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Provide standalone version of ssl_decrypt_buf

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Analogous to the previous commit, but concerning the record decryption
routine `ssl_decrypt_buf`.

An important change regards the checking of CBC padding:
Prior to this commit, the CBC padding check always read 256 bytes at
the end of the internal record buffer, almost always going past the
boundaries of the record under consideration. In order to stay within
the bounds of the given record, this commit changes this behavior by
always reading the last min(256, plaintext_len) bytes of the record
plaintext buffer and taking into consideration the last `padlen` of
these for the padding check. With this change, the memory access
pattern and runtime of the padding check is entirely determined by
the size of the encrypted record, in particular not giving away
any information on the validity of the padding.

The following depicts the different behaviors:

1) Previous CBC padding check

1.a) Claimed padding length <= plaintext length

  +----------------------------------------+----+
  |   Record plaintext buffer   |          | PL |
  +----------------------------------------+----+
                                 \__ PL __/

                                +------------------------------------...
                                |  read for padding check            ...
                                +------------------------------------...
                                                |
                                                 contents discarded
                                                 from here

1.b) Claimed padding length > plaintext length

  +----------------------------------------+----+
  |   Record plaintext buffer              | PL |
  +----------------------------------------+----+
                                           +-------------------------...
                                           |  read for padding check ...
                                           +-------------------------...
                                                |
                                                 contents discarded
                                                 from here

2) New CBC padding check

  +----------------------------------------+----+
  |   Record plaintext buffer   |          | PL |
  +----------------------------------------+----+
                                 \__ PL __/

        +---------------------------------------+
        |        read for padding check         |
        +---------------------------------------+
                                |
                                 contents discarded
                                 until here
This commit is contained in:
Hanno Becker 2017-12-27 21:28:58 +00:00 • committed by Manuel Pégourié-Gonnard
parent 3307b53413
commit 4c6876b134
2 changed files with 291 additions and 242 deletions
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6
include/mbedtls/md.h
View file

@ -74,6 +74,12 @@ typedef enum {
#define MBEDTLS_MD_MAX_SIZE 32 /* longest known is SHA256 or less */
#endif
#if defined(MBEDTLS_SHA512_C)
#define MBEDTLS_MD_MAX_BLOCK_SIZE 128
#else
#define MBEDTLS_MD_MAX_BLOCK_SIZE 64
#endif
/**
* Opaque struct defined in md_internal.h.
*/