CODEWITHSUNDEEP
mysqlhashtypesstoragebcrypt
helpermethod
helpermethod

Reputation: 62135

What column type/length should I use for storing a Bcrypt hashed password in a Database?

I want to store a hashed password (using BCrypt) in a database. What would be a good type for this, and which would be the correct length? Are passwords hashed with BCrypt always of same length?

EDIT

Example hash:

$2a$10$KssILxWNR6k62B7yiX0GAe2Q7wwHlrzhF3LqtVvpyvHZf0MwvNfVu

After hashing some passwords, it seems that BCrypt always generates 60 character hashes.

EDIT 2

Sorry for not mentioning the implementation. I am using jBCrypt.

Upvotes: 372

Views: 194336

Answers (5)

Daro from Poland
Daro from Poland

Reputation: 59

I think best choice is nonbinary type, because in comparison is less combination and should be faster. If data is encoded with base64_encode then each position, each byte have only 64 possible values. If encoded with bin2hex each byte have only 16 possible values, but string is much longer. In binary byte have 256 position on each. I use for hashes in form of encode64 VARCHAR(255) column with ascii character set and the same collation. VARBINARY causes comparison problem as described in MySQL documentation. I don't know why answers advice to use VARBINARY have so many positives. I checked this on my author site, where measure time (just refresh to see).

Upvotes: 1

Gumbo
Gumbo

Reputation: 655129

The modular crypt format for bcrypt consists of

  • $2$, $2a$ or $2y$ identifying the hashing algorithm and format
  • a two digit value denoting the cost parameter, followed by $
  • a 53 characters long base-64-encoded value (they use the alphabet ., /, 0–9, A–Z, a–z that is different to the standard Base 64 Encoding alphabet) consisting of:
    • 22 characters of salt (effectively only 128 bits of the 132 decoded bits)
    • 31 characters of encrypted output (effectively only 184 bits of the 186 decoded bits)

Thus the total length is 59 or 60 bytes respectively.

As you use the 2a format, you’ll need 60 bytes. And thus for MySQL I’ll recommend to use the CHAR(60) BINARYor BINARY(60) (see The _bin and binary Collations for information about the difference).

CHAR is not binary safe and equality does not depend solely on the byte value but on the actual collation; in the worst case A is treated as equal to a. See The _bin and binary Collations for more information.

Upvotes: 422

Andre D
Andre D

Reputation: 4703

A Bcrypt hash can be stored in a BINARY(40) column.

BINARY(60), as the other answers suggest, is the easiest and most natural choice, but if you want to maximize storage efficiency, you can save 20 bytes by losslessly deconstructing the hash. I've documented this more thoroughly on GitHub: https://github.com/ademarre/binary-mcf

Bcrypt hashes follow a structure referred to as modular crypt format (MCF). Binary MCF (BMCF) decodes these textual hash representations to a more compact binary structure. In the case of Bcrypt, the resulting binary hash is 40 bytes.

Gumbo did a nice job of explaining the four components of a Bcrypt MCF hash:

$<id>$<cost>$<salt><digest>

Decoding to BMCF goes like this:

  1. $<id>$ can be represented in 3 bits.
  2. <cost>$, 04-31, can be represented in 5 bits. Put these together for 1 byte.
  3. The 22-character salt is a (non-standard) base-64 representation of 128 bits. Base-64 decoding yields 16 bytes.
  4. The 31-character hash digest can be base-64 decoded to 23 bytes.
  5. Put it all together for 40 bytes: 1 + 16 + 23

You can read more at the link above, or examine my PHP implementation, also on GitHub.

Upvotes: 62

Mike
Mike

Reputation: 24363

If you are using PHP's password_hash() with the PASSWORD_DEFAULT algorithm to generate the bcrypt hash (which I would assume is a large percentage of people reading this question) be sure to keep in mind that in the future password_hash() might use a different algorithm as the default and this could therefore affect the length of the hash (but it may not necessarily be longer).

From the manual page:

Note that this constant is designed to change over time as new and stronger algorithms are added to PHP. For that reason, the length of the result from using this identifier can change over time. Therefore, it is recommended to store the result in a database column that can expand beyond 60 characters (255 characters would be a good choice).

Using bcrypt, even if you have 1 billion users (i.e. you're currently competing with facebook) to store 255 byte password hashes it would only ~255 GB of data - about the size of a smallish SSD hard drive. It is extremely unlikely that storing the password hash is going to be the bottleneck in your application. However in the off chance that storage space really is an issue for some reason, you can use PASSWORD_BCRYPT to force password_hash() to use bcrypt, even if that's not the default. Just be sure to stay informed about any vulnerabilities found in bcrypt and review the release notes every time a new PHP version is released. If the default algorithm is ever changed it would be good to review why and make an informed decision whether to use the new algorithm or not.

Upvotes: 29

James C
James C

Reputation: 14149

I don't think that there are any neat tricks you can do storing this as you can do for example with an MD5 hash.

I think your best bet is to store it as a CHAR(60) as it is always 60 chars long

Upvotes: 25

Related Questions