diff --git a/inc/qcbor/qcbor_decode.h b/inc/qcbor/qcbor_decode.h
index bf4a042..e9c40fc 100644
--- a/inc/qcbor/qcbor_decode.h
+++ b/inc/qcbor/qcbor_decode.h
@@ -291,6 +291,16 @@
 /** Type for a double floating-point number. Data is in @c val.double. */
 #define QCBOR_TYPE_DOUBLE        27
 
+/** Special type for integers between -2^63 - 1 to -2^64 that
+ * can't be returned as @ref QCBOR_TYPE_INT64 because they don't fit
+ * in an int64_t. The value is returned in @c val.uint64, but this
+ * isn't the number transmitted. Do this arithmatic (carefully to
+ * avoid over/underflow) to get the value transmitted: - val.uint64 - 1.
+ * See QCBOREncode_AddNegativeUInt64() for a longer explanation
+ * and warning. */
+#define QCBOR_TYPE_65BIT_NEG_INT 28
+
+
 #define QCBOR_TYPE_BREAK         31 /* Used internally; never returned */
 
 /** For @ref QCBOR_DECODE_MODE_MAP_AS_ARRAY decode mode, a map that is
diff --git a/inc/qcbor/qcbor_encode.h b/inc/qcbor/qcbor_encode.h
index 6012b25..0aa28f6 100644
--- a/inc/qcbor/qcbor_encode.h
+++ b/inc/qcbor/qcbor_encode.h
@@ -544,6 +544,51 @@
 
 
 /**
+ * @brief Add a negative 64-bit integer to encoded output
+ *
+ * @param[in] pCtx  The encoding context to add the integer to.
+ * @param[in] uNum  The integer to add.
+ *
+ * QCBOREncode_AddInt64() is much better to encode negative integers
+ * than this.  What this can do is add integers with one more
+ * significant bit than an int64_t (a "65-bit" integer if you count
+ * the sign as a bit) which is possible because CBOR happens to
+ * support such integers.
+ *
+ * The actual value encoded is -uNum - 1. That is, give 0 for uNum to
+ * transmit -1, give 1 to transmit -2 and give UINT64_MAX to transmit
+ * -UINT64_MAX-1 (18446744073709551616). The interface is odd like
+ * this so all negative values CBOR can represent can be encoded by
+ * QCBOR (making this a complete CBOR implementation).
+ *
+ * The most negative value QCBOREncode_AddInt64() can encode is
+ * -9223372036854775808 which is -2^63 or negative
+ * 0x800000000000.  This can encode from -9223372036854775809 to
+ * -18446744073709551616 or -2^63 - 1 to -2^64. Note that
+ * it is not possible to represent plus or minus 18446744073709551616
+ * in any standard C data type.
+ *
+ * Negative integers are normally decoded in QCBOR with type
+ * @ref QCBOR_TYPE_INT64.  Integers in the range of -9223372036854775809
+ * to -18446744073709551616 are returned as @ref QCBOR_TYPE_65BIT_NEG_INT.
+ *
+ * WARNING: some CBOR decoders will be unable to decode -2^63 - 1 to
+ * -2^64.  Also, most CPUs do not have registers that can represent
+ * this range.  If you need 65-bit negative integers, you likely need
+ * negative 66, 67 and 68-bit negative integers so it is likely better
+ * to use CBOR big numbers where you can have any number of bits. See
+ * QCBOREncode_AddTNegativeBignum() and TODO: also xxxx
+ */
+void
+QCBOREncode_AddNegativeUInt64(QCBOREncodeContext *pCtx, uint64_t uNum);
+
+static void
+QCBOREncode_AddNegativeUInt64ToMap(QCBOREncodeContext *pCtx, const char *szLabel, uint64_t uNum);
+
+static void
+QCBOREncode_AddNegativeUInt64ToMapN(QCBOREncodeContext *pCtx, int64_t nLabel, uint64_t uNum);
+
+/**
  * @brief  Add a UTF-8 text string to the encoded output.
  *
  * @param[in] pCtx   The encoding context to add the text to.
@@ -2458,6 +2503,24 @@
 
 
 static inline void
+QCBOREncode_AddNegativeUInt64ToMap(QCBOREncodeContext *pMe, const char *szLabel, uint64_t uNum)
+{
+   /* Use _AddBuffer() because _AddSZString() is defined below, not above */
+   QCBOREncode_Private_AddBuffer(pMe,
+                                 CBOR_MAJOR_TYPE_TEXT_STRING,
+                                 UsefulBuf_FromSZ(szLabel));
+   QCBOREncode_AddNegativeUInt64(pMe, uNum);
+}
+
+static inline void
+QCBOREncode_AddNegativeUInt64ToMapN(QCBOREncodeContext *pMe, int64_t nLabel, uint64_t uNum)
+{
+   QCBOREncode_AddInt64(pMe, nLabel);
+   QCBOREncode_AddNegativeUInt64(pMe, uNum);
+}
+
+
+static inline void
 QCBOREncode_AddText(QCBOREncodeContext *pMe, const UsefulBufC Text)
 {
    QCBOREncode_Private_AddBuffer(pMe, CBOR_MAJOR_TYPE_TEXT_STRING, Text);