HF_ST25_TEAROFF — ST25TB Tear-Off / Counter Restore

Author: Doegox, Iceman Frequency: HF (13.56 MHz) Hardware: RDV4 with flash (required)

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What

Store and restore ST25TB/SRx tags using a power tear-off technique on their decrementing counters (blocks 5 and 6). These counters normally can only count down, but a precisely timed tear-off can corrupt and reset them.

Why

ST25TB and SRx NFC tags (used in transport tickets, access systems, etc.) contain one-way counters that decrement on each use. Normally these counters cannot be reset. The tear-off technique exploits the fact that if the RF field drops at the precise moment a counter write is completing, the write may fail or partially complete — potentially restoring a previous counter value. This enables research into counter-based anti-replay mechanisms.

How

1. LEARN mode: Reads up to 8 ST25TB tags and stores their complete memory dump (including counter blocks 5 & 6) to Proxmark3 flash memory.
2. RESTORE mode: Reads a previously-learned tag, compares current counter values to stored values, and if the counters have decremented, attempts a tear-off write to restore the original counter values.
3. Tear-off mechanism: Rapidly toggles the RF field at the precise moment the counter write completes, attempting to corrupt the write. Retries with varied timing until the counter is restored or the maximum attempt count is reached.

LED Indicators

Button Controls

State Machine

stateDiagram-v2
    [*] --> LEARN : Startup

    state LEARN {
        WaitTag_L --> ReadTag : Tag detected
        ReadTag --> StoreFlash : Read all blocks
        StoreFlash --> WaitTag_L : Saved (up to 8 tags)
    }

    state RESTORE {
        WaitTag_R --> MatchTag : Tag detected
        MatchTag --> CompareCounters : UID matched to stored
        CompareCounters --> TearOff : Counter decremented
        CompareCounters --> SkipRestore : Counter matches stored
        TearOff --> VerifyRestore : RF tear-off attempt
        VerifyRestore --> Success : Counter restored
        VerifyRestore --> TearOff : Counter still low, retry
        VerifyRestore --> Fail : Max attempts reached
        Success --> WaitTag_R
        SkipRestore --> WaitTag_R
        Fail --> WaitTag_R
    }

    LEARN --> RESTORE : Button click
    RESTORE --> LEARN : Button click

    LEARN --> [*] : Long hold
    RESTORE --> [*] : Long hold

Stored Data

Compilation

make clean
make STANDALONE=HF_ST25_TEAROFF -j
./pm3-flash-fullimage

Related

• ST25TA / IKEA Rothult — Similar ST25 family, different attack