A cryptographic hash (sometimes called ‘digest’) is a kind of ‘signature’ for a text or a data file. SHA-256 generates an almost-unique 256-bit (32-byte) signature for a text. See below for the source code.
A hash is not ‘encryption’ – it cannot be decrypted back to the original text (it is a ‘one-way’ cryptographic function, and is a fixed size for any size of source text). This makes it suitable when it is appropriate to compare ‘hashed’ versions of texts, as opposed to decrypting the text to obtain the original version.
Such applications include hash tables, integrity verification, challenge handshake authentication, digital signatures, etc.
Note that hash functions are not appropriate for storing encrypted passwords, as they are designed to be fast to compute, and hence would be candidates for brute-force attacks. Key derivation functions such as bcrypt or scrypt are designed to be slow to compute, and are more appropriate for password storage (npm has bcrypt and scrypt libraries, and PHP has a bcrypt implementation with password_hash).
SHA-256 is one of the successor hash functions to SHA-1 (collectively referred to as SHA-2), and is one of the strongest hash functions available. SHA-256 is not much more complex to code than SHA-1, and has not yet been compromised in any way. The 256-bit key makes it a good partner-function for AES. It is defined in the NIST (National Institute of Standards and Technology) standard ‘FIPS 180-4’. NIST also provide a number of test vectors to verify correctness of implementation. There is a good description at Wikipedia.
This script is oriented toward hashing text messages rather than binary data. The standard considers hashing byte-stream (or bit-stream) messages only. Text which contains (multi-byte) characters outside ISO 8859-1 (i.e. accented characters outside Latin-1 or non-European character sets – anything with Unicode code-point above U+FF), can’t be encoded 4-per-word, so the script defaults to encoding the text as UTF-8 before hashing it.
Notes on the implementation of the preprocessing stage:
M[N-1] = ((msg.length-1)*8) >>> 32;
M[N-1] = ((msg.length-1)*8) & 0xffffffff;
Note that what is returned is the textual hexadecimal representation of the binary hash. This can be useful for instance for storing hashed passwords, but if you want to use the hash as a key to an encryption routine, for example, you will want to use the binary value not this textual representation.
Using Chrome on a low-to-middling Core i5 PC, in timing tests this script will hash a short message in around 0.03 – 0.06 ms; longer messages will be hashed at a speed of around 2 – 3 MB/sec.
Note that these scripts are intended to assist in studying the algorithms, not for production use. For production use, I would recommend the Web Cryptography API for the browser (see example), or the crypto library in Node.js. For password hashing, I have a WebCrypto example using PBKDF2.
also available on GitHub.
§ection numbers relate the code back to sections in the standard.
Note I use Greek letters in the ‘logical functions’, as presented in the spec (if you encounter
any problems, ensure your
I offer these scripts for free use and adaptation to balance my debt to the open-source info-verse. You are welcome to re-use these scripts [under an MIT licence, without any warranty express or implied] provided solely that you retain my copyright notice and a link to this page.
If you would like to show your appreciation and support continued development of these scripts, I would most gratefully accept donations.
If you have any queries or find any problems, contact me at ku.oc.epyt-elbavom@cne-stpircs.
© 2005-2017 Chris Veness