/*
- * RSA implementation just sufficient for ssh client-side
- * initialisation step
- *
- * Rewritten for more speed by Joris van Rantwijk, Jun 1999.
+ * RSA implementation for PuTTY.
*/
#include <stdio.h>
freebn(b2);
}
-Bignum rsadecrypt(Bignum input, struct RSAKey *key)
+/*
+ * This function is a wrapper on modpow(). It has the same effect
+ * as modpow(), but employs RSA blinding to protect against timing
+ * attacks.
+ */
+static Bignum rsa_privkey_op(Bignum input, struct RSAKey *key)
{
+ Bignum random, random_encrypted, random_inverse;
+ Bignum input_blinded, ret_blinded;
Bignum ret;
- ret = modpow(input, key->private_exponent, key->modulus);
+
+ /*
+ * Start by inventing a random number chosen uniformly from the
+ * range 2..modulus-1. (We do this by preparing a random number
+ * of the right length and retrying if it's greater than the
+ * modulus, to prevent any potential Bleichenbacher-like
+ * attacks making use of the uneven distribution within the
+ * range that would arise from just reducing our number mod n.
+ * There are timing implications to the potential retries, of
+ * course, but all they tell you is the modulus, which you
+ * already knew.)
+ */
+ while (1) {
+ int bits, byte, bitsleft, v;
+ random = copybn(key->modulus);
+ /*
+ * Find the topmost set bit. (This function will return its
+ * index plus one.) Then we'll set all bits from that one
+ * downwards randomly.
+ */
+ bits = bignum_bitcount(random);
+ byte = 0;
+ bitsleft = 0;
+ while (bits--) {
+ if (bitsleft <= 0)
+ bitsleft = 8, byte = random_byte();
+ v = byte & 1;
+ byte >>= 1;
+ bitsleft--;
+ bignum_set_bit(random, bits, v);
+ }
+
+ /*
+ * Now check that this number is strictly greater than
+ * zero, and strictly less than modulus.
+ */
+ if (bignum_cmp(random, Zero) <= 0 ||
+ bignum_cmp(random, key->modulus) >= 0) {
+ freebn(random);
+ continue;
+ } else {
+ break;
+ }
+ }
+
+ /*
+ * RSA blinding relies on the fact that (xy)^d mod n is equal
+ * to (x^d mod n) * (y^d mod n) mod n. We invent a random pair
+ * y and y^d; then we multiply x by y, raise to the power d mod
+ * n as usual, and divide by y^d to recover x^d. Thus an
+ * attacker can't correlate the timing of the modpow with the
+ * input, because they don't know anything about the number
+ * that was input to the actual modpow.
+ *
+ * The clever bit is that we don't have to do a huge modpow to
+ * get y and y^d; we will use the number we just invented as
+ * _y^d_, and use the _public_ exponent to compute (y^d)^e = y
+ * from it, which is much faster to do.
+ */
+ random_encrypted = modpow(random, key->exponent, key->modulus);
+ random_inverse = modinv(random, key->modulus);
+ input_blinded = modmul(input, random_encrypted, key->modulus);
+ ret_blinded = modpow(input_blinded, key->private_exponent, key->modulus);
+ ret = modmul(ret_blinded, random_inverse, key->modulus);
+
+ freebn(ret_blinded);
+ freebn(input_blinded);
+ freebn(random_inverse);
+ freebn(random_encrypted);
+ freebn(random);
+
return ret;
}
+Bignum rsadecrypt(Bignum input, struct RSAKey *key)
+{
+ return rsa_privkey_op(input, key);
+}
+
int rsastr_len(struct RSAKey *key)
{
Bignum md, ex;
length = (ssh1_bignum_length(key->modulus) +
ssh1_bignum_length(key->exponent) + 4);
- ret = smalloc(length);
+ ret = snewn(length, unsigned char);
PUT_32BIT(ret, bignum_bitcount(key->modulus));
pos = 4;
int slen;
struct RSAKey *rsa;
- rsa = smalloc(sizeof(struct RSAKey));
+ rsa = snew(struct RSAKey);
if (!rsa)
return NULL;
getstring(&data, &len, &p, &slen);
int len;
len = rsastr_len(rsa);
- p = smalloc(len);
+ p = snewn(len, char);
rsastr_fmt(p, rsa);
return p;
}
* (three length fields, 12+7=19).
*/
bloblen = 19 + elen + mlen;
- blob = smalloc(bloblen);
+ blob = snewn(bloblen, unsigned char);
p = blob;
PUT_32BIT(p, 7);
p += 4;
* sum of lengths.
*/
bloblen = 16 + dlen + plen + qlen + ulen;
- blob = smalloc(bloblen);
+ blob = snewn(bloblen, unsigned char);
p = blob;
PUT_32BIT(p, dlen);
p += 4;
char **b = (char **) blob;
struct RSAKey *rsa;
- rsa = smalloc(sizeof(struct RSAKey));
+ rsa = snew(struct RSAKey);
if (!rsa)
return NULL;
rsa->comment = NULL;
for (i = 0; i < 16; i++)
sprintf(buffer + strlen(buffer), "%s%02x", i ? ":" : "",
digest[i]);
- ret = smalloc(strlen(buffer) + 1);
+ ret = snewn(strlen(buffer) + 1, char);
if (ret)
strcpy(ret, buffer);
return ret;
return ret;
}
-unsigned char *rsa2_sign(void *key, char *data, int datalen, int *siglen)
+static unsigned char *rsa2_sign(void *key, char *data, int datalen,
+ int *siglen)
{
struct RSAKey *rsa = (struct RSAKey *) key;
unsigned char *bytes;
SHA_Simple(data, datalen, hash);
nbytes = (bignum_bitcount(rsa->modulus) - 1) / 8;
- bytes = smalloc(nbytes);
+ bytes = snewn(nbytes, unsigned char);
bytes[0] = 1;
for (i = 1; i < nbytes - 20 - ASN1_LEN; i++)
in = bignum_from_bytes(bytes, nbytes);
sfree(bytes);
- out = modpow(in, rsa->private_exponent, rsa->modulus);
+ out = rsa_privkey_op(in, rsa);
freebn(in);
nbytes = (bignum_bitcount(out) + 7) / 8;
- bytes = smalloc(4 + 7 + 4 + nbytes);
+ bytes = snewn(4 + 7 + 4 + nbytes, unsigned char);
PUT_32BIT(bytes, 7);
memcpy(bytes + 4, "ssh-rsa", 7);
PUT_32BIT(bytes + 4 + 7, nbytes);