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Authentication & Authorization

Password Hashing

Store passwords with bcrypt or Argon2id, per-user salts, cost factors, and constant-time comparison.

7 min readBuilding Backend ApplicationsBuild

Why Hash Passwords

Never store passwords in plaintext. If your database is compromised, every user account is exposed. Hashing transforms passwords into irreversible strings. Even if attackers get the hashes, they cannot recover the original passwords without brute force.

Hashing is not encryption

Encryption is reversible with a key. Hashing is one-way. You verify a password by hashing the input and comparing it to the stored hash. You never decrypt the hash back to the password.

bcrypt

bcrypt is the standard password hashing algorithm. It includes a per-password salt, is memory-hard to resist GPU attacks, and has a configurable cost factor. Go's crypto/x509 package includes bcrypt support.

Gohashing-and-verifying-passwords.go

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GenerateFromPassword automatically generates a salt and hashes the password. CompareHashAndPassword handles constant-time comparison internally.

Argon2id

Argon2id is the winner of the Password Hashing Competition. It is memory-hard and resistant to both GPU and side-channel attacks. It is the recommended algorithm for new applications.

Goargon2id-hashing.go

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Argon2id uses three parameters: time (iterations), memory (KiB), and parallelism (threads). The encoded string includes the parameters, salt, and hash.

Cost Factors

The cost factor determines how slow the hash computation is. Higher cost means slower hashing, which makes brute force attacks impractical. Balance security with response time.

Algorithm	Recommended Parameters	Time per Hash
bcrypt	Cost 12	~250ms
Argon2id	m=65536, t=1, p=4	~100ms
Argon2id (high security)	m=256000, t=3, p=4	~500ms

Benchmark on your hardware

Run a benchmark to find the right cost factor. Hash 1000 passwords and aim for 100-300ms per hash. This is slow enough to resist brute force but fast enough to not impact login response time.

Constant-Time Comparison

Regular string comparison short-circuits on the first differing byte. This leaks timing information that attackers can use to guess the hash one character at a time. Constant-time comparison takes the same time regardless of where strings differ.

Gousing-subtle.constanttimecompare.go

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subtle.ConstantTimeCompare returns 1 if equal, 0 if not. The comparison always examines every byte, preventing timing attacks.

Password Validation

Enforce password requirements at signup and password change. Minimum length is the most important rule. Complexity requirements often hurt more than they help.

Gopassword-validation.go

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Length is the primary security factor. A 20-character passphrase is more secure than a complex 8-character password. Check against leaked password databases.

Never truncate passwords

Some systems truncate passwords at a maximum length. This silently reduces security. Set a generous maximum (128 characters) and hash the full input.

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