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How long (in letters) are encryption keys for AES?

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How long is the encryption/decryption key for an assymetric algorithm, such as AES? If I use AES 128-bit, how many characters should I type in for my key? What about AES 256-bit?

Edit: Here is why I am asking: I am trying to use OpenSSL to encrypt some data using Node.js, PHP and command-line. I need to give in the key. When I tried 32 letter-key for AES128, it returned key length error. When I tried 32 for AES256 it returned general key error. I have no idea how I am supposed to enter the encryption key...

asked Nov 11, 2013 at 23:10

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An AES 128-bit key can be expressed as a hexadecimal string with 32 characters. It will require 24 characters in base64.

An AES 256-bit key can be expressed as a hexadecimal string with 64 characters. It will require 44 characters in base64.

answered Nov 12, 2013 at 6:15

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Encryption keys for AES are not expressed in characters or letters. Encryption keys are a series of 128 (or 256) bits.

If you are using some kind of interface to enter a text-based password, internally it is turning your typed password into bits. It may be using a complex routine like PBKDF2, or a single iteration of SHA-1.

In that case, you need to understand the unpredictability of someone guessing your password, instead of thinking of a minimum number of characters. Much has been written on the topic. I suggest you google for "correct horse battery staple" for an excellent primer on the topic.

answered Nov 12, 2013 at 6:11

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For a 128-bit key:

8-bits per character raw (base-256, i.e. where a character can take any byte value)
6-bits per character base-64 encoded

answered Nov 11, 2013 at 23:27

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