From 593c447d9637eb9c92d602a5ad3491ee74c8e6b7 Mon Sep 17 00:00:00 2001 From: Owain Cleaver Date: Tue, 16 Oct 2018 18:34:38 +0100 Subject: [PATCH 1/2] Fix to AES-CMAC bug that caused an extra block of padding to be created if the second to last call to update `leftover` and the final update call `remaining` equalled the size of the AES block. --- lib/source/cmac_mode.c | 44 +++++++++++++++++++++++++++++++----------- tests/test_cmac_mode.c | 44 ++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 77 insertions(+), 11 deletions(-) diff --git a/lib/source/cmac_mode.c b/lib/source/cmac_mode.c index 22adf20..90f2db2 100644 --- a/lib/source/cmac_mode.c +++ b/lib/source/cmac_mode.c @@ -175,7 +175,20 @@ int tc_cmac_update(TCCmacState_t s, const uint8_t *data, size_t data_length) s->countdown--; + restartUpdate: if (s->leftover_offset > 0) { + + if(s->leftover_offset == TC_AES_BLOCK_SIZE) { + /*the left over data is a full block; encrypt and restart the update */ + for (i = 0; i < TC_AES_BLOCK_SIZE; ++i) { + s->iv[i] ^= s->leftover[i]; + } + tc_aes_encrypt(s->iv, s->iv, s->sched); + s->leftover_offset = 0; + + goto restartUpdate; + } + /* last data added to s didn't end on a TC_AES_BLOCK_SIZE byte boundary */ size_t remaining_space = TC_AES_BLOCK_SIZE - s->leftover_offset; @@ -185,34 +198,43 @@ int tc_cmac_update(TCCmacState_t s, const uint8_t *data, size_t data_length) s->leftover_offset += data_length; return TC_CRYPTO_SUCCESS; } - /* leftover block is now full; encrypt it first */ + /* leftover block is now full */ _copy(&s->leftover[s->leftover_offset], remaining_space, data, remaining_space); data_length -= remaining_space; data += remaining_space; - s->leftover_offset = 0; - for (i = 0; i < TC_AES_BLOCK_SIZE; ++i) { - s->iv[i] ^= s->leftover[i]; - } - tc_aes_encrypt(s->iv, s->iv, s->sched); + /* encrypted or copied below */ + s->leftover_offset = TC_AES_BLOCK_SIZE; } /* CBC encrypt each (except the last) of the data blocks */ - while (data_length > TC_AES_BLOCK_SIZE) { - for (i = 0; i < TC_AES_BLOCK_SIZE; ++i) { - s->iv[i] ^= data[i]; + while (data_length + s->leftover_offset > TC_AES_BLOCK_SIZE) { + + if(s->leftover_offset > 0) { + for (i = 0; i < TC_AES_BLOCK_SIZE; ++i) { + s->iv[i] ^= s->leftover[i]; + } + s->leftover_offset = 0; + } + else { + for (i = 0; i < TC_AES_BLOCK_SIZE; ++i) { + s->iv[i] ^= data[i]; + } } + tc_aes_encrypt(s->iv, s->iv, s->sched); data += TC_AES_BLOCK_SIZE; data_length -= TC_AES_BLOCK_SIZE; } if (data_length > 0) { - /* save leftover data for next time */ - _copy(s->leftover, data_length, data, data_length); + /* save leftover data for next time, unless this last block is already leftover */ + if(s->leftover_offset != TC_AES_BLOCK_SIZE) { + _copy(s->leftover, data_length, data, data_length); + } s->leftover_offset = data_length; } diff --git a/tests/test_cmac_mode.c b/tests/test_cmac_mode.c index dc757fe..cfbbbda 100644 --- a/tests/test_cmac_mode.c +++ b/tests/test_cmac_mode.c @@ -247,6 +247,50 @@ static int verify_cmac_512_bit_msg(TCCmacState_t s) return result; } +static int verify_cmac_512_bit_msg_split_update(TCCmacState_t s) +{ + int result = TC_PASS; + + TC_PRINT("Performing CMAC test #5.1 (SP 800-38B test vector #4)\n"); + + const uint8_t msg1[63] = { + 0x6b, 0xc1, 0xbe, 0xe2, 0x2e, 0x40, 0x9f, 0x96, + 0xe9, 0x3d, 0x7e, 0x11, 0x73, 0x93, 0x17, 0x2a, + 0xae, 0x2d, 0x8a, 0x57, 0x1e, 0x03, 0xac, 0x9c, + 0x9e, 0xb7, 0x6f, 0xac, 0x45, 0xaf, 0x8e, 0x51, + 0x30, 0xc8, 0x1c, 0x46, 0xa3, 0x5c, 0xe4, 0x11, + 0xe5, 0xfb, 0xc1, 0x19, 0x1a, 0x0a, 0x52, 0xef, + 0xf6, 0x9f, 0x24, 0x45, 0xdf, 0x4f, 0x9b, 0x17, + 0xad, 0x2b, 0x41, 0x7b, 0xe6, 0x6c, 0x37 + }; + const uint8_t msg2[1] = { + 0x10 + }; + const uint8_t tag[BUF_LEN] = { + 0x51, 0xf0, 0xbe, 0xbf, 0x7e, 0x3b, 0x9d, 0x92, + 0xfc, 0x49, 0x74, 0x17, 0x79, 0x36, 0x3c, 0xfe + }; + uint8_t Tag[BUF_LEN]; + + (void)tc_cmac_init(s); + (void)tc_cmac_update(s, msg1, sizeof(msg1)); + (void)tc_cmac_update(s, msg2, sizeof(msg2)); + (void)tc_cmac_final(Tag, s); + + if (memcmp(Tag, tag, BUF_LEN) != 0) { + TC_ERROR("%s: aes_cmac failed with 512 bit msg split update\n", __func__); + show("aes_cmac failed with 512 bit msg1 =", msg1, sizeof(msg1)); + show("and msg2 =", msg2, sizeof(msg2)); + show("expected Tag =", tag, sizeof(tag)); + show("computed Tag =", Tag, sizeof(Tag)); + return TC_FAIL; + } + + TC_END_RESULT(result); + return result; +} + + /* * Main task to test CMAC * effects: returns 1 if all tests pass From 157989e6d980c95c1039471f0ec44867b566beae Mon Sep 17 00:00:00 2001 From: Owain Cleaver Date: Wed, 17 Oct 2018 10:55:58 +0100 Subject: [PATCH 2/2] Fix for introduced bug. --- lib/source/cmac_mode.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/lib/source/cmac_mode.c b/lib/source/cmac_mode.c index 90f2db2..0210b95 100644 --- a/lib/source/cmac_mode.c +++ b/lib/source/cmac_mode.c @@ -223,11 +223,11 @@ int tc_cmac_update(TCCmacState_t s, const uint8_t *data, size_t data_length) for (i = 0; i < TC_AES_BLOCK_SIZE; ++i) { s->iv[i] ^= data[i]; } + data += TC_AES_BLOCK_SIZE; + data_length -= TC_AES_BLOCK_SIZE; } tc_aes_encrypt(s->iv, s->iv, s->sched); - data += TC_AES_BLOCK_SIZE; - data_length -= TC_AES_BLOCK_SIZE; } if (data_length > 0) {