Excerpt
While no real-world attacks have yet compromised SHA1 in practice, academic research shows it is vulnerable to collisions. Developers should transition to more secure hashes like SHA256 and SHA3 to minimize future exploitation risks.
SHA1 has been one of the most widely used cryptographic hash functions since its introduction in 1995. However, in recent years SHA1 has shown vulnerabilities against collision attacks, raising concerns about its security. This blog examines the current status of SHA1 and the likelihood of practical attacks breaking its security.
Introduction
SHA1 (Secure Hash Algorithm 1) is a 160-bit cryptographic hash function designed by the NSA and published by NIST as a US federal standard. It generates a 160-bit hash value for an input of up to 264 bits in length. Due to its speed and simplicity, SHA1 has been extensively used for digital signatures and data integrity verification.
Background of SHA1
SHA1 was designed by the NSA and submitted to NIST in 1993 as part of the Digital Signature Standard. It was revised in 1995 and published as FIPS 180 becoming the first SHA standard. It quickly gained popularity due to performance improvements over prior standards like MD5.
SHA1 outputs a condensed representation of the input message which can be used to efficiently verify its integrity. Hashing is an integral part of security protocols like SSL, TLS, SSH, S/MIME, and PGP which have relied on SHA1 for years.
How SHA1 Works
The SHA1 algorithm takes input messages less than 2^64 bits and produces a 160-bit message digest. It processes blocks of 512 bits using a series of logical functions and bitwise operations.
Some key steps are:
- Padding the input to be 448 mod 512 bits
- Dividing padded input into 512-bit blocks
- Setting initial hash value to scrambled hex digits
- Iteratively processing each 512-bit message block
- Using bitwise logic and constant values
- Updating intermediate hash value
- After all blocks, outputting the final 160-bit hash
The hash value serves as the digital fingerprint of the input message.
Vulnerabilities of SHA1
Research over the years has revealed two major vulnerabilities in SHA1:
Collision Attacks
- Collisions involve two inputs hashing to the same value.
- Complexity of finding collisions in SHA1 is 2^80 operations.
- Academics have demonstrated real-world SHA1 collision attacks.
Security Decline Due to Computing Advances
- Increasing computing power has weakened SHA1’s security.
- Brute force and analytical attacks against SHA1 are more feasible.
- Hash rate estimates for breaking SHA1 are now in the single-digit billions of dollars range.
These vulnerabilities mean SHA1 is no longer considered secure against well-funded adversaries.
Real-World Exploits of SHA1
Some notable collisions attacks against SHA1 include:
In 2017, Google revealed the first real-world SHA1 collision with two PDF documents with different content but the same SHA1 hash.
Researchers also showed a SHA1 collision in a P2P digital signature standard used by bitcoin and other cryptocurrencies.
In 2022, CIA documents from 2010 regarding the Osama bin Laden raid were shown to use recycled SHA1 hashes pointing to potential malware exploitation.
Current Status of SHA1
Due to growing risks from SHA1 hashes, software vendors and agencies have recommended withdrawing support:
Microsoft disabled SHA1 by default in 2013 citing collision risks.
NIST deprecated use of SHA1 for digital signatures in 2011 due to threats of misuse.
Major web browsers have removed support for SHA1 signed TLS certificates, instead requiring SHA256 or better.
Cybersecurity agencies advise that SHA1 should be retired except to verify old digital signatures.
Alternatives to SHA1
Some more secure hash functions that can replace SHA1 include:
SHA2 algorithms (SHA256, SHA512) with longer digest sizes.
SHA3 with resistance against length extension and cryptanalytic attacks.
BLAKE2 hash function optimized for speed without compromising security.
Conclusion
In summary, while there are no known real-world collisions compromising applications yet, SHA1 is considered vulnerable to practical attacks based on academic research and computing advances. Developers and enterprises should transition away from SHA1 usage to more secure hash functions like SHA256 and SHA3 to minimize risks as exploitation capabilities improve.
