Add (back) OpenPGP-S2K

It was removed somewhere along the line in 1.11, with the logic
that it is a funky PGP-specific scheme and (quoting the commit
that removed it) "not really useful outside of a full PGP implementation".

This assumed that the PGP implementation would be in Botan itself, but
PGP is implemented in https://github.com/evpo/EncryptPad/ (which is
a PGP implementation using 1.10), and RNP (https://github.com/riboseinc/rnp)
would like to use it also.

This work was sponsored by Ribose Inc (@riboseinc).

4 files changed, 176 insertions, 0 deletions

diff --git a/src/lib/pbkdf/pbkdf.cpp b/src/lib/pbkdf/pbkdf.cpp
index a60c93d5c..40610998b 100644
--- a/src/lib/pbkdf/pbkdf.cpp
+++ b/src/lib/pbkdf/pbkdf.cpp
@@ -16,6 +16,10 @@
 #include <botan/pbkdf2.h>
 #endif
 
+#if defined(BOTAN_HAS_PGP_S2K)
+#include <botan/pgp_s2k.h>
+#endif
+
 namespace Botan {
 
 std::unique_ptr<PBKDF> PBKDF::create(const std::string& algo_spec,
@@ -50,6 +54,14 @@ std::unique_ptr<PBKDF> PBKDF::create(const std::string& algo_spec,
       }
 #endif
 
+#if defined(BOTAN_HAS_PGP_S2K)
+   if(req.algo_name() == "OpenPGP-S2K" && req.arg_count() == 1)
+      {
+      if(auto hash = HashFunction::create(req.arg(0)))
+         return std::unique_ptr<PBKDF>(new OpenPGP_S2K(hash.release()));
+      }
+#endif
+
    BOTAN_UNUSED(req);
    BOTAN_UNUSED(provider);
 
diff --git a/src/lib/pbkdf/pgp_s2k/info.txt b/src/lib/pbkdf/pgp_s2k/info.txt
new file mode 100644
index 000000000..5aabc0919
--- /dev/null
+++ b/src/lib/pbkdf/pgp_s2k/info.txt
@@ -0,0 +1,7 @@
+<defines>
+PGP_S2K -> 20170527
+</defines>
+
+<requires>
+hash
+</requires>
diff --git a/src/lib/pbkdf/pgp_s2k/pgp_s2k.cpp b/src/lib/pbkdf/pgp_s2k/pgp_s2k.cpp
new file mode 100644
index 000000000..d8b5d770e
--- /dev/null
+++ b/src/lib/pbkdf/pgp_s2k/pgp_s2k.cpp
@@ -0,0 +1,90 @@
+/*
+* OpenPGP S2K
+* (C) 1999-2007,2017 Jack Lloyd
+*
+* Distributed under the terms of the Botan license
+*/
+
+#include <botan/pgp_s2k.h>
+
+namespace Botan {
+
+//static
+uint8_t OpenPGP_S2K::encode_count(size_t desired_iterations)
+   {
+   /*
+   Only 256 different iterations are actually representable in OpenPGP format ...
+   */
+   for(size_t c = 0; c < 256; ++c)
+      {
+      size_t decoded_iter = OpenPGP_S2K::decode_count(c);
+      if(decoded_iter >= desired_iterations)
+         return c;
+      }
+   }
+
+//static
+size_t OpenPGP_S2K::decode_count(uint8_t iter)
+   {
+   // See RFC 4880 section 3.7.1.3
+   return (16 + (iter & 0x0F)) << ((iter >> 4) + 6);
+   }
+
+size_t OpenPGP_S2K::pbkdf(uint8_t output_buf[], size_t output_len,
+                          const std::string& passphrase,
+                          const uint8_t salt[], size_t salt_len,
+                          size_t iterations,
+                          std::chrono::milliseconds msec) const
+   {
+   if(iterations == 0 && msec.count() > 0) // FIXME
+      throw Not_Implemented("OpenPGP_S2K does not implemented timed KDF");
+
+   secure_vector<uint8_t> input_buf(salt_len + passphrase.size());
+   if(salt_len > 0)
+      {
+      copy_mem(&input_buf[0], salt, salt_len);
+      }
+   if(passphrase.empty() == false)
+      {
+      copy_mem(&input_buf[salt_len],
+               reinterpret_cast<const uint8_t*>(passphrase.data()),
+               passphrase.size());
+      }
+
+   /*
+   The input is always fully processed even if iterations is very small
+   */
+   const size_t to_hash = std::max(iterations, input_buf.size());
+
+   secure_vector<uint8_t> hash_buf(m_hash->output_length());
+
+   size_t pass = 0;
+   size_t generated = 0;
+
+   while(generated != output_len)
+      {
+      const size_t output_this_pass =
+         std::min(hash_buf.size(), output_len - generated);
+
+      // Preload some number of zero bytes (empty first iteration)
+      std::vector<uint8_t> zero_padding(pass);
+      m_hash->update(zero_padding);
+
+      size_t left = to_hash;
+      while(left > 0)
+         {
+         const size_t input_to_take = std::min(left, input_buf.size());
+         m_hash->update(input_buf.data(), input_to_take);
+         left -= input_to_take;
+         }
+
+      m_hash->final(hash_buf.data());
+      copy_mem(output_buf + generated, hash_buf.data(), output_this_pass);
+      generated += output_this_pass;
+      ++pass;
+      }
+
+   return iterations;
+   }
+
+}
diff --git a/src/lib/pbkdf/pgp_s2k/pgp_s2k.h b/src/lib/pbkdf/pgp_s2k/pgp_s2k.h
new file mode 100644
index 000000000..3d85ba306
--- /dev/null
+++ b/src/lib/pbkdf/pgp_s2k/pgp_s2k.h
@@ -0,0 +1,67 @@
+/*
+* OpenPGP PBKDF
+* (C) 1999-2007,2017 Jack Lloyd
+*
+* Distributed under the terms of the Botan license
+*/
+
+#ifndef BOTAN_OPENPGP_S2K_H__
+#define BOTAN_OPENPGP_S2K_H__
+
+#include <botan/pbkdf.h>
+#include <botan/hash.h>
+
+namespace Botan {
+
+/**
+* OpenPGP's S2K
+*
+* See RFC 4880 sections 3.7.1.1, 3.7.1.2, and 3.7.1.3
+* If the salt is empty and iterations == 1, "simple" S2K is used
+* If the salt is non-empty and iterations == 1, "salted" S2K is used
+* If the salt is non-empty and iterations > 1, "iterated" S2K is used
+*
+* If iterations == 0 and msec.count() > 0, "iterated" S2K is assumed,
+* and the number of iterations performed is returned.
+*
+* Note that unlike PBKDF2, OpenPGP S2K's "iterations" are defined as
+* the number of bytes hashed.
+*/
+class BOTAN_DLL OpenPGP_S2K final : public PBKDF
+   {
+   public:
+      /**
+      * @param hash_in the hash function to use
+      */
+      explicit OpenPGP_S2K(HashFunction* hash) : m_hash(hash) {}
+
+      std::string name() const override
+         {
+         return "OpenPGP-S2K(" + m_hash->name() + ")";
+         }
+
+      PBKDF* clone() const
+         {
+         return new OpenPGP_S2K(m_hash->clone());
+         }
+
+      size_t pbkdf(uint8_t output_buf[], size_t output_len,
+                   const std::string& passphrase,
+                   const uint8_t salt[], size_t salt_len,
+                   size_t iterations,
+                   std::chrono::milliseconds msec) const override;
+
+      /**
+      * RFC 4880 encodes the iteration count to a single-byte value
+      */
+      static uint8_t encode_count(size_t iterations);
+
+      static size_t decode_count(uint8_t encoded_iter);
+
+   private:
+      std::unique_ptr<HashFunction> m_hash;
+   };
+
+}
+
+#endif