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Merged
ps2 merged 1 commit into from
Oct 11, 2026
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Identify genuine vs simulated G7 sensors from their identity certificate #86

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8 changes: 8 additions & 0 deletions G7SensorKit.xcodeproj/project.pbxproj
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Original file line number Diff line number Diff line change
Expand Up @@ -32,6 +32,7 @@
69AE26EEA16A596AA47DF83A /* G7DexcomAppWarningView.swift in Sources */ = {isa = PBXBuildFile; fileRef = 11ED0ADBD1E5197198A0B215 /* G7DexcomAppWarningView.swift */; };
6F0232BD04ED7FD3626E0D52 /* G7ShareTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = BA54130B258935826428FB7C /* G7ShareTests.swift */; };
70DD3285A1241531F66439E3 /* G7DexcomCredentials.swift in Sources */ = {isa = PBXBuildFile; fileRef = 31D1424E75ED6303B966EF89 /* G7DexcomCredentials.swift */; };
FB2B6214AAB554E3EE99A456 /* G7SensorIdentity.swift in Sources */ = {isa = PBXBuildFile; fileRef = E9DD3001208B28CF3DE59FB6 /* G7SensorIdentity.swift */; };
72A6F9D68F93AE44814C3C3D /* G7StartupView.swift in Sources */ = {isa = PBXBuildFile; fileRef = 090FCA4FBE539EA494492C16 /* G7StartupView.swift */; };
74F6EF71F734A96B0DB47F52 /* G7PeripheralManager+Handshake.swift in Sources */ = {isa = PBXBuildFile; fileRef = 6180916611D0990A46355025 /* G7PeripheralManager+Handshake.swift */; };
77A93A0B05D5ADEDF217770B /* G7PairingView.swift in Sources */ = {isa = PBXBuildFile; fileRef = BF7CEC5E1884364C2F6DCCF4 /* G7PairingView.swift */; };
Expand Down Expand Up @@ -111,6 +112,7 @@
EE80FBB62E9FF65D5946BBD7 /* G7ChallengeSigner.swift in Sources */ = {isa = PBXBuildFile; fileRef = B1A771C4DCFFA924E278E352 /* G7ChallengeSigner.swift */; };
EF41639FF51B727324A04A49 /* G7NotificationPermissionsView.swift in Sources */ = {isa = PBXBuildFile; fileRef = F3BAC6D6BA20FCF904FA1465 /* G7NotificationPermissionsView.swift */; };
F5F8174E48028D9B111B8C59 /* G7JPAKETests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 6888359A74F9F636FBDCA8BB /* G7JPAKETests.swift */; };
CE1322361390E1AAD4E94A46 /* G7SensorIdentityTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 051E43BB8C8DAC1C57549FF0 /* G7SensorIdentityTests.swift */; };
F998BB980898C160A2440473 /* G7AuthCryptoTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 94A7A4E8FF66BE03B02B2E01 /* G7AuthCryptoTests.swift */; };
FD0DF29C00B05DECD53A4E2C /* G7CalibrationMessage.swift in Sources */ = {isa = PBXBuildFile; fileRef = DF238DACCD930310363D3EB5 /* G7CalibrationMessage.swift */; };
/* End PBXBuildFile section */
Expand Down Expand Up @@ -173,6 +175,7 @@
21F6EDAE3E7F2B5FF3E87922 /* G7PairingPlannerTests.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = G7PairingPlannerTests.swift; sourceTree = "<group>"; };
2A301AE5854C83E5BD9C6539 /* G7SensorModel.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = G7SensorModel.swift; sourceTree = "<group>"; };
31D1424E75ED6303B966EF89 /* G7DexcomCredentials.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = G7DexcomCredentials.swift; sourceTree = "<group>"; };
E9DD3001208B28CF3DE59FB6 /* G7SensorIdentity.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = G7SensorIdentity.swift; sourceTree = "<group>"; };
324F8C967DDB24EB158A1A45 /* G7PCert.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = G7PCert.swift; sourceTree = "<group>"; };
3B0FD2A42D803BF000E5E921 /* LoopKitUI.framework */ = {isa = PBXFileReference; explicitFileType = wrapper.framework; path = LoopKitUI.framework; sourceTree = BUILT_PRODUCTS_DIR; };
43AE2890ABEAE0FB78B276F1 /* G7SensorPackageTests.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = G7SensorPackageTests.swift; sourceTree = "<group>"; };
Expand All @@ -185,6 +188,7 @@
64134474341A32F8DFDA266F /* G7P256Tests.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = G7P256Tests.swift; sourceTree = "<group>"; };
641A515C9511A961C9C88318 /* G7ShareUploader.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = G7ShareUploader.swift; sourceTree = "<group>"; };
6888359A74F9F636FBDCA8BB /* G7JPAKETests.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = G7JPAKETests.swift; sourceTree = "<group>"; };
051E43BB8C8DAC1C57549FF0 /* G7SensorIdentityTests.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = G7SensorIdentityTests.swift; sourceTree = "<group>"; };
6C50AD3678079467EE110CD1 /* G7AES.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = G7AES.swift; sourceTree = "<group>"; };
7028829033065F083529D6A8 /* G7PairingPlanner.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = G7PairingPlanner.swift; sourceTree = "<group>"; };
724358361262BEA3A8F375AA /* G7LifecycleAlertTests.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = G7LifecycleAlertTests.swift; sourceTree = "<group>"; };
Expand Down Expand Up @@ -324,6 +328,7 @@
6C50AD3678079467EE110CD1 /* G7AES.swift */,
B1A771C4DCFFA924E278E352 /* G7ChallengeSigner.swift */,
31D1424E75ED6303B966EF89 /* G7DexcomCredentials.swift */,
E9DD3001208B28CF3DE59FB6 /* G7SensorIdentity.swift */,
63E50F6710E2494A0E14A6D8 /* G7JPAKE.swift */,
324F8C967DDB24EB158A1A45 /* G7PCert.swift */,
);
Expand Down Expand Up @@ -423,6 +428,7 @@
64134474341A32F8DFDA266F /* G7P256Tests.swift */,
94A7A4E8FF66BE03B02B2E01 /* G7AuthCryptoTests.swift */,
6888359A74F9F636FBDCA8BB /* G7JPAKETests.swift */,
051E43BB8C8DAC1C57549FF0 /* G7SensorIdentityTests.swift */,
588CB2A449236A6A47F4579B /* G7SessionModeMigrationTests.swift */,
742542EDF25C573BA2E8B09A /* G7AdvertisementTests.swift */,
21F6EDAE3E7F2B5FF3E87922 /* G7PairingPlannerTests.swift */,
Expand Down Expand Up @@ -818,6 +824,7 @@
2ED1C8E18DD717A8FB98CD3C /* G7AES.swift in Sources */,
EE80FBB62E9FF65D5946BBD7 /* G7ChallengeSigner.swift in Sources */,
70DD3285A1241531F66439E3 /* G7DexcomCredentials.swift in Sources */,
FB2B6214AAB554E3EE99A456 /* G7SensorIdentity.swift in Sources */,
8BB9F821830FA00C1939FFE3 /* G7JPAKE.swift in Sources */,
E18DCCF3754672CBF129B139 /* G7PCert.swift in Sources */,
576A7AA8FC9457FE0F540A86 /* G7Authenticator.swift in Sources */,
Expand Down Expand Up @@ -852,6 +859,7 @@
8C80F065A36E33C97219C570 /* G7P256Tests.swift in Sources */,
F998BB980898C160A2440473 /* G7AuthCryptoTests.swift in Sources */,
F5F8174E48028D9B111B8C59 /* G7JPAKETests.swift in Sources */,
CE1322361390E1AAD4E94A46 /* G7SensorIdentityTests.swift in Sources */,
D3DA1DF230DBADA4CB37F401 /* G7SessionModeMigrationTests.swift in Sources */,
D2F6C2D8999636C3D4F31DB1 /* G7AdvertisementTests.swift in Sources */,
97F1FB097E42B4F376FA206F /* G7PairingPlannerTests.swift in Sources */,
Expand Down
115 changes: 115 additions & 0 deletions G7SensorKit/Crypto/G7SensorIdentity.swift
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@@ -0,0 +1,115 @@
//
// G7SensorIdentity.swift
// G7SensorKit
//
// Copyright © 2026 LoopKit Authors. All rights reserved.
//

import CryptoKit
import Foundation
import Security

/// Whether the sensor on the other end of a direct connection is a genuine
/// Dexcom sensor, the g7sim development emulator, or could not be verified.
///
/// A real G7 proves itself during pairing: it streams an X.509 P-256
/// certificate chain whose leaf is signed by Dexcom's sensor intermediate
/// `DEX01PG2`, and signs the display's key-challenge nonce with that leaf key
/// (ECDSA-P256 over SHA256(nonce)). The g7sim emulator mirrors this with its
/// own published CA, so the two are distinguishable. This is a safety marker —
/// it never blocks a connection, and `.unverified` is treated like a real
/// sensor (older sensors, or a capture we cannot parse, simply are not
/// flagged). The emulator's key is public, so `.simulated` only ever errs
/// toward "this is simulated data", never the reverse for a real sensor.
public enum G7SensorAuthenticity: String, Equatable {
case genuine
case simulated
case unverified
}

enum G7SensorIdentity {

/// Dexcom's sensor intermediate (`CN=DEX01PG2`), captured from a real G7 on
/// 2026-10-10. A genuine sensor's leaf is signed by this.
static let dexcomSensorAnchor = Data(hexadecimalString: [
"308201e93082018fa00302010202145bb833870d8ff2cb8bfb33c39c1ba24f96",
"c8b9cd300a06082a8648ce3d04030230133111300f06035504030c0844455830",
"30504731301e170d3233303931333137323830355a170d333830393039313732",
"3830355a30133111300f06035504030c0844455830315047323059301306072a",
"8648ce3d020106082a8648ce3d0301070342000432959b9acf8602090669f3be",
"8834790b9635ccf48bcc92e072c0bad63ba823cd5a84d4e9ea415d3ed64efabd",
"b3a04f9c4ba723af4a751d020ce745700ad6c4a8a381c03081bd300f0603551d",
"130101ff040530030101ff301f0603551d230418301680149e0f1e36f3f276a7",
"01fe8e883a6e26a635bd6afc305a0603551d1f04533051304fa034a032863068",
"7474703a2f2f63726c2e64702e736161732e7072696d656b65792e636f6d2f63",
"726c2f44455830305047312e63726ca217a41530133111300f06035504030c08",
"4445583030504731301d0603551d0e04160414d0d56feb7d94f4535aa0cadd41",
"283536cb3aa2bf300e0603551d0f0101ff040403020186300a06082a8648ce3d",
"0403020348003045022100d023f2223574705f40ab3d04a658a367392167c35c",
"c22be4e7788c1de0c370350220532ef909c220944665bd9ac13cf0f4980d7b1f",
"5bea1f2ca0de1c63be876ae566"
].joined())!

/// The g7sim emulator's CA. Its signing key is public (a development
/// marker), so this proves "simulated", not authenticity.
static let emulatorAnchor = Data(hexadecimalString: [
"308201573081fda003020102020101300a06082a8648ce3d0403023013311130",
"0f06035504031308473753494d2d4341301e170d323630313031303030303030",
"5a170d3436303130313030303030305a30133111300f06035504031308473753",
"494d2d43413059301306072a8648ce3d020106082a8648ce3d03010703420004",
"4ebb9d03cece8ddd5d96d6a350615ea4b6e4ca6817bee3bac0938fb9045da5a9",
"cfda605269304703d2ebc1658d791be44831a3691dc27b3e83af202ed94ac673",
"a3423040300e0603551d0f0101ff040403020284300f0603551d130101ff0405",
"30030101ff301d0603551d0e0416041461e808e54c75334680ca4d6a2642a4c1",
"3ed34aed300a06082a8648ce3d0403020349003046022100d432d52df4713636",
"ecff04699d54cdce48946a0ed30dec1ab65dea4e22317b2c022100aecf28201b",
"e53e58221a1623de554be8a2b5c71e9497a6177fb8c3edf4871027"
].joined())!

/// Classifies a sensor from its leaf certificate, its 64-byte key-challenge
/// signature (raw r‖s), and the nonce the display sent in the `0C` request.
/// Never throws; anything it cannot verify is `.unverified`.
static func authenticity(leafCertificate: Data, challengeSignature: Data, nonce: Data) -> G7SensorAuthenticity {
guard challengeSignature.count == 64,
let leaf = SecCertificateCreateWithData(nil, leafCertificate as CFData),
let leafKey = SecCertificateCopyKey(leaf),
let leafPublicKey = SecKeyCopyExternalRepresentation(leafKey, nil) as Data?,
let publicKey = try? P256.Signing.PublicKey(x963Representation: leafPublicKey),
let signature = try? P256.Signing.ECDSASignature(rawRepresentation: challengeSignature)
else {
return .unverified
}

// The sensor must have signed our nonce with the leaf key: proof it
// holds the private key of the certificate it presented.
guard publicKey.isValidSignature(signature, for: nonce) else {
return .unverified
}

if chains(leaf, toAnchor: dexcomSensorAnchor) {
return .genuine
}
if chains(leaf, toAnchor: emulatorAnchor) {
return .simulated
}
return .unverified
}

/// Whether `leaf` is signed by the single pinned `anchor` (used as the only
/// trust anchor, so neither system roots nor the sensor's own streamed
/// intermediate can substitute for it).
private static func chains(_ leaf: SecCertificate, toAnchor anchorDER: Data) -> Bool {
guard let anchor = SecCertificateCreateWithData(nil, anchorDER as CFData) else {
return false
}
var trust: SecTrust?
guard SecTrustCreateWithCertificates([leaf, anchor] as CFArray, SecPolicyCreateBasicX509(), &trust) == errSecSuccess,
let trust = trust
else {
return false
}
SecTrustSetAnchorCertificates(trust, [anchor] as CFArray)
SecTrustSetAnchorCertificatesOnly(trust, true)
return SecTrustEvaluateWithError(trust, nil)
}
}
46 changes: 39 additions & 7 deletions G7SensorKit/G7CGMManager/G7Authenticator.swift
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Expand Up @@ -111,6 +111,10 @@ final class G7Authenticator {
/// Whether this run performed the EC-JPAKE exchange (as opposed to
/// reusing a stored key).
let didExchangeKeys: Bool
/// Whether the sensor proved itself genuine Dexcom, the g7sim emulator,
/// or could not be verified. Only meaningful after a fresh pairing (a
/// reconnect does not re-exchange certificates); `.unverified` then.
let authenticity: G7SensorAuthenticity
}

/// Per-step response deadline for a reconnect. Short: the link is up and
Expand All @@ -134,6 +138,13 @@ final class G7Authenticator {
/// Which of the sensor's display slots this client takes.
private let displayType: G7DisplayType

/// Captured during a fresh pairing to classify the sensor afterwards: its
/// leaf certificate, the nonce we sent in the key challenge, and the
/// signature it returned.
private var sensorLeafCertificate: Data?
private var keyChallengeNonce: Data?
private var keyChallengeSignature: Data?

/// Receives a short description of each step as it happens, for the
/// in-app device communication log. A tester reporting "it got stuck"
/// is only diagnosable if these are visible outside Xcode. Never carries
Expand Down Expand Up @@ -226,7 +237,7 @@ final class G7Authenticator {
// pairing; so do we.
if !didExchangeKeys, status.isAuthenticated, status.isBonded {
report("Already authenticated and bonded")
return Result(sharedKey: sharedKey, deviceName: peripheral.peripheral.name, didExchangeKeys: didExchangeKeys)
return Result(sharedKey: sharedKey, deviceName: peripheral.peripheral.name, didExchangeKeys: didExchangeKeys, authenticity: .unverified)
}

if !didExchangeKeys {
Expand All @@ -244,13 +255,27 @@ final class G7Authenticator {
try answerKeyChallenge(peripheral, authentication: authentication)
try finalizeAndBond(peripheral, authentication: authentication)

let authenticity = evaluateAuthenticity()
report("Sensor authenticity: \(authenticity.rawValue)")
return Result(
sharedKey: sharedKey,
deviceName: peripheral.peripheral.name,
didExchangeKeys: didExchangeKeys
didExchangeKeys: didExchangeKeys,
authenticity: authenticity
)
}

/// Classifies the sensor from what it presented during this fresh pairing.
private func evaluateAuthenticity() -> G7SensorAuthenticity {
guard let leaf = sensorLeafCertificate,
let nonce = keyChallengeNonce,
let signature = keyChallengeSignature
else {
return .unverified
}
return G7SensorIdentity.authenticity(leafCertificate: leaf, challengeSignature: signature, nonce: nonce)
}

// MARK: - EC-JPAKE

private func exchangeKeys(_ peripheral: G7PeripheralManager) throws -> Data {
Expand Down Expand Up @@ -389,6 +414,11 @@ final class G7Authenticator {
}

let theirs = try waitForCertificateBytes(peripheral, buffer: buffer, count: expectedLength, step: step)
// Index 1 is the sensor's leaf certificate (index 0 is the issuing
// intermediate), which classifies the sensor after the handshake.
if index == 1 {
sensorLeafCertificate = theirs
}
report("\(step): received \(theirs.count) bytes, sending ours (\(certificate.count) bytes)")
try peripheral.writeCertificateBytes(certificate)
}
Expand All @@ -400,8 +430,10 @@ final class G7Authenticator {
report("\(step): sending a nonce")

let buffer = installCertificateBuffer(peripheral)
let nonce = G7JPAKE.secureRandomBytes(16)
keyChallengeNonce = nonce
try peripheral.writeValue(
Data([0x0C]) + G7JPAKE.secureRandomBytes(16),
Data([0x0C]) + nonce,
for: .authentication,
type: .withResponse
)
Expand All @@ -412,10 +444,10 @@ final class G7Authenticator {
throw G7AuthenticatorError.unexpectedResponse(step: step, response: acknowledgement)
}

// The sensor's own 64-byte block arrives alongside. We do not verify
// it: the sensor is checking us, not the other way round, and the
// signing key it would be checked against is the one we just sent.
_ = try waitForCertificateBytes(peripheral, buffer: buffer, count: 64, step: step)
// The sensor's own 64-byte block arrives alongside: an ECDSA-P256
// signature over SHA256(our nonce) with its leaf key, which classifies
// the sensor (G7SensorIdentity). It is not needed to complete pairing.
keyChallengeSignature = try waitForCertificateBytes(peripheral, buffer: buffer, count: 64, step: step)

let signature = try G7ChallengeSigner.sign(challengeAcknowledgement: acknowledgement)
report("\(step): sending our signature")
Expand Down
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