diff --git a/libraries/Crypto/BigNumberUtil.cpp b/libraries/Crypto/BigNumberUtil.cpp
new file mode 100644
index 00000000..b058e45d
--- /dev/null
+++ b/libraries/Crypto/BigNumberUtil.cpp
@@ -0,0 +1,365 @@
+/*
+ * Copyright (C) 2015 Southern Storm Software, Pty Ltd.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included
+ * in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+ * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ */
+
+#include "BigNumberUtil.h"
+#include "utility/EndianUtil.h"
+#include <string.h>
+
+/**
+ * \class BigNumberUtil BigNumberUtil.h <BigNumberUtil.h>
+ * \brief Utilities to assist with implementing big number arithmetic.
+ *
+ * Big numbers are represented as arrays of limb_t words, which may be
+ * 8 bits, 16 bits, or 32 bits in size depending upon how the library
+ * was configured.  For AVR, 16 bit limbs usually give the best performance.
+ *
+ * Limb arrays are ordered from the least significant word to the most
+ * significant.
+ */
+
+/**
+ * \brief Unpacks the little-endian byte representation of a big number
+ * into a limb array.
+ *
+ * \param limbs The limb array, starting with the least significant word.
+ * \param count The number of elements in the \a limbs array.
+ * \param bytes The bytes to unpack.
+ * \param len The number of bytes to unpack.
+ *
+ * If \a len is shorter than the length of \a limbs, then the high bytes
+ * will be filled with zeroes.  If \a len is longer than the length of
+ * \a limbs, then the high bytes will be truncated and lost.
+ *
+ * \sa packLE(), unpackBE()
+ */
+void BigNumberUtil::unpackLE(limb_t *limbs, size_t count,
+                             const uint8_t *bytes, size_t len)
+{
+#if BIGNUMBER_LIMB_8BIT
+    if (len < count) {
+        memcpy(limbs, bytes, len);
+        memset(limbs + len, 0, count - len);
+    } else {
+        memcpy(limbs, bytes, count);
+    }
+#elif CRYPTO_LITTLE_ENDIAN
+    count *= sizeof(limb_t);
+    if (len < count) {
+        memcpy(limbs, bytes, len);
+        memset(limbs + len, 0, count - len);
+    } else {
+        memcpy(limbs, bytes, count);
+    }
+#elif BIGNUMBER_LIMB_16BIT
+    while (count > 0 && len >= 2) {
+        *limbs++ = ((limb_t)(bytes[0])) |
+                  (((limb_t)(bytes[1])) << 8);
+        bytes += 2;
+        --count;
+        len -= 2;
+    }
+    if (count > 0 && len == 1) {
+        *limbs++ = ((limb_t)(bytes[0]));
+        --count;
+    }
+    while (count > 0) {
+        *limbs++ = 0;
+        --count;
+    }
+#elif BIGNUMBER_LIMB_32BIT
+    while (count > 0 && len >= 4) {
+        *limbs++ = ((limb_t)(bytes[0])) |
+                  (((limb_t)(bytes[1])) <<  8) |
+                  (((limb_t)(bytes[2])) << 16) |
+                  (((limb_t)(bytes[3])) << 24);
+        bytes += 4;
+        --count;
+        len -= 4;
+    }
+    if (count > 0) {
+        if (len == 3) {
+            *limbs++ = ((limb_t)(bytes[0])) |
+                      (((limb_t)(bytes[1])) <<  8) |
+                      (((limb_t)(bytes[2])) << 16);
+        } else if (len == 2) {
+            *limbs++ = ((limb_t)(bytes[0])) |
+                      (((limb_t)(bytes[1])) <<  8);
+        } else if (len == 1) {
+            *limbs++ = ((limb_t)(bytes[0]));
+        }
+        --count;
+    }
+    while (count > 0) {
+        *limbs++ = 0;
+        --count;
+    }
+#endif
+}
+
+/**
+ * \brief Unpacks the big-endian byte representation of a big number
+ * into a limb array.
+ *
+ * \param limbs The limb array, starting with the least significant word.
+ * \param count The number of elements in the \a limbs array.
+ * \param bytes The bytes to unpack.
+ * \param len The number of bytes to unpack.
+ *
+ * If \a len is shorter than the length of \a limbs, then the high bytes
+ * will be filled with zeroes.  If \a len is longer than the length of
+ * \a limbs, then the high bytes will be truncated and lost.
+ *
+ * \sa packBE(), unpackLE()
+ */
+void BigNumberUtil::unpackBE(limb_t *limbs, size_t count,
+                             const uint8_t *bytes, size_t len)
+{
+#if BIGNUMBER_LIMB_8BIT
+    while (count > 0 && len > 0) {
+        --count;
+        --len;
+        *limbs++ = bytes[len];
+    }
+    memset(limbs, 0, count);
+#elif BIGNUMBER_LIMB_16BIT
+    bytes += len;
+    while (count > 0 && len >= 2) {
+        --count;
+        bytes -= 2;
+        len -= 2;
+        *limbs++ = ((limb_t)(bytes[1])) |
+                  (((limb_t)(bytes[0])) << 8);
+    }
+    if (count > 0 && len == 1) {
+        --count;
+        --bytes;
+        *limbs++ = (limb_t)(bytes[0]);
+    }
+    memset(limbs, 0, count * sizeof(limb_t));
+#elif BIGNUMBER_LIMB_32BIT
+    bytes += len;
+    while (count > 0 && len >= 4) {
+        --count;
+        bytes -= 4;
+        len -= 4;
+        *limbs++ = ((limb_t)(bytes[3])) |
+                  (((limb_t)(bytes[2])) << 8) |
+                  (((limb_t)(bytes[1])) << 16) |
+                  (((limb_t)(bytes[0])) << 24);
+    }
+    if (count > 0) {
+        if (len == 3) {
+            --count;
+            bytes -= 3;
+            *limbs++ = ((limb_t)(bytes[2])) |
+                      (((limb_t)(bytes[1])) << 8) |
+                      (((limb_t)(bytes[0])) << 16);
+        } else if (len == 2) {
+            --count;
+            bytes -= 2;
+            *limbs++ = ((limb_t)(bytes[1])) |
+                      (((limb_t)(bytes[0])) << 8);
+        } else if (len == 1) {
+            --count;
+            --bytes;
+            *limbs++ = (limb_t)(bytes[0]);
+        }
+    }
+    memset(limbs, 0, count * sizeof(limb_t));
+#endif
+}
+
+/**
+ * \brief Packs the little-endian byte representation of a big number
+ * into a byte array.
+ *
+ * \param bytes The byte array to pack into.
+ * \param len The number of bytes in the destination \a bytes array.
+ * \param limbs The limb array representing the big number, starting with
+ * the least significant word.
+ * \param count The number of elements in the \a limbs array.
+ *
+ * If \a len is shorter than the length of \a limbs, then the number will
+ * be truncated to the least significant \a len bytes.  If \a len is longer
+ * than the length of \a limbs, then the high bytes will be filled with zeroes.
+ *
+ * \sa unpackLE(), packBE()
+ */
+void BigNumberUtil::packLE(uint8_t *bytes, size_t len,
+                           const limb_t *limbs, size_t count)
+{
+#if BIGNUMBER_LIMB_8BIT
+    if (len <= count) {
+        memcpy(bytes, limbs, len);
+    } else {
+        memcpy(bytes, limbs, count);
+        memset(bytes + count, 0, len - count);
+    }
+#elif CRYPTO_LITTLE_ENDIAN
+    count *= sizeof(limb_t);
+    if (len <= count) {
+        memcpy(bytes, limbs, len);
+    } else {
+        memcpy(bytes, limbs, count);
+        memset(bytes + count, 0, len - count);
+    }
+#elif BIGNUMBER_LIMB_16BIT
+    limb_t word;
+    while (count > 0 && len >= 2) {
+        word = *limbs++;
+        bytes[0] = (uint8_t)word;
+        bytes[1] = (uint8_t)(word >> 8);
+        --count;
+        len -= 2;
+        bytes += 2;
+    }
+    if (count > 0 && len == 1) {
+        bytes[0] = (uint8_t)(*limbs);
+        --len;
+        ++bytes;
+    }
+    memset(bytes, 0, len);
+#elif BIGNUMBER_LIMB_32BIT
+    limb_t word;
+    while (count > 0 && len >= 4) {
+        word = *limbs++;
+        bytes[0] = (uint8_t)word;
+        bytes[1] = (uint8_t)(word >> 8);
+        bytes[2] = (uint8_t)(word >> 16);
+        bytes[3] = (uint8_t)(word >> 24);
+        --count;
+        len -= 4;
+        bytes += 4;
+    }
+    if (count > 0) {
+        if (len == 3) {
+            word = *limbs;
+            bytes[0] = (uint8_t)word;
+            bytes[1] = (uint8_t)(word >> 8);
+            bytes[2] = (uint8_t)(word >> 16);
+            len -= 3;
+            bytes += 3;
+        } else if (len == 2) {
+            word = *limbs;
+            bytes[0] = (uint8_t)word;
+            bytes[1] = (uint8_t)(word >> 8);
+            len -= 2;
+            bytes += 2;
+        } else if (len == 1) {
+            bytes[0] = (uint8_t)(*limbs);
+            --len;
+            ++bytes;
+        }
+    }
+    memset(bytes, 0, len);
+#endif
+}
+
+/**
+ * \brief Packs the big-endian byte representation of a big number
+ * into a byte array.
+ *
+ * \param bytes The byte array to pack into.
+ * \param len The number of bytes in the destination \a bytes array.
+ * \param limbs The limb array representing the big number, starting with
+ * the least significant word.
+ * \param count The number of elements in the \a limbs array.
+ *
+ * If \a len is shorter than the length of \a limbs, then the number will
+ * be truncated to the least significant \a len bytes.  If \a len is longer
+ * than the length of \a limbs, then the high bytes will be filled with zeroes.
+ *
+ * \sa unpackLE(), packBE()
+ */
+void BigNumberUtil::packBE(uint8_t *bytes, size_t len,
+                           const limb_t *limbs, size_t count)
+{
+#if BIGNUMBER_LIMB_8BIT
+    if (len > count) {
+        size_t size = len - count;
+        memset(bytes, 0, size);
+        len -= size;
+        bytes += size;
+    } else if (len < count) {
+        count = len;
+    }
+    limbs += count;
+    while (count > 0) {
+        --count;
+        *bytes++ = *(--limbs);
+    }
+#elif BIGNUMBER_LIMB_16BIT
+    size_t countBytes = count * sizeof(limb_t);
+    limb_t word;
+    if (len >= countBytes) {
+        size_t size = len - countBytes;
+        memset(bytes, 0, size);
+        len -= size;
+        bytes += size;
+        limbs += count;
+    } else {
+        count = len / sizeof(limb_t);
+        limbs += count;
+        if ((len & 1) != 0)
+            *bytes++ = (uint8_t)(*limbs);
+    }
+    while (count > 0) {
+        --count;
+        word = *(--limbs);
+        *bytes++ = (uint8_t)(word >> 8);
+        *bytes++ = (uint8_t)word;
+    }
+#elif BIGNUMBER_LIMB_32BIT
+    size_t countBytes = count * sizeof(limb_t);
+    limb_t word;
+    if (len >= countBytes) {
+        size_t size = len - countBytes;
+        memset(bytes, 0, size);
+        len -= size;
+        bytes += size;
+        limbs += count;
+    } else {
+        count = len / sizeof(limb_t);
+        limbs += count;
+        if ((len & 3) == 3) {
+            word = *limbs;
+            *bytes++ = (uint8_t)(word >> 16);
+            *bytes++ = (uint8_t)(word >> 8);
+            *bytes++ = (uint8_t)word;
+        } else if ((len & 3) == 2) {
+            word = *limbs;
+            *bytes++ = (uint8_t)(word >> 8);
+            *bytes++ = (uint8_t)word;
+        } else if ((len & 3) == 1) {
+            *bytes++ = (uint8_t)(*limbs);
+        }
+    }
+    while (count > 0) {
+        --count;
+        word = *(--limbs);
+        *bytes++ = (uint8_t)(word >> 24);
+        *bytes++ = (uint8_t)(word >> 16);
+        *bytes++ = (uint8_t)(word >> 8);
+        *bytes++ = (uint8_t)word;
+    }
+#endif
+}
diff --git a/libraries/Crypto/BigNumberUtil.h b/libraries/Crypto/BigNumberUtil.h
index 65719478..575b57ca 100644
--- a/libraries/Crypto/BigNumberUtil.h
+++ b/libraries/Crypto/BigNumberUtil.h
@@ -24,6 +24,7 @@
 #define CRYPTO_BIGNUMBERUTIL_h
 
 #include <inttypes.h>
+#include <stddef.h>
 
 // Define exactly one of these to 1 to set the size of the basic limb type.
 // 16-bit limbs seem to give the best performance on 8-bit AVR micros.
@@ -48,4 +49,22 @@ typedef uint64_t dlimb_t;
 #error "limb_t must be 8, 16, or 32 bits in size"
 #endif
 
+class BigNumberUtil
+{
+public:
+    static void unpackLE(limb_t *limbs, size_t count,
+                         const uint8_t *bytes, size_t len);
+    static void unpackBE(limb_t *limbs, size_t count,
+                         const uint8_t *bytes, size_t len);
+    static void packLE(uint8_t *bytes, size_t len,
+                       const limb_t *limbs, size_t count);
+    static void packBE(uint8_t *bytes, size_t len,
+                       const limb_t *limbs, size_t count);
+
+private:
+    // Constructor and destructor are private - cannot instantiate this class.
+    BigNumberUtil() {}
+    ~BigNumberUtil() {}
+};
+
 #endif
diff --git a/libraries/Crypto/Curve25519.cpp b/libraries/Crypto/Curve25519.cpp
index 9dde379f..9f8986e4 100644
--- a/libraries/Crypto/Curve25519.cpp
+++ b/libraries/Crypto/Curve25519.cpp
@@ -101,7 +101,9 @@ bool Curve25519::eval(uint8_t result[32], const uint8_t s[32], const uint8_t x[3
     // Unpack the "x" argument into the limb representation
     // which also masks off the high bit.  NULL means 9.
     if (x) {
-        unpack(x_1, x);                 // x_1 = x
+        // x1 = x
+        BigNumberUtil::unpackLE(x_1, NUM_LIMBS, x, 32);
+        x_1[NUM_LIMBS - 1] &= ((((limb_t)1) << (LIMB_BITS - 1)) - 1);
     } else {
         memset(x_1, 0, sizeof(x_1));    // x_1 = 9
         x_1[0] = 9;
@@ -176,7 +178,7 @@ bool Curve25519::eval(uint8_t result[32], const uint8_t s[32], const uint8_t x[3
     mul(x_2, x_2, z_3);
 
     // Pack the result into the return array.
-    pack(result, x_2);
+    BigNumberUtil::packLE(result, 32, x_2, NUM_LIMBS);
 
     // Clean up and exit.
     clean(x_1);
@@ -797,66 +799,3 @@ void Curve25519::recip(limb_t *result, const limb_t *x)
     // Clean up and exit.
     clean(t1);
 }
-
-/**
- * \brief Unpacks the little-endian byte representation of a field element
- * into a limb array.
- *
- * \param result The limb array.
- * \param x The byte representation.
- *
- * The top-most bit of \a result will be set to zero so that the value
- * is guaranteed to be 255 bits rather than 256.
- *
- * \sa pack()
- */
-void Curve25519::unpack(limb_t *result, const uint8_t *x)
-{
-#if BIGNUMBER_LIMB_8BIT
-    memcpy(result, x, 32);
-    result[31] &= 0x7F;
-#elif BIGNUMBER_LIMB_16BIT
-    for (uint8_t posn = 0; posn < 16; ++posn) {
-        result[posn] = ((limb_t)x[posn * 2]) | (((limb_t)x[posn * 2 + 1]) << 8);
-    }
-    result[15] &= 0x7FFF;
-#elif BIGNUMBER_LIMB_32BIT
-    for (uint8_t posn = 0; posn < 8; ++posn) {
-        result[posn] = ((limb_t)x[posn * 4]) |
-                      (((limb_t)x[posn * 4 + 1]) << 8) |
-                      (((limb_t)x[posn * 4 + 2]) << 16) |
-                      (((limb_t)x[posn * 4 + 3]) << 24);
-    }
-    result[7] &= 0x7FFFFFFF;
-#endif
-}
-
-/**
- * \brief Packs the limb array representation of a field element into a
- * byte array.
- *
- * \param result The byte array.
- * \param x The limb representation.
- *
- * \sa unpack()
- */
-void Curve25519::pack(uint8_t *result, const limb_t *x)
-{
-#if BIGNUMBER_LIMB_8BIT
-    memcpy(result, x, 32);
-#elif BIGNUMBER_LIMB_16BIT
-    for (uint8_t posn = 0; posn < 16; ++posn) {
-        limb_t value = x[posn];
-        result[posn * 2]     = (uint8_t)value;
-        result[posn * 2 + 1] = (uint8_t)(value >> 8);
-    }
-#elif BIGNUMBER_LIMB_32BIT
-    for (uint8_t posn = 0; posn < 8; ++posn) {
-        limb_t value = x[posn];
-        result[posn * 4]     = (uint8_t)value;
-        result[posn * 4 + 1] = (uint8_t)(value >> 8);
-        result[posn * 4 + 2] = (uint8_t)(value >> 16);
-        result[posn * 4 + 3] = (uint8_t)(value >> 24);
-    }
-#endif
-}
diff --git a/libraries/Crypto/Curve25519.h b/libraries/Crypto/Curve25519.h
index aa183f92..c88301db 100644
--- a/libraries/Crypto/Curve25519.h
+++ b/libraries/Crypto/Curve25519.h
@@ -24,7 +24,6 @@
 #define CRYPTO_CURVE25519_h
 
 #include "BigNumberUtil.h"
-#include <stddef.h>
 
 class Curve25519
 {
@@ -59,9 +58,6 @@ private:
 
     static void recip(limb_t *result, const limb_t *x);
 
-    static void unpack(limb_t *result, const uint8_t *x);
-    static void pack(uint8_t *result, const limb_t *x);
-
     // Constructor and destructor are private - cannot instantiate this class.
     Curve25519() {}
     ~Curve25519() {}