On Dec 1, 2011, at 11:48 PM, Marsh Ray wrote:
> On 12/01/2011 04:37 PM, Jerrie Union wrote:
>>
>> public boolean check(digest, secret) {
>> hash = md5(secret);
>>
>> if (digest.length != hash.length) {
>> return false;
>> }
>>
>> for (i = 0; i< digest.length; i++) {
>> if (digest[i] != hash[i]) {
>> return false;
>> }
>> }
>>
>> I’m wondering, if it’s running as some authenticated server application, if
>> it should be considered as resistant to time attacks nowadays.
>
> Not resistant. It's a timing oracle. Very dangerous.
How should the attacker mount the attack after hash[0] has been recovered?
I guess for a given digest D if the attacker guess the character at position 1
(D[1])
by supplying the secret S there’s no easy way to recover D[2] because the md5
function will introduce noise in every single bit of the output as you change a
single
bit in the input.
Maybe, by having a huge precomputed table the attacker can attempt to mount a
timing attack
in this way:
1. guess the first byte of the digest by exploiting the timing attack
2. for every digest in the rainbow table starting with the previously guessed
byte:
3. try to send the plaintext and time the response to recover the second byte
The same process could be iterated until the fully string is recovered.
Does it make sense?
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