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How To Convert .cer to .pem on Ubuntu Using OpenSSL

To convert a .cer certificate to a .pem format on Ubuntu, you can follow the steps below. This guide will use OpenSSL, a widely used tool for working with SSL/TLS certificates.

Using OpenSSL to Convert .cer to .pem

To convert a .cer certificate to a .pem format, you can use the following OpenSSL command:

openssl x509 -inform der -in certificate.cer -out certificate.pem

Replace certificate.cer with the actual filename of your .cer certificate.

This command will convert the binary encoded .cer certificate to a PEM-formatted certificate, which is more user-friendly and can be read and modified using standard text editors.

Note that the resulting PEM file will include the certificate and key (and possibly certificate chain).

Common errors that may occur when converting a .cer certificate to .pem

When converting a .cer certificate to a .pem format, some common errors may occur.

These errors are often related to the encoding and formatting of the certificates. Here are a few common errors that users may encounter:

  1. ASN.1 Encoding Errors: This error can occur during the conversion process and is related to the Abstract Syntax Notation One (ASN.1) encoding. It may manifest as “wrong tag” or “nested ASN.1 error” and is indicative of a problem with the certificate’s encoding.
  2. PEM Read Bio Error: This error indicates a problem with reading the PEM file, often due to an absence of the expected “—–BEGIN” line at the start of the file. It can occur when attempting to view a .cer file or convert a DER-encoded .cer file to a .pem format.
  3. Exported Private Key: When attempting to convert a .cer file to a .pem file, some users may inadvertently export the private key instead of the public key. This can lead to the presence of “—–BEGIN PRIVATE KEY—–” in the resulting .pem file, which is not the intended outcome.

To mitigate these errors, ensure that the input files are correctly formatted and that the appropriate OpenSSL commands are used.

Additionally, verifying the content of the original .cer file and the resulting .pem file can help identify any unexpected issues during the conversion process.


By following this guide on how to convert a .cer certificate to a .pem format on Ubuntu, you can effectively manage your SSL/TLS certificates and ensure smooth and secure data transmission.

For further information, you can refer to the following resources:

  1. Server Fault
  2. ClickSSL
  3. Stack Overflow
  4. TurnKey Linux

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