5.3. File Extension

5.3. File Extension

Although textual encodings of PKIX structures can occur anywhere, many tools are known to offer an option to output this encoding when serializing PKIX structures. To promote interoperability and to separate DER encodings from textual encodings, the extension ".crt" SHOULD be used for the textual encoding of a certificate. Implementations should be aware that in spite of this recommendation, many tools still default to encode certificates in this textual encoding with the extension ".cer".

This section does not disturb the official application/pkix-cert registration [RFC2585] in any way (which states that "each '.cer' file contains exactly one certificate, encoded in DER format"), but merely articulates a widespread, de facto alternative.