From c1d9742df3cf20c3b235ecdd4f4d7a6f71037f1d Mon Sep 17 00:00:00 2001
From: Karen Arutyunov <karen@codesynthesis.com>
Date: Wed, 20 Apr 2016 13:25:18 +0300
Subject: Add sha256_to_fingerprint (), fingerprint_to_sha256 ()

---
 NEWS                    |  2 ++
 butl/sha256             | 15 ++++++++++++
 butl/sha256.cxx         | 64 +++++++++++++++++++++++++++++++++++++++++++++++++
 tests/sha256/driver.cxx |  9 +++++++
 4 files changed, 90 insertions(+)

diff --git a/NEWS b/NEWS
index 2c5645d..bce5db3 100644
--- a/NEWS
+++ b/NEWS
@@ -15,6 +15,8 @@ Version 0.3.0
 
   * Implement timestamp from_string().
 
+  * Implement sha256_to_fingerprint (), fingerprint_to_sha256 ().
+
 Version 0.2.0
 
   * First public release.
diff --git a/butl/sha256 b/butl/sha256
index d583701..a1d0c38 100644
--- a/butl/sha256
+++ b/butl/sha256
@@ -79,6 +79,21 @@ namespace butl
     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.
+  //
+  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.
+  //
+  std::string
+  fingerprint_to_sha256 (const std::string&);
 };
 
 #endif // BUTL_SHA256
diff --git a/butl/sha256.cxx b/butl/sha256.cxx
index feca9a7..7c0b21e 100644
--- a/butl/sha256.cxx
+++ b/butl/sha256.cxx
@@ -9,6 +9,9 @@
 #include <stdint.h>
 #include <stddef.h> // size_t
 
+#include <cctype>    // isxdigit(), toupper(), tolower()
+#include <stdexcept> // invalid_argument
+
 using SHA256_CTX = butl::sha256::context;
 
 extern "C"
@@ -24,6 +27,8 @@ using namespace std;
 
 namespace butl
 {
+  // sha256
+  //
   sha256::
   sha256 ()
       : done_ (false)
@@ -73,4 +78,63 @@ namespace butl
 
     return str_;
   }
+
+  // Conversion functions
+  //
+  string
+  sha256_to_fingerprint (const string& s)
+  {
+    auto bad = []() {throw invalid_argument ("invalid SHA256 string");};
+
+    size_t n (s.size ());
+    if (n != 64)
+      bad ();
+
+    string f;
+    f.reserve (n + 31);
+    for (size_t i (0); i < n; ++i)
+    {
+      char c (s[i]);
+      if (!isxdigit (c))
+        bad ();
+
+      if (i > 0 && i % 2 == 0)
+        f += ":";
+
+      f += toupper (c);
+    }
+
+    return f;
+  }
+
+  string
+  fingerprint_to_sha256 (const string& f)
+  {
+    auto bad = []() {throw invalid_argument ("invalid fingerprint");};
+
+    size_t n (f.size ());
+    if (n != 32 * 3 - 1)
+      bad ();
+
+    string s;
+    s.reserve (64);
+    for (size_t i (0); i < n; ++i)
+    {
+      char c (f[i]);
+      if ((i + 1) % 3 == 0)
+      {
+        if (c != ':')
+          bad ();
+      }
+      else
+      {
+        if (!isxdigit (c))
+          bad ();
+
+        s += tolower (c);
+      }
+    }
+
+    return s;
+  }
 }
diff --git a/tests/sha256/driver.cxx b/tests/sha256/driver.cxx
index 8c2e525..ae2bd4a 100644
--- a/tests/sha256/driver.cxx
+++ b/tests/sha256/driver.cxx
@@ -37,4 +37,13 @@ main ()
   string s (h.string ());
   assert (s ==
           "204d9db65789fbede7829ed77f72ba1f0fe21a833d95abad4849b82f33a69b9d");
+
+  string fp ("F4:9D:C0:02:C6:B6:62:06:A5:48:AE:87:35:32:95:64:C2:B8:C9:6D:9B:"
+             "28:85:6D:EF:CA:FA:7F:04:B5:4F:A6");
+
+  string sh (
+    "f49dc002c6b66206a548ae8735329564c2b8c96d9b28856defcafa7f04b54fa6");
+
+  assert (fingerprint_to_sha256 (fp) == sh);
+  assert (sha256_to_fingerprint (sh) == fp);
 }
-- 
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