diff --git a/library/cipher.c b/library/cipher.c
index a883438..e9e0b22 100644
--- a/library/cipher.c
+++ b/library/cipher.c
@@ -326,9 +326,9 @@
          * If there is not enough data for a full block, cache it.
          */
         if( ( ctx->operation == MBEDTLS_DECRYPT &&
-                ilen + ctx->unprocessed_len <= block_size ) ||
+                ilen <= block_size - ctx->unprocessed_len ) ||
              ( ctx->operation == MBEDTLS_ENCRYPT &&
-                ilen + ctx->unprocessed_len < block_size ) )
+                ilen < block_size - ctx->unprocessed_len ) )
         {
             memcpy( &( ctx->unprocessed_data[ctx->unprocessed_len] ), input,
                     ilen );
diff --git a/library/ctr_drbg.c b/library/ctr_drbg.c
index 386f8ad..55612c7 100644
--- a/library/ctr_drbg.c
+++ b/library/ctr_drbg.c
@@ -290,7 +290,8 @@
     unsigned char seed[MBEDTLS_CTR_DRBG_MAX_SEED_INPUT];
     size_t seedlen = 0;
 
-    if( ctx->entropy_len + len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT )
+    if( ctx->entropy_len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT ||
+        len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT - ctx->entropy_len )
         return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG );
 
     memset( seed, 0, MBEDTLS_CTR_DRBG_MAX_SEED_INPUT );
diff --git a/library/md2.c b/library/md2.c
index 8976701..95cbcce 100644
--- a/library/md2.c
+++ b/library/md2.c
@@ -158,7 +158,7 @@
 
     while( ilen > 0 )
     {
-        if( ctx->left + ilen > 16 )
+        if( ilen > 16 - ctx->left )
             fill = 16 - ctx->left;
         else
             fill = ilen;