MD5

MD5 (Message Digest Algorithm 5) is a cryptographic hash function that transforms data (file, password, etc.) into a unique 128-bit digital fingerprint, typically represented by a 32-character hexadecimal string.

This fingerprint allows you to verify the integrity of the data: any modification to the original data produces a different fingerprint.

MD5 hashing transforms data into a 128-bit hash using nonlinear operations combined with binary rotations and additions.

MD5 processes the message in 512-bit blocks and modifies four registers A, B, C, and D (initialized to 0x67452301, 0xEFCDAB89, 0x98BADCFE, and 0x10325476, respectively).

Each block undergoes 64 steps where MD5 combines:

— one of the following nonlinear functions: F(B,C,D) = (B & C) | ((~B) & D), G(B,C,D) = (B & D) | (C & (~D)), H(B,C,D) = B ^ C ^ D or I(B,C,D) = C ^ (B | (~D))

— an addition (modulo 2^32) with A, a constant K, and a word from the message M,

— a left-bitwise rotation,

— then another addition with B.

These operations shuffle the bits using logical functions, carry-overs from additions, and rotations. After all the blocks, the concatenated registers A, B, C, and D form the 128-bit hash.

MD5 is a one-way function: there is no md5decrypt() method to recover the original data from its hash.

However, brute-force attacks (testing all possible combinations) or attacks using rainbow tables (databases of pre-calculated hashes) can recover the entry if it is simple or present in a database.

dCode uses its word dictionaries (10 million potential passwords) from which the MD5 hash is already pre-calculated.

The hash is composed of 32 hexadecimal characters 0123456789abcdef, so 128 bits.

The characters are close to random, therefore unpredictable with high entropy.

MD5 is used for file integrity verification (as a checksum), creating digital signatures, and in certain security protocols.

However, its use is now discouraged for the security of sensitive data such as passwords.

Statistically speaking, for any string (and there is an infinite number), the MD5 associates for a given value a 128-bit fingerprint (a finite number of possibilities). It is therefore mandatory that there are collisions (2 strings with the same hash). Several research works on the subject have demonstrated that the MD5 algorithm, although creating a large entropy of data, could be attacked, and that it was possible to generate chains with the same fingerprints (after several hours of neat calculations).

Since this publication in 2005, MD5 encryption is no longer considered cryptographically, giving way to its successors: SHA1 then SHA256.

The MD5 is threatened by the growing computing capabilities of supercomputers and processors capable of parallelizing hash functions. Thus, to complicate the search by the rainbow tables (passwords databases), it is recommended to add salt (a prefix or a suffix) to the password. With this salting step, the precalculated tables must be calculated again to take account of the salt which systematically modifies all the fingerprints.

Another variant is the application of DOUBLE MD5, which consists in applying the hash algorithm twice.

MD5 is not the only hash function, it also exists SHA1, SHA256, SHA512 etc.

MD5 stands for Message Digest 5:

— Message Digest: A unique summary (hint) of a piece of data.

— 5: The 5th version of the algorithm (successor to MD4).

The PHP language has a default functionality: the type juggling which allows to not define the type of variable used, the PHP engine tries to automatically detect if the variable is a string, an integer, etc.

However this functionality can become a flaw when handling MD5 string whose value has the form 0e followed by digits between 0 and 9. Indeed, in this case, the PHP engine will convert the string into a floating number having the value 0.

Here is a list of magic MD5 hashes:

Bonus strings that can also be evaluated at 0: 0e215962017, 0e730083352, 0e807097110, 0e840922711

The MD5 was invented by Ronald Rivest in 1991 to replace its predecessor, MD4.