SIM OTA Secured-Packet Decoder

What a SIM OTA secured packet is

SIM cards can be updated remotely by an operator using Over-The-Air (OTA) messages carried inside binary SMS. Those messages are wrapped in the secured-packet format defined by GSM 03.48 (3GPP TS 23.048, later ETSI TS 102 225). Each command packet starts with a fixed header that says how the update is protected — before any encrypted payload begins. This tool reads that header structure so a tester can see how a campaign was secured; it does not access the payload content and needs no keys.

The header fields

The header opens with two length fields, CPL (command packet length) and CHL (command header length), followed by the two-byte SPI (Security Parameter Indicator). The SPI bits state whether the payload is ciphered, which integrity mechanism is used (none, a redundancy check, a cryptographic checksum, or a digital signature), how the replay counter is checked, and whether a Proof of Receipt is requested. Next come KIc and KID — the key-set identifiers and algorithm family (DES/3DES or AES) for ciphering and for integrity — the three-byte TAR (the target applet on the card), a five-byte anti-replay counter, and a padding-counter byte. Only lengths are read for the secured data that follows.

Structure only — never the content

This is a read-only inspector for the security flags of an OTA campaign. It reports the SPI, KIc/KID, TAR, counter and lengths, and nothing more: no key material is entered, no cipher runs, and the protected payload is never read. Everything happens in your browser — nothing is uploaded. If a trace also carries subscriber identifiers, run it through the masker first.