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diff --git a/libbutl/sha256.hxx b/libbutl/sha256.hxx
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+++ b/libbutl/sha256.hxx
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+// file      : libbutl/sha256.hxx -*- C++ -*-
+// copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+#ifndef LIBBUTL_SHA256_HXX
+#define LIBBUTL_SHA256_HXX
+
+#include <string>
+#include <cstring> // strlen()
+#include <cstdint>
+#include <cstddef> // size_t
+
+#include <libbutl/export.hxx>
+
+namespace butl
+{
+  // SHA256 checksum calculator.
+  //
+  // For a single chunk of data a sum can be obtained in one line, for
+  // example:
+  //
+  // cerr << sha256 ("123").string () << endl;
+  //
+  class LIBBUTL_EXPORT sha256
+  {
+  public:
+    sha256 ();
+
+    // Append binary data.
+    //
+    void
+    append (const void*, std::size_t);
+
+    sha256 (const void* b, std::size_t n): sha256 () {append (b, n);}
+
+    // Append string.
+    //
+    // Note that the hash includes the '\0' terminator. Failed that, a call
+    // with an empty string will be indistinguishable from no call at all.
+    //
+    void
+    append (const std::string& s) {append (s.c_str (), s.size () + 1);}
+
+    void
+    append (const char* s) {append (s, std::strlen (s) + 1);}
+
+    explicit
+    sha256 (const std::string& s): sha256 () {append (s);}
+
+    explicit
+    sha256 (const char* s): sha256 () {append (s);}
+
+    // Extract result.
+    //
+    // It can be obtained as either a 32-byte binary digest or as a 64-
+    // character hex-encoded C-string.
+    //
+    using digest_type = std::uint8_t[32];
+
+    const digest_type&
+    binary () const;
+
+    const char*
+    string () const;
+
+  public:
+    struct context
+    {
+      std::uint32_t state[8];
+      std::uint64_t count;
+      std::uint8_t buf[64];
+    };
+
+  private:
+    union
+    {
+      mutable context ctx_;
+      mutable char str_[65];
+    };
+
+    mutable digest_type bin_;
+    mutable bool done_;
+  };
+
+  // Convert a SHA256 string representation (64 hex digits) to the fingerprint
+  // canonical representation (32 colon-separated upper case hex digit pairs,
+  // like 01:AB:CD:...). Throw invalid_argument if the argument is not a valid
+  // SHA256 string.
+  //
+  LIBBUTL_EXPORT std::string
+  sha256_to_fingerprint (const std::string&);
+
+  // Convert a fingerprint (32 colon-separated hex digit pairs) to the SHA256
+  // string representation (64 lower case hex digits). Throw invalid_argument
+  // if the argument is not a valid fingerprint.
+  //
+  LIBBUTL_EXPORT std::string
+  fingerprint_to_sha256 (const std::string&);
+}
+
+#endif // LIBBUTL_SHA256_HXX