Matroska / WebM signature (EBML 1A 45 DF A3)

Matroska (.mkv) and WebM files are EBML documents; they begin with the 4-byte EBML header magic 0x1A 0x45 0xDF 0xA3. The DocType element later in the header distinguishes 'matroska' from 'webm'.

file signature

magic: 1A 45 DF A3 @ offset 0

ascii: \x1aE\xdf\xa3

.mkv.mka.mks.webm video/x-matroska

spec: RFC 8794 (EBML) + Matroska/WebM specifications

1A 45 DF A3 is the EBML Header element ID, shared by all EBML documents (Matroska, WebM).
The DocType child element holds the ASCII string 'matroska' or 'webm'; that is what tells them apart.
WebM is a constrained Matroska profile (VP8/VP9/AV1 video, Vorbis/Opus audio).

provenance

source: https://www.rfc-editor.org/rfc/rfc8794#section-4.1
version: RFC 8794 (EBML), Section 4.1 EBML Header (EBML element ID 0x1A45DFA3)
retrieved: 2026-05-29 · updated: 2026-05-29T00:00:00Z

per-fact attribution:

ext.magic_hex — https://www.rfc-editor.org/rfc/rfc8794#section-4.1 Every EBML document starts with the EBML Header element whose ID is 0x1A45DFA3.
ext.notes — https://www.matroska.org/technical/elements.html DocType element ('matroska' vs 'webm') inside the EBML header distinguishes the two profiles; both share the 1A 45 DF A3 magic.

agent: curl -H 'accept: application/json' wire.phall.io/magic/matroska-webm or /magic/matroska-webm.json

Matroska / WebM signature (EBML 1A 45 DF A3)

file signature

provenance

see also