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What BER does
ASN.1 describes data types; BER specifies how values of those types are represented. As Recommendation ITU-T X.690 | ISO/IEC 8825-1 puts it, “Application of these encoding rules produces a transfer syntax for such values.” The standard defines BER alongside two more constrained rule sets: Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER).
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BER applies when an ASN.1-based protocol or application calls for it. It is not safe to assume that every ASN.1 protocol uses BER or that BER is interchangeable with CER or DER in every context; the relevant protocol profile or standard determines what is permitted.
What a BER-encoded value looks like
At an introductory level, X.690 describes a data-value encoding as an ordered sequence of components:
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- Identifier octets: identify the ASN.1 tag, indicating the kind of value.
- Length octets: indicate the length form and, where applicable, the size of the contents.
- Contents octets: carry the encoded value.
- End-of-contents octets: appear when the length form requires them; they are omitted otherwise.
This outline explains the outer structure, not all the rules needed to implement a decoder. Primitive and constructed forms and detailed length rules add important distinctions, so exact behavior should be checked in the standard and the applicable protocol profile.
How BER differs from CER and DER
BER allows alternative encodings in cases permitted by the basic rules. A conforming receiver must support those alternatives. CER and DER remove such alternatives, requiring a single permitted encoding for a value. The X.690 summary distinguishes them by permitted length form and offers a general suitability note:
| Rule set | Permitted alternatives | Length form | X.690 summary |
|---|---|---|---|
| BER | Alternatives may be available where the basic rules permit them. | Not limited to a single length form by the summary. | Basic encoding rules; conforming receivers support permitted alternatives. |
| CER | One permitted alternative for each value. | Indefinite length. | Described as more suitable for large encoded values. |
| DER | One permitted alternative for each value. | Definite length. | Described as more suitable for small encoded values. |
The suitability descriptions are the recommendation’s summary, not a universal performance result or a quantified comparison. Choose the rule set required by the application or protocol rather than treating the three as freely interchangeable.
Which standard defines BER?
The current edition identified by ITU-T is Recommendation X.690 (02/2021), approved on 13 February 2021 and listed as in force. ITU-T identifies ISO/IEC 8825-1:2021 as the identical international standard. The recommendation is international rather than specific to one country or region.
For exact conformance requirements, consult the official ITU-T X.690 edition page and the official English PDF.
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