Hash Generator

Yes, it uses the Web Crypto API (crypto.subtle.digest) which provides cryptographically secure hash functions.

SHA-256 is the sensible default for integrity checking and general use. MD5 and SHA-1 are both cryptographically broken — practical collision attacks exist for each — so restrict them to non-security uses such as cache keys or checksums against accidental corruption. Never select a hash for password storage on this basis; see the next answer.

No, and this is important. General-purpose hashes are designed to be fast, which is exactly wrong for passwords because it lets an attacker try billions of guesses per second. Password storage needs a deliberately slow, salted algorithm such as bcrypt, scrypt or Argon2id, with Argon2id the current recommendation.

Not mathematically — a hash discards information, so there is no decrypt operation. It can still be defeated by guessing: common passwords and short strings are readily found in precomputed rainbow tables. Hashing is not encryption and provides no confidentiality for low-entropy input.

A collision is two different inputs producing the same hash. Because the output is a fixed size, collisions must exist; what matters is whether anyone can construct one deliberately. For MD5 and SHA-1 they can, cheaply, which is why those algorithms are unfit for signatures or certificates. No practical collision is known for SHA-256.

Usually one of three reasons: a different algorithm than the one published, hashing the text of a file rather than its raw bytes, or an invisible difference in the input such as a trailing newline or Windows line endings. Confirm the algorithm and hash the exact bytes.

No. Each algorithm always produces a fixed-size output: MD5 gives 128 bits (32 hex characters), SHA-1 gives 160 bits (40 characters), SHA-256 gives 256 bits (64 characters) and SHA-512 gives 512 bits (128 characters), whether you hash one letter or a gigabyte.