The Call of the Open Sidewalk

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pgpfan:mdc [2022/07/18 22:58] – [The OpenPGP Modification Detection Code is Actually Good] b.walzerpgpfan:mdc [2022/07/22 15:50] – Readers might not know what a hash is. b.walzer
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 {{mdc1.svg}} {{mdc1.svg}}
  
-We create this by hashing the message. Then we append the hash to the end of the message. After that we encrypt everything; message and hash. To check for modification we hash the message and compare that hash to the hash appended to the message.+We create this by [[wp>Cryptographic_hash_function|hashing the message]]. Then we append the hash to the end of the message. After that we encrypt everything; message and hash. To check for modification we hash the message and compare that hash to the hash appended to the message.
  
 Let's consider the easiest situation for the attacker and assume they know the entire message. Then the attacker can hash that known message and will then know what the hash was before encryption. As a result they can modify the hash to any value they want by flipping bits as required. So the attacker can change the message to anything they want without restriction and can change the hash so that their changes would not be detected. If their target message is shorter than the original they can just generate the hash early and drop the extra part. So this is not entirely secure. Let's consider the easiest situation for the attacker and assume they know the entire message. Then the attacker can hash that known message and will then know what the hash was before encryption. As a result they can modify the hash to any value they want by flipping bits as required. So the attacker can change the message to anything they want without restriction and can change the hash so that their changes would not be detected. If their target message is shorter than the original they can just generate the hash early and drop the extra part. So this is not entirely secure.
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 {{mdc3.svg}} {{mdc3.svg}}
  
-We have added some random data to the start of the message. The random data prefix is included in the hash. That means that the attacker can never know the entire message and as a result will not know what the hash is to start with. As a result they will not be able to change the hash in a rational way by flipping bits.+We have added some random data to the start of the message. The random data prefix is included in the hash. That means that the attacker can never know the entire message and as a result will not know what the hash is to start with. As a result they will not be able to change the hash in a rational way by flipping bits. So the hash is protected by first randomizing it and then encrypting it.
  
 There are some mostly theoretical attacks that involve getting the victim to encrypt messages created by the attacker so that the attacker then can modify them by chopping off the start and/or the end of the message without detection. The version of cipher feedback used by OpenPGP((See the [[pgpfan:ocfb|OpenPGP's Improved Cipher Feedback Mode]] article for some more detail.)) (OCFB) prevents that sort of attack by preventing attacker knowledge of the random prefix data and requiring the key to create a new random prefix. This is the OCFB-MDC (OpenPGP Cipher FeedBack - Modification Detection Code) mode used by OpenPGP (irrelevant detail omitted): There are some mostly theoretical attacks that involve getting the victim to encrypt messages created by the attacker so that the attacker then can modify them by chopping off the start and/or the end of the message without detection. The version of cipher feedback used by OpenPGP((See the [[pgpfan:ocfb|OpenPGP's Improved Cipher Feedback Mode]] article for some more detail.)) (OCFB) prevents that sort of attack by preventing attacker knowledge of the random prefix data and requiring the key to create a new random prefix. This is the OCFB-MDC (OpenPGP Cipher FeedBack - Modification Detection Code) mode used by OpenPGP (irrelevant detail omitted):
  
 {{mdc4.svg}} {{mdc4.svg}}
 +
 +Now both the hash and the random data are protected by first randomizing them and then encrypting them.
  
 If you want to attack OCFB-MDC and modify a message without triggering the MDC you will have to deal with the following challenges: If you want to attack OCFB-MDC and modify a message without triggering the MDC you will have to deal with the following challenges:
pgpfan/mdc.txt · Last modified: 2023/12/11 13:30 by b.walzer