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What hash function does OpenSSL use to generate a key for AES-256? I can't find it anywhere in their documentation.

$ touch file
$ openssl aes-256-cbc -nosalt -P -in file

enter aes-256-cbc encryption password: (I type "a" and hit enter)
Verifying - enter aes-256-cbc encryption password: (I type "a" and hit enter)

iv =4FA92C5873672E20FB163A0BCB2BB4A4

Which hash algorithm generates the unsalted hash after key= on the second last line, for the input "a"?

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4 Answers 4

up vote 4 down vote accepted

Fairly sure it's an SHA1 digest algorithm but in all honesty I can't say with 100% certainty.

And who would have thought that something designed to increase obtuseness would have obtuse instructions ;)

EDIT: This may not be helpful in your circumstances but I guess you could always know by doing

openssl enc -d -a -md sha1 -aes-256-cbc -nosalt -p 
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I have determined that it uses MD5 by default, as when I use your command (sidenote: none of those options are documented in the mage page...) with md5 instead of sha1, I get the same results as I originally posted. The question is, how does it get 256 bits from MD5 (a 128-bit hashing algorithm)? – Mk12 Sep 8 '12 at 0:30
One way this is done is by concatenating two disparate MD5's in binary form which results in a true 256 bit key. There are a few other methods for this as well. You might check out the php package "md5_base64". Even if you're not a php guy, the docs are pretty informative. – Snesticle Sep 8 '12 at 1:20

It's a concatenation of two MD5 hashes.

It's derived like this:

128bit_Key = MD5(Passphrase + Salt)
256bit_Key = 128bit_Key + MD5(128bit_Key + Passphrase + Salt)

You can check this by doing:

$ echo Testing > file
$ openssl enc -aes-256-cbc -p -in file -out file.aes -salt
: enter aes-256-cbc encryption password: abc
: Verifying - enter aes-256-cbc encryption password: abc
: salt=3025373CA0530C93
: key=E165475C6D8B9DD0B696EE2A37D7176DFDF4D7B510406648E70BAE8E80493E5E
: iv =B030394C16C76C7A94DC22FDDB6B0744
$ perl -e 'print pack "H*", "3025373CA0530C93"' > salt
$ echo -n abc > passphrase
$ cat passphrase > key.128.tmp
$ cat salt >> key.128.tmp
$ md5sum key.128.tmp 
: e165475c6d8b9dd0b696ee2a37d7176d  key.128.tmp
$ perl -e 'print pack "H*", "e165475c6d8b9dd0b696ee2a37d7176d"' > key.128
$ cat key.128 > key.256.tmp
$ cat passphrase >> key.256.tmp
$ cat salt >> key.256.tmp
$ md5sum key.256.tmp 
: fdf4d7b510406648e70bae8e80493e5e  key.256.tmp

Notice how both MD5's of 'key.128.tmp' and 'key.256.tmp' concatenated together form the same key as output at the initial command.

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And if you do a third iteration you'll get the IV. This case is convenient because the key and data=IV sizes (256 and 128 bits) are both exact multiples of the hash output; in general you concatenate the hash outputs and take the first K bits for the key and the next D bits for the IV. – dave_thompson_085 Sep 10 at 21:52

I don't know the answer, but you could probably find it easily enough in the OpenSSL source code.

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"Easily enough"—The main function (where the password-asking functionality lives) is ~500 lines long and littered with gotos. – Mk12 Jul 30 '12 at 18:24
Wow. I just looked at the source code. It's virtually unreadable. No comments. One-letter variable names. Ugh. I'm sorry I suggested that. – Fran Jul 30 '12 at 18:37

OpenSSL uses AES with SHA1.

If you wish to examine better-written source than OpenSSL, have a look at the article
C++ class that interfaces to OpenSSL ciphers.

The article includes very simple source code that :

allows you to encrypt and decrypt files or strings using the OpenSSL AES-256-CBC cipher and SHA1 digest algorithms. It is interoperable with the openssl command line tool which makes it a good introduction to using OpenSSL for ciphers.

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