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CTR_DRBG: improve the discussion of entropy length vs strength

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This commit is contained in:
Gilles Peskine 2019-09-25 20:22:40 +02:00
parent 223deea86b
commit 6fdf0b3a47
1 changed files with 24 additions and 9 deletions
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33
include/mbedtls/ctr_drbg.h
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@ -24,10 +24,6 @@
* and #MBEDTLS_CTR_DRBG_ENTROPY_LEN is set to 24 or more (which is
* always the case unless it is explicitly set to a different value
* in `config.h`).
*
* \warning Using 128-bit keys for CTR_DRBG or using SHA-256 as the entropy
* compression function limits the security of generated
* keys and operations that use random values generated to 128-bit security.
*/
/*
* Copyright (C) 2006-2019, Arm Limited (or its affiliates), All Rights Reserved
@ -278,11 +274,30 @@ void mbedtls_ctr_drbg_set_prediction_resistance( mbedtls_ctr_drbg_context *ctx,
* #MBEDTLS_CTR_DRBG_ENTROPY_LEN.
*
* \note For compliance with NIST SP 800-90A, the entropy length
* must be at least 1.5 times security strength, since
* the entropy source is used both as the entropy input
* and to provide the initial nonce:
* - 24 bytes if using AES-128;
* - 48 bytes if using AES-256.
* (\p len bytes = \p len * 8 bits)
* must be at least the security strength.
* Furthermore, if the entropy input is used to provide
* the nonce, the entropy length must be 1.5 times
* the security strength.
* Per NIST SP 800-57A table 2, the achievable security
* strength is 128 bits if using AES-128 and
* 256 bits if using AES-256.
* Therefore, to provide full security,
* the entropy input must be at least:
* - 24 bytes if using AES-128 and the \p custom
* argument to mbedtls_ctr_drbg_seed() may repeat
* (for example because it is empty, or more generally
* constant);
* - 48 bytes if using AES-256 and the \p custom
* argument to mbedtls_ctr_drbg_seed() may repeat
* (for example because it is empty, or more generally
* constant);
* - 16 bytes if using AES-128 and the \p custom
* argument to mbedtls_ctr_drbg_seed() includes
* a nonce;
* - 32 bytes if using AES-256 and the \p custom
* argument to mbedtls_ctr_drbg_seed() includes
* a nonce.
*
* \param ctx The CTR_DRBG context.
* \param len The amount of entropy to grab, in bytes.