Improved modular exponentiation (automatic window optimisation if there is enough...

[BearSSL] / src / rsa / rsa_i15_pub.c

diff --git a/src/rsa/rsa_i15_pub.c b/src/rsa/rsa_i15_pub.c
index 37ebbd1..09f558a 100644 (file)
--- a/src/rsa/rsa_i15_pub.c
+++ b/src/rsa/rsa_i15_pub.c
@@ -24,6 +24,12 @@
 
 #include "inner.h"
 
+/*
+ * As a strict minimum, we need four buffers that can hold a
+ * modular integer.
+ */
+#define TLEN   (4 * (1 + ((BR_MAX_RSA_SIZE + 14) / 15)))
+
 /* see bearssl_rsa.h */
 uint32_t
 br_rsa_i15_public(unsigned char *x, size_t xlen,
@@ -31,10 +37,10 @@ br_rsa_i15_public(unsigned char *x, size_t xlen,
 {
        const unsigned char *n;
        size_t nlen;
-       uint16_t m[1 + ((BR_MAX_RSA_SIZE + 14) / 15)];
-       uint16_t a[1 + ((BR_MAX_RSA_SIZE + 14) / 15)];
-       uint16_t t1[1 + ((BR_MAX_RSA_SIZE + 14) / 15)];
-       uint16_t t2[1 + ((BR_MAX_RSA_SIZE + 14) / 15)];
+       uint16_t tmp[TLEN];
+       uint16_t *m, *a, *t;
+       size_t fwlen;
+       long z;
        uint16_t m0i;
        uint32_t r;
 
@@ -51,6 +57,25 @@ br_rsa_i15_public(unsigned char *x, size_t xlen,
        if (nlen == 0 || nlen > (BR_MAX_RSA_SIZE >> 3) || xlen != nlen) {
                return 0;
        }
+       z = (long)nlen << 3;
+       fwlen = 1;
+       while (z > 0) {
+               z -= 15;
+               fwlen ++;
+       }
+
+       /*
+        * The modulus gets decoded into m[].
+        * The value to exponentiate goes into a[].
+        * The temporaries for modular exponentiations are in t[].
+        */
+       m = tmp;
+       a = tmp + fwlen;
+       t = tmp + 2 * fwlen;
+
+       /*
+        * Decode the modulus.
+        */
        br_i15_decode(m, n, nlen);
        m0i = br_i15_ninv15(m[1]);
 
@@ -68,7 +93,7 @@ br_rsa_i15_public(unsigned char *x, size_t xlen,
        /*
         * Compute the modular exponentiation.
         */
-       br_i15_modpow(a, pk->e, pk->elen, m, m0i, t1, t2);
+       br_i15_modpow_opt(a, pk->e, pk->elen, m, m0i, t, TLEN - 2 * fwlen);
 
        /*
         * Encode the result.