't care */

    /* Read the Public-Lines header line and the public blob. */
    if (!read_header(fp, header) || 0 != strcmp(header, "Public-Lines"))
        goto error;
    if ((b = read_body(fp)) == NULL)
        goto error;
    i = atoi(b);
    sfree(b);
    if ((public_blob = read_blob(fp, i, &public_blob_len)) == NULL)
        goto error;

    fclose(fp);
    *pub_blob_len = public_blob_len;
    *algorithm = alg->name;
    return public_blob;

    /*
     * Error processing.
     */
  error:
    if (fp)
        fclose(fp);
    if (public_blob)
        sfree(public_blob);
    return NULL;
}

int ssh2_userkey_encrypted(char *filename, char **commentptr)
{
    FILE *fp;
    char header[40], *b, *comment;
    int ret;

    if (commentptr)
        *commentptr = NULL;

    fp = fopen(filename, "rb");
    if (!fp)
        return 0;
    if (!read_header(fp, header)
        || (0 != strcmp(header, "PuTTY-User-Key-File-2") &&
            0 != strcmp(header, "PuTTY-User-Key-File-1"))) {
        fclose(fp);
        return 0;
    }
    if ((b = read_body(fp)) == NULL) {
        fclose(fp);
        return 0;
    }
    sfree(b);                          /* we don't care about key type here */
    /* Read the Encryption header line. */
    if (!read_header(fp, header) || 0 != strcmp(header, "Encryption")) {
        fclose(fp);
        return 0;
    }
    if ((b = read_body(fp)) == NULL) {
        fclose(fp);
        return 0;
    }

    /* Read the Comment header line. */
    if (!read_header(fp, header) || 0 != strcmp(header, "Comment")) {
        fclose(fp);
        sfree(b);
        return 1;
    }
    if ((comment = read_body(fp)) == NULL) {
        fclose(fp);
        sfree(b);
        return 1;
    }

    if (commentptr)
        *commentptr = comment;

    fclose(fp);
    if (!strcmp(b, "aes256-cbc"))
        ret = 1;
    else
        ret = 0;
    sfree(b);
    return ret;
}

int base64_lines(int datalen)
{
    /* When encoding, we use 64 chars/line, which equals 48 real chars. */
    return (datalen + 47) / 48;
}

void base64_encode_atom(unsigned char *data, int n, char *out)
{
    static const char base64_chars[] =
        "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

    unsigned word;

    word = data[0] << 16;
    if (n > 1)
        word |= data[1] << 8;
    if (n > 2)
        word |= data[2];
    out[0] = base64_chars[(word >> 18) & 0x3F];
    out[1] = base64_chars[(word >> 12) & 0x3F];
    if (n > 1)
        out[2] = base64_chars[(word >> 6) & 0x3F];
    else
        out[2] = '=';
    if (n > 2)
        out[3] = base64_chars[word & 0x3F];
    else
        out[3] = '=';
}

void base64_encode(FILE * fp, unsigned char *data, int datalen)
{
    int linelen = 0;
    char out[4];
    int n;

    while (datalen > 0) {
        if (linelen >= 64) {
            linelen = 0;
            fputc('\n', fp);
        }
        n = (datalen < 3 ? datalen : 3);
        base64_encode_atom(data, n, out);
        data += n;
        datalen -= n;
        fwrite(out, 1, 4, fp);
        linelen += 4;
    }
    fputc('\n', fp);
}

int ssh2_save_userkey(char *filename, struct ssh2_userkey *key,
                      char *passphrase)
{
    FILE *fp;
    unsigned char *pub_blob, *priv_blob, *priv_blob_encrypted;
    int pub_blob_len, priv_blob_len, priv_encrypted_len;
    int passlen;
    int cipherblk;
    int i;
    char *cipherstr;
    unsigned char priv_mac[20];

    /*
     * Fetch the key component blobs.
     */
    pub_blob = key->alg->public_blob(key->data, &pub_blob_len);
    priv_blob = key->alg->private_blob(key->data, &priv_blob_len);
    if (!pub_blob || !priv_blob) {
        sfree(pub_blob);
        sfree(priv_blob);
        return 0;
    }

    /*
     * Determine encryption details, and encrypt the private blob.
     */
    if (passphrase) {
        cipherstr = "aes256-cbc";
        cipherblk = 16;
    } else {
        cipherstr = "none";
        cipherblk = 1;
    }
    priv_encrypted_len = priv_blob_len + cipherblk - 1;
    priv_encrypted_len -= priv_encrypted_len % cipherblk;
    priv_blob_encrypted = smalloc(priv_encrypted_len);
    memset(priv_blob_encrypted, 0, priv_encrypted_len);
    memcpy(priv_blob_encrypted, priv_blob, priv_blob_len);
    /* Create padding based on the SHA hash of the unpadded blob. This prevents
     * too easy a known-plaintext attack on the last block. */
    SHA_Simple(priv_blob, priv_blob_len, priv_mac);
    assert(priv_encrypted_len - priv_blob_len < 20);
    memcpy(priv_blob_encrypted + priv_blob_len, priv_mac,
           priv_encrypted_len - priv_blob_len);

    /* Now create the MAC. */
    {
        unsigned char *macdata;
        int maclen;
        unsigned char *p;
        int namelen = strlen(key->alg->name);
        int enclen = strlen(cipherstr);
        int commlen = strlen(key->comment);
        SHA_State s;
        unsigned char mackey[20];
        char header[] = "putty-private-key-file-mac-key";

        maclen = (4 + namelen +
                  4 + enclen +
                  4 + commlen +
                  4 + pub_blob_len +
                  4 + priv_encrypted_len);
        macdata = smalloc(maclen);
        p = macdata;
#define DO_STR(s,len) PUT_32BIT(p,(len));memcpy(p+4,(s),(len));p+=4+(len)
        DO_STR(key->alg->name, namelen);
        DO_STR(cipherstr, enclen);
        DO_STR(key->comment, commlen);
        DO_STR(pub_blob, pub_blob_len);
        DO_STR(priv_blob_encrypted, priv_encrypted_len);

        SHA_Init(&s);
        SHA_Bytes(&s, header, sizeof(header)-1);
        if (passphrase)
            SHA_Bytes(&s, passphrase, strlen(passphrase));
        SHA_Final(&s, mackey);
        hmac_sha1_simple(mackey, 20, macdata, maclen, priv_mac);
        memset(macdata, 0, maclen);
        sfree(macdata);
        memset(mackey, 0, sizeof(mackey));
        memset(&s, 0, sizeof(s));
    }

    if (passphrase) {
        char key[40];
        SHA_State s;

        passlen = strlen(passphrase);

        SHA_Init(&s);
        SHA_Bytes(&s, "\0\0\0\0", 4);
        SHA_Bytes(&s, passphrase, passlen);
        SHA_Final(&s, key + 0);
        SHA_Init(&s);
        SHA_Bytes(&s, "\0\0\0\1", 4);
        SHA_Bytes(&s, passphrase, passlen);
        SHA_Final(&s, key + 20);
        aes256_encrypt_pubkey(key, priv_blob_encrypted,
                              priv_encrypted_len);

        memset(key, 0, sizeof(key));
        memset(&s, 0, sizeof(s));
    }

    fp = fopen(filename, "w");
    if (!fp)
        return 0;
    fprintf(fp, "PuTTY-User-Key-File-2: %s\n", key->alg->name);
    fprintf(fp, "Encryption: %s\n", cipherstr);
    fprintf(fp, "Comment: %s\n", key->comment);
    fprintf(fp, "Public-Lines: %d\n", base64_lines(pub_blob_len));
    base64_encode(fp, pub_blob, pub_blob_len);
    fprintf(fp, "Private-Lines: %d\n", base64_lines(priv_encrypted_len));
    base64_encode(fp, priv_blob_encrypted, priv_encrypted_len);
    fprintf(fp, "Private-MAC: ");
    for (i = 0; i < 20; i++)
        fprintf(fp, "%02x", priv_mac[i]);
    fprintf(fp, "\n");
    fclose(fp);

    sfree(pub_blob);
    memset(priv_blob, 0, priv_blob_len);
    sfree(priv_blob);
    sfree(priv_blob_encrypted);
    return 1;
}

/* ----------------------------------------------------------------------
 * A function to determine which version of SSH to try on a private
 * key file. Returns 0 on failure, 1 or 2 on success.
 */
int keyfile_version(char *filename)
{
    FILE *fp;
    int i;

    fp = fopen(filename, "r");
    if (!fp)
        return 0;
    i = fgetc(fp);
    fclose(fp);
    if (i == 'S')
        return 1;                      /* "SSH PRIVATE KEY FORMAT" etc */
    if (i == 'P')                      /* "PuTTY-User-Key-File" etc */
        return 2;
    return 0;                          /* unrecognised or EOF */
}