+/**
+ * \brief ChaCha20 implementation (SSE2 code, constant-time).
+ *
+ * This implementation is available only on x86 platforms, depending on
+ * compiler support. Moreover, in 32-bit mode, it might not actually run,
+ * if the underlying hardware does not implement the SSE2 opcode (in
+ * 64-bit mode, SSE2 is part of the ABI, so if the code could be compiled
+ * at all, then it can run). Use `br_chacha20_sse2_get()` to safely obtain
+ * a pointer to that function.
+ *
+ * \see br_chacha20_run
+ *
+ * \param key secret key (32 bytes).
+ * \param iv IV (12 bytes).
+ * \param cc initial counter value.
+ * \param data data to encrypt or decrypt.
+ * \param len data length (in bytes).
+ */
+uint32_t br_chacha20_sse2_run(const void *key,
+ const void *iv, uint32_t cc, void *data, size_t len);
+
+/**
+ * \brief Obtain the `sse2` ChaCha20 implementation, if available.
+ *
+ * This function returns a pointer to `br_chacha20_sse2_run`, if
+ * that implementation was compiled in the library _and_ the SSE2
+ * opcodes are available on the currently running CPU. If either of
+ * these conditions is not met, then this function returns `0`.
+ *
+ * \return the `sse2` ChaCha20 implementation, or `0`.
+ */
+br_chacha20_run br_chacha20_sse2_get(void);
+