diff --git a/library/pem.c b/library/pem.c
index ac83116..b2c16c2 100644
--- a/library/pem.c
+++ b/library/pem.c
@@ -250,7 +250,7 @@
 
     enc = 0;
 
-    if( memcmp( s1, "Proc-Type: 4,ENCRYPTED", 22 ) == 0 )
+    if( s2 - s1 >= 22 && memcmp( s1, "Proc-Type: 4,ENCRYPTED", 22 ) == 0 )
     {
 #if defined(POLARSSL_MD5_C) && defined(POLARSSL_CIPHER_MODE_CBC) &&         \
     ( defined(POLARSSL_DES_C) || defined(POLARSSL_AES_C) )
@@ -263,22 +263,22 @@
 
 
 #if defined(POLARSSL_DES_C)
-        if( memcmp( s1, "DEK-Info: DES-EDE3-CBC,", 23 ) == 0 )
+        if( s2 - s1 >= 23 && memcmp( s1, "DEK-Info: DES-EDE3-CBC,", 23 ) == 0 )
         {
             enc_alg = POLARSSL_CIPHER_DES_EDE3_CBC;
 
             s1 += 23;
-            if( pem_get_iv( s1, pem_iv, 8 ) != 0 )
+            if( s2 - s1 < 16 || pem_get_iv( s1, pem_iv, 8 ) != 0 )
                 return( POLARSSL_ERR_PEM_INVALID_ENC_IV );
 
             s1 += 16;
         }
-        else if( memcmp( s1, "DEK-Info: DES-CBC,", 18 ) == 0 )
+        else if( s2 - s1 >= 18 && memcmp( s1, "DEK-Info: DES-CBC,", 18 ) == 0 )
         {
             enc_alg = POLARSSL_CIPHER_DES_CBC;
 
             s1 += 18;
-            if( pem_get_iv( s1, pem_iv, 8) != 0 )
+            if( s2 - s1 < 16 || pem_get_iv( s1, pem_iv, 8) != 0 )
                 return( POLARSSL_ERR_PEM_INVALID_ENC_IV );
 
             s1 += 16;
@@ -286,9 +286,11 @@
 #endif /* POLARSSL_DES_C */
 
 #if defined(POLARSSL_AES_C)
-        if( memcmp( s1, "DEK-Info: AES-", 14 ) == 0 )
+        if( s2 - s1 >= 14 && memcmp( s1, "DEK-Info: AES-", 14 ) == 0 )
         {
-            if( memcmp( s1, "DEK-Info: AES-128-CBC,", 22 ) == 0 )
+            if( s2 - s1 < 22 )
+                return( POLARSSL_ERR_PEM_UNKNOWN_ENC_ALG );
+            else if( memcmp( s1, "DEK-Info: AES-128-CBC,", 22 ) == 0 )
                 enc_alg = POLARSSL_CIPHER_AES_128_CBC;
             else if( memcmp( s1, "DEK-Info: AES-192-CBC,", 22 ) == 0 )
                 enc_alg = POLARSSL_CIPHER_AES_192_CBC;
@@ -298,7 +300,7 @@
                 return( POLARSSL_ERR_PEM_UNKNOWN_ENC_ALG );
 
             s1 += 22;
-            if( pem_get_iv( s1, pem_iv, 16 ) != 0 )
+            if( s2 - s1 < 32 || pem_get_iv( s1, pem_iv, 16 ) != 0 )
                 return( POLARSSL_ERR_PEM_INVALID_ENC_IV );
 
             s1 += 32;
@@ -317,7 +319,7 @@
           ( POLARSSL_AES_C || POLARSSL_DES_C ) */
     }
 
-    if( s1 == s2 )
+    if( s1 >= s2 )
         return( POLARSSL_ERR_PEM_INVALID_DATA );
 
     len = 0;