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Fix additional data length field check for CCM

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The CCM specification (NIST SP 800-38C) mandates that the formatting of
the additional data length l(a) changes when it is greater _or equal_ to
2^16 - 2^8 (>= 0xFF00). Since such lengths are not supported in mbed TLS,
the operation should fail in such cases.

This commit fixes an off-by-one error which allowed encryption/decryption
to be executed when l(a) was equal to 0xFF00, resulting in an
incorrect/non-standard length format being used.

Fixes #3719.

Signed-off-by: Fredrik Strupe <fredrik.strupe@silabs.com>
This commit is contained in:
Fredrik Strupe 2020-10-08 11:52:50 +02:00
parent 7829748cd4
commit 5e940c6068
3 changed files with 8 additions and 3 deletions
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5
ChangeLog.d/fix_ccm_add_length_check.txt Normal file
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@ -0,0 +1,5 @@
Bugfix
* Fix an off-by-one error in the additional data length check for
CCM, which allowed encryption with a non-standard length field.
Fixes #3719.

2
library/ccm.c
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@ -175,7 +175,7 @@ static int ccm_auth_crypt( mbedtls_ccm_context *ctx, int mode, size_t length,
if( iv_len < 7 || iv_len > 13 )
return( MBEDTLS_ERR_CCM_BAD_INPUT );
if( add_len > 0xFF00 )
if( add_len >= 0xFF00 )
return( MBEDTLS_ERR_CCM_BAD_INPUT );
q = 16 - 1 - (unsigned char) iv_len;

4
tests/suites/test_suite_ccm.data
View File

@ -41,9 +41,9 @@ ccm_lengths:5:10:5:18:MBEDTLS_ERR_CCM_BAD_INPUT
CCM lengths #6 tag length not even
ccm_lengths:5:10:5:7:MBEDTLS_ERR_CCM_BAD_INPUT
CCM lengths #7 AD too long (2^16 - 2^8 + 1)
CCM lengths #7 AD too long (2^16 - 2^8)
depends_on:!MBEDTLS_CCM_ALT
ccm_lengths:5:10:65281:8:MBEDTLS_ERR_CCM_BAD_INPUT
ccm_lengths:5:10:65280:8:MBEDTLS_ERR_CCM_BAD_INPUT
CCM lengths #8 msg too long for this IV length (2^16, q = 2)
ccm_lengths:65536:13:5:8:MBEDTLS_ERR_CCM_BAD_INPUT