diff --git a/gcm.cpp b/gcm.cpp
index 6d86bac1..6d389a1b 100644
--- a/gcm.cpp
+++ b/gcm.cpp
@@ -325,7 +325,13 @@ void GCM_Base::SetKeyWithoutResync(const byte *userKey, size_t keylength, const
     BlockCipher &blockCipher = AccessBlockCipher();
     blockCipher.SetKey(userKey, keylength, params);
 
-    if (blockCipher.BlockSize() != REQUIRED_BLOCKSIZE)
+    // GCM is only defined for 16-byte block ciphers at the moment.
+    // However, variable blocksize support means we have to defer
+    // blocksize checks to runtime after the key is set. Also see
+    // https://github.com/weidai11/cryptopp/issues/408.
+    const unsigned int blockSize = blockCipher.BlockSize();
+    CRYPTOPP_ASSERT(blockSize == REQUIRED_BLOCKSIZE);
+    if (blockSize != REQUIRED_BLOCKSIZE)
         throw InvalidArgument(AlgorithmName() + ": block size of underlying block cipher is not 16");
 
     int tableSize, i, j, k;
@@ -335,7 +341,7 @@ void GCM_Base::SetKeyWithoutResync(const byte *userKey, size_t keylength, const
     {
         // Avoid "parameter not used" error and suppress Coverity finding
         (void)params.GetIntValue(Name::TableSize(), tableSize);
-        tableSize = s_clmulTableSizeInBlocks * REQUIRED_BLOCKSIZE;
+        tableSize = s_clmulTableSizeInBlocks * blockSize;
     }
     else
 #elif CRYPTOPP_BOOL_ARM_PMULL_AVAILABLE
@@ -343,7 +349,7 @@ void GCM_Base::SetKeyWithoutResync(const byte *userKey, size_t keylength, const
     {
         // Avoid "parameter not used" error and suppress Coverity finding
         (void)params.GetIntValue(Name::TableSize(), tableSize);
-        tableSize = s_clmulTableSizeInBlocks * REQUIRED_BLOCKSIZE;
+        tableSize = s_clmulTableSizeInBlocks * blockSize;
     }
     else
 #endif
@@ -359,10 +365,10 @@ void GCM_Base::SetKeyWithoutResync(const byte *userKey, size_t keylength, const
 #endif
     }
 
-    m_buffer.resize(3*REQUIRED_BLOCKSIZE + tableSize);
+    m_buffer.resize(3*blockSize + tableSize);
     byte *table = MulTable();
     byte *hashKey = HashKey();
-    memset(hashKey, 0, REQUIRED_BLOCKSIZE);
+    memset(hashKey, 0, blockSize);
     blockCipher.ProcessBlock(hashKey);
 
 #if CRYPTOPP_BOOL_AESNI_INTRINSICS_AVAILABLE
@@ -537,6 +543,13 @@ void GCM_Base::Resync(const byte *iv, size_t len)
     BlockCipher &cipher = AccessBlockCipher();
     byte *hashBuffer = HashBuffer();
 
+    // GCM is only defined for 16-byte block ciphers at the moment.
+    // However, variable blocksize support means we have to defer
+    // blocksize checks to runtime after the key is set. Also see
+    // https://github.com/weidai11/cryptopp/issues/408.
+    const unsigned int blockSize = cipher.BlockSize();
+    CRYPTOPP_ASSERT(blockSize == REQUIRED_BLOCKSIZE);
+
     if (len == 12)
     {
         memcpy(hashBuffer, iv, len);
@@ -568,7 +581,7 @@ void GCM_Base::Resync(const byte *iv, size_t len)
     }
 
     if (m_state >= State_IVSet)
-        m_ctr.Resynchronize(hashBuffer, REQUIRED_BLOCKSIZE);
+        m_ctr.Resynchronize(hashBuffer, blockSize);
     else
         m_ctr.SetCipherWithIV(cipher, hashBuffer);
 
diff --git a/gcm.h b/gcm.h
index de5fe8b5..478b95b1 100644
--- a/gcm.h
+++ b/gcm.h
@@ -74,9 +74,9 @@ protected:
 	virtual GCM_TablesOption GetTablesOption() const =0;
 
 	const BlockCipher & GetBlockCipher() const {return const_cast<GCM_Base *>(this)->AccessBlockCipher();};
-	byte *HashBuffer() {return m_buffer+REQUIRED_BLOCKSIZE;}
-	byte *HashKey() {return m_buffer+2*REQUIRED_BLOCKSIZE;}
-	byte *MulTable() {return m_buffer+3*REQUIRED_BLOCKSIZE;}
+	byte *HashBuffer() {return m_buffer+GetBlockCipher().BlockSize();}
+	byte *HashKey() {return m_buffer+2*GetBlockCipher().BlockSize();}
+	byte *MulTable() {return m_buffer+3*GetBlockCipher().BlockSize();}
 	inline void ReverseHashBufferIfNeeded();
 
 	class CRYPTOPP_DLL GCTR : public CTR_Mode_ExternalCipher::Encryption