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Comment on hardcoding of maximum HKDF key expansion of 255 Bytes

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Signed-off-by: Hanno Becker <hanno.becker@arm.com>
This commit is contained in:
Hanno Becker 2020-09-16 09:45:27 +01:00
parent 61baae7c9f
commit 531fe3054c
2 changed files with 14 additions and 2 deletions
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9
library/ssl_tls13_keys.c
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@ -103,7 +103,14 @@ static void ssl_tls1_3_hkdf_encode_label(
unsigned char *p = dst;
/* Add total length. */
/* Add the size of the expanded key material.
* We're hardcoding the high byte to 0 here assuming that we never use
* TLS 1.3 HKDF key expansion to more than 255 Bytes. */
#if MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_EXPANSION_LEN > 255
#error "The implementation of ssl_tls1_3_hkdf_encode_label() is not fit for the \
value of MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_EXPANSION_LEN"
#endif
*p++ = 0;
*p++ = (unsigned char)( ( desired_length >> 0 ) & 0xFF );

7
library/ssl_tls13_keys.h
View File

@ -71,7 +71,12 @@ extern const struct mbedtls_ssl_tls1_3_labels_struct mbedtls_ssl_tls1_3_labels;
MBEDTLS_MD_MAX_SIZE
/* Maximum desired length for expanded key material generated
* by HKDF-Expand-Label. */
* by HKDF-Expand-Label.
*
* Warning: If this ever needs to be increased, the implementation
* ssl_tls1_3_hkdf_encode_label() in ssl_tls13_keys.c needs to be
* adjusted since it currently assumes that HKDF key expansion
* is never used with more than 255 Bytes of output. */
#define MBEDTLS_SSL_TLS1_3_KEY_SCHEDULE_MAX_EXPANSION_LEN 255
/**