What is password shadowing?

Question: What is password shadowing?
Answer: Password shadowing is a security system where the encrypted password field of /etc/passwd is replaced with a special token and the encrypted password is stored in a separate file (or files) which is not readable by normal system users

what is substr_replace?

Question: what is substr_replace?
Answer: It is a function in php it takes four arguments and is used for substituting the a string

Difference between hashing and encryption?

Question: What is the difference between hashing and encryption?

Answer:
Encryption is a scheme where an intelligible text (plaintext in crypto terms) is made unintelligible (ciphertext in crypto terms) using a secure key. Block and stream ciphers and public key systems do this work. The security of the ciphers reside in the key length and decryption process is a difficult without proper knowledge of the key. But in hashing‚ they are one-way functions that compress arbitrary length strings into fixed short strings (message digests). Hash Functions can be designed using block ciphers using a secret key as a parameter along with the message that has to be hashed or with out them (dedicated hash functions MD4‚MD5‚SHA-1 etc..).