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All about SHA1, SHA2 and SHA256 hash algorithms

SHA256, provided by TBS Internet since 2008, will in the coming few years replace SHA1. But what is SHA?


SHA - standing for secure hash algorithm - is a hash algorithm used by certification authorities to sign certificates and CRL (certificates revocation list). Introduced in 1993 by NSA with SHA0, it is used to generate unique hash values from files.

Example: A file hashed with SHA1 could look like:

As for any cryptographic solution, SHA must evolve along with our computers' calculation capacities in order to avoid any weakness.

There are, therefore, several versions of SHA: SHA0 (obsolete because vulnerable), SHA1 (the most popular one), SHA2 (the one we are interested in) and finally SHA3 introduced in 2012.


SHA2, not often used for now, is the successor of SHA1 and gathered 4 kinds of hash functions: SHA224, SHA256, SHA384 and SHA512.

It works the same way than SHA1 but is stronger and generate a longer hash.

Hash attacks, SHA1 and SHA2

There are 2 kinds of attacks specific to hash:

  • A collision: there is collision when 2 different files produce an identical hash. It is then possible to substitute a file for an other. In our domain of expertise we could then imagine to replace an official certificate by a fraudulent one having the same hash values. SHA0 is not resistant to collision attacks, that is the reason why it is not used anymore.
  • the preimage: one needs to distinguish preimage from first-preimage. The first one consists of 'guessing' a file value from its hash. The other one uses a hash to create a value different from the one that has been used to generate the hash.

What about SHA1 and SHA2?

Even if there has never been a successful complete collision with SHA1, the evolution of our computers' calculation capacities will soon make it possible. SHA2 is totally collision-resistant, its time has then come!

New standards / recommandations from certification authorities: Depreciation of SHA1

Note that SHA1 is being depraciated by browsers (Internet Explorer, Google Chrome, ...) and that the deadline for its scheduled disappearance is 2016-12-31. Therefore, the certification authorities that we work with do not issue certificates expiring after this date anymore.

Learn more

SHA256 hash algorithm does not intervene in the encryption / authentication process but tools (browsers, email clients, servers...) must be able to read / decipher this kind of hash during the connection / authentication process.
  • if you install a SHA256 certificate on a client (strong authentication by certificate),
    make the client (browser, webservice...) and the servers are compatible
    even if the server keep using a SHA1/MD5 signed certificate.

  • if you install a SHA256 certificate on a server then all the clients connecting to it and the server must be SHA256-compatible.

  • If you are using a SHA256 certificate to sign emails or documents, only the readers have to be compatible.

OpenSSL and SHA256

By default, OpenSSL cryptographic tools are configured to make SHA1 signatures.
for example, if you want to generate a SHA256-signed certificate request (CSR) , add in the command line: -sha256, as:

openssl req -new -newkey rsa:2048 -nodes -sha256 -out -keyout www.mydomain.key -subj "/C=FR/ST=Calvados/L=CAEN/O=TBS INTERNET/"

Note: To do so, you'll need OpenSSL version 0.9.8o at least.

Decipher a SSL certificate:
openssl x509 -text -in [path-to-your-certificate]
Check the field "Signature Algorithm". It should indicate either "sha1WithRSAEncryption" or "sha256WithRSAEncryption".

Useful links

Check your certificate installation with Co-Pibot:

In your Certificates' center, on your certificate's status page you'll see a "check your certificate" button. Click it to make sure your certificate has correctly been installed.