Remove the last lingering knowledge, outside sshbn.c, of the

internal structure of the Bignum type. Bignum is now a fully opaque
type unless you're inside sshbn.c.

git-svn-id: svn://svn.tartarus.org/sgt/putty@960 cda61777-01e9-0310-a592-d414129be87e

diff --git a/ssh.c b/ssh.c
index 033a0d2..a6433db 100644 (file)
--- a/ssh.c
+++ b/ssh.c
@@ -869,20 +869,18 @@ static void ssh2_pkt_addstring(char *data) {
 }
 static char *ssh2_mpint_fmt(Bignum b, int *len) {
     unsigned char *p;
-    int i, n = b[0];
-    p = smalloc(n * 2 + 1);
+    int i, n = (ssh1_bignum_bitcount(b)+7)/8;
+    p = smalloc(n + 1);
     if (!p)
         fatalbox("out of memory");
     p[0] = 0;
-    for (i = 0; i < n; i++) {
-        p[i*2+1] = (b[n-i] >> 8) & 0xFF;
-        p[i*2+2] = (b[n-i]     ) & 0xFF;
-    }
+    for (i = 1; i <= n; i++)
+        p[i] = bignum_byte(b, n-i);
     i = 0;
-    while (p[i] == 0 && (p[i+1] & 0x80) == 0)
+    while (i <= n && p[i] == 0 && (p[i+1] & 0x80) == 0)
         i++;
-    memmove(p, p+i, n*2+1-i);
-    *len = n*2+1-i;
+    memmove(p, p+i, n+1-i);
+    *len = n+1-i;
     return p;
 }
 static void ssh2_pkt_addmp(Bignum b) {
@@ -1041,7 +1039,7 @@ static void ssh2_pkt_getstring(char **p, int *length) {
 }
 static Bignum ssh2_pkt_getmp(void) {
     char *p;
-    int i, j, length;
+    int length;
     Bignum b;
 
     ssh2_pkt_getstring(&p, &length);
@@ -1051,15 +1049,7 @@ static Bignum ssh2_pkt_getmp(void) {
         bombout(("internal error: Can't handle negative mpints"));
         return NULL;
     }
-    b = newbn((length+1)/2);
-    for (i = 0; i < length; i++) {
-        j = length - 1 - i;
-        if (j & 1)
-            b[j/2+1] |= ((unsigned char)p[i]) << 8;
-        else
-            b[j/2+1] |= ((unsigned char)p[i]);
-    }
-    while (b[0] > 1 && b[b[0]] == 0) b[0]--;
+    b = bignum_from_bytes(p, length);
     return b;
 }
 
@@ -1753,9 +1743,8 @@ static int do_ssh1_login(unsigned char *in, int inlen, int ispkt)
             response = rsadecrypt(challenge, &pubkey);
             freebn(pubkey.private_exponent);   /* burn the evidence */
 
-            for (i = 0; i < 32; i += 2) {
-                buffer[i] = response[16-i/2] >> 8;
-                buffer[i+1] = response[16-i/2] & 0xFF;
+            for (i = 0; i < 32; i++) {
+                buffer[i] = bignum_byte(response, 31-i);
             }
 
             MD5Init(&md5c);
@@ -2388,6 +2377,8 @@ static int do_ssh2_transport(unsigned char *in, int inlen, int ispkt)
     /*
      * Now we begin the fun. Generate and send e for Diffie-Hellman.
      */
+    dh_setup_group1();
+
     e = dh_create_e();
     ssh2_pkt_init(SSH2_MSG_KEXDH_INIT);
     ssh2_pkt_addmp(e);
@@ -2410,6 +2401,8 @@ static int do_ssh2_transport(unsigned char *in, int inlen, int ispkt)
     sha_mpint(&exhash, K);
     SHA_Final(&exhash, exchange_hash);
 
+    dh_cleanup();
+
 #if 0
     debug(("Exchange hash is:\r\n"));
     for (i = 0; i < 20; i++)

diff --git a/ssh.h b/ssh.h
index 725569c..320c84e 100644 (file)
--- a/ssh.h
+++ b/ssh.h
@@ -20,12 +20,9 @@
 #define APIEXTRA 0
 #endif
 
-/*
- * A Bignum is stored as a sequence of `unsigned short' words. The
- * first tells how many remain; the remaining ones are digits, LS
- * first.
- */
-typedef unsigned short *Bignum;
+#ifndef BIGNUM_INTERNAL
+typedef void *Bignum;
+#endif
 
 struct RSAKey {
     int bits;
@@ -161,14 +158,16 @@ void random_add_heavynoise(void *noise, int length);
 
 void logevent (char *);
 
-Bignum newbn(int length);
 Bignum copybn(Bignum b);
+Bignum bn_power_2(int n);
+void bn_restore_invariant(Bignum b);
 Bignum bignum_from_short(unsigned short n);
 void freebn(Bignum b);
 Bignum modpow(Bignum base, Bignum exp, Bignum mod);
 Bignum modmul(Bignum a, Bignum b, Bignum mod);
 void decbn(Bignum n);
 extern Bignum Zero, One;
+Bignum bignum_from_bytes(unsigned char *data, int nbytes);
 int ssh1_read_bignum(unsigned char *data, Bignum *result);
 int ssh1_bignum_bitcount(Bignum bn);
 int ssh1_bignum_length(Bignum bn);
@@ -181,10 +180,13 @@ unsigned short bignum_mod_short(Bignum number, unsigned short modulus);
 Bignum bignum_add_long(Bignum number, unsigned long addend);
 Bignum bigmul(Bignum a, Bignum b);
 Bignum modinv(Bignum number, Bignum modulus);
+Bignum bignum_bitmask(Bignum number);
 Bignum bignum_rshift(Bignum number, int shift);
 int bignum_cmp(Bignum a, Bignum b);
 char *bignum_decimal(Bignum x);
 
+void dh_setup_group1(void);
+void dh_cleanup(void);
 Bignum dh_create_e(void);
 Bignum dh_find_K(Bignum f);
 

diff --git a/sshbn.c b/sshbn.c
index 6615d41..aea5a82 100644 (file)
--- a/sshbn.c
+++ b/sshbn.c
@@ -6,6 +6,9 @@
 #include <stdlib.h>
 #include <string.h>
 
+#define BIGNUM_INTERNAL
+typedef unsigned short *Bignum;
+
 #include "ssh.h"
 
 unsigned short bnZero[1] = { 0 };
@@ -27,7 +30,7 @@ unsigned short bnOne[2] = { 1, 1 };
 
 Bignum Zero = bnZero, One = bnOne;
 
-Bignum newbn(int length) {
+static Bignum newbn(int length) {
     Bignum b = smalloc((length+1)*sizeof(unsigned short));
     if (!b)
        abort();                       /* FIXME */
@@ -36,6 +39,10 @@ Bignum newbn(int length) {
     return b;
 }
 
+void bn_restore_invariant(Bignum b) {
+    while (b[0] > 1 && b[b[0]] == 0) b[0]--;
+}
+
 Bignum copybn(Bignum orig) {
     Bignum b = smalloc((orig[0]+1)*sizeof(unsigned short));
     if (!b)
@@ -52,6 +59,12 @@ void freebn(Bignum b) {
     sfree(b);
 }
 
+Bignum bn_power_2(int n) {
+    Bignum ret = newbn((n+15)/16);
+    bignum_set_bit(ret, n, 1);
+    return ret;
+}
+
 /*
  * Compute c = a * b.
  * Input is in the first len words of a and b.
@@ -410,41 +423,47 @@ void decbn(Bignum bn) {
     bn[i]--;
 }
 
+Bignum bignum_from_bytes(unsigned char *data, int nbytes) {
+    Bignum result;
+    int w, i;
+
+    w = (nbytes+1)/2;                 /* bytes -> words */
+
+    result = newbn(w);
+    for (i=1; i<=w; i++)
+        result[i] = 0;
+    for (i=nbytes; i-- ;) {
+        unsigned char byte = *data++;
+        if (i & 1)
+            result[1+i/2] |= byte<<8;
+        else
+            result[1+i/2] |= byte;
+    }
+
+    while (result[0] > 1 && result[result[0]] == 0) result[0]--;
+    return result;
+}
+
 /*
  * 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 */
 
     if (!result)                       /* just return length */
         return b + 2;
 
-    bn = newbn(w);
+    *result = bignum_from_bytes(p, b);
 
-    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;
+    return p + b - data;
 }
 
 /*
@@ -452,7 +471,6 @@ int ssh1_read_bignum(unsigned char *data, Bignum *result) {
  */
 int ssh1_bignum_bitcount(Bignum bn) {
     int bitcount = bn[0] * 16 - 1;
-
     while (bitcount >= 0 && (bn[bitcount/16+1] >> (bitcount % 16)) == 0)
         bitcount--;
     return bitcount + 1;
@@ -620,6 +638,30 @@ Bignum bigmul(Bignum a, Bignum b) {
 }
 
 /*
+ * Create a bignum which is the bitmask covering another one. That
+ * is, the smallest integer which is >= N and is also one less than
+ * a power of two.
+ */
+Bignum bignum_bitmask(Bignum n) {
+    Bignum ret = copybn(n);
+    int i;
+    unsigned short j;
+
+    i = ret[0];
+    while (n[i] == 0 && i > 0)
+        i--;
+    if (i <= 0)
+        return ret;                    /* input was zero */
+    j = 1;
+    while (j < n[i])
+        j = 2*j+1;
+    ret[i] = j;
+    while (--i > 0)
+        ret[i] = 0xFFFF;
+    return ret;
+}
+
+/*
  * Convert a (max 16-bit) short into a bignum.
  */
 Bignum bignum_from_short(unsigned short n) {
@@ -667,7 +709,7 @@ unsigned short bignum_mod_short(Bignum number, unsigned short modulus) {
     return (unsigned short) r;
 }
 
-static void diagbn(char *prefix, Bignum md) {
+void diagbn(char *prefix, Bignum md) {
     int i, nibbles, morenibbles;
     static const char hex[] = "0123456789ABCDEF";
 

diff --git a/sshdh.c b/sshdh.c
index dabed52..35e6690 100644 (file)
--- a/sshdh.c
+++ b/sshdh.c
@@ -7,57 +7,57 @@ struct ssh_kex ssh_diffiehellman = {
 /*
  * The prime p used in the key exchange. 
  */
-static unsigned short P[] = {
-    64,
-    0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0x5381, 0xECE6, 0x6651, 0x4928,
-    0x1FE6, 0x7C4B, 0x2411, 0xAE9F, 0x9FA5, 0x5A89, 0x6BFB, 0xEE38,
-    0xB7ED, 0xF406, 0x5CB6, 0x0BFF, 0xED6B, 0xA637, 0x42E9, 0xF44C,
-    0x7EC6, 0x625E, 0xB576, 0xE485, 0xC245, 0x6D51, 0x356D, 0x4FE1,
-    0x1437, 0xF25F, 0x0A6D, 0x302B, 0x431B, 0xCD3A, 0x19B3, 0xEF95,
-    0x04DD, 0x8E34, 0x0879, 0x514A, 0x9B22, 0x3B13, 0xBEA6, 0x020B,
-    0xCC74, 0x8A67, 0x4E08, 0x2902, 0x1CD1, 0x80DC, 0x628B, 0xC4C6,
-    0xC234, 0x2168, 0xDAA2, 0xC90F, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF,
+static unsigned char P[] = {
+    0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xC9, 0x0F, 0xDA, 0xA2,
+    0x21, 0x68, 0xC2, 0x34, 0xC4, 0xC6, 0x62, 0x8B, 0x80, 0xDC, 0x1C, 0xD1,
+    0x29, 0x02, 0x4E, 0x08, 0x8A, 0x67, 0xCC, 0x74, 0x02, 0x0B, 0xBE, 0xA6,
+    0x3B, 0x13, 0x9B, 0x22, 0x51, 0x4A, 0x08, 0x79, 0x8E, 0x34, 0x04, 0xDD,
+    0xEF, 0x95, 0x19, 0xB3, 0xCD, 0x3A, 0x43, 0x1B, 0x30, 0x2B, 0x0A, 0x6D,
+    0xF2, 0x5F, 0x14, 0x37, 0x4F, 0xE1, 0x35, 0x6D, 0x6D, 0x51, 0xC2, 0x45,
+    0xE4, 0x85, 0xB5, 0x76, 0x62, 0x5E, 0x7E, 0xC6, 0xF4, 0x4C, 0x42, 0xE9,
+    0xA6, 0x37, 0xED, 0x6B, 0x0B, 0xFF, 0x5C, 0xB6, 0xF4, 0x06, 0xB7, 0xED,
+    0xEE, 0x38, 0x6B, 0xFB, 0x5A, 0x89, 0x9F, 0xA5, 0xAE, 0x9F, 0x24, 0x11,
+    0x7C, 0x4B, 0x1F, 0xE6, 0x49, 0x28, 0x66, 0x51, 0xEC, 0xE6, 0x53, 0x81,
+    0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF
 };
 
 /*
- * The order q of the group: (p-1)/2.
+ * The generator g = 2.
  */
-static unsigned short Q[] = {
-    64,
-    0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0x29C0, 0xF673, 0x3328, 0x2494,
-    0x8FF3, 0xBE25, 0x9208, 0xD74F, 0xCFD2, 0xAD44, 0x35FD, 0xF71C,
-    0x5BF6, 0x7A03, 0xAE5B, 0x85FF, 0xF6B5, 0xD31B, 0x2174, 0x7A26,
-    0x3F63, 0x312F, 0xDABB, 0xF242, 0xE122, 0xB6A8, 0x9AB6, 0xA7F0,
-    0x8A1B, 0xF92F, 0x8536, 0x9815, 0x218D, 0xE69D, 0x8CD9, 0xF7CA,
-    0x026E, 0xC71A, 0x043C, 0x28A5, 0xCD91, 0x1D89, 0xDF53, 0x0105,
-    0xE63A, 0x4533, 0x2704, 0x9481, 0x0E68, 0xC06E, 0x3145, 0x6263,
-    0x611A, 0x10B4, 0xED51, 0xE487, 0xFFFF, 0xFFFF, 0xFFFF, 0x7FFF,
-};
+static unsigned char G[] = { 2 };
 
 /*
- * The bitmask covering q (for ease of generation of x).
+ * Variables.
  */
-static unsigned short Qmask[] = {
-    64,
-    0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF,
-    0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF,
-    0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF,
-    0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF,
-    0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF,
-    0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF,
-    0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF,
-    0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0xFFFF, 0x7FFF,
-};
+static Bignum x, e, p, q, qmask, g;
+static int need_to_free_pg;
 
 /*
- * The generator g = 2.
+ * Common DH initialisation.
  */
-static unsigned short G[] = { 1, 2 };
+static void dh_init(void) {
+    q = bignum_rshift(p, 1);
+    qmask = bignum_bitmask(q);
+}
 
 /*
- * Variables.
+ * Initialise DH for the standard group1.
  */
-static Bignum x, e;
+void dh_setup_group1(void) {
+    p = bignum_from_bytes(P, sizeof(P));
+    g = bignum_from_bytes(G, sizeof(G));
+    dh_init();
+}
+
+/*
+ * Clean up.
+ */
+void dh_cleanup(void) {
+    freebn(p);
+    freebn(g);
+    freebn(q);
+    freebn(qmask);
+}
 
 /*
  * DH stage 1: invent a number x between 1 and q, and compute e =
@@ -66,21 +66,28 @@ static Bignum x, e;
 Bignum dh_create_e(void) {
     int i;
 
-    x = newbn(Q[0]);
+    int nbytes;
+    unsigned char *buf;
+
+    nbytes = ssh1_bignum_length(qmask);
+    buf = smalloc(nbytes);
 
     do {
        /*
         * Create a potential x, by ANDing a string of random bytes
-        * with Qmask.
+        * with qmask.
         */
-       for (i = 1; i <= x[0]; i++)
-           x[i] = ((random_byte() << 8) + random_byte()) & Qmask[i];
-    } while (bignum_cmp(x, One) <= 0 || bignum_cmp(x, Q) >= 0);
+        if (x) freebn(x);
+        ssh1_write_bignum(buf, qmask);
+       for (i = 2; i < nbytes; i++)
+            buf[i] &= random_byte();
+        ssh1_read_bignum(buf, &x);
+    } while (bignum_cmp(x, One) <= 0 || bignum_cmp(x, q) >= 0);
 
     /*
      * Done. Now compute e = g^x mod p.
      */
-    e = modpow(G, x, P);
+    e = modpow(g, x, p);
 
     return e;
 }
@@ -89,5 +96,5 @@ Bignum dh_create_e(void) {
  * DH stage 2: given a number f, compute K = f^x mod p.
  */
 Bignum dh_find_K(Bignum f) {
-    return modpow(f, x, P);
+    return modpow(f, x, p);
 }

diff --git a/sshdss.c b/sshdss.c
index bacf68d..cf7fc3f 100644 (file)
--- a/sshdss.c
+++ b/sshdss.c
@@ -16,23 +16,6 @@
     (cp)[3] = (unsigned char)(value); }
 
 #if 0
-/*
- * Condition this section in for debugging of DSS.
- */
-static void diagbn(char *prefix, Bignum md) {
-    int i, nibbles, morenibbles;
-    static const char hex[] = "0123456789ABCDEF";
-
-    printf("%s0x", prefix ? prefix : "");
-
-    nibbles = (3 + ssh1_bignum_bitcount(md))/4; if (nibbles<1) nibbles=1;
-    morenibbles = 4*md[0] - nibbles;
-    for (i=0; i<morenibbles; i++) putchar('-');
-    for (i=nibbles; i-- ;)
-        putchar(hex[(bignum_byte(md, i/2) >> (4*(i%2))) & 0xF]);
-
-    if (prefix) putchar('\n');
-}
 #define DEBUG_DSS
 #else
 #define diagbn(x,y)
@@ -51,7 +34,7 @@ static void getstring(char **data, int *datalen, char **p, int *length) {
 }
 static Bignum getmp(char **data, int *datalen) {
     char *p;
-    int i, j, length;
+    int length;
     Bignum b;
 
     getstring(data, datalen, &p, &length);
@@ -59,37 +42,16 @@ static Bignum getmp(char **data, int *datalen) {
         return NULL;
     if (p[0] & 0x80)
         return NULL;                   /* negative mp */
-    b = newbn((length+1)/2);
-    for (i = 0; i < length; i++) {
-        j = length - 1 - i;
-        if (j & 1)
-            b[j/2+1] |= ((unsigned char)p[i]) << 8;
-        else
-            b[j/2+1] |= ((unsigned char)p[i]);
-    }
-    while (b[0] > 1 && b[b[0]] == 0) b[0]--;
+    b = bignum_from_bytes(p, length);
     return b;
 }
 
 static Bignum get160(char **data, int *datalen) {
-    char *p;
-    int i, j, length;
     Bignum b;
 
-    p = *data;
+    b = bignum_from_bytes(*data, 20);
     *data += 20; *datalen -= 20;
 
-    length = 20;
-    while (length > 0 && !p[0])
-        p++, length--;
-    b = newbn((length+1)/2);
-    for (i = 0; i < length; i++) {
-        j = length - 1 - i;
-        if (j & 1)
-            b[j/2+1] |= ((unsigned char)p[i]) << 8;
-        else
-            b[j/2+1] |= ((unsigned char)p[i]);
-    }
     return b;
 }
 
@@ -145,7 +107,10 @@ static char *dss_fmtkey(void *key) {
     if (!dss->p)
         return NULL;
     len = 8 + 4 + 1;                   /* 4 x "0x", punctuation, \0 */
-    len += 4 * (dss->p[0] + dss->q[0] + dss->g[0] + dss->y[0]);   /* digits */
+    len += 4 * (ssh1_bignum_bitcount(dss->p)+15)/16;
+    len += 4 * (ssh1_bignum_bitcount(dss->q)+15)/16;
+    len += 4 * (ssh1_bignum_bitcount(dss->g)+15)/16;
+    len += 4 * (ssh1_bignum_bitcount(dss->y)+15)/16;
     p = smalloc(len);
     if (!p) return NULL;
 
@@ -222,7 +187,7 @@ static int dss_verifysig(void *key, char *sig, int siglen,
         int i;
         printf("sig:");
         for (i=0;i<siglen;i++)
-            printf("  %02xf", (unsigned char)(sig[i]));
+            printf("  %02x", (unsigned char)(sig[i]));
         printf("\n");
     }
 #endif

diff --git a/sshprime.c b/sshprime.c
index 61488a0..03f3e6f 100644 (file)
--- a/sshprime.c
+++ b/sshprime.c
@@ -674,7 +674,7 @@ Bignum primegen(int bits, int modulus, int residue,
     /*
      * Generate a k-bit random number with top and bottom bits set.
      */
-    p = newbn((bits+15)/16);
+    p = bn_power_2(bits-1);
     for (i = 0; i < bits; i++) {
         if (i == 0 || i == bits-1)
             v = 1;
@@ -754,7 +754,7 @@ Bignum primegen(int bits, int modulus, int residue,
          * Invent a random number between 1 and p-1 inclusive.
          */
         while (1) {
-            w = newbn((bits+15)/16);
+            w = bn_power_2(bits-1);
             for (i = 0; i < bits; i++) {
                 if (bitsleft <= 0)
                     bitsleft = 8; byte = random_byte();
@@ -763,6 +763,7 @@ Bignum primegen(int bits, int modulus, int residue,
                 bitsleft--;
                 bignum_set_bit(w, i, v);
             }
+            bn_restore_invariant(w);
             if (bignum_cmp(w, p) >= 0 || bignum_cmp(w, Zero) == 0) {
                 freebn(w);
                 continue;

diff --git a/sshrsa.c b/sshrsa.c
index 14cf09a..537d0f1 100644 (file)
--- a/sshrsa.c
+++ b/sshrsa.c
@@ -48,7 +48,7 @@ int makeprivate(unsigned char *data, struct RSAKey *result) {
 
 void rsaencrypt(unsigned char *data, int length, struct RSAKey *key) {
     Bignum b1, b2;
-    int w, i;
+    int i;
     unsigned char *p;
 
     memmove(data+key->bytes-length, data, length);
@@ -62,31 +62,13 @@ void rsaencrypt(unsigned char *data, int length, struct RSAKey *key) {
     }
     data[key->bytes-length-1] = 0;
 
-    w = (key->bytes+1)/2;
-
-    b1 = newbn(w);
-
-    p = data;
-    for (i=1; i<=w; i++)
-       b1[i] = 0;
-    for (i=key->bytes; i-- ;) {
-       unsigned char byte = *p++;
-       if (i & 1)
-           b1[1+i/2] |= byte<<8;
-       else
-           b1[1+i/2] |= byte;
-    }
+    b1 = bignum_from_bytes(data, key->bytes);
 
     b2 = modpow(b1, key->exponent, key->modulus);
 
     p = data;
     for (i=key->bytes; i-- ;) {
-       unsigned char b;
-       if (i & 1)
-           b = b2[1+i/2] >> 8;
-       else
-           b = b2[1+i/2] & 0xFF;
-       *p++ = b;
+        *p++ = bignum_byte(b2, i);
     }
 
     freebn(b1);
@@ -101,10 +83,13 @@ Bignum rsadecrypt(Bignum input, struct RSAKey *key) {
 
 int rsastr_len(struct RSAKey *key) {
     Bignum md, ex;
+    int mdlen, exlen;
 
     md = key->modulus;
     ex = key->exponent;
-    return 4 * (ex[0]+md[0]) + 20;
+    mdlen = (ssh1_bignum_bitcount(md)+15) / 16;
+    exlen = (ssh1_bignum_bitcount(ex)+15) / 16;
+    return 4 * (mdlen+exlen) + 20;
 }
 
 void rsastr_fmt(char *str, struct RSAKey *key) {