X-Git-Url: https://asedeno.scripts.mit.edu/gitweb/?a=blobdiff_plain;f=sshbn.c;h=b40781ff2996dc65fe9659e4d15ef8921d1f52f3;hb=d23c0972cd850c77871f9a314e0520d7023c8b62;hp=b1ea5d63c488cde28e8744285a2368601863f56b;hpb=eac7e041f100291169304a99cc9c7300093f2c78;p=PuTTY.git

diff --git a/sshbn.c b/sshbn.c
index b1ea5d63..b40781ff 100644
--- a/sshbn.c
+++ b/sshbn.c
@@ -1092,6 +1092,38 @@ Bignum modmul(Bignum p, Bignum q, Bignum mod)
     return result;
 }
 
+Bignum modsub(const Bignum a, const Bignum b, const Bignum n)
+{
+    Bignum a1, b1, ret;
+
+    if (bignum_cmp(a, n) >= 0) a1 = bigmod(a, n);
+    else a1 = a;
+    if (bignum_cmp(b, n) >= 0) b1 = bigmod(b, n);
+    else b1 = b;
+
+    if (bignum_cmp(a1, b1) >= 0) /* a >= b */
+    {
+        ret = bigsub(a1, b1);
+    }
+    else
+    {
+        /* Handle going round the corner of the modulus without having
+         * negative support in Bignum */
+        Bignum tmp = bigsub(n, b1);
+        if (tmp) {
+            ret = bigadd(tmp, a1);
+            freebn(tmp);
+        } else {
+            ret = NULL;
+        }
+    }
+
+    if (a != a1) freebn(a1);
+    if (b != b1) freebn(b1);
+
+    return ret;
+}
+
 /*
  * Compute p % mod.
  * The most significant word of mod MUST be non-zero.
@@ -1201,6 +1233,37 @@ Bignum bignum_from_bytes(const unsigned char *data, int nbytes)
     return result;
 }
 
+Bignum bignum_random_in_range(const Bignum lower, const Bignum upper)
+{
+    Bignum ret = NULL;
+    int upper_len = bignum_bitcount(upper);
+    int upper_bytes = upper_len / 8;
+    int upper_bits = upper_len % 8;
+    if (upper_bits) ++upper_bytes;
+
+    do {
+        unsigned char *bytes;
+        int i;
+
+        if (ret) freebn(ret);
+
+        bytes = snewn(upper_bytes, unsigned char);
+        for (i = 0; i < upper_bytes; ++i)
+        {
+            bytes[i] = (unsigned char)random_byte();
+        }
+        /* Mask the top to reduce failure rate to 50/50 */
+        if (upper_bits)
+        {
+            bytes[i - 1] &= 0xFF >> (8 - upper_bits);
+        }
+
+        ret = bignum_from_bytes(bytes, upper_bytes);
+    } while (bignum_cmp(ret, lower) < 0 || bignum_cmp(ret, upper) > 0);
+
+    return ret;
+}
+
 /*
  * Read an SSH-1-format bignum from a data buffer. Return the number
  * of bytes consumed, or -1 if there wasn't enough data.
@@ -1375,6 +1438,45 @@ Bignum bignum_rshift(Bignum a, int shift)
     return ret;
 }
 
+/*
+ * Left-shift one bignum to form another.
+ */
+Bignum bignum_lshift(Bignum a, int shift)
+{
+    Bignum ret;
+    int bits, shiftWords, shiftBits;
+
+    assert(shift >= 0);
+
+    bits = bignum_bitcount(a) + shift;
+    ret = newbn((bits + BIGNUM_INT_BITS - 1) / BIGNUM_INT_BITS);
+    if (!ret) return NULL;
+
+    shiftWords = shift / BIGNUM_INT_BITS;
+    shiftBits = shift % BIGNUM_INT_BITS;
+
+    if (shiftBits == 0)
+    {
+        memcpy(&ret[1 + shiftWords], &a[1], sizeof(BignumInt) * a[0]);
+    }
+    else
+    {
+        int i;
+        BignumInt carry = 0;
+
+        /* Remember that Bignum[0] is length, so add 1 */
+        for (i = shiftWords + 1; i < ((int)a[0]) + shiftWords + 1; ++i)
+        {
+            BignumInt from = a[i - shiftWords];
+            ret[i] = (from << shiftBits) | carry;
+            carry = from >> (BIGNUM_INT_BITS - shiftBits);
+        }
+        if (carry) ret[i] = carry;
+    }
+
+    return ret;
+}
+
 /*
  * Non-modular multiplication and addition.
  */