Comprehensive Explanation

proem

Definition

In KERI's cryptographic architecture, a proem is a prepended derivation code that qualifies a cryptographic primitive by explicitly indicating the cryptographic algorithm or suite used for that primitive's derivation. The term "proem" literally means "introductory statement" or "preamble," which perfectly describes its function: it introduces and contextualizes the cryptographic data that follows.

When a cryptographic primitive (such as a digest, signature, or public key) is qualified with a proem, it becomes self-describing—the primitive carries its own metadata about how it was generated. This eliminates ambiguity about which cryptographic operations were applied and enables automated processing without external context.

Context in KERI/ACDC

Proems are fundamental to CESR (Composable Event Streaming Representation), KERI's dual text-binary encoding scheme. All cryptographic primitives expressed in CESR are qualified by definition, meaning they include a proem whether represented in text or binary format. This qualification is cited as an essential property of CESR in the IETF-KERI specification.

The proem serves two critical purposes:

1. Simplification: By embedding cryptographic metadata directly within the primitive, proems eliminate the need for separate schema definitions or external lookup tables to interpret the data. A verifier can immediately determine which algorithm was used simply by reading the proem.

2. Compactification: Rather than requiring verbose metadata structures, the proem provides a compact, standardized way to convey essential cryptographic information. This is particularly important for KERI's streaming protocols where bandwidth efficiency matters.

In practice, proems enable KERI's self-framing property—a parser can determine exactly how many characters (in text domain) or bytes (in binary domain) to extract from a stream for a given element without parsing the entire element. This supports and enables across text and binary domains.