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How to Encrypt Text with OpenSSL

Encrypt text with OpenSSL using a salted PBKDF2 recipe, preserve the settings needed for decryption, and verify the exact result.

Encrypt Online Editorial Team5 min read
How to Encrypt Text with OpenSSL guide cover

Before you start

Run the workflow once with a disposable value, then do a decrypt or restore check before you share anything real.

An OpenSSL command can produce ciphertext in seconds. Recovering that text later depends on something less obvious: keeping the cipher and password-derivation settings with the encrypted data.

The salt can travel inside normal openssl enc output, but the command is not a self-describing package. Save the recipe, not just the ciphertext.

In brief

Use a salted password-based workflow, select PBKDF2 explicitly, and record the cipher, digest, iteration count, and Base64 flags. The same choices must be supplied during decryption.

This guide uses AES-256-CBC because it is available through openssl enc. CBC encryption does not authenticate the ciphertext, and OpenSSL does not support GCM or CCM through this command.

Start with a repeatable command

This example prompts for a passphrase instead of placing it in shell history:

Shell
printf '%s' 'hello' > message.txt
openssl enc -aes-256-cbc -salt -pbkdf2 -iter 100000 -md sha256 \
  -a -A -in message.txt -out message.enc

The result in message.enc is Base64 text on one line. Run the command twice with the same message and passphrase and the ciphertext should still differ because OpenSSL generates a new random salt.

Know what each setting controls

  • -aes-256-cbc selects the cipher and mode. Decryption must use the same choice.
  • -salt asks OpenSSL to use a salt when deriving the key and IV. It is the default, but writing it down makes the recipe clearer.
  • -pbkdf2 selects PBKDF2 instead of the older password derivation behavior.
  • -iter 100000 sets the PBKDF2 work factor. This is an example value, not a universal policy; measure an acceptable delay for your environment.
  • -md sha256 makes the PBKDF2 digest choice explicit.
  • -a Base64-encodes the encrypted bytes. -A keeps that output on one line.

When OpenSSL generates the salt, the encrypted bytes normally begin with the ASCII marker Salted__, followed by the salt and ciphertext. This recipe does not set -saltlen, so it uses the eight-byte compatibility default.

OpenSSL 3.2 added -saltlen for PBKDF2, with a current maximum of 16 bytes. If you set it, record the value and use the same value for decryption. The Salted__ envelope does not store the selected salt length as a separate field.

Save the decrypt recipe with the output

Test the exact artifact before you archive or send it:

Shell
openssl enc -d -aes-256-cbc -pbkdf2 -iter 100000 -md sha256 \
  -a -A -in message.enc -out message.dec
cmp -s message.txt message.dec && echo 'exact match'

Store the command in documentation or a small README beside the encrypted file. Do not store the passphrase there.

Treat the format boundary as part of the recipe

PBKDF2-based openssl enc output is not accepted by the Encrypt Online homepage or /decrypt. Those interfaces use a different envelope and password-derivation scheme. Decrypt this output with a matching openssl enc -d command.

The OpenSSL AES Decrypt Checker can inspect the envelope and test a documented recipe locally. It cannot recover a missing cipher, KDF, digest, iteration count, or passphrase. A Base64 decoder can inspect the transport text, but Base64 decoding does not decrypt the underlying bytes.

Decide whether openssl enc fits the job

OpenSSL documents an important limit: enc does not support authenticated-encryption modes such as GCM and CCM. A successful CBC decryption is not proof that the ciphertext was not modified.

For a durable data format, recipient-based encryption, or built-in integrity handling, choose a format designed for that job. OpenSSL specifically recommends CMS for bulk encryption because it supplies a standard container and manages key, IV, and nonce details.

Mistakes that make the ciphertext hard to recover

  • Keeping only the ciphertext and forgetting the iteration count or digest.
  • Setting a non-default -saltlen in OpenSSL 3.2 or later without recording it.
  • Encrypting with -a -A but omitting those settings when decrypting a single-line value.
  • Using -nosalt outside a controlled compatibility test.
  • Supplying an explicit -S salt without preserving it. OpenSSL 3.0 also changed how explicit salts are stored and read.
  • Treating a padding error, or the lack of one, as a reliable integrity check.
  • Passing a real passphrase directly on the command line where shell history or process inspection may expose it.

Questions that come up later

Why does the ciphertext change every time?

The random salt changes the derived key and IV even when the message and passphrase stay the same. That is expected and helps prevent identical passphrases from producing identical encryption inputs.

Can OpenSSL recover the settings automatically?

It can read a normally embedded salt when the compatible salt length is known, but it cannot infer the full recipe. You still need the cipher, KDF, digest, iteration count, salt length, and Base64 choices.

Is a larger iteration count always better?

It raises the cost of password guessing, but it also raises the cost of every legitimate encrypt and decrypt operation. Pick a value based on measured performance and document it as part of the format.

References

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