diff --git a/wolfcrypt/src/sp_arm32.c b/wolfcrypt/src/sp_arm32.c
index 40481a80e..bd4a31b2b 100644
--- a/wolfcrypt/src/sp_arm32.c
+++ b/wolfcrypt/src/sp_arm32.c
@@ -4373,6 +4373,10 @@ static int sp_2048_mod_exp_32(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -4540,6 +4544,10 @@ static int sp_2048_mod_exp_32(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -6797,6 +6805,10 @@ static int sp_2048_mod_exp_64(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -6947,6 +6959,10 @@ static int sp_2048_mod_exp_64(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -8057,6 +8073,10 @@ static int sp_2048_mod_exp_2_64(sp_digit* r, const sp_digit* e, int bits,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -14439,6 +14459,10 @@ static int sp_3072_mod_exp_48(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -14606,6 +14630,10 @@ static int sp_3072_mod_exp_48(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -17695,6 +17723,10 @@ static int sp_3072_mod_exp_96(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -17845,6 +17877,10 @@ static int sp_3072_mod_exp_96(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -19203,6 +19239,10 @@ static int sp_3072_mod_exp_2_96(sp_digit* r, const sp_digit* e, int bits,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -25320,6 +25360,10 @@ static int sp_4096_mod_exp_128(sp_digit* r, const sp_digit* a, const sp_digit* e
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -25470,6 +25514,10 @@ static int sp_4096_mod_exp_128(sp_digit* r, const sp_digit* a, const sp_digit* e
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -27076,6 +27124,10 @@ static int sp_4096_mod_exp_2_128(sp_digit* r, const sp_digit* e, int bits,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
diff --git a/wolfcrypt/src/sp_arm64.c b/wolfcrypt/src/sp_arm64.c
index ba0367cfb..33941598b 100644
--- a/wolfcrypt/src/sp_arm64.c
+++ b/wolfcrypt/src/sp_arm64.c
@@ -3322,6 +3322,10 @@ static int sp_2048_mod_exp_16(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -3489,6 +3493,10 @@ static int sp_2048_mod_exp_16(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -4901,6 +4909,10 @@ static int sp_2048_mod_exp_32(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -5051,6 +5063,10 @@ static int sp_2048_mod_exp_32(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -5915,6 +5931,10 @@ static int sp_2048_mod_exp_2_32(sp_digit* r, const sp_digit* e, int bits,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -10944,6 +10964,10 @@ static int sp_3072_mod_exp_24(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -11111,6 +11135,10 @@ static int sp_3072_mod_exp_24(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -12947,6 +12975,10 @@ static int sp_3072_mod_exp_48(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -13097,6 +13129,10 @@ static int sp_3072_mod_exp_48(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -14085,6 +14121,10 @@ static int sp_3072_mod_exp_2_48(sp_digit* r, const sp_digit* e, int bits,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -18156,6 +18196,10 @@ static int sp_4096_mod_exp_64(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -18306,6 +18350,10 @@ static int sp_4096_mod_exp_64(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -19418,6 +19466,10 @@ static int sp_4096_mod_exp_2_64(sp_digit* r, const sp_digit* e, int bits,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
diff --git a/wolfcrypt/src/sp_armthumb.c b/wolfcrypt/src/sp_armthumb.c
index ab1cb855a..d3bc95a4a 100644
--- a/wolfcrypt/src/sp_armthumb.c
+++ b/wolfcrypt/src/sp_armthumb.c
@@ -23860,6 +23860,10 @@ static int sp_2048_mod_exp_32(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -24027,6 +24031,10 @@ static int sp_2048_mod_exp_32(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -27304,6 +27312,10 @@ static int sp_2048_mod_exp_64(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -27454,6 +27466,10 @@ static int sp_2048_mod_exp_64(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -29673,6 +29689,10 @@ static int sp_2048_mod_exp_2_64(sp_digit* r, const sp_digit* e, int bits,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -74892,6 +74912,10 @@ static int sp_3072_mod_exp_48(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -75059,6 +75083,10 @@ static int sp_3072_mod_exp_48(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -79167,6 +79195,10 @@ static int sp_3072_mod_exp_96(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -79317,6 +79349,10 @@ static int sp_3072_mod_exp_96(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -82334,6 +82370,10 @@ static int sp_3072_mod_exp_2_96(sp_digit* r, const sp_digit* e, int bits,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -91522,6 +91562,10 @@ static int sp_4096_mod_exp_128(sp_digit* r, const sp_digit* a, const sp_digit* e
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -91672,6 +91716,10 @@ static int sp_4096_mod_exp_128(sp_digit* r, const sp_digit* a, const sp_digit* e
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -95482,6 +95530,10 @@ static int sp_4096_mod_exp_2_128(sp_digit* r, const sp_digit* e, int bits,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
diff --git a/wolfcrypt/src/sp_cortexm.c b/wolfcrypt/src/sp_cortexm.c
index 8f2a94edf..e829f2a91 100644
--- a/wolfcrypt/src/sp_cortexm.c
+++ b/wolfcrypt/src/sp_cortexm.c
@@ -3264,6 +3264,10 @@ static int sp_2048_mod_exp_32(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -3431,6 +3435,10 @@ static int sp_2048_mod_exp_32(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -4237,6 +4245,10 @@ static int sp_2048_mod_exp_64(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -4387,6 +4399,10 @@ static int sp_2048_mod_exp_64(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -5390,6 +5406,10 @@ static int sp_2048_mod_exp_2_64(sp_digit* r, const sp_digit* e, int bits,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -8415,6 +8435,10 @@ static int sp_3072_mod_exp_48(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -8582,6 +8606,10 @@ static int sp_3072_mod_exp_48(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -9472,6 +9500,10 @@ static int sp_3072_mod_exp_96(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -9622,6 +9654,10 @@ static int sp_3072_mod_exp_96(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -10821,6 +10857,10 @@ static int sp_3072_mod_exp_2_96(sp_digit* r, const sp_digit* e, int bits,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -13189,6 +13229,10 @@ static int sp_4096_mod_exp_128(sp_digit* r, const sp_digit* a, const sp_digit* e
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -13339,6 +13383,10 @@ static int sp_4096_mod_exp_128(sp_digit* r, const sp_digit* a, const sp_digit* e
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
@@ -14735,6 +14783,10 @@ static int sp_4096_mod_exp_2_128(sp_digit* r, const sp_digit* e, int bits,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 32 - c;
diff --git a/wolfcrypt/src/sp_x86_64.c b/wolfcrypt/src/sp_x86_64.c
index afc94f4d3..0ef833fce 100644
--- a/wolfcrypt/src/sp_x86_64.c
+++ b/wolfcrypt/src/sp_x86_64.c
@@ -587,6 +587,10 @@ static int sp_2048_mod_exp_16(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -810,6 +814,10 @@ static int sp_2048_mod_exp_avx2_16(sp_digit* r, const sp_digit* a, const sp_digi
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -1272,6 +1280,10 @@ static int sp_2048_mod_exp_32(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -1479,6 +1491,10 @@ static int sp_2048_mod_exp_avx2_32(sp_digit* r, const sp_digit* a, const sp_digi
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -2190,6 +2206,10 @@ static int sp_2048_mod_exp_2_avx2_32(sp_digit* r, const sp_digit* e, int bits,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -2325,6 +2345,10 @@ static int sp_2048_mod_exp_2_32(sp_digit* r, const sp_digit* e, int bits,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -3112,6 +3136,10 @@ static int sp_3072_mod_exp_24(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -3335,6 +3363,10 @@ static int sp_3072_mod_exp_avx2_24(sp_digit* r, const sp_digit* a, const sp_digi
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -3797,6 +3829,10 @@ static int sp_3072_mod_exp_48(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -4004,6 +4040,10 @@ static int sp_3072_mod_exp_avx2_48(sp_digit* r, const sp_digit* a, const sp_digi
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -4715,6 +4755,10 @@ static int sp_3072_mod_exp_2_avx2_48(sp_digit* r, const sp_digit* e, int bits,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -4850,6 +4894,10 @@ static int sp_3072_mod_exp_2_48(sp_digit* r, const sp_digit* e, int bits,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -5687,6 +5735,10 @@ static int sp_4096_mod_exp_64(sp_digit* r, const sp_digit* a, const sp_digit* e,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -5894,6 +5946,10 @@ static int sp_4096_mod_exp_avx2_64(sp_digit* r, const sp_digit* a, const sp_digi
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -6605,6 +6661,10 @@ static int sp_4096_mod_exp_2_avx2_64(sp_digit* r, const sp_digit* e, int bits,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;
@@ -6740,6 +6800,10 @@ static int sp_4096_mod_exp_2_64(sp_digit* r, const sp_digit* e, int bits,
             n <<= c;
             c = 64 - c;
         }
+        else if (c == 0) {
+            /* All bits in top word used. */
+            y = (byte)n;
+        }
         else {
             y = (byte)(n >> c);
             n <<= 64 - c;