Question 1

Is SHA-1 still safe to use?

Accepted Answer

SHA-1 is collision-broken since 2017 (Google's SHAttered attack). Any use case where collisions matter — digital signatures, TLS certificates, code signing — has migrated to SHA-256 or stronger. SHA-1 remains in use for legacy compatibility (Git's content-addressed storage until v2.42+, some HMAC contexts) and for non-adversarial checksums where collision attacks don't apply. NIST formally deprecated SHA-1 in 2011 and removed all approval in 2030. If you have a choice, use SHA-256.

Question 2

Why is Git still using SHA-1?

Accepted Answer

Git used SHA-1 from 2005 because it was the standard cryptographic hash at the time. Migrating to SHA-256 is a major undertaking because every Git commit hash, every object reference, every signed tag — billions of identifiers across the world's Git repositories — would change. Git 2.42+ supports SHA-256 as an opt-in alternative for new repos, but most existing repos remain SHA-1 indefinitely. The practical risk is low because Git's threat model assumes a developer-trusted environment — collision attacks would have to target a specific Git repo and convince the maintainer to accept a malicious object.

Question 3

Is HMAC-SHA-1 safe?

Accepted Answer

HMAC-SHA-1 is more robust than plain SHA-1 because the secret key disrupts collision-finding strategies. RFC 6194 notes HMAC-SHA-1 'is not affected by collision attacks' but recommends HMAC-SHA-256 for new protocols. Existing HMAC-SHA-1 deployments (some legacy TLS suites, AWS Signature V2) are not actively broken but are deprecated. New systems should use HMAC-SHA-256 minimum.

Question 4

What's the difference between SHA-1 and SHA-2?

Accepted Answer

SHA-1 is the original 1995 Secure Hash Algorithm — a 160-bit output. SHA-2 is the 2001 successor family with variants SHA-224, SHA-256, SHA-384, SHA-512, producing 224 to 512 bits. SHA-2 uses similar Merkle-Damgård construction but with larger state, more rounds, and bigger words. SHA-3 (2015, also called Keccak) is a completely different algorithm based on a sponge construction — not a replacement for SHA-2 but a diversification in case both fall to the same attack.

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