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block ciphers: improve CTR nonce warning

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Manuel Pégourié-Gonnard 2018-02-28 12:29:41 +01:00
parent 5aa4e3b1d0
commit 22997b7200
4 changed files with 73 additions and 5 deletions
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19
include/mbedtls/aes.h
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@ -300,7 +300,24 @@ int mbedtls_aes_crypt_cfb8( mbedtls_aes_context *ctx,
* must use the context initialized with mbedtls_aes_setkey_enc()
* for both #MBEDTLS_AES_ENCRYPT and #MBEDTLS_AES_DECRYPT.
*
* \warning You must keep the maximum use of your counter in mind.
* \warning You must never reuse a nonce value with the same key. Doing so
* would void the encryption for the two messages encrypted with
* the same nonce and key.
*
* There are two common strategies for managing nonces with CTR:
*
* 1. Use a counter starting at 0 or a random value. With this
* strategy, this function will increment the counter for you, so
* you only need to preserve the \p nonce_counter buffer between
* calls. With this strategy, you must not encrypt more than
* 2**128 blocks of data.
* 2. Use a randomly-generated \p nonce_counter for each call.
* With this strategy, you need to ensure the nonce is generated
* in an unbiased way and you must not encrypt more than 2**64
* block of data.
*
* Note that for both stategies, the limit is in number of blocks
* and that an AES block is 16 bytes.
*
* \param ctx The AES context to use for encryption or decryption.
* \param length The length of the input data.