Editing PBKDF2 (section)

==HMAC collisions == 
PBKDF2 has an interesting property when using HMAC as its pseudo-random function. It is possible to trivially construct any number of different password pairs with collisions within each pair.<ref>{{Cite web |url=https://mathiasbynens.be/notes/pbkdf2-hmac |title=PBKDF2+HMAC hash collisions explained |first=Mathias |last=Bynens |website=mathiasbynens.be}}</ref> If a supplied password is longer than the block size of the underlying HMAC hash function, the password is first pre-hashed into a digest, and that digest is instead used as the password. For example, the following password is too long:

* '''Password:''' <code>{{not a typo|plnlrtfpijpuhqylxbgqiiyipieyxvfsavzgxbbcfusqkozwpngsyejqlmjsytrmd}}</code>

therefore, when using HMAC-SHA1, it is pre-hashed using SHA-1 into:

* '''SHA1''' (hex): <code>65426b585154667542717027635463617226672a</code>

Which can be represented in ASCII as:

* '''SHA1''' (ASCII): <code>eBkXQTfuBqp'cTcar&g*</code>

This means regardless of the salt or iterations, PBKDF2-HMAC-SHA1 will generate the same key bytes for the passwords:

* "plnlrtfpijpuhqylxbgqiiyipieyxvfsavzgxbbcfusqkozwpngsyejqlmjsytrmd"
* "eBkXQTfuBqp'cTcar&g*"

For example, using:

* '''PRF''': HMAC-SHA1
* '''Salt:''' A009C1A485912C6AE630D3E744240B04
* '''Iterations:''' 1,000
* '''Derived key length:''' 16 bytes

The following two function calls:
<syntaxhighlight lang="python">
PBKDF2-HMAC-SHA1("plnlrtfpijpuhqylxbgqiiyipieyxvfsavzgxbbcfusqkozwpngsyejqlmjsytrmd", ...)
PBKDF2-HMAC-SHA1("eBkXQTfuBqp'cTcar&g*", ...) 
</syntaxhighlight>
will generate the same derived key bytes (<code>17EB4014C8C461C300E9B61518B9A18B</code>). These derived key collisions do not represent a security vulnerability – as one still must know the original password in order to generate the ''hash'' of the password.<ref>{{Cite web|url=https://crypto.stackexchange.com/questions/26510/why-is-hmac-sha1-still-considered-secure|title = Collision resistance - Why is HMAC-SHA1 still considered secure? |website=crypto.stackexchange.com}}</ref>