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I read at crackstation not to use these variants of bcrypt* ($1$, $2$, $2a$, $2x$, $3$),but I've used bcrypt ($2a$) in various sensitive implementations recently.
Can any security expert clarify why recommending ($2y$, $5$, $6$) instead of ($1$, $2$, $2a$, $2x$, $3$), what is the original version proposed by Niels Provos, and in what they differ


bcrypt is a key derivation function for passwords designed by Niels Provos and David Mazières, based on the Blowfish cipher, and presented at USENIX in 1999. Besides incorporating a salt to protect against rainbow table attacks, bcrypt is an adaptive function: over time, the iteration count can be increased to make it slower, so it remains resistant to brute-force search attacks even with increasing computation power.

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I don't have enough reputation to leave a comment so I have to put this as an answer. >Read this, please: php.net/security/crypt_blowfish.php. $2a$ is secure in PHP>=5.3.7 – Andrey Botalov Sep 26 '12 at 12:36 Serious crypto bug in php 5.3.7 PHP 5.3.8 Fixes Serious Crypt Bug –  Dave Albert Mar 22 '13 at 9:52

1 Answer 1

up vote 12 down vote accepted
  • 2 - the original BCrypt, which has been deprecated because of a security issue a long time before BCrypt became popular.

  • 2a - the official BCrypt algorithm and a insecure implementation in crypt_blowfish

  • 2x - suggested for hashes created by the insecure algorithm for compatibility
  • 2y - suggested new marker for the fixed crypt_blowfish

So 2a hashes created by the original algorithm or the java port are fine, and identical to 2y-hashes created by crypt_blowfish. But 2a hashes created by crypt_blowfish are insecure.

  • 5 is sha256crypt
  • 6 is sha512crypt

The shaXXXcrypt algorithm are inspired by bcrypt but use sha2 instead of blowfish as hash functions in order to satisfy compliance requirements in the USA.

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can you further explain what you mean by insecure implementation, and in what area they are particularly insecure ? –  sarepta Sep 23 '12 at 7:51
    
@sarepta, if the password contains characters outside the 7-bit range, some character are ignored. I added a link to the report, which explains it in detail. –  Hendrik Brummermann Sep 23 '12 at 8:52
    
can you (if possible) provide a link to download secure implementation ($2y$, $5$, $6$) in .net, it seems as if all the libraries in the web are $2a$ and its getting in my nerves! –  sarepta Sep 23 '12 at 14:23
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Read this, please: php.net/security/crypt_blowfish.php. $2a$ is secure in PHP>=5.3.7 –  Andrey Botalov Sep 26 '12 at 12:36
    
@HendrikBrummermann can you comment further on how $6 would work in bcrypt for password hashing, vs. $2y$? –  Shackrock May 23 '13 at 11:34

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