Convert CER to PEM — Free Online Certificate Converter
Convert a binary DER-encoded CER certificate to PEM format for Nginx, Apache, and Linux servers — 100% in-browser.
Frequently Asked Questions
CER vs PEM Certificate Formats
SSL/TLS certificates are stored in two primary encoding formats: DER (Distinguished Encoding Rules) binary and PEM (Privacy Enhanced Mail) Base64 text. The .cer extension typically indicates a DER-encoded binary certificate, although Windows sometimes exports .cer as PEM — this tool handles both cases.
CER / DER files are compact binary representations of the ASN.1 certificate structure. They cannot be opened in a text editor. Windows Certificate Manager, Active Directory Certificate Services (AD CS), and IIS commonly use this format.
PEM certificates are the same ASN.1 data encoded in Base64 and wrapped with -----BEGIN CERTIFICATE----- and -----END CERTIFICATE----- headers. Nginx, Apache, OpenSSL, and virtually every Linux tool expects PEM format.
This conversion simply Base64-encodes the raw DER bytes and adds the correct PEM headers. The conversion is completely lossless — the certificate data is identical; only the representation changes. You can verify the result by decoding the PEM back to DER and comparing the byte-for-byte output.