Remove support for 8 or 16 bit BigInt words

It turned out 8 bit was very broken (failed to compile, due to
overload problems with functions taking uint8_t vs word).
16 bit words work aside from a test failure, but is really slow.

Practically speaking we are not in a position to support 16-bit CPUs
very well. And being able to assume sizeof(word) >= sizeof(uint32_t)
allows simplifying some code.

7 files changed, 52 insertions, 29 deletions

diff --git a/src/lib/ffi/ffi.h b/src/lib/ffi/ffi.h
index 0b61693ce..58d446d55 100644
--- a/src/lib/ffi/ffi.h
+++ b/src/lib/ffi/ffi.h
@@ -646,6 +646,9 @@ BOTAN_PUBLIC_API(2,1) int botan_mp_is_zero(const botan_mp_t mp);
 BOTAN_PUBLIC_API(2,1) int botan_mp_is_odd(const botan_mp_t mp);
 BOTAN_PUBLIC_API(2,1) int botan_mp_is_even(const botan_mp_t mp);
 
+BOTAN_PUBLIC_API(2,8) int botan_mp_add_u32(botan_mp_t result, const botan_mp_t x, uint32_t y);
+BOTAN_PUBLIC_API(2,8) int botan_mp_sub_u32(botan_mp_t result, const botan_mp_t x, uint32_t y);
+
 BOTAN_PUBLIC_API(2,1) int botan_mp_add(botan_mp_t result, const botan_mp_t x, const botan_mp_t y);
 BOTAN_PUBLIC_API(2,1) int botan_mp_sub(botan_mp_t result, const botan_mp_t x, const botan_mp_t y);
 BOTAN_PUBLIC_API(2,1) int botan_mp_mul(botan_mp_t result, const botan_mp_t x, const botan_mp_t y);
diff --git a/src/lib/ffi/ffi_mp.cpp b/src/lib/ffi/ffi_mp.cpp
index c7854ce9a..38a83e91b 100644
--- a/src/lib/ffi/ffi_mp.cpp
+++ b/src/lib/ffi/ffi_mp.cpp
@@ -138,17 +138,52 @@ int botan_mp_destroy(botan_mp_t mp)
 
 int botan_mp_add(botan_mp_t result, const botan_mp_t x, const botan_mp_t y)
    {
-   return BOTAN_FFI_DO(Botan::BigInt, result, res, { res = safe_get(x) + safe_get(y); });
+   return BOTAN_FFI_DO(Botan::BigInt, result, res, {
+      if(result == x)
+         res += safe_get(y);
+      else
+         res = safe_get(x) + safe_get(y);
+      });
    }
 
 int botan_mp_sub(botan_mp_t result, const botan_mp_t x, const botan_mp_t y)
    {
-   return BOTAN_FFI_DO(Botan::BigInt, result, res, { res = safe_get(x) - safe_get(y); });
+   return BOTAN_FFI_DO(Botan::BigInt, result, res, {
+      if(result == x)
+         res -= safe_get(y);
+      else
+         res = safe_get(x) - safe_get(y);
+      });
+   }
+
+int botan_mp_add_u32(botan_mp_t result, const botan_mp_t x, uint32_t y)
+   {
+   return BOTAN_FFI_DO(Botan::BigInt, result, res, {
+      if(result == x)
+         res += static_cast<Botan::word>(y);
+      else
+         res = safe_get(x) + static_cast<Botan::word>(y);
+      });
+   }
+
+int botan_mp_sub_u32(botan_mp_t result, const botan_mp_t x, uint32_t y)
+   {
+   return BOTAN_FFI_DO(Botan::BigInt, result, res, {
+      if(result == x)
+         res -= static_cast<Botan::word>(y);
+      else
+         res = safe_get(x) - static_cast<Botan::word>(y);
+      });
    }
 
 int botan_mp_mul(botan_mp_t result, const botan_mp_t x, const botan_mp_t y)
    {
-   return BOTAN_FFI_DO(Botan::BigInt, result, res, { res = safe_get(x) * safe_get(y); });
+   return BOTAN_FFI_DO(Botan::BigInt, result, res, {
+      if(result == x)
+         res *= safe_get(y);
+      else
+         res = safe_get(x) * safe_get(y);
+      });
    }
 
 int botan_mp_div(botan_mp_t quotient,
diff --git a/src/lib/math/mp/mp_madd.h b/src/lib/math/mp/mp_madd.h
index 4807fcd04..4f34efe39 100644
--- a/src/lib/math/mp/mp_madd.h
+++ b/src/lib/math/mp/mp_madd.h
@@ -14,15 +14,10 @@
 
 namespace Botan {
 
-#if (BOTAN_MP_WORD_BITS == 8)
-  typedef uint16_t dword;
-  #define BOTAN_HAS_MP_DWORD
-#elif (BOTAN_MP_WORD_BITS == 16)
-  typedef uint32_t dword;
-  #define BOTAN_HAS_MP_DWORD
-#elif (BOTAN_MP_WORD_BITS == 32)
+#if (BOTAN_MP_WORD_BITS == 32)
   typedef uint64_t dword;
   #define BOTAN_HAS_MP_DWORD
+
 #elif (BOTAN_MP_WORD_BITS == 64)
   #if defined(BOTAN_TARGET_HAS_NATIVE_UINT128)
     typedef uint128_t dword;
@@ -32,7 +27,7 @@ namespace Botan {
   #endif
 
 #else
-  #error BOTAN_MP_WORD_BITS must be 8, 16, 32, or 64
+  #error BOTAN_MP_WORD_BITS must be 32 or 64
 #endif
 
 #if defined(BOTAN_TARGET_ARCH_IS_X86_32) && (BOTAN_MP_WORD_BITS == 32)
diff --git a/src/lib/math/numbertheory/curve_nistp.h b/src/lib/math/numbertheory/curve_nistp.h
index c9936a338..710b06dec 100644
--- a/src/lib/math/numbertheory/curve_nistp.h
+++ b/src/lib/math/numbertheory/curve_nistp.h
@@ -23,8 +23,6 @@ namespace Botan {
 BOTAN_PUBLIC_API(2,0) const BigInt& prime_p521();
 BOTAN_PUBLIC_API(2,0) void redc_p521(BigInt& x, secure_vector<word>& ws);
 
-#if (BOTAN_MP_WORD_BITS == 32) || (BOTAN_MP_WORD_BITS == 64)
-
 #define BOTAN_HAS_NIST_PRIME_REDUCERS_W32
 
 BOTAN_PUBLIC_API(2,0) const BigInt& prime_p384();
@@ -39,8 +37,6 @@ BOTAN_PUBLIC_API(2,0) void redc_p224(BigInt& x, secure_vector<word>& ws);
 BOTAN_PUBLIC_API(2,0) const BigInt& prime_p192();
 BOTAN_PUBLIC_API(2,0) void redc_p192(BigInt& x, secure_vector<word>& ws);
 
-#endif
-
 }
 
 #endif
diff --git a/src/lib/math/numbertheory/nistp_redc.cpp b/src/lib/math/numbertheory/nistp_redc.cpp
index b74a2f9c6..1c2855784 100644
--- a/src/lib/math/numbertheory/nistp_redc.cpp
+++ b/src/lib/math/numbertheory/nistp_redc.cpp
@@ -91,10 +91,8 @@ inline uint32_t get_uint32_t(const BigInt& x, size_t i)
    {
 #if (BOTAN_MP_WORD_BITS == 32)
    return x.word_at(i);
-#elif (BOTAN_MP_WORD_BITS == 64)
-   return static_cast<uint32_t>(x.word_at(i/2) >> ((i % 2)*32));
 #else
-  #error "Not implemented"
+   return static_cast<uint32_t>(x.word_at(i/2) >> ((i % 2)*32));
 #endif
    }
 
@@ -103,10 +101,8 @@ inline void set_words(BigInt& x, size_t i, uint32_t R0, uint32_t R1)
 #if (BOTAN_MP_WORD_BITS == 32)
    x.set_word_at(i, R0);
    x.set_word_at(i+1, R1);
-#elif (BOTAN_MP_WORD_BITS == 64)
-   x.set_word_at(i/2, (static_cast<uint64_t>(R1) << 32) | R0);
 #else
-  #error "Not implemented"
+   x.set_word_at(i/2, (static_cast<uint64_t>(R1) << 32) | R0);
 #endif
    }
 
diff --git a/src/lib/utils/types.h b/src/lib/utils/types.h
index e25682a2f..476701a1f 100644
--- a/src/lib/utils/types.h
+++ b/src/lib/utils/types.h
@@ -92,16 +92,12 @@ using u32bit = std::uint32_t;
 using u64bit = std::uint64_t;
 using s32bit = std::int32_t;
 
-#if (BOTAN_MP_WORD_BITS == 8)
-  typedef uint8_t word;
-#elif (BOTAN_MP_WORD_BITS == 16)
-  typedef uint16_t word;
-#elif (BOTAN_MP_WORD_BITS == 32)
+#if (BOTAN_MP_WORD_BITS == 32)
   typedef uint32_t word;
 #elif (BOTAN_MP_WORD_BITS == 64)
   typedef uint64_t word;
 #else
-  #error BOTAN_MP_WORD_BITS must be 8, 16, 32, or 64
+  #error BOTAN_MP_WORD_BITS must be 32 or 64
 #endif
 
 }
diff --git a/src/tests/test_ffi.cpp b/src/tests/test_ffi.cpp
index eb0eacdf4..680e70f8b 100644
--- a/src/tests/test_ffi.cpp
+++ b/src/tests/test_ffi.cpp
@@ -1121,7 +1121,7 @@ class FFI_Unit_Tests final : public Test
          TEST_FFI_OK(botan_mp_num_bytes, (x, &bn_bytes));
          result.test_eq("Expected size for MP 5", bn_bytes, 1);
 
-         botan_mp_set_from_int(x, 80);
+         botan_mp_add_u32(x, x, 75);
          TEST_FFI_OK(botan_mp_num_bytes, (x, &bn_bytes));
          result.test_eq("Expected size for MP 80", bn_bytes, 1);
 
@@ -1129,7 +1129,9 @@ class FFI_Unit_Tests final : public Test
          TEST_FFI_OK(botan_mp_to_str, (x, 10, str_buf, &str_len));
          result.test_eq("botan_mp_add", std::string(str_buf), "80");
 
-         botan_mp_set_from_int(x, 259);
+         botan_mp_sub_u32(x, x, 80);
+         TEST_FFI_RC(1, botan_mp_is_zero, (x));
+         botan_mp_add_u32(x, x, 259);
          TEST_FFI_OK(botan_mp_num_bytes, (x, &bn_bytes));
          result.test_eq("Expected size for MP 259", bn_bytes, 2);