RSA key authentication in ssh1 works; SSH2 is nearly there

[u/mdw/putty] / sshbn.c

/*
 * Bignum routines for RSA and DH and stuff.
 */

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

#include <stdio.h> /* FIXME */
#include <stdarg.h> /* FIXME */
#include <windows.h> /* FIXME */
#include "putty.h" /* FIXME */

#include "ssh.h"

unsigned short bnZero[1] = { 0 };
unsigned short bnOne[2] = { 1, 1 };

Bignum Zero = bnZero, One = bnOne;

Bignum newbn(int length) {
    Bignum b = malloc((length+1)*sizeof(unsigned short));
    if (!b)
        abort();                       /* FIXME */
    memset(b, 0, (length+1)*sizeof(*b));
    b[0] = length;
    return b;
}

Bignum copybn(Bignum orig) {
    Bignum b = malloc((orig[0]+1)*sizeof(unsigned short));
    if (!b)
        abort();                       /* FIXME */
    memcpy(b, orig, (orig[0]+1)*sizeof(*b));
    return b;
}

void freebn(Bignum b) {
    /*
     * Burn the evidence, just in case.
     */
    memset(b, 0, sizeof(b[0]) * (b[0] + 1));
    free(b);
}

/*
 * Compute c = a * b.
 * Input is in the first len words of a and b.
 * Result is returned in the first 2*len words of c.
 */
static void bigmul(unsigned short *a, unsigned short *b, unsigned short *c,
                   int len)
{
    int i, j;
    unsigned long ai, t;

    for (j = len - 1; j >= 0; j--)
        c[j+len] = 0;

    for (i = len - 1; i >= 0; i--) {
        ai = a[i];
        t = 0;
        for (j = len - 1; j >= 0; j--) {
            t += ai * (unsigned long) b[j];
            t += (unsigned long) c[i+j+1];
            c[i+j+1] = (unsigned short)t;
            t = t >> 16;
        }
        c[i] = (unsigned short)t;
    }
}

/*
 * Compute a = a % m.
 * Input in first len2 words of a and first len words of m.
 * Output in first len2 words of a
 * (of which first len2-len words will be zero).
 * The MSW of m MUST have its high bit set.
 */
static void bigmod(unsigned short *a, unsigned short *m,
                   int len, int len2)
{
    unsigned short m0, m1;
    unsigned int h;
    int i, k;

    /* Special case for len == 1 */
    if (len == 1) {
        a[1] = (((long) a[0] << 16) + a[1]) % m[0];
        a[0] = 0;
        return;
    }

    m0 = m[0];
    m1 = m[1];

    for (i = 0; i <= len2-len; i++) {
        unsigned long t;
        unsigned int q, r, c;

        if (i == 0) {
            h = 0;
        } else {
            h = a[i-1];
            a[i-1] = 0;
        }

        /* Find q = h:a[i] / m0 */
        t = ((unsigned long) h << 16) + a[i];
        q = t / m0;
        r = t % m0;

        /* Refine our estimate of q by looking at
         h:a[i]:a[i+1] / m0:m1 */
        t = (long) m1 * (long) q;
        if (t > ((unsigned long) r << 16) + a[i+1]) {
            q--;
            t -= m1;
            r = (r + m0) & 0xffff; /* overflow? */
            if (r >= (unsigned long)m0 &&
                t > ((unsigned long) r << 16) + a[i+1])
                q--;
        }

        /* Substract q * m from a[i...] */
        c = 0;
        for (k = len - 1; k >= 0; k--) {
            t = (long) q * (long) m[k];
            t += c;
            c = t >> 16;
            if ((unsigned short) t > a[i+k]) c++;
            a[i+k] -= (unsigned short) t;
        }

        /* Add back m in case of borrow */
        if (c != h) {
            t = 0;
            for (k = len - 1; k >= 0; k--) {
                t += m[k];
                t += a[i+k];
                a[i+k] = (unsigned short)t;
                t = t >> 16;
            }
        }
    }
}

/*
 * Compute (base ^ exp) % mod.
 * The base MUST be smaller than the modulus.
 * The most significant word of mod MUST be non-zero.
 * We assume that the result array is the same size as the mod array.
 */
void modpow(Bignum base, Bignum exp, Bignum mod, Bignum result)
{
    unsigned short *a, *b, *n, *m;
    int mshift;
    int mlen, i, j;

    /* Allocate m of size mlen, copy mod to m */
    /* We use big endian internally */
    mlen = mod[0];
    m = malloc(mlen * sizeof(unsigned short));
    for (j = 0; j < mlen; j++) m[j] = mod[mod[0] - j];

    /* Shift m left to make msb bit set */
    for (mshift = 0; mshift < 15; mshift++)
        if ((m[0] << mshift) & 0x8000) break;
    if (mshift) {
        for (i = 0; i < mlen - 1; i++)
            m[i] = (m[i] << mshift) | (m[i+1] >> (16-mshift));
        m[mlen-1] = m[mlen-1] << mshift;
    }

    /* Allocate n of size mlen, copy base to n */
    n = malloc(mlen * sizeof(unsigned short));
    i = mlen - base[0];
    for (j = 0; j < i; j++) n[j] = 0;
    for (j = 0; j < base[0]; j++) n[i+j] = base[base[0] - j];

    /* Allocate a and b of size 2*mlen. Set a = 1 */
    a = malloc(2 * mlen * sizeof(unsigned short));
    b = malloc(2 * mlen * sizeof(unsigned short));
    for (i = 0; i < 2*mlen; i++) a[i] = 0;
    a[2*mlen-1] = 1;

    /* Skip leading zero bits of exp. */
    i = 0; j = 15;
    while (i < exp[0] && (exp[exp[0] - i] & (1 << j)) == 0) {
        j--;
        if (j < 0) { i++; j = 15; }
    }

    /* Main computation */
    while (i < exp[0]) {
        while (j >= 0) {
            bigmul(a + mlen, a + mlen, b, mlen);
            bigmod(b, m, mlen, mlen*2);
            if ((exp[exp[0] - i] & (1 << j)) != 0) {
                bigmul(b + mlen, n, a, mlen);
                bigmod(a, m, mlen, mlen*2);
            } else {
                unsigned short *t;
                t = a;  a = b;  b = t;
            }
            j--;
        }
        i++; j = 15;
    }

    /* Fixup result in case the modulus was shifted */
    if (mshift) {
        for (i = mlen - 1; i < 2*mlen - 1; i++)
            a[i] = (a[i] << mshift) | (a[i+1] >> (16-mshift));
        a[2*mlen-1] = a[2*mlen-1] << mshift;
        bigmod(a, m, mlen, mlen*2);
        for (i = 2*mlen - 1; i >= mlen; i--)
            a[i] = (a[i] >> mshift) | (a[i-1] << (16-mshift));
    }

    /* Copy result to buffer */
    for (i = 0; i < mlen; i++)
        result[result[0] - i] = a[i+mlen];

    /* Free temporary arrays */
    for (i = 0; i < 2*mlen; i++) a[i] = 0; free(a);
    for (i = 0; i < 2*mlen; i++) b[i] = 0; free(b);
    for (i = 0; i < mlen; i++) m[i] = 0; free(m);
    for (i = 0; i < mlen; i++) n[i] = 0; free(n);
}

/*
 * Compute (p * q) % mod.
 * The most significant word of mod MUST be non-zero.
 * We assume that the result array is the same size as the mod array.
 */
void modmul(Bignum p, Bignum q, Bignum mod, Bignum result)
{
    unsigned short *a, *n, *m, *o;
    int mshift;
    int pqlen, mlen, i, j;

    /* Allocate m of size mlen, copy mod to m */
    /* We use big endian internally */
    mlen = mod[0];
    m = malloc(mlen * sizeof(unsigned short));
    for (j = 0; j < mlen; j++) m[j] = mod[mod[0] - j];

    /* Shift m left to make msb bit set */
    for (mshift = 0; mshift < 15; mshift++)
        if ((m[0] << mshift) & 0x8000) break;
    if (mshift) {
        for (i = 0; i < mlen - 1; i++)
            m[i] = (m[i] << mshift) | (m[i+1] >> (16-mshift));
        m[mlen-1] = m[mlen-1] << mshift;
    }

    pqlen = (p[0] > q[0] ? p[0] : q[0]);

    /* Allocate n of size pqlen, copy p to n */
    n = malloc(pqlen * sizeof(unsigned short));
    i = pqlen - p[0];
    for (j = 0; j < i; j++) n[j] = 0;
    for (j = 0; j < p[0]; j++) n[i+j] = p[p[0] - j];

    /* Allocate o of size pqlen, copy q to o */
    o = malloc(pqlen * sizeof(unsigned short));
    i = pqlen - q[0];
    for (j = 0; j < i; j++) o[j] = 0;
    for (j = 0; j < q[0]; j++) o[i+j] = q[q[0] - j];

    /* Allocate a of size 2*pqlen for result */
    a = malloc(2 * pqlen * sizeof(unsigned short));

    /* Main computation */
    bigmul(n, o, a, pqlen);
    bigmod(a, m, mlen, 2*pqlen);

    /* Fixup result in case the modulus was shifted */
    if (mshift) {
        for (i = 2*pqlen - mlen - 1; i < 2*pqlen - 1; i++)
            a[i] = (a[i] << mshift) | (a[i+1] >> (16-mshift));
        a[2*pqlen-1] = a[2*pqlen-1] << mshift;
        bigmod(a, m, mlen, pqlen*2);
        for (i = 2*pqlen - 1; i >= 2*pqlen - mlen; i--)
            a[i] = (a[i] >> mshift) | (a[i-1] << (16-mshift));
    }

    /* Copy result to buffer */
    for (i = 0; i < mlen; i++)
        result[result[0] - i] = a[i+2*pqlen-mlen];

    /* Free temporary arrays */
    for (i = 0; i < 2*pqlen; i++) a[i] = 0; free(a);
    for (i = 0; i < mlen; i++) m[i] = 0; free(m);
    for (i = 0; i < pqlen; i++) n[i] = 0; free(n);
    for (i = 0; i < pqlen; i++) o[i] = 0; free(o);
}

/*
 * Decrement a number.
 */
void decbn(Bignum bn) {
    int i = 1;
    while (i < bn[0] && bn[i] == 0)
        bn[i++] = 0xFFFF;
    bn[i]--;
}

/*
 * Read an ssh1-format bignum from a data buffer. Return the number
 * of bytes consumed.
 */
int ssh1_read_bignum(unsigned char *data, Bignum *result) {
    unsigned char *p = data;
    Bignum bn;
    int i;
    int w, b;

    w = 0;
    for (i=0; i<2; i++)
        w = (w << 8) + *p++;

    b = (w+7)/8;                       /* bits -> bytes */
    w = (w+15)/16;                     /* bits -> words */

    bn = newbn(w);

    for (i=1; i<=w; i++)
        bn[i] = 0;
    for (i=b; i-- ;) {
        unsigned char byte = *p++;
        if (i & 1)
            bn[1+i/2] |= byte<<8;
        else
            bn[1+i/2] |= byte;
    }

    *result = bn;

    return p - data;
}