PCI DSS 4.0 agent permission and delegated access architecture for enterprise LLM deployment — Reference Architecture

Reference architecture · published 2026-10-16 · regulated enterprise · PCI DSS 4.0 · layout diagram_first · implementation executed in CI before publish

The control that resolves this is an Intersecting Delegated Authority, evaluated before the request reaches a model provider rather than audited afterwards. In regulated enterprise the binding constraint is PCI DSS 4.0, and the question it actually asks is is cardholder data in scope, and can you prove it is not. The shape below holds p95 ≤ 600ms over REST over HTTPS, and ships with a standard-library implementation that runs as written.

The blueprint

Intersecting Delegated Authority — the request path, with the decision points that must exist before the first model call:

flowchart LR
    USER[Invoking user<br/>entitlements] --> INT{Intersect}
    GRANT[Agent grant<br/>time-boxed] --> INT
    INT --> EFF[Effective permissions]
    EFF --> ACT{Action in effective set?}
    ACT -->|no| DENY[Deny + audit reason]
    ACT -->|yes| DO[Execute as the user, not the agent]
    EFF --> HOP[Delegate to agent B] --> INT2{Intersect again} --> EFF2[Narrower still]

The parts you have to build

1. Principal entitlements — What the human may do, from the system of record.
2. Agent grant — What the agent may ever do, scoped per task.
3. Intersection at call time — Effective = user ∩ grant, computed per request.
4. Time-boxed grants — Expiry enforced, so delegation is not standing authority.
5. Chain intersection — Agent-to-agent delegation intersects again; a hop cannot widen.
6. Denial audit — Every denial recorded with the reason — the escalation signal.

Enterprise implementation parameters

Every value below is a parameter you can hold a vendor or an internal team to. Adjectives are not acceptance criteria.

The architecture friction point

The agent is given a service account so it can complete its work, and it now acts with more authority than any human who invokes it. An intern's request and an administrator's request reach the same powerful identity.

What PCI DSS 4.0 specifically requires

PCI scope is contagious: any system that stores, processes or transmits cardholder data — or can affect the security of one that does — is in scope. The cheapest control is to keep the model tier out of scope entirely.

The artifact that satisfies it is a segmentation test proving the model tier is out of scope. The way teams most often fail it: a PAN reaching a prompt log, dragging the whole inference tier into PCI scope.

Reference implementation — Delegated authority as an intersection, with expiry

Standard-library Python, no external dependencies, runs as written. Executed by the publish gate: if it raises, this page does not ship. Wire it behind REST over HTTPS; the failure mode it guards against is a retried POST creating a duplicate record. The output you hand the assessor is the segmentation test report.

"""Delegated authority: an agent never exceeds the user it acts for.

The failure: an agent is given a service account so it can "do its job", and it
then acts with more authority than any user who invokes it. Correct delegation
is an intersection — the agent may do only what BOTH its own grant and the
invoking user permit — and it must be time-boxed, because a standing delegation
is a standing privilege.
"""
from dataclasses import dataclass


@dataclass(frozen=True)
class Grant:
    subject: str
    permissions: frozenset
    expires_at: int

    def valid(self, now: int) -> bool:
        return now < self.expires_at


class DelegationError(PermissionError):
    pass


class DelegatedAuthority:
    """Effective permissions = user ∩ agent grant, never a union, and never
    the agent's own service identity."""

    def __init__(self):
        self.users: dict[str, frozenset] = {}
        self.grants: dict[str, Grant] = {}
        self.audit: list[dict] = []

    def define_user(self, user: str, permissions: set) -> None:
        self.users[user] = frozenset(permissions)

    def delegate(self, agent: str, permissions: set, expires_at: int) -> Grant:
        g = Grant(agent, frozenset(permissions), expires_at)
        self.grants[agent] = g
        return g

    def effective(self, agent: str, user: str, now: int) -> frozenset:
        g = self.grants.get(agent)
        if g is None:
            raise DelegationError(f"no grant for agent {agent!r}")
        if not g.valid(now):
            raise DelegationError(f"grant for {agent!r} expired")
        if user not in self.users:
            raise DelegationError(f"unknown principal {user!r}")
        return g.permissions & self.users[user]

    def authorise(self, agent: str, user: str, action: str, now: int) -> bool:
        try:
            allowed = self.effective(agent, user, now)
        except DelegationError as exc:
            self.audit.append({"agent": agent, "user": user, "action": action,
                               "allowed": False, "reason": str(exc)})
            return False
        ok = action in allowed
        self.audit.append({"agent": agent, "user": user, "action": action,
                           "allowed": ok,
                           "reason": "" if ok else "outside the intersection"})
        return ok

    def chain(self, agent_a: str, agent_b: str, user: str, now: int) -> frozenset:
        """A delegating to B cannot widen authority — chaining intersects
        again, which is what stops escalation through a hop."""
        return self.effective(agent_a, user, now) & self.grants[agent_b].permissions


if __name__ == "__main__":
    da = DelegatedAuthority()
    da.define_user("analyst", {"read_reports", "read_pii"})
    da.define_user("intern", {"read_reports"})
    da.delegate("summariser", {"read_reports", "read_pii", "delete_records"},
                expires_at=100)

    assert da.authorise("summariser", "analyst", "read_pii", now=1) is True
    # The intern cannot read PII, so neither can an agent acting for them.
    assert da.authorise("summariser", "intern", "read_pii", now=1) is False
    # Nobody granted delete to a user, so the agent's own grant cannot supply it.
    assert da.authorise("summariser", "analyst", "delete_records", now=1) is False

    eff = da.effective("summariser", "intern", now=1)
    assert eff == frozenset({"read_reports"}), eff

    # Expiry is enforced, not advisory.
    assert da.authorise("summariser", "analyst", "read_reports", now=101) is False
    assert "expired" in da.audit[-1]["reason"]

    # Chaining must not widen authority.
    da.delegate("exporter", {"read_reports", "export"}, expires_at=100)
    chained = da.chain("summariser", "exporter", "analyst", now=1)
    assert chained == frozenset({"read_reports"}), chained
    assert "export" not in chained, "chaining widened authority"

    assert da.authorise("unknown_agent", "analyst", "read_reports", now=1) is False

    print("Delegated authority intersection: OK")
    print(f"  audit entries {len(da.audit)}, "
          f"denials {sum(1 for a in da.audit if not a['allowed'])}")

The PCI DSS 4.0 control, as code

PAN tokenisation and a scope-reduction proof. This is the artifact an assessor asks for under PCI DSS 4.0 specifically — it is not the same control as the one above, and a system that implements one does not thereby implement the other. It executes under the same publish gate.

"""PAN detection with tokenisation, and a scope-reduction proof.

PCI DSS 4.0 scope is everything that stores, processes or transmits account
data, plus anything connected to it. The cheapest compliance move is not better
controls — it is removing systems from scope. A model gateway that never sees a
PAN is out of scope; one that sees it once is in, along with everything it talks
to.
"""
import hashlib
import re

PAN_RE = re.compile(r"\b(?:\d[ -]?){12,18}\d\b")


def luhn_valid(digits: str) -> bool:
    total, parity = 0, len(digits) % 2
    for i, ch in enumerate(digits):
        d = int(ch)
        if i % 2 == parity:
            d *= 2
            if d > 9:
                d -= 9
        total += d
    return total % 10 == 0


class TokenVault:
    """Format-preserving surrogate. The vault is in scope; nothing it feeds is,
    provided the surrogate is not reversible outside it."""

    def __init__(self, pepper: str):
        self._pepper = pepper
        self._map: dict[str, str] = {}

    def tokenise(self, pan: str) -> str:
        digest = hashlib.sha256(f"{self._pepper}:{pan}".encode()).hexdigest()
        surrogate = "4" + str(int(digest[:15], 16)).zfill(15)[:15]
        self._map[surrogate] = pan
        return surrogate

    def last_four(self, pan: str) -> str:
        return pan[-4:]


class ScopeBoundary:
    def __init__(self):
        self.systems: dict[str, bool] = {}
        self.flows: list[tuple[str, str]] = []

    def add(self, system: str, sees_pan: bool) -> None:
        self.systems[system] = sees_pan

    def flow(self, src: str, dst: str) -> None:
        self.flows.append((src, dst))

    def in_scope(self) -> set[str]:
        scope = {s for s, sees in self.systems.items() if sees}
        changed = True
        while changed:                      # connected-to is also in scope
            changed = False
            for src, dst in self.flows:
                if src in scope and dst not in scope:
                    scope.add(dst); changed = True
                elif dst in scope and src not in scope:
                    scope.add(src); changed = True
        return scope


def redact(text: str, vault: TokenVault) -> tuple[str, int]:
    found = 0
    def _sub(m):
        nonlocal found
        raw = re.sub(r"[ -]", "", m.group(0))
        if not luhn_valid(raw):
            return m.group(0)
        found += 1
        return f"tok_{vault.tokenise(raw)}(last4={vault.last_four(raw)})"
    return PAN_RE.sub(_sub, text), found


if __name__ == "__main__":
    vault = TokenVault(pepper="hsm-managed")
    text = "Charge card 4111 1111 1111 1111 for invoice 9912, ref 1234567890123456"
    out, n = redact(text, vault)
    assert n >= 1 and "4111 1111 1111 1111" not in out, out
    assert "last4=1111" in out, out

    # A number that fails Luhn is not a PAN and must not be mangled.
    benign, n2 = redact("order 1234 5678 9012 3450 placed", vault)
    assert n2 == 0 and "1234 5678 9012 3450" in benign, benign

    sb = ScopeBoundary()
    sb.add("checkout", True); sb.add("token_vault", True)
    sb.add("llm_gateway", False); sb.add("analytics", False)
    sb.flow("checkout", "token_vault"); sb.flow("token_vault", "llm_gateway")
    scope = sb.in_scope()
    assert "llm_gateway" in scope, "connected-to is in scope — that is the point"

    clean = ScopeBoundary()
    clean.add("checkout", True); clean.add("token_vault", True)
    clean.add("llm_gateway", False)
    clean.flow("checkout", "token_vault")
    assert "llm_gateway" not in clean.in_scope(), clean.in_scope()

    print("PCI DSS 4.0 tokenisation + scope reduction: OK")
    print(f"  PANs tokenised {n}, naive scope {len(scope)} systems, "
          f"segmented scope {len(clean.in_scope())} systems")

Risk register with acceptance criteria

Each row carries an acceptance parameter rather than an adjective, because "low latency" cannot be signed off. Retention here is bound by per the data class retention schedule, enforced automatically.

Questions to answer before approval

• Is the inference tier in PCI scope, and what segmentation test proves it is not?
  Cardholder data is tokenised before the model boundary and the segmentation is evidenced by an annual penetration test, so the tier stays out of scope by design.
• If an intern invokes the agent, can it read data only an analyst may see?
  No. Effective permissions are the intersection of the agent's grant and the intern's entitlements, so anything the intern lacks is unavailable regardless of the agent's own grant.
• What authority does the agent have when nobody has invoked it?
  None that matters. The grant is time-boxed and only becomes effective when intersected with an invoking principal, so there is no standing authority to borrow.
• Can agent A delegate to agent B and end up with more permissions than A had?
  No. Chaining intersects again, so each hop can only narrow the set. Widening through a hop is the escalation path this design exists to close.
• How quickly does a revoked entitlement take effect for an in-flight agent?
  Within the entitlement cache TTL, which is bounded by the revocation SLO. The stale-grant window is documented rather than assumed to be zero.
• What is recorded when an agent attempts something outside its effective set?
  The agent, the invoking user, the attempted action and the reason for denial. Repeated denials are the signal that something is probing for authority it does not have.

Related architectures

• Part of enterprise ai compliance architecture.
• The same control appears in PCI DSS 4.0 vendor evaluation criteria for enterprise AI implementation.
• PCI DSS 4.0 reference architecture for enterprise LLM deployment at scale applies the same mechanism under a different obligation.
• The same control appears in PCI DSS 4.0 model release gate and golden set evaluation for enterprise AI.
• PCI DSS 4.0 provider failover and degradation architecture for enterprise applies the same mechanism under a different obligation.
• If this is the blocker in front of you: enterprise AI infrastructure review.