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Digital signature mini-tutorial

3 min read • tagged encryption

I’ve been doing a lot of reading into blockchain recently, and one of the foundational components of this technology is utilizing asymmetric encryption to produce a “digital signature”. These signatures accompany a message sent across the wire, and can be used to verify the authenticity of the sender and the veracity of the message (i.e. ensuring it was not modified maliciously during transmission).

Below is a short demonstration of creating and verifying a digital signature using the openssl CLI tool.

  1. Generate a private key, also called the sk or “secure key” for short. The key will be 2048 bits in length, as specified by the rsa_keygen_bits option.
$ openssl genpkey -algorithm RSA -out private_key.pem -pkeyopt rsa_keygen_bits:2048

  1. Generate the corresponding public key, also called the pk for short.
$ openssl rsa -pubout -in private_key.pem -out public_key.pem

  1. Create a secret message.
$ echo “Rock Chalk Jayhawk, go KU!> secret_message.txt

  1. Produce the digital signature for the secret message. This command is two separate operations: first, it produces a hash of secret_message.txt (commonly referred to as the message “digest”) using the SHA256 hash function. Then, it encrypts that digest using the pk. The process of encrypting the digest is what actually produces the digital signature.
openssl dgst -sha256 -sign private_key.pem -out digital_signature secret_message.txt

  1. (Optional) the produced digital_signature contains binary content by default. To get a human-readable version use the enc command.
$ openssl enc -base64 -in digital_signature -out digital_signature.base64

The digital_signature.base64 will look like:

$ cat digital_signature.base64
$ tjisvQHiIte5wf5J+ZTNYaXk1a/90eQG8Haxu16qSlO/MhP6XjRE1P87XxvbtrOt

  1. (Imagination required) Send the secret_message.txt, corresponding digital_signature and a copy of our public_key.pem off to a friend. The friend receives the message, and would like to verify its contents and that it actually came from us. To verify, they attempt to decrypt the digital signature using our pk:
$ openssl dgst -sha256 -verify public_key.pem -signature digital_signature secret_message.txt
$ Verified OK

  1. Now imagine that our secret_message.txt was actually intercepted and tampered with by an attacker. You can simulate this by making any change to secret_message.txt, e.g. change the message string to “Rock Chalk Jayhawk, go KSU!” Running verification on this modified message will now fail:
$ echo “Rock Chalk Jayhawk, go KSU!> secret_message.txt
$ openssl dgst -sha256 -verify public_key.pem -signature digital_signature secret_memo.txt
$ Verification failure

That’s it! Digital signatures provide a convenient way to validate the origin and authenticity of a message.

Thanks for reading!


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