Add a function for checking if poly_double_n supports a particular size

3 files changed, 10 insertions, 6 deletions

diff --git a/src/lib/mac/cmac/cmac.cpp b/src/lib/mac/cmac/cmac.cpp
index 18f7c151c..bd2a53a72 100644
--- a/src/lib/mac/cmac/cmac.cpp
+++ b/src/lib/mac/cmac/cmac.cpp
@@ -121,8 +121,7 @@ CMAC::CMAC(BlockCipher* cipher) :
    m_cipher(cipher),
    m_block_size(m_cipher->block_size())
    {
-   if(m_block_size !=  8 && m_block_size != 16 &&
-      m_block_size != 32 && m_block_size != 64)
+   if(poly_double_supported_size(m_block_size) == false)
       {
       throw Invalid_Argument("CMAC cannot use the " +
                              std::to_string(m_block_size * 8) +
diff --git a/src/lib/modes/xts/xts.cpp b/src/lib/modes/xts/xts.cpp
index 496b71c5f..0b10faa92 100644
--- a/src/lib/modes/xts/xts.cpp
+++ b/src/lib/modes/xts/xts.cpp
@@ -13,10 +13,7 @@ namespace Botan {
 
 XTS_Mode::XTS_Mode(BlockCipher* cipher) : m_cipher(cipher)
    {
-   if(m_cipher->block_size() != 8 &&
-      m_cipher->block_size() != 16 &&
-      m_cipher->block_size() != 32 &&
-      m_cipher->block_size() != 64)
+   if(poly_double_supported_size(m_cipher->block_size()) == false)
       {
       throw Invalid_Argument("Cannot use " + cipher->name() + " with XTS");
       }
diff --git a/src/lib/utils/poly_dbl/poly_dbl.h b/src/lib/utils/poly_dbl/poly_dbl.h
index 10c85c567..8f9c0d3aa 100644
--- a/src/lib/utils/poly_dbl/poly_dbl.h
+++ b/src/lib/utils/poly_dbl/poly_dbl.h
@@ -16,6 +16,14 @@ namespace Botan {
 */
 void BOTAN_PUBLIC_API(2,3) poly_double_n(uint8_t out[], const uint8_t in[], size_t n);
 
+/**
+* Returns true iff poly_double_n is implemented for this size.
+*/
+inline bool poly_double_supported_size(size_t n)
+   {
+   return (n == 8 || n == 16 || n == 24 || n == 32 || n == 64);
+   }
+
 inline void poly_double_n(uint8_t buf[], size_t n)
    {
    return poly_double_n(buf, buf, n);