+/** \brief PRF implementation for TLS 1.2, with SHA-256.
+ *
+ * This PRF is the one specified by TLS 1.2, when the underlying hash
+ * function is SHA-256.
+ *
+ * \param dst destination buffer.
+ * \param len output length (in bytes).
+ * \param secret secret value (key) for this computation.
+ * \param secret_len length of "secret" (in bytes).
+ * \param label PRF label (zero-terminated ASCII string).
+ * \param seed seed for this computation (usually non-secret).
+ * \param seed_len length of "seed" (in bytes).
+ */