64 lines
2.1 KiB
Python
64 lines
2.1 KiB
Python
"""
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Sequence-pattern classifier for TCP/IP fields that are useful as a tooling
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fingerprint when sampled across multiple packets from the same source.
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Two callers today:
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- IP-ID sequence per attacker (random/incremental/zero/constant).
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- TCP ISN sequence per attacker; modern stacks randomise, so a non-random
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result is itself a strong signal (legacy stacks, custom raw-socket tools).
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Pure stdlib so it stays trivially unit-testable.
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"""
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from __future__ import annotations
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import statistics
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# Minimum samples needed for a meaningful classification. Below this we
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# return "unknown" rather than guess from 1-3 noisy values.
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_MIN_SAMPLES = 4
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# Max plausible delta for an "incremental" classification. The IP-ID field
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# is 16-bit so kernel-emitted increments wrap rapidly under load — anything
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# over 4096 between consecutive SYNs from the same host is almost certainly
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# random rather than a counter we just happen to be sampling sparsely.
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_INCREMENTAL_MAX_DELTA = 0x1000
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# Coefficient-of-variation threshold above which we call a sequence random.
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# stddev/mean > 0.5 is well past anything a counter would produce.
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_RANDOM_CV_THRESHOLD = 0.5
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def classify_sequence(samples: list[int]) -> str:
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"""
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Classify an integer sequence as one of:
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- "zero": every sample is 0
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- "constant": every sample is the same non-zero value
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- "incremental": strictly monotonic with small positive deltas
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- "random": high coefficient of variation, no monotonic pattern
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- "unknown": fewer than _MIN_SAMPLES samples
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Order is preserved — pass the deque/list in arrival order.
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"""
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if len(samples) < _MIN_SAMPLES:
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return "unknown"
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if all(s == 0 for s in samples):
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return "zero"
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first = samples[0]
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if all(s == first for s in samples):
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return "constant"
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deltas = [b - a for a, b in zip(samples, samples[1:])]
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if all(0 < d <= _INCREMENTAL_MAX_DELTA for d in deltas):
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return "incremental"
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mean = statistics.fmean(samples)
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if mean > 0:
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stdev = statistics.pstdev(samples)
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if stdev / mean > _RANDOM_CV_THRESHOLD:
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return "random"
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return "random"
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