diff --git a/library/constant_time_internal.h b/library/constant_time_internal.h
index e085478..d1e3755 100644
--- a/library/constant_time_internal.h
+++ b/library/constant_time_internal.h
@@ -20,6 +20,9 @@
 #ifndef MBEDTLS_CONSTANT_TIME_INTERNAL_H
 #define MBEDTLS_CONSTANT_TIME_INTERNAL_H
 
+#include <stdint.h>
+#include <stddef.h>
+
 #include "common.h"
 
 #if defined(MBEDTLS_BIGNUM_C)
@@ -30,8 +33,6 @@
 #include "ssl_misc.h"
 #endif
 
-#include <stddef.h>
-
 
 /** Turn a value into a mask:
  * - if \p value == 0, return the all-bits 0 mask, aka 0
@@ -220,33 +221,6 @@
 
 #endif /* MBEDTLS_SSL_SOME_SUITES_USE_MAC */
 
-#if defined(MBEDTLS_BASE64_C)
-
-/** Constant-flow char selection
- *
- * \param low   Bottom of range
- * \param high  Top of range
- * \param c     Value to compare to range
- * \param t     Value to return, if in range
- *
- * \return      \p t if \p low <= \p c <= \p high, 0 otherwise.
- */
-static inline unsigned char mbedtls_ct_uchar_in_range_if(unsigned char low,
-                                                         unsigned char high,
-                                                         unsigned char c,
-                                                         unsigned char t)
-{
-    /* low_mask is: 0 if low <= c, 0x...ff if low > c */
-    unsigned low_mask = ((unsigned) c - low) >> 8;
-    /* high_mask is: 0 if c <= high, 0x...ff if c > high */
-    unsigned high_mask = ((unsigned) high - c) >> 8;
-    return (unsigned char)
-           mbedtls_ct_uint_if(~mbedtls_ct_mpi_uint_mask(low_mask | high_mask), t, 0);
-}
-
-#endif /* MBEDTLS_BASE64_C */
-
-
 #if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
 
 /** Constant-flow "greater than" comparison:
@@ -284,4 +258,447 @@
 
 #endif /* defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT) */
 
+
+
+/* The constant-time interface provides various operations that are likely
+ * to result in constant-time code that does not branch or use conditional
+ * instructions for secret data (for secret pointers, this also applies to
+ * the data pointed to).
+ *
+ * It has three main parts:
+ *
+ * - boolean operations (and a few non-boolean operations)
+ *   These are all named mbedtls_ct_bool_<operation>, and operate over
+ *   mbedtls_ct_condition_t.
+ *   All arguments to these operations are considered secret.
+ *   example: bool x = y | z          =>    x = mbedtls_ct_bool_or(y, z)
+ *
+ * - conditional data selection
+ *   These are all named mbedtls_ct_<type>_if and mbedtls_ct_<type>_if0
+ *   All arguments are considered secret.
+ *   example: size_t a = x ? b : c    =>    a = mbedtls_ct_size_if(x, b, c)
+ *   example: unsigned a = x ? b : 0  =>    a = mbedtls_ct_uint_if0(x, b)
+ *
+ * - block memory operations
+ *   Only some arguments are considered secret, as documented for each
+ *   function.
+ *   example: if (x) memcpy(...)      =>    mbedtls_ct_memcpy_if(x, ...)
+ *
+ * mbedtls_ct_condition_t should be treated as opaque and only manipulated
+ * via the functions in this header.
+ *
+ * mbedtls_ct_uint_t is an unsigned integer type over which constant time
+ * operations may be performed via the functions in this header. It is as big
+ * as the larger of size_t and mbedtls_mpi_uint, i.e. it is safe to cast
+ * to/from "unsigned int", "size_t", and "mbedtls_mpi_uint" (and any other
+ * not-larger integer types).
+ *
+ * For Arm (32-bit, 64-bit and Thumb), assembly implementations are used
+ * to ensure that the generated code is constant time. For other architectures,
+ * a plain C fallback designed to yield constant-time code (this has been
+ * observed to be constant-time on latest gcc, clang and MSVC as of May 2023).
+ */
+
+#if (SIZE_MAX > 0xffffffffffffffffULL)
+/* Pointer size > 64-bit */
+typedef size_t    mbedtls_ct_condition_t;
+typedef size_t    mbedtls_ct_uint_t;
+typedef ptrdiff_t mbedtls_ct_int_t;
+#define MBEDTLS_CT_TRUE  ((mbedtls_ct_condition_t) SIZE_MAX)
+#elif (SIZE_MAX > 0xffffffff) || defined(MBEDTLS_HAVE_INT64)
+/* 32-bit < pointer size < 64-bit, or 64-bit MPI */
+typedef uint64_t  mbedtls_ct_condition_t;
+typedef uint64_t  mbedtls_ct_uint_t;
+typedef int64_t   mbedtls_ct_int_t;
+#define MBEDTLS_CT_TRUE  ((mbedtls_ct_condition_t) UINT64_MAX)
+#else
+/* Pointer size < 32-bit, and no 64-bit MPIs */
+typedef uint32_t  mbedtls_ct_condition_t;
+typedef uint32_t  mbedtls_ct_uint_t;
+typedef int32_t   mbedtls_ct_int_t;
+#define MBEDTLS_CT_TRUE  ((mbedtls_ct_condition_t) UINT32_MAX)
+#endif
+#define MBEDTLS_CT_FALSE ((mbedtls_ct_condition_t) 0)
+
+/* constant_time_impl.h contains all the static inline implementations,
+ * so that constant_time_internal.h is more readable.
+ */
+#include "constant_time_impl.h"
+
+
+/* ============================================================================
+ * Boolean operations
+ */
+
+/** Convert a number into a mbedtls_ct_condition_t.
+ *
+ * \param x Number to convert.
+ *
+ * \return MBEDTLS_CT_TRUE if \p x != 0, or MBEDTLS_CT_FALSE if \p x == 0
+ *
+ */
+static inline mbedtls_ct_condition_t mbedtls_ct_bool(mbedtls_ct_uint_t x);
+
+/** Boolean "not equal" operation.
+ *
+ * Functionally equivalent to:
+ *
+ * \p x != \p y
+ *
+ * \param x     The first value to analyze.
+ * \param y     The second value to analyze.
+ *
+ * \return      MBEDTLS_CT_TRUE if \p x != \p y, otherwise MBEDTLS_CT_FALSE.
+ */
+static inline mbedtls_ct_condition_t mbedtls_ct_bool_ne(mbedtls_ct_uint_t x, mbedtls_ct_uint_t y);
+
+/** Boolean "equals" operation.
+ *
+ * Functionally equivalent to:
+ *
+ * \p x == \p y
+ *
+ * \param x     The first value to analyze.
+ * \param y     The second value to analyze.
+ *
+ * \return      MBEDTLS_CT_TRUE if \p x == \p y, otherwise MBEDTLS_CT_FALSE.
+ */
+static inline mbedtls_ct_condition_t mbedtls_ct_bool_eq(mbedtls_ct_uint_t x,
+                                                        mbedtls_ct_uint_t y);
+
+/** Boolean "less than" operation.
+ *
+ * Functionally equivalent to:
+ *
+ * \p x < \p y
+ *
+ * \param x     The first value to analyze.
+ * \param y     The second value to analyze.
+ *
+ * \return      MBEDTLS_CT_TRUE if \p x < \p y, otherwise MBEDTLS_CT_FALSE.
+ */
+static inline mbedtls_ct_condition_t mbedtls_ct_bool_lt(mbedtls_ct_uint_t x, mbedtls_ct_uint_t y);
+
+/** Boolean "greater than" operation.
+ *
+ * Functionally equivalent to:
+ *
+ * \p x > \p y
+ *
+ * \param x     The first value to analyze.
+ * \param y     The second value to analyze.
+ *
+ * \return      MBEDTLS_CT_TRUE if \p x > \p y, otherwise MBEDTLS_CT_FALSE.
+ */
+static inline mbedtls_ct_condition_t mbedtls_ct_bool_gt(mbedtls_ct_uint_t x,
+                                                        mbedtls_ct_uint_t y);
+
+/** Boolean "greater or equal" operation.
+ *
+ * Functionally equivalent to:
+ *
+ * \p x >= \p y
+ *
+ * \param x     The first value to analyze.
+ * \param y     The second value to analyze.
+ *
+ * \return      MBEDTLS_CT_TRUE if \p x >= \p y,
+ *              otherwise MBEDTLS_CT_FALSE.
+ */
+static inline mbedtls_ct_condition_t mbedtls_ct_bool_ge(mbedtls_ct_uint_t x,
+                                                        mbedtls_ct_uint_t y);
+
+/** Boolean "less than or equal" operation.
+ *
+ * Functionally equivalent to:
+ *
+ * \p x <= \p y
+ *
+ * \param x     The first value to analyze.
+ * \param y     The second value to analyze.
+ *
+ * \return      MBEDTLS_CT_TRUE if \p x <= \p y,
+ *              otherwise MBEDTLS_CT_FALSE.
+ */
+static inline mbedtls_ct_condition_t mbedtls_ct_bool_le(mbedtls_ct_uint_t x,
+                                                        mbedtls_ct_uint_t y);
+
+/** Boolean "xor" operation.
+ *
+ * Functionally equivalent to:
+ *
+ * \p x ^ \p y
+ *
+ * \param x     The first value to analyze.
+ * \param y     The second value to analyze.
+ *
+ * \note        This is more efficient than mbedtls_ct_bool_ne if both arguments are
+ *              mbedtls_ct_condition_t.
+ *
+ * \return      MBEDTLS_CT_TRUE if \p x ^ \p y,
+ *              otherwise MBEDTLS_CT_FALSE.
+ */
+static inline mbedtls_ct_condition_t mbedtls_ct_bool_xor(mbedtls_ct_condition_t x,
+                                                         mbedtls_ct_condition_t y);
+
+/** Boolean "and" operation.
+ *
+ * Functionally equivalent to:
+ *
+ * \p x && \p y
+ *
+ * \param x     The first value to analyze.
+ * \param y     The second value to analyze.
+ *
+ * \return      MBEDTLS_CT_TRUE if \p x && \p y,
+ *              otherwise MBEDTLS_CT_FALSE.
+ */
+static inline mbedtls_ct_condition_t mbedtls_ct_bool_and(mbedtls_ct_condition_t x,
+                                                         mbedtls_ct_condition_t y);
+
+/** Boolean "or" operation.
+ *
+ * Functionally equivalent to:
+ *
+ * \p x || \p y
+ *
+ * \param x     The first value to analyze.
+ * \param y     The second value to analyze.
+ *
+ * \return      MBEDTLS_CT_TRUE if \p x || \p y,
+ *              otherwise MBEDTLS_CT_FALSE.
+ */
+static inline mbedtls_ct_condition_t mbedtls_ct_bool_or(mbedtls_ct_condition_t x,
+                                                        mbedtls_ct_condition_t y);
+
+/** Boolean "not" operation.
+ *
+ * Functionally equivalent to:
+ *
+ * ! \p x
+ *
+ * \param x     The value to invert
+ *
+ * \return      MBEDTLS_CT_FALSE if \p x, otherwise MBEDTLS_CT_TRUE.
+ */
+static inline mbedtls_ct_condition_t mbedtls_ct_bool_not(mbedtls_ct_condition_t x);
+
+
+/* ============================================================================
+ * Data selection operations
+ */
+
+/** Choose between two size_t values.
+ *
+ * Functionally equivalent to:
+ *
+ * condition ? if1 : if0.
+ *
+ * \param condition     Condition to test.
+ * \param if1           Value to use if \p condition == MBEDTLS_CT_TRUE.
+ * \param if0           Value to use if \p condition == MBEDTLS_CT_FALSE.
+ *
+ * \return  \c if1 if \p condition == MBEDTLS_CT_TRUE, otherwise \c if0.
+ */
+static inline size_t mbedtls_ct_size_if(mbedtls_ct_condition_t condition,
+                                        size_t if1,
+                                        size_t if0);
+
+/** Choose between two unsigned values.
+ *
+ * Functionally equivalent to:
+ *
+ * condition ? if1 : if0.
+ *
+ * \param condition     Condition to test.
+ * \param if1           Value to use if \p condition == MBEDTLS_CT_TRUE.
+ * \param if0           Value to use if \p condition == MBEDTLS_CT_FALSE.
+ *
+ * \return  \c if1 if \p condition == MBEDTLS_CT_TRUE, otherwise \c if0.
+ */
+static inline unsigned mbedtls_ct_uint_if_new(mbedtls_ct_condition_t condition,
+                                          unsigned if1,
+                                          unsigned if0);
+
+#if defined(MBEDTLS_BIGNUM_C)
+
+/** Choose between two mbedtls_mpi_uint values.
+ *
+ * Functionally equivalent to:
+ *
+ * condition ? if1 : if0.
+ *
+ * \param condition     Condition to test.
+ * \param if1           Value to use if \p condition == MBEDTLS_CT_TRUE.
+ * \param if0           Value to use if \p condition == MBEDTLS_CT_FALSE.
+ *
+ * \return  \c if1 if \p condition == MBEDTLS_CT_TRUE, otherwise \c if0.
+ */
+static inline mbedtls_mpi_uint mbedtls_ct_mpi_uint_if(mbedtls_ct_condition_t condition, \
+                                                      mbedtls_mpi_uint if1, \
+                                                      mbedtls_mpi_uint if0);
+
+#endif
+
+/** Choose between an unsigned value and 0.
+ *
+ * Functionally equivalent to:
+ *
+ * condition ? if1 : 0.
+ *
+ * Functionally equivalent tombedtls_ct_uint_if(condition, if1, 0) but
+ * results in smaller code size.
+ *
+ * \param condition     Condition to test.
+ * \param if1           Value to use if \p condition == MBEDTLS_CT_TRUE.
+ *
+ * \return  \c if1 if \p condition == MBEDTLS_CT_TRUE, otherwise 0.
+ */
+static inline unsigned mbedtls_ct_uint_if0(mbedtls_ct_condition_t condition, unsigned if1);
+
+#if defined(MBEDTLS_BIGNUM_C)
+
+/** Choose between an mbedtls_mpi_uint value and 0.
+ *
+ * Functionally equivalent to:
+ *
+ * condition ? if1 : 0.
+ *
+ * Functionally equivalent tombedtls_ct_mpi_uint_if(condition, if1, 0) but
+ * results in smaller code size.
+ *
+ * \param condition     Condition to test.
+ * \param if1           Value to use if \p condition == MBEDTLS_CT_TRUE.
+ *
+ * \return  \c if1 if \p condition == MBEDTLS_CT_TRUE, otherwise 0.
+ */
+static inline mbedtls_mpi_uint mbedtls_ct_mpi_uint_if0(mbedtls_ct_condition_t condition,
+                                                       mbedtls_mpi_uint if1);
+
+#endif
+
+/** Constant-flow char selection
+ *
+ * \param low   Secret. Bottom of range
+ * \param high  Secret. Top of range
+ * \param c     Secret. Value to compare to range
+ * \param t     Secret. Value to return, if in range
+ *
+ * \return      \p t if \p low <= \p c <= \p high, 0 otherwise.
+ */
+static inline unsigned char mbedtls_ct_uchar_in_range_if(unsigned char low,
+                                                         unsigned char high,
+                                                         unsigned char c,
+                                                         unsigned char t);
+
+
+/* ============================================================================
+ * Block memory operations
+ */
+
+#if defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT)
+
+/** Conditionally set a block of memory to zero.
+ *
+ * Regardless of the condition, every byte will be read once and written to
+ * once.
+ *
+ * \param condition     Secret. Condition to test.
+ * \param buf           Secret. Pointer to the start of the buffer.
+ * \param len           Number of bytes to set to zero.
+ *
+ * \warning Unlike mbedtls_platform_zeroize, this does not have the same guarantees
+ * about not being optimised away if the memory is never read again.
+ */
+void mbedtls_ct_zeroize_if(mbedtls_ct_condition_t condition, void *buf, size_t len);
+
+/** Shift some data towards the left inside a buffer.
+ *
+ * Functionally equivalent to:
+ *
+ * memmove(start, start + offset, total - offset);
+ * memset(start + (total - offset), 0, offset);
+ *
+ * Timing independence comes at the expense of performance.
+ *
+ * \param start     Secret. Pointer to the start of the buffer.
+ * \param total     Total size of the buffer.
+ * \param offset    Secret. Offset from which to copy \p total - \p offset bytes.
+ */
+void mbedtls_ct_memmove_left(void *start,
+                             size_t total,
+                             size_t offset);
+
+#endif /* defined(MBEDTLS_PKCS1_V15) && defined(MBEDTLS_RSA_C) && !defined(MBEDTLS_RSA_ALT) */
+
+/** Conditional memcpy.
+ *
+ * Functionally equivalent to:
+ *
+ * if (condition) {
+ *      memcpy(dest, src1, len);
+ * } else {
+ *      if (src2 != NULL)
+ *          memcpy(dest, src2, len);
+ * }
+ *
+ * It will always read len bytes from src1.
+ * If src2 != NULL, it will always read len bytes from src2.
+ * If src2 == NULL, it will instead read len bytes from dest (as if src2 == dest).
+ *
+ * \param condition The condition
+ * \param dest      Secret. Destination pointer.
+ * \param src1      Secret. Pointer to copy from (if \p condition == MBEDTLS_CT_TRUE). Shouldn't overlap with \p dest.
+ * \param src2      Secret (contents only - may branch to test if src2 == NULL).
+ *                  Pointer to copy from (if \p condition == MBEDTLS_CT_FALSE and \p src2 is not NULL). Shouldn't overlap with \p dest. May be NULL.
+ * \param len       Number of bytes to copy.
+ */
+void mbedtls_ct_memcpy_if(mbedtls_ct_condition_t condition,
+                          unsigned char *dest,
+                          const unsigned char *src1,
+                          const unsigned char *src2,
+                          size_t len
+                          );
+
+/** Copy data from a secret position.
+ *
+ * Functionally equivalent to:
+ *
+ * memcpy(dst, src + offset, len)
+ *
+ * This function copies \p len bytes from \p src_base + \p offset to \p
+ * dst, with a code flow and memory access pattern that does not depend on
+ * \p offset, but only on \p offset_min, \p offset_max and \p len.
+ *
+ * \note                This function reads from \p dest, but the value that
+ *                      is read does not influence the result and this
+ *                      function's behavior is well-defined regardless of the
+ *                      contents of the buffers. This may result in false
+ *                      positives from static or dynamic analyzers, especially
+ *                      if \p dest is not initialized.
+ *
+ * \param dest          Secret. The destination buffer. This must point to a writable
+ *                      buffer of at least \p len bytes.
+ * \param src           Secret. The base of the source buffer. This must point to a
+ *                      readable buffer of at least \p offset_max + \p len
+ *                      bytes. Shouldn't overlap with \p dest.
+ * \param offset        Secret. The offset in the source buffer from which to copy.
+ *                      This must be no less than \p offset_min and no greater
+ *                      than \p offset_max.
+ * \param offset_min    The minimal value of \p offset.
+ * \param offset_max    The maximal value of \p offset.
+ * \param len           The number of bytes to copy.
+ */
+void mbedtls_ct_memcpy_offset(unsigned char *dest,
+                              const unsigned char *src,
+                              size_t offset,
+                              size_t offset_min,
+                              size_t offset_max,
+                              size_t len);
+
+/* Documented in include/mbedtls/constant_time.h. a and b are secret. */
+int mbedtls_ct_memcmp(const void *a,
+                      const void *b,
+                      size_t n);
+
 #endif /* MBEDTLS_CONSTANT_TIME_INTERNAL_H */