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Asked: 2026-05-20T20:04:12+00:00

I need to generate a safe prime which has the form 2p + 1

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I need to generate a safe prime which has the form 2p + 1 where p is also prime of a certain
bit length (lets say 1024 bits). It is to be used in a DH key exchange.

I believe openssl can do this via

openssl gendh 1024

however this return’s a base64 pem format 

-----BEGIN DH PARAMETERS-----
MIGHAoGBANzQ1j1z7RGB8XUagrGWK5a8AABecNrovcIgalv1hQdkna2PZorHtbOa
wYe1eDy1t/EztsM2Cncwvj5LBO3Zqsd5tneehKf8JoT35/q1ZznfLD8s/quBgrH8
es2xjSD/9syOMMiSv7/72GPJ8hzhLrbTgNlZ+kYBAPw/GcTlYjc7AgEC
-----END DH PARAMETERS-----

  • How can I extract the safe prime number from this base64 pem?

  • is it easier to generate my own safe prime with my own code?

how can i test that a prime is ‘safe’ and of a certain bit length.

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1 Answer

  1. Editorial Team

    @GregS has an approach that will probably work for you. Based on what you have told me, I would just create a C binary and leverage the BN_generate_prime(...) function in OpenSSL. That cuts out all of the intermediate parsing and despite adding a separate binary into the mix, it’s probably easier than the road you are headed down.

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