<p>It's well known that using a hash function as message authentication is vulnerable to length extension attacks.</p>

<p>ie. H(key+message) is a bad idea. H(message+key), H(key+message+key) have their issues as well.</p>

<p>However, what if valid messages are a known length? Does length extension still apply?</p>

CC BY-SA 3.0
2012-12-06T07:34:15.523Z
I think this is more suitable for [cs.se] or [crypto.se].
Kaveh
CC BY-SA 3.0
2012-10-03T19:19:20.367Z
Welcome to cstheory, a Q&A site for *research-level* questions in *theoretical computer science* (TCS). Your question does not appear to be a research-level question in TCS. Please see the [FAQ] for more information on what is meant by this and suggestions for sites that might welcome your question. Finally, if your question is closed for being out of scope, and you believe you can edit the question to make it a research-level question, please feel free to do so. Closing is not permanent and questions can be reopened, check the [FAQ] for more information.
Kaveh
<cr.crypto-security><hash-function>
Length Extension Attack with a fixed length message
kels
# Length Extension Attack with a fixed length message

(Asked by: username_0 on 2012-10-03)

It's well known that using a hash function as message authentication is vulnerable to length extension attacks.

ie. H(key+message) is a bad idea. H(message+key), H(key+message+key) have their issues as well.

However, what if valid messages are a known length? Does length extension still apply?

**Tags:** cr.crypto-security, hash-function

> **Comments:**
> **username_1**: Welcome to cstheory, a Q&A site for *research-level* questions in *theoretical computer science* (TCS). Your question does not appear to be a research-level question in TCS. Please see the [FAQ] for more information on what is meant by this and suggestions for sites that might welcome your question. Finally, if your question is closed for being out of scope, and you believe you can edit the question to make it a research-level question, please feel free to do so. Closing is not permanent and questions can be reopened, check the [FAQ] for more information.  
> **username_1**: I think this is more suitable for [cs.se] or [crypto.se].  

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