diff --git a/ChangeLog b/ChangeLog
index 58d20aa..77e99e3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -18,6 +18,11 @@
      Found independently by Florian in the mbed TLS forum and by Mishamax.
      #878, #1019.
    * Fix build problems on Windows. Contributed by Nicholas Wilson.
+   * Fix unchecked return codes from AES, DES and 3DES functions in
+     pem_aes_decrypt(), pem_des_decrypt() and pem_des3_decrypt() respectively.
+     If a call to one of the functions of the cryptographic primitive modules
+     failed, the error may not be noticed by the function pem_read_buffer()
+     causing it to return invalid values. Found by Guido Vranken. #756
 
 = mbed TLS 1.3.21 branch released 2017-08-10
 
diff --git a/library/pem.c b/library/pem.c
index b2c16c2..08c182f 100644
--- a/library/pem.c
+++ b/library/pem.c
@@ -135,45 +135,53 @@
 /*
  * Decrypt with DES-CBC, using PBKDF1 for key derivation
  */
-static void pem_des_decrypt( unsigned char des_iv[8],
-                               unsigned char *buf, size_t buflen,
-                               const unsigned char *pwd, size_t pwdlen )
+static int pem_des_decrypt( unsigned char des_iv[8],
+                            unsigned char *buf, size_t buflen,
+                            const unsigned char *pwd, size_t pwdlen )
 {
     des_context des_ctx;
     unsigned char des_key[8];
+    int ret;
 
     des_init( &des_ctx );
 
     pem_pbkdf1( des_key, 8, des_iv, pwd, pwdlen );
 
-    des_setkey_dec( &des_ctx, des_key );
-    des_crypt_cbc( &des_ctx, DES_DECRYPT, buflen,
-                     des_iv, buf, buf );
+    if( ( ret = des_setkey_dec( &des_ctx, des_key ) ) != 0 )
+        goto exit;
+    ret = des_crypt_cbc( &des_ctx, DES_DECRYPT, buflen, des_iv, buf, buf );
 
+exit:
     des_free( &des_ctx );
     polarssl_zeroize( des_key, 8 );
+
+    return( ret );
 }
 
 /*
  * Decrypt with 3DES-CBC, using PBKDF1 for key derivation
  */
-static void pem_des3_decrypt( unsigned char des3_iv[8],
-                               unsigned char *buf, size_t buflen,
-                               const unsigned char *pwd, size_t pwdlen )
+static int pem_des3_decrypt( unsigned char des3_iv[8],
+                             unsigned char *buf, size_t buflen,
+                             const unsigned char *pwd, size_t pwdlen )
 {
     des3_context des3_ctx;
     unsigned char des3_key[24];
+    int ret;
 
     des3_init( &des3_ctx );
 
     pem_pbkdf1( des3_key, 24, des3_iv, pwd, pwdlen );
 
-    des3_set3key_dec( &des3_ctx, des3_key );
-    des3_crypt_cbc( &des3_ctx, DES_DECRYPT, buflen,
-                     des3_iv, buf, buf );
+    if( ( ret = des3_set3key_dec( &des3_ctx, des3_key ) ) != 0 )
+        goto exit;
+    ret = des3_crypt_cbc( &des3_ctx, DES_DECRYPT, buflen, des3_iv, buf, buf );
 
+exit:
     des3_free( &des3_ctx );
     polarssl_zeroize( des3_key, 24 );
+
+    return( ret );
 }
 #endif /* POLARSSL_DES_C */
 
@@ -181,23 +189,27 @@
 /*
  * Decrypt with AES-XXX-CBC, using PBKDF1 for key derivation
  */
-static void pem_aes_decrypt( unsigned char aes_iv[16], unsigned int keylen,
-                               unsigned char *buf, size_t buflen,
-                               const unsigned char *pwd, size_t pwdlen )
+static int pem_aes_decrypt( unsigned char aes_iv[16], unsigned int keylen,
+                            unsigned char *buf, size_t buflen,
+                            const unsigned char *pwd, size_t pwdlen )
 {
     aes_context aes_ctx;
     unsigned char aes_key[32];
+    int ret;
 
     aes_init( &aes_ctx );
 
     pem_pbkdf1( aes_key, keylen, aes_iv, pwd, pwdlen );
 
-    aes_setkey_dec( &aes_ctx, aes_key, keylen * 8 );
-    aes_crypt_cbc( &aes_ctx, AES_DECRYPT, buflen,
-                     aes_iv, buf, buf );
+    if( ( ret = aes_setkey_dec( &aes_ctx, aes_key, keylen * 8 ) ) != 0 )
+        goto exit;
+    ret = aes_crypt_cbc( &aes_ctx, AES_DECRYPT, buflen, aes_iv, buf, buf );
 
+exit:
     aes_free( &aes_ctx );
     polarssl_zeroize( aes_key, keylen );
+
+    return( ret );
 }
 #endif /* POLARSSL_AES_C */
 
@@ -347,22 +359,30 @@
             return( POLARSSL_ERR_PEM_PASSWORD_REQUIRED );
         }
 
+        ret = 0;
+
 #if defined(POLARSSL_DES_C)
         if( enc_alg == POLARSSL_CIPHER_DES_EDE3_CBC )
-            pem_des3_decrypt( pem_iv, buf, len, pwd, pwdlen );
+            ret = pem_des3_decrypt( pem_iv, buf, len, pwd, pwdlen );
         else if( enc_alg == POLARSSL_CIPHER_DES_CBC )
-            pem_des_decrypt( pem_iv, buf, len, pwd, pwdlen );
+            ret = pem_des_decrypt( pem_iv, buf, len, pwd, pwdlen );
 #endif /* POLARSSL_DES_C */
 
 #if defined(POLARSSL_AES_C)
         if( enc_alg == POLARSSL_CIPHER_AES_128_CBC )
-            pem_aes_decrypt( pem_iv, 16, buf, len, pwd, pwdlen );
+            ret = pem_aes_decrypt( pem_iv, 16, buf, len, pwd, pwdlen );
         else if( enc_alg == POLARSSL_CIPHER_AES_192_CBC )
-            pem_aes_decrypt( pem_iv, 24, buf, len, pwd, pwdlen );
+            ret = pem_aes_decrypt( pem_iv, 24, buf, len, pwd, pwdlen );
         else if( enc_alg == POLARSSL_CIPHER_AES_256_CBC )
-            pem_aes_decrypt( pem_iv, 32, buf, len, pwd, pwdlen );
+            ret = pem_aes_decrypt( pem_iv, 32, buf, len, pwd, pwdlen );
 #endif /* POLARSSL_AES_C */
 
+        if( ret != 0 )
+        {
+            polarssl_free( buf );
+            return( ret );
+        }
+
         /*
          * The result will be ASN.1 starting with a SEQUENCE tag, with 1 to 3
          * length bytes (allow 4 to be sure) in all known use cases.
diff --git a/tests/suites/test_suite_pem.data b/tests/suites/test_suite_pem.data
index b5f63e5..416cf84 100644
--- a/tests/suites/test_suite_pem.data
+++ b/tests/suites/test_suite_pem.data
@@ -17,10 +17,22 @@
 pem_write_buffer:"-----START TEST-----\n":"-----END TEST-----\n":"000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F00":"-----START TEST-----\nAAECAwQFBgcICQoLDA0ODwABAgMEBQYHCAkKCwwNDg8AAQIDBAUGBwgJCgsMDQ4P\nAAECAwQFBgcICQoLDA0ODwABAgMEBQYHCAkKCwwNDg8AAQIDBAUGBwgJCgsMDQ4P\nAA==\n-----END TEST-----\n"
 
 PEM read (DES-EDE3-CBC + invalid iv)
-pem_read_buffer:"^":"$":"^\nProc-Type\: 4,ENCRYPTED\nDEK-Info\: DES-EDE3-CBC,00$":POLARSSL_ERR_PEM_INVALID_ENC_IV
+pem_read_buffer:"^":"$":"^\nProc-Type\: 4,ENCRYPTED\nDEK-Info\: DES-EDE3-CBC,00$":"pwd":POLARSSL_ERR_PEM_INVALID_ENC_IV
 
 PEM read (DES-CBC + invalid iv)
-pem_read_buffer:"^":"$":"^\nProc-Type\: 4,ENCRYPTED\nDEK-Info\: DES-CBC,00$":POLARSSL_ERR_PEM_INVALID_ENC_IV
+pem_read_buffer:"^":"$":"^\nProc-Type\: 4,ENCRYPTED\nDEK-Info\: DES-CBC,00$":"pwd":POLARSSL_ERR_PEM_INVALID_ENC_IV
 
 PEM read (unknown encryption algorithm)
-pem_read_buffer:"^":"$":"^\nProc-Type\: 4,ENCRYPTED\nDEK-Info\: AES-,00$":POLARSSL_ERR_PEM_UNKNOWN_ENC_ALG
+pem_read_buffer:"^":"$":"^\nProc-Type\: 4,ENCRYPTED\nDEK-Info\: AES-,00$":"pwd":POLARSSL_ERR_PEM_UNKNOWN_ENC_ALG
+
+PEM read (malformed PEM DES-CBC)
+depends_on:POLARSSL_DES_C:POLARSSL_CIPHER_MODE_CBC
+pem_read_buffer:"-----BEGIN EC PRIVATE KEY-----":"-----END EC PRIVATE KEY-----":"-----BEGIN EC PRIVATE KEY-----\nProc-Type\: 4,ENCRYPTED\nDEK-Info\: DES-CBC,AA94892A169FA426\n\nMAAA\n-----END EC PRIVATE KEY-----":"pwd":POLARSSL_ERR_DES_INVALID_INPUT_LENGTH
+
+PEM read (malformed PEM DES-EDE3-CBC)
+depends_on:POLARSSL_DES_C:POLARSSL_CIPHER_MODE_CBC
+pem_read_buffer:"-----BEGIN EC PRIVATE KEY-----":"-----END EC PRIVATE KEY-----":"-----BEGIN EC PRIVATE KEY-----\nProc-Type\: 4,ENCRYPTED\nDEK-Info\: DES-EDE3-CBC,AA94892A169FA426\n\nMAAA\n-----END EC PRIVATE KEY-----":"pwd":POLARSSL_ERR_DES_INVALID_INPUT_LENGTH
+
+PEM read (malformed PEM AES-128-CBC)
+depends_on:POLARSSL_AES_C:POLARSSL_CIPHER_MODE_CBC
+pem_read_buffer:"-----BEGIN EC PRIVATE KEY-----":"-----END EC PRIVATE KEY-----":"-----BEGIN EC PRIVATE KEY-----\nProc-Type\: 4,ENCRYPTED\nDEK-Info\: AES-128-CBC,AA94892A169FA426AA94892A169FA426\n\nMAAA\n-----END EC PRIVATE KEY-----":"pwd":POLARSSL_ERR_AES_INVALID_INPUT_LENGTH
diff --git a/tests/suites/test_suite_pem.function b/tests/suites/test_suite_pem.function
index e0b7679..e96c83f 100644
--- a/tests/suites/test_suite_pem.function
+++ b/tests/suites/test_suite_pem.function
@@ -1,6 +1,8 @@
 /* BEGIN_HEADER */
 #include "polarssl/base64.h"
 #include "polarssl/pem.h"
+#include "polarssl/des.h"
+#include "polarssl/aes.h"
 /* END_HEADER */
 
 /* BEGIN_CASE depends_on:POLARSSL_PEM_WRITE_C */
@@ -35,16 +37,19 @@
 /* END_CASE */
 
 /* BEGIN_CASE depends_on:POLARSSL_PEM_PARSE_C:POLARSSL_AES_C:POLARSSL_DES_C:POLARSSL_MD5_C:POLARSSL_CIPHER_MODE_CBC */
-void pem_read_buffer( char *header, char *footer, char *data, int ret )
+void pem_read_buffer( char *header, char *footer, char *data, char *pwd,
+                      int res )
 {
     pem_context ctx;
+    int ret;
     size_t use_len = 0;
+    size_t pwd_len = strlen( pwd );
 
     pem_init( &ctx );
 
-    TEST_ASSERT( pem_read_buffer( &ctx, header, footer,
-                                  (const unsigned char *)data, NULL, 0,
-                                  &use_len ) == ret );
+    ret = pem_read_buffer( &ctx, header, footer, (const unsigned char *)data,
+                           (unsigned char *)pwd, pwd_len, &use_len );
+    TEST_ASSERT( ret == res );
 
 exit:
     pem_free( &ctx );