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diff --git a/src/contrib/string.h b/src/contrib/string.h
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--- /dev/null
+++ b/src/contrib/string.h
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+/*  Copyright (C) 2020 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/*!
+ * \brief String manipulations.
+ */
+
+#pragma once
+
+#include <stddef.h>
+#include <stdint.h>
+
+/*!
+ * \brief Create a copy of a binary buffer.
+ *
+ * Like \c strdup, but for binary data.
+ */
+uint8_t *memdup(const uint8_t *data, size_t data_size);
+
+/*!
+ * \brief Format string and take care of allocating memory.
+ *
+ * \note sprintf(3) manual page reference implementation.
+ *
+ * \param fmt Message format.
+ * \return formatted message or NULL.
+ */
+char *sprintf_alloc(const char *fmt, ...);
+
+/*!
+ * \brief Create new string from a concatenation of s1 and s2.
+ *
+ * \param s1 First string.
+ * \param s2 Second string.
+ *
+ * \retval Newly allocated string on success.
+ * \retval NULL on error.
+ */
+char *strcdup(const char *s1, const char *s2);
+
+/*!
+ * \brief Create a copy of a string skipping leading and trailing white spaces.
+ *
+ * \return Newly allocated string, NULL in case of error.
+ */
+char *strstrip(const char *str);
+
+/*!
+ * \brief Convert upper-case letters to lower-case in a string.
+ */
+void strtolower(char *str);
+
+/*!
+ * \brief Compare data in time based on string length.
+ *        This function just checks for (in)equality not for relation
+ *
+ * \param s1 The first address to compare.
+ * \param s2 The second address to compare.
+ * \param n The size of memory to compare.
+ *
+ * \return Non zero on difference and zero if the buffers are identical.
+ */
+int const_time_memcmp(const void *s1, const void *s2, size_t n);
+
+/*!
+ * \brief Fill memory with zeroes.
+ *
+ * Inspired by OPENSSL_cleanse. Such a memset shouldn't be optimized out.
+ *
+ * \param s The address to fill.
+ * \param n The size of memory to fill.
+ *
+ * \return Pointer to the memory.
+ */
+void *memzero(void *s, size_t n);
+
+/*!
+ * \brief Convert binary data to hexadecimal string.
+ */
+char *bin_to_hex(const uint8_t *bin, size_t bin_len);
+
+/*!
+ * \brief Convert hex encoded string to binary data.
+ */
+uint8_t *hex_to_bin(const char *hex, size_t *out_len);