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DER (Distinguished Encoding Rules) is the binary format for encoding X.509 certificates and keys. While PEM wraps the same data in Base64 text with headers, DER stores it as raw binary. The data is identical — DER is just the compact, non-text representation.
You'll encounter DER files in Java keystores (Java prefers binary formats), embedded systems (where text overhead matters), and some Windows certificate workflows. DER files are smaller than their PEM equivalents (no Base64 overhead) but can't be concatenated, pasted into text fields, or included in configuration files that expect text.
Converting between DER and PEM is trivial with OpenSSL: `openssl x509 -in cert.der -inform der -out cert.pem` converts to PEM. Most certificate operations work with either format — the tools just need to know which encoding to expect.