The advantages of this method have been that the hashtext can be stored and copied among Unix systems without exposing the corresponding plaintext password to the system administrators or other users. Thus the password is never "decrypted": it is merely used to compute a result, and the matching results are presumed to be proof that the passwords were "the same." It is theoretically possible that two different passwords could result in exactly the same hash. However, one of the properties of DES is that it's very resistant to key recovery even in the face of known plaintext situations. This is technically not encryption since the data (all bits zero) is not being kept secret it's widely known to all in advance. The salt and the final ciphertext are encoded into a printable string in a form of base64. A 12-bit salt is used to perturb the encryption algorithm, so standard DES implementations can't be used to implement crypt(). That key is then used to encrypt an all-bits-zero block, and then the ciphertext is encrypted again with the same key, and so on for a total of 25 DES encryptions. In detail, the user's password is truncated to eight characters, and those are coerced down to only 7-bits each this forms the 56-bit DES key. In addition, the algorithm incorporated a 12-bit salt in order to ensure that an attacker would be forced to crack each password independently as opposed to being able to target the entire password database simultaneously. A goal of this change was to make encryption slower. In Seventh Edition Unix, the scheme was changed to a modified form of the DES algorithm. The original password encryption scheme was found to be too fast and thus subject to brute force enumeration of the most likely passwords. Rather than encrypting the password with a key, which would have allowed the password to be recovered from the encrypted value and the key, it used the password itself as a key, and the password database contained the result of encrypting the password with this key. The original implementation of the crypt() library function in Third Edition Unix mimicked the M-209 cipher machine. Original implementation using the password as a key A number of extra application-defined methods exist. The PHC subset covers a majority of MCF hashes. $gy$jCT$HM87v.7RwpQLba8fDjNSk1$VgqS7k2OZWhFbAJVBye2vaA7ex/1VtU3a5fmL8Wv/26 $md5,rounds=5000$GUBv0xjJ$$mSwgIswdjlTY0YxV7HBVm0 $7$DU./.2Q9obwLhin8qvQl6sisAO/$sHayJj/JBdcuD4lJ1AxiwCo9e5XSi8TcINcmyID12i8 $6$qoE2letU$wWPRl.PVczjzeMVgjiA8LLy2nOyZbf7Amj3qLIL978o18gbMySdKZ7uepq9tmMQXxyTIrS12Pln.2Q/6Xscao0 $5$9ks3nNEqv31FX.F$gdEoLFsCRsn/WRN3wxUnzfeZLoooVlzeF4WjLomTRFD ![]() $2a$10$VIhIOofSMqgdGlL4wzE//e.77dAQGqntF/1dT7bqCrVtquInWy2qi hash: Base64-like encoded result of hashing the password and salt.param name and its value: hash complexity parameters, like rounds/iterations count.id: an identifier representing the hashing algorithm (such as 1 for MD5, 5 for SHA-256 etc.).A well-defined subset of the Modular Crypt Format was created during the Password Hashing Competition. Old crypt(3) hashes generated before the de facto MCF standard may vary from scheme to scheme. To enable backward compatibility, each scheme started using some convention of serializing the password hashes that was later called the Modular Crypt Format (MCF). Over time various algorithms have been introduced. Key derivation functions supported by crypt The crypt() library function is also included in the Perl, PHP, Pike, Python (although it is now deprecated as of 3.11), and Ruby programming languages. ![]() The particular hash algorithm used can be identified by a unique code prefix in the resulting hashtext, following a de facto standard called Modular Crypt Format. Modern Unix implementations of the crypt library routine support a variety of hash schemes. This same crypt function is used both to generate a new hash for storage and also to hash a proffered password with a recorded salt for comparison. To distinguish between the two, writers often refer to the utility program as crypt(1), because it is documented in section 1 of the Unix manual pages, and refer to the C library function as crypt(3), because its documentation is in manual section 3. There is an unrelated crypt utility in Unix, which is often confused with the C library function. ![]()
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