it would be nice to have mp_sqrtmod_prime function - it is necessary for handling compressed ECC keys as mentioned in this issue libtom/libtomcrypt#34
here is my suggestion (I'll send it as pull request if you find it worth considering):
diff --git a/src/ltm/bn_mp_sqrtmod_prime.c b/src/ltm/bn_mp_sqrtmod_prime.c
--- /dev/null
+++ b/src/ltm/bn_mp_sqrtmod_prime.c
@@ -0,0 +1,122 @@
+#include <tommath.h>
+#ifdef BN_MP_SQRTMOD_PRIME_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis
+ *
+ * LibTomMath is a library that provides multiple-precision
+ * integer arithmetic as well as number theoretic functionality.
+ *
+ * The library is free for all purposes without any express
+ * guarantee it works.
+ */
+
+/* Tonelli–Shanks algorithm
+ * https://en.wikipedia.org/wiki/Tonelli%E2%80%93Shanks_algorithm
+ * https://gmplib.org/list-archives/gmp-discuss/2013-April/005300.html
+ *
+ */
+
+int mp_sqrtmod_prime(mp_int *n, mp_int *prime, mp_int *ret)
+{
+ int res, legendre;
+ mp_int t1, C, Q, S, Z, M, T, R, two;
+ unsigned long i;
+
+ /* first handle the simple cases */
+ if (mp_cmp_d(n, 0) == MP_EQ) {
+ mp_zero(ret);
+ return MP_OKAY;
+ }
+ if (mp_cmp_d(prime, 2) == MP_EQ) return MP_VAL; /* prime must be odd */
+ if ((res = mp_jacobi(n, prime, &legendre)) != MP_OKAY) return res;
+ if (legendre == -1) return MP_VAL; /* quadratic non-residue mod prime */
+
+ mp_init_multi(&t1, &C, &Q, &S, &Z, &M, &T, &R, &two, NULL);
+
+ /* SPECIAL CASE: if prime mod 4 == 3
+ * compute directly: res = n^(prime+1)/4 mod prime
+ * Handbook of Applied Cryptography algorithm 3.36
+ */
+ if ((res = mp_mod_d(prime, 4, &i)) != MP_OKAY) goto cleanup;
+ if (i == 3) {
+ if ((res = mp_add_d(prime, 1, &t1)) != MP_OKAY) goto cleanup;
+ if ((res = mp_div_2(&t1, &t1)) != MP_OKAY) goto cleanup;
+ if ((res = mp_div_2(&t1, &t1)) != MP_OKAY) goto cleanup;
+ if ((res = mp_exptmod(n, &t1, prime, ret)) != MP_OKAY) goto cleanup;
+ res = MP_OKAY;
+ goto cleanup;
+ }
+
+ /* NOW: Tonelli–Shanks algorithm */
+
+ /* factor out powers of 2 from prime-1, defining Q and S as: prime-1 = Q*2^S */
+ if ((res = mp_copy(prime, &Q)) != MP_OKAY) goto cleanup;
+ if ((res = mp_sub_d(&Q, 1, &Q)) != MP_OKAY) goto cleanup;
+ /* Q = prime - 1 */
+ mp_zero(&S);
+ /* S = 0 */
+ while (mp_iseven(&Q)) {
+ if ((res = mp_div_2(&Q, &Q)) != MP_OKAY) goto cleanup;
+ /* Q = Q / 2 */
+ if ((res = mp_add_d(&S, 1, &S)) != MP_OKAY) goto cleanup;
+ /* S = S + 1 */
+ }
+
+ /* find a Z such that the Legendre symbol (Z|prime) == -1 */
+ mp_set_int(&Z, 2);
+ /* Z = 2 */
+ while(1) {
+ if ((res = mp_jacobi(&Z, prime, &legendre)) != MP_OKAY) goto cleanup;
+ if (legendre == -1) break;
+ if ((res = mp_add_d(&Z, 1, &Z)) != MP_OKAY) goto cleanup;
+ /* Z = Z + 1 */
+ }
+
+ if ((res = mp_exptmod(&Z, &Q, prime, &C)) != MP_OKAY) goto cleanup;
+ /* C = Z ^ Q mod prime */
+ if ((res = mp_add_d(&Q, 1, &t1)) != MP_OKAY) goto cleanup;
+ if ((res = mp_div_2(&t1, &t1)) != MP_OKAY) goto cleanup;
+ /* t1 = (Q + 1) / 2 */
+ if ((res = mp_exptmod(n, &t1, prime, &R)) != MP_OKAY) goto cleanup;
+ /* R = n ^ ((Q + 1) / 2) mod prime */
+ if ((res = mp_exptmod(n, &Q, prime, &T)) != MP_OKAY) goto cleanup;
+ /* T = n ^ Q mod prime */
+ if ((res = mp_copy(&S, &M)) != MP_OKAY) goto cleanup;
+ /* M = S */
+ if ((res = mp_set_int(&two, 2)) != MP_OKAY) goto cleanup;
+
+ while (1) {
+ if ((res = mp_copy(&T, &t1)) != MP_OKAY) goto cleanup;
+ i = 0;
+ while (1) {
+ if (mp_cmp_d(&t1, 1) == MP_EQ) break;
+ if ((res = mp_exptmod(&t1, &two, prime, &t1)) != MP_OKAY) goto cleanup;
+ i++;
+ }
+ if (i == 0) {
+ mp_copy(&R, ret);
+ res = MP_OKAY;
+ goto cleanup;
+ }
+ if ((res = mp_sub_d(&M, i, &t1)) != MP_OKAY) goto cleanup;
+ if ((res = mp_sub_d(&t1, 1, &t1)) != MP_OKAY) goto cleanup;
+ if ((res = mp_exptmod(&two, &t1, prime, &t1)) != MP_OKAY) goto cleanup;
+ /* t1 = 2 ^ (M - i - 1) */
+ if ((res = mp_exptmod(&C, &t1, prime, &t1)) != MP_OKAY) goto cleanup;
+ /* t1 = C ^ (2 ^ (M - i - 1)) mod prime */
+ if ((res = mp_sqrmod(&t1, prime, &C)) != MP_OKAY) goto cleanup;
+ /* C = (t1 * t1) mod prime */
+ if ((res = mp_mulmod(&R, &t1, prime, &R)) != MP_OKAY) goto cleanup;
+ /* R = (R * t1) mod prime */
+ if ((res = mp_mulmod(&T, &C, prime, &T)) != MP_OKAY) goto cleanup;
+ /* T = (T * C) mod prime */
+ mp_set(&M, i);
+ /* M = i */
+ }
+
+ res = MP_VAL;
+cleanup:
+ mp_clear_multi(&t1, &C, &Q, &S, &Z, &M, &T, &R, &two, NULL);
+ return res;
+}
+
+#endif
diff --git a/src/ltm/tommath.h b/src/ltm/tommath.h
index 58a26c8..14da465 100644
--- a/src/ltm/tommath.h
+++ b/src/ltm/tommath.h
@@ -394,6 +394,9 @@ int mp_n_root(mp_int *a, mp_digit b, mp_int *c);
/* special sqrt algo */
int mp_sqrt(mp_int *arg, mp_int *ret);
+/* special sqrt (mod prime) */
+int mp_sqrtmod_prime(mp_int *arg, mp_int *prime, mp_int *ret);
+
/* is number a square? */
int mp_is_square(mp_int *arg, int *ret);
diff --git a/src/ltm/tommath_class.h b/src/ltm/tommath_class.h
index 68b88b9..a82fa44 100644
--- a/src/ltm/tommath_class.h
+++ b/src/ltm/tommath_class.h
@@ -104,6 +104,7 @@
#define BN_MP_SQR_C
#define BN_MP_SQRMOD_C
#define BN_MP_SQRT_C
+#define BN_MP_SQRTMOD_PRIME_C
#define BN_MP_SUB_C
#define BN_MP_SUB_D_C
#define BN_MP_SUBMOD_C
@@ -823,6 +824,18 @@
#define BN_MP_CLEAR_C
#endif
+#if defined(BN_MP_SQRTMOD_PRIME_C)
+ #define BN_MP_JACOBI_C
+ #define BN_MP_ZERO_C
+ #define BN_MP_SET_INT_C
+ #define BN_MP_COPY_C
+ #define BN_MP_SUB_C
+ #define BN_MP_SUB_D_C
+ #define BN_MP_DIV_2_C
+ #define BN_MP_ADD_D_C
+ #define BN_MP_EXPTMOD_C
+#endif
+
#if defined(BN_MP_SUB_C)
#define BN_S_MP_ADD_C
#define BN_MP_CMP_MAG_C
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