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From 2232d3d5f188b65dbb6c823ac62175412739eb16 Mon Sep 17 00:00:00 2001
From: Jouni Malinen <j@w1.fi>
Date: Fri, 7 Jan 2022 13:47:16 +0200
Subject: [PATCH 2/4] dragonfly: Add sqrt() helper function
This is a backport of "SAE: Move sqrt() implementation into a helper
function" to introduce the helper function needed for the following
patches.
Signed-off-by: Jouni Malinen <j@w1.fi>
---
src/common/dragonfly.c | 34 ++++++++++++++++++++++++++++++++++
src/common/dragonfly.h | 2 ++
2 files changed, 36 insertions(+)
--- a/src/common/dragonfly.c
+++ b/src/common/dragonfly.c
@@ -213,3 +213,37 @@ int dragonfly_generate_scalar(const stru
"dragonfly: Unable to get randomness for own scalar");
return -1;
}
+
+
+/* res = sqrt(val) */
+int dragonfly_sqrt(struct crypto_ec *ec, const struct crypto_bignum *val,
+ struct crypto_bignum *res)
+{
+ const struct crypto_bignum *prime;
+ struct crypto_bignum *tmp, *one;
+ int ret = 0;
+ u8 prime_bin[DRAGONFLY_MAX_ECC_PRIME_LEN];
+ size_t prime_len;
+
+ /* For prime p such that p = 3 mod 4, sqrt(w) = w^((p+1)/4) mod p */
+
+ prime = crypto_ec_get_prime(ec);
+ prime_len = crypto_ec_prime_len(ec);
+ tmp = crypto_bignum_init();
+ one = crypto_bignum_init_uint(1);
+
+ if (crypto_bignum_to_bin(prime, prime_bin, sizeof(prime_bin),
+ prime_len) < 0 ||
+ (prime_bin[prime_len - 1] & 0x03) != 3 ||
+ !tmp || !one ||
+ /* tmp = (p+1)/4 */
+ crypto_bignum_add(prime, one, tmp) < 0 ||
+ crypto_bignum_rshift(tmp, 2, tmp) < 0 ||
+ /* res = sqrt(val) */
+ crypto_bignum_exptmod(val, tmp, prime, res) < 0)
+ ret = -1;
+
+ crypto_bignum_deinit(tmp, 0);
+ crypto_bignum_deinit(one, 0);
+ return ret;
+}
--- a/src/common/dragonfly.h
+++ b/src/common/dragonfly.h
@@ -27,5 +27,7 @@ int dragonfly_generate_scalar(const stru
struct crypto_bignum *_rand,
struct crypto_bignum *_mask,
struct crypto_bignum *scalar);
+int dragonfly_sqrt(struct crypto_ec *ec, const struct crypto_bignum *val,
+ struct crypto_bignum *res);
#endif /* DRAGONFLY_H */
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