From d0f143b1c9be651c08a9b16bc8f21aeeb589eca6 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Manuel=20P=C3=A9gouri=C3=A9-Gonnard?= Date: Thu, 24 May 2018 12:01:58 +0200 Subject: [PATCH] Update CTR doc for the 64-bit block cipher - constants need adjustment - don't mention "random nonces" as the space is too small --- include/mbedtls/blowfish.h | 40 +++++++++++++++++++++++++++----------- 1 file changed, 29 insertions(+), 11 deletions(-) diff --git a/include/mbedtls/blowfish.h b/include/mbedtls/blowfish.h index 0318db697..fb16782aa 100644 --- a/include/mbedtls/blowfish.h +++ b/include/mbedtls/blowfish.h @@ -180,18 +180,36 @@ int mbedtls_blowfish_crypt_cfb64( mbedtls_blowfish_context *ctx, * * 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**64 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**32 - * blocks of data. + * 1. You can handle everything as a single message processed over + * successive calls to this function. In that case, you want to + * set \p nonce_counter and \p nc_off to 0 for the first call, and + * then preserve the values of \p nonce_counter, \p nc_off and \p + * stream_block across calls to this function as they will be + * updated by this function. * - * Note that for both stategies, the limit is in number of blocks - * and that a Blowfish block is 8 bytes. + * With this strategy, you must not encrypt more than 2**64 + * blocks of data with the same key. + * + * 2. You can encrypt separate messages by dividing the \p + * nonce_counter buffer in two areas: the first one used for a + * per-message nonce, handled by yourself, and the second one + * updated by this function internally. + * + * For example, you might reserve the first 4 bytes for the + * per-message nonce, and the last 4 bytes for internal use. In that + * case, before calling this function on a new message you need to + * set the first 4 bytes of \p nonce_counter to your chosen nonce + * value, the last 4 to 0, and \p nc_off to 0 (which will cause \p + * stream_block to be ignored). That way, you can encrypt at most + * 2**32 messages of up to 2**32 blocks each with the same key. + * + * The per-message nonce (or information sufficient to reconstruct + * it) needs to be communicated with the ciphertext and must be unique. + * The recommended way to ensure uniqueness is to use a message + * counter. + * + * Note that for both stategies, sizes are measured in blocks and + * that a Blowfish block is 8 bytes. * * \param ctx Blowfish context * \param length The length of the data