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Introduce public macro for maximum symmetric cipher key length

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This commit introduces the public macro MBEDTLS_MAX_KEY_LENGTH,
which evaluates to an upper bound for the key lengths of all enabled
ciphers, in Bytes.

This is analogous to the already existing macros MBEDTLS_MAX_IV_LENGTH
and MBEDTLS_MAX_BLOCK_LENGTH, which provide upper bounds for the IV
and block length, respectively.

For now, MBEDTLS_MAX_KEY_LENGTH is 32 Bytes by default, and 64 in case
XTS is enabled. This is a strict overapproximation for some restricted
configurations. Ideally, the upper bound should be calculated exactly
and automatically from the list of enabled ciphers. The same applies
to the existing macros MBEDTLS_MAX_IV_LENGTH and MBEDTLS_MAX_BLOCK_LENGTH,
though, and is left for future work.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
This commit is contained in:
Hanno Becker 2020-08-07 11:30:05 +01:00
parent 853f9bd65e
commit 27a2688fbb
1 changed files with 13 additions and 0 deletions
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13
include/mbedtls/cipher.h
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@ -227,10 +227,23 @@ enum {
};
/** Maximum length of any IV, in Bytes. */
/* This should ideally be derived automatically from list of ciphers. */
#define MBEDTLS_MAX_IV_LENGTH 16
/** Maximum block size of any cipher, in Bytes. */
/* This should ideally be derived automatically from list of ciphers. */
#define MBEDTLS_MAX_BLOCK_LENGTH 16
/** Maximum key length, in Bytes. */
/* This should ideally be derived automatically from list of ciphers.
* For now, only check whether XTS is enabled which uses 64 Byte keys,
* and use 32 Bytes as an upper bound for the maximum key length otherwise. */
#if defined(MBEDTLS_CIPHER_MODE_XTS)
#define MBEDTLS_MAX_KEY_LENGTH 64
#else
#define MBEDTLS_MAX_KEY_LENGTH 32
#endif /* MBEDTLS_CIPHER_MODE_XTS */
/**
* Base cipher information (opaque struct).
*/